Document:

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                                                                    EXHIBIT 4.20

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                               INDENTURE OF TRUST

                                     between

                              CLARK COUNTY, NEVADA

                                       and

                         HARRIS TRUST AND SAVINGS BANK,

                                   as Trustee

                         -------------------------------

                                   relating to

                              CLARK COUNTY, NEVADA
                      INDUSTRIAL DEVELOPMENT REVENUE BONDS
                       (SOUTHWEST GAS CORPORATION PROJECT)
                                  SERIES 1999A

                                       and

                              CLARK COUNTY, NEVADA
                      INDUSTRIAL DEVELOPMENT REVENUE BONDS
                       (SOUTHWEST GAS CORPORATION PROJECT)
                              TAXABLE SERIES 1999B

                          -----------------------------

                           Dated as of October 1, 1999

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<TABLE>
<S>                                                                                                             <C>
GRANTING CLAUSE FIRST ...........................................................................................2

GRANTING CLAUSE SECOND ..........................................................................................3

GRANTING CLAUSE THIRD ...........................................................................................3

ARTICLE I             DEFINITIONS AND RULES OF INTERPRETATION....................................................3

         Section 1.01.        RULES OF INTERPRETATION............................................................3

         Section 1.02.        DEFINITIONS........................................................................4

         Section 1.03.        NUMBER AND GENDER.................................................................14

         Section 1.04.        CONTENT OF CERTIFICATES AND OPINIONS..............................................14

ARTICLE II            THE BONDS.................................................................................14

         Section 2.01.        AUTHORIZED AMOUNT OF BONDS........................................................14

         Section 2.02.        ISSUANCE OF BONDS.................................................................15

         Section 2.03.        DETERMINATION OF RATE PERIODS AND INTEREST RATES..................................17

         Section 2.04.        OWNERSHIP, TRANSFER, EXCHANGE AND REGISTRATION OF BONDS...........................30

         Section 2.05.        EXECUTION OF BONDS................................................................31

         Section 2.06.        AUTHENTICATION....................................................................31

         Section 2.07.        FORM OF BONDS.....................................................................31

         Section 2.08.        MUTILATED, DESTROYED, LOST OR STOLEN BONDS........................................33

         Section 2.09.        TEMPORARY BONDS...................................................................33

         Section 2.10.        CANCELLATION AND DISPOSITION OF SURRENDERED BONDS.................................33

         Section 2.11.        USE OF CERTAIN MONEYS IN THE BOND FUND UPON REFUNDING.............................34

         Section 2.12.        DELIVERY OF THE BONDS.............................................................34

         Section 2.13.        BOOK-ENTRY SYSTEM.................................................................34

         Section 2.14.        DELIVERY OF THE BONDS.  DESIGNATION OF THE BONDS AS BOOK-ENTRY BONDS;
                              APPOINTMENT OF INITIAL SECURITIES DEPOSITORY FOR THE BONDS........................38

         Section 2.15.        CONVERSION OF SERIES 1999B BONDS TO TAX-EXEMPT SERIES.............................39

ARTICLE III           REDEMPTION OF BONDS BEFORE MATURITY.......................................................40

         Section 3.01.        REDEMPTION DATES AND PRICES.......................................................40

         Section 3.02.        NOTICE OF REDEMPTION..............................................................43

         Section 3.03.        DEPOSIT OF FUNDS..................................................................44
</TABLE>

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<TABLE>
<S>                                                                                                             <C>
         Section 3.04.        PARTIAL REDEMPTION OF BONDS.......................................................44

         Section 3.05.        SELECTION OF BONDS FOR REDEMPTION.................................................45

ARTICLE IV            TENDER AND PURCHASE OF BONDS; REMARKETING; REMARKETING AGENT..............................45

         Section 4.01.        PURCHASE OF BONDS AT OPTION OF OWNERS.............................................45

         Section 4.02.        MANDATORY PURCHASE OF BONDS.......................................................47

         Section 4.03.        OBLIGATION TO SURRENDER BONDS.....................................................48

         Section 4.04.        REMARKETING OF BONDS..............................................................48

         Section 4.05.        PURCHASE OF BONDS TENDERED TO TRUSTEE.............................................50

         Section 4.06.        DELIVERY OF PURCHASED BONDS.......................................................51

         Section 4.07.        NO SALES AFTER DEFAULT............................................................51

         Section 4.08.        REMARKETING AGENT.................................................................51

         Section 4.09.        QUALIFICATIONS OF REMARKETING AGENT...............................................52

         Section 4.10.        TENDER AND PURCHASE OF BOOK-ENTRY BONDS...........................................52

         Section 4.11.        DRAWS UPON THE LIQUIDITY FACILITY.................................................53

ARTICLE V             PAYMENT; FURTHER ASSURANCES...............................................................54

         Section 5.01.        PAYMENT OF PRINCIPAL OR REDEMPTION PRICE OF AND INTEREST ON BONDS.................54

         Section 5.02.        EXTENSION OR FUNDING OF CLAIMS FOR INTEREST.......................................54

         Section 5.03.        PRESERVATION OF REVENUES..........................................................54

         Section 5.04.        OTHER LIENS.......................................................................54

         Section 5.05.        COMPLIANCE WITH THE INDENTURE.....................................................54

         Section 5.06.        PERFORMANCE OF COVENANTS..........................................................54

         Section 5.07.        RIGHT TO PAYMENTS UNDER AGREEMENT; INSTRUMENTS OF FURTHER ASSURANCE...............55

         Section 5.08.        TAX COVENANTS.....................................................................55

         Section 5.09.        INSPECTION OF PROJECT BOOKS.......................................................56

         Section 5.10.        RIGHTS UNDER AGREEMENT............................................................56

         Section 5.11.        CONTINUING DISCLOSURE.............................................................56

ARTICLE VI            REVENUES AND FUNDS........................................................................56

         Section 6.01.        SOURCE OF PAYMENT OF BONDS; LIABILITY OF ISSUER LIMITED TO REVENUES...............56

         Section 6.02.        CREATION OF THE BOND FUND.........................................................57

         Section 6.03.        PAYMENTS INTO THE BOND FUND.......................................................57
</TABLE>

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<TABLE>
<S>                                                                                                             <C>
         Section 6.04.        USE OF MONEYS IN THE BOND FUND AND CERTAIN OTHER MONEYS...........................58

         Section 6.05.        CUSTODY OF THE BOND FUND..........................................................58

         Section 6.06.        CREATION OF THE CONSTRUCTION FUND; DISBURSEMENTS..................................58

         Section 6.07.        COSTS OF ISSUANCE FUND; DISBURSEMENTS.............................................59

         Section 6.08.        USE OF MONEYS IN CONSTRUCTION FUND UPON DEFAULT...................................59

         Section 6.09.        USE OF MONEYS IN CONSTRUCTION FUND UPON REDEMPTION................................60

         Section 6.10.        USE OF MONEYS IN CONSTRUCTION FUND UPON PAYMENT OF BONDS..........................60

         Section 6.11.        NON-PRESENTMENT OF BONDS..........................................................60

         Section 6.12.        TRUSTEE FEES, CHARGES AND EXPENSES................................................60

         Section 6.13.        MONEYS TO BE HELD IN TRUST........................................................60

         Section 6.14.        REPAYMENT TO THE BORROWER FROM THE BOND FUND......................................61

         Section 6.15.        REVENUES TO BE PAID OVER TO TRUSTEE...............................................61

         Section 6.16.        PAYMENTS OF PRINCIPAL AND INTEREST................................................61

         Section 6.17.        REVENUES TO BE HELD FOR ALL BONDHOLDERS; CERTAIN EXCEPTIONS.......................61

         Section 6.18.        REBATE FUND.......................................................................61

         Section 6.19.        BOND INSURANCE PAYMENTS...........................................................61

ARTICLE VII           INVESTMENT OF MONEYS......................................................................63

         Section 7.01.        INVESTMENT OF MONEYS..............................................................63

         Section 7.02.        INVESTMENTS; ARBITRAGE............................................................64

ARTICLE VIII          DEFEASANCE................................................................................64

         Section 8.01.        DEFEASANCE........................................................................64

ARTICLE IX            DEFAULT PROVISIONS AND REMEDIES OF TRUSTEE AND BONDHOLDERS................................66

         Section 9.01.        DEFAULTS; EVENTS OF DEFAULT.......................................................66

         Section 9.02.        ACCELERATION......................................................................67

         Section 9.03.        REMEDIES; RIGHTS OF BONDHOLDERS AND BOND INSURER..................................67

         Section 9.04.        RIGHT OF BONDHOLDERS TO DIRECT PROCEEDINGS........................................68
</TABLE>

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<TABLE>
<S>                                                                                                             <C>
         Section 9.05.        APPLICATION OF MONEYS.............................................................68

         Section 9.06.        REMEDIES VESTED IN TRUSTEE........................................................69

         Section 9.07.        RIGHTS AND REMEDIES OF BONDHOLDERS................................................70

         Section 9.08.        TERMINATION OF PROCEEDINGS........................................................70

         Section 9.09.        WAIVERS OF EVENTS OF DEFAULT......................................................70

         Section 9.10.        NOTICE OF EVENT OF DEFAULT UNDER SECTION 9.01(e) HEREOF; OPPORTUNITY OF
                              BORROWER TO CURE DEFAULTS.........................................................71

ARTICLE X             THE TRUSTEE...............................................................................72

         Section 10.01.       ACCEPTANCE OF THE TRUSTS BY TRUSTEE...............................................72

         Section 10.02.       CORPORATE TRUSTEE REQUIRED; ELIGIBILITY...........................................75

         Section 10.03.       FEES, CHARGES AND EXPENSES OF TRUSTEE.............................................76

         Section 10.04.       NOTICE TO BONDHOLDERS IF DEFAULT OCCURS...........................................76

         Section 10.05.       INTERVENTION BY TRUSTEE...........................................................76

         Section 10.06.       SUCCESSOR TRUSTEE.................................................................76

         Section 10.07.       RESIGNATION BY THE TRUSTEE........................................................76

         Section 10.08.       REMOVAL OF THE TRUSTEE............................................................77

         Section 10.09.       APPOINTMENT OF SUCCESSOR TRUSTEE..................................................77

         Section 10.10.       CONCERNING ANY SUCCESSOR TRUSTEES.................................................78

         Section 10.11.       TRUSTEE PROTECTED IN RELYING UPON RESOLUTION......................................78

         Section 10.12.       SUCCESSOR TRUSTEE AS THE TRUSTEE, PAYING AGENT, TENDER AGENT AND REGISTRAR........78

         Section 10.13.       NOTICES TO BE GIVEN BY TRUSTEE....................................................79

         Section 10.14.       NOTICES TO RATING AGENCY AND LIQUIDITY PROVIDER; NOTICES TO BOND INSURER..........79

ARTICLE XI            SUPPLEMENTAL INDENTURES...................................................................80

         Section 11.01.       SUPPLEMENTAL INDENTURES NOT REQUIRING CONSENT OF BONDHOLDERS (BUT
                              REQUIRING CONSENT OF THE BORROWER)................................................80

         Section 11.02.       SUPPLEMENTAL INDENTURES REQUIRING CONSENT OF BONDHOLDERS AND THE BORROWER.........82

         Section 11.03.       CONSENT OF BORROWER AND BOND INSURER..............................................82

         Section 11.04.       CONSENT OF REMARKETING AGENT AND LIQUIDITY PROVIDER...............................83

         Section 11.05.       CONSENT OF TRUSTEE................................................................83
</TABLE>

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<TABLE>
<S>                                                                                                             <C>
         Section 11.06.       REQUIRED AND PERMITTED OPINIONS OF COUNSEL........................................83

         Section 11.07.       NOTATION OF MODIFICATION ON BONDS; PREPARATION OF MODIFIED BONDS..................83

ARTICLE XII           AMENDMENT OF AGREEMENT....................................................................83

         Section 12.01.       AMENDMENTS TO AGREEMENT NOT REQUIRING CONSENT OF BONDHOLDERS......................83

         Section 12.02.       AMENDMENTS TO AGREEMENT REQUIRING CONSENT OF BONDHOLDERS..........................84

         Section 12.03.       CONSENT OF TRUSTEE................................................................85

         Section 12.04.       RELIANCE ON OPINIONS OF COUNSEL...................................................85

ARTICLE XIII          MISCELLANEOUS.............................................................................85

         Section 13.01.       SUCCESSORS OF THE ISSUER..........................................................85

         Section 13.02.       CONSENTS OF BONDHOLDERS...........................................................85

         Section 13.03.       LIMITATION OF RIGHTS..............................................................86

         Section 13.04.       WAIVER OF NOTICE..................................................................86

         Section 13.05.       SEVERABILITY......................................................................86

         Section 13.06.       NOTICES...........................................................................86

         Section 13.07.       WAIVER OF PERSONAL LIABILITY OF ISSUER MEMBERS, ETC...............................87

         Section 13.08.       HOLIDAYS..........................................................................88

         Section 13.09.       OPINIONS OF BOND COUNSEL..........................................................88

         Section 13.10.       COUNTERPARTS......................................................................88

         Section 13.11.       APPLICABLE LAW....................................................................88

         Section 13.12.       CAPTIONS..........................................................................88

         Section 13.13.       DEALING IN BONDS..................................................................88

         Section 13.14.       IMMUNITY OF INCORPORATORS.........................................................88

         Section 13.15.       BORROWER MAY ACT THROUGH AGENTS...................................................89

         Section 13.16.       RECORD DATE FOR DETERMINATION OF OWNERS ENTITLED TO VOTE..........................89

EXHIBIT A1            FORM OF SERIES 1999A BOND...............................................................A1-1

EXHIBIT A2            FORM OF TAXABLE SERIES 1999B BOND.......................................................A2-1

EXHIBIT B             FORM OF TAX-EXEMPT SERIES BOND...........................................................B-1

EXHIBIT C             TRUSTEE CERTIFICATE......................................................................C-1

EXHIBIT D             FORM OF COST OF ISSUANCE FUND REQUISITION................................................D-1
</TABLE>

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                  THIS INDENTURE OF TRUST, made and entered into as of October
1, 1999 (this "Indenture"), by and between CLARK COUNTY, NEVADA, a political
subdivision of the State of Nevada (the "Issuer"), and HARRIS TRUST AND SAVINGS
BANK, not in its individual capacity, but solely as Trustee (the "Trustee"), a
banking corporation organized, existing and authorized to accept and execute
trusts of the character herein set out under the laws of the State of Illinois.

                              W I T N E S S E T H:
                               - - - - - - - - - -

                  WHEREAS, the Issuer is a public instrumentality and political
subdivision of the State of Nevada organized and existing under the County
Economic Development Revenue Bond Law Sections 244A.669 to 244A.763, inclusive,
of the Nevada Revised Statutes, as supplemented and amended (the "Act"), and is
authorized by the Act to issue its revenue bonds for the purpose of paying all
or any part of the costs of a "project" as defined in the Act; and

                  WHEREAS, Southwest Gas Corporation, a California corporation
(the "Borrower"), has duly caused an application to be filed with the Issuer and
has requested that the Issuer issue bonds to finance or refinance a portion of
the cost of the acquisition, construction and installation of a "project" within
the meaning of the Act consisting of the upgrade, improvement, addition and
replacement of facilities for local furnishing of natural gas located in Clark
County, Nevada as more particularly described in Exhibit A of the Agreement (the
"Project"); and

                  WHEREAS, the Issuer, after due investigation and deliberation,
has adopted a resolution approving said request and authorizing the issuance of
bonds to finance or refinance a portion of the cost of the acquisition,
construction, and installation of the Project; and

                  WHEREAS, concurrently with the execution and delivery of this
Indenture, the Issuer is entering into a Financing Agreement, dated the date
hereof (the "Agreement"), with the Borrower specifying the terms and conditions
of the financing or refinancing of a portion of the cost of the acquisition and
construction of the Project by the Borrower, the lending of the proceeds of the
Issuer's Industrial Development Revenue Bonds (Southwest Gas Corporation
Project) Series 1999A (the "Series 1999A Bonds") and the Issuer's Industrial
Development Revenue Bonds (Southwest Gas Corporation Project) Taxable Series
1999B (the "Series 1999B Bonds") to the Borrower for such purpose, and the
repayment by the Borrower of such loan; and

                  WHEREAS, in order to provide for the authentication and
delivery of the Series 1999A Bonds and the Series 1999B Bonds (as more fully
defined herein, the "Bonds"), to establish and declare the terms and conditions
upon which the Bonds are to be issued and secured and to secure the payment of
the principal thereof and of the interest and premium, if any, thereon, the
Issuer has authorized the execution and delivery of this Indenture; and

                  WHEREAS, the Bonds are to be issued in a total aggregate
principal amount not to exceed $35,000,000 and are to be sold and delivered to
provide proceeds, as a loan to the Borrower, to finance or refinance a portion
of the cost of the acquisition, construction, and installation of the Project,
including the refinancing of such cost by refunding the Refunded Bonds, as
defined herein; and

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                  WHEREAS, the Bonds and the Trustee's certificate of
authentication and the form of assignment to be endorsed thereon shall be in
substantially the forms set forth in Exhibit A to this Indenture, with necessary
and appropriate variations, omissions and insertions as permitted or required by
this Indenture;

                  WHEREAS, in order to further enhance the payments of principal
of and interest on the Bonds, the Borrower has obtained municipal bond insurance
or financial guaranty insurance with respect to the Bonds, and in order to
provide for the purchase of tendered Bonds, the Borrower has arranged for the
provision of a liquidity facility with respect to certain of the Bonds;

                  WHEREAS, all acts and proceedings required by law necessary to
make the Bonds when executed by the Issuer, authenticated and delivered by the
Trustee and duly issued, the valid, binding and legal limited obligations of the
Issuer, and to constitute this Indenture a valid and binding agreement for the
uses and purposes herein set forth, in accordance with its terms, have been done
and taken; and the execution and delivery of this Indenture have been in all
respects duly authorized;

                  NOW, THEREFORE, THIS INDENTURE OF TRUST WITNESSETH:

                  That the Issuer in consideration of the premises, the
acceptance by the Trustee of the trust hereby created, the purchase and
acceptance of the Bonds by the purchasers thereof, one dollar duly paid to the
Issuer by the Trustee at or before the execution and delivery of these presents
and of other good and valuable considerations, the receipt of which is hereby
acknowledged, and in order to secure the payment of the principal of and
premium, if any, and interest on all Bonds outstanding hereunder from time to
time, according to their tenor and effect, and to secure the observance and
performance by the Issuer of all the covenants expressed or implied herein and
in the Bonds, does hereby pledge and assign unto the Trustee and unto its
successors and assigns forever;

                              GRANTING CLAUSE FIRST

                  The Agreement, including all extensions and renewals of the
term thereof, if any, together with all right, title and interest of the Issuer
therein (except for the right of the Issuer to the payment of costs, expenses
and indemnification pursuant to Sections 4.2(d), 4.2(e), 4.2(h) and 6.4 of the
Agreement and any rights of the Issuer to receive any notices, certificates,
requests, requisitions or other communications, to make inspections and to give
consents under the Agreement) including, but without limiting the generality of
the foregoing, the present and continuing right to receive, collect or make
claim for any of the moneys, income, revenues, issues, profits and other amounts
payable or receivable thereunder, including payments made by the Borrower under
the Agreement (excepting only payments made by the Borrower pursuant to the Tax
Certificate (as defined herein) in order to make rebate payments to the United
States), to bring actions and proceedings thereunder or for the enforcement
thereof, and to do any and all things which the Issuer or any other person is or
may become entitled to do under the Agreement;

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                             GRANTING CLAUSE SECOND

                  All Revenues (as defined herein) received by the Issuer under
the Agreement and all moneys and earnings thereon held by the Trustee in the
Construction Fund, the Costs of Issuance Fund, or in the Bond Fund under the
terms of this Indenture; and

                              GRANTING CLAUSE THIRD

                  Any and all other property of each name and nature from time
to time hereafter by delivery or by writing of any kind pledged or assigned as
and for additional security hereunder, by the Issuer or by anyone on its behalf
or with its written consent, to the Trustee, which are hereby authorized to
receive any and all such property at any and all times and to hold and apply the
same subject to the terms hereof.

                  TO HAVE AND TO HOLD all and singular the Trust Estate, whether
now owned or hereafter acquired, unto the Trustee and its successors in said
trusts and assigns forever.

                  IN TRUST NEVERTHELESS, upon the terms and trusts herein set
forth for the equal and proportionate benefit, security and protection of all
present and future owners of the Bonds, from time to time issued under and
secured by this Indenture without privilege, priority or distinction as to the
lien or otherwise of any of the Bonds over any of the other Bonds (except only
as otherwise expressly stated herein).

                  PROVIDED HOWEVER, that if the Issuer, its successors or
assigns, shall cause to be paid, the principal of the Bonds and the interest and
premium, if any, due or to become due thereon, at the times and in the manner
mentioned in the Bonds, according to the true intent and meaning thereof, and
shall cause the payments to be made into the Bond Fund as required under Article
VI hereof or shall provide, as permitted by Article VIII hereof, for the payment
thereof, and shall keep, perform and observe all the covenants and conditions
pursuant to the terms of this Indenture to be kept, performed and observed by
it, and shall pay or cause to be paid to the Trustee all sums of money due or to
become due in accordance with the terms and provisions hereof, then this
Indenture and the rights hereby granted shall cease, determine and be void
except as set forth in such Article VIII.

                  THIS INDENTURE OF TRUST FURTHER WITNESSETH, and it is
expressly declared, that all Bonds issued and secured hereunder are to be
issued, authenticated and delivered and the Trust Estate hereby assigned and
pledged are to be dealt with and disposed of under, upon and subject to the
terms, conditions, stipulations, covenants, agreements, trusts, uses and
purposes as hereinafter expressed, and the Issuer has agreed and covenanted, and
does hereby agree and covenant, with the Trustee and with the respective Owners
from time to time of the Bonds, as follows:

                                   ARTICLE I

                     DEFINITIONS AND RULES OF INTERPRETATION

         SECTION 1.01. RULES OF INTERPRETATION. For all purposes of this
Indenture, except as otherwise expressly provided or unless the context
otherwise requires:

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                  (A) All references in this Indenture to designated "Articles",
         "Sections" and other subdivisions are to the designated Articles,
         Sections and other subdivisions of this Indenture.

                  (B) The words "herein", "hereof", "hereto", "hereby", and
         "hereunder" and other words of similar import refer to this Indenture
         as a whole and not to any particular Article, Section or other
         subdivision.

                  (C) The terms defined in this Article have the meanings
         assigned to them in this Article, and include the plural as well as the
         singular.

                  (D) All accounting terms not otherwise defined herein have the
         meanings assigned to them in accordance with generally accepted
         accounting principles as in effect in the United States from time to
         time.

                  (E) Every "request", "order", "demand", "application",
         "appointment", "notice", "statement", "certificate" , "consent",
         "direction", "instruction" or similar action hereunder by the Issuer
         shall, unless the form thereof is specifically provided, be in writing
         signed by the Authorized Issuer Representative.

                  (F) All other terms used herein which are defined in the
         Agreement shall have the same meanings assigned them in the Agreement
         unless the context otherwise requires.

         SECTION 1.02. DEFINITIONS. In addition to the terms defined in the
recitals hereto, for all purposes of this Indenture and the Agreement, except as
otherwise expressly provided or unless the context otherwise requires:

         "Act" means the County Economic Development Revenue Bond Law Sections
244A.669 to 244A.763, inclusive, of the Nevada Revised Statutes, as amended and
supplemented.

         "Administrative Expenses" means any and all reasonable and necessary
expenses (including the reasonable and necessary out-of-pocket expenses and fees
of Counsel) incurred by the Issuer in connection with the Bonds, the Agreement,
this Indenture and any transaction or event contemplated by the Agreement or
this Indenture, and any agency of the State selected by the Issuer to act on its
behalf in connection with the Bonds, including any and all reasonable expenses
incurred by the Attorney General of the State in connection with any litigation
which may at any time be instituted involving the Bonds.

         "Agreement" means the Financing Agreement of even date herewith by and
between the Issuer and the Borrower, as from time to time amended and
supplemented.

         "Alternate Liquidity Facility" means any standby bond purchase
agreement or other liquidity facility meeting the requirements of Section 5.14
of the Agreement.

         "Authorized Borrower Representative" means the President, the Chief
Financial Officer, Treasurer or any Assistant Treasurer of the Borrower or any
person at the time

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designated to act on behalf of the Borrower by a written certificate furnished
to the Issuer, the Remarketing Agent, if any, and the Trustee containing the
specimen signature of such person and signed on behalf of the Borrower by any
officer of the Borrower. Such certificate may designate an alternate or
alternates.

         "Authorized Denominations" means (i) with respect to any Term Rate
Period or Taxable Term Rate Period, $5,000 and any integral multiple thereof;
and (ii) with respect to any other Rate Period, $100,000 and any integral
multiple of $5,000 in excess thereof, except that one Bond may be in a
denomination of any amount in excess of $100,000.

         "Authorized Issuer Representative" means the Chair of the Board of
Commissioners of the Issuer or any person at the time designated to act on
behalf of the Issuer by a written certificate furnished to the Borrower and the
Trustee containing the specimen signature of such person and signed on behalf of
the Issuer by the Chair of the Board of Commissioners of the Issuer.

         "Available Moneys" has meaning herein with respect only to Taxable
Weekly Bonds and Taxable Term Bonds (except Taxable Term Bonds where the term
ends at maturity thereof) and means (1) moneys derived from drawings under a
Liquidity Facility that are not commingled with any other moneys, (2) moneys
held by the Trustee (other than in the Rebate Fund) and subject to a
first-priority perfected lien under the Indenture for a period of at least
ninety-one (91) days and not commingled with any moneys so held for less than
said period and during and prior to which period no petition in bankruptcy was
filed by or against, and no receivership, insolvency, assignment for the benefit
of creditors or other similar proceeding has been commenced by or against the
Borrower or the Issuer, or (3) moneys for which there has been delivered to the
Trustee an opinion of nationally recognized bankruptcy counsel to the effect
that the payment of such moneys to the holders of such Bonds would not
constitute transfers avoidable under 11 U.S.C. Section 547(b) and recoverable
from the holders of such Bonds under 11 U.S.C. 550(a) should the Issuer or the
Borrower be the debtor in a case under the United States Bankruptcy Code.

         "Bond" or "Bonds" means any one or more of the bonds authorized,
authenticated and delivered under this Indenture.

         "Bond Counsel" means nationally recognized municipal bond counsel
mutually acceptable to the Issuer, the Trustee and the Borrower, but shall not
include Counsel to the Borrower.

         "Bond Fund" means the fund created by Section 6.02 hereof.

         "Bond Insurance" means, collectively, a Municipal Bond Insurance Policy
and any endorsements thereto with respect to the Series 1999A Bonds and any
Tax-Exempt Series into which any Series 1999B Bonds may be converted, and a
Financial Guaranty Insurance Policy and any endorsements thereto with respect to
the Series 1999B Bonds, each issued by the Bond Insurer on the date of issuance
of the Bonds.

         "Bondholder" or "holder" or "Owner" or "owner of Bonds" means the
Person or Persons in whose name or names a Bond shall be registered on books of
the Issuer kept by the

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Registrar for that purpose in accordance with the terms of this Indenture;
provided, however, with respect to Book-Entry Bonds the term "Owner" shall mean
the beneficial owners of the Bonds as the context may require.

         "Bond Insurer" shall mean Ambac Assurance Corporation, a
Wisconsin-domiciled stock insurance company, or any successor thereto.

         "Book-Entry Bond" means a Bond authorized to be issued to and, except
as provided in subsections (c) or (d) of Section 2.13 of this Indenture,
restricted to being registered in the name of a Securities Depository.

         "Borrower" means Southwest Gas Corporation, a California corporation
qualified to do business in the State, and its successors and assigns and any
surviving, resulting or transferee corporation as permitted in Section 5.2 of
the Agreement.

         "Business Day" means a day on which banking institutions located in New
York, New York, or in the city in which the principal corporate trust office of
the Trustee is located or the payment office for the Bond Insurer or the office
of the Liquidity Provider, if any, at which demands for payment of the Liquidity
Facility are to be presented, are not required or authorized to remain closed
and on which the New York Stock Exchange is not closed.

         "Code" means the Internal Revenue Code of 1986, and regulations
promulgated or proposed thereunder or (to the extent applicable) under prior
law, including temporary regulations.

         "Completion Date" means the date of completion of the Project as
certified under Section 5.3 of the Agreement.

         "Construction Fund" means the fund created by Section 6.06 hereof.

         "Cost" or "Cost of the Project" means the sum of the items authorized
to be paid from the Construction Fund pursuant to the provisions of paragraphs
(a) to (g), inclusive, of Section 3.3 of the Agreement.

         "Costs of Issuance" means all items of expense directly or indirectly
payable by or reimbursable to the Issuer or the Borrower and related to the
authorization, issuance, sale and delivery of the Bonds, including but not
limited to costs of preparation and reproduction of documents, printing
expenses, filing and recording fees, initial fees and charges of the Trustee,
legal fees and charges, fees and disbursements of consultants and professionals,
rating agency fees, fees and charges for preparation, execution and safekeeping
of the Bonds and any other cost, charge or fee in connection with the original
issuance of the Bonds which constitutes a "cost of issuance" within the meaning
of Section 147(g) of the Code.

         "Costs of Issuance Fund" means the fund by that name established
pursuant to Section 6.07 hereof.

         "Counsel" means an attorney at law or a firm of attorneys (who may be
an employee of or counsel to the Issuer or the Borrower or the Trustee, as
applicable) duly admitted

                                       6
<PAGE>   13

to the practice of law before the highest court of any state of the United
States of America or of the District of Columbia.

         "Daily Rate" means the interest rate on the Bonds of a Tax-Exempt
Series established in accordance with Section 2.03(a) hereof.

         "Daily Rate Period" means each period during which Bonds of a
Tax-Exempt Series bear interest at Daily Rates.

         "Dated Date" means (i) with respect to the Series 1999A Bonds, the
first day of the month in which such Bonds are issued, (ii) with respect to the
Series 1999B Bonds, the date of issuance and delivery of such Bonds to the
Initial Purchaser thereof, or (iii) with respect to Bonds of a Tax-Exempt Series
other than the Series 1999A Bonds, the Tax-Exempt Conversion Date.

         "Default" or "default" means any event which with the giving of notice,
the passage of time, or both, becomes an "event of default".

         "DTC" means The Depository Trust Company, a limited purpose trust
company organized under the laws of the State of New York, and its successors
and assigns.

         "Event of Default" or "event of default" means an occurrence or event
specified in and defined as such by Section 9.01 hereof.

         "Exempt Facilities" means facilities for the local furnishing of
electricity or natural gas within the meaning of Section 142(a) of the Code.

         "Expiration Date" means the earliest of (i) the stated expiration date
of the Liquidity Facility, (ii) the date on which the Liquidity Facility is
terminated pursuant to the terms of the Agreement or (iii) a Termination Date
under Section 7.2(b) of the Standby Bond Purchase Agreement.

         "Flexible Rate" means the interest rate on any Bonds of a Tax-Exempt
Series established in accordance with Section 2.03(d) hereof.

         "Flexible Rate Period" means each period, comprised of Flexible
Segments, during which Bonds of a Tax-Exempt Series bear interest at Flexible
Rates.

         "Flexible Segment" means, with respect to each Bond of a Tax-Exempt
Series bearing interest at a Flexible Rate, the period established in accordance
with Section 2.03(d) hereof.

         "Force Majeure" means acts of God, strikes, lockouts or other
industrial disturbances; acts of public enemies; orders or restraints of any
kind of the governments of the United States or of the State, or any of their
departments, agencies or officials, or any civil or military authority;
insurrections; riots; landslides; lightning; earthquakes; fires; tornadoes;
volcanoes; storms; droughts; floods; explosions, breakage, or malfunction or
accident to

                                       7
<PAGE>   14

machinery, transmission lines, pipes or canals, even if resulting from
negligence; civil disturbances; or any other cause not reasonably within the
control of the Borrower.

         The term "government obligations" means the obligations described in
Section 8.01(B)(a)(iii)(2) hereof.

         "Indenture" means this Indenture of Trust, including any indentures
supplemental hereto or amendatory hereof.

         "Initial Purchaser" means, collectively, Lehman Brothers Inc. and
Ramirez & Co., Inc.

         "Insurer Default" means any of the following events: (i) the Bond
Insurer shall fail, wholly or partially, to make a payment when and as required
under the provisions of the Bond Insurance; (ii) the Bond Insurance is
surrendered, cancelled or terminated, or amended or modified in any material
respect, without the Trustee's prior written consent; (iii) a court of competent
jurisdiction enters a final nonappealable judgment that the Bond Insurance is
not valid and binding on or enforceable against the Bond Insurer; or (iv) the
occurrence and continuation of one or more of the following: (A) the liquidation
or dissolution of the Bond Insurer; (B) the commencement by the Bond Insurer of
a voluntary case or other proceeding seeking liquidation, reorganization or
other relief with respect to itself or its debts under any bankruptcy,
insolvency or other similar law now or hereafter in effect, including without
limitation the appointment of a trustee, receiver, liquidator, custodian or
other similar official for itself or any substantial part of its property; (C)
the consent of the Bond Insurer to or the acquiescence by the Bond Insurer in
any case or proceeding described in the preceding clause (B) that is commenced
against it; (D) the making by the Bond Insurer of an assignment for the benefit
of creditors; (E) the failure of the Bond Insurer or the admission by the Bond
Insurer in writing of its inability generally to pay its debts or claims as they
become due; (F) the initiation by the Bond Insurer of any actions to authorize
any of the foregoing; (G) the commencement of an involuntary case or other
proceeding against the Bond Insurer seeking liquidation, reorganization or other
relief with respect to it or its debts under any bankruptcy, insolvency or other
similar law now or hereafter in effect or seeking the appointment of a trustee,
receiver, liquidator, custodian or other similar official of it or any
substantial part of its property, and such involuntary case remaining
undismissed and unstayed for a period of 60 days; or (H) the entering of an
order for relief against the Bond Insurer under the federal bankruptcy law as
now or hereafter in effect.

         "Interest Payment Date" means (a) in the case of the Series 1999A
Bonds, each April 1 and October 1, commencing April 1, 2000, and (b) in the case
of all other Bonds (i) with respect to any Daily Rate Period, Weekly Rate Period
or Taxable Weekly Rate Period, the first Business Day of each calendar month,
(ii) with respect to any Term Rate Period or Taxable Term Rate Period, the first
Business Day of the sixth calendar month following the effective date of such
Term Rate Period or Taxable Term Rate Period and the first Business Day of each
successive sixth calendar month, if any, of such Term Rate Period or Taxable
Term Rate Period, (iii) with respect to any Taxable Flexible Rate Segment or
Flexible Segment, the Business Day next succeeding the last day thereof, and
(iv) with respect to each Rate Period, the Business Day next succeeding the last
day thereof.

                                       8
<PAGE>   15

         "Investment Securities" means any of the following: (1) Permitted
Investments; (2) obligations, debentures, notes or other evidence of
indebtedness issued or guaranteed by any of the following: Farm Credit System
Financial Assistance Corporation, Export-Import Bank, Federal Financing Bank,
Government National Mortgage Association, Farmers' Home Administration, or
Federal Housing Administration; (3) obligations of any state government the
interest on which is exempt from federal income taxation for which a nationally
recognized rating service is maintaining a rating within the top two ratings of
such rating service; (4) repurchase agreements with reputable financial
institutions fully secured by collateral security actually delivered to the
Trustee described in clauses (1) or (2) of this definition continuously having a
market value at least equal to 102% of the amount so invested and approved in
writing by the Bond Insurer and S&P and Moody's at the time of such investment;
(5) bankers' acceptances maturing not more than 360 days from the date of
issuance issued by a bank (including the Trustee and its affiliates) which are
rated at least Aa by Moody's or rated AA by S&P and eligible for purchase by the
Federal Reserve Bank; (6) interest-bearing demand or time deposits (including
certificates of deposit) in banks (including the Trustee and its affiliates),
provided such deposits are (a) secured at all times, and are issued by a bank
which has a rating on its short-term certificates of deposit of "A-1" or better
by S&P or P-1 or better by Moody's and mature no more than 360 days after the
date of purchase or (b) fully insured by Federal deposit insurance; and (7)
commercial paper (including both non-interest-bearing discount obligations and
interest-bearing obligations payable on demand or on a specified date not more
than 270 days after the date of issuance thereof) which have been assigned the
rating of at least A-1 or better by S&P or P-1 or better by Moody's and approved
in writing by the Bond Insurer and S&P and Moody's at the time of such
investment.

         "Issuer" means Clark County, Nevada, as issuer of the Bonds.

         "Liquidity Facility" means, at any time as applicable, (i) the Standby
Bond Purchase Agreement among the Borrower, Bank One, NA and the Trustee, dated
as of October 1, 1999, as the same may be amended pursuant to its terms, and
(ii) in the event of delivery of an Alternate Liquidity Facility, such Alternate
Liquidity Facility, in each case acceptable to the Bond Insurer.

         "Liquidity Provider" means at any time any commercial bank, savings
association or other financial institution providing a Liquidity Facility then
in effect.

         "Liquidity Provider Bonds" shall have the meaning ascribed thereto in
Section 4.06(a)(i) hereof.

         "Moody's" means Moody's Investors Service, a corporation organized and
existing under the laws of the State of California, its successors and their
assigns, and, if such corporation shall be dissolved or liquidated or shall no
longer perform the functions of a securities rating agency, "Moody's" shall be
deemed to refer to any other nationally recognized securities rating agency
designated by the Borrower, with notice to the Trustee and the Liquidity
Provider, if any.

         "Outstanding" or "outstanding" or "Bonds Outstanding", in connection
with the Bonds means, as of the time in question, all Bonds authenticated and
delivered under this

                                       9
<PAGE>   16

Indenture, including, without limitation, Bonds deemed not defeased or satisfied
after the payment by the Bond Insurer of principal and interest on such Bonds in
accordance with Section 8.01 hereof, except:

                  A. Bonds theretofore canceled or required to be canceled under
         Section 2.10 or 6.16 hereof;

                  B. Bonds which are deemed to have been paid in accordance with
         Article VIII hereof; and

                  C. Bonds (including Bonds which are deemed to have been
         purchased pursuant to Section 4.03 hereof) in substitution for which
         other Bonds have been authenticated and delivered pursuant to Article
         II hereof.

In determining whether the Owners of a requisite aggregate principal amount of
outstanding Bonds have concurred in any request, demand, authorization,
direction, notice, consent or waiver under the provisions of this Indenture,
Bonds which are owned of record by the Borrower or any affiliate thereof or held
by the Trustee for the account of the Borrower shall be disregarded and deemed
not to be Outstanding hereunder for the purpose of any such determination
(except that, in determining whether the Trustee shall be protected in relying
upon any such request, demand, authorization, direction, notice, consent or
waiver, only Bonds which a Responsible Officer of the Trustee actually knows to
be so owned or held shall be disregarded) unless all Bonds are owned by the
Borrower or any affiliate thereof and/or held by the Trustee for the account of
the Borrower, in which case such Bonds shall be considered outstanding for the
purpose of such determination. For the purpose of this definition, an
"affiliate" of any specified Person means any other Person directly or
indirectly controlling or controlled by or under direct or indirect common
control with such specified Person and "control", when used with respect to any
specified Person, means the power to direct the management and policies of such
Person, directly or indirectly, whether through the ownership of voting
securities, by contract or otherwise; and the terms "controlling" and
"controlled" have meanings correlative to the foregoing.

         "Owner" is defined under the term "Bondholder."

         "Paying Agent" means the Trustee, acting as paying agent for the Bonds.

         "Permitted Investments" means (i) direct obligations of the United
States of America (including obligations issued or held in book-entry form on
the books of the Department of the Treasury of the United States of America);
(ii) obligations the timely payment of the principal of and interest on which
are fully guaranteed by the United States of America; (iii) money market funds
registered under the Investment Company Act of 1940, as amended, whose shares
are registered under the Securities Act of 1933, as amended, which invest only
in securities of the type described in clause (i) or (ii) of this definition and
having a rating by S&P of at least Aam-G or AAAm, and by Moody's of at least Aaa
or P-1; or (iv) certificates or receipts representing direct ownership interests
in future interest or principal payments on obligations described in clause (i)
or (ii) of this definition which are held by a custodian in safekeeping on
behalf of the holders of such certificates or receipts and approved in writing
by the Bond Insurer and S&P and Moody's at the time of such investments.

                                       10
<PAGE>   17

         "Person" means natural persons, firms, partnerships, associations,
corporations, trusts, limited liability companies and public bodies.

         "Plans and Specifications" means the plans and specifications for the
Project as prepared by the Borrower and heretofore approved by the Issuer.

         "Principal Office" means, with respect to the Trustee, the principal
corporate trust office of the Trustee, which office at the date of acceptance of
the Trustee of the duties and obligations imposed upon it hereunder is located
for the purposes and at the addresses specified in Section 13.06 hereof.

         "Project" means the facilities described in Exhibit A to the Agreement.

         "Rate Period" means any Daily Rate Period, Weekly Rate Period, Flexible
Rate Period, Taxable Flexible Rate Period, Taxable Weekly Rate Period , Taxable
Term Rate Period or Term Rate Period.

         "Rebate Fund" means the fund created by Section 6.18 hereof.

         "Record Date" means with respect to any Interest Payment Date in
respect of a Daily Rate Period, a Weekly Rate Period, a Taxable Flexible Rate
Segment, a Taxable Weekly Rate Period or a Flexible Segment, the Business Day
next preceding such Interest Payment Date and, with respect to any Interest
Payment Date in respect of a Term Rate Period or Taxable Term Rate Period, the
fifteenth day of the calendar month next preceding such Interest Payment Date.

         "Refunded Bonds" means the Issuer's Industrial Development Revenue
Bonds (Southwest Gas Corporation Project) Series 1998A.

         "Registrar" means the Trustee, acting as registrar for the Bonds.

         "Remarketing Agent" means the remarketing agent or agents appointed in
accordance with Section 4.08 hereof and any permitted successor or successors
thereto.

         "Resolution" means the resolution duly adopted and approved by the
governing body of the Issuer on March 2, 1999, authorizing the issuance and sale
of the Bonds and the execution and delivery of this Indenture and the Agreement
and the other documents and transactions contemplated herein or therein, and any
subsequent resolution relating to any of the Bonds which have not been issued as
of the date of such subsequent resolution.

         "Responsible Officer" means when used with respect to the Trustee, any
officer within the Principal Office of the Trustee including any Vice President,
Assistant Vice President, Secretary, Assistant Secretary or any other officer of
the Trustee customarily performing functions similar to those performed by any
of the above designated officers and also, with respect to a particular matter,
any other officer to whom such matter is referred because of such officer's
knowledge of and familiarity with the particular subject.

         "Revenues" means the amounts pledged hereunder to the payment of
principal of, and premium, if any, and interest on the Bonds, consisting of the
following: (i) all amounts

                                       11
<PAGE>   18

payable from time to time by the Borrower under Section 4.2(a) of the Agreement,
and all receipts of the Trustee credited under the provisions of this Indenture
against said amounts payable, (ii) any accrued interest on the Bonds deposited
with the Trustee under Section 6.03 hereof and (iii) any amounts paid into the
Bond Fund from the Construction Fund, including income or revenue derived from
the investment of moneys therein.

         "S.E.C." means the Securities and Exchange Commission of the United
States of America.

         "S&P" means Standard & Poor's Ratings Group, a division of The McGraw
Hill Companies, its successors and their assigns, and, if such securities rating
agency shall be dissolved or liquidated or shall no longer perform the functions
of a securities rating agency, "S&P" shall be deemed to refer to any other
nationally recognized securities rating agency designated by the Borrower, with
notice to the Trustee, the Bond Insurer and the Liquidity Provider, if any.

         "Series" means, as applicable, (i) the Series 1999A Bonds, (ii) the
Series 1999B Bonds or (iii) a Tax-Exempt Series other than the Series 1999A
Bonds.

         "Securities Depository" means, with respect to a Book-Entry Bond, DTC
or any person, firm, association or corporation constituting a "clearing agency"
(securities depository) registered under Section 17A of the Securities Exchange
Act of 1934, as amended, which may at any time be substituted in its place to
act as Securities Depository for the Bonds, or its successors, or any nominee
therefor.

         "State" means the State of Nevada.

         "Tax Certificate" means the Tax Certificate and Agreement, dated as of
the Dated Date of the Series 1999A Bonds, executed by the Issuer and the
Borrower, as the same may be amended and supplemented from time to time, and
with respect to any Series 1999B Bonds, converted to a Tax-Exempt Series of
Bonds on a particular Tax-Exempt Conversion Date, the Tax Certificate and
Agreement, dated such Tax-Exempt Conversion Date, executed by the Issuer and the
Borrower, as the same may be amended and supplemented from time to time.

         "Tax-Exempt" means, with respect to interest on any obligations of a
state or local government, including any Tax-Exempt Series of Bonds, that such
interest is excluded from the gross income of the Owners thereof (other than any
Owner who is a "substantial user" of facilities financed with such obligations
or a "related person" within the meaning of Section 147(a) of the Code) for
federal income tax purposes, whether or not such interest is includable as an
item of tax preference or otherwise includable directly or indirectly for
purposes of calculating other tax liabilities, including any alternative minimum
tax or environmental tax under the Code.

         "Tax-Exempt Conversion Date" means any date on which all or a portion
of the Series 1999B Bonds are converted to a Tax-Exempt Series as described in
Section 2.15 hereof.

         "Tax-Exempt Series" means, as applicable, the Series 1999A Bonds and
any Series 1999B Bonds as to which a Tax-Exempt Conversion Date has occurred.

                                       12
<PAGE>   19

         "Taxable Flexible Bonds" means any Bond that bears interest at a
Taxable Flexible Rate.

         "Taxable Flexible Rate" means with respect to any Taxable Flexible
Bond, the rate of interest on such Bond established in accordance with Section
2.03(e) hereof.

         "Taxable Flexible Rate Period" means with respect to each Taxable
Flexible Bond, each period, comprised of Taxable Flexible Rate Segments, during
which such Bond bears interest at a Taxable Flexible Rate.

         "Taxable Flexible Rate Segment" means, with respect to any Bond bearing
interest at the Taxable Flexible Rate, the period established in accordance with
Section 2.03(e) hereof.

         "Taxable Term Bond" means any Bond that bears interest at a Taxable
Term Rate.

         "Taxable Term Rate" means, as to any Bond, the interest rate on such
Bond established in accordance with Section 2.03(g) hereof.

         "Taxable Term Rate Period" means, as to any Taxable Term Bond, each
period during which a Taxable Term Rate is in effect.

         "Taxable Weekly Bonds" means any Bond that bears interest at a Taxable
Weekly Rate.

         "Taxable Weekly Rate" means with respect to any Taxable Weekly Bond,
the rate of interest on such Bond established in accordance with Section 2.03(f)
hereof.

         "Taxable Weekly Rate Period" means each period during which Bonds bear
interest at Taxable Weekly Rates.

         "TBMA Swap Index" means the TBMA Municipal Swap Index most recently
published in the Bond Buyer, or, if the Bond Buyer no longer publishes such
index or is no longer published, the variable rate index published in a
comparable periodical selected by the Remarketing Agent.

         "Tender Agent" means the Trustee, acting as tender agent for the Bonds.

         "Term Bond" means any Bond that bears interest at a Term Rate.

         "Term Rate" means the interest rate on the Bonds of a Tax-Exempt Series
established in accordance with Section 2.03(c) hereof.

         "Term Rate Period" means each period during which Bonds of a Tax-Exempt
Series bear interest at a Term Rate.

                                       13
<PAGE>   20

         "Trust Estate" means the property conveyed to the Trustee pursuant to
the Granting Clauses hereof.

         "Trustee" means Harris Trust and Savings Bank, an Illinois banking
corporation, and any successor trustee appointed and qualified pursuant to
Sections 10.02, 10.06 and 10.09 hereof at the time serving as successor Trustee
hereunder.

         "Weekly Rate" means the interest rate on the Bonds of a Tax-Exempt
Series established in accordance with Section 2.03(b) hereof.

         "Weekly Rate Period" means each period during which Bonds bear interest
at Weekly Rates.

         SECTION 1.03. NUMBER AND GENDER. The singular form of any word used
herein, including the terms defined in Section 1.02, shall include the plural,
and vice versa. The use herein of a word of any gender shall include all
genders.

         SECTION 1.04. CONTENT OF CERTIFICATES AND OPINIONS. Every certificate
or opinion with respect to compliance with a condition or covenant provided for
in this Indenture or the Agreement shall include (a) a statement that the person
or persons making or giving such certificate or opinion have read such covenant
or condition and the definitions herein relating thereto; (b) a brief statement
as to the nature and scope of the examination or investigation upon which the
statements or opinions contained in such certificate or opinion are based; (c) a
statement that, in the opinion of the signers, they have made or caused to be
made such examination or investigation as is necessary to enable them to express
an informed opinion as to whether or not such covenant or condition has been
complied with; and (d) a statement as to whether, in the opinion of the signers,
such condition or covenant has been complied with.

         Any such certificate or opinion made or given by an officer of the
Issuer or the Borrower may be based, insofar as it relates to legal matters,
upon a certificate or opinion of or representations by counsel, unless such
officer knows that the certificate or opinion or representations with respect to
the matters upon which his certificate or opinion may be based as aforesaid are
erroneous, or in the exercise of reasonable care should have known that the same
were erroneous. Any such certificate or opinion made or given by counsel may be
based, insofar as it relates to factual matters (with respect to which
information is in the possession of the Issuer or the Borrower), upon the
certificate or opinion of or representations by an officer of the Issuer or the
Borrower, as applicable, unless such counsel knows that the certificate or
opinion or representations with respect to the matters upon which his opinion
may be based as aforesaid are erroneous, or in the exercise of reasonable care
should have known that the same were erroneous.

                                   ARTICLE II

                                    THE BONDS

         SECTION 2.01. AUTHORIZED AMOUNT OF BONDS. No Bonds may be issued under
the provisions of this Indenture except in accordance with this Article. Except
as provided in Section 2.08 hereof, the total principal amount of Bonds that may
be issued

                                       14
<PAGE>   21

hereunder is hereby expressly limited to $35,000,000, consisting of Series 1999A
Bonds in an amount not to exceed $12,410,000 and Series 1999B Bonds in an amount
not to exceed $22,590,000. It is hereby recognized that the Issuer and the
Borrower have reserved the right to provide for the issuance of other
obligations not constituting Bonds pursuant to Section 4.1(b) of the Agreement.

         SECTION 2.02. ISSUANCE OF BONDS.

         (a) Authorization of Issuance. The Bonds are hereby authorized to be
issued, and upon such issuance the Trustee shall authenticate the Bonds and
deliver them as specified in a written request of the Issuer, as more fully
provided in Sections 2.06 and 2.12. Series 1999A Bonds shall be designated
"Clark County, Nevada Industrial Development Revenue Bonds (Southwest Gas
Corporation Project) Series 1999A." Until converted on a Tax-Exempt Conversion
Date, the Series 1999B Bonds shall be designated "Clark County, Nevada
Industrial Development Revenue Bonds (Southwest Gas Corporation Project) Taxable
Series 1999B." The Bonds shall be issuable as fully registered bonds without
coupons, in Authorized Denominations. Unless the Issuer shall otherwise direct,
the Series 1999A Bonds shall be numbered A-1 and upwards and the Series 1999B
Bonds shall be numbered TB-1 and upwards. It is anticipated that all or a
portion of the Series 1999B Bonds will be converted from time to time to Bonds
of one or more Tax-Exempt Series at the option of the Borrower. On and after any
Tax-Exempt Conversion Date with respect to Bonds of a Tax-Exempt Series, such
Bonds, at the option of the Borrower, may either (i) have the same designation
except that the Series designation shall be changed from "Taxable Series 1999B"
to "Series 1999_," with the blank completed by the next consecutive letter
designation or (ii) be combined with any Bonds of a Tax-Exempt Series (other
than Series 1999A Bonds) that are Outstanding, such that such Bonds shall have
the same Series designation as the Outstanding Tax-Exempt Series. Unless the
Issuer shall otherwise direct, the Bonds of each Tax-Exempt Series shall be
numbered separately from 1 and upwards and shall bear a letter designation
corresponding to the series of such Bonds. Bonds of each Tax-Exempt Series shall
bear a distinct CUSIP number from the Series 1999A Bonds, the Series 1999B Bonds
and from Bonds of any other Tax-Exempt Series.

         (b) General Terms. The Bonds shall be issued as fully registered bonds,
without coupons, in Authorized Denominations and shall be dated the Dated Date
and shall mature, subject to prior redemption or purchase upon the terms and
conditions hereinafter set forth, on December 1, 2038; provided, however, that
on any Tax-Exempt Conversion Date on which any Series 1999B Bonds may be
converted to a new Tax-Exempt Series, the Borrower may designate a new maturity
date for the Bonds of such Tax-Exempt Series, which maturity date shall be any
date on or prior to December 1, 2038. The Trustee shall insert the date of
authentication of each Bond in the place provided for such purpose in the form
of the certificate of authentication of the Trustee to be printed on each Bond.

         (c) Manner of Payment. The principal of and premium, if any, and
interest on the Bonds shall be payable in any coin or currency of the United
States of America which, at the respective dates of payment thereof, is legal
tender for the payment of public and private debts (which shall be in
immediately available funds), and, except as otherwise provided in Section 2.13
hereof with respect to Book-Entry Bonds, such principal and premium, if any, and
interest thereon shall be payable at the Principal Office of the Trustee, as
Paying Agent. Payment of

                                       15
<PAGE>   22

interest on any Interest Payment Date on any Bond shall be made to the Owner
thereof as of the close of business on the Record Date immediately prior thereto
and, except as otherwise provided in Section 2.13 hereof with respect to
Book-Entry Bonds, shall be (i) made by check or draft of the Trustee, as Paying
Agent, mailed on the Interest Payment Date to the Owner as of the close of
business on the Record Date immediately preceding the Interest Payment Date, at
the Owner's address as it appears on the registration books of the Issuer kept
by the Trustee or at such other address as is furnished to the Trustee in
writing by such Owner not later than the close of business on the Record Date
for such Interest Payment Date, or (ii) transmitted by wire transfer to the
account with a member of the Federal Reserve System located within the
continental United States of America of any Owner which owns at least $1,000,000
in aggregate principal amount of the Bonds of any Series and which shall have
provided wire transfer instructions to the Trustee prior to the close of
business on such Record Date, but, in the case of interest payable in respect of
a Flexible Segment or a Taxable Flexible Rate Segment, only upon presentation of
such Bond (if such Bond is not a Book-Entry Bond) at the Principal Office of the
Trustee for exchange or transfer in accordance with the provisions hereof,
except, in each case, that, if and to the extent that there shall be a default
in the payment of the interest due on such Interest Payment Date, such defaulted
interest shall be paid to the Owners in whose names any such Bonds are
registered at the close of business on the fifth (5th) Business Day next
preceding the date of payment of such defaulted interest. All payments will be
made in immediately available funds.

         (d) Interest. The Bonds shall bear interest from and including the
Dated Date until payment of the principal or the redemption or purchase price
thereof shall have been made or provided for in accordance with the provisions
hereof, whether at maturity, upon redemption or otherwise, at the rate or rates
per annum determined pursuant to Section 2.03 hereof. Interest on the Bonds
shall be paid on each applicable Interest Payment Date and at maturity or prior
redemption or purchase for the period commencing on the immediately preceding
Interest Payment Date (or if no interest has been paid thereon commencing on the
Dated Date) to but excluding such Interest Payment Date; provided, however, that
if, as shown by the records of the Trustee, interest on the Bonds shall be in
default, Bonds shall bear interest from the last date to which interest has been
paid in full or duly provided for on the Bonds or, if no interest has been paid
or duly provided for on the Bonds, from the Dated Date thereof. Each Bond shall
bear interest on overdue principal at the rate borne by the Bonds on the date on
which such principal became due and payable. During any Daily Rate Period,
Weekly Rate Period, Taxable Weekly Rate Period, Flexible Rate Period or Taxable
Flexible Rate Period, interest on the related Bonds shall be computed upon the
basis of a 365 or 366-day year, as applicable, for the number of days actually
elapsed. During any Term Rate Period or Taxable Term Rate Period, interest on
the Bonds shall be computed upon the basis of a 360-day year, consisting of
twelve (12) thirty (30) day months.

         (e) Proceeds of Sale. The proceeds received by the Issuer from the sale
of the Bonds shall be deposited with the Trustee, who shall forthwith deposit
such proceeds as set forth in a written request of the Issuer acknowledged by
the Borrower. Such request shall provide for such proceeds to be deposited as
set forth in this Section, as follows:

                  (i) to the Bond Fund, the amount of interest accrued on the
         Bonds from the Dated Date thereof to the date of issuance, such accrued
         interest on the Series 1999A

                                       16
<PAGE>   23

         Bonds to be deposited in the Series 1999A Account in the Bond Fund, and
         such accrued interest on the Series 1999B Bonds, if any, to be
         deposited in the Taxable Series 1999B Account in the Bond Fund;

                  (ii) to the Costs of Issuance Fund, the sum of $-0-,
         consisting of $-0- from proceeds of the Series 1999A Bonds to be
         deposited into the Series 1999A Costs of Issuance Account, and $-0-
         from proceeds of the Series 1999B Bonds to be deposited into the Series
         1999B Costs of Issuance Account;

                  (iii) to the Refunding Account created pursuant to this
         Section, the sum of $720,000 from proceeds of the Series 1999A Bonds;
         and

                  (iv) to the Construction Fund, the balance of such proceeds,
         consisting of $11,248,343.37 from proceeds of the Series 1999A Bonds to
         be deposited in the Series 1999A Construction Account and
         $21,839,339.13 from proceeds of the Series 1999B Bonds to be deposited
         in the Series 1999B Construction Account.

         The Trustee shall create a separate account, designated the Refunding
Account, into which the Trustee shall deposit the amount specified in (iii)
above. Upon the deposit of such amount, the Trustee shall immediately transfer
such amount to Harris Trust and Savings Bank, as trustee for the Refunded Bonds,
and shall close such account.

         SECTION 2.03. DETERMINATION OF RATE PERIODS AND INTEREST RATES. In the
manner hereinafter provided, the term of the Bonds will be divided into
consecutive Rate Periods during which the Bonds shall bear interest at the Daily
Rate, the Weekly Rate, the Flexible Rate, the Taxable Flexible Rate, the Taxable
Weekly Rate, the Term Rate or the Taxable Term Rate as the case may be. The
first Rate Period (i) with respect to the Series 1999A Bonds shall be a Term
Rate Period commencing on the Dated Date of such Bonds and ending on the date
prior to the maturity date of such bonds at the Term Rate indicated thereon per
annum; and (ii) with respect to the Series 1999B Bonds shall be the Taxable
Weekly Rate Period with the initial Taxable Weekly Rates established on or
before the Dated Date in accordance with the provisions of this Indenture. Each
Series of Bonds shall bear interest at the rate or rates per annum established
from time to time in accordance with the provisions of this Indenture. Prior to
the Tax-Exempt Conversion Date for any Series 1999B Bonds, all of the Series
1999B Bonds shall be in the Taxable Flexible Rate Period, Taxable Term Rate
Period or the Taxable Weekly Rate Period, and such Bonds shall bear interest
only at a Taxable Flexible Rate, Taxable Term Rate or a Taxable Weekly Rate;
however, Taxable Flexible Bonds may be in different Taxable Flexible Rate
Segments and may bear interest at different Taxable Flexible Rates.

                  (a) (i) Determination of Daily Rate. During each Daily Rate
         Period for any Tax-Exempt Series of Bonds, such Bonds shall bear
         interest at the Daily Rate, which shall be determined by the
         Remarketing Agent not later than 10:00 a.m., New York time, on each
         Business Day for such Business Day. The Daily Rate shall be the lowest
         rate determined by the Remarketing Agent to be the interest rate which
         would enable the Remarketing Agent to sell such Bonds on the effective
         date of such rate at a price equal to 100% of the principal amount
         thereof (without regard to accrued interest); provided,

                                       17
<PAGE>   24

         however, that (1) with respect to any day which is not a Business Day,
         the Daily Rate shall be the Daily Rate determined for the immediately
         preceding Business Day, and (2) with respect to any other day for which
         the Remarketing Agent shall not have determined a Daily Rate, the Daily
         Rate for such day shall be 105% of the most recent TBMA Swap Index. In
         no event shall the Daily Rate exceed the lesser of 15% per annum or the
         maximum rate per annum then permitted by applicable law. The
         Remarketing Agent shall provide the Trustee with immediate telephonic
         notice by noon New York time (promptly confirmed in writing) of each
         Daily Rate, as so determined; provided, however that no such notice
         need be given if the Daily Rate so determined is the same Daily Rate
         for the immediately preceding day.

                  (ii) Adjustment to Daily Rate. The Borrower, by written
         direction to the Issuer, the Trustee, the Liquidity Provider, and the
         Remarketing Agent, may elect that the Bonds of a Tax-Exempt Series
         (including Series 1999B Bonds on a Tax-Exempt Conversion Date) shall
         bear interest at a Daily Rate. Such direction shall (A) specify the
         Series of Bonds and the effective date of such adjustment to a Daily
         Rate, which shall be a Business Day not earlier than the fifteenth
         (15th) day after the date of such direction (or such shorter period of
         time to which the Trustee agrees), and shall be (1) in the case of an
         adjustment from a Term Rate Period or Taxable Term Rate Period, the day
         immediately following the last day of the then current Term Rate Period
         or Taxable Term Rate Period or on any day on which the Issuer at the
         direction of the Borrower would be permitted to redeem such Bonds
         pursuant to, and at the redemption price described in, Section
         3.01(A)(3) hereof, and (2) in the case of an adjustment from a Flexible
         Rate Period or Taxable Flexible Rate Period shall be either the day
         immediately following the last day of the then current Flexible Rate
         Period or Taxable Flexible Rate Period or the day immediately following
         the last day of the last Flexible Segment or Taxable Flexible Segment
         for each such Bond in the then current Flexible Rate Period or Taxable
         Flexible Rate Period all as determined in accordance with Section
         2.03(d)(iv) or 2.03(e)(iv) hereof; and (B) in the case of an adjustment
         from a Term Rate Period having a duration in excess of one year, be
         accompanied by a form of opinion of Bond Counsel to the effect that
         such adjustment (1) is authorized or permitted by the Act and this
         Indenture and (2) will not adversely affect the Tax-Exempt status of
         such Bonds. During each Daily Rate Period commencing on the date so
         specified or determined (provided that an opinion of Bond Counsel
         described in clause (B) above, if required, is delivered on such date)
         and ending on the day immediately preceding the effective date of the
         next succeeding Rate Period, the interest rate borne by the Bonds shall
         be a Daily Rate.

                  (iii) Notice of Adjustment to Daily Rate. Except with respect
         to an adjustment to a Daily Rate Period from a Flexible Rate or an
         adjustment occurring on a Tax-Exempt Conversion Date with respect to
         Taxable Flexible Bonds, the Trustee shall give notice of an adjustment
         to a Daily Rate Period to Owners of such Bonds, by first class mail,
         postage prepaid, not less than twelve (12) days prior to the effective
         date of such Daily Rate Period. Such notice shall state (1) that the
         interest rate on such Bonds will be adjusted to a Daily Rate (subject
         to receipt of the opinion of Bond Counsel referred to in the
         immediately preceding paragraph (a)(ii) if required, and to the
         Borrower's ability to rescind its election as described in Section
         2.03(i) hereof), (2) the effective date of such Daily Rate Period, (3)
         that all such Bonds are subject to mandatory purchase on such

                                       18
<PAGE>   25

         effective date, (4) the procedures of such purchase and the payment of
         the purchase price and (5) if applicable, that the adjustment will also
         be a Tax-Exempt Conversion Date.

                  (b) (i) Determination of Weekly Rate. During each Weekly Rate
         Period for any Tax-Exempt Series of Bonds, such Bonds shall bear
         interest at the Weekly Rate, which, in the case of the first Weekly
         Rate determined for each Weekly Rate Period, shall be determined by the
         Remarketing Agent not later than 10:00 a.m. New York time on the first
         day of such Weekly Rate Period and thereafter no later than the
         Business Day next preceding Wednesday of each week during such Weekly
         Rate Period. The Weekly Rate shall be the rate determined by the
         Remarketing Agent to be the lowest interest rate which would enable the
         Remarketing Agent to sell such Bonds on the effective date of such rate
         at a price equal to 100% of the principal amount thereof (without
         regard to accrued interest); provided, however, that if the Remarketing
         Agent shall not have determined a Weekly Rate for any period, the
         Weekly Rate for such period shall be the same as 105% of the most
         recent TBMA Swap Index. In no event shall the Weekly Rate exceed the
         lesser of 15% per annum or the maximum rate per annum then permitted by
         applicable law. The first Weekly Rate determined for each Weekly Rate
         Period shall apply to the period commencing on the first day of such
         period and ending on the next succeeding Tuesday. Thereafter, each
         Weekly Rate shall apply to the period commencing on each Wednesday and
         ending on the next succeeding Tuesday; provided, however, if a Weekly
         Rate Period shall end on a day other than Tuesday, the last Weekly Rate
         for such Weekly Rate Period shall apply to the period commencing on the
         Wednesday preceding the last day of such Weekly Rate Period and ending
         on such last day. The Remarketing Agent shall provide the Trustee with
         written notification on the first day of each Weekly Rate Period of
         each Weekly Rate as so determined.

                  (ii) Adjustment to Weekly Rate. The Borrower, by written
         direction to the Issuer, the Trustee, the Liquidity Provider, and the
         Remarketing Agent, may elect that the Bonds of a Tax-Exempt Series
         (including Series 1999B Bonds on a Tax-Exempt Conversion Date) shall
         bear interest at a Weekly Rate. Such direction shall (A) specify the
         Series of Bonds and the effective date of such adjustment to a Weekly
         Rate, which shall be a Business Day not earlier than the 15th day after
         the date of such direction (or such shorter period of time to which the
         Trustee agrees), and (1) in the case of an adjustment from a Term Rate
         Period or Taxable Term Rate Period, shall be the day immediately
         following the last day of the then current Term Rate Period or Taxable
         Term Rate Period or on any day on which the Issuer at the direction of
         the Borrower would be permitted to redeem such Bonds pursuant to, and
         at the redemption price described in, Section 3.01(A)(3) hereof, and
         (2) in the case of an adjustment from a Flexible Rate Period or Taxable
         Flexible Rate Period, shall be either the day immediately following the
         last day of the then current Flexible Rate Period or Taxable Flexible
         Rate Period or the day immediately following the last day of the last
         Flexible Segment or Taxable Flexible Segment for each such Bond in the
         then-current Flexible Rate Period, all as determined in accordance with
         Section 2.03(d)(iv) or 2.03(e)(iv) hereof; and (B) in the case of an
         adjustment from a Term Rate Period having a duration in excess of one
         year, be accompanied by a form of opinion of Bond Counsel to the effect
         that such adjustment (1) is authorized or permitted by the Act and this
         Indenture and (2) will not adversely affect the Tax-Exempt status of
         such Bonds. During each Weekly Rate Period commencing on

                                       19
<PAGE>   26

         the date so specified or determined (provided that an opinion of Bond
         Counsel described in clause (B) above, if required, is delivered on
         such date) and ending on the day immediately preceding the effective
         date of the next succeeding Rate Period, the interest rate borne by the
         Bonds shall be a Weekly Rate.

                  (iii) Notice of Adjustment to Weekly Rate. Except with respect
         to an adjustment to a Weekly Rate Period from a Flexible Rate or an
         adjustment occurring on a Tax-Exempt Conversion Date with respect to
         Taxable Flexible Bonds, the Trustee shall give notice of an adjustment
         to a Weekly Rate Period to Owners of the Bonds of a Tax-Exempt Series,
         by first class mail, postage prepaid, not less than twelve (12) days
         prior to the effective date of such Weekly Rate Period. Such notice
         shall state (1) that the interest rate on such Bonds will be adjusted
         to a Weekly Rate (subject to receipt of the opinion of Bond Counsel
         referred to in the immediately preceding paragraph (b)(ii), if
         required, and to the Borrower's ability to rescind its election as
         described in Section 2.03(i) hereof), (2) the effective date of such
         Weekly Rate Period, (3) that all such Bonds are subject to mandatory
         purchase on such effective date, (4) the procedures of such purchase
         and the payment of the purchase price, and (5) if applicable, that the
         adjustment will also be a Tax-Exempt Conversion Date.

                  (c) (i) Determination of Term Rate. During each Term Rate
         Period for any Tax-Exempt Series of Bonds (which shall be at least 180
         days in duration), such Bonds shall bear interest at the Term Rate
         determined by the Remarketing Agent on a Business Day selected by the
         Remarketing Agent, but not more than sixty (60) days prior to the first
         day of such Term Rate Period. The Term Rate shall be the rate
         determined by the Remarketing Agent on such date, and filed on such
         date with the Trustee and the Borrower, by written notice or by
         telephone promptly confirmed by telecopy or other writing, as being the
         lowest rate which would enable the Remarketing Agent to sell such Bonds
         on the effective date of such Term Rate at a price equal to 100% of the
         principal amount thereof; provided, however, that if, for any reason, a
         Term Rate for any Term Rate Period shall not be determined or become
         effective, then the Rate Period for such Bonds shall automatically
         adjust to a Daily Rate Period. If a Daily Rate for the first day of any
         such Daily Rate Period is not determined as provided in Section
         2.03(a)(i) hereof, the Daily Rate for the first day of such Daily Rate
         Period shall be 105% of the most recent TBMA Swap Index. In no event
         shall any Term Rate exceed the lesser of 15% per annum or the maximum
         rate per annum then permitted by applicable law.

                  (ii) Adjustment to or Continuation of Term Rate. The Borrower,
         by written direction to the Issuer, the Liquidity Provider, the Trustee
         and the Remarketing Agent, may elect that the Bonds of a Tax-Exempt
         Series (including Series 1999B Bonds on a Tax-Exempt Conversion Date)
         shall bear, or continue to bear, interest at a Term Rate, and if it
         shall so elect that such Bonds shall bear or continue to bear, interest
         at a Term Rate, then the Borrower shall determine the duration of the
         Term Rate Period during which the Bonds shall bear interest at such
         Term Rate. As a part of such election, the Borrower also may determine
         that the initial Term Rate Period shall be followed by successive Term
         Rate Periods and, if the Borrower so elects, shall specify the duration
         of each such successive Term Rate Period as provided in this paragraph
         (ii). Such direction shall (A) specify the Series of Bonds and the
         effective date of each Term Rate Period,

                                       20
<PAGE>   27

         which shall be a Business Day not earlier than the 15th day after the
         date of such written direction (or such shorter period of time to which
         the Trustee agrees), and (1) in the case of an adjustment from a
         Flexible Rate Period or Taxable Flexible Rate Period, shall be either
         the day immediately following the last day of the then current Flexible
         Rate Period or Taxable Flexible Rate Period or the day immediately
         following the last day of the last Flexible Segment or Taxable Flexible
         Segment for such Bond in the then-current Flexible Rate Period or
         Taxable Flexible Rate Period, all as determined in accordance with
         Section 2.03(d)(iv) or 2.03(e)(iv) hereof and (2) in the case of an
         adjustment from a Term Rate Period or Taxable Term Rate Period, shall
         be the day immediately following the last day of the then current Term
         Rate Period or Taxable Term Rate Period or any day on which the Issuer
         at the direction of the Borrower would be permitted to redeem such
         Bonds pursuant to, and at the redemption price described in, Section
         3.01(A)(3) hereof; (B) specify the last day of such Term Rate Period
         or, if successive Term Rate Periods shall have been designated, the
         last day of each such Term Rate Period (which shall be for each Term
         Rate Period either the date immediately preceding the final maturity
         date of the Bonds, or a day which both immediately precedes a Business
         Day and is at least 180 days after the effective date thereof); and (C)
         unless the adjustment is from a Term Rate Period of equal duration, be
         accompanied by a form of opinion of Bond Counsel to the effect that
         such adjustment (1) is authorized or permitted by the Act and this
         Indenture and (2) will not adversely affect the Tax-Exempt status of
         such Bonds. The opinion of Bond Counsel described in clause (C) above,
         if required, must be delivered on the effective date of such
         adjustment. Notwithstanding the stated date of termination of any Term
         Rate Period, the Borrower may elect to cause an adjustment to any other
         Rate Period as of any date on which the affected Bonds are subject to
         redemption pursuant to Section 3.01(A)(3) hereof.

         If, by the fourteenth (14th) day prior to the last day of any Term Rate
Period, the Trustee shall not have received notice of the Borrower's election
that, during the next succeeding Rate Period, the Bonds of a Tax-Exempt Series
shall bear interest at a Daily Rate, a Weekly Rate, a Flexible Rate or a Term
Rate, the next succeeding Rate Period of such Bonds shall be a Weekly Rate
Period until the interest rate on such Bonds is adjusted to another Rate Period,
and the Bonds shall be subject to purchase pursuant to Section 4.02.

                  (iii) Notice of Adjustment to or Continuation of Term Rate.
         Except with respect to an adjustment to a Term Rate Period from a
         Flexible Rate or an adjustment occurring on a Tax-Exempt Conversion
         Date with respect to Taxable Flexible Bonds, the Trustee shall give
         notice of an adjustment to (or the continuation of) a Term Rate Period
         to Owners of such Bonds, by first class mail, postage prepaid, not less
         than twelve (12) days prior to the effective date of such Term Rate
         Period. Such notice shall state (1) that the interest rate on such
         Bonds will be adjusted to, or continue to be, a Term Rate (subject to
         receipt of the opinion of Bond Counsel referred to in the immediately
         preceding paragraph (ii), if required, and to the Borrower's ability to
         rescind its election as described in Section 2.03(i) hereof), (2) the
         effective date and the last day of such Term Rate Period, (3) that the
         Term Rate for such Term Rate Period will be determined on or prior to
         the effective date thereof, (4) how such Term Rate may be obtained from
         the Remarketing Agent, (5) the Interest Payment Dates after such
         effective date, (6) that all such Bonds are subject to mandatory
         purchase on such effective date, (7) the

                                       21
<PAGE>   28

         procedures of such purchase and the payment of the purchase price, (8)
         the redemption provisions set forth in Section 3.01 hereof which will
         apply during such Term Rate Period, and (9) if applicable, that the
         adjustment will also be a Tax-Exempt Conversion Date.

                  (d) (i) Determination of Flexible Segments and Flexible Rates.
         During each Flexible Rate Period for any Tax-Exempt Series of Bonds,
         each Bond shall bear interest during each Flexible Segment for such
         Bond as described herein. Different Flexible Segments and Flexible
         Rates may apply to different Bonds of such Tax-Exempt Series at any
         time and from time to time. The Flexible Segment for each such Bond
         shall be a period of at least one day and not more than 270 days ending
         on a day that immediately precedes a Business Day, determined by the
         Remarketing Agent to be the period which, together with all such other
         Flexible Segments for all Bonds of such Tax-Exempt Series then
         Outstanding, will result in the lowest overall interest expense on the
         Bonds of such Tax-Exempt Series over the next succeeding 270 days. The
         Flexible Rate for each Flexible Segment for each Bond shall be
         determined by the Remarketing Agent no later than 1:00 p.m., New York
         time on the Business Day next preceding the first day of such Flexible
         Segment (and in time to enable the Remarketing Agent to give to the
         Trustee the notice required by Section 4.04(c) hereof) to be the lowest
         interest rate which would enable the Remarketing Agent to sell such
         Bonds on the effective date of such rate at a price equal to 100% of
         the principal amount thereof. If a Flexible Segment or a Flexible Rate
         for a Flexible Segment is not determined or effective, the Flexible
         Segment for such Bond shall be a Flexible Segment of one day, and the
         interest rate for such Flexible Segment of one day shall be 105% of the
         most recent TBMA Swap Index. In no event shall the Flexible Rate for
         any Flexible Segment exceed the lesser of 15% per annum or the maximum
         rate per annum then permitted by applicable law. The Remarketing Agent
         shall provide the Trustee with facsimile or telephonic notice of each
         Flexible Segment and Flexible Rate, as provided in Section 4.04(c)
         hereof.

                  (ii) Adjustment to Flexible Rates. The Borrower, by written
         direction to the Issuer, the Trustee, the Liquidity Provider, and the
         Remarketing Agent, may elect that the Bonds of a Tax-Exempt Series
         (including Series 1999B Bonds on a Tax- Exempt Conversion Date) shall
         bear interest at Flexible Rates. Such direction shall (A) specify the
         Series of Bonds and the effective date of the Flexible Rate Period
         during which such Bonds shall bear interest at Flexible Rates, which
         shall be a Business Day not earlier than the fifteenth (15th) day after
         the date of such direction (or such shorter period of time to which the
         Trustee agrees), and shall be, in the case of an adjustment from a Term
         Rate Period or Taxable Term Rate Period, the day immediately following
         the last day of the then current Term Rate Period or Taxable Term Rate
         Period or on any day on which the Issuer at the direction of the
         Borrower would be permitted to redeem such Bonds pursuant to, and at
         the redemption price described in, Section 3.01(A)(3) hereof; and (B)
         in the case of an adjustment from a Term Rate Period having a duration
         in excess of one year, be accompanied by a form of opinion of Bond
         Counsel to the effect that such adjustment (1) is authorized or
         permitted by the Indenture and the Act and (2) will not adversely
         affect the Tax-Exempt status of such Bonds. During each Flexible Rate
         Period commencing on the date so specified (provided that the opinion
         of Bond Counsel described in clause (B) above, if required, is
         delivered on such date) and ending on the

                                       22
<PAGE>   29

         day immediately preceding the effective date of the next succeeding
         Rate Period, each Bond shall bear interest at a Flexible Rate during
         each Flexible Segment for such Bond.

                  (iii) Notice of Adjustment to Flexible Rates. Except with
         respect to adjustment to a Flexible Rate Period from a Flexible Rate
         Period or an adjustment occurring on a Tax-Exempt Conversion Date with
         respect to Taxable Flexible Bonds, the Trustee shall give notice of an
         adjustment to a Flexible Rate Period to Owners of the Bonds of a
         Tax-Exempt Series, by first class mail, postage prepaid, not less than
         twelve (12) days prior to the effective date of such Flexible Rate
         Period. Such notice shall state (1) that the interest rate on such
         Bonds will be adjusted to the Flexible Rate (subject to receipt of the
         opinion of Bond Counsel referred to in the immediately preceding
         paragraph (ii), if required, and to the Borrower's ability to rescind
         its election as described in Section 2.03(i) hereof), (2) the effective
         date of such Flexible Rate Period, (3) that all such Bonds are subject
         to mandatory purchase on such effective date, (4) the procedures of
         such purchase and the payment of the purchase price, and (5) if
         applicable, that the adjustment will also be a Tax-Exempt Conversion
         Date.

                  (iv) Adjustment from Flexible Rates. At any time during a
         Flexible Rate Period, the Borrower may elect that the Bonds of a
         Tax-Exempt Series shall no longer bear interest at Flexible Rates and
         shall instead bear interest as otherwise permitted under this
         Indenture. The Borrower shall give written notice to the Issuer, the
         Trustee, the Liquidity Provider and the Remarketing Agent of such
         election and shall specify the Rate Period to follow with respect to
         such Bonds upon cessation of the Flexible Rate Period and instruct the
         Remarketing Agent to (1) determine Flexible Segments of such duration
         that, as soon as possible, all Flexible Segments shall end on the same
         date, not earlier than the fourteenth (14th) day (or such shorter
         period of time to which the Trustee agrees) after the date of such
         written notice from the Borrower, and upon the establishment of such
         Flexible Segments the day next succeeding the last day of all such
         Flexible Segments shall be the effective date of the Rate Period
         elected by the Borrower; or (2) determine Flexible Segments that will
         best promote an orderly transition to the next succeeding Rate Period
         to apply to such Bonds, beginning not earlier than the fourteenth
         (14th) day (or such shorter period of time to which the Trustee agrees)
         after the date of such written notice from the Borrower. If the
         Borrower elects the alternative in clause (2) above, the day next
         succeeding the last day of the Flexible Segment for each such Bond
         shall be with respect to such Bond the effective date of the new Rate
         Period elected by the Borrower. The Remarketing Agent, promptly upon
         the determination thereof, shall give written notice of such last day
         and such effective dates to the Borrower, the Liquidity Provider and
         the Trustee. During any transitional period from a Flexible Rate Period
         to the next succeeding Rate Period in accordance with clause (2) above,
         the provisions of this Indenture shall be deemed to apply to such Bonds
         as follows: such Bonds continuing to bear interest at Flexible Rates
         shall have applicable to them the provisions hereunder theretofore
         applicable to such Bonds as if all Bonds were continuing to bear
         interest at Flexible Rates and such Bonds bearing interest in the Rate
         Period to which the transition is being made will have applicable to
         them the provisions hereunder as if all such Bonds were bearing
         interest in such Rate Period.

                                       23
<PAGE>   30

                  (e) (i) Determination of Taxable Flexible Rates and Taxable
         Flexible Rate Segments. During the Taxable Flexible Rate Period, each
         Taxable Flexible Bond shall bear interest during each Taxable Flexible
         Rate Segment for such Bond at the Taxable Flexible Rate for such Bond
         as described herein. Different Taxable Flexible Rate Segments and
         Taxable Flexible Rates may apply to different Taxable Flexible Bonds at
         any time and from time to time. The Taxable Flexible Rate Segment for
         each Taxable Flexible Bond shall be a period of at least one day and
         not more than 270 days, ending on a day that immediately precedes a
         Business Day, determined by the Remarketing Agent to be the period
         which, together with all other Taxable Flexible Segments for all other
         Taxable Flexible Bonds then Outstanding, will result in the lowest
         overall interest expense on the Taxable Flexible Bonds over the next
         succeeding 270 days. The Taxable Flexible Rate for each Taxable
         Flexible Rate Segment for each such Bond shall be determined by the
         Remarketing Agent no later than 1:00 p.m., New York time on the
         Business Day next preceding the first day of such Taxable Flexible Rate
         Segment (and in time to enable the Remarketing Agent to give to the
         Trustee the notice required by Section 4.04(c) hereof) to be the lowest
         interest rate which would enable the Remarketing Agent to sell such
         Bonds on the effective date of such rate at a price equal to 100% of
         the principal amount thereof. If a Taxable Flexible Segment or a
         Taxable Flexible Rate for a Taxable Flexible Rate Segment is not
         determined or effective, the Taxable Flexible Rate Segment for such
         Bond shall be a Taxable Flexible Rate Segment of one day, and the
         interest rate for such Taxable Flexible Rate Segment of one day shall
         be 105% of the Federal Reserve H.15 Non-Financial Commercial Paper
         7-day rate until a new Taxable Flexible Rate and Taxable Flexible Rate
         Segment shall be established. In no event shall the Taxable Flexible
         Rate for any Taxable Flexible Rate Segment exceed the lesser of 15% per
         annum or the maximum rate per annum then permitted by applicable law.
         The Remarketing Agent shall provide the Trustee with facsimile or
         telephonic notice of each Taxable Flexible Rate Segment and Taxable
         Flexible Rate, as provided in Section 4.04(c) hereof.

         If the Borrower has given notice of a Tax-Exempt Conversion Date with
respect to any Taxable Flexible Bonds, no new Taxable Flexible Rate Segment for
such Bonds shall be established unless the last day of such Taxable Flexible
Rate Segment occurs prior to such Tax-Exempt Conversion Date.

                  (ii) Adjustment to Taxable Flexible Rates. At any time while
         the Taxable Weekly Bonds are bearing interest at Taxable Weekly Rates
         or while the Taxable Term Bonds bear interest at Taxable Term Rates,
         the Borrower, by written direction to the Issuer, the Liquidity
         Provider, the Trustee and the Remarketing Agent, may elect that the
         Taxable Term Bonds or Taxable Weekly Bonds shall bear interest at
         Taxable Flexible Rates. Such direction shall specify the effective date
         of the Taxable Flexible Rate Period during which such Bonds shall bear
         interest at Taxable Flexible Rates, which shall be a Business Day not
         earlier than the fifteenth (15th) day after the date such direction (or
         such shorter period of time to which the Trustee agrees) and shall be,
         in the case of adjustment from a Taxable Term Rate Period, the day
         immediately following the last day of the then current Taxable Term
         Rate Period, or on any day on which the Issuer at the direction of the
         Borrower would be permitted to redeem such Bonds pursuant to, and at
         the redemption price described in, Section 3.01(A)(3) hereof. During
         each Taxable

                                       24
<PAGE>   31

         Flexible Rate Period commencing on the date so specified and ending on
         the day immediately preceding the effective date of the next succeeding
         Rate Period, each Bond shall bear interest at a Taxable Flexible Rate
         during each Taxable Flexible Segment for such Bond.

                  (iii) Notice of Adjustment to Taxable Flexible Rates. The
         Trustee shall give notice of an adjustment to a Taxable Flexible Rate
         Period to the Owners of Taxable Weekly Bonds or Taxable Term Bonds, by
         first class mail, postage prepaid, not less than twelve (12) days prior
         to the effective date of such Taxable Flexible Rate Period. Such notice
         shall state (1) that the interest rate on such Taxable Weekly Bonds or
         Taxable Term Bonds will be adjusted to a Taxable Flexible Rate (subject
         to the Borrower's ability to rescind its election as described in
         Section 2.03(i) hereof), (2) the effective date of such Taxable
         Flexible Rate Period, (3) that all such Bonds to be adjusted are
         subject to mandatory purchase on such effective date, and (4) the
         procedures of such purchase and the payment of the purchase price.

                  (iv) Adjustment from Taxable Flexible Rates. At any time
         during a Taxable Flexible Rate Period, the Borrower may elect that the
         Taxable Flexible Bonds shall no longer bear interest at Taxable
         Flexible Rates and shall instead bear interest at Taxable Weekly Rates
         or Taxable Term Rates. The Borrower shall give written notice to the
         Issuer, the Liquidity Provider, the Trustee and the Remarketing Agent
         of such election and shall specify the Taxable Weekly Rate Period or
         Taxable Term Period to follow with respect to such Bonds upon cessation
         of the Taxable Flexible Rate Period and instruct the Remarketing Agent
         to (1) determine Taxable Flexible Rate Segments of such duration that,
         as soon as possible, all Taxable Flexible Rate Segments shall end on
         the same date, not earlier than the fourteenth (14th) day (or such
         shorter period of time to which the Trustee agrees) after the date of
         such written notice from the Borrower, and upon the establishment of
         such Taxable Flexible Rate Segments the day next succeeding the last
         day of all such Taxable Flexible Rate Segments shall be the effective
         date of the Taxable Weekly Rate Period or Taxable Term Rate Period, as
         applicable; or (2) determine Taxable Flexible Rate Segments that will
         best promote an orderly transition to the next succeeding Rate Period
         to apply to such Bonds, beginning not earlier than the fourteenth
         (14th) day (or such shorter period of time to which the Trustee agrees)
         after the date of such written notice from the Borrower. If the
         Borrower elects the alternative in clause (2) above, the day next
         succeeding the last day of the Taxable Flexible Rate Segment for each
         such Bond shall be, with respect to such Bond, the effective date of
         the new Taxable Weekly Rate Period or Taxable Term Rate Period, as
         applicable. The Remarketing Agent, promptly upon the determination
         thereof, shall give written notice of such last day and such effective
         dates to the Borrower, the Liquidity Provider and the Trustee. During
         any transitional period from a Taxable Flexible Period in accordance
         with clause (2) above, the provisions of this Indenture shall apply as
         follows: such Bonds continuing to bear interest at Taxable Flexible
         Rates shall have applicable to them the provisions hereunder
         theretofore applicable to such Bonds as if all such Bonds were
         continuing to bear interest at Taxable Flexible Rates and such Bonds
         bearing interest in the Rate Period to which the transition being made
         will have applicable to them the provisions hereunder as if all such
         Bonds were bearing interest in such Rate Period.

                                       25
<PAGE>   32

                  (v) Adjustment from Taxable Flexible Rate Period; Failure to
         Convert to Tax-Exempt Series. All or a portion of the Taxable Flexible
         Bonds may be converted to Bonds of a Tax-Exempt Series in accordance
         with Section 2.15 hereof. If the conditions to such conversion are not
         satisfied on a Tax-Exempt Conversion Date with respect to any Taxable
         Flexible Bonds, such Bonds shall automatically adjust to a Taxable
         Weekly Rate Period and shall bear interest at Taxable Weekly Rates from
         and after the date of such failed conversion until a Tax-Exempt
         Conversion Date, if any, occurs with respect to such Bonds, and shall
         be in a Taxable Weekly Rate Period and bear interest at the Taxable
         Weekly Rates established in accordance with paragraph (f)(i) below,
         until adjusted to another Rate Period.

                  (f) (i) Determination of Taxable Weekly Rate. During each
         Taxable Weekly Rate Period for any Taxable Weekly Bonds, such Bonds
         shall bear interest at the Taxable Weekly Rate, which, in the case of
         the first Taxable Weekly Rate determined for each Taxable Weekly Rate
         Period, shall be determined by the Remarketing Agent not later than
         10:00 a.m. New York time on the first day of such Taxable Weekly Rate
         Period and thereafter no later than the Business Day next preceding
         Wednesday of each week during such Taxable Weekly Rate Period. The
         Taxable Weekly Rate shall be the rate determined by the Remarketing
         Agent to be the lowest interest rate which would enable the Remarketing
         Agent to sell the Taxable Weekly Bonds on the effective date of such
         rate at a price equal to 100% of the principal amount thereof (without
         regard to accrued interest); provided, however, that if the Remarketing
         Agent shall not have determined a Taxable Weekly Rate for any period or
         a Taxable Weekly Rate is not effective, the Taxable Weekly Rate for
         such period shall be a rate per annum equal to 100% of the 30-day
         London Interbank Offered Rate until a new Taxable Weekly Rate is
         established. In no event shall the Taxable Weekly Rate exceed the
         lesser of 15% per annum or the maximum rate per annum then permitted by
         applicable law. The first Taxable Weekly Rate determined for each
         Taxable Weekly Rate Period shall apply to the period commencing on the
         first day of such period and ending on the next succeeding Tuesday.
         Thereafter, each Taxable Weekly Rate shall apply to the period
         commencing on each Wednesday and ending on the next succeeding Tuesday;
         provided, however, if a Taxable Weekly Rate Period shall end on a day
         other than Tuesday, the last Taxable Weekly Rate for such Taxable
         Weekly Rate Period shall apply to the period commencing on the
         Wednesday preceding the last day of such Taxable Weekly Rate Period and
         ending on such last day. The Remarketing Agent shall provide the
         Trustee with written notification on the first day of each Taxable
         Weekly Rate Period of each Taxable Weekly Rate as so determined.

                  (ii) Adjustment to Taxable Weekly Rate. At any time while the
         Taxable Flexible Bonds are bearing interest at Taxable Flexible Rates
         or Taxable Term Bonds are bearing interest at Taxable Term Rates, the
         Borrower, by written direction to the Issuer, the Trustee, the
         Liquidity Provider and the Remarketing Agent, may elect that such Bonds
         shall bear interest at a Taxable Weekly Rate. Such direction shall
         specify the effective date of such adjustment to a Taxable Weekly Rate
         Period, which shall be a Business Day not earlier than the fifteenth
         (15th) day after the date of such written direction (or such shorter
         period of time to which the Trustee agrees), and (1) in the case of an
         adjustment from a Taxable Term Rate Period, shall be the day
         immediately

                                       26
<PAGE>   33

         following the last date of the then current Taxable Term Rate Period or
         any day on which such Taxable Term Bonds are subject to redemption by
         the Issuer hereunder at the direction of the Borrower, and (2) shall be
         either the day immediately following the last day of the then current
         Taxable Flexible Rate Period or the day immediately following the last
         day of the last Taxable Flexible Rate Segment for each such Bond in the
         then-current Taxable Flexible Rate Period, all as determined in
         accordance with Section 2.03(e)(iv) hereof.

                  (iii) Notice of Adjustment to Taxable Weekly Rate. The Trustee
         shall give notice of an adjustment to a Taxable Weekly Rate Period to
         the Owners of Taxable Flexible Bonds or Taxable Term Bonds, by first
         class mail, postage prepaid, not less than twelve (12) days prior to
         the effective date of such Taxable Weekly Rate Period. Such notice
         shall state (1) that the interest rate on the Taxable Flexible Bonds or
         Taxable Term Bonds will be adjusted to a Taxable Weekly Rate (subject
         to the Borrower's ability to rescind its election as described in
         Section 2.03(i) hereof), (2) the effective date of such Taxable Weekly
         Rate Period, (3) that all Taxable Flexible Bonds or Taxable Term Bonds
         are subject to mandatory purchase on such effective date and (4) the
         procedures of such purchase and the payment of the purchase price.

                  (iv) Adjustment from Taxable Weekly Rate; Failure to Convert
         to Tax-Exempt Series. All or a portion of the Taxable Weekly Bonds may
         be converted to Bonds of a Tax-Exempt Series in accordance with Section
         2.15 hereof. If the conditions to such conversion do not occur on a
         Tax-Exempt Conversion Date with respect to any Taxable Weekly Bonds, on
         and after the date of such failed conversion until a Tax-Exempt
         Conversion Date, if any, occurs with respect to such Bonds, such Bonds
         shall continue in a Taxable Weekly Rate Period and bear interest at the
         Taxable Weekly Rates established in accordance with paragraph (f)(i)
         above until adjusted to another Rate Period.

                  (g) (i) Determination of Taxable Term Rate. During each
         Taxable Term Rate Period for any Taxable Term Bonds (which shall be at
         least 180 days in duration), such Bonds shall bear interest at the
         Taxable Term Rate determined by the Remarketing Agent on a Business Day
         selected by the Remarketing Agent, but not more than sixty (60) days
         prior to the first day of such Taxable Term Rate Period. The Taxable
         Term Rate shall be the rate determined by the Remarketing Agent on such
         date, and filed on such date with the Trustee and the Borrower, by
         written notice or by telephone promptly confirmed by telecopy or other
         writing, as being the lowest rate which would enable the Remarketing
         Agent to sell such Bonds on the effective date of such Taxable Term
         Rate at a price equal to 100% of the principal amount thereof;
         provided, however, that if, for any reason, a Taxable Term Rate for any
         Taxable Term Rate Period shall not be determined or become effective,
         then the Rate Period for the Bonds of such Series shall automatically
         adjust to a Taxable Weekly Rate Period, and bear interest at a Taxable
         Weekly Rate established in accordance with (f)(i) above until adjusted
         to another Rate Period. In no event shall any Taxable Term Rate exceed
         the lesser of 15% per annum or the maximum rate per annum then
         permitted by applicable law.

                  (ii) Adjustment to or Continuation of Taxable Term Rate. At
         any time the Borrower, upon written direction to the Issuer, the
         Trustee, the Liquidity Provider, and

                                       27
<PAGE>   34

         the Remarketing Agent, may elect that the interest rate borne by the
         Taxable Flexible Bonds, Taxable Weekly Bonds, or Taxable Term Bonds
         shall be adjusted to or continued as a Taxable Term Rate, and if it
         shall so elect that such Bonds shall bear or continue to bear, interest
         at a Taxable Term Rate, then the Borrower shall determine the duration
         of the Taxable Term Rate Period during which the Bonds shall bear
         interest at such Taxable Term Rate. As a part of such election, the
         Borrower also may determine that the initial Taxable Term Rate Period
         shall be followed by successive Taxable Term Rate Periods and, if the
         Borrower so elects, shall specify the duration of each such successive
         Taxable Term Rate Period as provided in this paragraph (ii). Such
         direction shall (A) specify the Series of Bonds and the effective date
         of each Taxable Term Rate Period, which shall be a Business Day not
         earlier than the fifteenth (15th) day after the date of such written
         direction (or such shorter period of time to which the Trustee agrees),
         and (1) in the case of an adjustment from a Taxable Flexible Rate
         Period, shall be either the day immediately following the last day of
         the then current Taxable Flexible Rate Period or the day immediately
         following the last day of the last Taxable Flexible Segment for each
         Bond in the then-current Taxable Flexible Rate Period, all as
         determined in accordance with Section 2.03(e)(iv) hereof and (2) in the
         case of adjustment or continuation of a Taxable Term Rate Period, shall
         be the day immediately following the last day of the then current
         Taxable Term Rate Period or any day on which the Issuer at the
         direction of the Borrower would be permitted to redeem such Bonds
         pursuant to, and at the redemption price described in, Section
         3.01(A)(3) hereof and; (B) specify the last day of such Taxable Term
         Rate Period or, if successive Taxable Term Rate Periods shall have been
         designated, the last day of each such Taxable Term Rate Period (which
         shall be for each Taxable Term Rate Period either the date immediately
         preceding the final maturity of the Series 1999B Bonds, or a day which
         both immediately precedes a Business Day and is at least 180 days after
         the effective date thereof). Notwithstanding the stated date of
         termination of any Taxable Term Rate Period, the Borrower may elect to
         cause an adjustment to any other Rate Period as of any date on which
         the affected Bonds are subject to redemption pursuant to Section
         3.01(A)(3) hereof.

         If, by the fourteenth (14th) day prior to the last day of any Taxable
Term Rate Period, the Trustee shall not have received notice of the Borrower's
election that, during the next succeeding Rate Period, such Bonds shall bear
interest at a Taxable Flexible Rate, Taxable Term Rate or a Taxable Weekly Rate,
the next succeeding Rate Period of such Bonds shall be a Taxable Weekly Rate
Period until the interest rate on such Bonds is adjusted to another Rate Period,
and the Bonds shall be subject to purchase pursuant to Section 4.02.

                  (iii) Notice of Adjustment to or Continuation of Taxable Term
         Rates. The Trustee shall give notice of an adjustment to or
         continuation of a Taxable Term Rate Period to the Owners of Taxable
         Weekly Bonds, Taxable Term Bonds, or Taxable Flexible Bonds, by first
         class mail, postage prepaid, not less than twelve (12) days prior to
         the effective date of such Taxable Term Rate Period. Such notice shall
         state (1) that the interest rate on such Bonds will be adjusted to or
         continued in a Taxable Term Rate (subject to the Borrower's ability to
         rescind its election as described in Section 2.03(i) hereof), (2) the
         effective date and last date of such Term Rate Period, (3) that the
         Taxable Term Rate for such Taxable Term Rate Period will be determined
         on or prior to the effective date thereof, (4) how such Taxable Term
         Rate may be obtained from the

                                       28
<PAGE>   35

         Remarketing Agent, (5) the Interest Payment Dates after such effective
         date, (6) that all such Taxable Term Bonds, Taxable Flexible Bonds or
         Taxable Weekly Bonds are subject to mandatory purchase on such
         effective date, (7) the procedures of such purchase and the payment of
         the purchase price, and (8) the redemption provisions that will pertain
         to the Taxable Term Bonds during such Taxable Term Rate Period.

                  (iv) Adjustment from Taxable Term Rate Period; Failure to
         Convert to Tax-Exempt Series. All or a portion of the Taxable Term
         Bonds may be converted to Bonds of a Tax-Exempt Series in accordance
         with Section 2.15 hereof. If the conditions to such conversion do not
         occur on a Tax-Exempt Conversion Date with respect to any Taxable Term
         Bonds, such Bonds shall bear interest at a Taxable Weekly Rate from and
         after the date of such failed conversion until a Tax-Exempt Conversion
         Date, if any, occurs with respect to such Bonds at Taxable Weekly Rates
         established in accordance with paragraph (f)(i) above until adjusted to
         another Rate Period.

         (h) Determinations Binding. The establishment and determination by the
Remarketing Agent of each Daily Rate, Weekly Rate, Taxable Weekly Rate, Taxable
Term Rate and Term Rate, each Taxable Flexible Rate Segment and Taxable Flexible
Rate and each Flexible Segment and Flexible Rate shall, absent manifest error,
be conclusive and binding upon the Remarketing Agent, the Liquidity Provider,
the Trustee, the Issuer, the Borrower and the Owners of the Bonds.

         (i) Rescission of Election; Automatic Adjustment. Notwithstanding
anything herein to the contrary, the Borrower may rescind any election by it to
adjust to or, in the case of a Term Rate Period or Taxable Term Rate Period,
continue a Rate Period pursuant to Section 2.03(a)(ii), (b)(ii), (c)(ii),
(d)(ii), (e)(ii), (f)(ii) or (g)(ii) hereof prior to the effective date of such
adjustment or continuation by giving written notice thereof to the Issuer, the
Trustee, the Liquidity Provider, and the Remarketing Agent prior to such
effective date. At the time the Borrower gives notice to rescind any election by
it to adjust to, or in the case of a Term Rate Period or Taxable Term Rate
Period, continue a Rate Period pursuant to Section 2.03(a)(ii), (b)(ii),
(c)(ii), (d)(ii), (e)(ii), (f)(ii) or (g)(ii) hereof, it may also elect to
continue the then effective Rate Period; provided, however, if the Rate Period
then in effect is a Term Rate Period, the subsequent Term Rate Period shall not
be of different duration than the Term Rate Period then in effect unless the
Borrower, prior to the expiration of the then-current Term Rate Period, provides
to the Trustee an opinion of Bond Counsel to the effect that the continuation in
a Term Rate Period of a different duration does not adversely affect the
Tax-Exempt status of the affected Bonds. If the notice of such rescission does
not become effective for any reason, and the Borrower does not elect to continue
the Rate Period then in effect, the Rate Period for (i) the Taxable Flexible
Bonds, the Taxable Term Bonds, or the Taxable Weekly Bonds shall be the Taxable
Weekly Rate Period, and (ii) the Bonds of a Tax-Exempt Series shall
automatically adjust to or continue in a Daily Rate Period. If a Daily Rate for
the first day of any Daily Rate Period to which a Rate Period is adjusted under
this Section 2.03(i) is not determined as provided in Section 2.03(a)(i) hereof
the Daily Rate for the first day of such Daily Rate Period shall be 105% of the
most recent TBMA Swap Index. The Trustee shall immediately give written notice
of each such automatic adjustment to a Rate Period pursuant to this Section
2.03(i) to the Owners in the form provided in Section 2.03(a)(iii) hereof.

                                       29
<PAGE>   36

         Notwithstanding the rescission of any notice to adjust or continue a
Rate Period, if notice has been given to Bondholders pursuant to Section
2.03(a)(iii), (b)(iii), (c)(iii), (d)(iii), (e)(iii), (f)(iii) or (g)(iii), the
Bonds shall be subject to mandatory purchase as specified in such notice.

         (j) Liquidity Provider Bonds. Notwithstanding any other provision of
this Indenture, including any provision of this Section 2.03 relating to the
determination of interest rates on the Bonds, any Bond which is acquired by a
Liquidity Provider pursuant to a Liquidity Facility shall bear interest at the
rate, payable at the times and in the manner, specified by such Liquidity
Facility.

         SECTION 2.04. OWNERSHIP, TRANSFER, EXCHANGE AND REGISTRATION OF BONDS.
The Issuer shall cause books for the registration and for the transfer of the
Bonds as provided herein to be kept by the Trustee, which is hereby constituted
and appointed the Registrar and transfer agent for the Bonds. The Issuer shall
prepare and deliver to the Trustee, and the Trustee shall keep custody of, a
supply of unauthenticated Bonds of each Series duly executed by the Issuer, as
provided in Section 2.05 hereof, for use in the transfer and exchange of Bonds
of such Series. The Trustee is hereby authorized and directed to complete such
forms of Bonds as to principal amounts and registered owners, in accordance with
the provisions hereof, in effecting transfers and exchanges of Bonds as provided
herein.

         Upon surrender for transfer of any Bond at the Principal Office of the
Trustee, duly endorsed for transfer or accompanied by a written instrument or
instruments of transfer in form satisfactory to the Trustee duly executed by the
registered owner or his attorney duly authorized in writing, the Trustee shall
date and execute the certificate of authentication on and deliver in the name of
the transferee or transferees a new Bond or Bonds of the applicable Series duly
executed by the Issuer of Authorized Denominations and for a like aggregate
principal amount.

         Any Bond or Bonds may be exchanged at the Principal Office of the
Trustee for a new Bond or Bonds of like aggregate principal amount and Series in
Authorized Denominations. Upon surrender of any Bond or Bonds for exchange, the
Trustee shall date and execute the certificate of authentication on and deliver
a new Bond or Bonds duly executed by the Issuer which the Bondholder making the
exchange is entitled to receive.

         Except in connection with the remarketing of any Bonds, the Trustee
shall not be required to transfer or exchange any Bond after the mailing of
notice calling such Bond or portion thereof for redemption, nor during the
period of ten days next preceding the mailing of such notice of redemption.

         Except as provided in Section 4.03, hereof, the person in whose name
any Bond shall be registered shall be deemed and regarded as the absolute owner
thereof for all purposes, and payment of the principal of, premium, if any, or
interest on any Bond shall be made only to or upon the written order of the
registered Owner thereof or his legal representative, but such registration may
be changed as hereinabove provided. All such payments shall be valid and
effective to satisfy and discharge the liability upon such Bond to the extent of
the sum or sums so paid.

                                       30
<PAGE>   37

         The Issuer and the Trustee shall require the payment by the Bondholder
requesting exchange or transfer (other than an exchange upon a partial
redemption of a Bond) of any tax, fee or other governmental charge required to
be paid with respect to such exchange or transfer, but otherwise no charge shall
be made to the Bondholder for such exchange or transfer.

         SECTION 2.05. EXECUTION OF BONDS. The Bonds shall be signed in the name
and on behalf of the Issuer with the manual or facsimile signature of the
Chairman of its Board of Commissioners and its Treasurer and attested by the
manual or facsimile signature of its Clerk or Deputy Clerk. The Bonds shall then
be delivered to the Trustee for authentication by it. In case any officer who
shall have signed any of the Bonds shall cease to be such officer before the
Bonds so signed or attested shall have been authenticated or delivered by the
Registrar or issued by the Issuer, such Bonds may nevertheless be authenticated,
delivered and issued and, upon such authentication, delivery and issuance, shall
be as binding upon the Issuer as though those who signed and attested the same
had continued to be such officers of the Issuer. Also, any Bond may be signed on
behalf of the Issuer by such persons as on the actual date of the execution of
such Bond shall be the proper officers although on the nominal date of such Bond
any such person shall not have been such officer.

         SECTION 2.06. AUTHENTICATION. No Bond shall be valid for any purpose
until the certificate of authentication on such Bond shall have been duly
executed by the Trustee, and such authentication shall be conclusive proof that
such Bond has been duly authenticated and delivered under this Indenture and
that the Owner thereof is entitled to the benefits of the trust hereby created.
The Trustee's certificate of authentication on any Bond shall be deemed to have
been executed by it if manually signed by an authorized signatory of the
Trustee, but it shall not be necessary that the same signatory sign the
certificate of authentication on all of the Bonds issued hereunder.

         Upon authentication of any Bond, the Trustee shall set forth on such
Bond (1) the date of such authentication and (2) if the Bonds are not Book-Entry
Bonds, in the case of a Bond bearing interest at a Flexible Rate or Taxable
Flexible Rate, such Flexible Rate or Taxable Flexible Rate, the day next
succeeding the last day of the applicable Flexible Segment or Taxable Flexible
Segment, the number of days comprising such Flexible Segment or Taxable Flexible
Segment and the amount of interest to accrue during such Flexible Segment or
Taxable Flexible Segment.

         SECTION 2.07. FORM OF BONDS. The Series 1999A Bonds and the certificate
of authentication to be executed thereon and the Series 1999B Bonds and the
certificate of authentication to be executed thereon shall be in substantially
the forms attached hereto as Exhibit A1 and Exhibit A2, respectively, with such
appropriate variations, omissions and insertions as are permitted or required by
this Indenture. The Bonds of any Tax-Exempt Series other than the Series 1999A
Bonds and the certificate of authentication to be executed thereon shall be in
substantially the form attached hereto as Exhibit B, with such appropriate
variations, omissions and insertion as are permitted or required by this
Indenture.

         Prior to conversion to a Tax-Exempt Series, each Taxable Flexible Bond,
Taxable Term Bond and Taxable Weekly Bond shall contain a legend substantially
to the following effect:

                                       31
<PAGE>   38

                  "By its acceptance of this Bond, each purchaser of this Bond
         will be deemed to have represented and agreed as follows: (1) the
         purchaser understands that the Bonds are being issued only in
         transactions not involving any public offering within the meaning of
         the Securities Act; (2) the purchaser is (A) a sophisticated
         institutional investor who (i) is an "Accredited Investor" as that term
         is defined in Rule 501(a) of Regulation D under the Securities Act (or
         is a fiduciary or agent (other than a U.S. bank or savings and loan
         association) that is purchasing the Bonds for the account of an
         Accredited Investor), (ii) has such knowledge and experience (or is a
         fiduciary or agent with sole investment discretion having such
         knowledge and experience) in financial and business matters that it (or
         such fiduciary or agent) is capable of evaluating the merits and risks
         of investing in such Bond, (iii) has had access to such information as
         the purchaser deems necessary in order to make an informed investment
         decision, and (iv) is purchasing the Bond for investment and not with a
         view to distribution; or (B) in the case of sale of Bonds pursuant to
         Rule 144A under the Securities Act, a "Qualified Institutional Buyer"
         as defined in Rule 144A (or is a Qualified Institutional Buyer
         purchasing the Bonds on behalf of one or more other Qualified
         Institutional Buyers); (3) if in the future the purchaser (or any such
         other investor or any other fiduciary or agent representing such
         investor) decides to sell such Bond prior to the maturity date, any
         redemption date, or the date fixed for mandatory purchase, it will be
         sold only in a transaction exempt from registration under or are not
         subject to the Securities Act and only to (i) [name of Remarketing
         Agent] or through [name of Remarketing Agent] to an institutional
         investor approved by [name of Remarketing Agent] as an institutional
         Accredited Investor or a Qualified Institutional Buyer, or (ii) a
         Qualified Institutional Buyer in a transaction made pursuant to Rule
         144A under the Securities Act; (4) the purchaser understands that,
         although [name of Remarketing Agent] may repurchase Bonds, [name of
         Remarketing Agent] is not obligated to do so, and accordingly the
         purchaser (or any such other investor) should be prepared to hold such
         Bond until the mandatory purchase thereof at the end of the Taxable
         Flexible Rate Segment, Taxable Weekly Rate Period or Taxable Term Rate
         Period, as applicable or until a Tax-Exempt Conversion Date, if any;
         and (5) the purchaser acknowledges that the Bond sold to the purchaser
         by [name of Remarketing Agent] may be sold to it pursuant to Rule
         144A."

         The Trustee shall not transfer any such Bond to a new owner if a
Responsible Officer of the Trustee has actual knowledge that such transfer would
violate the terms of the legend set forth above. The Trustee may request
documentation from a transferor or transferee thereof, including opinions or
certificates, to ensure compliance with the terms of such legend prior to
effecting any such transfer.

         Upon adjustment to a Term Rate Period or Taxable Term Rate Period, the
form of Bond may include a summary of the mandatory and optional redemption
provisions to apply to the Bonds during such Term Rate Period or Taxable Term
Rate Period, or a statement to the effect that the Bonds will not be optionally
redeemed during such Term Rate Period or Taxable Term Rate Period, and a
statement indicating the applicable Term Rate or Taxable Term Rate and the
duration of the applicable Term Rate Period or Taxable Term Rate Period,
provided that the Registrar shall not authenticate such a revised Bond form
prior to receiving an opinion of Bond Counsel that such Bond form conforms to
the terms of the Act and of this Indenture and

                                       32
<PAGE>   39

that authentication thereof will not adversely affect the Tax-Exempt status of
such Bonds if such Bonds are Bonds of a Tax-Exempt Series.

         SECTION 2.08. MUTILATED, DESTROYED, LOST OR STOLEN BONDS. In the event
any Bond or temporary Bond is mutilated, lost, stolen or destroyed, the Trustee
may authenticate a new Bond duly executed by the Issuer of like date and
denomination as that mutilated, lost, stolen or destroyed; provided that, in the
case of any mutilated Bond, such mutilated Bond shall first be surrendered to
the Trustee, and in the case of any lost, stolen or destroyed Bond, there shall
be first furnished to the Trustee evidence of such loss, theft or destruction
satisfactory to the Trustee, together with indemnity to the Issuer and the
Trustee satisfactory to them. In the event any such Bond shall have matured,
instead of issuing a duplicate Bond, the Trustee on behalf of the Issuer may pay
the same without surrender thereof. The Issuer and the Trustee may charge the
Owner of such Bond with their reasonable fees and expenses in this connection.
The Issuer shall cooperate with the Trustee in connection with the issue of
replacement Bonds, but nothing in this Section shall be construed in derogation
of any rights which the Issuer, the Borrower or the Trustee may have to receive
indemnification against liability, or payment or reimbursement of expenses, in
connection with the issue of a replacement Bond.

         If, after the delivery of such new Bond, a bona fide purchaser of the
original Bond in lieu of which such new Bond was issued presents for payment or
registration such original Bond, the Trustee shall be entitled to recover such
new Bond from the person to whom it was delivered or any person taking
therefrom, except a bona fide purchaser, and shall be entitled to recover upon
the security or indemnity provided therefor to the extent of any loss, damage,
cost or expense incurred by the Trustee or the Issuer in connection therewith.

         Each duplicate Bond delivered in accordance with this Section, except
as otherwise provided herein, shall constitute an original additional
contractual obligation of the Issuer and shall be entitled to the benefit and
security of this Indenture to the same extent as the Bond in lieu of which such
duplicate Bond was delivered.

         All Bonds shall be held and owned upon the express condition that the
foregoing provisions are, to the extent permitted by law, exclusive with respect
to the replacement or payment of mutilated, destroyed, lost or stolen Bonds, and
shall preclude any and all other rights or remedies.

         SECTION 2.09. TEMPORARY BONDS. Pending preparation of definitive Bonds,
or by agreement with the purchasers of all Bonds, the Issuer may issue and, upon
its request, the Trustee shall authenticate, in lieu of definitive Bonds, one or
more temporary printed or typewritten Bonds in Authorized Denominations of the
same Series and of substantially the tenor recited above. Upon request of the
Issuer, the Trustee shall authenticate definitive Bonds in exchange for and upon
surrender of an equal principal amount of temporary Bonds. Until so exchanged,
temporary Bonds shall have the same rights, remedies and security hereunder as
definitive Bonds.

         SECTION 2.10. CANCELLATION AND DISPOSITION OF SURRENDERED BONDS.
Whenever any Outstanding Bond shall be delivered to the Trustee

                                       33
<PAGE>   40

for transfer, exchange or cancellation pursuant to this Indenture, upon payment
of the principal amount represented thereby, or for replacement pursuant to
Section 2.08 hereof, such Bond shall be promptly canceled and disposed of by the
Trustee in accordance with its ordinary customs and practices.

         SECTION 2.11. USE OF CERTAIN MONEYS IN THE BOND FUND UPON REFUNDING. In
the event that refunding bonds shall be issued by the Issuer to pay the
principal of or premium, if any, on all or any portion of the Bonds, the net
proceeds of the refunding bonds remaining after payment of expenses incident to
the refunding shall be deposited by the Issuer into the Bond Fund as provided in
Section 6.03 hereof. All moneys remaining in the Bond Fund on the date of the
refunding to be used to pay interest on the Bonds to be refunded shall be held,
as collateral for the payment of the Bonds to be refunded, by the Trustee, in
trust for and on behalf of the Owners of the Bonds to be refunded, together with
the portion of the proceeds of the sale of the refunding bonds so deposited and
any investments or reinvestments of such proceeds, in one or more separate
subaccounts in the Bond Fund irrevocably in trust for the respective holders of
Bonds to be refunded, and upon defeasance of the Bonds to be refunded as
provided in Article VIII hereof shall be held, invested and used as provided in
Article VIII hereof. Investment income or profit on any such investments or
reinvestments shall remain in the Bond Fund.

         SECTION 2.12. DELIVERY OF THE BONDS. Upon or at any time after the
execution and delivery of this Indenture, the Issuer shall execute and deliver
to the Trustee and the Trustee shall authenticate the Series 1999A and Series
1999B Bonds and deliver them to the purchasers as directed by the Issuer as
hereinafter in this Section provided.

         Prior to the delivery by the Trustee of any of the Bonds there shall be
filed with the Trustee:

                  (1) A copy of the Resolution, duly certified by the Clerk or
         Deputy Clerk of the Issuer, authorizing issuance of such Bonds.

                  (2) Original executed counterparts of the Agreement, this
         Indenture, the Tax Certificate, the Bond Insurance and, in the case of
         any Bonds that may be remarketed from time to time, a Liquidity
         Facility.

                  (3) A request and authorization to the Trustee on behalf of
         the Issuer, signed by the Chairman and the Clerk or Deputy Clerk of the
         Issuer and acknowledged by the Borrower, to authenticate and deliver
         the Bonds pursuant to Section 2.14 hereof, registered in the names and
         in the Authorized Denominations specified to the Trustee by the Initial
         Purchaser, upon payment by the Initial Purchaser to the Trustee of the
         sum specified in such request and authorization for deposit in the
         Construction Fund and Costs of Issuance Fund, plus accrued interest, if
         any, on the Bonds to the date of delivery.

         SECTION 2.13. BOOK-ENTRY SYSTEM. (a) Anything in this Indenture to the
contrary notwithstanding, any Bond may be authorized and issued as a Book-Entry
Bond.

                                       34
<PAGE>   41

         (b) For all purposes of this Indenture, the Owner of a Book-Entry Bond
shall be the Securities Depository therefor and neither the Issuer, the Trustee,
the Paying Agent, the Tender Agent, the Remarketing Agent nor the Registrar
shall have any responsibility or obligation to the beneficial owner of such Bond
or to any direct or indirect participant in such Securities Depository, except
as expressly provided in this Indenture. Without limiting the generality of the
foregoing, neither the Issuer, the Trustee, the Paying Agent, the Tender Agent,
the Remarketing Agent nor the Registrar shall have any responsibility or
obligation to any such participant or to the beneficial owner of a Book-Entry
Bond with respect to (i) the accuracy of the records of the Securities
Depository or any participant with respect to any beneficial ownership interest
in such Bond, (ii) the delivery to any participant of the Securities Depository,
the beneficial owner of such Bond or any other person, other than the Securities
Depository, of any notice with respect to such Bond, including any notice of the
redemption or purchase thereof, or (iii) the payment to any participant of the
Securities Depository, the beneficial owner of such Bond or any other person,
other than the Securities Depository, of any amount with respect to the
principal, redemption price, if applicable, or purchase price of, or interest
on, such Bond. The Issuer, the Trustee, the Paying Agent, the Tender Agent, the
Remarketing Agent and the Registrar may treat the Securities Depository therefor
as, and deem such Securities Depository to be, the absolute owner of a
Book-Entry Bond for all purposes whatsoever, including, but not limited to, (1)
payment of the principal, redemption price, if applicable, or purchase price of,
and interest on, such Bond, (2) giving notices of redemption or purchase and of
other matters with respect to such Bond, (3) registering transfers with respect
to such Bond as permitted hereby and (4) except as expressly provided in this
Indenture, giving to the Issuer, the Trustee, the Paying Agent, the Tender
Agent, the Remarketing Agent or the Registrar any notice, consent, request or
demand pursuant to the Indenture for any purpose whatsoever. The Trustee, acting
as Paying Agent, shall pay the principal or redemption price, if applicable, of,
and interest on, a Book-Entry Bond, and the Trustee, acting as Tender Agent,
shall pay the purchase price of a Book-Entry Bond, only to or upon the order of
the Securities Depository therefor, and all such payments shall be valid and
effective to satisfy fully and discharge the Issuer's obligations with respect
to such principal or redemption price or purchase price, and interest, to the
extent of the sum or sums so paid. Except as otherwise provided in subsection
(d) of this Section 2.13, no person other than the Securities Depository shall
receive a Bond or other instrument evidencing the Issuer's obligation to make
payments of the principal, redemption price or purchase price thereof, and
interest thereon.

         (c) The Issuer, by notice to the Trustee, the Paying Agent, the Tender
Agent, the Registrar, the Remarketing Agent, if any, and a Securities
Depository, may, with the prior written consent of the Borrower, and shall, at
the written direction of an Authorized Borrower Representative, terminate the
services of such Securities Depository with respect to the Book-Entry Bonds for
which such Securities Depository serves as securities depository if the Issuer
determines that (i) the Securities Depository is unable to discharge its
responsibilities with respect to such Bond or (ii) a continuation of the
requirement that all of the Bonds issued as Book-Entry Bonds be registered in
the registration books of the Issuer kept by the Trustee in the name of the
Securities Depository is not in the best interests of the beneficial owners of
such Bonds or of the Issuer.

         (d) Upon the termination of the services of a Securities Depository
with respect to a Book-Entry Bond pursuant to clause (ii) of subsection (c) of
this Section 2.13, such

                                       35
<PAGE>   42

Bond no longer shall be restricted to being registered in the registration books
kept by the Registrar in the name of a Securities Depository. Upon the
termination of the services of a Securities Depository with respect to a
Book-Entry Bond pursuant to clause (i) of subsection (c) of this Section 2.13,
the Issuer may, with the prior written consent of the Borrower, and shall, at
the written direction of an Authorized Borrower Representative, within ninety
(90) days thereafter appoint a substitute securities depository which, in the
opinion of the Issuer, is willing and able to undertake the functions of
Securities Depository under this Indenture upon reasonable and customary terms.
If no such successor can be found within such period, such Book-Entry Bond shall
no longer be restricted to being registered in the registration books of the
Issuer kept by the Trustee in the name of a Securities Depository. In the event
that a Book-Entry Bond shall no longer be restricted to being registered in the
registration books of the Issuer kept by the Trustee in the name of a Securities
Depository, (i) the Issuer shall execute and the Trustee shall authenticate and
deliver, upon presentation and surrender of the Book-Entry Bond, Bond
certificates as requested by the Securities Depository so terminated of like
principal amount, maturity and interest rate, in Authorized Denominations, to
the identifiable beneficial owners in replacement of such beneficial owners'
beneficial ownership interests in such Book-Entry Bond and (ii) the Trustee
shall notify the Remarketing Agent, if any, and the Borrower that the Bonds are
no longer restricted to being registered in the registration books of the Issuer
kept by the Trustee in the name of a Securities Depository; provided, however
that such registration shall not be terminated by the Issuer or the Borrower
without an opinion of Bond Counsel confirming that such termination of
registration will not adversely affect the Tax-Exempt status of any Bonds of a
Tax-Exempt Series.

         (e) Anything in this Indenture to the contrary notwithstanding, payment
of the redemption price of a Book-Entry Bond, or portion thereof, called for
redemption prior to maturity may be paid to the Securities Depository by wire
transfer of immediately available funds. Anything in the Indenture to the
contrary notwithstanding, such redemption price may be paid without presentation
and surrender to the Trustee, as Paying Agent, of the Book-Entry Bond, or
portion thereof, called for redemption; provided, however, that payment of (a)
the principal payable at maturity of a Book-Entry Bond and (b) the redemption
price of a Book-Entry Bond as to which the entire principal amount thereof has
been called for redemption shall be payable only upon presentation and surrender
of such Book-Entry Bond to the Trustee, as Paying Agent; and provided, further,
that no such redemption price shall be so payable without presentation and
surrender unless such Book-Entry Bond shall contain or have endorsed thereon a
legend substantially to the following effect (or such other legend(s) of similar
content as may be determined to be necessary or desirable by the Issuer or the
Securities Depository):

         "AS PROVIDED IN THE INDENTURE REFERRED TO HEREIN, UNTIL THE TERMINATION
         OF THE SYSTEM OF BOOK-ENTRY-ONLY TRANSFERS THROUGH [NAME OF SECURITIES
         DEPOSITORY] (TOGETHER WITH ANY SUCCESSOR SECURITIES DEPOSITORY
         APPOINTED PURSUANT TO THE INDENTURE, "[NAME OF SECURITIES
         DEPOSITORY]"), AND NOTWITHSTANDING ANY OTHER PROVISION OF THE INDENTURE
         TO THE CONTRARY, (A) THIS BOND MAY BE TRANSFERRED, IN WHOLE BUT NOT IN
         PART, ONLY TO A NOMINEE OF [NAME OF SECURITIES DEPOSITORY], OR BY A
         NOMINEE OF [NAME OF SECURITIES DEPOSITORY] TO [NAME OF SECURITIES

                                       36
<PAGE>   43

         DEPOSITORY] OR A NOMINEE OF [NAME OF SECURITIES DEPOSITORY], OR BY
         [NAME OF SECURITIES DEPOSITORY] OR A NOMINEE OF [NAME OF SECURITIES
         DEPOSITORY] TO ANY SUCCESSOR SECURITIES DEPOSITORY OR ANY NOMINEE
         THEREOF AND (B) A PORTION OF THE PRINCIPAL AMOUNT OF THIS BOND MAY BE
         PAID OR REDEEMED WITHOUT SURRENDER HEREOF TO THE PAYING AGENT. [NAME OF
         SECURITIES DEPOSITORY] OR A NOMINEE, TRANSFEREE OR ASSIGNEE OF [NAME OF
         SECURITIES DEPOSITORY] OF THIS BOND MAY NOT RELY UPON THE PRINCIPAL
         AMOUNT INDICATED HEREON AS THE PRINCIPAL AMOUNT HEREOF OUTSTANDING AND
         UNPAID. THE PRINCIPAL AMOUNT HEREOF OUTSTANDING AND UNPAID SHALL FOR
         ALL PURPOSES BE THE AMOUNT DETERMINED IN THE MANNER PROVIDED IN THE
         INDENTURE."

Anything in this Indenture to the contrary notwithstanding, upon any such
payment to the Securities Depository without presentation and surrender, for all
purposes of (i) the Book-Entry Bond as to which such payment has been made and
(ii) this Indenture, the unpaid principal amount of such Book-Entry Bond
Outstanding shall be reduced automatically by the principal amount so paid. In
such event, the Trustee shall notify forthwith the Remarketing Agent, if any, as
to the particular Book-Entry Bond as to which such payment has been made, and
the principal amount of such Bond so paid, and the Trustee shall note such
payment on the registration books of the Issuer kept by it, but failure to make
any such notation shall not affect the automatic reduction of the principal
amount of such Book-Entry Bond Outstanding as provided in this subsection.

         (f) For all purposes of this Indenture authorizing or permitting the
purchase of Bonds, or portions thereof, by, or for the account of, the Issuer
for cancellation, and anything in the Indenture to the contrary notwithstanding,
a portion of a Book-Entry Bond may be deemed to have been purchased and
cancelled without surrender thereof upon delivery to the Trustee of a
certificate executed by the Issuer and a participant of the Securities
Depository therefor to the effect that a beneficial ownership interest in such
Bond, in the principal amount stated therein, has been purchased by, or for the
account of, the Issuer through the participant of the Securities Depository
executing such certificate; provided, however, that any purchase for
cancellation of the entire principal amount of a Book-Entry Bond shall be
effective for purposes of the Indenture only upon surrender of such Book-Entry
Bond to the Paying Agent; and provided, further, that no portion of a Book-Entry
Bond may be deemed to have been so purchased and cancelled without surrender
thereof unless such Book-Entry Bond shall contain or have endorsed thereon the
legend referred to in subsection (e) of this Section 2.13. Anything in the
Indenture to the contrary notwithstanding, upon delivery of any such certificate
to the Trustee, for all purposes of (i) the Book-Entry Bond to which such
certificate relates and (ii) this Indenture, the unpaid principal amount of such
Book-Entry Bond Outstanding shall be reduced automatically by the principal
amount so purchased. In such event, the Trustee shall immediately notify the
Remarketing Agent, if any, as to the particular Book-Entry Bond as to which such
payment has been made and the amount thereof and shall note such reduction in
principal amount of such Book-Entry Bond Outstanding on the registration books
of the Issuer kept by it, but failure to

                                       37
<PAGE>   44

make any such notation shall not affect the automatic reduction of the principal
amount of such Book-Entry Bond Outstanding as provided in this subsection.

         (g) Anything in this Indenture to the contrary notwithstanding, a
Securities Depository may make a notation on a Book-Entry Bond (i) redeemed in
part or (ii) purchased by, or for the account of, the Issuer in part for
cancellation, to reflect, for informational purposes only, the date of such
redemption or purchase and the principal amount thereof redeemed or deemed
cancelled, but failure to make any such notation shall not affect the automatic
reduction of the principal amount of such Book-Entry Bond Outstanding as
provided in subsection (e) or (f) of this Section 2.13, as the case may be.

         (h) Anything in this Indenture to the contrary notwithstanding, in the
case of a Book-Entry Bond, the Issuer shall be authorized to redeem or purchase
(by or for the account of the Issuer) less than all of the entire Outstanding
principal amount thereof, and in the event of such partial defeasance,
redemption, purchase or refunding, the provisions of the Indenture relating to
the defeasance, redemption, purchase or refunding of a Bond or Bonds shall be
deemed to refer to the defeasance, redemption, purchase or refunding of a
portion of a Bond.

         (i) The Issuer, the Trustee, the Paying Agent, the Tender Agent and the
Remarketing Agent may enter into an agreement with a Securities Depository for
the Bonds providing for procedures for the registration, payment, tender and
delivery of notices relating to the Bonds, provided that the terms of such
agreement shall not be inconsistent with the terms of this Indenture. Any such
agreement may provide that (i) such Securities Depository is not required to
present a Bond to the Trustee in order to receive a partial payment of
principal; (ii) a Bond need not be delivered to the Trustee in order for a
tender of such Bond pursuant to Article IV of this Indenture to be effective or
in order for the purchase price of such tendered Bond to be paid and that notice
of tender of a Bond for purchase pursuant to Article IV hereof may be given to
the Trustee by a beneficial owner of a Bond or a direct participant of the
Securities Depository; (iii) a legend with respect to the registration of the
Bond in the name of the Securities Depository shall appear on each Bond so long
as the Bonds are subject to such agreement; and (iv) different provisions for
notices to such Securities Depository may be set forth therein; and such
provisions shall be binding on the Issuer, the Trustee, the Paying Agent, the
Tender Agent and the Remarketing Agent for so long as such Securities Depository
is the Securities Depository for Book-Entry Bonds hereunder.

         SECTION 2.14. DELIVERY OF THE BONDS. DESIGNATION OF THE BONDS AS
BOOK-ENTRY BONDS; APPOINTMENT OF INITIAL SECURITIES DEPOSITORY FOR THE BONDS.
(a) The Bonds are hereby authorized to be and shall be issued initially, subject
to the provisions of this Indenture, as Book-Entry Bonds within the meaning of
and subject to Section 2.13 hereof.

         (b) DTC is hereby appointed as the initial Securities Depository for
the Bonds.

         (c) The Bonds of each Series (including any Bond issued on a Tax-Exempt
Conversion Date) shall be initially issued in the form of a separate single,
fully registered Bond in the aggregate principal amount thereof. So long as DTC
serves as Securities Depository for

                                       38
<PAGE>   45

the Bonds, the Owner of all Bonds shall be, and each of the Bonds shall be
registered in the name of, Cede & Co. ("Cede"), as nominee for DTC. Upon
delivery by DTC to the Trustee of written notice to the effect that DTC has
determined to substitute a new nominee in place of Cede, and subject to the
transfer provisions of the Indenture, the word "Cede" in the Indenture shall
refer to such new nominee of DTC. So long as any Bond is registered in the name
of Cede, as nominee for DTC in its capacity as Securities Depository for the
Bonds, all payments with respect to the principal, redemption price, if
applicable, or purchase price of, and interest on, such Bond and all notices
with respect to such Bond shall be made or given, as the case may be, to DTC as
provided in the Indenture and in the representation letter of the Issuer, the
Trustee, the Paying Agent and the Remarketing Agent, if any, delivered in
connection with the issuance of the Bonds and addressed to DTC, as such
representation letter may be amended and supplemented from time to time.

         SECTION 2.15. CONVERSION OF SERIES 1999B BONDS TO TAX-EXEMPT SERIES.

         The Borrower may give written notice at any time to the Issuer, the
Remarketing Agent, the Liquidity Provider, the Bond Insurer and the Trustee that
it intends to effect a conversion of all or a portion of the Series 1999B Bonds
to Bonds of a new or existing Tax-Exempt Series (other than Series 1999A Bonds)
on the day specified in such notice, which date shall be the Tax-Exempt
Conversion Date for such Bonds and shall be not less than 15 days after the date
of such notice (or such shorter period of time as to which the Trustee agrees)
and shall be the day following the end of a Taxable Flexible Rate Segment with
respect to any Taxable Flexible Bonds, the day specified in such notice with
respect to any Taxable Weekly Bonds, or the day following the end of the Taxable
Term Rate Period or a day on which such Bonds are subject to redemption pursuant
to Section 3.01(A)(3) with respect to any Taxable Term Bonds to be converted. If
less than all of the Taxable Flexible Bonds, Taxable Term Bonds or Taxable
Weekly Bonds are being converted to a Tax-Exempt Series, the notice shall state
the amount and the identity of such Bonds to be so converted as provided in
Section 2.02(a). Together with such notice, the Borrower shall file with the
Issuer, the Remarketing Agent, and the Trustee a form of opinion of Bond Counsel
acceptable to the Issuer, the Remarketing Agent, and the Trustee (which opinion
may be based on rulings of the Internal Revenue Service) to the effect that (i)
the conversion of such Bonds will not adversely affect the validity thereof,
(ii) the interest on such Bonds shall be Tax-Exempt, and (iii) that such
conversion is permitted by the terms hereof. No proposed conversion of such
Bonds to a Tax-Exempt Series shall become effective unless (i) the Borrower
shall also file with the Issuer, the Remarketing Agent, the Liquidity Provider,
and the Trustee such an opinion dated the Tax-Exempt Conversion Date, and (ii)
except in the case of a conversion to a Term Rate with a Term Rate Period fixed
to maturity, a Liquidity Facility meeting the requirements of Section 5.14 of
the Agreement will be in full force and effect after the Tax-Exempt Conversion
Date and the Borrower is not in default thereunder. Upon compliance with the
applicable provisions of Section 2.03 of this Indenture simultaneously with the
provisions of this Section 2.15, the Borrower may, on the Tax-Exempt Conversion
Date, either (i) convert the Rate Period of the Series 1999B Bonds to be
converted to a Tax-Exempt Series to a new Rate Period on the Tax-Exempt
Conversion Date, or (ii) convert the Rate Period of such Bonds to be converted
to an Outstanding Tax-Exempt Series (other than the Series 1999A Bonds) to the
existing Rate Period of such Outstanding Bonds, all as stated in the notice from
the Borrower to the Trustee, the Issuer, the Liquidity Provider and the
Remarketing Agent.

                                       39
<PAGE>   46

                                  ARTICLE III

                       REDEMPTION OF BONDS BEFORE MATURITY

         SECTION 3.01. REDEMPTION DATES AND PRICES. The Bonds shall be subject
to redemption prior to maturity in the amounts, at the times and in the manner
provided in this Article III.

         (A) Optional Redemption.

         (1) On any Business Day during a Daily Rate Period, a Weekly Rate
Period or a Taxable Weekly Rate Period for any Bonds, and on the day after the
last day of any such Rate Period, such Bonds shall be subject to redemption by
the Issuer, at the written direction of the Borrower to the Issuer and the
Trustee, in whole or in part, at 100% of their principal amount, plus accrued
interest, if any, to the redemption date (but, with respect to Taxable Weekly
Bonds, only to the extent of Available Moneys).

         (2) On the day next succeeding the last day of any Flexible Segment or
of any Taxable Flexible Rate Segment with respect to any Bonds, such Bonds shall
be subject to redemption by the Issuer, at the written direction of the Borrower
to the Issuer and the Trustee, in whole or in part, at 100% of their principal
amount, plus accrued interest, if any, to the redemption date.

         (3) During any Term Rate Period or Taxable Term Rate Period for any
Bonds, such Bonds shall be subject to redemption by the Issuer, at the written
direction of the Borrower to the Issuer and the Trustee, during the periods
specified below, in whole at any time or in part from time to time on any date
(but, with respect to Taxable Term Bonds, only to the extent of Available
Moneys), at the redemption prices (expressed as percentages of principal amount)
set forth below plus accrued interest, if any, to the redemption date:

                                       40
<PAGE>   47

             Length of Term
               Rate Period
          (expressed in years)                Redemption Prices
          --------------------                -----------------
       greater than or equal to 15        after 10 years at 102%,
                                          declining by 1% annually to
                                          100%

       less than 15 and 10 or greater     after 8 years at 101.5%,
                                          declining by 0.75% annually
                                          to 100%

       less than 10 and 5 or greater      after 5 years at 101%,
                                          declining by 0.5% annually
                                          to 100%

       less than 5                        only on commencement of next
                                          Term Rate Period or Taxable
                                          Term Rate Period at 100%

With respect to any Term Rate Period or Taxable Term Rate Period, the Borrower
may specify in its notice of adjustment to or continuation of a Term Rate Period
or Taxable Term Rate Period redemption prices and periods other than those set
forth above for Bonds in such Rate Period not then called for redemption;
provided, however, that such notice shall be accompanied by an opinion of Bond
Counsel to the effect that such changes in redemption prices and periods (i) are
authorized or permitted by the Act and this Indenture, and (ii) if applicable,
will not adversely affect the Tax-Exempt status of the Bonds of any Tax-Exempt
Series.

         (4) The Bonds shall be redeemed in whole at any time at a redemption
price equal to 100% of the principal amount thereof plus accrued interest, if
any, to the redemption date (but, as applicable, only to the extent of Available
Moneys) upon receipt by the Trustee of a written notice from the Borrower
stating that any of the following events has occurred and that it therefore
intends to exercise its option to prepay the payments due under the Agreement in
whole pursuant to Section 7.1 of the Agreement and thereby effect the redemption
of the Bonds in whole:

         (a) all or substantially all of the Project shall be damaged or
destroyed and it is not practicable or desirable to rebuild, repair and restore
the Project;

         (b) all or substantially all of the Project shall be condemned or such
use or control thereof shall be taken by eminent domain so as to render the
Project unsatisfactory for continued operation;

         (c) unreasonable burdens or excessive liabilities shall be imposed upon
the Issuer or the Borrower with respect to the Project or the operation thereof;

                                       41
<PAGE>   48

         (d) changes that cannot reasonably be controlled or overcome in the
economic availability of materials, supplies, labor, equipment and other
properties and things necessary for the efficient operation of the Project for
the purposes contemplated by the Agreement shall have occurred or technological
changes that cannot reasonably be overcome shall have occurred which, in the
judgment of the Borrower, render the continued operation of the Project
uneconomic; or

         (e) legal curtailment of the use and occupancy of all or substantially
all of the Project for any reason, which curtailment shall prevent the carrying
on of normal operations at the Project for a period of three consecutive months.

         (B) Mandatory Redemption.

         (1) The Bonds are subject to mandatory redemption, at any time, at a
redemption price equal to 100% of the principal amount thereof plus accrued
interest, if any, to the redemption date not more than 180 days after the
occurrence of the following event (of which a Responsible Officer of the Trustee
shall be given notice in writing by an Authorized Issuer Representative), upon
fulfillment by the Borrower of its obligation to prepay the payments due under
the Agreement in accordance with Section 7.2 of the Agreement, if, as a result
of any changes in the Constitution of the State or in the Constitution of the
United States of America or of legislative or administrative action (whether
state or Federal), or by final decree, judgment or order of any court or
administrative body (whether state or Federal) entered after the contest thereof
by the Borrower in good faith, the Agreement shall have become impossible of
performance in accordance with the intent and purposes of the parties as
expressed in the Agreement.

         (2) The Bonds of any Tax-Exempt Series are subject to mandatory
redemption, at any time, at a redemption price equal to 100% of the principal
amount thereof plus accrued interest, if any, to the redemption date not more
than 180 days after the occurrence of the following event (of which a
Responsible Officer of the Trustee shall be given notice in writing by an
Authorized Issuer Representative), upon fulfillment by the Borrower of its
obligation to prepay the payments due under the Agreement in accordance with
Section 7.2 of the Agreement, in the event a final determination by an
administrative agency or a court of competent jurisdiction occurs to the effect
that, solely as a result of failure by the Borrower to observe any covenant,
agreement or representation by the Borrower in the Agreement, the interest
payable on the Bonds of such Tax-Exempt Series or any of them is no longer
Tax-Exempt. No determination by any court or administrative agency will be
considered final unless the Borrower has participated in the proceeding which
resulted in such determination, either directly or, at the option of the
Borrower, through an Owner to a degree it reasonably deems sufficient and until
the conclusion of any appellate review sought by any party to such proceeding or
the expiration of the time for seeking such review. Subject to the foregoing,
the Tax-Exempt Series will be redeemed in whole unless, in the opinion of Bond
Counsel delivered to the Trustee, the redemption of a portion of such Tax-Exempt
Series would have the result that interest payable on the Tax-Exempt Series
remaining outstanding after such redemption would be Tax-Exempt.

                                       42
<PAGE>   49

         (C) Liquidity Provider Bonds. In addition to the foregoing provisions
for the redemption of Bonds, any Bond which is acquired by a Liquidity Provider
pursuant to a Liquidity Facility shall be subject to redemption at the time and
in the amount and at the price specified by such Liquidity Facility.

         SECTION 3.02. NOTICE OF REDEMPTION. Notice of the call for any
redemption of Bonds or any portion thereof (which shall be in Authorized
Denominations) pursuant to Section 3.01 hereof identifying the Bonds or portions
thereof to be redeemed, specifying the Series of Bonds to be redeemed, the
redemption date, the redemption price, the place and manner of payment and that
from the redemption date interest will cease to accrue, shall be given by the
Trustee by mailing a copy of the redemption notice by first-class mail, postage
prepaid, to the Owner of each Bond to be redeemed in whole or in part, at the
address shown on the registration books, with a copy to the Tender Agent, if
any, the Bond Insurer, and the Liquidity Provider, if any. Such notice shall be
given at least thirty (30) days but not more than sixty (60) days prior to the
date fixed for redemption; provided, however, that failure to duly give such
notice, or any defect therein, shall not affect the validity of any proceedings
for the redemption of Bonds with respect to which no such failure or defect
occurred, and provided further that no notice of redemption shall be required
for any Bonds which are otherwise subject to mandatory purchase pursuant to
Section 4.02(a). Upon presentation and surrender of Bonds so called for
redemption in whole or in part at the place or places of payment, except as
otherwise provided in Section 2.13 hereof with respect to Book-Entry Bonds, such
Bonds or portions thereof shall be redeemed.

         With respect to any notice of redemption of Bonds at the written
direction of the Borrower, unless upon the giving of such notice such Bonds
shall be deemed to have been paid within the meaning of Article VIII hereof,
such notice may state (if so directed by the Borrower in writing) that such
redemption shall be conditional upon the receipt by the Trustee, on or before
the date fixed for such redemption, of moneys sufficient to pay the principal of
and premium, if any, and interest on such Bonds to be redeemed, and that if such
moneys shall not have been so received said notice shall be of no further force
and effect and the Issuer shall not be required to redeem such Bonds. In the
event that such notice of redemption contains such a condition and such moneys
are not so received, the redemption shall not be made and the Trustee shall
within a reasonable time thereafter give notice to such Owners, in the manner in
which the notice of redemption was given, that such moneys were not so received.

         Any notice mailed as provided in this Section shall be conclusively
presumed to have been duly given, whether or not the Owner receives the notice.

         If a Bond is presented to the Trustee for transfer after notice of
redemption of such Bond has been mailed as herein provided, the Trustee shall
deliver a copy of such notice of redemption to the new Owner of such Bond.

         In addition to the foregoing notice, further notice may be given by the
Trustee as set out below, but no defect in said further notice nor any failure
to give all or any portion of such further notice shall in any manner (i) defeat
the effectiveness of a call for redemption if notice thereof is given as above
prescribed or (ii) give rise to any liability on the part of the

                                       43
<PAGE>   50

Issuer, the Borrower, the Liquidity Provider, the Bond Insurer, the Trustee or
the Remarketing Agent:

                  A. Each further notice of redemption given hereunder may
         contain the information required above for an official notice of
         redemption plus (i) the CUSIP number of the Bonds; (ii) the date of
         issue of the Bonds; (iii) the rate or rates of interest borne by the
         Bonds; (iv) the maturity date of the Bonds; and (v) any other
         descriptive information needed to identify accurately the Bonds being
         redeemed.

                  B. Each further notice of redemption may be sent to all
         registered securities depositories then in the business of holding
         substantial amounts of obligations of types comprising the Bonds (such
         depositories as of the date hereof being The Depository Trust Company,
         New York, New York and Midwest Securities Trust Company, Chicago,
         Illinois).

                  C. Each further notice of redemption may be published one time
         in The Bond Buyer of New York, New York or, if such publication is
         impractical, in some other financial newspaper or journal which
         regularly carries notices of redemption of other obligations similar to
         the Bonds, such publication to be made at the time the redemption
         notice to the Owners is required to be given as provided in the first
         paragraph of this Section 3.02.

                  D. Each further notice of redemption may be given to two of
         the following services selected by the Borrower and at the address
         provided to the Trustee by the Borrower:

                  (1)      Financial Information, Inc.'s Financial Daily Called
                           Bond Service;

                  (2)      Interactive Data Corporation's Bond Service;

                  (3)      Kenny Information Service's Called Bond Service;

                  (4)      Moody's Municipal and Government Called Bond Service;
                           or

                  (5)      S&P's Called Bond Record.

         SECTION 3.03. DEPOSIT OF FUNDS. For the redemption of any of the Bonds,
the Issuer shall cause to be deposited in the Bond Fund out of the Revenues, to
the extent available therefor, moneys sufficient to pay when due the principal
of and premium, if any, and interest on the redemption date.

         SECTION 3.04. PARTIAL REDEMPTION OF BONDS. In case a Bond is of a
denomination larger than the minimum Authorized Denomination, all or a portion
of such Bond may be redeemed provided the principal amount not being redeemed is
in an Authorized Denomination. Upon surrender of any Bond for redemption in part
only, the Issuer shall execute and the Trustee shall authenticate and deliver to
the Owner thereof, without cost to the Owner, a new Bond or Bonds of Authorized
Denominations in aggregate principal amount equal to the unredeemed portion of
the Bond surrendered.

                                       44
<PAGE>   51

         SECTION 3.05. SELECTION OF BONDS FOR REDEMPTION. If less than all of
the Bonds of any Series are called for redemption, the Trustee shall select the
Bonds of such Series or portions thereof to be redeemed, from the Bonds of such
Series Outstanding not previously called for redemption, by lottery or in such
other manner as in the Trustee's sole discretion it shall deem appropriate and
fair, in either case in Authorized Denominations provided that Liquidity
Provider Bonds shall be the first Bonds selected for redemption and provided
further that the aggregate principal amount of each Bond remaining Outstanding
following such redemption shall be in an Authorized Denomination. The Trustee
shall promptly notify the Issuer and the Borrower in writing of the Bonds or
portions thereof selected for redemption, provided, however, that in connection
with any redemption of Bonds the Trustee shall select for redemption any Bonds
held by the Trustee for the account of the Borrower or held of record by the
Borrower prior to any Bonds other than Liquidity Provider Bonds. If, as
indicated in a certificate of an Authorized Borrower Representative delivered to
the Trustee, the Borrower shall have offered to purchase all Bonds of such
Series then outstanding and less than all such Bonds of such Series shall have
been tendered to the Borrower for such purchase, the Trustee, at the direction
of the Borrower, shall select for redemption all such Bonds of such Series
regardless of whether such Bonds have been so tendered. If it is determined that
one or more, but not all, of the units of principal amount represented by any
such Bond is to be called for redemption, then, upon notice of intention to
redeem such unit or units, the Owner of such Bond shall, except as provided in
Section 2.13 hereof with respect to Book-Entry Bonds, forthwith surrender such
Bond to the Trustee for (a) payment to such Owner of the redemption price of the
unit or units of principal amount called for redemption, and (b) delivery to
such Owner of a new Bond or Bonds in the aggregate principal amount of the
unredeemed balance of the principal amount of such Bond. New Bonds representing
the unredeemed balance of the principal amount of such Bond shall be issued to
the Owner thereof, without charge therefor. If the surrender of such Bonds is
required hereunder and the Owner of any such Bond shall fail to present such
Bond to the Trustee for payment and exchange as aforesaid, such Bond shall,
nevertheless, become due and payable, and interest thereon shall cease to
accrue, on the date fixed for redemption to the extent of the unit or units of
principal amount called for redemption (and to that extent only). Payment of the
redemption prices by the Borrower for any Series of Bonds called for redemption
constitutes full and complete payment of the Bonds of such Series.

                                   ARTICLE IV

                          TENDER AND PURCHASE OF BONDS;
                         REMARKETING; REMARKETING AGENT

         SECTION 4.01. PURCHASE OF BONDS AT OPTION OF OWNERS.

         (a) Daily Rate Period. On any Business Day during any Daily Rate Period
for any Tax-Exempt Series of Bonds, any such Bond (or portion thereof in an
Authorized Denomination provided that the principal amount to be retained by the
Owner shall be in an Authorized Denomination) shall be purchased by the Trustee,
acting as Tender Agent, at a purchase price equal to 100% of the principal
amount thereof plus accrued interest, if any, thereon to the date of purchase,
upon, subject to Section 4.10 hereof, (i) delivery by the Owner of such Bond to
the Trustee, acting as Tender Agent, at its Principal Office by no later than
11:00

                                       45
<PAGE>   52

a.m., New York time, on such Business Day, of an irrevocable written notice or
an irrevocable telephonic notice, promptly confirmed by telecopy or other
writing, which states the principal amount or portion thereof to be purchased
and number of such Bond (if such Bond is in certificated form) and the date on
which such Bond shall be purchased pursuant to this subsection (a), and (ii)
delivery of such Bond (if such Bond is in certificated form) to the Trustee,
acting as Tender Agent, at its Principal Office accompanied by an instrument of
transfer thereof, in form satisfactory to the Trustee, executed in blank by the
Owner thereof with the signature of such Owner guaranteed by a bank, trust
company or member firm of the New York Stock Exchange, at or prior to 1:00 p.m.,
New York time, on such Business Day.

         (b) Weekly Rate Period. On any Business Day, during any Weekly Rate
Period for any Tax-Exempt Series of Bonds, any such Bond (or portion thereof in
an Authorized Denomination provided that the principal amount to be retained by
the Owner shall be in an Authorized Denomination) shall be purchased from its
Owner by the Trustee, acting as Tender Agent, at a purchase price equal to 100%
of the principal amount thereof plus accrued interest, if any, thereon to the
date of purchase, upon, subject to Section 4.10 hereof, (i) delivery by the
Owner of such Bond to the Trustee, acting as Tender Agent, at its Principal
Office of an irrevocable written notice or an irrevocable telephonic notice
promptly confirmed by telecopy or other writing, which states the principal
amount or portion thereof to be purchased and number of such Bond (if such Bond
is in certificated form) and the date on which the same shall be purchased,
which date shall be a Business Day not prior to the seventh day next succeeding
the date of the delivery of such notice to the Trustee, and (ii) delivery of
such Bond (if such Bond is in certificated form) to the Trustee, acting as
Tender Agent, at its Principal Office, accompanied by an instrument of transfer
thereof, in form satisfactory to the Trustee, executed in blank by the Owner
thereof with the signature of such Owner guaranteed by a bank, trust company or
member firm of the New York Stock Exchange, at or prior to 10:00 a.m., New York
time, on the date specified in such notice.

         (c) Taxable Weekly Rate Period. On any Business Day, during any Taxable
Weekly Rate Period for any Taxable Weekly Bonds, any such Taxable Weekly Bond
(or portion thereof in an Authorized Denomination provided that the principal
amount to be retained by the Owner shall be in an Authorized Denomination) shall
be purchased from its Owner by the Trustee, acting as Tender Agent, at a
purchase price equal to 100% of the principal amount thereof plus accrued
interest, if any, thereon to the date of purchase, upon, subject to Section 4.10
hereof, (i) delivery by the Owner of such Taxable Weekly Bond to the Trustee,
acting as Tender Agent, at its Principal Office of an irrevocable written notice
or an irrevocable telephonic notice promptly confirmed by telecopy or other
writing, which states the principal amount or portion thereof to be purchased
and number of such Taxable Weekly Bond (if such Taxable Weekly Bond is in
certificated form) and the date on which the same shall be purchased, which date
shall be a Business Day not prior to the seventh day next succeeding the date of
the delivery of such notice to the Trustee, and (ii) delivery of such Taxable
Weekly Bond (if such Taxable Weekly Bond is in certificated form) to the
Trustee, acting as Tender Agent, at its Principal Office, accompanied by an
instrument of transfer thereof, in form satisfactory to the Trustee, executed in
blank by the Owner thereof with the signature of such Owner guaranteed by a
bank, trust company or member firm of the New York Stock Exchange, at or prior
to 10:00 a.m., New York time, on the date specified in such notice.

                                       46
<PAGE>   53

         SECTION 4.02. MANDATORY PURCHASE OF BONDS.

         (a) Any Bonds, other than the Series 1999A Bonds, shall be subject to
mandatory purchase at a purchase price equal to 100% of the principal amount
thereof plus accrued interest thereon, unless the Mandatory Purchase Date is an
Interest Payment Date, on the dates stated below:

                  (i) As to each such Bond in a Flexible Rate Period or Taxable
         Flexible Rate Period, on the day next succeeding the last day of each
         Flexible Segment or Taxable Flexible Segment thereof applicable to such
         Bond;

                  (ii) As to each Taxable Weekly Bond, on the effective date of
         change from a Taxable Weekly Rate Period to a new Rate Period other
         than a Taxable Weekly Rate Period;

                  (iii) As to each Term Bond or Taxable Term Bond, on the
         effective date of change from a Term Rate or Taxable Term Rate, as the
         case may be, to a new Rate Period, including a change from one Term
         Rate Period or Taxable Term Rate Period to another Term Rate Period or
         Taxable Term Rate Period, respectively, of the same duration; provided
         that Bonds in a Term Rate Period or Taxable Term Rate Period which are
         then redeemable pursuant to Section 3.01(A) hereof shall be purchased
         at a purchase price equal to 100% of the principal amount thereof plus
         a premium equal to the redemption premium, if any, that would have been
         payable if such Bonds were to be redeemed on the date such Bonds are to
         be purchased pursuant to the terms hereof, together with accrued
         interest, if any, thereon to the date of purchase;

                  (iv) On the Business Day prior to the Expiration Date of the
         applicable Liquidity Facility (including any such Expiration Date which
         occurs by reason of the delivery of an Alternate Liquidity Facility);
         and

                  (v) As to each Bond in a Daily Rate Period or Weekly Rate
         Period, on the effective date of change to a new Rate Period, other
         than a change from one Daily Rate Period to another Daily Rate Period
         or a change from one Weekly Rate Period to another Weekly Rate Period.

         Notwithstanding the foregoing, the Bonds will not be subject to
mandatory purchase under (iv) if the Borrower is no longer required to maintain
a Liquidity Facility under Section 5.15 of the Agreement. In addition,
notwithstanding the foregoing, the Bonds will not be subject to mandatory
purchase pursuant to Section 4.02(a)(iv) if at least 25 days before the
Expiration Date (i) the Trustee has received written notice from the Liquidity
Provider that the Liquidity Facility then in effect has been extended, or (ii)
an Alternate Liquidity Facility is delivered to the Trustee and is effective,
together with written evidence that the replacement of the Liquidity Facility
theretofore in effect with such Alternate Liquidity Facility will not result in
any suspension, withdrawal or reduction of the short-term ratings, if any,
assigned to the Bonds by S&P and Moody's.

         (b) Subject to Section 4.10 hereof, an Owner must deliver each such
Bond subject to mandatory purchase as provided in Section 4.02(a) hereof to the
Trustee, acting as

                                       47
<PAGE>   54

Tender Agent, at its Principal Office accompanied by an instrument of transfer
thereof, in form satisfactory to the Trustee, executed in blank by the Owner
thereof, with the signature of such Owner guaranteed by a bank, trust company or
member firm of the New York Stock Exchange at or prior to 10:00 a.m., New York
time, on the purchase date in order to receive payment of the purchase price on
such date.

         (c) Notice of each mandatory purchase pursuant to the provisions of
Section 4.02(a) hereof is hereby required by the provisions of Section
2.03(a)(iii), 2.03(b)(iii), 2.03(c)(iii), 2.03(d)(iii), 2.03(e)(iii),
2.03(f)(iii) or 2.03(g)(iii), as the case may be, to be included in the notice
given pursuant to such Section or by delivery of an Alternate Liquidity Facility
which does not require a mandatory purchase. No notice of any mandatory purchase
pursuant to the provisions of Section 4.02(a)(i) hereof shall be given to the
Owners of the Bonds.

         SECTION 4.03. OBLIGATION TO SURRENDER BONDS.

         The giving of notice as provided in Section 4.01 hereof shall
constitute the irrevocable tender for purchase of each such Bond or portion
thereof with respect to which such notice shall have been given, irrespective of
whether such Bond shall be delivered as provided in Section 4.01. The occurrence
of any event specified in Section 4.02(a) hereof shall constitute the mandatory
tender for purchase of each such Bond or portion thereof, irrespective of
whether such Bond shall be delivered as provided in Section 4.02(b). Upon the
purchase of each such Bond or portion thereof so deemed to be tendered, such
Bond or portion thereof shall cease to bear interest payable to the former Owner
thereof, who thereafter shall have no rights with respect thereto, other than
the right to receive the purchase price thereof upon surrender of such Bond to
the Trustee, acting as Tender Agent, and such Bond or portion thereof shall be
no longer outstanding. If such Bonds are no longer Book-Entry Bonds, the Trustee
shall authenticate, register and deliver new Bonds in replacement of such Bonds
or portions thereof deemed so tendered and not surrendered on the date of
purchase.

         SECTION 4.04. REMARKETING OF BONDS.

         (a) By 11:30 a.m., New York time, on the date the Trustee receives
notice from any Bondholder in accordance with Section 4.01(a) hereof, and
promptly, but in no event later than 11:30 a.m., New York time, on the Business
Day following the day on which the Trustee receives notice from any Bondholder
of its demand to have the Trustee purchase Bonds pursuant to Section 4.01(b) or
(c) hereof, the Trustee shall give facsimile or telephonic notice, confirmed in
writing thereafter, to the Remarketing Agent specifying the principal amount of
Bonds which such Bondholder has demanded to have purchased and the date on which
such Bonds are demanded to be purchased, with a copy of such notice to the
Liquidity Provider, if a Liquidity Facility is in effect with respect to such
Series of Bonds.

         (b) Upon the giving of notice to the Trustee by any Bondholder in
accordance with Section 4.01(a), (b) or (c) hereof and the giving of notice by
the Trustee to the Remarketing Agent as provided in Section 4.04(a) hereof with
respect to such notices, and on each date on which Bonds are to be purchased in
accordance with Section 4.02 hereof, the Remarketing Agent shall offer for sale
and use its reasonable best efforts to sell such Bonds on the date such Bonds
are to be purchased at a purchase price equal to 100% of the principal amount
thereof plus

                                       48
<PAGE>   55

accrued interest, if any, to the purchase date; provided that Bonds in a Term
Rate Period or Taxable Term Rate Period which are then redeemable pursuant to
Section 3.01(A) hereof shall be purchased at a purchase price equal to 100% of
the principal amount thereof plus a premium equal to the redemption premium, if
any, that would be payable if such Bonds were to be redeemed on the date they
are to be purchased, together with accrued interest, if any, thereon to the date
of purchase. The Remarketing Agent shall only remarket Taxable Flexible Bonds,
Taxable Weekly Bonds and Taxable Term Bonds to investors who meet the
requirements described in the legend set forth in Section 2.07 hereof. The
Remarketing Agent shall not sell any Bonds to the Issuer or the Borrower.

         (c) Not later than 1:00 p.m., New York time, on the Business Day next
preceding the date on which Bonds are to be purchased pursuant to Section 4.01
or Section 4.02 hereof, the Remarketing Agent shall give (i) facsimile or
telephonic notice to the Trustee, acting as Tender Agent, specifying the names,
addresses and taxpayer identification numbers of the purchasers of, and the
principal amount and denominations of, and, with respect to such Bonds which are
being purchased pursuant to Section 4.02(a)(i) hereof, the Flexible Segments and
the Flexible Rates for, or the Taxable Flexible Rate Segments and the Taxable
Flexible Rates for, as applicable, such Bonds remarketed by it pursuant to
subsection (b) hereof and shall transfer all remarketing proceeds it has
received to that time to the Trustee, acting as Tender Agent, and the amount of
remaining remarketing proceeds it will provide to the Trustee on the date on
which Bonds are to be purchased, as set forth in Section 4.04(d) hereof and (ii)
telephonic notice to the Borrower and the Trustee, acting as Tender Agent, of
the principal amount of and accrued interest on any such Bonds not remarketed by
such time.

         (d) Upon the giving of the notice specified in Section 4.04(c)(i)
hereof, the Remarketing Agent shall be obligated to deliver to the Trustee,
acting as Tender Agent, the remaining amount of remarketing proceeds specified
in such notice to be received, as follows:

                  (i) in the case of Bonds which are being purchased pursuant to
         Section 4.01 or 4.02(a)(ii), (iii), (iv) or (v) hereof, by 1:00 p.m.,
         New York time, on the purchase date; and

                  (ii) in the case of Bonds which are being purchased pursuant
         to Section 4.02(a)(i) hereof, by 3:00 p.m., New York time, on the
         purchase date, subject only to timely delivery of Bonds by the Trustee,
         acting as Tender Agent, as set forth in Section 4.04(e) hereof and
         verification by the Remarketing Agent that such Bonds conform to the
         instructions contained in the notice given by the Remarketing Agent to
         the Trustee pursuant to Section 4.04(c) hereof.

Any remarketing proceeds received by the Remarketing Agent in excess of such
amounts so transferred shall be delivered as provided in Section 4.06 as soon as
practicable after the receipt thereof.

         (e) Subject to Section 4.10 hereof, upon receipt by the Trustee, acting
as Tender Agent, of notice from the Remarketing Agent pursuant to Section
4.04(c) hereof, the Trustee shall authenticate and deliver new Bonds to the
Remarketing Agent, as follows:

                                       49
<PAGE>   56

                  (i) in the case of Bonds which are being purchased pursuant to
         Section 4.01 or Section 4.02(a)(ii), (iii), (iv) or (v) hereof, and
         provided that moneys derived from the sources specified in Section
         4.05(a) hereof in an amount equal to the purchase price therefor shall
         have been received by the Trustee, acting as Tender Agent, by 1:00
         p.m., New York time, such new Bonds shall be delivered by 2:00 p.m.,
         New York time; and

                  (ii) in the case of Bonds which are being purchased pursuant
         to Section 4.02(a)(i) hereof, such new Bonds shall be delivered by 4:00
         p.m., New York time.

         SECTION 4.05. PURCHASE OF BONDS TENDERED TO TRUSTEE.

         (a) By the close of business on the date Bonds or portions thereof are
to be purchased pursuant to Section 4.01 or 4.02 hereof by the Trustee, acting
as Tender Agent, such Trustee, acting as Tender Agent, shall purchase, but only
from the funds listed below, such Bonds or portions thereof (in Authorized
Denominations) from the Owners thereof at a purchase price equal to the
principal amount thereof plus accrued interest, if any, to the date of purchase;
provided that Bonds in a Term Rate Period or Taxable Term Rate Period which are
then redeemable pursuant to Section 3.01(A) hereof shall be purchased at a
purchase price equal to 100% of the principal amount thereof plus a premium
equal to the redemption premium, if any, that would be payable if such Bonds
were to be redeemed on the date they are to be purchased, together with accrued
interest, if any, thereon to the date of purchase. Funds for the payment of such
purchase price shall be derived from the following sources in the order of
priority indicated:

                  (i) proceeds of the remarketing of such Bonds pursuant to
         Section 4.04 hereof to any purchaser except the Issuer or the Borrower;

                  (ii) proceeds of a draw on the Liquidity Facility, if any;

                  (iii) moneys furnished by the Borrower to the Trustee, acting
         as Tender Agent, pursuant to Section 4.2(b) of the Agreement or Section
         4.5 of the Agreement, provided that such moneys shall not be used to
         pay accrued interest, if any, on such Bonds.

         (b) The Trustee, acting as Tender Agent, shall:

                  (i) hold all Bonds delivered to it pursuant to Section 4.01 or
         4.02 hereof in trust for the benefit of the respective Bondholders
         which shall have so delivered such Bonds until moneys representing the
         purchase price of such Bonds shall have been delivered to or for the
         account of or to the order of such Bondholders; and

                  (ii) hold all moneys delivered to it hereunder for the
         purchase of such Bonds in trust for the benefit of the person or entity
         which shall have so delivered such moneys in a separate and segregated
         fund, and not commingle such funds with any other funds or invest such
         funds, until such Bonds purchased with such moneys shall have been
         delivered or deemed delivered to or for the account of such person or
         entity; provided, that any moneys so deposited with and held by the
         Trustee not so applied to the purchase of Bonds within one (1) year
         after the date of purchase shall be paid by the Trustee to the Borrower
         upon the written direction of the Authorized Borrower Representative
         and thereafter the former Bondholders shall be entitled to look only to
         the Borrower for

                                       50
<PAGE>   57

         payment of such purchase price, and then only to the extent of the
         amount so repaid, and the Borrower shall not be liable for any interest
         thereon and shall not be regarded as a trustee of such moneys, and the
         Trustee shall have no further responsibility with respect to such
         moneys. To the extent any moneys are held by the Trustee for the
         payment of the purchase price of such Bonds which have not been
         presented for payment, such moneys shall not be invested.

         SECTION 4.06. DELIVERY OF PURCHASED BONDS.

         (a) Bonds sold by the Remarketing Agent pursuant to Section 4.04 hereof
shall be delivered to the Remarketing Agent, as specified in Section 4.04(e)
hereof.

         (b) Bonds purchased by the Trustee, acting as Tender Agent, hereunder:

                  (i) with moneys described in clause (ii) of Section 4.05(a)
         hereof ("Liquidity Provider Bonds"), shall be held by the Trustee, as
         Tender Agent (upon written directions from the Liquidity Provider) or
         registered in the name of the Liquidity Provider on the registration
         books of DTC, with respect to Book-Entry Bonds. The Remarketing Agent
         shall seek to remarket any Liquidity Provider Bonds prior to
         remarketing any other Bonds tendered for purchase. The proceeds of any
         remarketing of Liquidity Provider Bonds shall be transferred by the
         Trustee to the Liquidity Provider. Upon receipt by the Trustee of funds
         representing the proceeds of the remarketing of Liquidity Provider
         Bonds, Bonds in place of such Liquidity Provider Bonds so purchased
         shall be made available for pick-up by the Remarketing Agent for
         subsequent delivery to the purchasers thereof, or the ownership
         interest shall be transferred to the new direct participants on the
         books of DTC. Prior to or simultaneously with such delivery, the
         proceeds of such remarketing shall have been or shall be transferred to
         the Liquidity Provider, and the Trustee as Tender Agent shall have
         received written confirmation from the Liquidity Provider of the
         reinstatement of the Liquidity Facility; and

                  (ii) with moneys described in clause (iii) of Section 4.05(a)
         hereof shall, at the direction of the Borrower, be (A) held by the
         Trustee, acting as Tender Agent, for the account of the Borrower, (B)
         canceled or (C) delivered to the Borrower.

         SECTION 4.07. NO SALES AFTER DEFAULT. Anything in this Indenture to the
contrary notwithstanding, there shall be no remarketing of Bonds pursuant to
this Article IV if there shall have occurred and be continuing an event of
default under Section 9.01 hereof; provided, that nothing in this Section 4.07
shall be construed as prohibiting purchases of Bonds pursuant to Section 4.01 or
4.02 hereof.

         SECTION 4.08. REMARKETING AGENT. The initial Remarketing Agent shall be
Lehman Brothers Inc., provided, however, that the Issuer may at its own
discretion, and shall, at the written direction of the Borrower, remove the
Remarketing Agent upon at least five (5) Business Days' notice in writing;
provided, further, the appointment of any Remarketing Agent shall terminate upon
the defeasance of all Series 1999B Bonds (and all Bonds of Tax-Exempt

                                       51
<PAGE>   58

Series into which they may be converted) in accordance with the provisions of
Article VIII hereof, or the date in which all Series 1999B Bonds (and all Bonds
of Tax-Exempt Series into which they may be converted) are in a Term Rate Period
or Taxable Term Rate Period ending on the maturity date of such Bonds. The
Issuer shall, with the concurrence of the Borrower and the Liquidity Provider,
appoint a successor Remarketing Agent for such Bonds upon the removal or
resignation of a Remarketing Agent unless all Bonds are in a Term Rate Period or
Taxable Term Rate Period ending on the maturity date for such Bonds. The
Remarketing Agent shall signify its acceptance of the duties and obligations
imposed upon it hereunder by a written instrument of acceptance delivered to the
Issuer, the Trustee and the Borrower. No removal of or resignation by the
Remarketing Agent (whether at the direction of the Issuer or the Borrower, by
the Remarketing Agent as may be provided in the Remarketing Agreement, or by
automatic termination as described above) shall become effective until a
successor Remarketing Agent has delivered a written acceptance of appointment to
the Trustee and the Borrower has provided the notice required by Section 5.8 of
the Agreement.

         SECTION 4.09. QUALIFICATIONS OF REMARKETING AGENT. The Remarketing
Agent shall be a member of the National Association of Securities Dealers, Inc.
and authorized by law to perform all the duties imposed upon it by this
Indenture. The Remarketing Agent may at any time resign and be discharged of its
duties and obligations created by this Indenture by giving at least thirty (30)
Business Days' notice to the Issuer, the Borrower and the Trustee or such
shorter period as the Issuer, the Borrower, the Trustee and the Remarketing
Agent agree. No resignation of the Remarketing Agent, however, shall become
effective until a successor Remarketing Agent has been appointed and accepted
such appointment as provided in Section 4.08 hereof.

         SECTION 4.10. TENDER AND PURCHASE OF BOOK-ENTRY BONDS. Notwithstanding
any provisions of this Indenture to the contrary, at any time while any Series
of Bonds that are subject to tender are Book-Entry Bonds, the provisions of this
Article IV are modified as follows:

         (a) Any notice pursuant to SECTION 4.01(a)(i), 4.01(b)(i) or 4.01(c)(i)
hereof may be given by any direct participant in the Securities Depository
acting on behalf of either any owner of a beneficial interest in such Bonds or
any indirect participant in the Securities Depository acting on behalf of such
an owner, provided that any such notice shall not be required to contain the
bond number of Bonds to be tendered for purchase and the Trustee may
conclusively rely on any written certification or representation by a person,
firm, corporation or other entity that it is acting as a direct participant in
the Securities Depository for such Bonds for the purposes of giving any such
notice.

         (b) Delivery of such Bonds to the Trustee, as provided in Sections
4.01(a)(ii), 4.01(b)(ii), 4.01(c)(ii) and 4.02(b) hereof, shall be effected by
book-entry credit to the account of the Trustee on the records of the Securities
Depository, at or prior to 1:00 p.m., New York time, on the date such Bonds or
portions thereof are required to be tendered to the Trustee for purchase, of a
beneficial interest in such Bonds to be purchased on such date.

         (c) The Remarketing Agent shall give the information required by
Section 4.04(c) hereof to the Securities Depository instead of to the Trustee,
but shall at the same time give facsimile or telephonic notice to the Trustee
specifying the principal amount of such Bonds which it has been unable to
remarket (if such be the case).

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<PAGE>   59

         (d) The Remarketing Agent shall deliver remarketing proceeds in
accordance with the provisions of Section 4.04(d) hereof to the Securities
Depository instead of to the Trustee, acting as Tender Agent.

         (e) SECTION 4.04(e) hereof shall be inapplicable.

         (f) The provisions of Sections 4.05 and 4.06 hereof shall apply only if
Bonds are purchased with moneys described in clauses (i) and (iii) of Section
4.05(a) hereof; the beneficial interests in Bonds purchased with moneys
described in clause (ii) of Section 4.05(a) shall be transferred in accordance
with the procedures of the Securities Depository.

         SECTION 4.11. DRAWS UPON THE LIQUIDITY FACILITY.

         (a) The Trustee or Tender Agent, as applicable, shall draw funds under
any Liquidity Facility supporting a Series of the Bonds in an amount necessary
and in sufficient time (as set forth by the terms of such Liquidity Facility) so
as to provide to the Trustee the balance of the funds needed to purchase
tendered Bonds of such Series, taking into account the remarketing proceeds
received by the Trustee or Tender Agent, as applicable, in the Remarketing
Agent's notice pursuant to Section 4.04(c) hereof not later than 1:00 p.m., New
York City time, on the Business Day prior to the date on which Bonds are to be
purchased. If the Remarketing Agent remarkets Bonds after 1:00 p.m. New York
City time on the Business Day prior to the date on which Bonds are to be
purchased, the Trustee shall still draw on the Liquidity Facility for such Bonds
in an amount necessary and in sufficient time (as set forth by the terms of such
Liquidity Facility) so as to provide the balance of the funds needed to purchase
tendered Bonds, without taking into account any remarketing proceeds other than
those specified in the Remarketing Agent's notice pursuant to Section 4.04(c)
hereof. The Trustee shall transfer to the Liquidity Provider any excess moneys
received from a draw on the Liquidity Facility for such Bonds that are not
needed to pay the purchase price of such series of Bonds on the date on which
Bonds are to be purchased.

         (b) If a commitment to renew the Liquidity Facility for the Series
1999B Bonds (or any Tax-Exempt Series into which such Bonds may be converted) or
to provide an Alternate Liquidity Facility for such Bonds shall not be provided
prior to the 30th day before the scheduled expiration date of such Liquidity
Facility, then such Bonds shall not be remarketed after the 15th day prior to
such expiration.

         (c) (i) Whenever the Borrower has delivered to the Trustee a commitment
for the delivery of an Alternate Liquidity Facility for a Series of Bonds
pursuant to Section 5.14 of the Agreement, the Trustee shall mail by first class
mail a notice to all Bondholders (and the Bond Insurer) stating: (i) the name of
the issuer of the Alternate Liquidity Facility; (ii) the date on which the
Alternate Liquidity Facility will become effective, which date shall not be less
than five (5) calendar days prior to the stated expiration date of the existing
Liquidity Facility for the Bonds; and (iii) the rating expected to apply to such
series of Bonds after the Alternate Liquidity Facility is delivered. Such notice
shall be mailed at least ten (10) days prior to the effective date of the
Alternate Liquidity Facility.

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<PAGE>   60

                  (ii) Upon receipt of any Alternate Liquidity Facility, the
         Trustee shall provide notice thereof to the Issuer, each rating agency
         then rating the Bonds and the Borrower.

                                   ARTICLE V

                           PAYMENT; FURTHER ASSURANCES

         SECTION 5.01. PAYMENT OF PRINCIPAL OR REDEMPTION PRICE OF AND INTEREST
ON BONDS. The Issuer shall promptly pay or cause to be paid the principal of and
premium, if any, and interest on, every Bond issued hereunder according to the
terms thereof, but shall be required to make such payment or cause such payment
to be made only out of Revenues or the proceeds of Bond Insurance. The Issuer
hereby appoints the Trustee to act as the Paying Agent for the Bonds, and
designates the Principal Office of the Trustee as the place of payment for the
Bonds, such appointment and designation to remain in effect until notice of
change is filed with the Trustee.

         SECTION 5.02. EXTENSION OR FUNDING OF CLAIMS FOR INTEREST. In order to
prevent any accumulation of claims for interest after maturity, the Issuer shall
not, directly or indirectly, extend or assent to the extension of the time for
the payment of any claim for interest on any of the Bonds, and shall not,
directly or indirectly, be a party to or approve any such arrangement by
purchasing or funding such claims or in any other manner. In case any such claim
for interest shall be extended or funded, whether or not with the consent of the
Issuer, such claim for interest so extended or funded shall not be entitled, in
case of default hereunder, to the benefits of this Indenture, except subject to
the prior payment in full of the principal of all of the Bonds then outstanding
and of all claims for interest which shall not have been so extended or funded.

         SECTION 5.03. PRESERVATION OF REVENUES. The Issuer shall not take any
action to interfere with or impair the pledge and assignment hereunder of
Revenues and the assignment to the Trustee of rights under the Agreement, or the
Trustee's enforcement of any rights thereunder, without the prior written
consent of the Trustee. The Trustee may give such written consent only in
accordance with the provisions of Article IX hereof.

         SECTION 5.04. OTHER LIENS. So long as any Bonds are outstanding, the
Issuer shall not create or suffer to be created any pledge, lien or charge of
any type whatsoever upon all or any part of the Revenues, other than the lien of
this Indenture.

         SECTION 5.05. COMPLIANCE WITH THE INDENTURE. The Issuer shall not
issue, or permit to be issued, any Bonds secured or payable in any manner out of
Revenues in any manner other than in accordance with the provisions of this
Indenture, and shall not suffer or permit any default to occur under this
Indenture, but shall faithfully observe and perform all the covenants,
conditions and requirements hereof.

         SECTION 5.06. PERFORMANCE OF COVENANTS. The Issuer covenants that it
will faithfully perform at all times any and all covenants, undertakings,
stipulations and provisions contained in this Indenture, in any and every Bond
executed, authenticated and delivered hereunder and in all of its proceedings
pertaining thereto; provided,

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<PAGE>   61

however, that except for the matters set forth in Section 5.01 hereof the Issuer
shall not be obligated to take any action or execute any instrument pursuant to
any provision hereof until it shall have been requested to do so by the
Borrower, or shall have received the instrument to be executed and at the
Issuer's option shall have received from the Borrower assurance satisfactory to
the Issuer that the Issuer shall be reimbursed for its reasonable expenses
incurred or to be incurred in connection with taking such action or executing
such instrument. The Bonds and interest and premium, if any, thereon, and any
obligation of the Issuer under the Agreement or this Indenture, shall never
constitute a debt or indebtedness of the Issuer within the meaning of any
constitutional or statutory provision or limitation and shall not constitute nor
give rise to a pecuniary liability of the Issuer or a charge against its general
credit or taxing powers.

         SECTION 5.07. RIGHT TO PAYMENTS UNDER AGREEMENT; INSTRUMENTS OF FURTHER
ASSURANCE. The Issuer covenants that it will defend its right to the payment of
amounts due from the Borrower under the Agreement to the Trustee, for the
benefit of the Owners of the Bonds against the claims and demands of all persons
whomsoever. The Issuer covenants that it will do, execute, acknowledge and
deliver or cause to be done, executed, acknowledged and delivered, such
indentures supplemental hereto and such further acts, instruments and transfers
as the Trustee may reasonably require for the better assuring, transferring,
conveying, pledging, assigning and confirming unto the Trustee all and singular
the rights assigned hereby and the amounts pledged hereto, to the payment of the
principal of and premium, if any, and interest on the Bonds. The Issuer
covenants and agrees that, except as provided herein and in the Agreement, it
will not sell, convey, mortgage, encumber or otherwise dispose of any part of
the Revenues or its rights under the Agreement.

         SECTION 5.08. TAX COVENANTS. The provisions of this Section 5.08 shall
only apply to the Series 1999A Bonds or any portion of the Series 1999B Bonds
which have been converted to Bonds of a Tax-Exempt Series pursuant to Section
2.15.

         (a) Pursuant to the Agreement and the Tax Certificate, the Borrower
covenants to maintain the Tax-Exempt status of such Bonds pursuant to Section
103(a) of the Code, and will take, or require to be taken, such acts as may be
reasonably within its ability and as may from time to time be required under
applicable law and regulation to continue the Tax-Exempt status of such Bonds;
and in furtherance of such covenants, the Issuer agrees to comply with the Tax
Certificate.

         (b) Pursuant to the Agreement and the Tax Certificate, the Borrower
covenants that it will not take any action or fail to take any action with
respect to the Bonds which would cause such Bonds to be "arbitrage bonds" within
the meaning of such term as used in Section 148 of the Code.

         (c) Pursuant to the Agreement and the Tax Certificate, the Borrower
shall make any and all payments required to be made to the United States
Department of the Treasury in connection with such Bonds pursuant to Section
148(f) of the Code from amounts on deposit in the funds and accounts established
under this Indenture and available therefor.

         (d) Pursuant to the Agreement and the Tax Certificate, the Borrower
covenants that it will not use or permit the use of any property financed with
the proceeds of

                                       55
<PAGE>   62

such Bonds by any person (other than a state or local governmental unit) in such
manner or to such extent as would result in a loss of the Tax-Exempt status of
such Bonds.

         (e) Notwithstanding any other provisions of this Indenture to the
contrary, so long as necessary in order to maintain the Tax-Exempt status of
such Bonds under Section 103(a) of the Code, the covenants in this Section 5.08
shall survive the payment of such Bonds and the interest thereon, including any
payment or defeasance thereof pursuant to Section 8.01 hereof.

         SECTION 5.09. INSPECTION OF PROJECT BOOKS. The Issuer and the Trustee
covenant and agree that all books and documents in their possession relating to
the Project and the Revenues shall at all times be open to inspection by such
accountants or other agencies as the other party may from time to time
designate.

         SECTION 5.10. RIGHTS UNDER AGREEMENT. The Agreement, a duly executed
counterpart of which has been filed with the Trustee, sets forth the covenants
and obligations of the Issuer and the Borrower, and reference is hereby made to
the same for a detailed statement of said covenants and obligations of the
Borrower thereunder, and the Issuer agrees that the Trustee in its name or in
the name of the Issuer may enforce all rights of the Issuer and all obligations
of the Borrower under and pursuant to the Agreement for and on behalf of the
Bondholders, whether or not the Issuer is in default hereunder. Nothing herein
contained shall be construed to prevent the Issuer from enforcing directly any
and all of its rights under Sections 4.2(d), 4.2(e), 4.2(h) and 6.4 of the
Agreement.

         SECTION 5.11. CONTINUING DISCLOSURE. Pursuant to Section 5.12 of the
Agreement, the Borrower shall undertake to satisfy the continuing disclosure
requirements promulgated under S.E.C. Rule 15c2-12, as it may from time to time
hereafter be amended or supplemented, if applicable. The Issuer shall have no
liability to the holders of the Bonds or any other person with respect to such
disclosure matters. Notwithstanding any other provision of this Indenture,
failure of the Borrower to comply with the requirements of S.E.C. Rule 15c2-12,
as it may from time to time hereafter be amended or supplemented, shall not be
considered an Event of Default; however, the Trustee, subject to Article X, may
(and, at the request of the Remarketing Agent or the holders of at least 25%
aggregate principal amount of Outstanding Bonds, shall), or any Bondholder or
beneficial owner of any Bonds may, take such actions as may be necessary and
appropriate, including seeking mandate or specific performance by court order,
to cause the Borrower to comply with its obligations under Section 5.12 of the
Agreement.

                                   ARTICLE VI

                               REVENUES AND FUNDS

         SECTION 6.01. SOURCE OF PAYMENT OF BONDS; LIABILITY OF ISSUER LIMITED
TO REVENUES. The Bonds and all payments required of the Issuer hereunder are not
general obligations of the Issuer but are limited obligations of the Issuer. The
Trust Estate is pledged and assigned to the payment of the principal of and
interest and premium, if any, on the Bonds. The payments provided in subsection
(a) of Section 4.2 of the Agreement are to be remitted directly to the Trustee
for the account of the Issuer and deposited in the Bond

                                       56
<PAGE>   63

Fund. Such payments, sufficient in amount to insure the prompt payment of the
principal of and premium, if any, and interest on the Bonds, are pledged to such
payment.

         All Revenues shall be held in trust for the benefit of the holders from
time to time of the Bonds, but shall nevertheless be disbursed, allocated and
applied solely for the uses and purposes hereinafter set forth in this Article
VI. Notwithstanding the foregoing provisions of this Section 6.01, that portion
of the Revenues permitted to be returned to the Borrower under Section 6.14
hereof shall not be subject to the pledge and lien of this Indenture.

         The Bonds shall not constitute a debt or liability or a pledge of the
faith and credit of the Issuer or the State of Nevada, but shall be payable
solely from the funds herein provided therefor. The issuance of the Bonds shall
not directly or indirectly or contingently obligate the Issuer, the State of
Nevada or any political subdivision thereof to levy or to pledge any form of
taxation whatever therefor or to make any appropriation for their payment.

         Notwithstanding any provisions of the Agreement or this Indenture to
the contrary, the Issuer shall not be required to advance any moneys derived
from any source other than Revenues and other assets pledged under this
Indenture for any purposes mentioned in this Indenture, whether for the payment
of the principal or purchase price of, or redemption premium, if any, or
interest on the Bonds or otherwise; provided, however, the Issuer may, but shall
not be required to, advance funds of the Issuer which may be available to it for
such purposes.

         SECTION 6.02. CREATION OF THE BOND FUND. There is hereby created by the
Issuer and ordered established with the Trustee a trust fund to be designated
"Clark County, Nevada Industrial Development Revenue Bonds (Southwest Gas
Corporation Project) Series 1999 - Bond Fund" (the "Bond Fund"), which shall be
used to pay the principal of and premium, if any, and the interest on the Bonds
and accounts within the Bond Fund which shall be designated the "Series 1999A
Account" and the "Taxable Series 1999B Account" of the Bond Fund. The Trustee is
hereby authorized to create additional accounts as necessary upon conversion of
any Series 1999B Bonds to Bonds of a Tax-Exempt Series.

         SECTION 6.03. PAYMENTS INTO THE BOND FUND. There shall be deposited
into the corresponding Account of the Bond Fund from time to time the following:

         (a) all accrued interest, if any, paid by the Initial Purchaser of the
applicable Series of Bonds;

         (b) any amounts transferred from the Construction Fund pursuant to the
provisions of Sections 6.06, 6.08, 6.09 and 6.10 of this Indenture;

         (c) all payments specified in Section 4.2(a) of the Agreement; and

         (d) all other moneys received by the Trustee under and pursuant to the
provisions of Section 2.11 of this Indenture or the Bond Insurance or by any of
the provisions of the Agreement, when accompanied by directions from the person
depositing such moneys that such moneys are to be paid into the Bond Fund.

                                       57
<PAGE>   64

         The Issuer hereby covenants and agrees that so long as any of the Bonds
issued hereunder are outstanding it will cause to be deposited in the Bond Fund
sufficient amounts from Revenues promptly to meet and pay the principal of and
premium, if any, and interest on the Bonds as the same become due and payable.
Nothing herein shall be construed as requiring the Issuer to use any funds or
revenues from any source other than the Trust Estate.

         SECTION 6.04. USE OF MONEYS IN THE BOND FUND AND CERTAIN OTHER MONEYS.
Except as provided in Sections 4.11, 6.11, 6.14 and 10.03 hereof and subject to
the provisions of the Tax Certificate, moneys in the Bond Fund shall be used
solely for the payment of the principal of and premium, if any, and interest on
the respective Series of Bonds as the same shall become due and payable at
maturity, upon redemption or otherwise. Funds for such payments of the principal
of and premium, if any, and interest on the Bonds held by Owners other than the
Bond Insurer or the Borrower shall be derived from the following sources in the
order of priority indicated:

         (a) from moneys paid into the Bond Fund pursuant to Section 6.03(a)
hereof which shall be applied to the payment of interest on the Bonds;

         (b) from moneys held by the Trustee pursuant to Article VIII hereof,
such moneys to be applied only to the payment of the principal of and premium,
if any, and interest on Bonds which are deemed to be paid in accordance with
Article VIII hereof;

         (c) from proceeds of refunding bonds pursuant to the provisions of
Section 2.11 of this Indenture and from income from the investment of such
proceeds;

         (d) from moneys retained in the Construction Fund following the
Completion Date pursuant to Section 6.06 of the Indenture and amounts withdrawn
from the Construction Fund and deposited into the Bond Fund pursuant to the
provisions of Sections 6.06, 6.08, 6.09 or 6.10 of this Indenture; and

         (e) from all other amounts on deposit in the Bond Fund, including
amounts paid by the Borrower pursuant to the provisions of Section 4.2(a) or
Article VII of the Agreement, and proceeds from the investment thereof.

         SECTION 6.05. CUSTODY OF THE BOND FUND. The Bond Fund shall be in the
custody of the Trustee but in the name of the Issuer and the Issuer hereby
authorizes and directs the Trustee to withdraw in accordance with the provisions
of Section 6.04 of this Indenture sufficient funds from the Bond Fund to pay the
principal of, premium, if any, and interest on the Bonds as the same become due
and payable, which authorization and direction the Trustee hereby accepts.

         SECTION 6.06. CREATION OF THE CONSTRUCTION FUND; DISBURSEMENTS. There
is hereby created and established with the Trustee a trust fund in the name of
the Issuer but for the account of the Borrower, such fund to be designated
"Clark County, Nevada, Industrial Development Revenue Bonds (Southwest Gas
Corporation Project) Series 1999 - Construction Fund" (the "Construction Fund")
and accounts within such fund to be designated "Series 1999A Construction
Account" and "Series 1999B Construction Account". The Trustee shall create
further accounts as necessary upon conversions of the Series 1999B

                                       58
<PAGE>   65

Bonds from time to time. Proceeds from the sale of a Series of Bonds shall be
deposited in the account of the Construction Fund relating to such Series of
Bonds in the amount specified in a written request of the Issuer acknowledged by
the Borrower delivered to the Trustee in connection with the issuance of such
Series of Bonds.

         Moneys in the Construction Fund shall be disbursed to the Borrower, or
such other Person as may be designated, on requisitions signed by the Authorized
Borrower Representative and delivered to the Trustee, stating with respect to
each payment to be made:

                  (1) The amount of such disbursement and from which account it
         is to be paid; and

                  (2) That each obligation mentioned therein (i) has been
         properly incurred, (ii) is a proper charge against the indicated
         account of the Construction Fund in accordance with the provisions of
         the Agreement (including Section 3.3 thereof) and this Indenture, and
         (iii) has not been the basis of any previous requisition.

         A copy of each such requisition shall be furnished to the Authorized
Issuer Representative by the Borrower. The Trustee is hereby authorized and
directed to make each disbursement required by the provisions of the Agreement
and to issue its check therefor or to transfer funds (by wire or otherwise) in
accordance with directions contained in such requisition (or a supplement
thereto). The Trustee shall have no duty or obligation to verify the content of
any requisition provided to it hereunder. Moneys in the Construction Fund on the
Completion Date shall be transferred to the appropriate account of the Bond Fund
and the Construction Fund shall be closed.

         SECTION 6.07. COSTS OF ISSUANCE FUND; DISBURSEMENTS. The Trustee shall
establish the Southwest Gas Corporation Project Costs of Issuance Fund (the
"Southwest Gas Corporation Project Costs of Issuance Fund") and accounts within
such fund to be designated the "Series 1999A Costs of Issuance Account" (the
"Series 1999A Costs of Issuance Account") and another account within such Fund
to be designated the "Series 1999B Costs of Issuance Account" (the "Series 1999B
Costs of Issuance Account" and, collectively, the "Costs of Issuance Accounts").
The moneys in each account of the Costs of Issuance Fund shall be held by the
Trustee in trust and applied to the payment of Costs of Issuance for the related
Series of Bonds upon a requisition filed with the Trustee, in the form attached
hereto as Exhibit D, signed by an Authorized Borrower Representative. All
payments from the Costs of Issuance Fund shall be reflected in the Trustee's
regular accounting statements. Any amounts remaining in any account within the
Costs of Issuance Fund six months following the date of issuance of the Bonds
shall be transferred to the Construction Fund.

         SECTION 6.08. USE OF MONEYS IN CONSTRUCTION FUND UPON DEFAULT. Upon an
Event of Default pursuant to Article IX of this Indenture and an acceleration of
the Bonds pursuant to Section 9.02 hereof, any balance remaining in the
Construction Fund shall without further authorization be transferred to the Bond
Fund with advice to the Issuer and the Borrower of such action.

                                       59
<PAGE>   66

         SECTION 6.09. USE OF MONEYS IN CONSTRUCTION FUND UPON REDEMPTION. In
the event the Borrower shall be required under Section 7.2 of the Agreement, or
in the event the Borrower elects under Section 7.1 of the Agreement, to prepay
all the amounts due under the Agreement, the Trustee shall deposit in the Bond
Fund, on the date on which such prepayment is due, all amounts remaining in the
Construction Fund, except as otherwise provided in Section 6.10 of this
Indenture. Upon the making of any deposit pursuant to the provisions of this
Section, the Trustee shall advise the Issuer and the Borrower of such action.

         SECTION 6.10. USE OF MONEYS IN CONSTRUCTION FUND UPON PAYMENT OF BONDS.
Any balance remaining in the Construction Fund after the payment in full of all
Bonds issued under the provisions of this Indenture shall be deposited into the
Bond Fund, except that if the Issuer has issued a series of refunding bonds for
the purpose of refunding all of the Bonds at or prior to their stated maturity,
any moneys remaining in the Construction Fund at the time of such refunding may
be deposited by the Issuer and the Trustee into a special fund created in the
proceedings authorizing the issuance of the refunding bonds and used to pay
costs of the Project not paid out of the Construction Fund prior to such
refunding.

         SECTION 6.11. NON-PRESENTMENT OF BONDS. In the event any Bond shall not
be presented for payment when the principal thereof becomes due, either at
maturity or otherwise, or at the date fixed for redemption thereof or in the
event any interest payment thereon is unclaimed, if moneys sufficient to pay
such Bond or interest shall have been deposited in the Bond Fund, all liability
of the Issuer to the Owner thereof for the payment of such Bond or interest
shall forthwith cease, determine and be completely discharged, and thereupon it
shall be the duty of the Trustee to hold such moneys, without liability for
interest thereon, for the benefit of the Owner of such Bond who shall thereafter
be restricted exclusively to such moneys, for any claim of whatever nature on
his part under this Indenture or on, or with respect to, said Bond. Any moneys
so deposited with and held by the Trustee not so applied to the payment of Bonds
or such interest, if any, within one (1) year after the date on which the same
shall have become due shall be paid by the Trustee to the Borrower upon the
written direction of an Authorized Borrower Representative and thereafter
Bondholders shall be entitled to look only to the Borrower for payment, and then
only to the extent of the amount so repaid, and the Borrower shall not be liable
for any interest thereon and shall not be regarded as a trustee of such moneys
and the Trustee shall have no further responsibility with respect to such
moneys.

         SECTION 6.12. TRUSTEE FEES, CHARGES AND EXPENSES. The Trustee agrees
that the Issuer shall have no liability for any fees, charges and expenses of
the Trustee, and the Trustee agrees to look only to the Borrower for the payment
of all fees, charges and expenses of the Trustee as provided in the Agreement
and in this Indenture.

         SECTION 6.13. MONEYS TO BE HELD IN TRUST. All moneys required to be
deposited with or paid to the Trustee for deposit into the Bond Fund or into the
Construction Fund or Costs of Issuance Fund under any provision hereof, all
moneys withdrawn from the Bond Fund and held by the Trustee and any moneys
withdrawn from the Construction Fund or Costs of Issuance Fund and held by the
Trustee shall be held by the Trustee in trust, and such moneys (other than
moneys held pursuant to Section 6.11 hereof) shall, while so held, constitute
part of the Trust Estate and be subject to the lien hereof. Moneys held for the
payment

                                       60
<PAGE>   67

of the purchase price of Bonds pursuant to Article IV hereof shall not
constitute part of the Trust Estate and shall be held in a special purpose trust
account which will be established and maintained by the Trustee and held for the
exclusive benefit of the holders of Bonds with respect to which such purchase
was made.

         SECTION 6.14. REPAYMENT TO THE BORROWER FROM THE BOND FUND. Any amounts
remaining in the Bond Fund after payment in full of the principal of and
premium, if any, and interest on the Outstanding Bonds (or provision for payment
thereof as provided in this Indenture), the fees, charges and expenses of the
Issuer, the Trustee, the Liquidity Provider, if any, the Remarketing Agent, if
any, and all other amounts required to be paid under the Agreement, the Bond
Insurance, the Liquidity Facility, if any, and this Indenture shall be paid to
the Borrower as provided in Section 9.5 of the Agreement.

         SECTION 6.15. REVENUES TO BE PAID OVER TO TRUSTEE. The Issuer will
cause the Revenues to be paid to the Trustee for deposit in the Bond Fund in
accordance with the terms of this Indenture to effect payment of the principal
of and premium, if any, and interest on the Bonds as the same become due.

         SECTION 6.16. PAYMENTS OF PRINCIPAL AND INTEREST. The Trustee shall pay
from Revenues received by the Trustee, in the order of priority indicated in
Section 6.04 hereof, the principal of and premium, if any, and interest on, the
Bonds as the same become due and payable. If, prior to the maturity of any Bond,
the Borrower surrenders such Bond to the Trustee for cancellation, the Trustee
shall cancel such Bond.

         SECTION 6.17. REVENUES TO BE HELD FOR ALL BONDHOLDERS; CERTAIN
EXCEPTIONS. Revenues and investments thereof shall, until applied as provided in
this Indenture, be held by the Trustee for the benefit of the Owners of all
Outstanding Bonds, except as provided by Sections 2.11, 6.11 and 6.14 hereof and
except that any portion of the Revenues representing principal of and premium,
if any, and interest on, any Bonds previously called for redemption in
accordance with Article III of this Indenture or previously matured or
representing unclaimed interest on the Bonds shall be held for the benefit of
the Owners of such Bonds only and shall not be deposited or invested pursuant to
Article VII hereof, notwithstanding any provision of Article VII.

         SECTION 6.18. REBATE FUND. There is hereby created by the Issuer and
ordered established with the Trustee a custodial fund to be designated the
"Clark County, Nevada Industrial Development Revenue Bonds (Southwest Gas
Corporation Project) Series 1999 - Rebate Fund" (the "Rebate Fund"). The Trustee
shall establish within the Rebate Fund a separate account designated
"Tax-Exempt". The Trustee covenants and agrees to deposit to the Rebate Fund
amounts paid to the Trustee by the Borrower pursuant to Section 4.2(g) of the
Agreement and, at the written direction of an Authorized Borrower
Representative, to withdraw from such Rebate Fund such amounts at such times in
order to pay the Rebate Requirement (as defined in the Tax Certificate) in
accordance with the Tax Certificate. Funds on deposit in the Rebate Fund are not
part of the Trust Estate.

         SECTION 6.19. BOND INSURANCE PAYMENTS. As long as the Bond Insurance
shall be in full force and effect with respect to a Series of Bonds hereunder,
the Issuer,

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the Trustee and any Paying Agent for such Series of Bonds agree to comply with
the following provisions:

         (a) at least one (1) day prior to all Interest Payment Dates the
Trustee or Paying Agent, if any, will determine whether there will be sufficient
funds in the Bond Fund to pay the principal of or interest on the Bonds of such
Series on such Interest Payment Date. If the Trustee or Paying Agent, if any,
determines that there will be insufficient funds in such Bond Fund, the Trustee
or Paying Agent, if any, shall so notify the Bond Insurer. Such notice shall
specify the amount of the anticipated deficiency, the Series of Bonds to which
such deficiency is applicable and whether such Bonds will be deficient as to
principal or interest, or both. If the Trustee or Paying Agent, if any, has not
so notified the Bond Insurer at least one (1) day prior to an Interest Payment
Date, the Bond Insurer will make payments of principal or interest due on the
Bonds on or before the first (1st) day next following the date on which Bond
Insurer shall have received notice of nonpayment from the Trustee or Paying
Agent, if any.

         (b) the Trustee or Paying Agent, if any, shall, after giving notice to
the Bond Insurer as provided in (a) above, make available to the Bond Insurer
and, at the Bond Insurer's direction, to the United States Trust Company of New
York, as insurance trustee for the Bond Insurer or any successor insurance
trustee (the "Insurance Trustee"), the registration books of the Issuer
maintained by the Trustee or Paying Agent, if any, and all records relating to
the funds maintained under the Indenture.

         (c) the Trustee or Paying Agent, if any, shall provide the Bond Insurer
and the Insurance Trustee with a list of registered owners of Bonds entitled to
receive principal or interest payments from the Bond Insurer under the terms of
the Bond Insurance, and shall make arrangements with the Insurance Trustee (i)
to mail checks or drafts to the registered owners of such Bonds entitled to
receive full or partial interest payments from the Bond Insurer and (ii) to pay
principal upon such Bonds surrendered to the Insurance Trustee by the registered
owners of Bonds entitled to receive full or partial principal payments from the
Bond Insurer.

         (d) the Trustee or Paying Agent, if any, shall, at the time it provides
notice to the Bond Insurer pursuant to (a) above, notify registered owners of
Bonds entitled to receive the payment of principal or interest thereon from the
Bond Insurer (i) as to the fact of such entitlement, (ii) that the Bond Insurer
will remit to them all or a part of the interest payments next coming due upon
proof of Bondholder entitlement to interest payments and delivery to the
Insurance Trustee, in form satisfactory to the Insurance Trustee, of an
appropriate assignment of the registered owner's right to payment, (iii) that
should they be entitled to receive full payment of principal from the Bond
Insurer, they must surrender their Bonds (along with an appropriate instrument
of assignment in form satisfactory to the Insurance Trustee to permit ownership
of such Bonds to be registered in the name of the Bond Insurer) for payment to
the Insurance Trustee, and not the Trustee or Paying Agent, if any, and (iv)
that should they be entitled to receive partial payment of principal from the
Bond Insurer, they must surrender their Bonds for payment thereon first to the
Trustee or Paying Agent, if any, who shall note on such Bonds the portion of the
principal paid by the Trustee or Paying Agent, if any, and then, along with an
appropriate instrument of assignment in form satisfactory to the Insurance
Trustee, to the Insurance Trustee, which will then pay the unpaid portion of
principal.

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         (e) in the event that the Trustee or Paying Agent, if any, has notice
that any payment of principal of or interest on a Bond which has become due for
payment and which is made to a Bondholder by or on behalf of the Issuer has been
deemed a preferential transfer and theretofore recovered from its registered
owner pursuant to the United States Bankruptcy Code by a trustee in bankruptcy
in accordance with the final, nonappealable order of a court having competent
jurisdiction, the Trustee or Paying Agent, if any, shall, at the time the Bond
Insurer is notified pursuant to (a) above, notify all registered owners that in
the event that any registered owner's payment is so recovered, such registered
owner will be entitled to payment from the Bond Insurer to the extent of such
recovery if sufficient funds are not otherwise available, and the Trustee or
Paying Agent, if any, shall furnish to the Bond Insurer its records evidencing
the payments of principal of and interest on the Bonds which have been made by
the Trustee or Paying Agent, if any, and subsequently recovered from registered
owners and the dates on which such payments were made.

         (f) in addition to those rights granted the Bond Insurer hereunder, the
Bond Insurer shall, to the extent it makes payment of principal of or interest
on Bonds, become subrogated to the rights of the recipients of such payments in
accordance with the terms of the Bond Insurance, and to evidence such
subrogation (i) in the case of subrogation as to claims for past due interest,
the Trustee or Paying Agent, if any, shall note the Bond Insurer's rights as
subrogee on the registration books of the Issuer maintained by the Trustee or
Paying Agent, if any, upon receipt from the Bond Insurer of proof of the payment
of interest thereon to the registered owners of the Bonds, and (ii) in the case
of subrogation as to claims for past due principal, the Trustee or Paying Agent,
if any, shall note the Bond Insurer's rights as subrogee on the registration
books of the Issuer maintained by the Trustee or Paying Agent, if any, upon
surrender of the Bonds by the registered owners thereof together with proof of
the payment of principal thereof.

                                  ARTICLE VII

                              INVESTMENT OF MONEYS

         SECTION 7.01. INVESTMENT OF MONEYS. The Trustee shall invest and
reinvest any moneys held as part of the Bond Fund, the Costs of Issuance Fund,
and the Construction Fund upon the written direction of an Authorized Borrower
Representative in Investment Securities. Any such investments shall be held by
or under the control of the Trustee and shall be deemed at all times a part of
the fund for which they were made. The interest accruing thereon, any profit
realized from such investments and any loss resulting from such investments
shall be credited or charged to such fund in accordance with Section 3.4 of the
Agreement. The Trustee shall sell and reduce to cash a sufficient amount of such
investments of the Bond Fund, the Costs of Issuance Fund and the Construction
Fund, each in accordance with written directions or oral directions promptly
confirmed by telecopy or other writing of an Authorized Borrower Representative,
whenever the cash balance in the Bond Fund is insufficient to pay the principal
of and premium, if any, and interest on the Bonds when due and whenever the cash
balance in the Construction Fund is insufficient to pay amounts due from the
Construction Fund. Moneys comprising proceeds of a draw on a Liquidity Facility,
and other moneys held for the payment of the purchase price of Bonds pursuant to
Article IV hereof or the payment of Bonds pursuant to Section 6.11 and 6.17
hereof, shall be held uninvested. Moneys

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<PAGE>   70

that are "Available Moneys" under the definition thereof may be invested in (1)
cash (insured at all times by the Federal Deposit Insurance Corporation or
otherwise collateralized with obligations described in clause (2) following), or
(2) direct obligations of (including obligations issued or held in book entry
form on the books of) the Department of the Treasury of the United States of
America maturing as to principal and interest at such time as will insure
availability at the time of the required payments.

         SECTION 7.02. INVESTMENTS; ARBITRAGE. The Trustee may make any and all
investments permitted by the provisions of Section 7.01 hereof through its own
bond department. The Trustee may act as principal or agent in the making or
disposing of any investments, and may act as sponsor, advisor or manager in
connection with any such investments. The provisions of this subsection shall
apply to affiliates of the Trustee. As and when any amount invested pursuant to
this Article may be needed for disbursement, the Trustee may cause a sufficient
amount of such investments to be sold and reduced to cash to the credit of such
funds.

         The Borrower has covenanted in Section 5.11 of the Agreement that it
will not take any action or fail to take any action with respect to the Bonds to
cause the Bonds of any Tax-Exempt Series to be treated as "arbitrage bonds"
within the meaning of Section 148(a) of the Code.

                                  ARTICLE VIII

                                   DEFEASANCE

         SECTION 8.01. DEFEASANCE. (A) If the Issuer shall pay or cause to be
paid, or there shall be otherwise paid or provision for payment made to or for
the Owners of the Bonds the principal, premium, if any, and interest due or to
become due thereon at the times and in the manner stipulated therein, and if the
Issuer shall keep, perform and observe all the covenants and promises in the
Bonds and in this Indenture expressed as to be kept, performed and observed by
it or on its part, and shall pay or cause to be paid to the Trustee all sums of
money due or to become due according to the provisions hereof, then this
Indenture and the lien, rights and interests created hereby shall cease,
determine and become null and void (except as to any surviving rights of
payment, registration, transfer or exchange of Bonds herein provided for),
whereupon the Trustee upon written request of an Authorized Issuer
Representative shall cancel and discharge this Indenture, and execute and
deliver to the Issuer such instruments in writing as shall be requested by an
Authorized Issuer Representative and requisite to discharge this Indenture, and
release, assign and deliver unto the Issuer any and all the estate, right, title
and interest in and to any and all rights assigned or pledged to the Trustee or
otherwise subject to this Indenture, except amounts in the Bond Fund required to
be paid to the Borrower under Section 6.14 hereof and except moneys or
securities held by the Trustee for the payment of the principal of and premium,
if any, and interest on, and purchase prices of, the Bonds. Notwithstanding the
foregoing, under no circumstances may the Issuer or the Borrower receive any
funds derived from a draw on any Liquidity Facility, Bond Insurance or moneys
held for the payment of particular Bonds.

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<PAGE>   71

         (B) Any Bond or Authorized Denomination thereof shall be deemed to be
paid within the meaning of this Indenture when (a) payment of the principal of
and premium, if any, on such Bond or Authorized Denomination thereof, plus
interest thereon to the due date thereof (whether such due date is by reason of
maturity or upon redemption as provided herein) either (i) shall have been made
or caused to be made in accordance with the terms thereof, (ii) shall have been
provided for by depositing sufficient amounts as described in clause (1) and/or
(2) below for such payment with the Trustee and the due date of such principal,
interest and premium, if any, has occurred, or (iii) in the case of a Bond which
bears interest at a Flexible Rate, a Taxable Term Rate, a Taxable Flexible Rate
or a Term Rate, shall have been provided for by irrevocably depositing with the
Trustee in trust and irrevocably setting aside exclusively for such payment on
such due date (which due date shall be in the case of a Bond bearing interest at
a Flexible Rate or Taxable Flexible Rate no later than the Interest Payment Date
for the then current Flexible Segment or Taxable Segment, as applicable, for
such Bond and in the case of a Bond bearing interest at a Term Rate no later
than the last Interest Payment Date for the then current Term Rate Period or
Taxable Term Rate Period for such Bond) (1) cash (insured at all times by the
Federal Deposit Insurance Corporation or otherwise collateralized with
obligations described in clause (2) following), or (2) direct obligations of
(including obligations issued or held in book entry form on the books of) the
Department of the Treasury of the United States of America maturing as to
principal and interest in such amount and at such time as will insure the
availability of sufficient moneys to make such payment, and (b) all necessary
and proper fees, compensation and expenses of the Trustee pertaining to any such
deposit shall have been paid or the payment thereof provided for to the
satisfaction of the Trustee; provided, however, that no Bond shall be deemed
paid pursuant to this Article VIII prior to the due date for the payment of
principal, premium if any, and interest thereon unless there shall have been
delivered an opinion of Bond Counsel to the effect that such treatment will not
adversely affect the Tax-Exempt status of any Tax-Exempt Series of Bonds
hereunder and will not cause such Bonds to be treated as sold or otherwise
disposed of for the purposes of Section 1001 of the Code (or any successor
provision). At such times as a Bond or Authorized Denomination thereof shall be
deemed to be paid hereunder, as aforesaid, such Bond or Authorized Denomination
thereof shall no longer be secured by or entitled to the benefits of this
Indenture (other than Sections 2.04 and 2.08 hereof in the case of a deposit
under clause (a)(iii) above), except for the purposes of any such payment from
such moneys or government obligations referred to in clause (2) above.

         (C) Notwithstanding the foregoing paragraph, in the case of a Bond or
Authorized Denomination thereof which by its terms may be redeemed prior to the
stated maturity thereof, no deposit under clause (a)(iii) of the immediately
preceding paragraph shall be deemed a payment of such Bond or Authorized
Denomination thereof as aforesaid until: (a) proper notice of redemption of such
Bond or Authorized Denomination thereof shall have been previously given in
accordance with Article III of this Indenture, or in the event said Bond or
Authorized Denomination thereof is not to be redeemed within the next succeeding
seventy-five (75) days, until the Borrower shall have given the Trustee on
behalf of the Issuer, in form satisfactory to the Trustee, irrevocable
instructions to notify, as soon as practicable, the Owner of such Bond or
Authorized Denomination thereof in accordance with Article III hereof, that the
deposit required by (a)(iii) above has been made with the Trustee and that said
Bond or Authorized Denomination thereof is deemed to have been paid in
accordance with this Article and stating the maturity or redemption date upon
which moneys are to be available for the payment of the principal of and the
applicable premium, if any, on said Bond or Authorized

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<PAGE>   72

Denomination thereof, plus interest thereon to the due date thereof, or (b) the
maturity of such Bond or Authorized Denomination thereof.

         For purposes of paragraph (B) above, if the Bonds to be deemed paid are
Taxable Weekly Bonds which are rated by S&P, such Bonds shall not be deemed paid
unless the Trustee shall receive, prior to the defeasance thereof, written
evidence from S&P that the short-term rating on such Taxable Weekly Bonds will
not be reduced or withdrawn due to such defeasance pursuant to Section 8.01(B)
hereof.

         (D) Notwithstanding any provision of any other Article of this
Indenture which may be contrary to the provisions of this Article, all moneys or
government obligations set aside and held in trust pursuant to the provisions of
this Article for the payment of Bonds or Authorized Denominations thereof
(including interest and premium thereon, if any) shall be applied to and used
solely for the payment of the particular Bonds or Authorized Denominations
thereof (including interest and premium thereon, if any) with respect to which
such moneys and government obligations have been so set aside in trust.

         (E) Notwithstanding anything herein to the contrary, in the event that
the principal and/or interest due on the Bonds shall be paid by the Bond Insurer
pursuant to the Bond Insurance, the Bonds shall remain Outstanding for all
purposes, not be defeased or otherwise satisfied and not be considered paid by
the Issuer, and the assignment and pledge of the Trust Estate and all covenants,
agreements and other obligations of the Issuer to the registered owners shall
continue to exist and shall run to the benefit of the Bond Insurer, and the Bond
Insurer shall be subrogated to the rights of such registered owners.

         (F) Anything in Article XI hereof to the contrary notwithstanding, if
moneys or government obligations have been deposited or set aside with the
Trustee pursuant to this Article for the payment of Bonds or Authorized
Denominations thereof and the interest and premium, if any, thereon and such
Bonds or Authorized Denominations thereof and the interest and premium, if any,
thereon shall not have in fact been actually paid in full, no amendment to the
provisions of this Article shall be made without the consent of the Owner of
each of the Bonds affected thereby.

                                   ARTICLE IX

           DEFAULT PROVISIONS AND REMEDIES OF TRUSTEE AND BONDHOLDERS

         SECTION 9.01. DEFAULTS; EVENTS OF DEFAULT. If any of the following
events occurs, it is hereby defined as and declared to be and to constitute a
default or an Event of Default:

         (a) Failure to make payment of any installment of interest upon any
Bond after such payment has become due and payable;

         (b) Failure to make payment of the principal of and premium, if any, on
any Bond at the stated maturity thereof or upon the unconditional redemption
thereof;

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         (c) A failure to pay an amount due pursuant to Article IV hereof when
the same shall have become due and payable;

         (d) The occurrence of an "Event of Default" under the Agreement;

         (e) Failure on the part of the Issuer to perform or observe any of its
covenants, agreements or conditions in this Indenture or in the Bonds contained
and failure to remedy the same after notice thereof pursuant to Section 9.10
hereof; and

         (f) Receipt by the Trustee of notice of an Insurer Default.

         SECTION 9.02. ACCELERATION. Upon the occurrence and continuance of an
Event of Default under Section 9.01 hereof, the Trustee may with the consent of
the Bond Insurer, and upon the written request of the Owners of not less than a
majority in aggregate principal amount of Bonds then Outstanding with the
consent of the Bond Insurer, shall, by notice in writing delivered to the
Borrower, with copies to the Issuer, the Bond Insurer, the Liquidity Provider,
if any, and the Remarketing Agent, if any, declare the principal of all Bonds
then outstanding and the interest accrued thereon to the date of such
declaration immediately due and payable, and such principal and interest shall
thereupon become and be immediately due and payable. Upon any such declaration,
the Trustee shall declare all indebtedness payable under Section 4.2(a) of the
Agreement to be immediately due and payable in accordance with Section 6.2 of
the Agreement and may exercise and enforce such rights as exist under the
Agreement and this Indenture. The above provisions are subject to waiver,
rescission and annulment as provided in Section 9.09 hereof.

         SECTION 9.03. REMEDIES; RIGHTS OF BONDHOLDERS AND BOND INSURER. Upon
the occurrence and continuation of an Event of Default under Section 9.01
hereof, the Trustee may pursue any available remedy at law or in equity by suit,
action, mandamus or other proceeding to enforce the payment of the principal of
and premium, if any, and interest on the Bonds then outstanding and to enforce
and compel the performance of the duties and obligations of the Issuer as herein
set forth. In addition, the Trustee may, without notice to the Issuer or the
Borrower, exercise any and all remedies afforded the Issuer under Article VI of
the Agreement in its name or the name of the Issuer without the necessity of
joining the Issuer.

         If an Event of Default under Section 9.01 hereof shall have occurred
and be continuing and if requested so to do by the Owners of not less than a
majority in aggregate principal amount of Bonds then outstanding and indemnified
as provided in Section 10.01(i) hereof, the Trustee shall be obliged to exercise
such one or more of the rights and powers conferred by this Section 9.03 and
Section 9.02 hereof as the Trustee being advised by Counsel shall deem most
expedient in the interests of the Bondholders.

         No remedy by the terms of this Indenture conferred upon or reserved to
the Trustee (or to the Bondholders) is intended to be exclusive of any other
remedy, but each and every such remedy shall be cumulative and shall be in
addition to any other remedy given to the Trustee or to the Bondholders
hereunder or now or hereafter existing at law or in equity or by statute.

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         No delay or omission to exercise any right, power or remedy accruing
upon any Event of Default shall impair any such right, power or remedy or shall
be construed to be a waiver of any such Event of Default or acquiescence
therein; and every such right, power or remedy may be exercised from time to
time and as often as may be deemed expedient.

         No waiver of any Event of Default hereunder, whether by the Trustee or
by the Bondholders, shall extend to or shall affect any subsequent Event of
Default or shall impair any rights or remedies consequent thereon.

         Anything in this Indenture to the contrary notwithstanding, upon the
occurrence and continuance of an Event of Default other than an Insurer Default,
the Bond Insurer shall be entitled to control and direct the enforcement of all
rights and remedies granted to the Bondholders or the Trustee for the benefit of
the Bondholders hereunder including, without limitation: (i) the right to
accelerate the principal of the Bonds as described herein, and (ii) the right to
annul any declaration of acceleration, and the Bond Insurer shall also be
entitled to approve all waivers of Events of Default hereunder.

         SECTION 9.04. RIGHT OF BONDHOLDERS TO DIRECT PROCEEDINGS. Other than as
provided in Section 9.03, the Owners of not less than a majority in aggregate
principal amount of Bonds then Outstanding shall have the right, at any time, by
an instrument or instruments in writing executed and delivered to the Trustee,
to direct the time, method and place of conducting all proceedings to be taken
in connection with the enforcement of the terms and conditions of this
Indenture, or for the appointment of a receiver or any other proceedings
hereunder; provided, that such direction shall not be otherwise than in
accordance with the provisions of law and of this Indenture and could not
involve the Trustee in personal liability.

         SECTION 9.05. APPLICATION OF MONEYS. All moneys received by the Trustee
pursuant to any right given or action taken under the provisions of this Article
or pursuant to 6.08 hereof shall, after payment of the costs and expenses of the
proceedings resulting in the collection of such moneys and of the expenses,
liabilities and advances incurred or made by the Trustee and its Counsel, be
deposited in the Bond Fund and all such moneys in the Bond Fund shall be applied
as follows:

         (a) Unless the principal of all the Bonds shall have become or shall
have been declared due and payable, all such moneys shall be applied:

                  FIRST - To the payment to the persons entitled thereto of all
         interest then due on the Bonds (other than interest due on Bonds for
         the payment of which moneys are held pursuant to the provisions of this
         Indenture), and, if the amount available shall not be sufficient to pay
         said amount in full, then to the payment ratably, according to the
         amounts due, to the persons entitled thereto, without any
         discrimination or privilege;

                  SECOND - To the payment to the persons entitled thereto of the
         unpaid principal of and premium, if any, on any of the Bonds which
         shall have become due (other than Bonds matured or called for
         redemption for the payment of which

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<PAGE>   75

         moneys are held pursuant to the provisions of this Indenture), and, if
         the amount available shall not be sufficient to pay in full such unpaid
         principal and premium, then to the payment ratably to the persons
         entitled thereto without any discrimination or privilege; and

                  THIRD - To the payment to the persons entitled thereto of
         interest on overdue principal of and premium, if any, on any Bonds
         without preference or priority as between principal or premium or
         interest one over the others, or any installment of interest over any
         other installment of interest, or of any Bond over any other Bond, and
         if the amount available shall not be sufficient to pay such amounts in
         full, then ratably, without any discrimination or privilege.

         (b) If the principal of all the Bonds shall have become due or shall
have been declared due and payable, all such moneys shall be applied to the
payment of the principal and premium, if any, and interest then due and unpaid
upon the Bonds (other than Bonds matured or called for redemption or interest
due on Bonds for the payment of which moneys are held pursuant to the provisions
of this Indenture), without preference or priority of principal, premium or
interest one over the others, or of any installment of interest over any other
installment of interest, or of any Bond over any other Bond, ratably, according
to the amounts due respectively for principal and interest, to the persons
entitled thereto without any discrimination or privilege.

         (c) If the principal of all the Bonds shall have been declared due and
payable, and if such declaration shall thereafter have been rescinded and
annulled under the provisions of this Article, then, subject to the provisions
of Section 9.05(b) hereof in the event that the principal of all the Bonds shall
later become due or be declared due and payable, the moneys shall be applied in
accordance with the provisions of Section 9.05(a) hereof.

         Whenever moneys are to be applied pursuant to the provisions of this
Section, such moneys shall be applied at such times, and from time to time, as
the Trustee shall determine. Whenever the Trustee shall apply such moneys, it
shall fix the date upon which such application is to be made and upon such date
interest on the amounts of principal to be paid on such date shall cease to
accrue. The Trustee shall give such notice as it may deem appropriate of the
deposit with it of any such moneys and of the fixing of any such date.

         Whenever all principal of and premium, if any, and interest on all
Bonds have been paid under the provisions of this Section 9.05 and all expenses
and charges of the Trustee have been paid, any balance remaining in the Bond
Fund shall be paid to the Borrower as provided in Section 6.14 hereof.

         SECTION 9.06. REMEDIES VESTED IN TRUSTEE. All rights of action
(including the right to file proofs of claims) under this Indenture or under any
of the Bonds may be enforced by the Trustee without the possession of any of the
Bonds or the production thereof in any trial or other proceedings relating
thereto and any such suit or proceeding instituted by the Trustee shall be
brought in its name as Trustee without the necessity of joining as plaintiffs or
defendants any Owners of the Bonds, and any recovery of judgment shall be for
the equal and ratable benefit of the Owners of the Outstanding Bonds.

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         SECTION 9.07. RIGHTS AND REMEDIES OF BONDHOLDERS. No Owner of any Bond
shall have any right to institute any suit, action or proceeding in equity or at
law for the enforcement of this Indenture or for the execution of any trust
thereof or for the appointment of a receiver or any other remedy hereunder,
unless (i) a default has occurred of which a Responsible Officer of the Trustee
has actual knowledge or has been notified as provided in subsection (g) of
Section 10.01 hereof, (ii) such default shall have become an Event of Default
hereunder and be continuing, (iii) the Owners of more than a majority in
aggregate principal amount of Bonds then Outstanding shall have made written
request to the Trustee, either to proceed to exercise the powers herein granted
or to institute such action, suit or proceeding in its own name, and shall have
offered to the Trustee indemnity as provided in Section 10.01(i), and (iv) the
Trustee shall for sixty (60) days after such notice, request and offer of
indemnity fail or refuse to exercise the powers herein granted, or to institute
such action, suit or proceeding in its own name; and such notification, request
and offer of indemnity are hereby declared in every case at the option of the
Trustee to be conditions precedent to the execution of the powers and trusts of
this Indenture, and to any action or cause of action for the enforcement of this
Indenture, or for the appointment of a receiver or for any other remedy
hereunder. No one or more Owners of the Bonds shall have any right in any manner
whatsoever to enforce any right hereunder except in the manner herein provided,
and all proceedings at law or in equity shall be instituted, had and maintained
in the manner herein provided and for the equal and ratable benefit of the
Owners of all Bonds then outstanding. Nothing in this Indenture shall, however,
affect or impair the right of any Bondholder to enforce the payment of the
principal of and premium, if any, and interest on any Bond at and after the
maturity thereof.

         SECTION 9.08. TERMINATION OF PROCEEDINGS. In case the Trustee shall
have proceeded to enforce any right under this Indenture by the appointment of a
receiver or otherwise, and such proceedings shall have been discontinued or
abandoned for any reason, or shall have been determined adversely, then and in
every such case the Issuer, the Trustee and the Bondholders shall be restored to
their former positions and rights hereunder, respectively, and all rights,
remedies and powers of the Trustee shall continue as if no such proceedings had
been taken.

         SECTION 9.09. WAIVERS OF EVENTS OF DEFAULT. Subject to the next
paragraph and to the last paragraph of Section 9.03, the Trustee may in its
discretion waive any Event of Default hereunder and rescind its consequences and
shall do so upon the written request of the Owners of not less than a majority
in aggregate principal amount of all Bonds then Outstanding; provided, however,
that there shall not be waived any Event of Default in the payment of the
principal of, or premium on, any Outstanding Bonds when due (whether at maturity
or by redemption), or any Event of Default in the payment when due of the
interest on any such Bonds, unless prior to such waiver and rescission, all
arrears of principal of and interest upon such Bonds, and interest on overdue
principal at the rate borne by the Bonds on the date on which such principal
became due and payable, and all arrears of premium, if any, when due, together
with the reasonable expenses of the Trustee and of the Owners of such Bonds,
including reasonable attorneys' fees paid or incurred, shall have been

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paid or provided for; provided further, there shall not be waived any Event of
Default in the payment when due of any purchase prices of any Bonds pursuant to
Article IV hereof unless prior to such waiver and rescission all arrears of such
purchase prices, together with reasonable expenses of the Trustee and of the
Owners of such Bonds, including reasonable attorneys' fees paid or incurred,
shall have been paid or provision therefor made. In the case of any such waiver
and rescission, or in case any proceeding taken by the Trustee on account of any
such default shall have been discontinued or abandoned or determined adversely,
then and in every such case the Issuer, the Trustee and the Bondholders shall be
restored to their former positions and rights hereunder, respectively, but no
such waiver and rescission shall extend to any subsequent or other default, or
impair any right consequent thereon. All waivers under this Indenture shall be
in writing and a copy thereof shall be delivered to the Issuer, the Borrower and
the Remarketing Agent, if any.

         The provisions of Sections 9.01 and 9.02 hereof are subject to the
conditions that if, after the principal of all Bonds then Outstanding shall have
been declared to be due and payable, all arrears of principal of and interest
upon such Bonds, and the premium, if any, on all Bonds then outstanding which
shall have become due and payable otherwise than by acceleration, and all other
sums payable under this Indenture, except the principal of, and interest on, the
Bonds which by such declaration shall have become due and payable, shall have
been paid by or on behalf of the Issuer, together with the reasonable expenses
of the Trustee and of the Owners of such Bonds, including reasonable attorneys'
fees paid or incurred, and if no other defaults shall have occurred and be
continuing, then and in every such case, the Trustee shall annul such
declaration of maturity and its consequences, which waiver and annulment shall
be binding upon all Bondholders; but no such waiver, rescission and annulment
shall extend to or affect any subsequent default or impair any right or remedy
consequent thereon. In the case of any such annulment, the Borrower, the Issuer,
the Trustee and the Bondholders shall be restored to their former positions and
rights under this Indenture. All waivers and annulments under this Indenture
shall be in writing and a copy thereof shall be delivered to the Issuer, the
Bond Insurer, the Borrower and the Remarketing Agent, if any.

         SECTION 9.10. NOTICE OF EVENT OF DEFAULT UNDER SECTION 9.01(e) HEREOF;
OPPORTUNITY OF BORROWER TO CURE DEFAULTS. Anything herein to the contrary
notwithstanding, no default described in Section 9.01(e) hereof shall constitute
an Event of Default until actual notice of such default, requiring that it be
remedied and stating that such notice is a "Notice of Default" hereunder, by
registered or certified mail shall be given to the Issuer and the Borrower by
the Trustee or to the Issuer and the Borrower and the Trustee by the Owners of a
majority in aggregate principal amount of all Bonds Outstanding, and the Issuer
or the Borrower on behalf of the Issuer shall have had ninety days after receipt
of such notice to correct said default or cause said default to be corrected,
and shall not have corrected said default or caused said default to be corrected
within the applicable period; provided, however, if said default be such that it
cannot be corrected within the applicable period, it shall not constitute an
event of default if corrective action is instituted within the applicable period
and diligently pursued until the default is corrected and the fact of such
non-correction, corrective action and diligent pursuit is evidenced to the
Trustee by a certificate of an Authorized Borrower Representative or an
Authorized Issuer Representative.

         Whenever, so long as the Borrower is not in default under the
Agreement, after a reasonable request by the Borrower, the Issuer shall fail,
refuse or neglect to give any direction to the Trustee or to require the Trustee
to take any other action which the Issuer is required to have the Trustee take
pursuant to the provisions of the Agreement or this Indenture, the Borrower
instead of the Issuer may give any such direction to the Trustee or require the
Trustee to take any such action. Upon receipt by the Trustee of a written notice
signed by the Authorized Borrower

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<PAGE>   78

Representative stating that the Borrower has made reasonable request of the
Issuer, and that the Issuer has failed, refused or neglected to give any
direction to the Trustee or to require the Trustee to take any such action, the
Trustee is hereby irrevocably empowered and directed to accept such direction
from the Borrower as sufficient for all purposes of this Indenture. The Borrower
shall have the direct right to cause the Trustee to comply with any of the
Trustee's obligations under this Indenture to the same extent that the Issuer is
empowered so to do.

         Certain actions or failures to act by the Issuer under this Indenture
may create or result in an event of default under this Indenture and the Issuer
hereby grants the Borrower full authority, to the extent permitted by law, for
account of the Issuer to perform or observe any covenant or obligation of the
Issuer alleged in a written notice to the Issuer and the Borrower from the
Trustee not to have been performed or observed, in the name and stead of the
Issuer with full power to do any and all things and acts to remedy any default.

                                   ARTICLE X

                                   THE TRUSTEE

         SECTION 10.01. ACCEPTANCE OF THE TRUSTS BY TRUSTEE. The Trustee hereby
accepts the trusts imposed upon it by this Indenture, represents and covenants
that it is fully empowered under the applicable laws and regulations of the
State to accept said trusts, and agrees to perform said trusts, but only upon
and subject to the following express terms and conditions, and no implied
covenants or obligations shall be read into this Indenture against the Trustee:

         (a) The Trustee may execute any of the trusts or powers hereof and
perform any of its duties by or through attorneys, agents, custodians, nominees,
receivers or employees and shall not be responsible for the supervision of, or
the acts of any attorneys, agents or receivers appointed by it in good faith and
without negligence, and shall be entitled to advice of Counsel concerning all
matters of trusts hereof and the duties hereunder, and may in all cases pay such
reasonable compensation to all such attorneys, agents and receivers as may
reasonably be employed in connection with the trusts hereof. The Trustee may act
upon the opinion or advice of Counsel. The Trustee shall not be responsible for
any loss or damage resulting from any action or non-action in good faith in
reliance upon such opinion or advice.

         (b) Except for its certificate of authentication on the Bonds and the
other information the Trustee is required to set forth on the Bonds pursuant to
Section 2.06 hereof, the Trustee shall not be responsible for any recital herein
or in the Bonds, or the validity, priority, recording, or rerecording, filing,
or refiling of this Indenture or any financing statement, amendments to this
Indenture, or continuation statements, or for reviewing any annual reports,
financial statements or audits, or for insuring the Project or collecting any
insurance moneys, other than the Bond Insurance, or for the validity of the
execution by the Issuer of this Indenture or for any supplements hereto or
instruments of further assurance, or for the sufficiency of the security for the
Bonds issued hereunder or intended to be secured hereby, for the value or title
of the Project or as to the maintenance of the security hereof. The Trustee
shall not be bound to ascertain or inquire as to the performance or observance
of any covenants, conditions or agreements on the part of the Issuer or on the
part of the Borrower under the Agreement, except

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<PAGE>   79

as hereinafter set forth, but the Trustee may require of the Issuer or the
Borrower full information and advice as to the performance of the covenants,
conditions and agreements aforesaid. Except as otherwise provided in Sections
9.02 and 9.03 hereof, the Trustee shall have no obligation to perform any of the
rights or obligations of the Issuer under the Agreement. The Trustee shall not
be liable for participating in any act directed by the Issuer or the Borrower
which might cause the Bonds of any Tax-Exempt Series to be "arbitrage bonds"
within the meaning of Section 148(a) of the Code. The Trustee shall not be
responsible or liable for the selection of any investment or for any loss
suffered in connection with any investment of funds made by it in accordance
with Article VII hereof including, without limitation, any loss suffered in
connection with the sale of any investment pursuant to Article VII hereof.

         (c) The Trustee shall not be accountable for the use of any Bonds
authenticated or delivered hereunder. The Trustee may become the Owner of Bonds
and otherwise transact banking and trustee business with the Borrower with the
same rights which it would have if it were not Trustee.

         (d) The Trustee shall be fully protected in acting in good faith upon
any notice, request, resolution, consent, certificate, affidavit, letter,
telegram or other paper or document, or oral communication or direction,
believed to be genuine and correct and to have been signed or sent or given by
the proper person or persons. Any action taken by the Trustee pursuant to this
Indenture upon the request or authority or consent of any person who at the time
of making such request or giving such authority or consent is the Owner of any
Bond shall be conclusive and binding upon all future owners of the same Bond and
upon Bonds issued in exchange therefor or upon transfer or in place thereof.

         (e) As to the existence or non-existence of any fact or as to the
sufficiency or validity of any instrument, paper or proceeding, the Trustee
shall be entitled to conclusively rely upon a certificate signed on behalf of
the Issuer by an Authorized Issuer Representative as sufficient evidence of the
facts therein contained, and prior to the occurrence of a default of which the
Trustee has been notified as provided in subsection (g) of this Section, or
subsequent to the waiver, rescission or annulment of a default as provided in
Article IX hereof, shall also be at liberty to accept a similar certificate to
the effect that any particular dealing, transaction or action is necessary or
expedient, but may at its discretion secure such further evidence deemed
necessary or advisable, but shall in no case be bound to secure the same. The
Trustee may accept a certificate signed on behalf of the Issuer by an Authorized
Issuer Representative to the effect that a resolution or ordinance in the form
therein set forth has been adopted by the Issuer as conclusive evidence that
such resolution or ordinance has been duly adopted, and is in full force and
effect.

         (f) The permissive right of the Trustee to do things enumerated in this
Indenture shall not be construed as a duty and the Trustee shall not be liable
in the performance of its obligations hereunder except for its negligence or
willful misconduct.

         (g) The Trustee shall not be required to take notice or be deemed to
have notice of any default or Event of Default hereunder, except failure by the
Issuer to cause to be made any of the payments to the Trustee required to be
made by Article IV hereof and all defaults under Section 9.01(a), (b) or (c)
hereof, unless a Responsible Officer shall be

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specifically notified in writing of such default by the Issuer, the Bond Insurer
or the Owners of at least a majority in aggregate principal amount of all Bonds
then outstanding or the Remarketing Agent.

         (h) The Trustee shall not be required to give any bonds or surety in
respect of the execution of its trusts and powers hereunder.

         (i) Before taking any action under Article IX hereof at the request or
direction of the Bondholders, the Trustee may require that a satisfactory
indemnity bond be furnished by the Bondholders, for the reimbursement of all
expenses to which it may be put and to protect it against all liability, except
liability which is adjudicated to have resulted from its negligence or willful
default in connection with any action so taken.

         (j) All moneys received by the Trustee shall, until used or applied or
invested as herein provided, be held in trust for the purposes for which they
were received and shall not be commingled with the general funds of the Trustee
but need not be segregated from other funds except to the extent required or
permitted by law. Neither the Trustee nor any Paying Agent shall be under any
liability for interest on any moneys received hereunder except such as may be
agreed upon.

         (k) The Trustee, prior to the occurrence of an Event of Default
specified in Section 9.01 of this Indenture and after the curing or waiving of
all Events of Default which may have occurred, undertakes to perform such duties
and only such duties as are specifically set forth in this Indenture and, in the
absence of bad faith on its part, the Trustee may conclusively rely, as to the
truth of the statements and correctness of the opinions expressed therein, upon
certificates or opinions furnished to the Trustee and conforming to the
requirements of this Indenture; but in the case of any such certificates or
opinions which by any provision hereof are specifically required to be furnished
to the Trustee, the Trustee shall be under a duty to examine the same to
determine whether or not they conform to the requirements of this Indenture. In
case an event of default has occurred (which has not been cured or waived) the
Trustee shall exercise such of the rights and powers vested in it by this
Indenture, and use the same degree of care and skill in their exercise, as a
prudent person would exercise or use under the circumstances in the conduct of
his own affairs.

         (l) No provision of this Indenture shall be construed to relieve the
Trustee from liability for its own negligent action, its own negligent failure
to act, or its own willful misconduct, except that:

                  (i) This subsection shall not be construed to limit the effect
         of subsection (k) of this Section;

                  (ii) The Trustee shall not be liable for any error of judgment
         made in good faith by an officer of the Trustee, unless it shall be
         proved that the Trustee was negligent in ascertaining the pertinent
         facts;

                  (iii) The Trustee shall not be liable with respect to any
         action taken or omitted to be taken by it in good faith in accordance
         with the direction of the Owners of a majority in aggregate principal
         amount of the Bonds outstanding relating to the time,

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<PAGE>   81

         method and place of conducting any proceeding or any remedy available
         to the Trustee, or exercising any trust or power conferred upon the
         Trustee, under this Indenture; and

                  (iv) No provision of this Indenture shall require the Trustee
         to expend or risk its own funds or otherwise incur any financial
         liability in the performance of any of its duties hereunder, or in the
         exercise of any of its rights or powers, if it shall have reasonable
         grounds for believing that repayment of such funds or adequate
         indemnity against such risk or liability is not reasonably assured to
         it.

         (m) Notwithstanding anything elsewhere in this Indenture contained, the
Trustee shall have the right, but shall not be required, to demand, in respect
of the authentication of any Bonds, the withdrawal of any cash, the release of
any property, or any action whatsoever within the purview of this Indenture, any
showings, certificates, opinions, appraisals or other information, or corporate
action or evidence thereof, in addition to that by the terms hereof required as
a condition of such action by the Trustee, deemed desirable for the purpose of
establishing the right of the Issuer to the authentication of any Bonds, the
withdrawal of any cash, or as a condition to the taking of any action by the
Trustee.

         (n) In the event the Trustee incurs expenses or renders services in
connection with an event of bankruptcy, reorganization or insolvency, such
expenses (including the fees and expenses of its counsel) and the compensation
for such services are intended to constitute expenses of administration under
any bankruptcy, reorganization or insolvency.

         (o) To the extent that the Trustee is also acting as Paying Agent,
Registrar or Tender Agent hereunder, the rights and protections afforded to the
Trustee pursuant to this Article X shall also be afforded to such Paying Agent,
Registrar and Tender Agent.

         Notwithstanding any other provision of this Indenture, in determining
whether the rights of the Bondholders will be adversely affected by any action
taken pursuant to the terms and provisions hereof, the Trustee (or Paying Agent)
shall consider the effect on the Bondholders as if there were no Bond Insurance.

         SECTION 10.02. CORPORATE TRUSTEE REQUIRED; ELIGIBILITY. There shall at
all times be a Trustee hereunder which shall be a corporation organized and
doing business as a commercial bank or trust company in good standing under the
laws of the United States of America or any state or territory thereof, duly
authorized under such laws to exercise corporate trust powers, be authorized to
accept and exercise the trusts herein provided, have a combined reported capital
and surplus of at least $75,000,000, be subject to examination by Federal or
state authority and have such offices and agencies in such locations as are
required under the Act and as are required to enable it to perform the functions
herein contemplated and acceptable to the Bond Insurer. If such corporation
publishes reports of condition at least annually, pursuant to law or to the
requirements of the aforesaid supervising or examining authority, then for the
purposes of this Section 10.02, the combined capital and surplus of such
corporation shall be deemed to be its combined capital and surplus as set forth
in its most recent report of condition so published.

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         SECTION 10.03. FEES, CHARGES AND EXPENSES OF TRUSTEE. The Trustee shall
be entitled to payment and/or reimbursement from the Borrower for reasonable
fees for its services rendered hereunder and all advances, fees of counsel and
other expenses reasonably and necessarily made or incurred by the Trustee in
connection with such services hereunder; provided, that if such expenses are
occasioned by the negligence or willful misconduct of the Trustee it shall not
be entitled to compensation or reimbursement therefor. The Trustee shall have a
first lien with right of payment prior to payment on account of principal of or
premium, if any, or interest on any Bond upon all moneys in its possession under
any provisions hereof for the foregoing advances, fees, costs and expenses
incurred; provided, however, that the Trustee shall not have a first lien with
right of payment prior to payment on account of principal of or premium, if any,
or interest on any Bond with respect to moneys in the Bond Fund held for the
payment of the principal of and premium, if any, and interest on particular
Bonds or moneys held for the payment of the purchase price of particular Bonds.
The Trustee's rights under this Section 10.03 shall survive the termination or
expiration of this Indenture.

         SECTION 10.04. NOTICE TO BONDHOLDERS IF DEFAULT OCCURS. If a default
occurs of which the Trustee is by subsection (g) of Section 10.01 hereof
required to take notice or if notice of default be given as in said subsection
(g) provided, the Trustee shall within fifteen (15) days thereafter (unless such
default is cured or waived), give notice of such default to each Owner of Bonds
then Outstanding, provided that, except in the case of a default in the payment
of the principal of or premium, if any, or interest on any Bond, the Trustee may
withhold such notice to the Bondholders if and so long as a trust committee of
Responsible Officers of the Trustee in good faith determine that the withholding
of such notice is in the interests of the Bondholders and provided further that
nothing in this Section 10.04 shall be deemed to limit the notice required by
the second to last paragraph of Section 9.05 hereof.

         SECTION 10.05. INTERVENTION BY TRUSTEE. In any judicial proceeding to
which the Issuer or the Borrower is a party and which in the opinion of the
Trustee and its Counsel has a substantial bearing on the interests of Owners of
the Bonds, the Trustee may intervene on behalf of Bondholders and, subject to
the provisions of Section 10.01(i), shall do so if requested in writing by the
Owners of a majority in aggregate principal amount of all Bonds then
outstanding.

         SECTION 10.06. SUCCESSOR TRUSTEE. Any corporation or association into
which the Trustee may be merged or converted, or with which it may be
consolidated, or to which it may sell, lease or transfer its corporate trust
business and assets as a whole or substantially as a whole, shall be and become
successor Trustee hereunder and shall be vested with all the trusts, powers,
rights, obligations, duties, remedies, immunities and privileges hereunder as
was its predecessor, without the execution or filing of any instrument on the
part of any of the parties hereto.

         SECTION 10.07. RESIGNATION BY THE TRUSTEE. The Trustee may at any time
resign from the trusts hereby created by giving sixty (60) days written notice
by registered or certified mail to the Issuer, the Borrower, the Liquidity
Provider, if any, the Bond Insurer, the Owner of each Bond and the Remarketing
Agent, if any, and such resignation shall not take effect until the appointment
of a successor Trustee pursuant to the provisions of Section

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10.09 hereof and acceptance by the successor Trustee of the trusts created
hereby. If no successor Trustee shall have been so appointed and have accepted
appointment within forty-five (45) days of the giving of written notice by the
resigning Trustee as aforesaid, the resigning Trustee may petition any court of
competent jurisdiction for the appointment of a successor Trustee or any
Bondholder who has been a bona fide holder of a Bond for at least six months
may, on behalf of itself and others similarly situated, petition any such court
for the appointment of a successor trustee. Such court may thereupon, after such
notice, if any, as it may deem proper and may prescribe, appoint a successor
trustee meeting the qualifications set forth in Section 10.02.

         SECTION 10.08. REMOVAL OF THE TRUSTEE. (a) In case at any time either
of the following shall occur:

                  (1) the Trustee shall cease to be eligible in accordance with
         the provisions of Section 10.02 hereof and shall fail to resign after
         written request therefor by the Issuer or by any Bondholder who has
         been a bona fide holder of a Bond for at least six months, or

                  (2) the Trustee shall become incapable of acting, or shall be
         adjudged a bankrupt or insolvent, or a receiver of the Trustee or of
         its property shall be appointed, or any public officer shall take
         charge or control of the Trustee or of its property or affairs for the
         purpose of rehabilitation, conservation or liquidation,

then, in any such case, the Issuer may remove the Trustee and appoint a
successor Trustee (with the advice of the Borrower and the consent of the Bond
Insurer) by an instrument in writing, or any such Bondholder may, on behalf of
itself and all others similarly situated, petition any court of competent
jurisdiction for the removal of the Trustee and the appointment of a successor
Trustee, in each case, meeting the eligibility standards set forth in Section
10.02. Such court may thereupon, after such notice if any, as it may deem proper
and may prescribe, remove the Trustee and appoint a successor Trustee.

         (b) The Issuer, in its sole discretion or upon the written request of
an Authorized Borrower Representative, or the Owners of a majority in aggregate
principal amount of the Bonds at the time outstanding or the Bond Insurer, for
any failure by the Trustee to fulfill its obligations hereunder may at any time,
with or without cause, remove the Trustee and appoint a successor Trustee by an
instrument or concurrent instruments in writing signed by the Issuer (with the
advice of the Borrower and the consent of the Bond Insurer) or such Bondholders,
as the case may be.

         (c) Any resignation or removal of the Trustee and appointment of a
successor trustee, pursuant to any of the provisions of this Section shall not
become effective until the acceptance of appointment by the successor Trustee as
provided in Section 10.09 and acceptance by the successor Trustee of the trusts
created hereby.

         SECTION 10.09. APPOINTMENT OF SUCCESSOR TRUSTEE. In case the Trustee
hereunder shall:

         (a) resign pursuant to Section 10.07 hereof;

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<PAGE>   84

         (b) be removed pursuant to Section 10.08 hereof; or

         (c) be dissolved, taken under the control of any public officer or
officers or of a receiver appointed by a court, or otherwise become incapable of
acting hereunder,

a successor shall be appointed by the Issuer at the direction of the Borrower
and with the written consent of the Bond Insurer; provided, that if a successor
Trustee is not so appointed within ten (10) days after notice of resignation is
mailed or instrument of removal is delivered as provided under Sections 10.07
and 10.08 hereof, respectively, or within ten (10) days of the Issuer's
knowledge of any of the events specified in (c) hereinabove, then the Owners of
a majority in aggregate principal amount of Bonds then outstanding, by filing
with the Issuer, the Borrower and the Remarketing Agent, if any, an instrument
or concurrent instruments in writing signed by or on behalf of such Owners, may
designate a successor Trustee meeting the eligibility standards set forth in
Section 10.02.

         SECTION 10.10. CONCERNING ANY SUCCESSOR TRUSTEES. Every successor
Trustee appointed hereunder shall execute, acknowledge and deliver to its
predecessor and also to the Issuer and the Borrower, an instrument in writing
accepting such appointment hereunder, and thereupon such successor shall become
fully vested with all the trusts, powers, rights, obligations, duties, remedies,
immunities and privileges of its predecessor; but, nevertheless, and upon
payment of its charges (1) such predecessor shall, on the written request of the
Issuer, execute and deliver an instrument transferring to such successor Trustee
all the trusts, powers, rights, obligations, duties, remedies, immunities and
privileges of such predecessor hereunder and (2) such predecessor shall deliver
all securities and moneys held by it as Trustee hereunder to its successor and
transfer any Liquidity Facility to such successor. Should any instrument in
writing from the Issuer be required by any successor Trustee for more fully and
certainly vesting in such successor the trusts, powers, rights, obligations,
duties, remedies, immunities and privileges hereby vested in the predecessor any
and all such instruments in writing shall, on request, be executed, acknowledged
and delivered by the Issuer at the expense of the Borrower. The resignation of
any Trustee and the instrument or instruments removing any Trustee and
appointing a successor hereunder, together with all other instruments provided
for in this Article, shall be filed or recorded by the successor Trustee in each
recording office, if any, where the Indenture or a financing statement relating
thereto shall have been filed or recorded. No Trustee hereunder shall be liable
for the acts or omissions of any successor Trustee.

         SECTION 10.11. TRUSTEE PROTECTED IN RELYING UPON RESOLUTION. The
resolutions, ordinances, opinions, certificates and other instruments provided
for in this Indenture may be accepted by the Trustee as conclusive evidence of
the facts and conclusions stated therein and shall be full warrant, protection
and authority to the Trustee for the release of property and the withdrawal of
cash hereunder.

         SECTION 10.12. SUCCESSOR TRUSTEE AS THE TRUSTEE, PAYING AGENT, TENDER
AGENT AND REGISTRAR. In the event of a change in the office of the Trustee, the
predecessor Trustee which has resigned or been removed shall cease to be
Trustee, Tender Agent, Registrar and Paying Agent, and the successor Trustee
shall become such Trustee, Tender Agent, Registrar and Paying Agent.

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         SECTION 10.13. NOTICES TO BE GIVEN BY TRUSTEE. The Trustee shall
provide the Issuer, the Borrower and the Liquidity Provider, if any, with the
following:

         (A) On or before December 15 of each year during which any of the Bonds
are outstanding, commencing December 15, 1999, or upon any significant change
that occurs which would adversely impact the Trustee's ability to perform its
duties under the Indenture, a written disclosure of any such change, or if
applicable, of any conflicts that the Trustee may have as a result of other
business dealings between the Trustee and the Borrower. If there are no such
instances of a significant change, or of a conflict existing, then a statement
to that effect shall be provided on such date.

         (B) If there is a failure to pay any amount of principal of, premium,
if any, or interest on the Bonds when due; or if there is a failure of the
Borrower to provide any notice, certification or report specified in Section 5.3
of the Agreement; or if there is an occurrence of an Event of Default hereunder,
of which the Trustee has knowledge, the Trustee shall provide written notice to
the Issuer within five (5) Business Days of such occurrence and such notice
shall include a statement setting forth the steps the Trustee is taking to
remedy such failure or Event of Default, as applicable.

         (C) As of June 30 and December 31 of each year (beginning December 31,
1999) in which Bonds are Outstanding, a Trustee's report in the form of Exhibit
C attached hereto. This report shall be received no later than January 15 or
July 15 next following each such June 30 or December 31, as the case may be.

         SECTION 10.14. NOTICES TO RATING AGENCY AND LIQUIDITY PROVIDER; NOTICES
TO BOND INSURER. (a) The Trustee shall provide any rating agency then rating the
Bonds and the Liquidity Provider with written notice upon the occurrence of: (i)
the expiration, termination, extension of or substitution for any Liquidity
Facility; (ii) the discharge of liability on any Series of the Bonds pursuant to
the terms hereof; (iii) the resignation or removal of the Trustee, Tender Agent,
or Remarketing Agent; (iv) acceptance of appointment as successor Trustee,
Tender Agent, or Remarketing Agent hereunder; (v) the redemption of all Bonds;
(vi) any conversion of the Series 1999B Bonds to a Tax-Exempt Series, or to a
Rate Period not requiring a Liquidity Facility; (vii) a material change in the
Indenture, the Agreement, or Liquidity Facility; (viii) any mandatory tender of
a Series of Bonds hereunder and (ix) when a Series of Bonds is no longer
Outstanding. The Trustee shall also notify any rating agency then rating the
Bonds of any changes to any of the documents to which the Trustee is a party,
upon its receipt of notification of any such changes.

         (a) While the Bond Insurance is in effect with respect to a Series of
Bonds, the Trustee shall furnish to the Bond Insurer (to the attention of its
surveillance department, unless otherwise indicated):

                  (i) as soon as practicable after the filing thereof, a copy of
         any financial statement of the Issuer and a copy of any audit and
         annual report of the Issuer;

                  (ii) a copy of any notice to be given to the registered owners
         of such Series of Bonds, including, without limitation, notice of any
         redemption of or defeasance of such

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<PAGE>   86

         Series of Bonds, a copy of any notice provided to the Liquidity
         Provider, and any certificate rendered pursuant to this Indenture
         relating to the security for the Bonds; and

                  (iii) such additional information it may reasonably request.

         (c) The Trustee shall notify the Bond Insurer of any failure of the
Issuer to provide relevant notices or certificates required to be provided by
the Issuer hereunder.

         (d) Notwithstanding any other provision hereof, the Trustee shall
immediately notify the Bond Insurer if at any time there are insufficient moneys
to make any payments of principal and/or interest as required and immediately
upon the occurrence of any Event of Default hereunder.

         (e) The Issuer will permit the Bond Insurer to discuss the finances and
accounts of the Issuer or any information the Bond Insurer may reasonably
request regarding the security for the Bonds with appropriate officers of the
Issuer. The Trustee will permit the Bond Insurer to have access to and to make
copies of all books and records relating to the Bonds at any reasonable time.

         (f) The Bond Insurer shall have the right to direct an accounting at
the Issuer's expense, and the Issuer's failure to comply with such direction
within thirty (30) days after receipt of written notice to the Issuer and the
Borrower of the direction from the Bond Insurer shall be deemed a default
hereunder; provided, however, that if compliance cannot occur within such
period, then such period will be extended so long as compliance is begun within
such period and diligently pursued, but only if such extension would not
materially adversely affect the interests of any registered owner of the Bonds.

                                   ARTICLE XI

                             SUPPLEMENTAL INDENTURES

         SECTION 11.01. SUPPLEMENTAL INDENTURES NOT REQUIRING CONSENT OF
BONDHOLDERS (BUT REQUIRING CONSENT OF THE BORROWER). The Issuer and the Trustee
may without the consent of, or notice to, any of the Bondholders, but with the
consent of the Borrower and the Bond Insurer pursuant to Section 11.03 hereof,
enter into an indenture or indentures supplemental to this Indenture for any one
or more of the following purposes:

                  (i) to add to the covenants and agreements of, and limitations
         and restrictions upon, the Issuer in this Indenture, other covenants,
         agreements, limitations and restrictions to be observed by the Issuer
         which are not contrary to or inconsistent with this Indenture as
         theretofore in effect;

                  (ii) to grant to or confer or impose upon the Trustee for the
         benefit of the Bondholders any additional rights, remedies, powers,
         authority, security, liabilities or duties which may lawfully be
         granted, conferred or imposed and which are not contrary to or
         inconsistent with this Indenture as heretofore in effect;

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<PAGE>   87

                  (iii) to cure any ambiguity or omission or to cure, correct or
         supplement any defective provision of this Indenture in each case in
         such manner as shall not adversely affect the Bondholders;

                  (iv) to evidence the appointment of a separate trustee or a
         co-trustee or to evidence the succession of a new trustee or a new
         co-trustee hereunder;

                  (v) to comply with the requirements of the Trust Indenture Act
         of 1939, as from time to time amended;

                  (vi) to subject to this Indenture additional revenues,
         properties or collateral;

                  (vii) to provide for the issuance of coupon bonds (provided,
         however, that the Issuer and the Trustee have received an opinion of
         Bond Counsel to the effect that the issuance of such coupon bonds
         complies with all applicable laws and will not adversely affect the
         Tax-Exempt status of the Bonds of a Tax-Exempt Series);

                  (viii) to provide for the use of an uncertificated book entry
         system (provided, however, that the Issuer and the Trustee have
         received an opinion of Bond Counsel to the effect that the use of an
         uncertificated book entry system complies with all applicable laws and
         will not adversely affect the Tax-Exempt status of the Bonds of any
         Tax-Exempt Series);

                  (ix) to modify, alter, amend or supplement the Indenture in
         any other respect, if the effective date of such supplement or
         amendment is a date on which all the Bonds affected thereby are subject
         to mandatory purchase or if notice by mail of the proposed amendment or
         supplement is given to the Owners of the Bonds at least 30 days before
         the effective date thereof, and, on or before such effective date, such
         Owners have the right to require purchase of their Bonds (provided,
         however, that the Issuer and the Trustee have received an opinion of
         Bond Counsel to the effect that any such amendment complies with all
         applicable laws and will not adversely affect the Tax-Exempt status of
         the Bonds of any Tax-Exempt Series);

                  (x) to authorize different Authorized Denominations of the
         Bonds and to make correlative amendments and modifications to this
         Indenture regarding exchangeability of Bonds of different Authorized
         Denominations, redemptions of portions of Bonds of particular
         Authorized Denominations and similar amendments and modifications of a
         technical nature;

                  (xi) to modify, delete or supplement any provision, term or
         requirement relating to Bonds that may bear interest at Flexible Rates
         to the extent deemed necessary or desirable further to protect or
         assure the Tax-Exempt status of the Bonds; provided, however, that the
         effective date of any such modification, deletion or supplementation
         with respect to any Bond shall be no earlier than the day next
         succeeding the last day of any then current Flexible Segment with
         respect to such Bond;

                  (xii) to preserve the Tax-Exempt status of the Bonds of a
         Tax-Exempt Series; or

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<PAGE>   88

                  (xiii) to modify, alter, amend or supplement this Indenture in
         any other respect which is not adverse to the Bondholders and which
         does not involve a change described in clause (a), (b), (c), (d) or (e)
         of Section 11.02 hereof.

         SECTION 11.02. SUPPLEMENTAL INDENTURES REQUIRING CONSENT OF BONDHOLDERS
AND THE BORROWER. Exclusive of supplemental indentures covered by Section 11.01
hereof and subject to the terms and provisions contained in this Section, and
not otherwise, the holders of not less than a majority in aggregate principal
amount of the Bonds then Outstanding (and with the consent of the Borrower
pursuant to Section 11.03 hereof) shall have the right, from time to time,
anything contained in this Indenture to the contrary notwithstanding, to consent
to and approve the execution by the Issuer and the Trustee of such other
indenture or indentures supplemental hereto for the purpose of modifying,
amending, adding to or rescinding, in any particular manner, any of the terms or
provisions contained in this Indenture; provided, however, that nothing in this
Section contained shall permit or be construed as permitting amendments of this
Indenture, without the consent of the holders of 100% of such Series of the
Bonds then Outstanding affected by such amendment, to effect (a) an extension of
the maturity date of the principal of or the interest on any Bond of such Series
issued hereunder, or (b) a reduction in the principal amount of, premium, if
any, on any Bond of such Series or the rate of interest thereon, or (c) an
adverse change in the rights of the Owners of the Bonds of such Series to the
purchase thereof pursuant to Article IV hereof, or (d) a privilege or priority
of any Bond or Bonds of such Series over any other Bond or Bonds, or (e) a
reduction in the aggregate principal amount of the Bonds of such Series the
Owners of which are required to consent to such supplemental indenture.

         If at any time the Issuer shall request the Trustee to enter into any
such supplemental indenture for any of the purposes allowed by this Section, the
Trustee shall, at the request of the Issuer and upon being satisfactorily
indemnified with respect to expenses and upon receiving from the Borrower forms
of notices and any other related solicitation materials, cause notice of the
proposed execution of such supplemental indenture to be mailed in substantially
the manner provided in Section 3.02 hereof with respect to redemption of Bonds.
Such notice shall briefly set forth the nature of the proposed supplemental
indenture entitled to so consent and shall state that copies thereof are on file
at the Principal Office of the Trustee for inspection by all Bondholders. If,
within sixty (60) days or such longer period of time as shall be prescribed by
the Issuer following the mailing of such notice, the Owners of a majority or
100%, as the case may be, in aggregate principal amount of the Bonds Outstanding
at the time of the execution of any such supplemental indenture entitled to so
consent shall have consented to and approved the execution thereof as herein
provided, no Owner of any Bond shall have any right to object to any of the
terms and provisions contained therein, or the operation thereof, or in any
manner to question the propriety of the execution thereof, or to enjoin or
restrain the Trustee or the Issuer from executing the same or from taking any
action pursuant to the provisions thereof. The Issuer shall have the right to
extend, with the prior written consent of the Borrower, from time to time the
period within which such consent and approval may be obtained from Bondholders.
Upon the execution of any such supplemental indenture as in this Section
permitted and provided, this Indenture shall be and be deemed to be modified and
amended in accordance therewith.

         SECTION 11.03. CONSENT OF BORROWER AND BOND INSURER. Anything herein to
the contrary notwithstanding, a supplemental indenture under this Article XI

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<PAGE>   89

shall not become effective unless and until the Borrower and Bond Insurer shall
have consented to the execution and delivery of such supplemental indenture.

         SECTION 11.04. CONSENT OF REMARKETING AGENT AND LIQUIDITY PROVIDER.
Anything herein to the contrary notwithstanding, a supplemental indenture under
this Article XI which affects any rights, duties or obligations of the
Remarketing Agent or which affects any Liquidity Facility shall not become
effective unless and until the Remarketing Agent or the Liquidity Provider, as
applicable, shall have consented to the execution and delivery of such
supplemental indenture.

         SECTION 11.05. CONSENT OF TRUSTEE. The Trustee may, but shall not be
obligated to, enter into any supplemental indenture which adversely affects the
Trustee's own rights, liabilities, duties or immunities under this Indenture or
otherwise.

         SECTION 11.06. REQUIRED AND PERMITTED OPINIONS OF COUNSEL. The Issuer
and the Trustee may receive and rely on an opinion of Counsel to the effect that
any supplemental indenture entered into by the Issuer and the Trustee complies
with the provisions of this Article XI and an opinion of Bond Counsel that any
such supplemental indenture does not adversely affect the Tax-Exempt status of
the Bonds of a Tax-Exempt Series. No supplemental indenture or amendment or
modification to the Agreement or the Bonds shall be effective until the Issuer
and the Trustee shall have received an opinion of Bond Counsel to the effect
that such supplemental indenture or such amendment or modification is permitted
by the Act and will not adversely affect the Tax-Exempt status of the Bonds of a
Tax-Exempt Series.

         SECTION 11.07. NOTATION OF MODIFICATION ON BONDS; PREPARATION OF
MODIFIED BONDS. Bonds authenticated and delivered after the execution of any
supplemental indenture pursuant to the provisions of this Article XI may bear a
notation, in form approved by the Trustee, as to any matter provided for in such
supplemental indenture, and if such supplemental indenture shall so provide, new
Bonds, so modified as to conform, in the opinion of the Trustee and the Issuer,
to any modification of this Indenture contained in any such supplemental
indenture, may be prepared by the Issuer, authenticated by the Trustee and
delivered without cost to the holders of the Bonds then outstanding, upon
surrender for cancellation of such Bonds in equal aggregate principal amounts.

                                  ARTICLE XII

                             AMENDMENT OF AGREEMENT

         SECTION 12.01. AMENDMENTS TO AGREEMENT NOT REQUIRING CONSENT OF
BONDHOLDERS. The Issuer and the Borrower may, with the written consent of the
Trustee, the Bond Insurer and the Liquidity Provider, if a Liquidity Facility is
in effect, but without the consent of or notice to any of the Bondholders, enter
into any amendment, change or modification of the Agreement (a) as may be
required by the provisions of the Agreement or this Indenture or any Liquidity
Facility, (b) for the purpose of curing any ambiguity or formal defect or
omission, (c) so as to add additional rights acquired in accordance with the
provisions of the Agreement, (d) to preserve the Tax-Exempt status of the Bonds
of a Tax-Exempt Series, or any

                                       83
<PAGE>   90

of them, (e) to make any modification or amendment in any other respect if the
effective date of such amendment is a date on which all the affected Bonds are
subject to mandatory purchase pursuant to Section 4.02(a) hereof or if notice by
mail of the proposed amendment or supplement is given to the Owners of the Bonds
at least 30 days before the effective date, and prior to such effective date
such Owners have the right to require purchase of their Bonds pursuant to
Section 4.01 hereof, or (f) in connection with any other change therein which is
not materially adverse to the Bondholders and which does not involve a change
described in clauses (a) or (b) of Section 12.02 hereof and which in the
reasonable judgment of the Trustee is not to the prejudice of the Trustee;
provided that any amendment or supplement to Exhibit A to the Agreement
contemplated in Section 3.1 of the Agreement shall not be deemed to be an
amendment of the Agreement for any purpose of this Article XII.

         SECTION 12.02. AMENDMENTS TO AGREEMENT REQUIRING CONSENT OF
BONDHOLDERS. Unless otherwise specifically provided in this Section, the Bond
Insurer's consent shall be required in addition to required Bondholder consent,
when required, for the execution and delivery of any amendment, supplement or
change to or modification of the Agreement. Except for the amendments, changes
or modifications as provided in Section 12.01 hereof, neither the Issuer nor the
Borrower shall enter into any other amendment, change or modification of the
Agreement without mailing of notice and the written approval or consent of the
Owners of not less than a majority in aggregate principal amount of the Bonds at
the time Outstanding given and procured as provided in this Section; provided,
however, that nothing in this Section or Section 12.01 hereof shall permit or be
construed as permitting, without the consent of the holders of 100% of the Bonds
then Outstanding, (a) an extension of the time of the payment of any amounts
payable under Section 4.2(a) or Section 4.2(b) of the Agreement, or (b) a
reduction in the amount of any payment or in the total amount due under Section
4.2(a) or Section 4.2(b) of the Agreement. If at any time the Issuer and the
Borrower shall request the consent of the Trustee to any such proposed
amendment, change or modification of the Agreement, the Trustee shall, at the
request of the Issuer and upon being satisfactorily indemnified with respect to
expenses and upon receiving from the Borrower forms of notices and any other
related solicitation materials, cause notice of such proposed amendment, change
or modification to be mailed to the Owners of Bonds in the same manner as
provided by Section 3.02 hereof with respect to redemption of Bonds. Such notice
shall briefly set forth the nature of such proposed amendment, change or
modification and shall state that copies of the instrument embodying the same
are on file with the Trustee for inspection by all Bondholders. If, within sixty
(60) days, or such longer period as shall be prescribed by the Issuer, following
the mailing of such notice, the Owners of a majority or 100%, as the case may
be, in aggregate principal amount of the Bonds Outstanding at the time of the
execution of any such amendment, change or modification, as the case may be,
shall have consented to and approved the execution thereof as herein provided,
no Owner of any Bond shall have any right to object to any of the terms and
provisions contained therein, or the operation thereof, or in any manner to
question the propriety of the execution thereof, or to enjoin or restrain the
Borrower or the Issuer from executing the same or from taking any action
pursuant to the provisions thereof, or the Trustee from consenting thereto. The
Issuer shall have the right to extend from time to time the period within which
such consent and approval may be obtained from Bondholders. Upon the execution
of any such amendment, change or modification as in this Section permitted and
provided, the Agreement shall be and be deemed to be modified, changed and
amended in accordance therewith.

                                       84
<PAGE>   91

         SECTION 12.03. CONSENT OF TRUSTEE. The Trustee may, but shall not be
obligated to, consent to any amendment, change or modification of the Agreement
which adversely affects the Trustee's own rights, duties or immunities under
this Indenture or otherwise.

         SECTION 12.04. RELIANCE ON OPINIONS OF COUNSEL. The Issuer and the
Trustee may receive and rely upon an opinion of Counsel to the effect that any
such proposed amendment, change or modification will comply with the provisions
of this Article XII and an opinion of Bond Counsel that any such amendment,
change or modification does not adversely affect the Tax-Exempt status of the
Bonds of a Tax-Exempt Series.

                                  ARTICLE XIII

                                  MISCELLANEOUS

         SECTION 13.01. SUCCESSORS OF THE ISSUER. All the covenants,
stipulations, promises and agreements in this Indenture contained, by or on
behalf of the Issuer, shall bind and inure to the benefit of its successors and
assigns, whether so expressed or not. If any of the powers or duties of the
Issuer shall hereafter be transferred by any law of the State, and if such
transfer shall relate to any matter or thing permitted or required to be done
under this Indenture by the Issuer, then the body or official of the Issuer who
shall succeed to such powers or duties shall act and be obligated in the place
and stead of the Issuer as in this Indenture provided.

         SECTION 13.02. CONSENTS OF BONDHOLDERS. Any consent, approval,
direction or other instrument required by this Indenture to be signed and
executed by the Bondholders may be in any number of concurrent writings of
similar tenor and may be signed or executed by such Bondholders in person or by
agent appointed in writing. Proof of the execution of any such consent,
approval, direction or other instrument or of the writing appointing any such
agent, if made in the following manner, shall be sufficient for any of the
purposes of this Indenture, and shall be conclusive in favor of the Trustee with
regard to any action taken under such request or other instrument, namely:

         (a) The fact and date of the execution by any Person of any such
instrument or writing may be proved by the certificate of any officer in any
jurisdiction who by law has power to take acknowledgments within such
jurisdiction that the Person signing such instrument or writing acknowledged
before him the execution thereof, or by affidavit of any witness to such
execution or in any other manner satisfactory to the Trustee; or

         (b) The fact of ownership of Bonds and the amount or amounts, numbers
and other identification of such Bonds, and the date of acquiring the same shall
be proved by the registration books of the Issuer maintained by the Trustee
pursuant to Section 2.04 hereof.

         Any request, demand, authorization, direction, notice, consent, waiver
or other action by any Bondholder shall bind every future holder of the same
Bond in respect of anything done or suffered to be done by the Trustee or the
Issuer in reliance thereon, whether or not notation of such action is made upon
such Bond.

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<PAGE>   92

         SECTION 13.03. LIMITATION OF RIGHTS. With the exception of rights
herein expressly conferred, nothing expressed or mentioned in or to be implied
from this Indenture or the Bonds is intended or shall be construed to give to
any person other than the parties hereto, the Borrower, the Liquidity Provider,
if any, the Bond Insurer, the Remarketing Agent, if any, and the Owners of the
Bonds any legal or equitable right, remedy or claim under or in respect to this
Indenture. This Indenture and all of the covenants, conditions and provisions
hereof are intended to be and being for the sole and exclusive benefit of the
parties hereto, the Owners of the Bonds, the Remarketing Agent, if any, and the
Borrower as herein provided. To the extent that this Indenture confers upon or
gives or grants to the Bond Insurer any right, remedy or claim under or by
reason of this Indenture, the Bond Insurer is hereby explicitly recognized as
being a third-party beneficiary hereunder and may enforce any such right, remedy
or claim conferred, given or granted hereunder.

         SECTION 13.04. WAIVER OF NOTICE. Whenever in this Indenture the giving
of notice by mail or otherwise is required, the giving of such notice may be
waived in writing by the person entitled to receive such notice and in any such
case the giving or receipt of such notice shall not be a condition precedent to
the validity of any action taken in reliance upon such waiver.

         SECTION 13.05. SEVERABILITY. If any provision of this Indenture shall
be invalid, inoperative or unenforceable as applied in any particular case in
any jurisdiction or jurisdictions or in all jurisdictions, or in all cases
because it conflicts with any other provision or provisions hereof or any
constitution or statute or rule of public policy, or for any other reason, such
circumstances shall not have the effect of rendering the provision in question
inoperative or unenforceable in any other case or circumstance, or of rendering
any other provision or provisions herein contained invalid, inoperative, or
unenforceable to any extent whatever.

         The invalidity of any one or more phrases, sentences, clauses or
Sections in this Indenture contained, shall not affect the remaining portions of
this Indenture, or any part thereof.

         SECTION 13.06. NOTICES. Except as otherwise provided herein, all
notices, certificates or other communications hereunder shall be sufficiently
given if in writing and shall be deemed given when mailed by registered,
certified or first class mail, postage prepaid, or by qualified overnight
courier service mutually acceptable to the delivering and recipient parties,
courier charges prepaid, addressed as follows:

If to the Issuer:             Clark County Government Center
                              County Manager's Office
                              500 South Grand Central Parkway, 6th Floor
                              Las Vegas, NV 89155-1111
                              Phone:  (702) 455-3234
                              Telecopy:  (702) 455-6298

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<PAGE>   93

If to the Borrower:         Southwest Gas Corporation
                            5241 Spring Mountain Road
                            Las Vegas, NV 89192-8510
                            Phone: (702) 876-7252
                            Telecopy: (702) 876-7037

If to the Trustee,          Harris Trust and Savings Bank
the Paying Agent or the     Indenture Trust Administration
Tender Agent:               311 West Monroe, 12th Floor
                            Chicago, IL 60606
                            Phone: (312) 461-2908
                            Telecopy: (312) 461-3525

If to the Bond Insurer:     Ambac Assurance Corporation
                            Utilities Group
                            One State Street Plaza
                            New York, NY 10004
                            Telecopy: (212) 797-5725

If to the                   Lehman Brothers
Remarketing Agent:          3 World Financial Center, 9th Floor
                            New York, NY 10285
                            Attention:  Short-Term Municipal Desk
                            Telecopy: (212) 526-1386

If to any                   The address provided by such Liquidity Provider to
Liquidity Provider:         the Issuer, the Borrower, the Trustee, the Bond
                            Insurer and the Remarketing Agent

Unless specifically otherwise required by the terms of this Indenture, any
notice required to be given pursuant to any provision of this Indenture may be
given by any form of electronic transmission that is capable of producing a
written record, including, without limitation, telecopy transmissions, provided
that the deliverer of any such notice given by electronic transmission shall
verify receipt of such notice promptly upon the transmission thereof and such
notice shall not be deemed duly given unless full and legible receipt thereof
has been verified by the recipient of such notice. The Issuer, the Borrower, the
Trustee, the Bond Insurer, the Liquidity Provider, if any, and the Remarketing
Agent, if any, by notice pursuant to this Section 13.06, designate any different
addresses to which subsequent notices, certificates or other communications
shall be sent. A duplicate copy of each notice, approval, consent, request,
complaint, demand or other communication given hereunder by the Issuer, the
Borrower or the Trustee to any one of the others shall also be given to each one
of the others.

         SECTION 13.07. WAIVER OF PERSONAL LIABILITY OF ISSUER MEMBERS, ETC. No
member, officer, agent or employee of the Issuer, and no officer, official,
agent or employee of the State of Nevada or any department, board or agency of
the Issuer or the State of Nevada shall be individually or personally liable for
the payment of the principal of or

                                       87
<PAGE>   94

premium or interest on the Bonds or be subject to any personal liability or
accountability by reason of the issuance thereof; but nothing herein contained
shall relieve any such member, officer, agent or employee from the performance
of any official duty provided by law or by this Indenture.

         SECTION 13.08. HOLIDAYS. If the date for making any payment or the last
date for performance of any act or the exercising of any right, as provided in
this Indenture, is not a Business Day, such payment may be made or act performed
or right exercised on the next succeeding Business Day with the same force and
effect as if done on the nominal date provided in this Indenture and no interest
shall accrue on the payment so deferred during the intervening period.

         SECTION 13.09. OPINIONS OF BOND COUNSEL. For so long as Orrick,
Herrington & Sutcliffe LLP is nationally recognized bond counsel, whenever in
this Indenture it is required that prior to the taking of any action an opinion
of Bond Counsel is required to be delivered to the effect that such action will
not adversely affect the Tax-Exempt status of the Bonds of any Tax-Exempt
Series, and such opinion is not given by Orrick, Herrington & Sutcliffe LLP, the
opinion of Bond Counsel shall instead affirmatively state, in a manner
acceptable to the Issuer and the Trustee, that interest on the Bonds of any
Tax-Exempt Series is Tax-Exempt and will remain so after the action in question.
This Section shall apply in the same fashion with respect to the affirmative
opinion of any such successor Bond Counsel.

         SECTION 13.10. COUNTERPARTS. This Indenture may be simultaneously
executed in several counterparts, each of which shall be an original and all of
which shall constitute but one and the same instrument.

         SECTION 13.11. APPLICABLE LAW. This Indenture shall be governed
exclusively by and construed in accordance with the applicable laws of the State
of Nevada.

         SECTION 13.12. CAPTIONS. The captions or headings in this Indenture are
for convenience only and in no way define, limit, or describe the scope or
intent of any provisions or sections of this Indenture.

         SECTION 13.13. DEALING IN BONDS. The Trustee, or the Remarketing Agent,
in its individual capacity, may in good faith buy, sell, own, hold and deal in
any of the Bonds, and may join in any action which any Bondholder may be
entitled to take with like effect as if it did not act in any capacity
hereunder. The Trustee or the Remarketing Agent, in its individual capacity,
either as principal or agent, may also engage in or be interested in any
financial or other transaction with the Issuer or the Borrower, and may act as
depositary, trustee or agent for any committee or body of Bondholders secured
hereby or other obligations of the Issuer as freely as if it did not act in any
capacity hereunder.

         SECTION 13.14. IMMUNITY OF INCORPORATORS. No recourse under or upon any
obligations, covenants or agreements contained in the Agreement, this Indenture
or the Bonds, or for any claim based thereon or otherwise in respect thereof,
shall be had against any incorporator, stockholder, director, officer or
employee, as such, whether past, present, or future, of the Borrower or the
Issuer or of any successor Person, either directly or through the

                                       88
<PAGE>   95

Borrower or the Issuer, whether by virtue of any constitution, statute or rule
of law, or by the enforcement of any assessment or penalty or otherwise; it
being expressly understood that any such liability and any and all such claims
are hereby expressly waived and released as a condition of, and as a
consideration for, the execution of the Agreement.

         SECTION 13.15. BORROWER MAY ACT THROUGH AGENTS. In connection with any
and all actions permitted or required to be taken by the Borrower in connection
with the provisions hereof, including without limitation those set forth in
Section 2.03 hereof, the Borrower may by written instrument filed with the
Trustee appoint one or more agents (which may be the Remarketing Agent) to take
such actions on its behalf, which appointment may be revoked at any time by the
Borrower by written instrument filed with the Trustee.

         SECTION 13.16. RECORD DATE FOR DETERMINATION OF OWNERS ENTITLED TO
VOTE. The Borrower may set a record date for the purpose of determining the
Owners entitled to give or take any request, demand, authorization, direction,
notice, consent, waiver or other action, or to vote on any action, authorized or
permitted to be given or taken by Owners. If not set by the Borrower prior to
their first solicitation of an Owner made by any Person in respect of any such
action, or, in the case of any such vote, prior to such vote, the record date
for any such action or vote shall be the thirtieth (30th) day prior to such
first solicitation or vote, as the case may be. With regard to any record date,
only the Owners on such date (or their duly appointed proxies) shall be entitled
to give or take, or vote on the relevant action.

                     [REST OF PAGE INTENTIONALLY LEFT BLANK]

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<PAGE>   96

         IN WITNESS WHEREOF, the Issuer and the Trustee have caused this
Indenture of Trust to be signed by authorized officers, all as of the date first
above written.

                                        CLARK COUNTY, NEVADA

                                        By         /s/ Bruce L. Woodbury
                                            ------------------------------------
                                            Chair, Board of County Commissioners
(SEAL)

Attest:

        /s/ Shirley B Parraguirre
------------------------------------------
              County Clerk
                                        HARRIS TRUST AND SAVINGS BANK,
                                        as Trustee

                                        By            /s/ J. Bartolini
                                            ------------------------------------
                                                     Authorized Officer

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<PAGE>   97
                                   EXHIBIT A1

                           [FORM OF SERIES 1999A BOND]

No. A-1                                                          CUSIP: ________

AS PROVIDED IN THE INDENTURE REFERRED TO HEREIN, UNTIL THE TERMINATION OF THE
SYSTEM OF BOOK-ENTRY-ONLY TRANSFERS THROUGH THE DEPOSITORY TRUST COMPANY, A NEW
YORK CORPORATION (TOGETHER WITH ANY SUCCESSOR SECURITIES DEPOSITORY APPOINTED
PURSUANT TO THE INDENTURE, "DTC"), AND NOTWITHSTANDING ANY OTHER PROVISION OF
THE INDENTURE TO THE CONTRARY, (A) THIS BOND MAY BE TRANSFERRED, IN WHOLE BUT
NOT IN PART, ONLY TO A NOMINEE OF DTC, OR BY DTC OR A NOMINEE OF DTC TO ANY
SUCCESSOR SECURITIES DEPOSITORY OR ANY NOMINEE THEREOF AND (B) A PORTION OF THE
PRINCIPAL AMOUNT OF THIS BOND MAY BE PAID OR REDEEMED WITHOUT SURRENDER HEREOF
TO THE PAYING AGENT. DTC OR A NOMINEE, TRANSFEREE OR ASSIGNEE OF DTC AS OWNER OF
THIS BOND MAY NOT RELY UPON THE PRINCIPAL AMOUNT INDICATED HEREON AS THE
PRINCIPAL AMOUNT HEREOF OUTSTANDING AND UNPAID. THE PRINCIPAL AMOUNT HEREOF
OUTSTANDING AND UNPAID SHALL FOR ALL PURPOSES BE THE AMOUNT DETERMINED IN THE
MANNER PROVIDED IN THE INDENTURE.

UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF DTC, TO
ISSUER OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE, OR PAYMENT, AND ANY
CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME
AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE
TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE, OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL, INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

                            UNITED STATES OF AMERICA

                                 STATE OF NEVADA

                              CLARK COUNTY, NEVADA
                       INDUSTRIAL DEVELOPMENT REVENUE BOND
                (SOUTHWEST GAS CORPORATION PROJECT) SERIES 1999A

NEITHER THE FAITH AND CREDIT NOR THE TAXING POWER OF THE STATE OF NEVADA OR
CLARK COUNTY, NEVADA OR ANY POLITICAL SUBDIVISION THEREOF IS PLEDGED TO THE
PAYMENT OF THE PRINCIPAL OF, PREMIUM, IF ANY, OR ANY INTEREST ON THIS BOND.

Maturity Date:                      ____________,  ________

Registered Owner:                   Cede & Co.

Initial Principal Amount:           _____________  DOLLARS

Dated Date:                         ____________,  _______

Interest Rate:                      [TERM RATE OF _________ %]

         CLARK COUNTY, NEVADA (the "Issuer"), a public instrumentality and
political subdivision of the State of Nevada, for value received, hereby
promises to pay (but only out of the source hereinafter provided) to the
Registered Owner identified above, or registered assigns as hereinafter
provided, on the Maturity Date identified above, the Principal Amount identified
above, and to pay (but only out of the source hereinafter provided) interest on
the balance of said Principal Amount from time to time remaining unpaid until
payment of said

                                      A1-1
<PAGE>   98

Principal Amount has been made or duly provided for, at the rates and on the
dates determined as described herein and in the Indenture as hereinafter
defined, and to pay (but only out of the source hereinafter provided) interest
on overdue principal at the rate borne by this Bond on the date on which such
principal became due and payable, except as the provisions set forth in the
Indenture with respect to redemption or acceleration prior to maturity may
become applicable hereto, the principal of and premium, if any, and interest on
this Bond being payable in lawful money of the United States of America at the
Principal Office of Harris Trust and Savings Bank, an Illinois banking
corporation, as Paying Agent (the "Paying Agent"); provided, however, payment of
interest on any Interest Payment Date shall be made to the registered owner
hereof as of the close of business on the Record Date with respect to such
Interest Payment Date and shall be (i) paid by check or draft of the Paying
Agent mailed to such registered owner hereof at his address as it appears on the
registration books of the Issuer maintained by Harris Trust and Savings Bank, an
Illinois banking corporation, as Trustee (the "Trustee") or at such other
address as is furnished in writing by such registered owner to the Trustee not
later than the close of business on the Record Date or (ii) transmitted by wire
transfer to the account with a member of the Federal Reserve System located
within the continental United States of America of any owner which owns at least
$1,000,000 in aggregate principal amount of the Bonds and which shall have
provided wire transfer instructions to the Trustee prior to the close of
business on such Record Date. Notwithstanding the foregoing provisions, for so
long as this Bond is restricted to being registered on the registration books of
the Issuer kept by the Trustee in the name of a Securities Depository, the
provisions of the Indenture governing Book-Entry Bonds shall govern the manner
of the payment of the principal and purchase price (if applicable) of and
premium, if any, and interest on this Bond.

         This Bond is one of an authorized issue of bonds limited in the initial
aggregate principal amount of $35,000,000 (the "Bonds") issued pursuant to a
resolution duly adopted by the governing body of the Issuer on March 2, 1999,
and the applicable provisions of the County Economic Development Revenue Bond
Law, Sections 244A.669 to 244A.763, inclusive, of the Nevada Revised Statutes,
as amended and supplemented to the date hereof (the "Act"), and issued under an
Indenture of Trust, dated as of October 1, 1999 (the "Indenture"), between the
Issuer and the Trustee, for the purpose of financing or refinancing a portion of
the cost of the acquisition, construction and installation of a "project" within
the meaning of the Act consisting of the upgrade, improvement, addition and
replacement of facilities in Clark County, Nevada for the local furnishing of
natural gas (the "Project"). Proceeds from the sale of the Bonds are to be
loaned by the Issuer to Southwest Gas Corporation, a California corporation (the
"Borrower"), under the terms of a Financing Agreement, dated as of October 1,
1999 (the "Agreement"). The Bonds are all issued under and secured by and
entitled to the benefits of the Indenture, including the security of a pledge
and assignment of certain revenues and receipts derived by the Issuer pursuant
to the Agreement, and all receipts of the Trustee credited under the provisions
of the Indenture against such payments, including amounts realized by the
Trustee from repayments by the Borrower under the Agreement and from any other
moneys held by the Trustee under the Indenture for such purpose, and there shall
be no other recourse against the Issuer or any property now or hereafter owned
by it. Payments of principal of, and interest on the Bonds are guaranteed by
Ambac Assurance Corporation, a Wisconsin-domiciled stock insurance company.

         This Bond may be transferred or exchanged by the registered owner
hereof in person or by his attorney duly authorized in writing at the Principal
Office of the Trustee but

                                      A1-2
<PAGE>   99

only in the manner, subject to the limitations and upon payment of the charges
provided in the Indenture, and upon surrender and cancellation of this Bond.
Upon such transfer or exchange a new registered Bond or Bonds, of authorized
denomination or denominations, for the same aggregate principal amount will be
issued to the transferee in exchange herefor.

         IN NO EVENT SHALL THE INTEREST RATE ON ANY BOND EXCEED THE LESSER OF
15% PER ANNUM OR THE MAXIMUM RATE PER ANNUM THEN PERMITTED BY APPLICABLE LAW.

         Interest on this Bond shall be payable on each Interest Payment Date
for the period commencing on the next preceding Interest Payment Date (or if no
interest has been paid hereon, commencing on the Dated Date) and ending on the
day next preceding such Interest Payment Date; provided, however, that if, as
shown by the records of the Trustee, interest on the Bonds shall be in default,
Bonds shall bear interest from the last date to which interest has been paid in
full or duly provided for on the Bonds, or if no interest has been paid or duly
provided for on the Bonds, from the Dated Date. Interest shall be computed on
the basis of a 360-day year consisting of twelve 30-day months.

         The term "Interest Payment Date" means April 1 and October 1 of each
year, commencing April 1, 2000. The term "Record Date" means the fifteenth day
of the calendar month next preceding such Interest Payment Date.

         The Bonds shall be deliverable in the form of registered Bonds without
coupons in the denominations of $5,000 and any integral multiple thereof.

         THE BONDS ARE SUBJECT TO OPTIONAL AND MANDATORY REDEMPTION AS PROVIDED
IN THE INDENTURE.

         This Bond is issued pursuant to and in full compliance with the
Constitution and laws of the State of Nevada, particularly the Act, and pursuant
to further proceedings adopted by the governing authority of the Issuer, which
proceedings authorize the execution and delivery of the Indenture. This Bond and
the series of which it forms a part are limited obligations of the Issuer
payable solely from the amounts derived under the Agreement and pledged under
the Indenture, including all amounts payable from time to time by the Borrower
in respect of the indebtedness under the Agreement and all receipts of the
Trustee credited under the provisions of the Indenture against said amounts
payable. No owner of any Bond issued under the Act has the right to compel any
exercise of the taxing power of the Issuer to pay the Bonds, or the interest or
premium, if any, thereon. The Project is not security for the Bonds.

         No recourse shall be had for the payment of the principal of and
premium, if any, or interest on any of the Bonds or for any claim based thereon
or upon any obligation, covenant or agreement in the Indenture contained,
against any past, present or future member, director, officer, employee or agent
of the Issuer or the Borrower, or through the Issuer or the Borrower, or any
successor to the Issuer or the Borrower, under any rule of law or equity,
statute or constitution or by the enforcement of any assessment or penalty or
otherwise, and all such liability of any such member, director, officer,
employee or agent as such is hereby expressly

                                      A1-3
<PAGE>   100

waived and released as a condition of and in consideration for the execution of
the Indenture, the Agreement and the issuance of any of the Bonds.

         The owner of this Bond shall have no right to enforce the provisions of
the Indenture or to institute action to enforce the covenants therein, or to
take any action with respect to any event of default under the Indenture, or to
institute, appear in or defend any suit or other proceedings with respect
thereto, except as provided in the Indenture. If an Event of Default occurs and
is continuing, the principal of all Bonds then outstanding issued under the
Indenture and accrued interest thereon to the date of declaration may be
declared due and payable upon the conditions and in the manner and with the
effect provided in the Indenture.

         The Issuer, the Trustee, the Paying Agent, the Tender Agent, the
Registrar and any other agent of the Issuer or the Trustee may treat the person
in whose name this Bond is registered as the owner hereof for the purpose of
receiving payment as herein provided and for all other purposes, whether or not
this Bond be overdue, and neither the Issuer, the Trustee, the Paying Agent, the
Tender Agent, the Registrar, nor any such agent shall be affected by notice to
the contrary.

         The Indenture prescribes the manner in which it may be discharged and
after which the Bonds shall no longer be secured by or entitled to the benefits
of the Indenture, except for the purposes of payment, registration, transfer or
exchange of Bonds, including a provision that under certain circumstances the
Bonds shall be deemed to be paid if there shall have been deposited with the
Trustee certain government obligations, as provided in the Indenture, maturing
as to principal and interest in such amounts and at such times as to insure the
availability of sufficient moneys to pay the principal of and premium, if any,
and interest on the Bonds and all necessary and proper fees, compensation and
expenses of the Trustee.

         Modifications or alterations of the Indenture, or any supplements
thereto, and the Agreement may be made only to the extent and in the
circumstances permitted by the Indenture.

         Terms which are used herein as defined terms and which are not
otherwise defined shall have the meanings assigned to them in the Indenture.

         IT IS HEREBY CERTIFIED, RECITED AND DECLARED that all of the
conditions, things and acts required to exist, to have happened and to have been
performed precedent to and in the issuance of this Bond do exist, have happened
and have been performed in due time, form and manner as required by the Law (as
hereinafter defined) and by the Constitution and statutes of the State of Nevada
and that the amount of this Bond, together with all other indebtedness of the
Issuer, does not exceed any limit prescribed by the Constitution or statutes of
the State of Nevada.

         This Bond shall not be valid or become obligatory for any purpose or be
entitled to any security or benefit under the Indenture unless and until the
certificate of authentication hereon shall have been duly executed by the
Trustee.

                                      A1-4
<PAGE>   101

         IN WITNESS WHEREOF, CLARK COUNTY, NEVADA has caused this Bond to be
executed in its name by the manual or duly authorized facsimile signatures of
its Chair of the Board of County Commissioners and its Treasurer and attested by
the manual or duly authorized facsimile signature of its County Clerk.

                                      CLARK COUNTY, NEVADA

                                      By
                                            ------------------------------------
                                            Chair, Board of County Commissioners

                                      By
                                            ------------------------------------
                                                        Treasurer

Attest:

------------------------------------
            County Clerk

                                      A1-5
<PAGE>   102

                [FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION]

         This Bond is one of the Bonds referred to in the within-mentioned
Indenture of Trust.

                                    HARRIS TRUST AND SAVINGS BANK,
                                    not in its individual capacity,
                                    but solely as Trustee

                                    By:
                                        ----------------------------------------
                                                 Authorized Signatory

Date of Authentication:
                       ----------------------------

                       [BOND INSURANCE LEGEND TO BE ADDED]

                                      A1-6
<PAGE>   103

                              [FORM OF ASSIGNMENT]

         The following abbreviations, when used in the inscription on the face
of this Bond, shall be construed as though they were written out in full
according to applicable laws or regulations:

                               UNIF GIFT MIN ACT--

TEN COM -- as tenants in common __________________  Custodian __________________
TEN ENT -- as tenants by the entireties   (Cust)                   (Minor)
JT TEN  -- as joint tenants with right    under Uniform Gifts to Minors Act
           of survivorship and not as     ______________________________________
           tenants in common                             (State)

                Additional abbreviations may also be used though

                             not in the above list.

      FOR VALUE RECEIVED, the undersigned sells, assigns and transfers unto

================================================================================
                         (Name and Address of Assignee)

--------------------------------------------------------------------------------
       Social Security or Other Taxpayer Identification Number of Assignee

the within Bond of Clark County, Nevada and does hereby irrevocably constitute
and appoint ______________________________ to transfer the said Bond on the
books kept for registration thereof with full power of substitution in the
premises.

Dated:____________________

                                             ___________________________________
Signature Guaranteed:

_________________________________

NOTICE: The signature to this assignment must correspond with the name as it
appears upon the face of the within Bond in every particular, without alteration
or enlargement or any change whatever.

NOTICE: Signature(s) must be guaranteed by a member firm of the New York Stock
Exchange or a commercial bank or trust company.

                              [END OF FORM OF BOND]

                                      A1-7
<PAGE>   104

[At such time as the Bonds are no longer held in Book-Entry form, the references
herein to the book-entry system may be deleted.]

                                   EXHIBIT A2

                       [FORM OF TAXABLE SERIES 1999B BOND]

No. TB-______                                                 CUSIP: __________

         By its acceptance of this Bond, each purchaser of this Bond will be
deemed to have represented and agreed as follows: (1) the purchaser understands
that the Bonds are being issued only in transactions not involving any public
offering within the meaning of the Securities Act; (2) the purchaser is (A) a
sophisticated institutional investor who (i) is an "Accredited Investor" as that
term is defined in Rule 501(a) of Regulation D under the Securities Act (or is a
fiduciary or agent (other than a U.S. bank or savings and loan association) that
is purchasing the Bonds for the account of an Accredited Investor), (ii) has
such knowledge and experience (or is a fiduciary or agent with sole investment
discretion having such knowledge and experience) in financial and business
matters that it (or such fiduciary or agent) is capable of evaluating the merits
and risks of investing in such Bond, (iii) has had access to such information as
the purchaser deems necessary in order to make an informed investment decision,
and (iv) is purchasing the Bond for investment and not with a view to
distribution; or (B) in the case of sale of Bonds pursuant to Rule 144A under
the Securities Act, a "Qualified Institutional Buyer" as defined in Rule 144A
(or is a Qualified Institutional Buyer purchasing the Bonds on behalf of one or
more other Qualified Institutional Buyers); (3) if in the future the purchaser
(or any such other investor or any other fiduciary or agent representing such
investor) decides to sell such Bond prior to the maturity date, any redemption
date, or the date fixed for mandatory purchase, it will be sold only in a
transaction exempt from registration under, or not subject to, the Securities
Act and only to (i) the Remarketing Agent (as defined in the hereinafter
referred to Indenture) or through the Remarketing Agent to an institutional
investor approved by the Remarketing Agent as an institutional Accredited
Investor or a Qualified Institutional Buyer, or (ii) a Qualified Institutional
Buyer in a transaction made pursuant to Rule 144A under the Securities Act; (4)
the purchaser understands that, although the Remarketing Agent may repurchase
Bonds, the Remarketing Agent is not obligated to do so, and accordingly the
purchaser (or any such other investor) should be prepared to hold such Bond
until the maturity date or any earlier redemption date or date of mandatory
purchase thereof; (5) the purchaser acknowledges that the Bond sold to the
purchaser by the Remarketing Agent may be sold to it pursuant to Rule 144A; and
(6) the purchaser acknowledges that the Trustee or the Borrower may request
reasonable documentation from the purchaser (including opinions and
certificates) to ensure compliance with the terms of this legend in connection
with a transfer.

AS PROVIDED IN THE INDENTURE REFERRED TO HEREIN, UNTIL THE TERMINATION OF THE
SYSTEM OF BOOK-ENTRY-ONLY TRANSFERS THROUGH THE DEPOSITORY TRUST COMPANY, A NEW
YORK CORPORATION (TOGETHER WITH ANY SUCCESSOR SECURITIES DEPOSITORY APPOINTED
PURSUANT TO THE INDENTURE, "DTC"), AND NOTWITHSTANDING ANY OTHER PROVISION OF
THE INDENTURE TO THE CONTRARY, (A) THIS BOND MAY BE TRANSFERRED, IN WHOLE BUT
NOT IN PART, ONLY TO A NOMINEE OF DTC, OR BY DTC OR A NOMINEE OF DTC TO ANY
SUCCESSOR SECURITIES DEPOSITORY OR ANY NOMINEE THEREOF AND (B) A PORTION OF THE
PRINCIPAL AMOUNT OF THIS BOND MAY BE PAID OR REDEEMED WITHOUT SURRENDER HEREOF
TO THE PAYING AGENT. DTC OR A NOMINEE, TRANSFEREE OR ASSIGNEE OF DTC AS OWNER OF
THIS BOND MAY NOT RELY UPON THE PRINCIPAL AMOUNT INDICATED HEREON AS THE
PRINCIPAL AMOUNT HEREOF OUTSTANDING AND UNPAID. THE PRINCIPAL AMOUNT HEREOF
OUTSTANDING AND UNPAID SHALL FOR ALL PURPOSES BE THE AMOUNT DETERMINED IN THE
MANNER PROVIDED IN THE INDENTURE.

UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF DTC, TO
ISSUER OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE, OR PAYMENT, AND ANY
CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME
AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE
TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE, OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL, INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

                                      A2-1
<PAGE>   105

                            UNITED STATES OF AMERICA

                                 STATE OF NEVADA

                              CLARK COUNTY, NEVADA
                       INDUSTRIAL DEVELOPMENT REVENUE BOND
            (SOUTHWEST GAS CORPORATION PROJECT) TAXABLE SERIES 1999B

NEITHER THE FAITH AND CREDIT NOR THE TAXING POWER OF THE STATE OF NEVADA OR ANY
POLITICAL SUBDIVISION THEREOF IS PLEDGED TO THE PAYMENT OF THE PRINCIPAL OF,
PREMIUM, IF ANY, OR ANY INTEREST ON THIS BOND.

Maturity Date:             ____________, _______

Registered Owner:          Cede & Co.

Initial Principal Amount:  [____________] DOLLARS

Dated Date:                ____________, _______

Initial Interest Rate:     [TAXABLE WEEKLY RATE]

         CLARK COUNTY, NEVADA (the "Issuer"), a political subdivision of the
State of Nevada, for value received, hereby promises to pay (but only out of the
source hereinafter provided) to the Registered Owner identified above, or
registered assigns as hereinafter provided, on the Maturity Date identified
above, the Principal Amount identified above, and to pay (but only out of the
source hereinafter provided) interest on the balance of said Principal Amount
from time to time remaining unpaid until payment of said Principal Amount has
been made or duly provided for, at the rates and on the dates determined as
described herein and in the Indenture as hereinafter defined, and to pay (but
only out of the source hereinafter provided) interest on overdue principal at
the rate borne by this Bond on the date on which such principal became due and
payable, except as the provisions set forth in the Indenture with respect to
redemption or acceleration prior to maturity may become applicable hereto, the
principal of and premium, if any, and interest on this Bond being payable in
lawful money of the United States of America at the Principal Office of Harris
Trust and Savings Bank an Illinois banking corporation, as Paying Agent (the
"Paying Agent"); provided, however, payment of interest on any Interest Payment
Date shall be made to the registered owner hereof as of the close of business on
the Record Date with respect to such Interest Payment Date and shall be (i) paid
by check or draft of the Paying Agent mailed to such registered owner hereof at
his address as it appears on the registration books of the Issuer maintained by
Harris Trust and Savings Bank an Illinois banking corporation, as Trustee (the
"Trustee") or at such other address as is furnished in writing by such
registered owner to the Trustee not later than the close of business on the
Record Date or (ii) transmitted by wire transfer to the account with a member of
the Federal Reserve System located within the continental United States of
America of any owner which owns at least $1,000,000 in aggregate principal
amount of the Bonds and which shall have provided wire transfer instructions to
the Trustee prior to the close of business on such Record Date, but, in the case
of interest payable on the first Interest Payment Date with respect to a Taxable
Flexible Rate

                                      A2-2
<PAGE>   106

Segment, only upon presentation of such Bond (if such Bond is not a Book-Entry
Bond) at the Principal Office of the Trustee for exchange or transfer as
provided in the Indenture. Notwithstanding the foregoing provisions, for so long
as this Bond is restricted to being registered on the registration books of the
Issuer kept by the Trustee in the name of a Securities Depository, the
provisions of the Indenture governing Book-Entry Bonds shall govern the manner
of the payment of the principal and purchase price (if applicable) of and
premium, if any, and interest on this Bond.

         This Bond is one of an authorized issue and series of bonds limited in
aggregate principal amount to $35,000,000 (the "Bonds") issued pursuant to a
resolution duly adopted by the governing body of the Issuer on March 2, 1999,
and the applicable provisions of the County Economic Development Revenue Bond
Law, Sections 244A.669 to 244A.763, inclusive, of the Nevada Revised Statutes
(the "Act"), and issued under an Indenture of Trust, dated as of October 1, 1999
(the "Indenture"), between the Issuer and the Trustee, for the purpose of
financing or refinancing a portion of the cost of the acquisition, construction
and installation of a "project" within the meaning of the Act consisting of the
upgrade, improvement, addition and replacement of facilities in Clark County,
Nevada, for the purpose of local furnishing of natural gas (the "Project").
Proceeds from the sale of the Bonds are to be loaned by the Issuer to Southwest
Gas Corporation, a California corporation (the "Borrower"), under the terms of a
Financing Agreement, dated as of October 1, 1999 (the "Agreement"). The Bonds
are all issued under and secured by and entitled to the benefits of the
Indenture, including the security of a pledge and assignment of certain revenues
and receipts derived by the Issuer pursuant to the Agreement, and all receipts
of the Trustee credited under the provisions of the Indenture against such
payments, including amounts realized by the Trustee from repayments by the
Borrower under the Agreement and from any other moneys held by the Trustee under
the Indenture for such purpose, and there shall be no other recourse against the
Issuer or any property now or hereafter owned by it. Payments of principal of
and interest on the Bonds are guaranteed by Ambac Assurance Corporation. While
the Bonds are subject to tender for purchase, payments of the purchase price of
Bonds tendered for purchase and not remarketed are initially supported by a
Standby Bond Purchase Agreement (together with any replacement facility, the
"Liquidity Facility") between the Borrower and Bank One, NA (together with any
successor or the provider of any replacement Liquidity Facility, the "Liquidity
Provider"), pursuant to the terms of which the Trustee or Tender Agent, as
applicable, may draw for such payments hereunder. Notwithstanding any other
provision hereof or of the Indenture, if this Bond is acquired by the Liquidity
Provider pursuant to the provisions of the Liquidity Facility, such Bond will
bear interest and be subject to redemption as provided in the Liquidity
Facility.

         The Bonds initially have been issued as taxable weekly variable rate
bonds (defined in the Indenture as Taxable Weekly Bonds), the interest on which
is includable in gross income for federal income tax purposes, but all or any
portion of the Bonds may be converted to bonds, the interest on which is
excludable from gross income for federal income tax purposes as provided in the
Indenture.

         This Bond may be transferred or exchanged by the registered owner
hereof in person or by his attorney duly authorized in writing at the Principal
Office of the Trustee but only in the manner, subject to the limitations and
upon payment of the charges provided in the Indenture, and upon surrender and
cancellation of this Bond. Upon such transfer or exchange a

                                      A2-3
<PAGE>   107

new registered Bond or Bonds, of authorized denomination or denominations, for
the same aggregate principal amount and in the same series will be issued to the
transferee in exchange herefor.

         IN THE MANNER PROVIDED AND SUBJECT TO THE PROVISIONS OF THE INDENTURE,
THE TERM OF THE BONDS WILL BE DIVIDED INTO CONSECUTIVE RATE PERIODS DURING EACH
OF WHICH THE BONDS SHALL BEAR INTEREST AT TAXABLE WEEKLY RATES OR AT TAXABLE
FLEXIBLE RATES OR AT TAXABLE TERM RATES, EACH OF WHICH SHALL BE DETERMINED IN
THE MANNER PROVIDED IN THE INDENTURE, UNTIL CONVERSION OF ALL OR ANY PORTION OF
THE BONDS AS AFORESAID. IN NO EVENT SHALL THE INTEREST RATE ON ANY BOND EXCEED
THE LESSER OF 15% PER ANNUM OR THE MAXIMUM INTEREST RATE PERMITTED BY LAW. THE
DETERMINATION OF THE TAXABLE WEEKLY RATE, THE TAXABLE FLEXIBLE RATE FOR ANY
TAXABLE RATE SEGMENT AND THE TAXABLE TERM RATE SHALL BE BINDING ON THE
REMARKETING AGENT, THE TRUSTEE, THE ISSUER, THE BORROWER AND THE OWNER OF THIS
BOND, Absent manifest error.

         Interest on this Bond shall be payable on each Interest Payment Date
for the period commencing on the next preceding Interest Payment Date and ending
on the day next preceding such Interest Payment Date; provided, however, that
if, as shown by the records of the Trustee, interest on the Bonds shall be in
default, Bonds shall bear interest from the last date to which interest has been
paid in full or duly provided for on the Bonds, or if no interest has been paid
or duly provided for on the Bonds, from the Dated Date. Interest in the case of
any Taxable Weekly Rate Period or any Taxable Flexible Rate Period shall be
computed on the basis of a 365 or 366-day year, as applicable, for the number of
days actually elapsed, and in the case of a Taxable Term Rate shall be computed
on the basis of a year consisting of twelve 30-day months.

         The terms used herein but not otherwise defined, including "Interest
Payment Date," "Record Date," "Tax-Exempt Conversion Date," "Taxable Flexible
Rate Segment," and "Taxable Term Rate Period," have the meanings assigned to
such terms in the Indenture.

         The Bonds shall be deliverable in the form of registered Bonds without
coupons in the denominations authorized by the Indenture.

         THE BONDS ARE SUBJECT TO OPTIONAL AND MANDATORY TENDER AND PURCHASE AS
PROVIDED IN THE INDENTURE.

         THE BONDS ARE SUBJECT TO OPTIONAL AND MANDATORY REDEMPTION, IN SOME
INSTANCES AT PAR, AS PROVIDED IN THE INDENTURE.

         This Bond and all other Bonds of the series of which it forms a part
are issued pursuant to and in full compliance with the Constitution and laws of
the State of Nevada, particularly the Act, and pursuant to further proceedings
adopted by the governing authority of the Issuer, which proceedings authorize
the execution and delivery of the Indenture. This Bond and the series of which
it forms a part are limited obligations of the Issuer payable solely from the
amounts derived under the Agreement and pledged under the Indenture, including
all

                                      A2-4
<PAGE>   108

amounts payable from time to time by the Borrower in respect of the indebtedness
under the Agreement and all receipts of the Trustee credited under the
provisions of the Indenture against said amounts payable. No owner of any Bond
issued under the Act has the right to compel any exercise of the taxing power of
the Issuer to pay the Bonds, or the interest or premium, if any, thereon. The
Project is not security for the Bonds.

         No recourse shall be had for the payment of the principal of and
premium, if any, or interest on any of the Bonds or for any claim based thereon
or upon any obligation, covenant or agreement in the Indenture contained,
against any past, present or future member, director, officer, employee or agent
of the Issuer or the Borrower, or through the Issuer or the Borrower, or any
successor to the Issuer or the Borrower, under any rule of law or equity,
statute or constitution or by the enforcement of any assessment or penalty or
otherwise, and all such liability of any such member, director, officer,
employee or agent as such is hereby expressly waived and released as a condition
of and in consideration for the execution of the Indenture, the Agreement and
the issuance of any of the Bonds.

         The Owner of this Bond shall have no right to enforce the provisions of
the Indenture or to institute action to enforce the covenants therein, or to
take any action with respect to any event of default under the Indenture, or to
institute, appear in or defend any suit or other proceedings with respect
thereto, except as provided in the Indenture. If an Event of Default occurs and
is continuing, the principal of all Bonds then outstanding issued under the
Indenture and accrued interest thereon to the date of declaration may be
declared due and payable upon the conditions and in the manner and with the
effect provided in the Indenture.

         The Issuer, the Trustee, the Remarketing Agent, the Tender Agent, the
Paying Agent and any other agent of the Issuer or the Trustee may treat the
person in whose name this Bond is registered as the Owner hereof for the purpose
of receiving payment as herein provided and for all other purposes, whether or
not this Bond be overdue, and neither the Issuer, the Trustee, the Remarketing
Agent, the Paying Agent nor any such agent shall be affected by notice to the
contrary.

         This Bond is subject to, and is executed in accordance with, all of the
terms, conditions and provisions of the Indenture.

         The Indenture prescribes the manner in which it may be discharged and
after which the Bonds shall no longer be secured by or entitled to the benefits
of the Indenture, except for the purposes of payment, registration, transfer or
exchange of Bonds, including a provision that under certain circumstances the
Bonds shall be deemed to be paid if there shall have been deposited with the
Trustee certain government obligations, as provided in the Indenture, maturing
as to principal and interest in such amounts and at such times as to insure the
availability of sufficient moneys to pay the principal of and premium, if any,
and interest on the Bonds and all necessary and proper fees, compensation and
expenses of the Trustee.

         Modifications or alterations of the Indenture, or of any supplements
thereto, may be made only to the extent and in the circumstances permitted by
the Indenture.

                                      A2-5
<PAGE>   109

         Terms which are used herein as defined terms and which are not
otherwise defined shall have the meanings assigned to them in the Indenture.

         IT IS HEREBY CERTIFIED, RECITED AND DECLARED that all of the
conditions, things and acts required to exist, to have happened and to have been
performed precedent to and in the issuance of this Bond do exist, have happened
and have been performed in due time, form and manner as required by the Law (as
hereinafter defined) and by the Constitution and statutes of the State of Nevada
and that the amount of this Bond, together with all other indebtedness of the
Issuer, does not exceed any limit prescribed by the Constitution or statutes of
the State of Nevada.

         This Bond shall not be valid or become obligatory for any purpose or be
entitled to any security or benefit under the Indenture unless and until the
certificate of authentication hereon shall have been duly executed by the
Trustee.

                    [Remainder of page intentionally blank.]

                                      A2-6
<PAGE>   110

         IN WITNESS WHEREOF, CLARK COUNTY, NEVADA has caused this Bond to be
executed in its name by the manual or duly authorized facsimile signatures of
its Chair of the Board of County Commissioners and its Treasurer and attested by
the manual or duly authorized facsimile signature of its County Clerk.

                                       CLARK COUNTY, NEVADA

                                       By
                                            ------------------------------------
                                            Chair, Board of County Commissioners

                                       By
                                            ------------------------------------
                                                        Treasurer

Attest:

-------------------------------
         County Clerk

                                      A2-7
<PAGE>   111

                              FOR CONVERSIONS ONLY

<TABLE>
<CAPTION>
                                          Principal Amount of      Authorized
    Conversion        Principal Amount     Tax-Exempt Bonds       Signatory of
       Date          of Converted Bonds    After Conversion         Trustee
    ----------       ------------------    ----------------       ------------
<S>                  <C>                  <C>                  <C>

__________________   __________________   __________________   _________________

__________________   __________________   __________________   _________________

__________________   __________________   __________________   _________________

__________________   __________________   __________________   _________________
</TABLE>

                [FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION]

         This Bond is one of the Bonds referred to in the within-mentioned
Indenture of Trust.

                                                 HARRIS TRUST AND SAVINGS BANK,
                                                 not in its individual capacity,
                                                 but solely as Trustee

                                                 By:____________________________
                                                        Authorized Signatory

Date of Authentication:______________________________

                       [BOND INSURANCE LEGEND TO BE ADDED]

                                      A2-8
<PAGE>   112

                              [FORM OF ASSIGNMENT]

         The following abbreviations, when used in the inscription on the face
of this Bond, shall be construed as though they were written out in full
according to applicable laws or regulations:

                               UNIF GIFT MIN ACT--

TEN COM -- as tenants in common ___________________  Custodian _________________
TEN ENT -- as tenants by the entireties  (Cust)                    (Minor)
JT TEN  -- as joint tenants with right   under Uniform Gifts to Minors Act
           of survivorship and not as    _______________________________________
           tenants in common                              (State)

                Additional abbreviations may also be used though

                             not in the above list.

      FOR VALUE RECEIVED, the undersigned sells, assigns and transfers unto

================================================================================
                         (Name and Address of Assignee)

--------------------------------------------------------------------------------
       Social Security or Other Taxpayer Identification Number of Assignee

the within Bond of Clark County, Nevada and does hereby irrevocably constitute
and appoint ___________________________ to transfer the said Bond on the books
kept for registration thereof with full power of substitution in the premises.

Dated:__________________________

                                             ___________________________________
Signature Guaranteed:

___________________________________

NOTICE: The signature to this assignment must correspond with the name as it
appears upon the face of the within Bond in every particular, without alteration
or enlargement or any change whatever.

NOTICE: Signature(s) must be guaranteed by a member firm of the New York Stock
Exchange or a commercial bank or trust company.

                              [END OF FORM OF BOND]

                                      A2-9
<PAGE>   113

                                    EXHIBIT B

                        [FORM OF TAX-EXEMPT SERIES BOND]

AS PROVIDED IN THE INDENTURE REFERRED TO HEREIN, UNTIL THE TERMINATION OF THE
SYSTEM OF BOOK-ENTRY-ONLY TRANSFERS THROUGH THE DEPOSITORY TRUST COMPANY, A NEW
YORK CORPORATION (TOGETHER WITH ANY SUCCESSOR SECURITIES DEPOSITORY APPOINTED
PURSUANT TO THE INDENTURE, "DTC"), AND NOTWITHSTANDING ANY OTHER PROVISION OF
THE INDENTURE TO THE CONTRARY, (A) THIS BOND MAY BE TRANSFERRED, IN WHOLE BUT
NOT IN PART, ONLY TO A NOMINEE OF DTC, OR BY DTC OR A NOMINEE OF DTC TO ANY
SUCCESSOR SECURITIES DEPOSITORY OR ANY NOMINEE THEREOF AND (B) A PORTION OF THE
PRINCIPAL AMOUNT OF THIS BOND MAY BE PAID OR REDEEMED WITHOUT SURRENDER HEREOF
TO THE PAYING AGENT. DTC OR A NOMINEE, TRANSFEREE OR ASSIGNEE OF DTC AS OWNER OF
THIS BOND MAY NOT RELY UPON THE PRINCIPAL AMOUNT INDICATED HEREON AS THE
PRINCIPAL AMOUNT HEREOF OUTSTANDING AND UNPAID. THE PRINCIPAL AMOUNT HEREOF
OUTSTANDING AND UNPAID SHALL FOR ALL PURPOSES BE THE AMOUNT DETERMINED IN THE
MANNER PROVIDED IN THE INDENTURE.

UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF DTC, TO
ISSUER OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE, OR PAYMENT, AND ANY
CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME
AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE
TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC), ANY TRANSFER, PLEDGE, OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY OR TO ANY PERSON IS WRONGFUL, INASMUCH AS THE REGISTERED OWNER
HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

                            UNITED STATES OF AMERICA

                                 STATE OF NEVADA

                              CLARK COUNTY, NEVADA
                       INDUSTRIAL DEVELOPMENT REVENUE BOND
           (SOUTHWEST GAS CORPORATION PROJECT) TAX-EXEMPT SERIES 1999 ___

NEITHER THE FAITH AND CREDIT NOR THE TAXING POWER OF THE STATE OF NEVADA OR ANY
POLITICAL SUBDIVISION THEREOF IS PLEDGED TO THE PAYMENT OF THE PRINCIPAL OF,
PREMIUM, IF ANY, OR ANY INTEREST ON THIS BOND.

Registered                                                            Registered
No. __-__                                                               $

                         For Flexible Rate Periods Only

<TABLE>
<CAPTION>
                                              Mandatory
                                             Purchase and          Amount of
                          Number of            Interest        Interest Due for
     Interest         Days in Flexible         Payment             Flexible
       Rate                Segment              Date                Segment
     --------         ----------------       -------------     ----------------
<S>                  <C>                  <C>                  <C>

__________________   __________________   __________________   _________________

_________________%   __________________   __________________   ________________%
</TABLE>

                                       B-1
<PAGE>   114

Maturity Date:               _______________                    CUSIP: _________

Dated Date:                  _______________, ___________

Registered Owner:            Cede & Co.

Initial Principal Amount:    _______________ DOLLARS

[Initial Interest Rate:      ________________________]

         CLARK COUNTY, NEVADA (the "Issuer"), a public instrumentality and
political subdivision of the State of Nevada, for value received, hereby
promises to pay (but only out of the source hereinafter provided) to the
Registered Owner identified above, or registered assigns as hereinafter
provided, on the Maturity Date identified above, the Principal Amount identified
above, and to pay (but only out of the source hereinafter provided) interest on
the balance of said Principal Amount from time to time remaining unpaid until
payment of said Principal Amount has been made or duly provided for, at the
rates and on the dates determined as described herein and in the Indenture as
hereinafter defined, and to pay (but only out of the source hereinafter
provided) interest on overdue principal at the rate borne by this Bond on the
date on which such principal became due and payable, except as the provisions
set forth in the Indenture with respect to redemption or acceleration prior to
maturity may become applicable hereto, the principal of and premium, if any, and
interest on this Bond being payable in lawful money of the United States of
America at the Principal Office of Harris Trust and Savings Bank, an Illinois
banking corporation, as Paying Agent (the "Paying Agent"); provided, however,
payment of interest on any Interest Payment Date shall be made to the registered
owner hereof as of the close of business on the Record Date with respect to such
Interest Payment Date and shall be (i) paid by check or draft of the Paying
Agent mailed to such registered owner hereof at his address as it appears on the
registration books of the Issuer maintained by Harris Trust and Savings Bank, an
Illinois banking corporation, as Trustee (the "Trustee") or at such other
address as is furnished in writing by such registered owner to the Trustee not
later than the close of business on the Record Date or (ii) transmitted by wire
transfer to the account with a member of the Federal Reserve System located
within the continental United States of America of any owner which owns at least
$1,000,000 in aggregate principal amount of the Bonds and which shall have
provided wire transfer instructions to the Trustee prior to the close of
business on such Record Date, but, in the case of interest payable in respect of
a Flexible Segment, only upon presentation of such Bond (if such Bond is not a
Book-Entry Bond) at the Principal Office of the Trustee for exchange or transfer
as provided in the Indenture. Notwithstanding the foregoing provisions, for so
long as this Bond is restricted to being registered on the registration books of
the Issuer kept by the Trustee in the name of a Securities Depository, the
provisions of the Indenture governing Book-Entry Bonds shall govern the manner
of the payment of the principal and purchase price (if applicable) of and
premium, if any, and interest on this Bond.

         This Bond is one of an authorized issue of bonds limited in the initial
aggregate principal amount of $35,000,000 (the "Bonds") issued pursuant to a
resolution duly adopted by the governing body of the Issuer on March 2, 1999,
and the applicable provisions of the County

                                      B-2
<PAGE>   115

Economic Development Revenue Bond Law, Sections 244A.669 to 244A.763, inclusive,
of the Nevada Revised Statutes, as amended and supplemented to the date hereof
(the "Act"), and issued under an Indenture of Trust, dated as of October 1, 1999
(the "Indenture"), between the Issuer and the Trustee, for the purpose of
financing or refinancing a portion of the cost of the acquisition, construction
and installation of a "project" within the meaning of the Act consisting of the
upgrade, improvement, addition and replacement of facilities in Clark County,
Nevada for the local furnishing of natural gas (the "Project"). Proceeds from
the sale of the Bonds are to be loaned by the Issuer to Southwest Gas
Corporation, a California corporation (the "Borrower"), under the terms of a
Financing Agreement, dated as of October 1, 1999 (the "Agreement"). The Bonds
are all issued under and secured by and entitled to the benefits of the
Indenture, including the security of a pledge and assignment of certain revenues
and receipts derived by the Issuer pursuant to the Agreement, and all receipts
of the Trustee credited under the provisions of the Indenture against such
payments, including amounts realized by the Trustee from repayments by the
Borrower under the Agreement and from any other moneys held by the Trustee under
the Indenture for such purpose, and there shall be no other recourse against the
Issuer or any property now or hereafter owned by it. Payments of principal of
and interest on the Bonds are guaranteed by Ambac Assurance Corporation. [While
the Bonds are subject to tender for purchase, payments of the purchase price of
Bonds tendered for purchase and not remarketed are initially supported by a
Standby Bond Purchaser Agreement (together with any replacement facility, the
"Liquidity Facility") between the Borrower and ___________________ (together
with any successor or the provider of any replacement Liquidity Facility, the
"Liquidity Provider"), pursuant to the terms of which the Trustee or Tender
Agent, as applicable, may draw for such payments hereunder. Notwithstanding any
other provision hereof or of the Indenture, if this Bond is acquired by the
Liquidity Provider pursuant to the provisions of the Liquidity Facility, such
Bond will bear interest and be subject to redemption as provided in the
Liquidity Facility.]

         This Bond may be transferred or exchanged by the registered owner
hereof in person or by his attorney duly authorized in writing at the Principal
Office of the Trustee but only in the manner, subject to the limitations and
upon payment of the charges provided in the Indenture, and upon surrender and
cancellation of this Bond. Upon such transfer or exchange a new registered Bond
or Bonds, of authorized denomination or denominations, for the same aggregate
principal amount will be issued to the transferee in exchange herefor.

         IN THE MANNER PROVIDED AND SUBJECT TO THE PROVISIONS OF THE INDENTURE,
THE TERM OF THE BONDS WILL BE DIVIDED INTO CONSECUTIVE RATE PERIODS DURING EACH
OF WHICH THE BONDS SHALL BEAR INTEREST AT EITHER A DAILY RATE, A WEEKLY RATE, A
TERM RATE OR FLEXIBLE RATES FOR ANY FLEXIBLE RATE SEGMENT, EACH OF WHICH SHALL
BE DETERMINED IN THE MANNER PROVIDED IN THE INDENTURE. IN NO EVENT SHALL THE
INTEREST RATE ON ANY BOND EXCEED THE LESSER OF 15% PER ANNUM OR THE MAXIMUM RATE
PER ANNUM THEN PERMITTED BY APPLICABLE LAW. THE FIRST RATE PERIOD SHALL BE
SELECTED BY THE BORROWER IN ACCORDANCE WITH THE INDENTURE AND THE AGREEMENT ON
OR BEFORE THE DATE OF ISSUANCE OF THE BONDS.

                                      B-3
<PAGE>   116

         Interest on this Bond shall be payable on each Interest Payment Date
for the period commencing on the next preceding Interest Payment Date (or if no
interest has been paid hereon, commencing on the Dated Date) and ending on the
day next preceding such Interest Payment Date; provided, however, that if, as
shown by the records of the Trustee, interest on the Bonds shall be in default,
Bonds shall bear interest from the last date to which interest has been paid in
full or duly provided for on the Bonds, or if no interest has been paid or duly
provided for on the Bonds, from the Dated Date. Interest shall be computed (1)
in the case of a Term Rate Period, on the basis of a 360-day year consisting of
twelve 30-day months, and (2) in the case of any other Rate Period, on the basis
of a 365 or 366-day year, as appropriate, and the actual number of days elapsed.

         The terms used herein and not otherwise defined, including "Interest
Payment Date" and "Record Date," have the meanings assigned to such terms in the
Indenture.

         The Bonds shall be deliverable in the form of registered Bonds without
coupons in the denominations authorized by the Indenture.

         THE BONDS ARE SUBJECT TO OPTIONAL AND MANDATORY TENDER AND PURCHASE AS
PROVIDED IN THE INDENTURE.

         THE BONDS ARE SUBJECT TO OPTIONAL AND MANDATORY REDEMPTION AS PROVIDED
IN THE INDENTURE.

         This Bond and all other Bonds of the series of which it forms a part
are issued pursuant to and in full compliance with the Constitution and laws of
the State of Nevada, particularly the Act, and pursuant to further proceedings
adopted by the governing authority of the Issuer, which proceedings authorize
the execution and delivery of the Indenture. This Bond and the series of which
it forms a part are limited obligations of the Issuer payable solely from the
amounts derived under the Agreement and pledged under the Indenture, including
all amounts payable from time to time by the Borrower in respect of the
indebtedness under the Agreement and all receipts of the Trustee credited under
the provisions of the Indenture against said amounts payable. No owner of any
Bond issued under the Act has the right to compel any exercise of the taxing
power of the Issuer to pay the Bonds, or the interest or premium, if any,
thereon. The Project is not security for the Bonds.

         No recourse shall be had for the payment of the principal of and
premium, if any, or interest on any of the Bonds or for any claim based thereon
or upon any obligation, covenant or agreement in the Indenture contained,
against any past, present or future member, director, officer, employee or agent
of the Issuer or the Borrower, or through the Issuer or the Borrower, or any
successor to the Issuer or the Borrower, under any rule of law or equity,
statute or constitution or by the enforcement of any assessment or penalty or
otherwise, and all such liability of any such member, director, officer,
employee or agent as such is hereby expressly waived and released as a condition
of and in consideration for the execution of the Indenture, the Agreement and
the issuance of any of the Bonds.

         The owner of this Bond shall have no right to enforce the provisions of
the Indenture or to institute action to enforce the covenants therein, or to
take any action with respect

                                      B-4
<PAGE>   117

to any event of default under the Indenture, or to institute, appear in or
defend any suit or other proceedings with respect thereto, except as provided in
the Indenture. If an Event of Default occurs and is continuing, the principal of
all Bonds then outstanding issued under the Indenture and accrued interest
thereon to the date of declaration may be declared due and payable upon the
conditions and in the manner and with the effect provided in the Indenture.

         The Issuer, the Trustee, the Paying Agent, the Tender Agent, the
Registrar, the Remarketing Agent and any other agent of the Issuer or the
Trustee may treat the person in whose name this Bond is registered as the owner
hereof for the purpose of receiving payment as herein provided and for all other
purposes, whether or not this Bond be overdue, and neither the Issuer, the
Trustee, the Paying Agent, the Tender Agent, the Registrar, the Remarketing
Agent nor any such agent shall be affected by notice to the contrary.

         The Indenture prescribes the manner in which it may be discharged and
after which the Bonds shall no longer be secured by or entitled to the benefits
of the Indenture, except for the purposes of payment, registration, transfer or
exchange of Bonds, including a provision that under certain circumstances the
Bonds shall be deemed to be paid if there shall have been deposited with the
Trustee certain government obligations, as provided in the Indenture, maturing
as to principal and interest in such amounts and at such times as to insure the
availability of sufficient moneys to pay the principal of and premium, if any,
and interest on the Bonds and all necessary and proper fees, compensation and
expenses of the Trustee.

         Modifications or alterations of the Indenture, or any supplements
thereto, and the Agreement may be made only to the extent and in the
circumstances permitted by the Indenture.

         Terms which are used herein as defined terms and which are not
otherwise defined shall have the meanings assigned to them in the Indenture.

         IT IS HEREBY CERTIFIED, RECITED AND DECLARED that all of the
conditions, things and acts required to exist, to have happened and to have been
performed precedent to and in the issuance of this Bond do exist, have happened
and have been performed in due time, form and manner as required by the Law (as
hereinafter defined) and by the Constitution and statutes of the State of Nevada
and that the amount of this Bond, together with all other indebtedness of the
Issuer, does not exceed any limit prescribed by the Constitution or statutes of
the State of Nevada.

         This Bond shall not be valid or become obligatory for any purpose or be
entitled to any security or benefit under the Indenture unless and until the
certificate of authentication hereon shall have been duly executed by the
Trustee.

                                      B-5
<PAGE>   118

         IN WITNESS WHEREOF, CLARK COUNTY, NEVADA has caused this Bond to be
executed in its name by the manual or duly authorized facsimile signatures of
its Chair of the Board of County Commissioners and its Treasurer and attested by
the manual or duly authorized facsimile signature of its County Clerk.

                                       CLARK COUNTY, NEVADA

                                       By_______________________________________
                                            Chair, Board of County Commissioners

                                       By_______________________________________
                                                        Treasurer

Attest:

___________________________________
           County Clerk

                                      B-6
<PAGE>   119

                [FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION]

         This Bond is one of the Bonds referred to in the within-mentioned
Indenture of Trust.

                                     HARRIS TRUST AND SAVINGS BANK,
                                     not in its individual capacity,
                                     but solely, as Trustee

                                     By_________________________________________
                                                  Authorized Officer

Date of Authentication: ____________________

                       [BOND INSURANCE LEGEND TO BE ADDED]

                                      B-7
<PAGE>   120

                              [FORM OF ASSIGNMENT]

                  The following abbreviations, when used in the inscription on
the face of this Bond, shall be construed as though they were written out in
full according to applicable laws or regulations:

                             UNIF GIFT MIN ACT--

TEN COM -- as tenants in common ___________________ Custodian __________________
TEN ENT -- as tenants by the entireties   (Cust)                    (Minor)
JT TEN  -- as joint tenants with right    under Uniform Gifts to Minors Act
           of survivorship and not as     ______________________________________
           tenants in common                               (State)

                Additional abbreviations may also be used though

                             not in the above list.

      FOR VALUE RECEIVED, the undersigned sells, assigns and transfers unto

================================================================================
                         (Name and Address of Assignee)

--------------------------------------------------------------------------------
       Social Security or Other Taxpayer Identification Number of Assignee

the within Bond of Clark County, Nevada and does hereby irrevocably constitute
and appoint ________________________ to transfer the said Bond on the books kept
for registration thereof with full power of substitution in the premises.

Dated: ________________________

                                           _____________________________________
Signature Guaranteed:

________________________

NOTICE: The signature to this assignment must correspond with the name as it
appears upon the face of the within Bond in every particular, without alteration
or enlargement or any change whatever.

NOTICE: Signature(s) must be guaranteed by a member firm of the New York Stock
Exchange or a commercial bank or trust company.

                              [END OF FORM OF BOND]

                                      B-8
<PAGE>   121

                                    EXHIBIT C

                               TRUSTEE CERTIFICATE

Harris Trust and Savings Bank
311 West Monroe Street
Chicago, Illinois  60606

                                        Principal       Bonds outstanding
1.       Name of Bond Issue           Amount Issued    6/30/__ or 12/31/__
         ------------------           -------------    --------------------

         Clark County, Nevada
         Industrial Development
         Revenue Bonds (Southwest
         Gas Corporation Project)
         Series 1999A and Taxable
         Series 1999B

2. During the past six months, did the Borrower make all required payments to
   the Trustee at the proper time and in the manner required by the Indenture?

   Yes ____ No ____

   If no, please explain on a separate page.

3. If the Borrower failed to make required payments, please attach copies of any
   correspondence between the Trustee and the Borrower discussing the failure
   and any steps to correct such failure.

4. Has the Trustee received a copy of the latest annual financial statements of
   the Borrower as required by the Financing Agreement?

   Yes ____ No ____

If no, please explain what steps have been taken to secure them.

5. Has the Trustee received a certificate of an officer of the Borrower, signed
   within 120 days of the close of the fiscal year, stating whether there exists
   any default under the terms of the Indenture, and if a default exists, what
   steps will be taken to correct the default?

   Yes ____ No ____

   If no, please explain what steps have been taken to secure it.

6. If a credit enhancement supports this bond issue, will such credit
   enhancement continue in full force during the next 12 months?
   Yes ____ No ____ Not Applicable

   If no, please explain on a separate page.

7. Has the Trustee received a copy of any rebate calculations prepared by or on
   behalf of the borrower company? Yes ____ No ____

                                      C-1
<PAGE>   122

    If no, and the Trustee has any actual knowledge of why it did not receive
    such calculations, please explain on a separate page.

By:  _____________________________  Date __________________________
         Authorized Signature

Title:  __________________________  Phone No._____________________

                                      C-2
<PAGE>   123

                                    EXHIBIT D

                   [FORM OF COST OF ISSUANCE FUND REQUISITION]

              REQUISITION FOR MONEY FROM THE COSTS OF ISSUANCE FUND

                      [Series A Account][Series B Account]

To:      Harris Trust and Savings Bank

Re:      Clark County, Nevada
         Industrial Development Revenue Bonds
         Southwest Gas Corporation Project
         [Series 1999A][Taxable Series 1999B]
         (the "1999 Bonds")

         Requisition No. ________

         The undersigned, on behalf of Southwest Gas Corporation (the
"Borrower"), hereby requests payment, from the Account of the Costs of Issuance
Fund identified above (the "Account"), the total amount shown below to the order
of the payee or payees named below, as payment or reimbursement for costs
incurred or expenditures made in connection with the issuance of the 1999 Bonds.
The payee(s), the purpose and the amount of the disbursement requested are as
follows:

      Payee                      Purpose                            Amount
      -----                      -------                            ------
[name and address]
                                                          Total $

                                      D-1
<PAGE>   124

         The undersigned hereby certifies as follows:

         Each obligation mentioned herein is described in Section 3.3 of the
Financing Agreement relating to the Project, has been properly incurred and is a
proper charge against the Account, and each item for which payment is requested
is or was necessary in connection with the issuance of the 1999 Bonds. None of
the items for which payment is requested has been reimbursed previously from the
Account, and none of the payments herein requested will result in a breach of
the representations and agreements in Section 2.2 of the Financing Agreement
relating to the Project.

         Dated: _________________.

                                          SOUTHWEST GAS CORPORATION

                                          By:___________________________________
                                              Authorized Borrower Representative

                                      D-2<PAGE>   1

                                                                   EXHIBIT 10.12

                            SOUTHWEST GAS CORPORATION

                          SUPPLEMENTAL RETIREMENT PLAN

                            Effective October 7, 1980

                              Amended March 1, 1986

                            Amended December 7, 1987

                 Amended and Restated Effective January 1, 1989

                             Amended January 1, 1990

                  Amended and Restated Effective March 5, 1991

                  Amended and Restated Effective March 2, 1993

                   Amended and Restated Effective May 10, 1994

                  Amended and Restated Effective March 1, 1999

<PAGE>   2

                            SOUTHWEST GAS CORPORATION
                          SUPPLEMENTAL RETIREMENT PLAN

                                     PURPOSE

The principal objective of this Supplemental Retirement Plan (Plan) is to ensure
that a competitive level of retirement income is paid in order to attract,
retain, and motivate officers of the Company. The Plan is designed to provide a
benefit which, when added to an officer's other retirement income, will meet
that objective. All elected officers of the Company are Participants in the
Plan, subject to meeting the eligibility requirements for retirement under the
Plan.

The Plan is also designed to eliminate reductions in benefits under the Basic
Plan for those employees who have participated in the Company's Executive
Deferral Plan and do not qualify for the full scope of benefits under the Plan.

The original Plan was effective on October 7, 1980, and as restated or amended,
is effective with respect to each Participant starting on the effective date of
election to officer status or selection for Executive Deferral Plan
participation by the Board of Directors of Southwest Gas Corporation.

                    I. DEFINITIONS AND CONSTRUCTION OF TERMS

1.1      For purposes of the Plan, the following words and phrases will have the
         meanings stated below unless a different meaning is clearly required by
         the context. In the event there is a conflict in the meaning of any
         defined terms used in this Plan because of the reference to the Basic
         Plan, the definition contained in the Basic Plan shall prevail.

         "AFFILIATE" means any corporation, partnership, or other organization
         which, during any period of a Participant's employment, was at least 50
         percent controlled by the Company or an affiliate of the Company.

         "AVERAGE EARNINGS" means the 12-month average of the highest
         consecutive 36-months of Earnings with the Company and its successors
         and assigns.

         "BASIC PLAN" means the defined benefit plans of the Company and/or
         PriMerit Bank, its former Affiliate, in effect prior to a Change in
         Control, whether maintained by the Company, PriMerit Bank or their
         successor or assigns.

         "BASIC PLAN BENEFITS" means the amount of benefit payable from the
         Basic Plan to a Participant, including benefits payable from any
         employer funded defined benefit plans of any of the Company's
         successors or assigns, as if the form of a straight life annuity was
         selected by the Participant.

                                       1
<PAGE>   3

         "BOARD OF DIRECTORS" means the Board of Directors of the Company and
         its successors and assigns.

         "CHANGE IN CONTROL" means the first to occur of any of the following
         events:

         (a)      Any "person" (as the term is used in Sections 13 and 14(d)(2)
                  of the Securities Exchange Act of 1934 ("Exchange Act")) who
                  becomes a beneficial owner (as that term is used in Section
                  13(d) of the Exchange Act), directly or indirectly, of 50% or
                  more of the Company's capital stock entitled to vote in the
                  election of directors; or

         (b)      During any period of not more than two consecutive years, not
                  including any period prior to the adoption of this Plan,
                  individuals who, at the beginning of such period constitute
                  the board of directors of the Company, and any new director
                  (other than a director designated by a person who has entered
                  into an agreement with the Company to effect a transaction
                  described in clause (a) of this definition) whose election by
                  the board of directors or nomination for election by the
                  Company's shareholders was approved by a vote of at least
                  three-fourths (3/4ths) of the directors then still in office,
                  who either were directors at the beginning of the period or
                  whose election or nomination for election was previously
                  approved, cease for any reason to constitute at least a
                  majority thereof.

         "COMMITTEE" means the Compensation Committee of the Board of Directors,
         to which the Board of Directors has given the authority to administer
         this Plan. After a Change in Control, the Committee shall cease to have
         any powers under the Plan and all powers previously vested in the
         Committee under the Plan will then be vested in the Third Party
         Fiduciary.

         "COMPANY" means Southwest Gas Corporation and such of its Affiliates as
         the Board of Directors may select to become parties to the Plan.

         "CONTINUOUS SERVICE" means a Participant's Benefit Service with the
         Company and its successors or assigns, as defined in the Basic Plan.

         "EARNINGS" means the yearly compensation paid to a Participant,
         including salary deferrals, but excluding bonuses, commissions,
         overtime, special income (as defined in the Company's Executive
         Deferral Plan) and nonmonetary awards for employment services to the
         Company and its successors and assigns.

         "ELIGIBLE SPOUSE" means the surviving spouse of a Participant as
         defined in the Basic Plan.

                                       2
<PAGE>   4

         "PARTICIPANT" means an employee of the Company who is or was an
         officer, including a Senior Officer, of the Company and any participant
         in the Company's Executive Deferral Plan prior to a Change in Control.

         "PLAN" means the Company's Supplemental Retirement Plan.

         "RETIREMENT" means the termination of a Participant's employment with
         the Company under the provisions of Sections II and VI. Continued
         employment with a successor or assign of the Company will not qualify
         as Retirement, unless otherwise agreed by the Participant and such
         successors or assigns.

         "SENIOR OFFICER" means an officer of the Company with the title "Senior
         Vice President" or above.

         "THIRD PARTY FIDUCIARY" means an independent third party (a corporate
         entity with no other relationship with the Company) selected by the
         Company, or its successors or assigns, to take over the administration
         of the Plan upon and after a Change in Control and to determine appeals
         of claims denied under the Plan before and after a Change in Control
         pursuant to a Third Party Fiduciary Services Agreement.

         "THIRD PARTY FIDUCIARY SERVICES AGREEMENT" means the agreement with the
         Third Party Fiduciary to perform services with respect to the Plan.

         "TRUST AGREEMENT" means an agreement establishing a "grantor trust" of
         which the Company, or its successors or assigns, is the grantor, within
         the meaning of subpart E, part I, subchapter J, chapter 1, subtitle A
         of the Internal Revenue Code of 1986, as amended (the "Code").

         "TRUST FUND OR FUNDS" means the assets of every kind and description
         held under any Trust Agreement forming a part of the Plan.

         "TRUSTEE" means any person or entity selected by the Company, or its
         successors or assigns, to act as trustee under any Trust Agreement at
         any time of reference.

                 II. ELIGIBILITY FOR PARTICIPATION AND BENEFITS

2.1      An individual shall become a Participant in the Plan as of the
         effective date of his election by the Board of Directors as an officer
         of the Company (unless the Board of Directors determines, at that time,
         that such officer will not become eligible to participate in the Plan)
         or the effective date of his selection to participate in the Company's
         Executive Deferral Plan.

2.2      A Participant with 20 or more years of Continuous Service is eligible
         to retire and receive benefits under the Plan after attaining age 55.

                                       3
<PAGE>   5

2.3      A Senior Officer with 10 or more years of Continuous Service is
         eligible to retire and receive a benefit under this Plan after
         attaining age 65.

2.4      A Participant who is vested under the Basic Plan, but who fails to
         satisfy the requirements of Sections 2.2 or 2.3 of this Section, is
         eligible to receive benefits only under the provisions of Section 3.3
         of the Plan.

2.5      Anything herein to the contrary notwithstanding, if a Participant who
         is receiving, or may be entitled to receive, a benefit hereunder
         engages in competition with the Company (without the Committee's prior
         authorization in writing), or is discharged for cause, or performs acts
         of willful malfeasance or gross negligence in a matter of material
         importance to the Company, payments thereafter payable hereunder to
         such Participant or such Participant's Eligible Spouse will, at the
         Board of Directors' discretion, be forfeited and the Company will have
         no further obligation to such Participant or Eligible Spouse. This
         Section 2.5 shall not apply after a Change in Control.

                   III. AMOUNT AND FORM OF RETIREMENT BENEFIT

3.1      The annual retirement benefit payable will be 50 percent (60 percent
         for Senior Officers) of the Participant's Average Earnings, less any
         Basic Plan Benefits.

3.2      If a Participant qualifies for benefits under Section 2.2 of the Plan
         and retires before age 60, the benefits he receives under the
         provisions of Section 3.1 of this Section will be reduced in the same
         manner as the benefits under the Basic Plan are adjusted for early
         retirement.

3.3      The annual retirement benefit payable to a Participant who only
         satisfies the provisions of Section 2.4 of the Plan will be the benefit
         payable under the Basic Plan as if Compensation, as defined in the
         Basic Plan, includes compensation deferred under the Company's
         Executive Deferral Plan or other non-qualified deferral plan offered by
         any successors or assigns (but not any incentive or bonus award or
         special income (as defined in the Company's Executive Deferral Plan))
         and without regard to any statutory limitation on the compensation that
         can be considered under the Basic Plan, less any Basic Plan Benefits.

3.4      The benefits determined under this Plan will be payable in the form of
         a straight life annuity except as Section V otherwise provides.

                       IV. PAYMENT OF RETIREMENT BENEFITS

4.1      One-twelfth of the annual benefit determined in accordance with Section
         III will be payable beginning on the first day of the month following
         the date the Participant is eligible to retire and has retired.
         Benefits will continue to be paid on

                                       4
<PAGE>   6

         the first day of each succeeding month. The last benefit payment will
         be paid on the first day of the month in which the retired Participant
         dies unless otherwise provided in accordance with Section V of the
         Plan.

                            V. DEATH BENEFITS PAYABLE

5.1      If a Participant should die before retirement and after becoming
         eligible for retirement as provided for in Sections 2.2 and 2.3 of the
         Plan, the Eligible Spouse will receive a benefit equal to 50 percent of
         the amount of the Participant's benefit under the Plan, determined in
         accordance with Section III as if the Participant had retired and begun
         receiving a benefit in accordance with Section IV of the Plan on the
         first of the month before the date of his death.

5.2      If a Participant should die after retirement benefits under Section 3.1
         of the Plan have begun, the Participant's Eligible Spouse will receive
         a benefit equal to 50 percent of the benefit the Participant was
         receiving under the Plan.

5.3      If a Participant should die before becoming eligible for retirement as
         provided for in Sections 2.2 and 2.3 of the Plan, any benefits
         available to the Eligible Spouse under the Basic Plan will be
         determined using Compensation as defined in Section 3.3 of the Plan.

5.4      If a disabled Participant should die while receiving benefits in
         accordance with Section VI of the Plan, such Participant's Eligible
         Spouse will receive a benefit equal to 50 percent of the benefit the
         Participant was receiving under the Plan.

5.5      If an Eligible Spouse is under age 50 and is more than 5 years younger
         than the Participant, the Eligible Spouse's benefit described in this
         Section, except as provided for in Section 5.3 of this Section, will be
         reduced by 2 percent for each year over 5 by which such Eligible Spouse
         is younger than the Participant.

5.6      Eligible Spouse's benefits described herein will commence on the first
         day of the month following the Participant's death and continue on the
         first of each succeeding month, ending on the first day of the month in
         which the Eligible Spouse dies. No benefits under this Plan will be
         payable thereafter.

5.7      If, on the date of his death, a Participant has no Eligible Spouse, no
         further benefits are payable under this Plan.

                         VI. DISABILITY BENEFITS PAYABLE

6.1      Notwithstanding the provisions of Sections 2.2 or 2.3 of the Plan, if
         the Committee determines that a Participant has become totally disabled
         before

                                       5
<PAGE>   7

         attaining age 65, the Participant shall be entitled to retire and
         receive a benefit under this Plan.

6.2      The annual disability benefit will be 50 percent (60 percent for Senior
         Officers) of the Participant's Average Earnings, less any benefits
         payable under the Company's salary continuation and long-term
         disability plans or like plans offered by any of its successors and
         assigns, and any Basic Plan Benefits.

6.3      Disability benefits will be payable on the same basis as retirement
         benefits under Section IV of the Plan. The last payment will occur on
         the first of the month during which the disabled Participant either
         recovers, as determined solely by the Committee, or dies.

6.4      If a disabled Participant dies, a benefit will be paid to the Eligible
         Spouse as provided in Section 5.4 of the Plan.

6.5      The Committee may require, no more frequently than once in any calendar
         year, that a disabled Participant submit medical evidence of disability
         satisfactory to the Committee. The Committee may discontinue a
         disability benefit after considering such evidence or lack thereof.

6.6      If a Participant is determined to no longer be disabled, the period of
         time he was disabled will be added to his Continuous Service for the
         purposes of determining further eligibility for benefits under the
         Plan.

                                  VII. GENERAL

7.1      Amounts payable to a Participant or his Eligible Spouse shall be paid
         from the general assets of the Company, the general assets of its
         successors or assigns, or from the assets of a grantor trust within the
         meaning of subpart E, part I, subchapter J, chapter 1, subtitle A of
         the Code, established for use in funding executive compensation
         arrangements and commonly known as a "Rabbi Trust."

7.2      The Company, or its successors or assigns, shall have no obligation
         under the Plan to a Participant or his Eligible Spouse, except as
         provided in this Plan.

7.3      The Participant or his Eligible Spouse shall cooperate with the
         Committee in furnishing all information requested by the Company, or
         its successors or assigns, to facilitate the payment of his benefit.

7.4      Participants and their Eligible Spouses, heirs, successors, and assigns
         shall have no legal or equitable rights, claims, or interest in any
         specific property or assets of the Company, or its successors or
         assigns. No assets of the Company, or that of its successors or
         assigns, shall be held under any trust, or held in any way as
         collateral security for the fulfilling its obligations under the Plan.
         Any and all assets of the Company, or that of its successors or
         assigns, shall be, and

                                       6
<PAGE>   8

         remain, the general unpledged, unrestricted assets of such entities.
         The obligations of the Company, or its successors or assigns, under the
         Plan shall be merely that of an unfunded and unsecured promise of the
         Company, or its successors or assigns, to pay money in the future, and
         the rights of the Participants shall be no greater than those of
         unsecured general creditors. It is the intention of the Company, or its
         successors or assigns, that this Plan (and the Trust Funds described in
         Section VII) be unfunded for purposes of the Code and for the purposes
         of Title I of the Employee Retirement Income Security Act of 1974, as
         amended ("ERISA").

7.5      There shall be deducted from each payment made under the Plan or other
         compensation payable to the Participant or his Eligible Spouse all
         taxes which are required to be withheld by the Company, or its
         successors or assigns, in respect to such payment or this Plan. The
         Company, or its successors or assigns, shall have the right to reduce
         any payment (or other compensation) by the amount of cash sufficient to
         provide the payment amount of said taxes.

7.6      Nothing contained herein will confer on any Participant the right to be
         retained in the service of the Company, or its successors or assigns,
         nor will it interfere with the rights of such entities to discharge or
         otherwise deal with Participants without regard to the Plan's
         existence.

                                  VIII. TRUSTS

8.1      The Company, or its successors or assigns, may maintain one or more
         Trust Funds to finance all or a portion of the benefits under the Plan
         by entering into one or more Trust Agreement(s). Any Trust Agreement is
         designated as, and shall constitute, a part of the Plan, and all rights
         which may accrue to any person under the Plan shall be subject to all
         the terms and provisions of such Trust Agreement. A Trustee shall be
         appointed by the Committee or the Board of Directors and shall have
         such powers as provided in the Trust Agreement. The Committee or the
         Board of Directors may modify any Trust Agreement, in accordance with
         its terms, to accomplish the purposes of the Plan and appoint a
         successor Trustee under the provisions of such Trust Agreement. By
         entering into such Trust Agreement, the Committee or the Board of
         Directors may vest in the Trustee, or in one or more investment
         managers (as defined in ERISA), the power to manage and control the
         Trust Fund. The Committee's authority under the provisions of this
         Section 8.1 will cease with a Change in Control.

                    IX. TERMINATION, SUSPENSION OR AMENDMENT

 9.1     The Board of Directors may, at its sole discretion, amend or modify the
         Plan or by resolution reduce the eligibility requirements or increase
         the benefits for an individual Participant at any time or from time to
         time, in whole or in part.

                                       7
<PAGE>   9

         However: (i) no amendment or modification of the Plan will affect or
         reduce (a) the rights and benefits available to Participants under
         terms of the Plan as in effect at the time of their selection and
         during their participation in the Plan, (b) their Eligible Spouses'
         rights to receive death benefits in accordance with this Plan, (c) the
         continued accrual of benefits under the Plan on terms at least as
         favorable as the terms of the Plan applicable to each Participant in
         effect immediately prior to a Change in Control, taking into account
         service and compensation earned after such an event, or (d) a retired
         Participant's right or the right of an Eligible Spouse to continue to
         receive a benefit in accordance with this Plan (as in effect on the
         date such Participant began to receive a benefit under this Plan); and
         (ii) effective March 1, 1999, no amendment or modification of this
         Section IX, Section XI, or Section XII of the Plan shall be effective.

9.2      The Board shall not terminate the Plan until all benefits have been
         paid in full under the provisions of the Plan.

                    X. RESTRICTIONS ON ALIENATION OF BENEFITS

10.1     To the maximum extent permitted by law, no interest or benefit under
         the Plan shall be assignable or subject in any manner to alienation,
         sale, transfer, claims of creditors, pledge, attachment, or
         encumbrances of any kind.

                         XI. ADMINISTRATION OF THE PLAN

11.1     Except as otherwise provided in this Section XI, and subject to Section
         XII, the general administration of the Plan, as well as construction
         and interpretation thereof, shall be vested in the Committee.
         Specifically, the Committee shall have the discretion and authority to:
         (a) make, amend, interpret, and enforce all appropriate rules and
         regulations for the administration of the Plan; and (b) decide or
         resolve any and all questions including interpretations of the Plan.
         Any individual serving on the Committee who is a Participant shall not
         vote or act on any matter relating solely to himself or herself. The
         number of members of the Committee shall be established by, and the
         members shall be appointed from time to time by, and shall serve at the
         pleasure of, the Board of Directors. Members of the Committee may be
         Participants under the Plan.

11.2     Upon and after a Change in Control, the administration of the Plan
         shall be vested in a Third Party Fiduciary, as provided for herein and
         pursuant to the terms of a Third Party Fiduciary Services Agreement.
         Any Third Party Fiduciary Services Agreement is designated as, and
         shall constitute, a part of the Plan. The Third Party Fiduciary shall
         also have the discretion and authority to: (a) make, amend, interpret,
         and enforce all appropriate rules and regulations for the
         administration of the Plan; and (b) decide or resolve any and all
         questions including interpretation of the Plan and the Trust Agreement.
         Except as

                                       8
<PAGE>   10

         otherwise provided for in any Trust Agreement, the Third Party
         Fiduciary shall have no power to direct the investment of Plan or Trust
         Funds or select any investment manager or custodial firm for the Plan
         or Trust Agreement. The Company, or its successors or assigns, shall
         pay all reasonable administrative expenses and fees of the Third Party
         Fiduciary when it acts as the administrator of the Plan or pursuant to
         Section XII. The Third Party Fiduciary may not be terminated by the
         Company, or its successors or assigns, without the consent of 50% of
         the Participants in the Plan.

11.3     In the administration of the Plan, the Committee or the Third Party
         Fiduciary, as the case may be, may from time to time employ such
         agents, consultants, advisors, and managers as it deems necessary or
         useful in carrying out its duties as it sees fit (including acting
         through a duly authorized representative) and may from to time to time
         consult with counsel to the Company, or counsel of any of its
         successors or assigns.

11.4     The decision or action of the Committee or the Third Party Fiduciary,
         as the case may be, with respect to any question arising out of or in
         connection with the administration, interpretation, and application of
         the Plan (and the Trust Agreement to the extent provided for in Section
         11.2) and the rules and regulations promulgated hereunder shall be
         final and conclusive and binding upon all persons having any interest
         in the Plan.

11.5     The Company, or its successors or assigns, shall indemnify and save
         harmless each member of the Committee, the Third Party Fiduciary, and
         any employee of the Company, or its successors or assigns, to whom the
         duties of the Committee may be delegated against any and all claims,
         losses, damages, expenses, and liabilities arising from any action or
         failure to act with respect to the Plan, except in the case of fraud,
         gross negligence, or willful misconduct by the Committee, any of its
         members, the Third Party Fiduciary, or any such employee.

11.6     To enable the Committee and the Third Party Fiduciary to perform their
         functions, the Company, or its successors or assigns, shall supply full
         and timely information to the Committee and the Third Party Fiduciary,
         as the case may be, on all matters relating to the compensation of all
         Participants, their Retirement, death or other cause for termination of
         service, and such other pertinent facts as the Committee or the Third
         Party Fiduciary may require.

                          XII. BENEFIT CLAIMS PROCEDURE

12.1     Any Participant or Eligible Spouse (such being referred to below as a
         "Claimant") may deliver to the Committee a written claim for
         determination with respect to benefits available to such Claimant from
         the Plan. The claim must state with particularity the determination
         desired by the Claimant.

                                       9
<PAGE>   11

12.2     The Committee shall consider a claim and notify the Claimant within 90
         calendar days after receipt of a claim in writing:

         (a)      That the Claimant's requested determination has been made, and
                  that the claim has been allowed in full; or

         (b)      That the Committee has reached a conclusion contrary, in whole
                  or in part, to the Claimant's requested determination, and
                  such notice must set forth in a manner calculated to be
                  understood by the Claimant: (i) the specific reason(s) for the
                  denial of the claim, or any part thereof; (ii) the specific
                  reference(s) to pertinent provisions of the Plan upon which
                  the denial was based; (iii) a description of any additional
                  material or information necessary for the Claimant to perfect
                  the claim, and an explanation of why such material or
                  information is necessary; and (iv) an explanation of the claim
                  review procedure set forth in Section 12.3.

12.3     Within 60 days after receiving a notice from the Committee that a claim
         has been denied, in whole or in part, a Claimant (or the Claimant's
         duly authorized representative) may file with the Third Party Fiduciary
         a written request for a review of the denial of the claim. Thereafter,
         the Claimant (or the Claimant's duly authorized representative) may
         review pertinent documents, submit written comments or other documents,
         and request a hearing, which the Third Party Fiduciary, in its sole
         discretion, may grant.

12.4     The Third Party Fiduciary shall render its decision on review promptly,
         and not later than 60 days after the filing of a written request for
         review of a denial, unless a hearing is held or other special
         circumstances require additional time, in which case the Third Party
         Fiduciary's decision must be rendered within 120 calendar days after
         such date. Such decision must be written in a manner calculated to be
         understood by the Claimant, and it must contain: (i) the specific
         reason(s) for the decision; (ii) the specific reference(s) to the
         pertinent Plan provisions upon which the decision was based; and (iii)
         such other matters as the Third Party Fiduciary deems relevant.

12.5     A Claimant's compliance with the foregoing provisions of this Section
         XII is a mandatory prerequisite to a Claimant's right to commence any
         legal action with respect to any claim for benefits under the Plan.

                               XIII. MISCELLANEOUS

13.1     No Participant will participate in an action of the Committee or the
         Board of Directors on a matter that solely applies to that Participant.
         Such matters will be determined by a majority of the rest of the
         Committee or the Board of Directors.

                                       10
<PAGE>   12

13.2     Each Participant will receive a copy of this Plan and the Committee
         will make available for any Participant's inspection a copy of the
         rules and regulations the Committee uses in administering the Plan.

13.3     Except to the extent that federal law applies, the Plan shall be
         governed by and construed under the laws of the State of Nevada.

13.4     The Plan shall be binding upon the Company and its successors and
         assigns, and upon a Participant, his Eligible Spouse, and their
         assigns, heirs, executors, and administrators.

13.5     Masculine pronouns wherever used shall include feminine pronouns and
         when the context dictates, the singular shall include the plural.

13.6     In case any provision of the Plan shall be held illegal or invalid for
         any reason, said illegality or invalidity shall not affect the
         remaining parts hereof, but the Plan shall be construed and enforced as
         if such illegal and invalid provisions had never been inserted herein.

13.7     Any notice given under the Plan shall be in writing and shall be mailed
         or delivered to:

                                    SOUTHWEST GAS CORPORATION
                                    Supplemental Retirement Plan
                                    Compensation Committee
                                    5241 Spring Mountain Road
                                    Las Vegas, NV  89102

         and
                                    CRG Fiduciary Services, Inc.
                                    633 West Fifth Street, 53rd floor
                                    Los Angeles, CA 90071-2086
                                    Attn: Managing Director

IN WITNESS WHEREOF, Southwest Gas Corporation has caused this Amended and
Restated Plan to be executed this 30th day of July 1999.

                                    SOUTHWEST GAS CORPORATION

                                    By
                                       -----------------------------------------
                                                  Michael O. Maffie
                                         President & Chief Executive Officer

                                       11

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