<PAGE>
 
                                                                     EXHIBIT 4.5
________________________________________________________________________________

                                NGC CORPORATION

                         ______________________________



                         ______________________________


                        SUBORDINATED DEBENTURE INDENTURE

                            Dated as of May 28, 1997
                         ______________________________



                       THE FIRST NATIONAL BANK OF CHICAGO


                                   as Trustee


                         ______________________________


                  SUBORDINATED DEFERRABLE INTEREST DEBENTURES



_______________________________________________________________________________
<PAGE>
 
TIE-SHEET

     of provisions of Trust Indenture Act of 1939 with Indenture dated as of May
28, 1997 between NGC Corporation and The First National Bank of Chicago,
Trustee:

ACT SECTION                                      INDENTURE SECTION
<TABLE>
<CAPTION>
 
<S>                                                  <C>
310(a)(1)...................................................  6.09
   (a)(2)...................................................  6.09
310(a)(3)...................................................   N/A
   (a)(4)...................................................   N/A
310(a)(5)...................................................  6.09
310(b)................................................  6.08, 6.10
310(c)......................................................   N/A
311(a) and (b)..............................................  6.13
311(c).............................................  4.01, 4.02(a)
312(a)......................................................  4.01
312(b) and (c)..............................................  4.02
313(a)......................................................  4.04
313(b)(1)...................................................  4.04
313(b)(2)...................................................  4.04
313(c)......................................................  4.04
313(d)......................................................  4.04
314(a)......................................................  4.03
314(b)......................................................   N/A
314(c)(1) and (2)...........................................  6.07
314(c)(3)...................................................   N/A
314(d)......................................................   N/A
314(e)......................................................  6.07
314(f)......................................................   N/A
315(a)(c) and (d)...........................................  6.01
315(b)......................................................  5.08
315(e)......................................................  5.09
316(a)(1)...................................................  5.07
316(a)(2)...................................................   N/A
316(a) last sentence........................................  7.04
316(b)......................................................  9.02
317(a)......................................................  5.05
317(b)......................................................  6.05
318(a)...................................................... 13.08
-------------------------------------
</TABLE>
           THIS TIE-SHEET IS NOT PART OF THE INDENTURE AS EXECUTED.
<PAGE>
 


                                                                            PAGE
                                                                            ----

                               TABLE OF CONTENTS

                                   ARTICLE I
                                  DEFINITIONS

SECTION 1.01.    Definitions................................................   1
SECTION 1.02.    Business Day Certificate...................................  11
SECTION 1.03.    Form of Documents Delivered to Trustee.....................  11

                                  ARTICLE II
                                  SECURITIES

SECTION 2.01.    Forms Generally............................................  12
SECTION 2.02.    Execution and Authentication...............................  12
SECTION 2.03.    Form and Payment...........................................  13
SECTION 2.04.    Legends....................................................  13
SECTION 2.05.    Global Security............................................  14
SECTION 2.06.    Interest...................................................  16
SECTION 2.07.    Transfer and Exchange......................................  17
SECTION 2.08.    Replacement Securities.....................................  19
SECTION 2.09.    Temporary Securities.......................................  20
SECTION 2.10.    Cancellation...............................................  20
SECTION 2.11.    Defaulted Interest.........................................  20
SECTION 2.12.    CUSIP Numbers..............................................  21

                                  ARTICLE III
                      PARTICULAR COVENANTS OF THE COMPANY

SECTION 3.01.    Payment of Principal, Premium and Interest.................  22
SECTION 3.02.    Offices for Notices and Payments, etc......................  22
SECTION 3.03.    Appointments to Fill Vacancies in Trustee's Office.........  23
SECTION 3.04.    Provision as to Paying Agent...............................  23
SECTION 3.05.    Certificate to Trustee.....................................  24
SECTION 3.06.    Compliance with Consolidation Provisions...................  24
SECTION 3.07.    Limitation on Dividends....................................  24
SECTION 3.08.    Covenants as to NGC Corporation Capital Trust..............  25
SECTION 3.09.    Payment of Trust's Costs and Expenses......................  25
SECTION 3.10.    Payment Upon Resignation or Removal........................  26

                                  ARTICLE IV
                      SECURITYHOLDERS' LISTS AND REPORTS
                        BY THE COMPANY AND THE TRUSTEE

SECTION 4.01.    Securityholders' Lists.....................................  26
SECTION 4.02.    Preservation and Disclosure of Lists.......................  27


                                       i
<PAGE>



                                                                            PAGE
                                                                            ----

SECTION 4.03.    Reports by Company........................................   28
SECTION 4.04.    Reports by the Trustee....................................   29

                                   ARTICLE V
                REMEDIES OF THE TRUSTEE AND SECURITYHOLDERS ON
                               EVENT OF DEFAULT

SECTION 5.01.    Events of Default..........................................  30
SECTION 5.02.    Payment of Securities on Default; Suit Therefor............  32
SECTION 5.03.    Application of Moneys Collected by Trustee.................  33
SECTION 5.04.    Proceedings by Securityholders.............................  34
SECTION 5.05.    Proceedings by Trustee.....................................  35
SECTION 5.06.    Remedies Cumulative and Continuing.........................  35
SECTION 5.07.    Direction of Proceedings and Waiver of Defaults
                 by Majority of Securityholders.............................  35
SECTION 5.08.    Notice of Defaults.........................................  36
SECTION 5.09.    Undertaking to Pay Costs...................................  37
SECTION 5.10.    Waiver of Stay or Extension Laws...........................  37

                                  ARTICLE VI
                            CONCERNING THE TRUSTEE

SECTION 6.01.    Duties and Responsibilities of Trustee.....................  37
SECTION 6.02.    Reliance on Documents, Opinions, etc.......................  39
SECTION 6.03.    No Responsibility for Recitals, etc........................  40
SECTION 6.04.    Trustee, Authenticating Agent, Paying Agents, Transfer
                 Agents or Registrar May Own Securities.....................  40
SECTION 6.05.    Moneys to be Held in Trust.................................  40
SECTION 6.06.    Compensation and Expenses of Trustee.......................  41
SECTION 6.07.    Officers' Certificate as Evidence..........................  41
SECTION 6.08.    Conflicting Interest of Trustee............................  42
SECTION 6.09.    Eligibility of Trustee.....................................  42
SECTION 6.10.    Resignation or Removal of Trustee..........................  42
SECTION 6.11.    Acceptance by Successor Trustee............................  44
SECTION 6.12.    Succession by Merger, etc..................................  44
SECTION 6.13.    Limitation on Rights of Trustee as a Creditor..............  45
SECTION 6.14.    Authenticating Agents......................................  45

                                  ARTICLE VII
                        CONCERNING THE SECURITYHOLDERS

SECTION 7.01.    Action by Securityholders..................................  46
SECTION 7.02.    Proof of Execution by Securityholders......................  47
SECTION 7.03.    Who Are Deemed Absolute Owners.............................  47
SECTION 7.04.    Securities Owned by Company Deemed Not Outstanding.........  47
SECTION 7.05.    Revocation of Consents; Future Holders Bound...............  48


                                       ii
<PAGE>



 
                                                                            PAGE
                                                                            ----

                                 ARTICLE VIII
                           SECURITYHOLDERS' MEETINGS


SECTION 8.01.     Purposes of Meetings......................................  48
SECTION 8.02.     Call of Meetings by Trustee...............................  49
SECTION 8.03.     Call of Meetings by Company or Securityholders............  49
SECTION 8.04.     Qualifications for Voting.................................  49
SECTION 8.05.     Regulations...............................................  49
SECTION 8.06.     Voting....................................................  50

                                  ARTICLE IX
                            SUPPLEMENTAL INDENTURES

SECTION 9.01.     Without Consent of Securityholders........................  51
SECTION 9.02.     With Consent of Securityholders...........................  52
SECTION 9.03.     Compliance with Trust Indenture Act; Effect             
                  of Supplemental Indentures................................  53
SECTION 9.04.     Notation on Securities....................................  53
SECTION 9.05.     Evidence of Compliance of Supplemental Indenture        
                  to be Furnished Trustee...................................  54

                                   ARTICLE X
               CONSOLIDATION, MERGER, SALE, CONVEYANCE AND LEASE

SECTION 10.01.    Company May Consolidate, etc., on Certain Terms...........  54
SECTION 10.02.    Successor Corporation to be Substituted for Company.......  55
SECTION 10.03.    Opinion of Counsel to be Given Trustee....................  55

                                  ARTICLE XI
                    SATISFACTION AND DISCHARGE OF INDENTURE

SECTION 11.01.    Discharge of Indenture....................................  55
SECTION 11.02.    Deposited Moneys and U.S. Government Obligations              
                  to be Held in Trust by Trustee............................  56
SECTION 11.03.    Paying Agent to Repay Moneys Held.........................  56
SECTION 11.04.    Return of Unclaimed Moneys................................  57
SECTION 11.05.    Defeasance Upon Deposit of Moneys or
                  U.S. Government Obligations...............................  57

                                  ARTICLE XII
                          IMMUNITY OF INCORPORATORS,
                      STOCKHOLDERS,OFFICERS AND DIRECTORS

SECTION 12.01.    Indenture and Securities Solely Corporate Obligations.....  58

                                 ARTICLE XIII
                           MISCELLANEOUS PROVISIONS

SECTION 13.01.    Successors................................................  59
SECTION 13.02.    Official Acts by Successor Corporation....................  59


                                      iii
<PAGE>


 
                                                                            PAGE
                                                                            ----

SECTION 13.03.    Surrender of Company Powers...............................  59
SECTION 13.04.    Addresses for Notices, etc................................  59
SECTION 13.05.    GOVERNING LAW.............................................  60
SECTION 13.06.    Evidence of Compliance with Conditions Precedent..........  60
SECTION 13.07.    Business Days.............................................  60
SECTION 13.08.    Trust Indenture Act to Control............................  60
SECTION 13.09.    Table of Contents, Headings, etc..........................  61
SECTION 13.10.    Execution in Counterparts.................................  61
SECTION 13.11.    Separability..............................................  61
SECTION 13.12.    Assignment................................................  61
SECTION 13.13.    Acknowledgment of Rights..................................  61

                                  ARTICLE XIV
                           REDEMPTION OF SECURITIES

SECTION 14.01.    Special Event Redemption..................................  62
SECTION 14.02.    Optional Redemption by Company............................  62
SECTION 14.03.    No Sinking Fund...........................................  63
SECTION 14.04.    Notice of Redemption; Selection of Securities.............  63
SECTION 14.05.    Payment of Securities Called for Redemption...............  64

                                  ARTICLE XV
                          SUBORDINATION OF SECURITIES

SECTION 15.01.    Agreement to Subordinate..................................  64
SECTION 15.02.    Default on Senior Indebtedness............................  65
SECTION 15.03.    Liquidation; Dissolution; Bankruptcy......................  65
SECTION 15.04.    Subrogation...............................................  67
SECTION 15.05.    Trustee to Effectuate Subordination.......................  67
SECTION 15.06.    Notice by the Company.....................................  67
SECTION 15.07.    Rights of the Trustee; Holders of Senior Indebtedness.....  68
SECTION 15.08.    Subordination May Not Be Impaired.........................  69
SECTION 15.09.    Article Applicable to Paying Agents.......................  69
SECTION 15.10.    Trust Moneys Not Subordinated.............................  70

                                  ARTICLE XVI
                     EXTENSION OF INTEREST PAYMENT PERIOD

SECTION 16.01.    Extension of Interest Payment Period......................  70
SECTION 16.02.    Notice of Extension.......................................  71


                                       iv
<PAGE>
 
                                                                            PAGE
                                                                            ____


Testimonium
Signatures
Acknowledgments

                                       v
<PAGE>
 
     THIS SUBORDINATED DEBENTURE INDENTURE, dated as of May 28, 1997,
between NGC Corporation, a Delaware corporation (hereinafter sometimes called
the "Company"), and The First National Bank of Chicago, a Delaware banking
corporation, as trustee (hereinafter sometimes called the "Trustee"),

                             W I T N E S S E T H :

     In consideration of the premises, and the purchase of the Securities by the
holders thereof, the Company covenants and agrees with the Trustee for the equal
and proportionate benefit of the respective holders from time to time of the
Securities, as follows:


                                   ARTICLE I
                                  DEFINITIONS
                                  -----------

     SECTION 1.01. Definitions.

     The terms defined in this Section 1.01 (except as herein otherwise
expressly provided or unless the context otherwise requires) for all purposes of
this Indenture shall have the respective meanings specified in this Section
1.01. All other terms used in this Indenture which are defined in the Trust
Indenture Act of 1939, as amended (the "Trust Indenture Act"), or which are by
reference therein defined in the Securities Act, shall (except as herein
otherwise expressly provided or unless the context otherwise requires) have the
meanings assigned to such terms in said Trust Indenture Act and in said
Securities Act as in force at the date of this Indenture as originally executed.
The following terms have the meanings given to them in the Declaration: (i)
Clearing Agency; (ii) Delaware Trustee; (iii) Property Trustee; (iv)
Administrative Trustees; (v) Series A Capital Securities; (vi) Series B Capital
Securities; (vii) Direct Action; (viii) Distributions; and (ix) Private Exchange
Capital Securities. All accounting terms used herein and not expressly defined
shall have the meanings assigned to such terms in accordance with generally
accepted accounting principles and the term "generally accepted accounting
principles" means such accounting principles as are generally accepted at the
time of any computation. The words "herein", "hereof" 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. Headings are used for
convenience of reference only and do not affect interpretation. The singular
includes the plural and vice versa.

     "Additional Sums" shall have the meaning set forth in Section 2.06(c).

     "Adjusted Treasury Rate" means, with respect to any redemption date, the
rate per annum equal to (i) the yield, under the heading which represents the
average for the immediately prior week, appearing in the most recently published
statistical release designated "H.15(519)" or any successor publication which is
published weekly by the Federal Reserve and which establishes yields on actively
traded United States Treasury securities adjusted to constant maturity under the
caption "Treasury Constant Maturities," for the maturity date corresponding to
the Maturity Date (if no 
<PAGE>
 
maturity date is within three months before or after the Maturity Date, yields
for the two published maturities most closely corresponding to the Maturity Date
shall be interpolated and the Adjusted Treasury Rate shall be interpolated or
extrapolated from such yields on a straight-line basis, rounding to the nearest
month) or (ii) if such release (or any successor release) is not published
during the week preceding the calculation date or does not contain such yields,
the rate per annum equal to the semiannual equivalent yield to maturity of the
Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue
(expressed as a percentage of its principal amount) equal to the Comparable
Treasury Price for such redemption date plus (i) 1.03% if such redemption date
occurs prior to June 1, 1998 and (ii) 0.50% in all other cases.

     "Affiliate" shall have the meaning given to that term in Rule 405 under the
Securities Act or any successor rule thereunder.

     "Authenticating Agent" shall mean any agent or agents of the Trustee which
at the time shall be appointed and acting pursuant to Section 6.14.

     "Bankruptcy Law" shall mean Title 11, U.S. Code, or any similar federal or
state law for the relief of debtors.

     "Board of Directors" shall mean either the Board of Directors of the
Company or any duly authorized committee of that board.

     "Board Resolution" shall mean a copy of a resolution certified by the
Secretary or an Assistant Secretary of the Company to have been duly adopted by
the Board of Directors and to be in full force and effect on the date of such
certification, and delivered to the Trustee.

     "Business Day" shall mean, with respect to any series of Securities, any
day other than a Saturday or a Sunday or a day on which banking institutions in
Houston, Texas or Chicago, Illinois are authorized or required by law or
executive order to close.

     "Capital Securities" shall mean the subordinated capital income securities
representing undivided beneficial interests in the assets of NGC Corporation
Capital Trust which rank pari passu with the Common Securities issued by NGC
Corporation Capital Trust; provided, however, that if an Event of Default has
occurred and is continuing, no payments in respect of Distributions on, or
payments upon liquidation, redemption or otherwise with respect to, the Common
Securities shall be made until the holders of the Capital Securities shall be
paid in full the Distributions and the liquidation, redemption and other
payments to which they are entitled.  References to "Capital Securities" shall
include collectively any Series A Capital Securities, Series B Capital
Securities and Private Exchange Capital Securities.

     "Capital Securities Guarantee" shall mean any guarantee that the Company
may enter into with The First National Bank of Chicago or other Persons that
operates directly or indirectly for the 

                                       2
<PAGE>
 
benefit of holders of Capital Securities of NGC Corporation Capital Trust and
shall include a Series A Capital Securities Guarantee and a Series B Capital
Securities Guarantee with respect to the Series A Capital Securities and the
Series B Capital Securities, respectively.

     "Cedel" means Cedel, S.A.

     "Commission" shall mean the Securities and Exchange Commission, as from
time to time constituted, created under the Exchange Act, or if at any time
after the execution of this Indenture such Commission is not existing and
performing the duties now assigned to it under the Trust Indenture Act, then the
body performing such duties at such time.

     "Common Securities" shall mean the common securities of the Trust
representing undivided beneficial interests in the assets of the Trust which
rank pari passu with Capital Securities issued by the Trust; provided, however,
that if an Event of Default has occurred and is continuing, no payments in
respect of Distributions on, or payments upon liquidation, redemption or
otherwise with respect to, the Common Securities shall be made until the holders
of the Capital Securities shall be paid in full the Distributions and the
liquidation, redemption and other payments to which they are entitled.

     "Common Securities Guarantee" shall mean any guarantee that the Company may
enter into with any Person or Persons that operates directly or indirectly for
the benefit of holders of Common Securities of NGC Corporation Capital Trust.

     "Common Stock" shall mean the Common Stock, par value $.01 per share, of
the Company or any other class of stock resulting from changes or
reclassifications of such Common Stock consisting solely of changes in par
value, or from par value to no par value, or from no par value to par value.

     "Company" shall mean NGC Corporation, a Delaware corporation, and, subject
to the provisions of Article X, shall include its successors and assigns.

     "Company Request" or "Company Order" shall mean a written request or order
signed in the name of the Company by the Chairman of the Board, the Chief
Executive Officer, the President, a Vice Chairman of the Board or a Vice
President, and by its Treasurer, an Assistant Treasurer, its Secretary or an
Assistant Secretary, and delivered to the Trustee.

     "Comparable Treasury Issue" means the United States Treasury security
selected by the Reference Treasury Dealer as having a maturity comparable to the
remaining term of the Securities to be redeemed that would be utilized, at the
time of selection and in accordance with customary financial practice, in
pricing new issues of corporate debt securities of comparable maturity to the
remaining term of such Securities.

                                       3
<PAGE>
 
     "Comparable Treasury Price" means, with respect to any redemption date, (i)
the average of the bid and asked prices for the Comparable Treasury Issue
(expressed in each case as a percentage of its principal amount) on the third
business day preceding such redemption date, as set forth in the daily
statistical release (or any successor release) published by the Federal Reserve
Bank of New York and designated "Composite 3:30 p.m. Quotations for U.S.
Government Securities" or (ii) if such release (or any successor release) is not
published or does not contain such prices on such business day, the average of
the Reference Treasury Dealer Quotations for such redemption date.

     "Compounded Interest" shall have the meaning set forth in Section 16.01.

     "Custodian" shall mean any receiver, trustee, assignee, liquidator, or
similar official under any Bankruptcy Law.

     "Declaration" means the Amended and Restated Declaration of Trust of NGC
Corporation Capital Trust, dated as of May 28, 1997, as amended from time to
time.

     "Default" means any event, act or condition that with notice or lapse of
time, or both, would constitute an Event of Default.

     "Defaulted Interest" shall have the same meaning set forth in Section 2.11.

     "Deferred Interest" shall have the meaning set forth in Section 16.01.

     "Definitive Securities" shall mean those securities issued in fully
registered certificated form not otherwise in global form.

     "Depositary" shall mean, with respect to Securities issuable in whole or in
part in the form of one or more Global Securities, The Depository Trust Company,
New York, New York, another clearing agency, or any successor registered as a
clearing agency under the Exchange Act or other applicable statute or
regulation, which, in each case, shall be designated by the Company pursuant to
Section 2.05(d).

     "Dissolution Event" means the liquidation of NGC Corporation Capital Trust
pursuant to the Declaration, and the distribution of the Securities held by the
Property Trustee to the holders of the Trust Securities issued by NGC
Corporation Capital Trust pro rata in accordance with the Declaration.

     "Event of Default" shall mean any event specified in Section 5.01,
continued for the period of time, if any, and after the giving of the notice, if
any, therein designated.

     "Exchange Act" shall mean the Securities Exchange Act of 1934, as amended.

                                       4
<PAGE>
 
     "Exchange Offer" means the offer that may be made pursuant to the
Registration Rights Agreement (i) by the Company to exchange Series B Securities
for Series A Securities and to exchange a Series B Capital Securities Guarantee
for a Series A Capital Securities Guarantee and (ii) by NGC Corporation Capital
Trust to exchange Series B Capital Securities for Series A Capital Securities.

     "Extended Interest Payment Period" shall have the meaning set forth in
Section 16.01.

     "Global Security" means, with respect to the Securities, a Security
executed by the Company and delivered by the Trustee to the Depositary or
pursuant to the Depositary's instruction, all in accordance with the Indenture,
which shall be registered in the name of the Depositary or its nominee.

     "Indebtedness" shall mean (i) every obligation of the Company for money
borrowed; (ii) every obligation of the Company evidenced by bonds, debentures,
notes or other similar instruments, including obligations incurred in connection
with the acquisition of property, assets or businesses; (iii) every
reimbursement obligation of the Company with respect to letters of credit,
bankers' acceptances or similar facilities issued for the account of the
Company; (iv) every obligation of the Company issued or assumed as the deferred
purchase price of property or services (but excluding trade accounts payable or
accrued liabilities arising in the ordinary course of business); (v) every
capital lease obligation of the Company; (vi) all indebtedness of the Company
whether incurred on or prior to the date of the Indenture or thereafter
incurred, for claims in respect of derivative products, including interest rate,
foreign exchange rate and commodity forward contracts, options and swaps and
similar arrangements; and (vii) every obligation of the type referred to in
clauses (i) through (vi) of another Person and all dividends of another Person
the payment of which, in either case, the Company has guaranteed or is
responsible or liable, directly or indirectly, as obligor or otherwise.

     "Indebtedness Ranking on a Parity with the Securities" shall mean (i)
Indebtedness, whether outstanding on the date of execution of this Indenture or
hereafter created, assumed or incurred, to the extent such indebtedness
specifically by its terms ranks equally with and not prior to the Securities in
the right of payment upon the happening of any dissolution or winding up or
liquidation or reorganization of the Company, and (ii) all other debt
securities, and guarantees in respect of those debt securities (including Other
Debentures and Other Guarantees), issued to any trust other than NGC Corporation
Capital Trust, or a trustee of such trust, partnership or other entity
affiliated with the Company that is a financing vehicle of the Company (a
"financing entity") in connection with the issuance by such financing entity of
equity securities that are similar to the Capital Securities or other securities
guaranteed by the Company.  The securing of any Indebtedness, otherwise
constituting Indebtedness Ranking on a Parity with the Securities, shall not be
deemed to prevent such Indebtedness from constituting Indebtedness Ranking on a
Parity with the Securities.

                                       5
<PAGE>
 
     "Indebtedness Ranking Junior to the Securities" shall mean any
Indebtedness, whether outstanding on the date of execution of this Indenture or
hereafter created, assumed or incurred, to the extent such indebtedness
specifically by its terms ranks junior to and not equally with or prior to the
Securities (and any other Indebtedness Ranking on a Parity with the Securities)
in right of payment upon the happening of any dissolution or winding up or
liquidation or reorganization of the Company.  The securing of any Indebtedness,
otherwise constituting Indebtedness Ranking Junior to the Securities, shall not
be deemed to prevent such Indebtedness from constituting Indebtedness Ranking
Junior to the Securities.

     "Indenture" means this instrument as originally executed or as it may from
time to time be supplemented or amended by one or more indentures supplemental
hereto entered into pursuant to the applicable provisions hereof, including, for
all purposes of this Indenture and any such supplemental indenture, the
provisions of the Trust Indenture Act that are deemed to be a part of and govern
this instrument and any such supplemental indenture, respectively.

     "Interest Payment Date" shall have the meaning set forth in Section
2.06(a).

     "Investment Company Event" means the receipt by NGC Corporation Capital
Trust and the Company of an opinion of counsel experienced in such matters to
the effect that, as a result of the occurrence of a change in law or regulation
or a written change (including any announced prospective change) in
interpretation or application of law or regulation by any legislative body,
court, governmental agency or regulatory authority, which amendment or change is
effective or which pronouncement or decision is announced on or after May 22,
1997, there is more than an insubstantial risk that NGC Corporation Capital
Trust is or will be considered an "investment company" that is required to be
registered under the Investment Company Act of 1940, as amended (the "Investment
Company Act"), which change or prospective change becomes effective or would
become effective, as the case may be, on or after the date of the original
issuance of the Capital Securities.

     "Liquidated Damages" shall have the meaning set forth in the Registration
Rights Agreement.

     "Make-Whole Amount" shall be equal to the greater of (a) 100% of the
principal amount of the Securities or (b) the sum, as determined by the
Reference Treasury Dealer, of the present values of the remaining scheduled
payments of principal and interest on the Securities, discounted to the date of
redemption on a semi-annual basis (assuming a 360-day year consisting of twelve
30-day months) at the Adjusted Treasury Rate, plus, in each case, accrued and
unpaid interest thereon (including Additional Sums and Liquidated Damages, if
any), to the date of redemption.

     "Maturity Date" shall mean June 1, 2027.

     "Mortgage" shall mean and include any mortgage, pledge, lien, security
interest, conditional sale or other title retention agreement or other similar
encumbrance.

                                       6
<PAGE>
 
     "NGC Corporation Capital Trust" or the "Trust" shall mean NGC Corporation
Capital Trust I, a Delaware business trust created for the purpose of issuing
its undivided beneficial interests in connection with the issuance of Securities
under this Indenture.

     "Non Book-Entry Capital Securities" shall have the meaning set forth in
Section 2.05(a)(ii).

     "Officers" shall mean any of the Chairman, a Vice Chairman, the Chief
Executive Officer, the President, a Vice President, the Comptroller, the
Secretary or an Assistant Secretary of the Company.

     "Officers' Certificate" shall mean a certificate signed on behalf of the
Company, by the Chairman of the Board, Chief Executive Officer, a Vice Chairman
of the Board, the President or a Vice President, and by the Treasurer, an
Assistant Treasurer, the Secretary or an Assistant Secretary, of the Company,
and delivered to the Trustee.  Any Officers' Certificate delivered with respect
to compliance with a condition or a covenant provided for in this Indenture
shall include the statements specified to be so included by Section 13.06 of
this Indenture.

     "Opinion of Counsel" shall mean a written opinion of counsel, who may be an
employee of the Company, and who shall be acceptable to the Trustee.

     "Optional Redemption Price" means the greater of (i) 100% of the principal
amount of the Securities to be redeemed plus accrued and unpaid interest thereon
(including Additional Sums and Liquidated Damages, if any) to the date of
redemption and (ii) the sum of the present values of the remaining scheduled
payments of principal of the Securities to be redeemed and interest thereon
(including Additional Sums and Liquidated Damages, if any) discounted to the
date of redemption, on a semi-annual basis (assuming a 360-day year consisting
of twelve 30-day months), at the Treasury Rate plus 25 basis points plus accrued
and unpaid interest thereon to the date of redemption.

     "Other Debentures" means all junior subordinated debentures issued by the
Company from time to time and sold to trusts to be established by the Company
(if any), in each case similar to the Trust.

     "Other Guarantees" means all guarantees to be issued by the Company with
respect to capital securities (if any) and issued to other trusts to be
established by the Company (if any), in each case similar to the Trust.

     The term "outstanding" when used with reference to Securities, shall,
subject to the provisions of Section 7.04, mean, as of any particular time, all
Securities authenticated and delivered by the Trustee or the Authenticating
Agent under this Indenture, except

                                       7
<PAGE>
 
     (a)  Securities theretofore canceled by the Trustee or the Authenticating
          Agent or delivered to the Trustee for cancellation;

     (b)  Securities, or portions thereof, for the payment or redemption of
          which moneys in the necessary amount shall have been deposited in
          trust with the Trustee or with any paying agent (other than the
          Company) or shall have been set aside and segregated in trust by the
          Company (if the Company shall act as its own paying agent); provided
          that, if such Securities, or portions thereof, are to be redeemed
          prior to maturity thereof, notice of such redemption shall have been
          given as in Article XIV provided or provision satisfactory to the
          Trustee shall have been made for giving such notice; and

     (c)  Securities which have been paid or in lieu of or in substitution for
          which other Securities shall have been authenticated and delivered
          pursuant to the terms of Section 2.08 unless proof satisfactory to
          the Company and the Trustee is presented that any such Securities are
          held by bona fide holders in due course.

     "Person" shall mean any individual, corporation, estate, partnership, joint
venture, association, joint-stock company, limited liability company, trust,
unincorporated organization or government or any agency or political subdivision
thereof.

     "Predecessor Security" of any particular Security means every previous
Security evidencing all or a portion of the same debt and as that evidenced by
such particular Security; and, for the purposes of this definition, any Security
authenticated and delivered under Section 2.08 in lieu of a lost, destroyed or
stolen Security shall be deemed to evidence the same debt as the lost, destroyed
or stolen Security.

     "Principal Office of the Trustee", or other similar term, shall mean the
office of the Trustee, at which at any particular time its corporate trust
business shall be administered.

     "Private Exchange" has the meaning set forth in the Registration Rights
Agreement.

     "Private Exchange Securities" means the subordinated deferrable interest
debentures issued in a Private Exchange.

     "Purchase Agreement" shall mean the Purchase Agreement dated May 22, 1997
among the Company, NGC Corporation Capital Trust and the initial purchasers
named therein.

     "Property Trustee" shall have the same meaning as set forth in the
Declaration.

     "Redemption Price" means when used with respect to any Security to be
redeemed, the price at which it is to be redeemed pursuant to this Indenture.

                                       8
<PAGE>
 
     "Reference Treasury Dealer" means Lehman Brothers Inc. and its successors;
provided however, that if Lehman Brothers Inc. shall cease to be a primary U.S.
Government securities dealer in New York City (a "Primary Treasury Dealer"), the
Company shall substitute therefor another Primary Treasury Dealer.

     "Reference Treasury Dealer Quotations" means, with respect to the Reference
Treasury Dealer and any redemption date, the average, as determined by the
Trustee, of the bid and asked prices for the Comparable Treasury Issue
(expressed in each case as a percentage of its principal amount) quoted in
writing to the Trustee by the Reference Treasury Dealer at 5:00 p.m., New York
City time, on the third business day preceding such redemption date.

     "Registration Rights Agreement" means the Registration Rights Agreement,
dated as of May 28, 1997, by and among the Company, the Trust and the initial
purchasers named therein, as such agreement may be amended, modified or
supplemented from time to time.

     "Regulation S" means Regulation S under the Securities Act, as such
regulation may be amended from time to time, or any similar rule or regulation
hereafter adjusted by the Commission.

     "Responsible Officer" shall mean any officer of the Trustee with direct
responsibility for the administration of the Indenture and also means, with
respect to a particular corporate trust matter, any other officer to whom such
matter is referred because of his knowledge of and familiarity with the
particular subject.

     "Restricted Global Security" means any Global Security or Securities
evidencing Securities that are to be traded pursuant to Rule 144A.

     "Restricted Security" shall mean Securities that bear or are required to
bear the legends relating to transfer restrictions under the Securities Act set
forth in Exhibit A hereto.

     "Rule 144A" means Rule 144A under the Securities Act, as such Rule may be
amended from time to time, or under any similar rule or regulation hereafter
adopted by the Commission.

     "Securities" means, collectively, the Series A Securities, the Series B
Securities and Private Exchange Securities.

     "Securities Act" shall mean the Securities Act of 1933, as amended.

     "Securityholder", "holder of Securities", or other similar terms, shall
mean any Person in whose name at the time a particular Security is registered on
the register kept by the Company or the Trustee for that purpose in accordance
with the terms hereof.

                                       9
<PAGE>
 
     "Security Register" shall mean (i) prior to a Dissolution Event, the list
of holders provided to the Trustee pursuant to Section 4.01, and (ii) following
a Dissolution Event, any security register maintained by a security registrar
for the Securities appointed by the Company following the execution of a
supplemental indenture providing for transfer procedures as provided for in
Section 2.07(a).

     "Senior Indebtedness" shall mean all Indebtedness, whether outstanding on
the date of execution of this Indenture or hereafter created, assumed or
incurred, except Indebtedness Ranking on a Parity with the Securities or
Indebtedness Ranking Junior to the Securities, and any deferrals, renewals or
extensions of such Senior Indebtedness.

     "Series A Securities" means the Company's Series A 8.316% Subordinated
Deferrable Interest Debentures due June 1, 2027, as authenticated and issued
under this Indenture.

     "Series B Securities" means the Company's Series B 8.316% Subordinated
Deferrable Interest Debentures due June 1, 2027, as authenticated and issued
under this Indenture.

     "Special Event" means either an Investment Company Event or a Tax Event.

     "Special Event Redemption Price" shall mean, with respect to any redemption
of the Securities following a Special Event, an amount in cash equal to the
Make-Whole Amount.

     "Subsidiary" shall mean with respect to any Person, (i) any corporation at
least a majority of the outstanding voting stock of which is owned, directly or
indirectly, by such Person or by one or more of its Subsidiaries, or by such
Person and one or more of its Subsidiaries, (ii) any general partnership, joint
venture or similar entity, at least a majority of whose outstanding partnership
or similar interests shall at the time be owned by such Person, or by one or
more of its Subsidiaries, or by such Person and one or more of its Subsidiaries
and (iii) any limited partnership of which such Person or any of its
Subsidiaries is a general partner.  For the purposes of this definition, "voting
stock" means shares, interests, participations or other equivalents in the
equity interest (however designated) in such Person having ordinary voting power
for the election of a majority of the directors (or the equivalent) of such
Person, other than shares, interests, participations or other equivalents having
such power only by reason of the occurrence of a contingency.

     "Tax Event" shall mean the receipt by NGC Corporation Capital Trust and the
Company of an opinion of counsel experienced in such matters to the effect that,
as a result of any amendment to, or change (including any announced prospective
change) in, the laws or any regulations thereunder of the United States or any
political subdivision or taxing authority thereof or therein or as a result of
any official administrative pronouncement or judicial decision interpreting or
applying such laws or regulations, which amendment or change is effective or
which pronouncement or decision is announced on or after May 22, 1997, there is
more than an insubstantial risk that (i) NGC Corporation Capital Trust is, or
will be within 90 days of the date of such opinion, subject to  United States
Federal income tax with respect to income received or accrued on the Securities,
(ii) interest 

                                       10
<PAGE>
 
payable by the Company on the Securities is not, or within 90 days of the date
of such opinion, will not be, deductible by the Company, in whole or in part,
for United States Federal income tax purposes, or (iii) NGC Corporation Capital
Trust is, or will be within 90 days of the date of such opinion, subject to more
than a de minimis amount of other taxes, duties or other governmental charges.

     "Treasury Rate" means, with respect to any redemption date, the rate per
annum equal to the semiannual equivalent yield to maturity of the Comparable
Treasury Issue, assuming a price for the Comparable Treasury Issue (expressed as
a percentage of its principal amount) equal to the Comparable Treasury Price for
such redemption date.

     "Trust Indenture Act of 1939" shall mean the Trust Indenture Act of 1939 as
in force at the date of execution of this Indenture; provided, however, that, in
the event the Trust Indenture Act of 1939 is amended after such date, "Trust
Indenture Act of 1939" shall mean, to the extent required by any such amendment,
the Trust Indenture Act of 1939 as so amended.

     "Trustee" shall mean the Person identified as "Trustee" in the first
paragraph hereof, and, subject to the provisions of Article VI hereof, shall
also include its successors and assigns as Trustee hereunder.  The term
"Trustee" as used with respect to a particular series of the Securities shall
mean the trustee with respect to that series.

     "Trust Securities" shall mean the Capital Securities and the Common
Securities, collectively.

     "U.S. Government Obligations" shall mean securities that are (i) direct
obligations of the United States of America for the payment of which its full
faith and credit is pledged or (ii) obligations of a Person controlled or
supervised by and acting as an agency or instrumentality of the United States of
America the payment of which is unconditionally guaranteed as a full faith and
credit obligation by the United States of America, which, in either case under
clauses (i) or (ii) are not callable or redeemable at the option of the issuer
thereof, and shall also include a depository receipt issued by a bank or trust
company as custodian with respect to any such U.S. Government Obligation or a
specific payment of interest on or principal of any such U.S. Government
Obligation held by such custodian for the account of the holder of a depository
receipt; provided that (except as required by law) such custodian is not
authorized to make any deduction from the amount payable to the holder of such
depository receipt from any amount received by the custodian in respect of the
U.S. Government Obligation or the specific payment of interest on or principal
of the U.S. Government Obligation evidenced by such depository receipt.

      SECTION 1.02. Business Day Certificate.

       On the date of execution and delivery of this Indenture (with respect to
the remainder of calendar year 1997) and thereafter, within 15 days prior to the
end of each calendar year while this Indenture remains in effect (with respect
to the succeeding calendar years), the Company shall 

                                       11
<PAGE>
 
deliver to the Trustee an Officers' Certificate specifying the days on which
banking institutions or trust companies in Wilmington, Delaware or Chicago,
Illinois are authorized or obligated by law or executive order to be closed.

      SECTION 1.03. Form of Documents Delivered to Trustee.

     In any case where several matters are required to be certified by, or
covered by an opinion of, any specified Person, it is not necessary that all
such matters be certified by, or covered by the opinion of, only one such
Person, or that they be so certified or covered by only one document, but one
such Person may certify or give an opinion with respect to some matters and one
or more other such Persons as to other matters, and any such Person may certify
or give an opinion as to such matters in one or several documents.

     Any certificate or opinion of an officer of the Company may be based,
insofar as it relates to legal matters, upon a certificate or opinion of, or
representations by, counsel, unless such officer knows, or in the exercise of
reasonable care should know, that the certificate or opinion or representations
with respect to the matters upon which his certificate or opinion is based are
erroneous.  Any such certificate or opinion of counsel may be based, insofar as
it relates to factual matters, upon a certificate or opinion of, or
representations by, an officer or officers of the Company stating that the
information with respect to such factual matters is in the possession of the
Company, unless such counsel knows, or in the exercise of reasonable care should
know, that the certificate or opinion or representations with respect to such
matters are erroneous.

     Where any Person is required to make, give or execute two or more
applications, requests, consents, certificates, statements, opinions or other
instruments under this Indenture, they may, but need not, be consolidated and
form one instrument.


                                  ARTICLE II
                                  SECURITIES
                                  ----------

     SECTION 2.01. Forms Generally.

     The Securities and the Trustee's certificate of authentication shall be
substantially in the form of Exhibit A, the terms of which are incorporated in
and made a part of this Indenture.  The Securities may have notations, legends
or endorsements required by law, stock exchange rule, agreements to which the
Company is subject or usage.  Each Security shall be dated the date of its
authentication.  The Securities shall be issued in denominations of $1,000 and
integral multiples thereof.

     The Securities shall be redeemable as provided in Article XIV.

     The Securities shall be subordinated in right of payment to Senior
Indebtedness as provided in Article XV.

                                       12
<PAGE>
 
     SECTION 2.02. Execution and Authentication.

     The Securities shall be executed on behalf of the Company by its Chairman
of the Board, a Vice Chairman of the Board, its Chief Executive Officer, its
President or one of its Vice Presidents and attested by a Treasurer, Assistant
Treasurer, Secretary or an Assistant Secretary.  The signature of any such
person on the Securities may be manual or facsimile.  If an Officer whose
signature is on a Security no longer holds that office at the time the Security
is authenticated, the Security shall nevertheless be valid.

     A Security shall not be valid until authenticated by the manual signature
of the Trustee.  The signature of the Trustee shall be conclusive evidence that
the Security has been authenticated under this Indenture.  The form of Trustee's
certificate of authentication to be borne by the Securities shall be
substantially as set forth in Exhibit A hereto.

     The Trustee shall, upon a Company Order, authenticate for original issue up
to, and the aggregate principal amount of Securities outstanding at any time may
not exceed $206,200,000 aggregate principal amount of the Securities, except as
provided in Sections 2.07, 2.08, 2.09 and 14.05.  The series of Securities to be
initially issued hereunder shall be the Series A Securities.

     Each Security shall be dated the date of its authentication.

     SECTION 2.03. Form and Payment.

     Except as provided in Section 2.05, the Securities shall be issued in fully
registered certificated form without interest coupons.  Principal of, premium,
if any, and interest on the Securities issued in certificated form will be
payable, the transfer of such Securities will be registrable and such Securities
will be exchangeable for Securities bearing identical terms and provisions at
the office or agency of the Company maintained for such purpose under Section
3.02; provided, however, that payment of interest with respect to Securities
(other than a Global Security) may be made at the option of the Company (i) by
check mailed to the holder at such address as shall appear in the Security
Register or (ii) by transfer to an account maintained by the Person entitled
thereto; provided that proper transfer instructions have been received in
writing by the relevant record date.  Notwithstanding the foregoing, so long as
the holder of any Securities is the Property Trustee, the payment of the
principal of, premium, if any, and interest (including Compounded Interest and
Additional Sums, if any) and Liquidated Damages, if any, on such Securities held
by the Property Trustee will be made at such place and to such account as may be
designated by the Property Trustee.

     SECTION 2.04. Legends.

                                       13
<PAGE>
 
          (a) Except as permitted by subsection (b) of this Section 2.04 or as
     otherwise determined by the Company in accordance with applicable law, each
     Security shall bear the applicable legends relating to restrictions on
     transfer pursuant to the securities laws in substantially the form set
     forth on Exhibit A hereto.

          (b) In the event of an Exchange Offer, the Company shall issue and the
     Trustee, upon Company Order, shall authenticate Series B Securities in
     exchange for Series A Securities accepted for exchange in the Exchange
     Offer (and, if required pursuant to the Registration Rights Agreement, the
     Private Exchange Securities), which Series B Securities shall not bear the
     legends required by subsection (a) above (other than the legend dealing
     with the restriction referred to in Section 2.07(a)(ii) of this Indenture),
     in each case unless the holder of such Series A Securities is either (A) a
     broker dealer who purchased such Series A Securities directly from the
     Company for resale pursuant to Rule 144A or any other available exemption
     under the Securities Act, (B) a Person participating in the distribution of
     the Series A Securities or (C) a Person who is an Affiliate of the Company.

          (c) Upon any sale or transfer of any Restricted Security (including
     any interest in a Global Security) (i) that is effected pursuant to an
     effective registration statement under the Securities Act or (ii) in
     connection with which the Trustee receives certificates and other
     information (including an opinion of counsel, if requested) reasonably
     acceptable to the Company and the Trustee to the effect that such Security
     will no longer be subject to the resale restrictions under federal and
     state securities laws, then (A) in the case of a Restricted Security in
     definitive form, the Security Registrar or co-registrar shall permit the
     holder thereof to exchange such Restricted Security for a security that
     does not bear the legend (other than the legend dealing with the
     restriction referred to in Section 2.07(a)(ii) of this Indenture), and
     shall rescind any such restrictions on transfer and (B) in the case of
     Restricted Securities represented by a Global Security, such Security shall
     no longer be subject to the restrictions contained in the legend referenced
     in Section 2.04(a) (but still subject to the other provisions hereof).  In
     addition, any Security (or security issued in exchange or substitution
     therefor) as to which the restrictions on transfer described in the legend
     referenced in Section 2.04(a) have expired by their terms, may, upon
     surrender thereof (in accordance with the terms of this Indenture) together
     with such certifications and other information (including an opinion of
     counsel having substantial experience in practice under the Securities Act
     and otherwise reasonably acceptable to the Company, addressed to the
     Company and the Trustee and in a form acceptable to the Company, to the
     effect that the transfer of such Restricted Security has been made in
     compliance with Rule 144 or such successor provision) acceptable to the
     Company and the Trustee as either of them may reasonably require, be
     exchanged for a new Security or Securities of like tenor and aggregate
     principal amount, which shall not bear the restrictive legends referenced
     in Section 2.04(a).

     SECTION 2.05. Global Security.

                                       14
<PAGE>
 
          (a) In connection with a Dissolution Event,

               (i) if any Capital Securities are held in book-entry form, the
          related Definitive Securities shall be presented to the Trustee (if an
          arrangement with the Depositary has been maintained) by the Property
          Trustee in exchange for one or more Global Securities (as may be
          required pursuant to Section 2.07) in an aggregate principal amount
          equal to the aggregate principal amount of all outstanding Securities,
          to be registered in the name of the Depositary, or its nominee, and
          delivered by the Trustee to the Depositary for crediting to the
          accounts of its participants pursuant to the instructions of the
          Administrative Trustees; the Company upon any such presentation
          shall execute one or more Global Securities in such aggregate
          principal amount and deliver the same to the Trustee for
          authentication and delivery in accordance with this Indenture; and
          payments on the Securities issued as a Global Security will be made to
          the Depositary; and

               (ii) if any Capital Securities are held in certificated form, the
          related Definitive Securities may be presented to the Trustee by the
          Property Trustee and any Capital Security certificate which represents
          Capital Securities other than Capital Securities in book-entry form
          ("Non Book-Entry Capital Securities") will be deemed to represent
          beneficial interests in Securities presented to the Trustee by the
          Property Trustee having an aggregate principal amount equal to the
          aggregate liquidation amount of the Non Book-Entry Capital Securities
          until such Capital Security certificates are presented to the
          Security Registrar for transfer or reissuance, at which time such
          Capital Security certificates will be canceled and a Security,
          registered in the name of the holder of the Capital Security
          certificate or the transferee of the holder of such Capital Security
          certificate, as the case may be, with an aggregate principal amount
          equal to the aggregate liquidation amount of the Capital Security
          certificate canceled, will be executed by the Company and delivered to
          the Trustee for authentication and delivery in accordance with this
          Indenture.  Upon the issuance of such Securities, Securities with an
          equivalent aggregate principal amount that were presented by the
          Property Trustee to the Trustee will be canceled.

          (b) The Global Securities shall represent the aggregate amount of
     outstanding Securities from time to time endorsed thereon; provided, that
     the aggregate amount of outstanding Securities represented thereby may
     from time to time be reduced or increased, as appropriate, to reflect
     exchanges and redemptions.  Any endorsement of a Global Security to reflect
     the amount of any increase or decrease in the amount of outstanding
     Securities represented thereby shall be made by the Trustee, in accordance
     with instructions given by the Company as required by this Section 2.05.

          (c) The Global Securities may be transferred, in whole but not in
     part, only to the Depositary, another nominee of the Depositary, or to a
     successor Depositary selected or approved by the Company or to a nominee of
     such successor Depositary.

                                       15
<PAGE>
 
          (d) If at any time the Depositary notifies the Company that it is
     unwilling or unable to continue as Depositary or the Depositary has ceased
     to be a clearing agency registered under the Exchange Act or any applicable
     statute or regulation, and a successor Depositary is not appointed by the
     Company within 90 days after the Company receives such notice or becomes
     aware of such condition, as the case may be, the Company will execute, and
     the Trustee, upon receipt of a Company Order, will authenticate and make
     available for delivery the Definitive Securities, in authorized
     denominations, and in an aggregate principal amount equal to the principal
     amount of the Global Security in exchange for such Global Security.  If
     there is an Event of Default, the Depositary shall have the right to
     exchange the Global Securities for Definitive Securities.  In addition, the
     Company may at any time determine that the Securities shall no longer be
     represented by a Global Security.  In the event of such an Event of Default
     or such a determination, the Company shall execute, and subject to Section
     2.07, the Trustee, upon receipt of an Officers' Certificate evidencing such
     determination by the Company and a Company Order, will authenticate and
     make available for delivery the Definitive Securities, in authorized
     denominations, and in an aggregate principal amount equal to the principal
     amount of the Global Security in exchange for such Global Security.  Upon
     the exchange of the Global Security for such Definitive Securities, in
     authorized denominations, the Global Security shall be canceled by the
     Trustee.  Such Definitive Securities issued in exchange for the Global
     Security shall be registered in such names and in such authorized
     denominations as the Depositary, pursuant to instructions from its direct
     or indirect participants or otherwise, shall instruct the Trustee.  The
     Trustee shall deliver such Definitive Securities to the Depositary for
     delivery to the Persons in whose names such Definitive Securities are so
     registered.

     SECTION 2.06. Interest.

          (a) Each Security will bear interest at the rate of 8.316% per annum
     (the "Coupon Rate") from the most recent date to which interest has been
     paid or duly provided for or, if no interest has been paid or duly provided
     for, from May 28, 1997, until the principal thereof becomes due and
     payable, and at the Coupon Rate on any overdue principal (and premium, if
     any) and (to the extent that payment of such interest is enforceable under
     applicable law) on any overdue installment of interest, compounded semi-
     annually, payable (subject to the provisions of Article XVI) semi-annually
     in arrears on June 1 and December 1 of each year (each, an "Interest
     Payment Date") commencing on December 1, 1997, to the Person in whose name
     such Security or any predecessor Security is registered, at the close of
     business on the regular record date for such interest installment, which
     shall be the fifteenth day of the month immediately preceding the month in
     which the relevant Interest Payment Date falls.

          (b) Interest will be computed on the basis of a 360-day year
     consisting of twelve 30-day months and, for any period of less than a full
     calendar month, the number of days 

                                       16
<PAGE>
 
     lapsed in such month. In the event that any Interest Payment Date falls on
     a day that is not a Business Day, then payment of interest payable on such
     date will be made on the next succeeding day which is a Business Day (and
     without any interest or other payment in respect of any such delay), except
     that if such next succeeding Business Day falls in the next succeeding
     calendar year, then such payment shall be made on the immediately preceding
     Business Day, in each case with the same force and effect as if made on the
     date such payment is otherwise required.

          (c) During such time as the Property Trustee is the holder of any
     Securities, the Company shall pay any additional amounts on the Securities
     as may be necessary in order that the amount of Distributions then due and
     payable by NGC Corporation Capital Trust on the outstanding Trust
     Securities shall not be reduced as a result of any additional taxes, duties
     and other governmental charges to which NGC Corporation Capital Trust has
     become subject as a result of a Tax Event ("Additional Sums").

          (d) Interest on any Security which is payable, and which is punctually
     paid or duly provided for, on any Interest Payment Date shall be paid to
     the Person is whose name that Security (or one or more Predecessor
     Securities) is registered at the close of business on the record date for
     such interest installment.

     SECTION 2.07. Transfer and Exchange.

          (a) Transfer Restrictions.  (i) The Series A Securities, and those
     Series B Securities with respect to which any Person described in Section
     2.04(b)(A), (B) or (C) is the beneficial owner, may not be transferred
     except in compliance with the legends contained in Exhibit A unless
     otherwise determined by the Company in accordance with applicable law.
     Upon any distribution of the Securities following a Dissolution Event, the
     Company and the Trustee shall enter into a supplemental indenture pursuant
     to Section 9.01 to provide for the transfer restrictions and procedures
     with respect to the Securities substantially similar to those contained in
     the Declaration to the extent applicable in the circumstances existing at
     such time.

               (ii) The Securities will be issued and may be transferred only in
          blocks having an aggregate principal amount of not less than $100,000.
          Any such transfer of the Securities in a block having an aggregate
          principal amount of less than $100,000 shall be deemed to be void and
          of no legal effect whatsoever.  Any such transferee shall be deemed
          not to be a holder of such Securities for any purpose, including, but
          not limited to the receipt of payments on such Securities, and such
          transferee shall be deemed to have no interest whatsoever in such
          Securities.

          (b) General Provisions Relating to Transfers and Exchanges.  To permit
     registrations of transfers and exchanges, the Company shall execute and the
     Trustee shall 

                                       17
<PAGE>
 
     authenticate Definitive Securities and Global Securities in any authorized
     denomination and of a like aggregate principal amount at the Security
     Registrar's request. All Definitive Securities and Global Securities issued
     upon any registration of transfer or exchange of Definitive Securities or
     Global Securities shall be the valid obligations of the Company, evidencing
     the same debt, and entitled to the same benefits under this Indenture, as
     the Definitive Securities or Global Securities surrendered upon such
     registration of transfer or exchange.

     Every Security presented or surrendered for registration of transfer or for
exchange shall (if so required by the Company or the Trustee) be duly endorsed
or be accompanied by a written instrument of transfer in a form that is
satisfactory to the Company and the Security Registrar duly executed by the
holder thereof or his attorney duly authorized in writing.

     No service charge shall be made to a holder for any registration of
transfer or exchange, but the Company may require payment of a sum sufficient to
cover any transfer tax or similar governmental charge payable in connection
therewith.

     The Company shall not be required to (i) issue, register the transfer of or
exchange Securities during a period beginning at the opening of business 15 days
before the day of mailing of a notice of redemption or any notice of selection
of Securities for redemption under Article XIV hereof and ending at the close of
business on the day of such mailing; or (ii) register the transfer of or
exchange any Security so selected for redemption in whole or in part, except the
unredeemed portion of any Security being redeemed in part.

     So long as the Securities are eligible for book-entry settlement with the
Depositary, or unless otherwise required by law, all Securities to be traded in
the PORTAL Market shall be represented by a Restricted Global Security
registered in the name of the Depositary or the nominee of the Depositary.

     The transfer and exchange of beneficial interests in a Global Security,
which does not involve the issuance of a Definitive Security or the transfer of
an interest to another Global Security, shall be effected through the Depositary
(but not the Trustee) in accordance with the Indenture (including the
restrictions on transfers set forth herein) and the procedures of the Depositary
therefore.  The Trustee will not have any responsibility for the transfer and
exchange of beneficial interests in such Global Security that does not involve
the issuance of a Definitive Security or the transfer of interests to another
Global Security.

     Prior to due presentment for the registration of a transfer of any
Security, the Trustee, the Company and any agent of the Trustee or the Company
may deem and treat the Person in whose name any Security is registered as the
absolute owner of such Security for the purpose of receiving payment of
principal of and premium, if any, and interest on such Securities, neither the
Trustee, nor the Company nor any agent of the Trustee or the Company shall be
affected by notice to the contrary.

                                       18
<PAGE>
 
          (c) Exchange of Series A Securities for Series B Securities or Private
     Exchange Securities.  The Series A Securities may be exchanged for Series B
     Securities pursuant to the terms of the Exchange Offer.  In addition, the
     Series A Securities may be exchanged in a Private Exchange for Private
     Exchange Securities under the circumstances described in the Registration
     Rights Agreement.  The Trustee shall make such exchange as follows:

     The Company shall present the Trustee with an Officers' Certificate
     certifying the following:

               (i) upon issuance of the Series B Securities or the Private
          Exchange Securities, as the case may be, the transactions contemplated
          by the Exchange Offer or the Private Exchange have been consummated;
          and

               (ii) the principal amount of Series A Securities properly
          tendered in the Exchange Offer and the Private Exchange  that are
          represented by a Global Security and the principal amount of Series A
          Securities properly tendered in the Exchange Offer and the Private
          Exchange that are represented by Definitive Securities, the name of
          each holder of such Definitive Securities, the principal amount
          properly tendered in the Exchange Offer or Private Exchange, as the
          case may be, by each such holder and the name and address to which
          Definitive Securities for Series B Securities or the Private Exchange
          Securities, as the case may be, shall be registered and sent for each
          such holder.

     The Trustee, upon receipt of (i) such Officers' Certificate, (ii) an
Opinion of Counsel (x) to the effect that the Series B Securities or the Private
Exchange Securities, as the case may be, have been registered under Section 5 of
the Securities Act and the Indenture has been qualified under the Trust
Indenture Act and (y) with respect to the matters set forth in Section 3(p) of
the Registration Rights Agreement and (iii) a Company Order, shall authenticate
(A) a Global Security representing Series B Securities and the Private Exchange
Securities, as the case may be, in aggregate principal amount equal to the
aggregate principal amount of Series A Securities represented by a Global
Security indicated in such Officers' Certificate as having been properly
tendered and (B) Definitive Securities representing Series B Securities and the
Private Exchange Securities, as the case may be, registered in the names of, and
in the principal amounts indicated in, such Officers' Certificate.

     If the principal amount of the Global Security for the Series B Securities
or the Private Exchange Securities is less than the principal amount of the
Global Security for the Series A Securities, the Trustee shall make an
endorsement on such Global Security for Series A Securities indicating a
reduction in the principal amount represented thereby.

     The Trustee shall deliver such Definitive Securities representing Series B
Securities to the holders thereof as indicated in such Officers' Certificate.

                                       19
<PAGE>
 
     SECTION 2.08. Replacement Securities.

     If any mutilated Security is surrendered to the Trustee, or the Company and
the Trustee receive evidence to their satisfaction of the destruction, loss or
theft of any Security, the Company shall issue and the Trustee shall
authenticate a replacement Security if the Trustee's requirements for
replacements of Securities are met.  An indemnity bond must be supplied by the
holder that is sufficient in the judgment of the Trustee and the Company to
protect the Company, the Trustee, any agent thereof or any authenticating agent
from any loss that any of them may suffer if a Security is replaced.  The
Company or the Trustee may charge for its expenses in replacing a Security.

     Every replacement Security is an obligation of the Company and shall be
entitled to all of the benefits of this Indenture equally and proportionately
with all other Securities duly issued hereunder.

     SECTION 2.09. Temporary Securities.

     Pending the preparation of Definitive Securities, the Company may execute,
and upon Company Order the Trustee shall authenticate and make available for
delivery, temporary Securities that are printed, lithographed, typewritten,
mimeographed or otherwise reproduced, in any authorized denomination,
substantially of the tenor of the Definitive Securities in lieu of which they
are issued and with such appropriate insertions, omissions, substitutions and
other variations as the officers executing such Securities may determine, as
conclusively evidenced by their execution of such Securities.

     If temporary Securities are issued, the Company shall cause Definitive
Securities to be prepared without unreasonable delay.  The Definitive Securities
shall be printed, lithographed or engraved, or provided by any combination
thereof, or in any other manner permitted by the rules and regulations of any
applicable securities exchange, all as determined by the officers executing such
Definitive Securities.  After the preparation of Definitive Securities, the
temporary Securities shall be exchangeable for Definitive Securities upon
surrender of the temporary Securities at the office or agency maintained by the
Company for such purpose pursuant to Section 3.02 hereof, without charge to the
Holder.  Upon surrender for cancellation of any one or more temporary
Securities, the Company shall execute, and the Trustee shall authenticate and
make available for delivery, in exchange therefor the same aggregate principal
amount of Definitive Securities of authorized denominations.  Until so
exchanged, the temporary Securities shall in all respects be entitled to the
same benefits under this Indenture as Definitive Securities.

     SECTION 2.10. Cancellation.

     The Company at any time may deliver Securities to the Trustee for
cancellation.  The Trustee and no one else shall cancel all Securities
surrendered for registration of transfer, exchange, payment, replacement or
cancellation and shall retain or destroy canceled Securities in accordance 

                                       20
<PAGE>
 
with its normal practices (subject to the record retention requirement of the
Exchange Act) unless the Company directs them to be returned to it. The Company
may not issue new Securities to replace Securities that have been redeemed or
paid or that have been delivered to the Trustee for cancellation.

     SECTION 2.11. Defaulted Interest.

     Any interest on any Security that is payable, but is not punctually paid or
duly provided for, on any Interest Payment Date (herein called "Defaulted
Interest") shall forthwith cease to be payable to the holder on the relevant
regular record date by virtue of having been such holder; and such Defaulted
Interest shall be paid by the Company, at its election, as provided in clause
(a) or clause (b) below:

          (a) The Company may make payment of any Defaulted Interest on
     Securities to the Persons in whose names such Securities (or their
     respective Predecessor Securities) are registered at the close of business
     on a special record date for the payment of such Defaulted Interest, which
     shall be fixed in the following manner: the Company shall notify the
     Trustee in writing of the amount of Defaulted Interest proposed to be paid
     on each such Security and the date of the proposed payment, and at the same
     time the Company shall deposit with the Trustee an amount of money equal to
     the aggregate amount proposed to be paid in respect of such Defaulted
     Interest or shall make arrangements satisfactory to the Trustee for such
     deposit prior to the date of the proposed payment, such money when
     deposited to be held in trust for the benefit of the Persons entitled to
     such Defaulted Interest as in this clause provided.  Thereupon the Trustee
     shall fix a special record date for the payment of such Defaulted Interest
     which shall not be more than 15 nor less than 10 days prior to the date of
     the proposed payment and not less than 10 days after the receipt by the
     Trustee of the notice of the proposed payment.  The Trustee shall promptly
     notify the Company of such special record date and, in the name and at the
     expense of the Company, shall cause notice of the proposed payment of such
     Defaulted Interest and the special record date therefor to be mailed, first
     class postage prepaid, to each Securityholder at his or her address as it
     appears in the Security Register, not less than 10 days prior to such
     special record date.  Notice of the proposed payment of such Defaulted
     Interest and the special record date therefor having been mailed as
     aforesaid, such Defaulted Interest shall be paid to the Persons in whose
     names such Securities (or their respective Predecessor Securities) are
     registered on such special record date and shall be no longer payable
     pursuant to the following clause (b).

          (b) The Company may make payment of any Defaulted Interest on any
     Securities in any other lawful manner not inconsistent with the
     requirements of any securities exchange on which such Securities may be
     listed, and upon such notice as may be required by such exchange, if, after
     notice given by the Company to the Trustee of the proposed payment pursuant
     to this clause, such manner of payment shall be deemed practicable by the
     Trustee.

                                       21
<PAGE>
 
     SECTION 2.12. CUSIP Numbers.

     The Company in issuing the Securities may use "CUSIP" numbers (if then
generally in use), and, if so, the Trustee shall use "CUSIP" numbers in notices
of redemption as a convenience to Securityholders; provided that any such notice
may state that no representation is made as to the correctness of such numbers
either as printed on the Securities or as contained in any notice of a
redemption and that reliance may be placed only on the other identification
numbers printed on the Securities, and any such redemption shall not be affected
by any defect in or omission of such numbers.  The Company will promptly notify
the Trustee of any change in the CUSIP numbers.


                                  ARTICLE III
                      PARTICULAR COVENANTS OF THE COMPANY
                      -----------------------------------

     SECTION 3.01. Payment of Principal, Premium and Interest.

     The Company covenants and agrees for the benefit of the holders of the
Securities that it will duly and punctually pay or cause to be paid the
principal of, premium, if any, and interest on the Securities at the place, at
the respective times and in the manner provided herein.  The Company further
covenants to pay any and all amounts, including, without limitation, Additional
Sums, as may be required pursuant to Section 2.06(c), Liquidated Damages, if
any, on the dates and in the manner required under the Registration Rights
Agreement and Compounded Interest, as may be required pursuant to Section 16.01.

     SECTION 3.02. Offices for Notices and Payments, etc.

     So long as any of the Securities remain outstanding, the Company will
maintain in the Borough of Manhattan, The City of New York, an office or agency
where the Securities may be presented for payment, an office or agency where the
Securities may be presented for registration of transfer and for exchange as in
this Indenture provided and an office or agency where notices and demands to or
upon the Company in respect of the Securities or of this Indenture may be
served. The Company will give to the Trustee written notice of the location of
any such office or agency and of any change of location thereof.  Until
otherwise designated from time to time by the Company in a notice to the
Trustee, any such office or agency for all of the above purposes shall be the
Principal Office of the Trustee.  In case the Company shall fail to maintain any
such office or agency in the Borough of Manhattan, The City of New York, or
shall fail to give such notice of the location or of any change in the location
thereof, presentations and demands may be made and notices may be served at the
Principal Office of the Trustee.

     In addition to any such office or agency, the Company may from time to time
designate one or more offices or agencies outside the Borough of Manhattan, The
City of New York, where the Securities may be presented for payment,
registration of transfer and for exchange in the manner 

                                       22
<PAGE>
 
provided in this Indenture, and the Company may from time to time rescind such
designation, as the Company may deem desirable or expedient; provided, however,
that no such designation or rescission shall in any manner relieve the Company
of its obligation to maintain any such office or agency in the Borough of
Manhattan, The City of New York, for the purposes above mentioned. The Company
will give to the Trustee prompt written notice of any such designation or
rescission thereof.

     SECTION 3.03. Appointments to Fill Vacancies in Trustee's Office.

     The Company, whenever necessary to avoid or fill a vacancy in the office of
Trustee, will appoint, in the manner provided in Section 6.10, a Trustee, so
that there shall at all times be a Trustee hereunder.

     SECTION 3.04. Provision as to Paying Agent.

          (a) If the Company shall appoint a paying agent other than the Trustee
     with respect to the Securities, it will cause such paying agent to execute
     and deliver to the Trustee an instrument in which such agent shall agree
     with the Trustee, subject to the provision of this Section 3.04,

               (i) that it will hold all sums held by it as such agent for the
          payment of the principal of and premium, if any, or interest on the
          Securities (whether such sums have been paid to it by the Company or
          by any other obligor on the Securities) in trust for the benefit of
          the holders of the Securities and comply with all provisions of the
          Trust Indenture Act applicable to it as paying agent; and

               (ii) that it will give the Trustee notice of any failure by the
          Company (or by any other obligor on the Securities) to make any
          payment of the principal of and premium or interest (including
          Additional Sums and Compounded Interest, if any) and Liquidated
          Damages, if any, on the Securities when the same shall be due and
          payable.

          (b) If the Company shall act as its own paying agent, it will, on or
     before each due date of the principal of and premium, if any, or interest
     on the Securities, set aside, segregate and hold in trust for the benefit
     of the holders of the Securities a sum sufficient to pay such principal,
     premium or interest so becoming due and will notify the Trustee of any
     failure to take such action and of any failure by the Company (or by any
     other obligor under the Securities) to make any payment of the principal of
     and premium, if any, or interest on the Securities when the same shall
     become due and payable.

          (c) Anything in this Section 3.04 to the contrary notwithstanding, the
     Company may, at any time, for the purpose of obtaining a satisfaction and
     discharge with respect to the 

                                       23
<PAGE>
 
     Securities hereunder, or for any other reason, pay or cause to be paid to
     the Trustee all sums held in trust for such Securities by the Trustee or
     any paying agent hereunder, as required by this Section 3.04, such sums to
     be held by the Trustee upon the trusts herein contained.

          (d) Anything in this Section 3.04 to the contrary notwithstanding, the
     agreement to hold sums in trust as provided in this Section 3.04 is subject
     to Sections 11.03 and 11.04.

     SECTION 3.05. Certificate to Trustee.

     The Company will deliver to the Trustee on or before 120 days after the end
of each fiscal year in each year, commencing with the first fiscal year ending
after the date hereof, so long as Securities are outstanding hereunder, an
Officers' Certificate, one of the signers of which shall be the principal
executive, principal financial or principal accounting officer of the Company,
stating that in the course of the performance by the signers of their duties as
officers of the Company they would normally have knowledge of any default by the
Company in the performance of any covenants contained herein, stating whether or
not they have knowledge of any such default and, if so, specifying each such
default of which the signers have knowledge and the nature thereof.

     SECTION 3.06. Compliance with Consolidation Provisions.

     The Company will not, while any of the Securities remain outstanding,
consolidate with, or merge into, or merge into itself, or sell or convey all or
substantially all of its property to any other Person unless the provisions of
Article X hereof are complied with.

     SECTION 3.07. Limitation on Dividends.

     The Company will not, and will not permit any subsidiary of the Company to,
(i) declare or pay any dividends or distributions on, or redeem, purchase,
acquire, or make a liquidation payment with respect to, any of the Company's
capital stock (which includes common and preferred stock) (other than (a)
dividends or distributions in shares of, or options, warrants or rights to
subscribe for or purchase shares of, Common Stock of the Company, (b) any
declaration of a dividend in connection with the implementation of a
shareholder's rights plan, or the issuance of stock under any such plan in the
future, or the redemption or repurchase of any such rights pursuant thereto, (c)
as a result of a reclassification of the Company's capital stock or the exchange
or the conversion of one class or series of the Company's capital stock for
another class or series of the Company's capital stock, (d) the purchase of
fractional interests in shares of the Company's capital stock pursuant to the
conversion or exchange provisions of such capital stock or the security being
converted or exchanged, and (e) purchases of Common Stock related to the
issuance of Common Stock or rights under any of the Company's benefit plans for
its directors, officers, employees or consultants or any of the Company's
dividend reinvestment plans), (ii) make any payment of principal, premium, if
any, or interest on or repay or repurchase or redeem any debt securities of the
Company (including Other Debentures) that rank pari passu with or junior in
right of payment to the Securities or (iii) make any 

                                       24
<PAGE>
 
guarantee payments with respect to any guarantee by the Company (other than
payments under the Capital Securities Guarantee) of the debt securities of any
Subsidiary of the Company (including Other Guarantees) if such guarantee ranks
pari passu or junior in right of payment to the Securities if at such time (1)
there shall have occurred any event of which the Company has actual knowledge
that (a) with the giving of notice or the lapse of time, or both, would
constitute an Event of Default and (b) in respect of which the Company shall not
have taken reasonable steps to cure, (2) an Event of Default hereunder shall
have occurred and be continuing, (3) if such Securities are held by the Property
Trustee, the Company shall be in default with respect to its payment obligations
under the Capital Securities Guarantee or (4) the Company shall have given
notice of its election of the exercise of its right to extend the interest
payment period pursuant to Section 16.01 and any such extension shall be
continuing.

     SECTION 3.08. Covenants as to NGC Corporation Capital Trust

     In the event Securities are issued to NGC Corporation Capital Trust or a
trustee of such trust in connection with the issuance of Trust Securities by NGC
Corporation Capital Trust, for so long as such Trust Securities remain
outstanding, the Company (i) will maintain 100% direct or indirect ownership of
the Common Securities of NGC Corporation Capital Trust; provided, however, that
any successor of the Company, permitted pursuant to Article X, may succeed to
the Company's ownership of such Common Securities, (ii) will use its reasonable
efforts to cause NGC Corporation Capital Trust (a) to remain a business trust,
except in connection with a distribution of Securities to the holders of Trust
Securities in liquidation of the Trust, the redemption of all of the Trust
Securities of NGC Corporation Capital Trust or certain mergers, consolidations
or amalgamations, each as permitted by the Declaration of NGC Corporation
Capital Trust, and (b) to otherwise continue to be treated as a grantor trust
and not an association taxable as a corporation for United States federal income
tax purposes and (iii) use its reasonable efforts to cause each holder of Trust
Securities to be treated as owning an undivided beneficial interest in the
Securities.

     SECTION 3.09. Payment of Trust's Costs and Expenses.

     In connection with the offering, sale and issuance of the Securities to NGC
Corporation Capital Trust and in connection with the sale of the Trust
Securities by NGC Corporation Capital Trust, the Company, in its capacity as
borrower with respect to the Securities, shall:

          (a) pay all costs and expenses relating to the offering, sale and
     issuance of the Securities, including commissions to the initial purchaser
     payable pursuant to the Purchase Agreement, fees and expenses in connection
     with any exchange offer, filing of a shelf registration statement or other
     action to be taken pursuant to the Registration Rights Agreement and
     compensation of the Trustee in accordance with the provisions of Section
     6.06;

                                       25
<PAGE>
 
          (b) pay all costs and expenses of the Trust (including, but not
     limited to, costs and expenses relating to the organization of NGC
     Corporation Capital Trust, the offering, sale and issuance of the Trust
     Securities (including commissions to the initial purchaser in connection
     therewith), the fees and expenses of the Property Trustee and the Delaware
     Trustee, the costs and expenses relating to the operation of NGC
     Corporation Capital Trust, including without limitation, costs and expenses
     of accountants, attorneys, statistical or bookkeeping services, expenses
     for printing and engraving and computing or accounting equipment, paying
     agent(s), registrar(s), transfer agent(s), duplicating, travel and
     telephone and other telecommunications expenses and costs and expenses
     incurred in connection with the acquisition, financing, and disposition of
     assets of NGC Corporation Capital Trust;

          (c) be primarily and fully liable for any indemnification obligations
     arising with respect to the Declaration;

          (d) pay any and all taxes, duties, assessments or governmental charges
     of whatever nature (other than United States withholding taxes attributable
     to NGC Corporation Capital Trust or its assets) imposed on the Trust by the
     United States, or any other taxing authority, and all liabilities, costs
     and expenses with respect to such taxes of the Trust; and

          (e) pay all other fees, expenses, debts and obligations (other than in
     respect of principal, interest and premium, if any, on the Trust
     Securities) related to NGC Corporation Capital Trust.

     The foregoing obligations of the Company are for the benefit of, and shall
be enforceable by, any person to whom any such debt, obligations, costs,
expenses and taxes are owed (each, a "Creditor") whether or not such Creditor
has received notice thereof.  Any such Creditor may enforce such obligations of
the Company directly against the Company, and the Company irrevocably waives any
right or remedy to require that any such Creditor take any action against the
Trust or any person before proceeding against the Company.  The Company shall
execute such additional agreements as may be necessary or desirable to give full
effect to the foregoing.

     SECTION 3.10. Payment Upon Resignation or Removal.

     Upon termination of this Indenture or the removal or resignation of the
Trustee, unless otherwise stated, the Company shall pay to the Trustee all
amounts accrued and owing to the date of such termination, removal or
resignation.  Upon termination of the Declaration or the removal or resignation
of the Delaware Trustee or the Property Trustee, as the case may be, pursuant to
Section 5.7 of the Declaration, the Company shall pay to the Delaware Trustee or
the Property Trustee, as the case may be, all amounts accrued and owing to the
date of such termination, removal or resignation.


                                  ARTICLE IV

                                       26
<PAGE>
 
                   SECURITYHOLDERS' LISTS AND REPORTS BY THE
                   -----------------------------------------
                            COMPANY AND THE TRUSTEE
                            -----------------------

     SECTION 4.01. Securityholders' Lists.

     The Company covenants and agrees that it will furnish or cause to be
furnished to the Trustee:

          (a) on a semi-annual basis on each regular record date for the
     Securities, a list, in such form as the Trustee may reasonably require, of
     the names and addresses of the Securityholders as of such record date; and

          (b) at such other times as the Trustee may request in writing, within
     30 days after the receipt by the Company, of any such request, a list of
     similar form and content as of a date not more than 15 days prior to the
     time such list is furnished, except that, no such lists need be furnished
     so long as the Trustee is in possession thereof by reason of its acting as
     Security registrar.

     SECTION 4.02. Preservation and Disclosure of Lists.

          (a) The Trustee shall preserve, in as current a form as is reasonably
     practicable, all information as to the names and addresses of the holders
     of the Securities (1) contained in the most recent list furnished to it as
     provided in Section 4.01 or (2) received by it in the capacity of
     Securities registrar (if so acting) hereunder.  The Trustee may destroy any
     list furnished to it as provided in Section 4.01 upon receipt of a new list
     so furnished.
 
          (b) In case three or more holders of Securities (hereinafter referred
     to as "applicants") apply in writing to the Trustee and furnish to the
     Trustee reasonable proof that each such applicant has owned a Security for
     a period of at least six months preceding the date of such application, and
     such application states that the applicants desire to communicate with
     other holders of Securities or with holders of all Securities with respect
     to their rights under this Indenture and is accompanied by a copy of the
     form of proxy or other communication which such applicants propose to
     transmit, then the Trustee shall within five Business Days after the
     receipt of such application, at its election, either:

               (i) afford such applicants access to the information preserved at
          the time by the Trustee in accordance with the provisions of
          subsection (a) of this Section 4.02, or

               (ii) inform such applicants as to the approximate number of
          holders of all Securities, whose names and addresses appear in the
          information preserved at the time by the Trustee in accordance with
          the provisions of subsection (a) of this 

                                       27
<PAGE>
 
          Section 4.02, and as to the approximate cost of mailing to such
          Securityholders the form of proxy or other communication, if any,
          specified in such application.

               If the Trustee shall elect not to afford such applicants access
          to such information, the Trustee shall, upon the written request of
          such applicants, mail to each Securityholder whose name and address
          appear in the information preserved at the time by the Trustee in
          accordance with the provisions of subsection (a) of this Section 4.02
          a copy of the form of proxy or other communication which is specified
          in such request with reasonable promptness after a tender to the
          Trustee of the material to be mailed and of payment, or provision for
          the payment, of the reasonable expenses of mailing, unless within five
          days after such tender, the Trustee shall mail to such applicants and
          file with the Commission, together with a copy of the material to be
          mailed, a written statement to the effect that, in the opinion of the
          Trustee, such mailing would be contrary to the best interests of the
          holders of Securities of such series or all Securities, as the case
          may be, or would be in violation of applicable law. Such written
          statement shall specify the basis of such opinion.  If the Commission,
          after opportunity for a hearing upon the objections specified in the
          written statement so filed, shall enter an order refusing to sustain
          any of such objections or if, after the entry of an order sustaining
          one or more of such objections, the Commission shall find, after
          notice and opportunity for hearing, that all the objections so
          sustained have been met and shall enter an order so declaring, the
          Trustee shall mail copies of such material to all such Securityholders
          with reasonable promptness after the entry of such order and the
          renewal of such tender; otherwise the Trustee shall be relieved of any
          obligation or duty to such applicants respecting their application.

               (c) Each and every holder of Securities, by receiving and holding
          the same, agrees with the Company and the Trustee that neither the
          Company nor the Trustee nor any paying agent shall be held accountable
          by reason of the disclosure of any such information as to the names
          and addresses of the holders of Securities in accordance with the
          provisions of subsection (b) of this Section 4.02, regardless of the
          source from which such information was derived, and that the Trustee
          shall not be held accountable by reason of mailing any material
          pursuant to a request made under said subsection (b).

     SECTION 4.03. Reports by Company.

          (a) The Company covenants and agrees to file with the Trustee, within
     15 days after the date on which the Company is required to file the same
     with the Commission, copies of the annual reports and of the information,
     documents and other reports (or copies of such portions of any of the
     foregoing as said Commission may from time to time by rules and regulations
     prescribe) which the Company may be required to file with the Commission
     pursuant to Section 13 or Section 15(d) of the Exchange Act; or, if the
     Company is not 

                                       28
<PAGE>
 
     required to file information, documents or reports pursuant to either of
     such sections, then to file with the Trustee and the Commission, in
     accordance with rules and regulations prescribed from time to time by the
     Commission, such of the supplementary and periodic information, documents
     and reports which may be required pursuant to Section 13 of the Exchange
     Act in respect of a security listed and registered on a national securities
     exchange as may be prescribed from time to time in such rules and
     regulations.

          (b) The Company covenants and agrees to file with the Trustee and the
     Commission, in accordance with the rules and regulations prescribed from
     time to time by said Commission, such additional information, documents and
     reports with respect to compliance by the Company with the conditions and
     covenants provided for in this Indenture as may be required from time to
     time by such rules and regulations.

          (c) The Company covenants and agrees to transmit by mail to all
     holders of Securities, as the names and addresses of such holders appear
     upon the Security Register, within 30 days after the filing thereof with
     the Trustee, such summaries of any information, documents and reports
     required to be filed by the Company pursuant to subsections (a) and (b) of
     this Section 4.03 as may be required by rules and regulations prescribed
     from time to time by the Commission.

          (d) Delivery of such reports, information and documents to the Trustee
     is for informational purposes only and the Trustee's receipt of such shall
     not constitute constructive notice of any information contained therein or
     determinable from information contained therein, including the Company's
     compliance with any of its covenants hereunder (as to which the Trustee is
     entitled to rely exclusively on Officers' Certificates).

          (e) So long as is required for an offer or sale of the Securities to
     qualify for an exemption under Rule 144A under the Securities Act, the
     Company shall, upon request, provide the information required by clause
     (d)(4) thereunder to each Securityholder and to each beneficial owner and
     prospective purchaser of Securities identified by each Securityholder of
     Restricted Securities, unless such information is furnished to the
     Commission pursuant to Section 13 or 15(d) of the Exchange Act.

     SECTION 4.04. Reports by the Trustee.

          (a) The Trustee shall transmit to Securityholders such reports
     concerning the Trustee and its actions under this Indenture as may be
     required pursuant to the Trust Indenture Act at the times and in the manner
     provided pursuant thereto.  If required by Section 313(a) of the Trust
     Indenture Act, the Trustee shall, within sixty days after each May 15
     following the date of this Indenture, commencing May 15, 1998, deliver to
     Securityholders a brief report, dated as of such May 15, which complies
     with the provisions of such Section 313(a).

                                       29
<PAGE>
 
          (b) A copy of each such report shall, at the time of such transmission
     to Securityholders, be filed by the Trustee with each stock exchange, if
     any, upon which the Securities are listed, with the Commission and with the
     Company.  The Company will promptly notify the Trustee when the Securities
     are listed on any stock exchange.


                                   ARTICLE V
                  REMEDIES OF THE TRUSTEE AND SECURITYHOLDERS
                              ON EVENT OF DEFAULT


     SECTION 5.01. Events of Default.

     "Event of Default", wherever used herein with respect to Securities, means
any one of the following events (whatever the reason for such Event of Default
and whether it shall be voluntary or involuntary or be effected by operation of
law or pursuant to any judgment, decree or order of any court or any order, rule
or regulation of any administrative or governmental body):

          (a) default in the payment of any interest (including Compounded
     Interest or Additional Sums, if any) or Liquidated Damages, if any, upon
     any Security or any Other Debentures when it becomes due and payable, and
     continuance of such default for a period of 30 days; provided, however,
     that a valid extension of an interest payment period by the Company in
     accordance with the terms hereof shall not constitute a default in the
     payment of interest for this purpose; or

          (b) default in the payment of all or any part of the principal of (or
     premium, if any, on) any Security or any Other Debentures as and when the
     same shall become due and payable either at maturity, upon redemption, by
     declaration of acceleration of maturity or otherwise; or

          (c) default in the performance, or breach, of any covenant or warranty
     of the Company in this Indenture (other than a covenant or warranty a
     default in whose performance or whose breach is elsewhere in this Section
     specifically dealt with), and continuance of such default or breach for a
     period of 90 days after there has been given, by registered or certified
     mail, to the Company by the Trustee or to the Company and the Trustee by
     the holders of at least 25% in aggregate principal amount of the
     outstanding Securities a written notice specifying such default or breach
     and requiring it to be remedied and stating that such notice is a "Notice
     of Default" hereunder; or

          (d) a court having jurisdiction in the premises shall enter a decree
     or order for relief in respect of the Company in an involuntary case under
     any applicable bankruptcy, insolvency or other similar law now or hereafter
     in effect, or appointing a receiver, 

                                       30
<PAGE>
 
     liquidator, assignee, custodian, trustee, sequestrator (or similar
     official) of the Company or for any substantial part of its property, or
     ordering the winding-up or liquidation of its affairs and such decree or
     order shall remain unstayed and in effect for a period of 90 consecutive
     days; or

          (e) the Company shall commence a voluntary case under any applicable
     bankruptcy, insolvency, reorganization or other similar law now or
     hereafter in effect, shall consent to the entry of an order for relief in
     an involuntary case under any such law, or shall consent to the appointment
     of or taking possession by a receiver, liquidator, assignee, trustee,
     custodian, sequestrator (or other similar official) of the Company or of
     any substantial part of its property, or shall make any general assignment
     for the benefit of creditors, or shall fail generally to pay its debts as
     they become due.

     If an Event of Default with respect to Securities at the time outstanding
occurs and is continuing, then in every such case the Trustee or the holders
of not less than 25% in aggregate principal amount of the Securities then
outstanding may declare the principal amount of all Securities to be due and
payable immediately, by a notice in writing to the Company (and to the Trustee
if given by the holders of the outstanding Securities), and upon any such
declaration the same shall become immediately due and payable.  Payment of
principal and interest (including any Compounded Interest and Additional Sums to
the extent permitted by applicable law) on such Securities shall remain
subordinated to the extent provided in Article XV notwithstanding that such
amount shall become immediately due and payable as herein provided.  If an Event
of Default specified in Section 5.01(d) or (e) with respect to the Securities
occurs, the principal amount of all the Securities shall automatically, and
without any declaration or other action on the part of Trustee or any holder,
become immediately due and payable.

     The foregoing provisions, however, are subject to the condition that if, at
any time after the principal of the Securities shall have been so declared due
and payable, and before any judgment or decree for the payment of the moneys due
shall have been obtained or entered as hereinafter provided, (i) the Company
shall pay or shall deposit with the Trustee a sum sufficient to pay (A) all
matured installments of interest (including Compounded Interest and Additional
Sums, if any) and Liquidated Damages, if any, upon all the Securities and the
principal of and premium, if any, on any and all Securities which shall have
become due otherwise than by acceleration (with interest upon such principal and
premium, if any, and, to the extent that payment of such interest is enforceable
under applicable law, on overdue installments of interest, at the same rate as
the rate of interest specified in the Securities to the date of such payment or
deposit) and (B) such amount as shall be sufficient to cover compensation due to
the Trustee and each predecessor Trustee, their respective agents, attorneys and
counsel, pursuant to Section 6.06, and (ii) any and all Events of Default under
the Indenture, other than the non-payment of the principal of the Securities
which shall have become due solely by such declaration of acceleration, shall
have been cured, waived or otherwise remedied as provided herein, then, in every
such case, the holders of a majority in aggregate principal amount of the
Securities then outstanding, by written notice to the Company and to the
Trustee, may rescind 

                                       31
<PAGE>
 
and annul such declaration and its consequences, but no such waiver or
rescission and annulment shall extend to or shall affect any subsequent default
or shall impair any right consequent thereon.

     In case the Trustee or any holder shall have proceeded to enforce any right
under this Indenture and such proceedings shall have been discontinued or
abandoned because of such rescission or annulment or for any other reason or
shall have been determined adversely to the Trustee or such holder, then and in
every such case the Company, the Trustee and the holders of the Securities shall
be restored respectively to their several positions and rights hereunder, and
all rights, remedies and powers of the Company, the Trustee and the holders of
the Securities shall continue as though no such proceeding had been taken.

     SECTION 5.02. Payment of Securities on Default; Suit Therefor.

     The Company covenants that (a) in case default shall be made in the payment
of any installment of interest (including Compounded Interest and Additional
Sums, if any) and Liquidated Damages, if any, upon any of the Securities as and
when the same shall become due and payable, and such default shall have
continued for a period of 30 days, or (b) in case default shall be made in the
payment of the principal of or premium, if any, on any of the Securities as and
when the same shall have become due and payable, whether at maturity of the
Securities or upon redemption or by declaration or otherwise, then, upon demand
of the Trustee, the Company will pay to the Trustee, for the benefit of the
holders of the Securities, the whole amount that then shall have become due and
payable on all such Securities for principal and premium, if any, or interest
(including Compounded Interest and Additional Sums, if any) and Liquidated
Damages, if any, or both, as the case may be, with interest upon the overdue
principal and premium, if any, and (to the extent that payment of such interest
is enforceable under applicable law and, if the Securities are held by NGC
Corporation Capital Trust or a trustee of such trust, without duplication of any
other amounts paid by NGC Corporation Capital Trust or a trustee in respect
thereof) upon the overdue installments of interest (including Compounded
Interest and Additional Sums, if any) and Liquidated Damages, if any, at the
rate borne by the Securities; and, in addition thereto, such further amount as
shall be sufficient to cover the costs and expenses of collection, including a
reasonable compensation to the Trustee, its agents, attorneys and counsel, and
any other amount due to the Trustee pursuant to Section 6.06.

     In case the Company shall fail forthwith to pay such amounts upon such
demand, the Trustee, in its own name and as trustee of an express trust, shall
be entitled and empowered to institute any actions or proceedings at law or in
equity for the collection of the sums so due and unpaid, and may prosecute any
such action or proceeding to judgment or final decree, and may enforce any such
judgment or final decree against the Company or any other obligor on the
Securities and collect in the manner provided by law out of the property of the
Company or any other obligor on the Securities wherever situated the moneys
adjudged or decreed to be payable.

                                       32
<PAGE>
 
     In case there shall be pending proceedings for the bankruptcy or for the
reorganization of the Company or any other obligor on the Securities under Title
11, United States Code, or any other applicable law, or in case a receiver or
trustee shall have been appointed for the property of the Company or such other
obligor, or in the case of any other similar judicial proceedings relative to
the Company or other obligor upon the Securities, or to the creditors or
property of the Company or such other obligor, the Trustee, irrespective of
whether the principal of the Securities shall then be due and payable as therein
expressed or by declaration or otherwise and irrespective of whether the Trustee
shall have made any demand pursuant to the provisions of this Section 5.02,
shall be entitled and empowered, by intervention in such proceedings or
otherwise, to file and prove a claim or claims for the whole amount of principal
premium, if any, and interest (including Compounded Interest and Additional
Sums, if any) and Liquidated Damages, if any, owing and unpaid in respect of the
Securities and, in case of any judicial proceedings, to file such proofs of
claim and other papers or documents as may be necessary or advisable in order to
have the claims of the Trustee (including any claim for amounts due to the
Trustee pursuant to 6.06) and of the Securityholders allowed in such judicial
proceedings relative to the Company or any other obligor on the Securities, or
to the creditors or property of the Company or such other obligor, unless
prohibited by applicable law and regulations, to vote on behalf of the holders
of the Securities in any election of a trustee or a standby trustee in
arrangement, reorganization, liquidation or other bankruptcy or insolvency
proceedings or person performing similar functions in comparable proceedings,
and to collect and receive any moneys or other property payable or deliverable
on any such claims, and to distribute the same after the deduction of its
charges and expenses; and any receiver, assignee or trustee in bankruptcy or
reorganization is hereby authorized by each of the Securityholders to make such
payments to the Trustee, and, in the event that the Trustee shall consent to the
making of such payments directly to the Securityholders, to pay to the Trustee
such amounts as shall be sufficient to cover reasonable compensation to the
Trustee, each predecessor Trustee and their respective agents, attorneys and
counsel, and all other amounts due to the Trustee pursuant to Section 6.06.

     Nothing herein contained shall be construed to authorize the Trustee to
authorize or consent to or accept or adopt on behalf of any Securityholder any
plan of reorganization, arrangement, adjustment or composition affecting the
Securities or the rights of any holder thereof or to authorize the Trustee to
vote in respect of the claim of any Securityholder in any such proceeding.

     All rights of action and of asserting claims under this Indenture, or under
any of the Securities, may be enforced by the Trustee without the possession of
any of the Securities, or the production thereof on any trial or other
proceeding relative thereto, and any such suit or proceeding instituted by the
Trustee shall be brought in its own name as trustee of an express trust, and any
recovery of judgment shall be for the ratable benefit of the holders of the
Securities.

     In any proceedings brought by the Trustee (and also any proceedings
involving the interpretation of any provision of this Indenture to which the
Trustee shall be a party) the Trustee shall be held to represent all the holders
of the Securities, and it shall not be necessary to make any holders of the
Securities parties to any such proceedings.

                                       33
<PAGE>
 
     SECTION 5.03. Application of Moneys Collected by Trustee.

     Any moneys collected by the Trustee shall be applied in the order
following, at the date or dates fixed by the Trustee for the distribution of
such moneys, upon presentation of the Securities in respect of which moneys have
been collected, and stamping thereon the payment, if only partially paid, and
upon surrender thereof if fully paid:

     First:  To the payment of costs and expenses of collection applicable to
the Securities and all other amounts due to the Trustee under Section 6.06;

     Second:  To the payment of all Senior Indebtedness of the Company if and to
the extent required by Article XV;

     Third:  To the payment of the amounts then due and unpaid upon Securities
for principal of (and premium, if any) and interest (including Compounded
Interest and Additional Sums, if any) and Liquidated Damages, if any, on the
Securities, in respect of which or for the benefit of which money has been
collected, ratably, without preference of priority of any kind, according to the
amounts due on such Securities for principal (and premium, if any) and interest,
respectively; and

     Fourth:  To the Company.

     SECTION 5.04. Proceedings by Securityholders.

     No holder of any Security shall have any right by virtue of or by availing
of any provision of this Indenture to institute any suit, action or proceeding
in equity or at law upon or under or with respect to this Indenture or for the
appointment of a receiver or trustee, or for any other remedy hereunder, unless
such holder previously shall have given to the Trustee written notice of an
Event of Default and of the continuance thereof with respect to the Securities
specifying such Event of Default, as hereinbefore provided, and unless also the
holders of not less than 25% in aggregate principal amount of the Securities
then outstanding shall have made written request upon the Trustee to institute
such action, suit or proceeding in its own name as Trustee hereunder and shall
have offered to the Trustee such reasonable indemnity as it may require against
the costs, expenses and liabilities to be incurred therein or thereby, and the
Trustee for 60 days after its receipt of such notice, request and offer of
indemnity shall have failed to institute any such action, suit or proceeding, it
being understood and intended, and being expressly covenanted by the taker and
holder of every Security with every other taker and holder and the Trustee, that
no one or more holders of Securities shall have any right in any manner whatever
by virtue of or by availing of any provision of this Indenture to affect,
disturb or prejudice the rights of any other holder of Securities, or to obtain
or seek to obtain priority over or preference to any other such holder, or to
enforce any right under this Indenture, except in the manner herein provided and
for the equal, ratable and common benefit of all holders of Securities.

                                       34
<PAGE>
 
     Notwithstanding any other provisions in this Indenture, however, the right
of any holder of any Security, which right is absolute and unconditional, to
receive payment of the principal of (premium, if any) and interest (including
Compounded Interest and Additional Sums, if any) and Liquidated Damages, if any,
on such Security, on or after the same shall have become due and payable, or to
institute suit for the enforcement of any such payment, shall not be impaired or
affected without the consent of such holder and by accepting a Security
hereunder it is expressly understood, intended and covenanted by the taker and
holder of every Security with every other such taker and holder and the Trustee,
that no one or more holders of Securities shall have any right in any manner
whatsoever by virtue or by availing of any provision of this Indenture to
affect, disturb or prejudice the rights of the holders of any other Securities,
or to obtain or seek to obtain priority over or preference to any other such
holder, or to enforce any right under this Indenture, except in the manner
herein provided and for the equal, ratable and common benefit of all holders of
Securities.  For the protection and enforcement of the provisions of this
Section, each and every Securityholder and the Trustee shall be entitled to such
relief as can be given either at law or in equity.

     The Company and the Trustee acknowledge that pursuant to the Declaration,
the holders of Capital Securities are entitled, in the circumstances and subject
to the limitations set forth therein, to commence a Direct Action with respect
to any Event of Default under this Indenture and the Securities.

     SECTION 5.05. Proceedings by Trustee.

     In case an Event of Default occurs with respect to Securities and is
continuing, the Trustee may in its discretion proceed to protect and enforce the
rights vested in it by this Indenture by such appropriate judicial proceedings
as the Trustee shall deem most effectual to protect and enforce any of such
rights, either by suit in equity or by action at law or by proceeding in
bankruptcy or otherwise, whether for the specific enforcement of any covenant or
agreement contained in this Indenture or in aid of the exercise of any power
granted in this Indenture, or to enforce any other legal or equitable right
vested in the Trustee by this Indenture or by law.

     SECTION 5.06. Remedies Cumulative and Continuing.

     All powers and remedies given by this Article V to the Trustee or to the
Securityholders shall, to the extent permitted by law, be deemed cumulative and
not exclusive of any other powers and remedies available to the Trustee or the
holders of the Securities, by judicial proceedings or otherwise, to enforce the
performance or observance of the covenants and agreements contained in this
Indenture or otherwise established with respect to the Securities, and no delay
or omission of the Trustee or of any holder of any of the Securities to exercise
any right or power accruing upon any Event of Default occurring and continuing
as aforesaid shall impair any such right or power, or shall be construed to be a
waiver of any such default or an acquiescence therein; and, subject to the

                                       35
<PAGE>
 
provisions of Section 5.04, every power and remedy given by this Article V or by
law to the Trustee or to the Securityholders may be exercised from time to time,
and as often as shall be deemed expedient, by the Trustee or by the
Securityholders.

     SECTION 5.07.  Direction of Proceedings and Waiver of Defaults by
                    Majority of Securityholders.

     The holders of a majority in aggregate principal amount of the Securities
at the time outstanding shall have the right to direct the time, method, and
place of conducting any proceeding for any remedy available to the Trustee, or
exercising any trust or power conferred on the Trustee; provided, however, that
such direction shall not be in conflict with any rule or law or with this
Indenture and, (subject to the provisions of Section 6.01) the Trustee shall
have the right to decline to follow any such direction if the Trustee shall
determine that the action so directed would be unjustly prejudicial to the
holders not taking part in such direction or if the Trustee being advised by
counsel determines that the action or proceeding so directed may not lawfully be
taken or if the Trustee in good faith by its board of directors or trustees,
executive committee, or a trust committee of directors or trustees and/or
Responsible Officers shall determine that the action or proceedings so directed
would involve the Trustee in personal liability.  Prior to any declaration
accelerating the maturity of the Securities, the holders of a majority in
aggregate principal amount of the Securities at the time outstanding may on
behalf of the holders of all of the Securities waive any past default or Event
of Default and its consequences except a default (a) in the payment of principal
of or premium, if any, or interest (including Compounded Interest and Additional
Sums, if any) or Liquidated Damages, if any, on any of the Securities or (b) in
respect of covenants or provisions hereof which cannot be modified or amended
without the consent of the holder of each Security affected; provided, however,
that if the Securities are held by the Property Trustee, such waiver or
modification to such waiver shall not be effective until the holders of a
majority in aggregate liquidation amount of Trust Securities shall have
consented to such waiver or modification to such waiver; provided further, that
if the consent of the holder of each outstanding Security is required, such
waiver shall not be effective until each holder of the Trust Securities shall
have consented to such waiver.  Upon any such waiver, the default covered
thereby shall be deemed to be cured for all purposes of this Indenture and the
Company, the Trustee and the holders of the Securities shall be restored to
their former positions and rights hereunder, respectively; but no such waiver
shall extend to any subsequent or other default or impair any right consequent
thereon.  Whenever any default or Event of Default hereunder shall have been
waived as permitted by this Section 5.07, said default or Event of Default shall
for all purposes of the Securities and this Indenture be deemed to have been
cured and to be not continuing.

     SECTION 5.08. Notice of Defaults.

          (a) The Trustee shall, within 90 days after the occurrence of a
     default with respect to the Securities known to a Responsible Officer of
     the Trustee, mail to all Securityholders, as the names and addresses of
     such holders appear upon the Security 

                                       36
<PAGE>
 
     Register, notice of all defaults known to the Trustee, unless such defaults
     shall have been cured before the giving of such notice (the term "defaults"
     for the purpose of this Section 5.08 being hereby defined to be the events
     specified in clauses (a), (b), (c), (d) and (e) of Section 5.01, not
     including periods of grace, if any, provided for therein, and irrespective
     of the giving of written notice specified in clause (c) of Section 5.01);
     and provided that, except in the case of default in the payment of the
     principal of or premium, if any, or interest (including Compounded Interest
     or Additional Sums, if any) or Liquidated Damages, if any, on any of the
     Securities, the Trustee shall be protected in withholding such notice if
     and so long as the board of directors, the executive committee, or a trust
     committee of directors and/or Responsible Officers of the Trustee in good
     faith determines that the withholding of such notice is in the interests of
     the Securityholders; and provided further, that in the case of any default
     of the character specified in Section 5.01(c) no such notice to
     Securityholders shall be given until at least 60 days after the occurrence
     thereof but shall be given within 90 days after such occurrence.

          (b) Within five Business Days after the occurrence of any Event of
     Default actually known to the Trustee, the Trustee shall transmit notice of
     such Event of Default to all Securityholders, unless such Event of Default
     shall have been cured or waived.

     SECTION 5.09. Undertaking to Pay Costs.

     All parties to this Indenture agree, and each holder of any Security by his
acceptance thereof shall be deemed to have agreed, that any court may in its
discretion require, in any suit for the enforcement of any right or remedy under
this Indenture, or in any suit against the Trustee for any action taken or
omitted by it as Trustee, the filing by any party litigant in such suit of an
undertaking to pay the costs of such suit, and that such court may in its
discretion assess reasonable costs, including reasonable attorneys' fees and
expenses, against any party litigant in such suit, having due regard to the
merits and good faith of the claims or defenses made by such party litigant; but
the provisions of this Section 5.09 shall not apply to any suit instituted by
the Company or the Trustee; to any suit instituted by any Securityholder, or
group of Securityholders, holding in the aggregate more than 10% in aggregate
principal amount of the Securities outstanding;  or to any suit instituted by
any Securityholder for the enforcement of the payment of the principal of (or
premium, if any) or interest (including Compounded Interest and Additional Sums,
if any) or Liquidated Damages, if any, on any Security against the Company on or
after the same shall have become due and payable.

     SECTION 5.10. Waiver of Stay or Extension Laws.

     The Company covenants (to the extent that it may lawfully do so) that it
will not any time insist upon, or plead, or in any manner whatsoever claim or
take the benefit or advantage of, any stay or extension law wherever enacted,
now or at any time hereafter in force which may affect the covenants or the
performance of this Indenture; and the Company (to the extent that it may
lawfully 

                                       37
<PAGE>
 
do so) hereby expressly waives all benefit or advantage of any such law and
covenants that it will not hinder, delay or impede the execution of any power
herein granted to the Trustee, but will suffer and permit the execution of every
such power as though no such law had been enacted.


                                  ARTICLE VI
                            CONCERNING THE TRUSTEE
                            ----------------------

     SECTION 6.01. Duties and Responsibilities of Trustee.

     With respect to the holders of the Securities issued hereunder, the
Trustee, prior to the occurrence of an Event of Default 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.  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 man would exercise or use under the circumstances in the conduct of
his own affairs.

     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

          (a) prior to the occurrence of an Event of Default and after the
     curing or waiving of all Events of Default which may have occurred,

               (i) the duties and obligations of the Trustee shall be determined
          solely by the express provisions of this Indenture and the Trust
          Indenture Act, and the Trustee shall not be liable except for the
          performance of such duties and obligations as are specifically set
          forth in this Indenture or in the Trust Indenture Act, and no implied
          covenants or obligations shall be read into this Indenture against the
          Trustee; and

               (ii) in the absence of bad faith on the part of the Trustee, the
          Trustee may conclusively rely, as to the truth of the statements and
          the correctness of the opinions expressed therein, upon any
          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;

          (b) the Trustee shall not be liable for any error of judgment made in
     good faith by a Responsible Officer or Officers, unless it shall be proved
     that the Trustee was negligent in ascertaining the pertinent facts; and

                                       38
<PAGE>
 
          (c) 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 Securityholders pursuant to Section 5.07, relating to the
     time, method and place of conducting any proceeding for any remedy
     available to the Trustee, or exercising any trust or power conferred upon
     the Trustee, under this Indenture.

     None of the provisions contained in this Indenture shall require the
Trustee to expend or risk its own funds or otherwise incur personal financial
liability in the performance of any of its duties or in the exercise of any of
its rights or powers, if there is reasonable ground for believing that the
repayment of such funds or liability is not reasonably assured to it under the
terms of this Indenture or adequate indemnity against such risk is not
reasonably assured to it.

     SECTION 6.02. Reliance on Documents, Opinions, etc.

     Except as otherwise provided in Section 6.01:

          (a) the Trustee may rely and shall be protected in acting or
     refraining from acting upon any resolution, certificate, statement,
     instrument, opinion, report, notice, request, consent, order, bond, note,
     debenture or other paper or document believed by it to be genuine and to
     have been signed or presented by the proper party or parties;

          (b) any request, direction, order or demand of the Company mentioned
     herein may be sufficiently evidenced by an Officers' Certificate (unless
     other evidence in respect thereof be herein specifically prescribed); and
     any Board Resolution may be evidenced to the Trustee by a copy thereof
     certified by the Secretary or an Assistant Secretary of the Company;

          (c) the Trustee may consult with counsel of its selection and any
     advice or Opinion of Counsel shall be full and complete authorization and
     protection in respect of any action taken or suffered omitted by it
     hereunder in good faith and in accordance with such advice or Opinion of
     Counsel;

          (d) the Trustee shall be under no obligation to exercise any of the
     rights or powers vested in it by this Indenture at the request, order or
     direction of any of the Securityholders, pursuant to the provisions of this
     Indenture, unless such Securityholders shall have offered to the Trustee
     reasonable and sufficient security or indemnity against the costs, expenses
     and liabilities which may be incurred therein or thereby;

          (e) the Trustee shall not be liable for any action taken or omitted by
     it in good faith and believed by it to be authorized or within the
     discretion or rights or powers conferred upon it by this Indenture; nothing
     contained herein shall, however, relieve the Trustee of the obligation,
     upon the occurrence of an Event of Default (that has not been cured or
     waived), 

                                       39
<PAGE>
 
     to exercise such of the rights and powers vested in it by this Indenture,
     and to use the same degree of care and skill in their exercise, as a
     prudent man would exercise or use under the circumstances in the conduct of
     his own affairs;

          (f) the Trustee shall not be bound to make any investigation into the
     facts or matters stated in any resolution, certificate, statement,
     instrument, opinion, report, notice, request, consent, order, approval,
     bond, debenture, coupon or other paper or document, unless requested in
     writing to do so by the holders of a majority in aggregate principal amount
     of the outstanding Securities; provided, however, that if the payment
     within a reasonable time to the Trustee of the costs, expenses or
     liabilities likely to be incurred by it in the making of such investigation
     is, in the opinion of the Trustee, not reasonably assured to the Trustee by
     the security afforded to it by the terms of this Indenture, the Trustee may
     require reasonable indemnity against such expense or liability as a
     condition to so proceeding;

          (g) the Trustee may execute any of the trusts or powers hereunder or
     perform any duties hereunder either directly or by or through agents
     (including any Authenticating Agent) or attorneys, and the Trustee shall
     not be responsible for any misconduct or negligence on the part of any such
     agent or attorney appointed by it with due care;

          (h) the Trustee shall not be charged with knowledge of any Default or
     Event of Default with respect to the Securities unless (1) such default is
     a default under Sections 5.01(a) (other than a default with respect to the
     payment of Compounded Interest, Liquidated Damages or Additional Sums) and
     5.01(b) of the Indenture, (2) a Responsible Officer shall have actual
     knowledge of such Default or Event of Default or (3) written notice of such
     Default or Event of Default shall have been given to the Trustee by the
     Company or any other obligor on the Securities or by any holder of the
     Securities; and

          (i) the Trustee shall not be liable for any action taken, suffered or
     omitted by it in good faith, without negligence or willful misconduct and
     believed by it to be authorized or within the discretion or rights or
     powers conferred upon it by this Indenture.

     SECTION 6.03. No Responsibility for Recitals, etc.

     The recitals contained herein and in the Securities (except in the
certificate of authentication of the Trustee or the Authenticating Agent) shall
be taken as the statements of the Company and the Trustee and the Authenticating
Agent assume no responsibility for the correctness of the same.  The Trustee and
the Authenticating Agent make no representations as to the validity or
sufficiency of this Indenture or of the Securities.  The Trustee and the
Authenticating Agent shall not be accountable for the use or application by the
Company of any Securities or the proceeds of any Securities authenticated and
delivered by the Trustee or the Authenticating Agent in conformity with the
provisions of this Indenture.

                                       40
<PAGE>
 
     SECTION 6.04.  Trustee, Authenticating Agent, Paying Agents, Transfer
                    Agents or Registrar May Own Securities.

     The Trustee or any Authenticating Agent or any paying agent or any transfer
agent or any Security registrar, in its individual or any other capacity, may
become the owner or pledgee of Securities with the same rights it would have if
it were not Trustee, Authenticating Agent, paying agent, transfer agent or
Security registrar.

     SECTION 6.05. Moneys to be Held in Trust.

     Subject to the provisions of Section 11.04, all moneys received by the
Trustee or any paying agent shall, until used or applied as herein provided, be
held in trust for the purpose for which they were received, but need not be
segregated from other funds except to the extent required by law.  The Trustee
and any paying agent shall be under no liability for interest on any money
received by it hereunder except as otherwise agreed in writing with the Company.
So long as no Event of Default shall have occurred and be continuing, all
interest allowed on any such moneys shall be paid from time to time upon the
written order of the Company, signed by the Chairman of the Board of Directors,
the President, a Vice President, the Treasurer or an Assistant Treasurer of the
Company.

     SECTION 6.06. Compensation and Expenses of Trustee.

     The Company, as issuer of Securities under this Indenture, covenants and
agrees to pay to the Trustee from time to time, and the Trustee shall be
entitled to, such compensation as shall be agreed to in writing between the
Company and the Trustee (which shall not be limited by any provision of law in
regard to the compensation of a trustee of an express trust), and the Company
will pay or reimburse the Trustee upon its request for all reasonable expenses,
disbursements and advances incurred or made by the Trustee in accordance with
any of the provisions of this Indenture (including the reasonable compensation
and the expenses and disbursements of its counsel and of all persons not
regularly in its employ) except any such expense, disbursement or advance as may
arise from its negligence or bad faith.  The Company also covenants to indemnify
each of the Trustee or any predecessor Trustee (and its officers, agents,
directors and employees) for, and to hold it harmless against, any and all loss,
damage, claim, liability or expense including taxes (other than taxes based on
the income of the Trustee) incurred without negligence or bad faith on the part
of the Trustee and arising out of or in connection with the acceptance or
administration of this trust, including the costs and expenses of defending
itself against any claim of liability in the premises.  The obligations of the
Company under this Section 6.06 to compensate and indemnify the Trustee and to
pay or reimburse the Trustee for expenses, disbursements and advances shall
constitute additional indebtedness hereunder.  Such additional indebtedness
shall be secured by a lien prior to that of the Securities upon all property and
funds held or collected by the Trustee as such, except funds held in trust for
the benefit of the holders of particular Securities.

                                       41
<PAGE>
 
     When the Trustee incurs expenses or renders services in connection with an
Event of Default specified in Section 5.01(d) or Section 5.01(e), the expenses
(including the reasonable charges and expenses of its counsel) and the
compensation for the services are intended to constitute expenses of
administration under any applicable federal or state bankruptcy, insolvency or
other similar law.

     The provisions of this Section shall survive the resignation or removal of
the Trustee and the defeasance or other termination of this Indenture.

     SECTION 6.07. Officers' Certificate as Evidence.

     Except as otherwise provided in Sections 6.01 and 6.02, whenever in the
administration of the provisions of this Indenture the Trustee shall deem it
necessary or desirable that a matter be proved or established prior to taking or
omitting any action hereunder, such matter (unless other evidence in respect
thereof is herein specifically prescribed) may, in the absence of negligence or
bad faith on the part of the Trustee, be deemed to be conclusively proved and
established by an Officers' Certificate delivered to the Trustee, and such
certificate, in the absence of negligence or bad faith on the part of the
Trustee, shall be full warrant to the Trustee for any action taken or omitted by
it under the provisions of this Indenture upon the faith thereof.

     SECTION 6.08. Conflicting Interest of Trustee.

     If the Trustee has or shall acquire any "conflicting interest" within the
meaning of Section 310(b) of the Trust Indenture Act, the Trustee and the
Company shall in all respects comply with the provisions of Section 310(b) of
the Trust Indenture Act.

     SECTION 6.09. Eligibility of Trustee.

     The Trustee hereunder shall at all times be a corporation organized and
doing business under the laws of the United States of America or any state or
territory thereof or of the District of Columbia or a corporation or other
Person permitted to act as trustee by the Commission authorized under such laws
to exercise corporate trust powers, having a combined capital and surplus of at
least 50 million U.S. dollars ($50,000,000) and subject to supervision or
examination by federal, state, territorial, or District of Columbia authority.
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 6.09 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.

     The Company may not, nor may any Person directly or indirectly controlling,
controlled by, or under common control with the Company, serve as Trustee.

                                       42
<PAGE>
 
     In case at any time the Trustee shall cease to be eligible in accordance
with the provisions of this Section 6.09, the Trustee shall resign immediately
in the manner and with the effect specified in Section 6.10.

     SECTION 6.10. Resignation or Removal of Trustee.

          (a) The Trustee, or any trustee or trustees hereafter appointed, may
     at any time resign by giving written notice of such resignation to the
     Company and by mailing notice thereof to the holders of the Securities at
     their addresses as they shall appear on the Security register.  Upon
     receiving such notice of resignation, the Company shall promptly appoint a
     successor trustee or trustees by written instrument, in duplicate, one copy
     of which instrument shall be delivered to the resigning Trustee and one
     copy to the successor trustee. If no successor trustee shall have been so
     appointed and have accepted appointment within 60 days after the mailing of
     such notice of resignation to the affected Securityholders, the resigning
     Trustee may petition any court of competent jurisdiction for the
     appointment of a successor trustee, or any Securityholder who has been a
     bona fide holder of a Security for at least six months may, subject to the
     provisions of Section 5.09, on behalf of himself and all 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 prescribe, appoint a successor trustee.

          (b) In case at any time any of the following shall occur:

               (i) the Trustee shall fail to comply with the provisions of
          Section 6.08 after written request therefor by the Company or by any
          Securityholder who has been a bona fide holder of a Security or
          Securities for at least six months, or

               (ii) the Trustee shall cease to be eligible in accordance with
          the provisions of Section 6.09 and shall fail to resign after written
          request therefor by the Company or by any such Securityholder, or

               (iii)  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 Company may remove the Trustee and appoint a successor
          trustee by written instrument, in duplicate, one copy of which
          instrument shall be delivered to the Trustee so removed and one copy
          to the successor trustee, or, subject to the provisions of Section
          5.09, any Securityholder who has been a bona fide holder of a Security
          for at least six months may, on behalf of himself and all others
          similarly situated, petition any court of competent jurisdiction for
          the removal of the Trustee and the appointment of a successor trustee.
          Such court may thereupon, after such 

                                       43
<PAGE>
 
          notice, if any, as it may deem proper, remove the Trustee and appoint
          a successor trustee.

          (c) The holders of a majority in aggregate principal amount of the
     Securities at the time outstanding may at any time remove the Trustee and
     nominate a successor trustee, which shall be deemed appointed as successor
     trustee unless within 10 days after such nomination the Company objects
     thereto or if no successor trustee shall have been so appointed and shall
     have accepted appointment within 30 days after such removal, in which case
     the Trustee so removed or any Securityholder, upon the terms and conditions
     and otherwise as in subsection (a) of this Section 6.10 provided, may
     petition any court of competent jurisdiction for an appointment of a
     successor trustee.

          (d) Any resignation or removal of the Trustee and appointment of a
     successor trustee pursuant to any of the provisions of this Section 6.10
     shall become effective only upon acceptance of appointment by the successor
     trustee as provided in Section 6.11.

     SECTION 6.11. Acceptance by Successor Trustee.

     Any successor trustee appointed as provided in Section 6.10 shall execute,
acknowledge and deliver to the Company and to its predecessor trustee an
instrument accepting such appointment hereunder, and thereupon the resignation
or removal of the retiring trustee shall become effective and such successor
trustee, without any further act, deed or conveyance, shall become vested with
all the rights, powers, duties and obligations of its predecessor hereunder,
with like effect as if originally named as trustee herein; but, nevertheless, on
the written request of the Company or of the successor trustee, the trustee
ceasing to act shall, upon payment of any amounts then due it pursuant to the
provisions of Section 6.06, execute and deliver an instrument transferring to
such successor trustee all the rights and powers of the trustee so ceasing to
act and shall duly assign, transfer and deliver to such successor trustee all
property and money held by such retiring trustee thereunder.  Upon request of
any such successor trustee, the Company shall execute any and all instruments in
writing for more fully and certainly vesting in and confirming to such successor
trustee all such rights and powers.  Any trustee ceasing to act shall,
nevertheless, retain a lien upon all property or funds held or collected by such
trustee to secure any amounts then due it pursuant to the provisions of Section
6.06.

     No successor trustee shall accept appointment as provided in this Section
6.11 unless at the time of such acceptance such successor trustee shall be
qualified under the provisions of Section 6.08 and eligible under the provisions
of Section 6.09.

     Upon acceptance of appointment by a successor trustee as provided in this
Section 6.11, the Company shall mail notice of the succession of such trustee
hereunder to the holders of Securities at their addresses as they shall appear
on the Security register.  If the Company fails to mail such 

                                       44
<PAGE>
 
notice within 10 days after the acceptance of appointment by the successor
trustee, the successor trustee shall cause such notice to be mailed at the
expense of the Company.

     SECTION 6.12. Succession by Merger, etc.

     Any corporation into which the Trustee may be merged or converted or with
which it may be consolidated, or any corporation resulting from any merger,
conversion or consolidation to which the Trustee shall be a party, or any
corporation succeeding to all or substantially all of the corporate trust
business of the Trustee, shall be the successor of the Trustee hereunder without
the execution or filing of any paper or any further act on the part of any of
the parties hereto provided such corporations shall be otherwise qualified and
eligible under this Article.

     In case at the time such successor to the Trustee shall succeed to the
trusts created by this Indenture any Securities shall have been authenticated
but not delivered, any such successor to the Trustee may adopt the certificate
of authentication of any predecessor trustee, and deliver such Securities so
authenticated; and in case at that time any of the Securities shall not have
been authenticated, any successor to the Trustee may authenticate such
Securities either in the name of any predecessor hereunder or in the name of the
successor trustee; and in all such cases such certificates shall have the full
force which the Securities or this Indenture elsewhere provides that the
certificate of the Trustee shall have; provided, however, that the right to
adopt the certificate of authentication of any predecessor Trustee or
authenticate Securities in the name of any predecessor Trustee shall apply only
to its successor or successors by merger, conversion or consolidation.

     SECTION 6.13. Limitation on Rights of Trustee as a Creditor.

     The Trustee shall comply with Section 311(a) of the Trust Indenture Act,
excluding any creditor relationship described in Section 311(b) of the Trust
Indenture Act.  A Trustee who has resigned or been removed shall be subject to
Section 311(a) of the Trust Indenture Act to the extent included therein.

     SECTION 6.14. Authenticating Agents.

     There may be one or more Authenticating Agents appointed by the Trustee
upon the request of the Company with power to act on its behalf and subject to
its direction in the authentication and delivery of Securities issued upon
exchange or transfer thereof as fully to all intents and purposes as though any
such Authenticating Agent had been expressly authorized to authenticate and
deliver Securities; provided, that the Trustee shall have no liability to the
Company for any acts or omissions of the Authenticating Agent with respect to
the authentication and delivery of Securities.  Any such Authenticating Agent
shall at all times be a corporation organized and doing business under the laws
of the United States or of any state or territory thereof or of the District of
Columbia authorized under such laws to act as Authenticating Agent, having a
combined capital and surplus of at least $50,000,000 and being subject to
supervision or examination by federal, state, territorial or District 

                                       45
<PAGE>
 
of Columbia authority. If such corporation publishes reports of condition at
least annually pursuant to law or the requirements of such authority, then for
the purposes of this Section 6.14 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. If at any time an
Authenticating Agent shall cease to be eligible in accordance with the
provisions of this Section, it shall resign immediately in the manner and with
the effect herein specified in this Section.

     Any corporation into which any Authenticating Agent may be merged or
converted or with which it may be consolidated, or any corporation resulting
from any merger, consolidation or conversion to which any Authenticating Agent
shall be a party, or any corporation succeeding to the corporate trust business
of any Authenticating Agent, shall be the successor of such Authenticating Agent
hereunder, if such successor corporation is otherwise eligible under this
Section 6.14 without the execution or filing of any paper or any further act on
the part of the parties hereto or such Authenticating Agent.

     Any Authenticating Agent may at any time resign by giving written notice of
resignation to the Trustee and to the Company.  The Trustee may at any time
terminate the agency of any Authenticating Agent by giving written notice of
termination to such Authenticating Agent and to the Company.  Upon receiving
such a notice of resignation or upon such a termination, or in case at any time
any Authenticating Agent shall cease to be eligible under this Section 6.14, the
Trustee may, and upon the request of the Company shall, promptly appoint a
successor Authenticating Agent eligible under this Section 6.14, shall give
written notice of such appointment to the Company and shall mail notice of such
appointment to all Securityholders as the names and addresses of such holders
appear on the Security Register.  Any successor Authenticating Agent upon
acceptance of its appointment hereunder shall become vested with all rights,
powers, duties and responsibilities of its predecessor hereunder, with like
effect as if originally named as Authenticating Agent herein.

     The Company, as borrower, agrees to pay to any Authenticating Agent from
time to time reasonable compensation for its services.  Any Authenticating Agent
shall have no responsibility or liability for any action taken by it as such in
accordance with the directions of the Trustee.


                                  ARTICLE VII
                        CONCERNING THE SECURITYHOLDERS
                        ------------------------------

     SECTION 7.01. Action by Securityholders.

     Whenever in this Indenture it is provided that the holders of a specified
percentage in aggregate principal amount of the Securities may take any action
(including the making of any demand or request, the giving of any notice,
consent or waiver or the taking of any other action) the fact that at the time
of taking any such action the holders of such specified percentage have joined
therein may be evidenced (a) by any instrument or any number of instruments of
similar tenor 

                                       46
<PAGE>
 
executed by such Securityholders in person or by agent or proxy appointed in
writing, or (b) by the record of such holders of Securities voting in favor
thereof at any meeting of such Securityholders duly called and held in
accordance with the provisions of Article VIII, or (c) by a combination of such
instrument or instruments and any such record of such a meeting of such
Securityholders.

     If the Company shall solicit from the Securityholders any request, demand,
authorization, direction, notice, consent, waiver or other action, the Company
may, at its option, as evidenced by an Officers' Certificate, fix in advance a
record date for the determination of Securityholders entitled to give such
request, demand, authorization, direction, notice, consent, waiver or other
action, but the Company shall have no obligation to do so.  If such a record
date is fixed, such request, demand, authorization, direction, notice, consent,
waiver or other action may be given before or after the record date, but only
the Securityholders of record at the close of business on the record date shall
be deemed to be Securityholders for the purposes of determining whether
Securityholders of the requisite proportion of outstanding Securities have
authorized or agreed or consented to such request, demand, authorization,
direction, notice, consent, waiver or other action, and for that purpose the
outstanding Securities shall be computed as of the record date; provided,
however, that no such authorization, agreement or consent by such
Securityholders on the record date shall be deemed effective unless it shall
become effective pursuant to the provisions of this Indenture not later than six
months after the record date.

     SECTION 7.02. Proof of Execution by Securityholders.

     Subject to the provisions of Section 6.01, 6.02 and 8.05, proof of the
execution of any instrument by a Securityholder or his agent or proxy shall be
sufficient if made in accordance with such reasonable rules and regulations as
may be prescribed by the Trustee or in such manner as shall be satisfactory to
the Trustee.  The ownership of Securities shall be proved by the Security
Register or by a certificate of the Security registrar.  The Trustee may require
such additional proof of any matter referred to in this Section as it shall deem
necessary.

     The record of any Securityholders' meeting shall be proved in the manner
provided in Section 8.06.

     SECTION 7.03. Who Are Deemed Absolute Owners.

     Prior to due presentment for registration of transfer of any Security, the
Company, the Trustee, any Authenticating Agent, any paying agent, any transfer
agent and any Security registrar may deem the person in whose name such Security
shall be registered upon the Security Register to be, and may treat him as, the
absolute owner of such Security (whether or not such Security shall be overdue)
for the purpose of receiving payment of or on account of the principal of and
premium, if any, and (subject to Section 2.06) interest on such Security and for
all other purposes; and neither the Company nor the Trustee nor any
Authenticating Agent nor any paying agent nor any transfer agent nor any
Security registrar shall be affected by any notice to the contrary.  All such
payments 

                                       47
<PAGE>
 
so made to any holder for the time being or upon his order shall be valid, and,
to the extent of the sum or sums so paid, effectual to satisfy and discharge the
liability for moneys payable upon any such Security.

     SECTION 7.04. Securities Owned by Company Deemed Not Outstanding.

     In determining whether the holders of the requisite aggregate principal
amount of Securities have concurred in any direction, consent, notice or waiver
under this Indenture, Securities which are owned by the Company or any other
obligor on the Securities or any Affiliate of the Company (other than the Trust)
or any other obligor on the Securities shall be disregarded and deemed not to be
outstanding for the purpose of any such determination; provided that for the
purposes of determining whether the Trustee shall be protected in relying on any
such direction, consent or waiver, only Securities which a Responsible Officer
of the Trustee actually knows are so owned shall be so disregarded.
Securities so owned which have been pledged in good faith may be regarded as
outstanding for the purposes of this Section 7.04 if the pledgee shall establish
to the satisfaction of the Trustee the pledgee's right to vote such Securities
and that the pledgee is not the Company or any such other obligor or Affiliate
of the Company or any such other obligor.  In the case of a dispute as to such
right, any decision by the Trustee taken upon the advice of counsel shall be
full protection to the Trustee.

     SECTION 7.05. Revocation of Consents; Future Holders Bound.

     At any time prior to (but not after) the evidencing to the Trustee, as
provided in Section 7.01, of the taking of any action by the holders of the
percentage in aggregate principal amount of the Securities specified in this
Indenture in connection with such action, any holder of a Security (or any
Security issued in whole or in part in exchange or substitution therefor),
subject to Section 7.01, the serial number of which is shown by the evidence to
be included in the Securities the holders of which have consented to such action
may, by filing written notice with the Trustee at its principal office and upon
proof of holding as provided in Section 7.02, revoke such action so far as
concerns such Security (or so far as concerns the principal amount represented
by any exchanged or substituted Security).  Except as aforesaid any such action
taken by the holder of any Security shall be conclusive and binding upon such
holder and upon all future holders and owners of such Security, and of any
Security issued in exchange or substitution therefor, irrespective of whether or
not any notation in regard thereto is made upon such Security or any Security
issued in exchange or substitution therefor.


                                 ARTICLE VIII
                           SECURITYHOLDERS' MEETINGS
                           -------------------------

     SECTION 8.01. Purposes of Meetings.

                                       48
<PAGE>
 
     A meeting of Securityholders may be called at any time and from time to
time pursuant to the provisions of this Article VIII for any of the following
purposes:

          (a) to give any notice to the Company or to the Trustee, or to give
     any directions to the Trustee, or to consent to the waiving of any default
     hereunder and its consequences, or to take any other action authorized to
     be taken by Securityholders pursuant to any of the provisions of Article V;

          (b) to remove the Trustee and nominate a successor trustee pursuant to
     the provisions of Article VI;

          (c) to consent to the execution of an indenture or indentures
     supplemental hereto pursuant to the provisions of Section 9.02; or

          (d) to take any other action authorized to be taken by or on behalf of
     the holders of any specified aggregate principal amount of such Securities
     under any other provision of this Indenture or under applicable law.

     SECTION 8.02. Call of Meetings by Trustee.

     The Trustee may at any time call a meeting of Securityholders to take any
action specified in Section 8.01, to be held at such time and at such place in
the Borough of Manhattan, The City of New York, as the Trustee shall determine.
Notice of every meeting of the Securityholders, setting forth the time and the
place of such meeting and in general terms the action proposed to be taken at
such meeting, shall be mailed to holders of Securities at their addresses as
they shall appear on the Securities Register.  Such notice shall be mailed not
less than 20 nor more than 180 days prior to the date fixed for the meeting.

     SECTION 8.03. Call of Meetings by Company or Securityholders.

     In case at any time the Company pursuant to a resolution of the Board of
Directors, or the holders of at least 10% in aggregate principal amount of the
Securities then outstanding, shall have requested the Trustee to call a meeting
of Securityholders, by written request setting forth in reasonable detail the
action proposed to be taken at the meeting, and the Trustee shall not have
mailed the notice of such meeting within 20 days after receipt of such request,
then the Company or such Securityholders may determine the time and the place in
said Borough of Manhattan for such meeting and may call such meeting to take any
action authorized in Section 8.01, by mailing notice thereof as provided in
Section 8.02.

     SECTION 8.04. Qualifications for Voting.

                                       49
<PAGE>
 
     To be entitled to vote at any meeting of Securityholders a Person shall (a)
be a holder of one or more Securities or (b) a Person appointed by an instrument
in writing as proxy by a holder of one or more Securities.  The only Persons who
shall be entitled to be present or to speak at any meeting of Securityholders
shall be the Persons entitled to vote at such meeting and their counsel and any
representatives of the Trustee and its counsel and any representatives of the
Company and its counsel.

     SECTION 8.05. Regulations.

     Notwithstanding any other provisions of this Indenture, the Trustee may
make such reasonable regulations as it may deem advisable for any meeting of
Securityholders, in regard to proof of the holding of Securities and of the
appointment of proxies, and in regard to the appointment and duties of
inspectors of votes, the submission and examination of proxies, certificates and
other evidence of the right to vote, and such other matters concerning the
conduct of the meeting as it shall think fit.

     The Trustee shall, by an instrument in writing, appoint a temporary
chairman of the meeting, unless the meeting shall have been called by the
Company or by Securityholders as provided in Section 8.03, in which case the
Company or the Securityholders calling the meeting, as the case may be, shall in
like manner appoint a temporary chairman.  A permanent chairman and a permanent
secretary of the meeting shall be elected by majority vote of the meeting.

     Subject to the provisions of Section 8.04, at any meeting each holder of
Securities or proxy therefor shall be entitled to one vote for each $1,000
principal amount of Securities held or represented by him; provided, however,
that no vote shall be cast or counted at any meeting in respect of any Security
challenged as not outstanding and ruled by the chairman of the meeting to be not
outstanding.  The chairman of the meeting shall have no right to vote other than
by virtue of Securities held by him or instruments in writing as aforesaid duly
designating him as the person to vote on behalf of other Securityholders.  Any
meeting of Securityholders duly called pursuant to the provisions of Section
8.02 or 8.03 may be adjourned from time to time by a majority of those present,
and the meeting may be held as so adjourned without further notice.

     SECTION 8.06. Voting.

     The vote upon any resolution submitted to any meeting of holders of
Securities shall be by written ballots on which shall be subscribed the
signatures of such holders or of their representatives by proxy and the serial
number or numbers of the Securities held or represented by them.  The permanent
chairman of the meeting shall appoint two inspectors of votes who shall count
all votes cast at the meeting for or against any resolution and who shall make
and file with the secretary of the meeting their verified written reports in
triplicate of all votes cast at the meeting.  A record in duplicate of the
proceedings of each meeting of Securityholders shall be prepared by the
secretary of the meeting and there shall be attached to said record the original
reports of the inspectors of votes 

                                       50
<PAGE>
 
on any vote by ballot taken thereat and affidavits by one or more persons having
knowledge of the facts setting forth a copy of the notice of the meeting and
showing that said notice was mailed as provided in Section 8.02. The record
shall show the serial numbers of the Securities voting in favor of or against
any resolution. The record shall be signed and verified by the affidavits of the
permanent chairman and secretary of the meeting and one of the duplicates shall
be delivered to the Company and the other to the Trustee to be preserved by the
Trustee, the latter to have attached thereto the ballots voted at the meeting.
The holders of the Series A Capital Securities, the Series B Capital Securities
and the Private Exchange Securities shall vote for all purposes as a single
class.

     Any record so signed and verified shall be conclusive evidence of the
matters therein stated.


                                  ARTICLE IX
                            SUPPLEMENTAL INDENTURES
                            -----------------------

     SECTION 9.01. Without Consent of Securityholders.

     The Company, when authorized by a Board Resolution, and the Trustee may
from time to time and at any time enter into one or more indentures supplemental
hereto, in form satisfactory to the Trustee, without the consent of the
Securityholders, for one or more of the following purposes:

          (a) to evidence the succession of another Person to the Company, or
     successive successions, and the assumption by the successor Person of the
     covenants, agreements and obligations of the Company herein and in the
     Securities pursuant to Article X hereof;

          (b) to add to the covenants of the Company such further covenants,
     restrictions or conditions for the protection of the Securityholders as the
     Board of Directors and the Trustee shall consider to be for the protection
     of the Securityholders, and to make the occurrence, or the occurrence and
     continuance, of a default in any of such additional covenants, restrictions
     or conditions a default or an Event of Default permitting the enforcement
     of all or any of the remedies provided in this Indenture as herein set
     forth; provided, however, that in respect of any such additional covenant,
     restriction or condition such amendment may provide for a particular period
     of grace after default (which period may be shorter or longer than that
     allowed in the case of other defaults) or may provide for an immediate
     enforcement upon such default or may limit the remedies available to the
     Trustee upon such default;

          (c) to provide for the issuance under this Indenture of Securities in
     coupon form (including Securities registrable as to principal only) and to
     provide for exchangeability of such Securities with the Securities issued
     hereunder in fully registered form and to make all appropriate changes for
     such purpose;

                                       51
<PAGE>
 
          (d) to cure any ambiguity or to correct or supplement any provision
     contained herein or in any supplemental indenture which may be defective or
     inconsistent with any other provision contained herein or in any
     supplemental indenture, or to make such other provisions in regard to
     matters or questions arising under this Indenture; provided that any such
     action shall not materially adversely affect the interests of the holders
     of the Securities;

          (e) to evidence and provide for the acceptance of appointment
     hereunder by a successor trustee with respect to the Securities;

          (f) to make provision for transfer procedures, certification, book-
     entry provisions, the form of restricted securities legends, if any, to be
     placed on Securities, and all other matters required pursuant to Section
     2.07 or otherwise necessary, desirable or appropriate in connection with
     the issuance of Securities to holders of Capital Securities in the event of
     a distribution of Securities by NGC Corporation Capital Trust following a
     Dissolution Event;

          (g) to qualify or maintain qualification of this Indenture under the
     Trust Indenture Act; or

          (h) to make any change that does not adversely affect the rights of
     any Securityholder in any material respect.

     The Trustee is hereby authorized to join with the Company in the execution
of any supplemental indenture to effect such amendment, to make any further
appropriate agreements and stipulations which may be therein contained and to
accept the conveyance, transfer and assignment of any property thereunder, but
the Trustee shall not be obligated to, but may in its discretion, enter into any
such supplemental indenture which affects the Trustee's own rights, duties or
immunities under this Indenture or otherwise.

     Any amendment to the Indenture authorized by the provisions of this Section
9.01 may be executed by the Company and the Trustee without the consent of the
holders of any of the Securities at the time outstanding, notwithstanding any of
the provisions of Section 9.02.

     SECTION 9.02. With Consent of Securityholders.

     With the consent (evidenced as provided in Section 7.01) of the holders of
a majority in aggregate principal amount of the Securities at the time
outstanding, the Company, when authorized by a Board Resolution, and the Trustee
may from time to time and at any time enter into an indenture or indentures
supplemental hereto for the purpose of adding any provisions to or changing in
any manner or eliminating any of the provisions of this Indenture or of
modifying in any manner the rights of the holders of the Securities; provided,
however, that no such amendment shall without the 

                                       52
<PAGE>
 
consent of the holders of each Security then outstanding and affected thereby
(i) change the Maturity Date of any Security, or reduce the rate of interest
(including Compounded Interest) or extend the time of payment of interest
thereon (except as contemplated by Article XVI), or reduce the principal amount
thereof or the amount of premium thereon, if any, or reduce any amount payable
on redemp tion thereof, or make the principal thereof or any interest or premium
thereon payable in any coin or currency other than that provided in the
Securities, or impair or affect the right of any Securityholder to institute
suit for payment thereof, (ii) modify the provisions of this Indenture with
respect to the subordination of the Securities in a manner adverse to the
holders, (iii) reduce the aforesaid percentage of Securities the holders of
which are required to consent to any such amend ment to the Indenture, or the
consent of whose holders is required for any waiver (of compliance with certain
provisions of this Indenture or certain defaults hereunder and their
consequences) provided for in this Indenture or (iv) modify any of the
provisions of the first paragraph of this Section 9.02, or the second sentence
of Section 5.07, except to increase any such percentage or to provide that
certain other provisions of this Indenture cannot be modified or waived without
the consent of the holder of each outstanding Security affected thereby;
provided, however, that if the Securities are held by NGC Corporation Capital
Trust, such amendment shall not be effective until the holders of a majority in
liquidation amount of Trust Securities shall have consented to such amendment;
provided, further, that if the consent of the holder of each outstanding
Security is required, such amendment shall not be effective until each holder of
the Trust Securities shall have consented to such amendment.

     Upon the request of the Company accompanied by a copy of a resolution of
the Board of Directors certified by its Secretary or Assistant Secretary
authorizing the execution of any supplemental indenture affecting such
amendment, and upon the filing with the Trustee of evidence of the consent of
Securityholders as aforesaid, the Trustee shall join with the Company in the
execution of such supplemental indenture unless such supplemental indenture
affects the Trustee's own rights, duties or immunities under this Indenture or
otherwise, in which case the Trustee may in its discretion, but shall not be
obligated to, enter into such supplemental indenture.

     Promptly after the execution by the Company and the Trustee of any
supplemental indenture pursuant to the provisions of this Section, the Trustee
shall transmit by mail, first class postage prepaid, a notice, prepared by the
Company, setting forth in general terms the substance of such supplemental
indenture, to the Securityholders as their names and addresses appear upon the
Security Register.  Any failure of the Trustee to mail such notice, or any
defect therein, shall not, however, in any way impair or affect the validity of
any such supplemental indenture.

     It shall not be necessary for the consent of the Securityholders under this
Section 9.02 to approve the particular form of any proposed supplemental
indenture, but it shall be sufficient if such consent shall approve the
substance thereof.

     SECTION 9.03. Compliance with Trust Indenture Act; Effect of Supplemental
                   Indentures.

                                       53
<PAGE>
 
     Any supplemental indenture executed pursuant to the provisions of this
Article IX shall comply with the Trust Indenture Act.  Upon the execution of any
supplemental indenture pursuant to the provisions of this Article IX, this
Indenture shall be and be deemed to be modified and amended in accordance
therewith and the respective rights, limitations of rights, obligations, duties
and immunities under this Indenture of the Trustee, the Company and the holders
of Securities shall thereafter be determined, exercised and enforced hereunder
subject in all respects to such modifications and amendments and all the terms
and conditions of any such supplemental indenture shall be and be deemed to be
part of the terms and conditions of this Indenture for any and all purposes.

     SECTION 9.04. Notation on Securities.

     Securities authenticated and delivered after the execution of any
supplemental indenture affecting such series pursuant to the provisions of this
Article IX may bear a notation in form approved by the Trustee as to any matter
provided for in such supplemental indenture.  If the Company or the Trustee
shall so determine, new Securities so modified as to conform, in the opinion of
the Trustee and the Board of Directors, to any modification of this Indenture
contained in any such supplemental indenture may be prepared and executed by the
Company, authenticated by the Trustee or the Authenticating Agent and delivered
in exchange for the Securities then outstanding.

     SECTION 9.05. Evidence of Compliance of Supplemental Indenture to be
                   Furnished Trustee.

     The Trustee, subject to the provisions of Sections 6.01 and 6.02, may
receive, in addition to the document required by Section 13.06, an Officers'
Certificate and an Opinion of Counsel as conclusive evidence that any
supplemental indenture executed pursuant hereto complies with the requirements
of this Article IX.


                                   ARTICLE X
               CONSOLIDATION, MERGER, SALE, CONVEYANCE AND LEASE
               -------------------------------------------------

     SECTION 10.01.     Company May Consolidate, etc., on Certain Terms.

     Nothing contained in this Indenture or in any of the Securities shall
prevent any consolidation or merger of the Company with or into any other Person
(whether or not affiliated with the Company, as the case may be), or successive
consolidations or mergers in which the Company or its successor or successors,
as the case may be, shall be a party or parties, or shall prevent any sale,
conveyance, transfer or lease of the property of the Company, or its successor
or successors as the case may be, as an entirety, or substantially as an
entirety, to any other Person (whether or not affili  ated with the Company, or
its successor or successors, as the case may be) authorized to acquire 

                                       54
<PAGE>
 
and operate the same; provided, that (a) the Company is the surviving Person, or
the Person formed by or surviving any such consolidation or merger (if other
than the Company) or to which such sale, conveyance, transfer or lease of
property is made is a Person organized and existing under the laws of the United
States or any State thereof or the District of Columbia, and (b) upon any such
consolidation, merger, sale, conveyance, transfer or lease, the due and punctual
payment of the principal of (and premium, if any) and interest on the Securities
according to their tenor and the due and punctual performance and observance of
all the covenants and conditions of this Indenture to be kept or performed by
the Company shall be expressly assumed, by supplemental indenture (which shall
conform to the provisions of the Trust Indenture Act, as then in effect)
satisfactory in form to the Trustee executed and delivered to the Trustee by the
Person formed by such consolidation, or into which the Company shall have been
merged, or by the Person which shall have acquired such property, as the case
may be and (c) after giving effect to such consolidation, merger, sale,
conveyance, transfer or lease, no Default or Event of Default shall have
occurred and be continuing.

     SECTION 10.02.     Successor Corporation to be Substituted for Company.

     In case of any such consolidation, merger, conveyance or transfer and upon
the assumption by the successor corporation, by supplemental indenture, executed
and delivered to the Trustee and satisfactory in form to the Trustee, of the due
and punctual payment of the principal of and premium, if any, and interest
(including any Additional Sums or Compounded Interest or Liquidated Damages) on
all of the Securities and the due and punctual performance and observance of all
of the covenants and conditions of this Indenture to be performed or observed by
the Company, such successor Person shall succeed to and be substituted for the
Company, with the same effect as if it had been named herein as the party of the
first part, and the Company thereupon shall be relieved of any further liability
or obligation hereunder or upon the Securities.  Such successor Person thereupon
may cause to be signed, and may issue either in its own name or in the name of
NGC Corporation, any or all of the Securities issuable hereunder which
theretofore shall not have been signed by the Company and delivered to the
Trustee or the Authenticating Agent; and, upon the order of such successor
Person instead of the Company and subject to all the terms, conditions and
limitations in this Indenture prescribed, the Trustee or the Authenticating
Agent shall authenticate and deliver any Securities which previously shall have
been signed and delivered by the officers of the Company to the Trustee or the
Authenticating Agent for authentication, and any Securities which such successor
Person thereafter shall cause to be signed and delivered to the Trustee or the
Authenticating Agent for that purpose.  All the Securities so issued shall in
all respects have the same legal rank and benefit under this Indenture as the
Securities theretofore or thereafter issued in accordance with the terms of this
Indenture as though all of such Indentures had been issued at the date of the
execution hereof.

     SECTION 10.03.     Opinion of Counsel to be Given Trustee.

     The Trustee, subject to the provisions of Sections 6.01 and 6.02, may
receive an Opinion of Counsel as conclusive evidence that any consolidation,
merger, sale, conveyance, transfer or lease, 

                                       55
<PAGE>
 
and any assumption, permitted or required by the terms of this Article X
complies with the provisions of this Article X.


                                  ARTICLE XI
                    SATISFACTION AND DISCHARGE OF INDENTURE
                    ---------------------------------------

     SECTION 11.01.     Discharge of Indenture.

     When (a) the Company shall deliver to the Trustee for cancellation all
Securities theretofore authenticated (other than any Securities which shall have
been destroyed, lost or stolen and which shall have been replaced as provided in
Section 2.08) and not theretofore canceled, or (b) all the Securities not
theretofore canceled or delivered to the Trustee for cancellation shall have
become due and payable, or are by their terms to become due and payable within
one year or are to be called for redemption within one year under arrangements
satisfactory to the Trustee for the giving of notice of redemption, and the
Company shall deposit with the Trustee, in trust, funds sufficient to pay on the
Maturity Date or upon redemption all of the Securities (other than any
Securities which shall have been destroyed, lost or stolen and which shall have
been replaced as provided in Section 2.08) not theretofore canceled or delivered
to the Trustee for cancellation, including principal and premium, if any, and
interest (including Compounded Interest and Additional Sums, if any) and
Liquidated Damages, if any, due or to become due to the Maturity Date or
redemption date, as the case may be, but excluding, however, the amount of any
moneys for the payment of principal of or premium, if any, or interest
(including Compounded Interest and Additional Sums, if any) or Liquidated
Damages, if any, on the Securities (1) theretofore repaid to the Company in
accordance with the provisions of Section 11.04, or (2) paid to any State or to
the District of Columbia pursuant to its unclaimed property or similar laws, and
if in either case the Company shall also pay or cause to be paid all other sums
payable hereunder by the Company, then this Indenture shall cease to be of
further effect except for the provisions of Sections 2.02, 2.07, 2.08, 3.01,
3.02, 3.04, 6.06, 6.10 and 11.04 hereof, which shall survive until such
Securities shall mature and be paid.  Thereafter, Sections 6.06, 6.10 and 11.04
shall survive, and the Trustee, on demand of the Company accompanied by any
Officers' Certificate and an Opinion of Counsel and at the cost and expense of
the Company, shall execute proper instruments acknowledging satisfaction of and
discharging this Indenture, the Company, however, hereby agreeing to reimburse
the Trustee for any costs or expens  es thereafter reasonably and properly
incurred by the Trustee in connection with this Indenture or the Securities.

      SECTION 11.02.    Deposited Moneys and U.S. Government Obligations to be
                        Held in Trust by Trustee.

     Subject to the provisions of Section 11.04, all moneys and U.S. Government
Obligations deposited with the Trustee pursuant to Sections 11.01 or 11.05 shall
be held in trust and applied by it to the payment, either directly or through
any paying agent (including the Company if acting as 

                                       56
<PAGE>
 
its own paying agent), to the holders of the particular Securities for the
payment of which such moneys or U.S. Government Obligations have been deposited
with the Trustee, of all sums due and to become due thereon for principal,
premium, if any, and interest.

     The Company shall pay and indemnify the Trustee against any tax, fee or
other charge imposed on or assessed against the U.S. Government Obligations
deposited pursuant to Section 11.05 or the principal and interest received in
respect thereof other than any such tax, fee or other charge which by law is for
the account of the holders of outstanding Securities.

     SECTION 11.03.     Paying Agent to Repay Moneys Held.

     Upon the satisfaction and discharge of this Indenture all moneys then held
by any paying agent of the Securities (other than the Trustee) shall, upon
written demand of the Company, be repaid to it or paid to the Trustee, and
thereupon such paying agent shall be released from all further liability with
respect to such moneys.

     SECTION 11.04.     Return of Unclaimed Moneys.

     Any moneys deposited with or paid to the Trustee or any paying agent for
payment of the principal of or premium, if any, or interest on Securities and
not applied but remaining unclaimed by the holders of Securities for two years
after the date upon which the principal of or premium, if any, or interest
(including Compounded Interest and Additional Sums, if any) or Liquidated
Damages, if any, on such Securities, as the case may be, shall have become due
and payable, shall be repaid to the Company by the Trustee or such paying agent
on written demand; and the holder of any of the Securities shall thereafter look
only to the Company for any payment which such holder may be entitled to collect
and all liability of the Trustee or such paying agent with respect to such
moneys shall thereupon cease.

     SECTION 11.05.     Defeasance Upon Deposit of Moneys or U.S. Government
                        Obligations.

     The Company shall be deemed to have been Discharged (as defined below) from
its obligations with respect to the Securities on the 91st day after the
applicable conditions set forth below have been satisfied:

          (a) the Company shall have deposited or caused to be deposited
     irrevocably with the Trustee or the Defeasance Agent (as defined below) as
     trust funds in trust, specifically pledged as security for, and dedicated
     solely to, the benefit of the holders of the Securities (i) money in an
     amount, or (ii) U.S. Government Obligations which through the payment of
     interest and principal in respect thereof in accordance with their terms
     will provide, not later than one day before the due date of any payment,
     money in an amount, or (iii) a combination of (i) and (ii), sufficient, in
     the opinion (with respect to (ii) and (iii)) of a nationally 

                                       57
<PAGE>
 
     recognized firm of independent public accountants expressed in a written
     certification thereof delivered to the Trustee and the Defeasance Agent, if
     any, to pay and discharge each installment of principal of and interest and
     premium, if any, on the outstanding Securities on the dates such
     installments of principal, interest or premium are due;

          (b) if the Securities are then listed on any national securities
     exchange, the Company shall have delivered to the Trustee and the
     Defeasance Agent, if any, an Opinion of Counsel to the effect that the
     exercise of the option under this Section 11.05 would not cause such
     Securities to be delisted from such exchange;

          (c) no Default or Event of Default with respect to the Securities
     shall result from such deposit or shall have occurred and be continuing on
     the date of such deposit; and

          (d) the Company shall have delivered to the Trustee and the Defeasance
     Agent, if any, an Opinion of Counsel to the effect that holders of the
     Securities will not recognize income, gain or loss for United States
     federal income tax purposes as a result of the exercise of the option under
     this Section 11.05 and will be subject to United States federal income tax
     on the same amount and in the same manner and at the same times as would
     have been the case if such option had not been exercised, and such opinion
     shall be based on a statute so providing or be accompanied by a private
     letter ruling to that effect received from the United States Internal
     Revenue Service or a revenue ruling pertaining to a comparable form of
     transaction to that effect published by the United States Internal Revenue
     Service.

     "Discharged" means that the Company shall be deemed to have paid and
discharged the entire indebtedness represented by, and obligations under, the
Securities and to have satisfied all the obligations under this Indenture
relating to the Securities (and the Trustee, at the expense of the Company,
shall execute proper instruments acknowledging the same), except (A) the rights
of holders of Securities to receive, from the trust fund described in clause (1)
above, payment of the principal of and the interest and premium, if any, on the
Securities when such payments are due; (B) the Company's obligations with
respect to the Securities under Sections 2.07, 2.08, 5.02 and 11.04; and (C) the
rights, powers, trusts, duties and immunities of the Trustee hereunder.

     "Defeasance Agent" means another financial institution which is eligible to
act as Trustee hereunder and which assumes all of the obligations of the Trustee
necessary to enable the Trustee to act hereunder.  In the event such a
Defeasance Agent is appointed pursuant to this Section, the following conditions
shall apply:

          (a) The Trustee shall have approval rights over the document
     appointing such Defeasance Agent and the document setting forth such
     Defeasance Agent's rights and responsibilities;

                                       58
<PAGE>
 
          (b) The Defeasance Agent shall provide verification to the Trustee
     acknowledging receipt of sufficient money and/or U. S. Government
     Obligations to meet the applicable conditions set forth in this Section
     11.05.


                                  ARTICLE XII
                   IMMUNITY OF INCORPORATORS, STOCKHOLDERS,
                   ----------------------------------------
                            OFFICERS AND DIRECTORS
                            ----------------------

     SECTION 12.01.     Indenture and Securities Solely Corporate Obligations.

     No recourse for the payment of the principal of or premium, if any, or
interest on any Security, or for any claim based thereon or otherwise in respect
thereof, and no recourse under or upon any obligation, covenant or agreement of
the Company in this Indenture, or in any Security, or because of the creation of
any indebtedness represented thereby, shall be had against any incorporator,
stockholder, officer or director, as such, past, present or future, of the
Company or of any successor Person to the Company, either directly or through
the Company or any successor Person to the Company, 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 all such liability is
hereby expressly waived and released as a condition of, and as a consideration
for, the execution of this Indenture and the issue of the Securities.


                                 ARTICLE XIII
                           MISCELLANEOUS PROVISIONS
                           ------------------------

     SECTION 13.01.     Successors.

     All the covenants, stipulations, promises and agreements in this Indenture
contained by the Company shall bind its successors and assigns whether so
expressed or not.

     SECTION 13.02.     Official Acts by Successor Corporation.

     Any act or proceeding by any provision of this Indenture authorized or
required to be done or performed by any board, committee or officer of the
Company shall and may be done and performed with like force and effect by the
like board, committee or officer of any corporation that shall at the time be
the lawful sole successor of the Company.

     SECTION 13.03.     Surrender of Company Powers.

     The Company by instrument in writing executed by authority of 2/3 (two-
thirds) of its Board of Directors and delivered to the Trustee may surrender any
of the powers reserved to the Company, 

                                       59
<PAGE>
 
and thereupon such power so surrendered shall terminate both as to the Company,
as the case may be, and as to any successor Person.

     SECTION 13.04.     Addresses for Notices, etc.

     Any notice or demand which by any provision of this Indenture is required
or permitted to be given or served by the Trustee or by the holders of
Securities on the Company may be given or served by being deposited postage
prepaid by first class mail, registered or certified mail, overnight courier
service or conformed telecopy addressed (until another address is filed by the
Company with the Trustee for the purpose) to the Company at 1000 Louisiana,
Suite 5800, Houston, Texas 77002 Attention: Chief Financial Officer.  Any
notice, direction, request or demand by any Securityholder to or upon the
Trustee shall be deemed to have been sufficiently given or made, for all
purposes, if given or made in writing at the office of the Trustee, Rodney
Square North, 1100 North Market Street, Wilmington, Delaware 19890, Attention:
Corporate Trust Administration (unless another address is provided by the
Trustee to the Company for such purpose).  Any notice or communication to a
Securityholder shall be mailed by first class mail to his or her address shown
on the register kept by the Security Registrar.

     SECTION 13.05.     GOVERNING LAW.

     THIS INDENTURE AND EACH SECURITY SHALL BE DEEMED TO BE A CONTRACT MADE
UNDER THE LAWS OF THE STATE OF NEW YORK, AND FOR ALL PURPOSES SHALL BE GOVERNED
BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF SAID STATE.

     SECTION 13.06.     Evidence of Compliance with Conditions Precedent.

     Upon any application or request by the Company to the Trustee to take any
action under any of the provisions of this Indenture, the Company shall furnish
to the Trustee an Officers' Certificate stating that in the opinion of the
signers all conditions precedent, if any, provided for in this Indenture
relating to the proposed action have been complied with and an Opinion of
Counsel stating that, in the opinion of such counsel, all such conditions
precedent have been complied with.

     Each certificate or opinion provided for in this Indenture and delivered to
the Trustee with respect to compliance with a condition or covenant provided for
in this Indenture (except certificates delivered pursuant to Section 3.05) shall
include (1) a statement that the Person making such certificate or opinion has
read such covenant or condition; (2) 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; (3) a statement that, in the
opinion of such Person, he has made such examination or investigation as is
necessary to enable him to express an informed opinion as to whether or not such
covenant or condition has been complied with; and (4) a statement as to whether
or not, in the opinion of such person, such condition or covenant has been
complied with.

                                       60
<PAGE>
 
     SECTION 13.07.     Business Days.

     In any case where the date of payment of principal of or premium, if any,
or interest on the Securities will not be a Business Day, the payment of such
principal of or premium, if any, or interest on the Securities need not be made
on such date but may be made on the next succeeding Business Day, with the same
force and effect as if made on the date of payment and no interest shall accrue
for the period from and after such date, except that if such next succeeding
Business Day falls in the next succeeding calendar year, then such payment shall
be made on the immediately preceding Business Day, in each case with the same
force and effect as if made on such date.

     SECTION 13.08.     Trust Indenture Act to Control.

     If and to the extent that any provision of this Indenture limits, qualifies
or conflicts with the duties imposed by Sections 310 to 317, inclusive, of the
Trust Indenture Act of 1939, such imposed duties shall control.

     SECTION 13.09.     Table of Contents, Headings, etc.

     The table of contents and the titles and headings of the articles and
sections of this Indenture have been inserted for convenience of reference only,
are not to be considered a part hereof, and shall in no way modify or restrict
any of the terms or provisions hereof.

     SECTION 13.10.     Execution in Counterparts.

     This Indenture may be executed in any number of counterparts, each of which
shall be an original, but such counterparts shall together constitute but one
and the same instrument.

     SECTION 13.11.     Separability.

     In case any one or more of the provisions contained in this Indenture or in
the Securities shall for any reason be held to be invalid, illegal or
unenforceable in any respect, such invalidity, illegality or unenforceability
shall not affect any other provisions of this Indenture or of the Securities,
but this Indenture and the Securities shall be construed as if such invalid or
illegal or unenforceable provision had never been contained herein or therein.

     SECTION 13.12.     Assignment.

     The Company will have the right at all times to assign any of its
respective rights or obligations under this Indenture to a direct or indirect
wholly owned Subsidiary of the Company; provided that, in the event of any such
assignment, the Company will remain liable for all such obligations.  Subject to
the foregoing, the Indenture is binding upon and inures to the benefit of the

                                       61
<PAGE>
 
parties thereto and their respective successors and assigns.  This Indenture may
not otherwise be assigned by the parties thereto.

     SECTION 13.13.     Acknowledgment of Rights.

     The Company acknowledges that, with respect to any Securities held by NGC
Corporation Capital Trust or a trustee of such trust, if the Property Trustee of
such Trust fails to enforce its rights under this Indenture as the holder of the
Securities held as the assets of NGC Corporation Capital Trust any holder of
Capital Securities may, to the full extent permitted by law, institute legal
proceedings directly against the Company to enforce such Property Trustee's
rights under this Indenture without first instituting any legal proceedings
against such Property Trustee or any other person or entity.  Notwithstanding
the foregoing, if an Event of Default has occurred and is continuing and such
event is attributable to the failure of the Company to pay principal of or
premium, if any, or interest on the Securities when due, the Company
acknowledges that a holder of Capital Securities may directly institute a
proceeding for enforcement of payment to such holder of the principal of or
premium, if any, or interest on the Securities having a principal amount equal
to the aggregate liquidation amount of the Capital Securities of such holder on
or after the respective due date specified in the Securities.


                                  ARTICLE XIV
                           REDEMPTION OF SECURITIES
                           ------------------------

     SECTION 14.01.     Special Event Redemption.

     If, at any time, a Special Event has occurred and is continuing then,
notwithstanding Section 14.02(a), the Company shall have the right but not the
obligation, at any time within 90 days following the occurrence of such Special
Event, upon (i) not less than 45 days' written notice to the Trustee and (ii)
not less than 30 days' nor more than 60 days' written notice to the
Securityholders, to redeem the Securities, in whole (but not in part), at the
Special Event Redemption Price. Following a Special Event, if the Company wishes
to exercise its right of redemption pursuant to this Section 14.01, the Company
shall take such action as is necessary to promptly determine the Special Event
Redemption Price, including without limitation the appointment by the Company of
a Reference Treasury Dealer.  The Company shall provide the Trustee with written
notice of the Special Event Redemption Price promptly after the calculation
thereof, which notice shall include any calculation made by the Reference
Treasury Dealer in connection with the determination of the Special Event
Redemption Price.  The Special Event Redemption Price shall be paid prior to
12:00 noon, New York City time, on the date of such redemption or such earlier
time as the Company determines, provided that the Company shall deposit with the
Trustee an amount sufficient to pay the Special Event Redemption Price by 10:00
a.m., New York City time, on the date such Special Event Redemption Price is to
be paid.

     SECTION 14.02.     Optional Redemption by Company.

                                       62
<PAGE>
 
     Subject to the provisions of this Article XIV, the Company shall have the
right to redeem the Securities, in whole or in part, from time to time, at a
redemption price equal to the Optional Redemption Price.

     If the Securities are only partially redeemed pursuant to this Section
14.02, the Securities to be redeemed shall be selected on a pro rata basis not
more than 60 days prior to the date fixed for redemption from the outstanding
Securities not previously called for redemption; provided, however, that with
respect to Securityholders that would be required to hold Securities with an
aggregate principal amount of less than $100,000 but more than an aggregate
principal amount of zero as a result of such pro rata redemption, the Company
shall redeem Securities of each such Securityholder so that after such
redemption such Securityholder shall hold Securities either with an aggregate
principal amount of at least $100,000 or such Securityholder no longer holds any
Securities and shall use such method (including, without limitation, by lot) as
the Company shall deem fair and appropriate; provided, further, that any such
proration may be made on the basis of the aggregate principal amount of
Securities held by each Securityholder and may be made by making such
adjustments as the Company deems fair and appropriate in order that only
Securities in denominations of $1,000 or integral multiples thereof shall be
redeemed.  The Optional Redemption Price shall be paid prior to 12:00 noon, New
York City time, on the date of such redemption or at such earlier time as the
Company determines; provided that the Company shall deposit with the Trustee an
amount sufficient to pay the Optional Redemption Price by 10:00 a.m., New York
City time, on the date such Optional Redemption Price is to be paid.

     SECTION 14.03.     No Sinking Fund.

     The Securities are not entitled to the benefit of any sinking fund.

     SECTION 14.04.     Notice of Redemption; Selection of Securities.

     In case the Company shall desire to exercise the right to redeem all, or,
as the case may be, any part of the Securities in accordance with their terms,
it shall fix a date for redemption and shall mail a notice of such redemption at
least 30 and not more than 60 days prior to the date fixed for redemption to the
holders of Securities so to be redeemed as a whole or in part at their last
addresses as the same appear on the Security Register.  Such mailing shall be by
first class mail.  The notice if mailed in the manner herein provided shall be
conclusively presumed to have been duly given, whether or not the holder
receives such notice.  In any case, failure to give such notice by mail or any
defect in the notice to the holder of any Security designated for redemption as
a whole or in part shall not affect the validity of the proceedings for the
redemption of any other Security.

     Each such notice of redemption shall specify the CUSIP number of the
Securities to be redeemed, the date fixed for redemption, the redemption price
at which the Securities are to be redeemed (or the method by which such
redemption price is to be calculated), the place or places of 

                                       63
<PAGE>
 
payment that payment will be made upon presentation and surrender of the
Securities, that interest accrued to the date fixed for redemption will be paid
as specified in said notice, and that on and after said date interest thereon or
on the portions thereof to be redeemed will cease to accrue. If less than all
the Securities are to be redeemed the notice of redemption shall specify the
numbers of the Securities to be redeemed. In case any Security is to be redeemed
in part only, the notice of redemption shall state the portion of the principal
amount thereof to be redeemed and shall state that on and after the date fixed
for redemption, upon surrender of such Security, a new Security or Securities in
principal amount equal to the unredeemed portion thereof will be issued.

     By 10:00 a.m. New York City time on the redemption date specified in the
notice of redemption given as provided in this Section, the Company will deposit
with the Trustee or with one or more paying agents an amount of money sufficient
to redeem on the redemption date all the Securities so called for redemption at
the appropriate Redemption Price, together with accrued interest to the date
fixed for redemption.

     The Company will give the Trustee notice not less than 45 days prior to the
redemption date as to the aggregate principal amount of Securities to be
redeemed and the Trustee shall select, in such manner as in its sole discretion
it shall deem appropriate and fair, the Securities or portions thereof (in
integral multiples of $1,000, except as otherwise set forth in the applicable
form of Security) to be redeemed.

     SECTION 14.05.     Payment of Securities Called for Redemption.

     If notice of redemption has been given as provided in Section 14.04, the
Securities or portions of Securities with respect to which such notice has been
given shall become due and payable on the date and at the place or places stated
in such notice at the applicable Redemption Price, together with interest
accrued to the date fixed for redemption (subject to the rights of holders of
Securities on the close of business on a regular record date in respect of an
Interest Payment Date occurring on or prior to the redemption date), and on and
after said date (unless the Company shall default in the payment of such
Securities at the Redemption Price, together with interest accrued to said date)
interest (including Compounded Interest and Additional Sums, if any) and
Liquidated Damages, if any, on the Securities or portions of Securities so
called for redemption shall cease to accrue.  On presentation and surrender of
such Securities at a place of payment specified in said notice, the said
Securities or the specified portions thereof shall be paid and redeemed by the
Company at the applicable Redemption Price, together with interest (including
Compounded Interest and Additional Sums, if any) and Liquidated Damages, if any,
accrued thereon to the date fixed for redemption (subject to the rights of
holders of Securities on the close of business on a regular record date in
respect of an Interest Payment Date occurring on or prior to the redemption
date).

     Upon presentation of any Security redeemed in part only, the Company shall
execute and the Trustee shall authenticate and make available for delivery to
the holder thereof, at the expense of the Company, a new Security or Securities
of authorized denominations, in principal amount equal to the unredeemed portion
of the Security so presented.

                                       64
<PAGE>
 
                                  ARTICLE XV
                          SUBORDINATION OF SECURITIES
                          ---------------------------

     SECTION 15.01.     Agreement to Subordinate.

     The Company covenants and agrees, and each holder of Securities issued
hereunder likewise covenants and agrees, that the Securities shall be issued
subject to the provisions of this Article XV; and each holder of a Security,
whether upon original issue or upon transfer or assignment thereof, accepts and
agrees to be bound by such provisions.

     The payment by the Company of the principal of, premium, if any, and
interest (including Compounded Interest and Additional Sums, if any) and
Liquidated Damages, if any, on all Securities issued hereunder shall, to the
extent and in the manner hereinafter set forth, be subordinated and junior in
right of payment to all Senior Indebtedness, whether outstanding at the date of
this Indenture or thereafter incurred.

     No provision of this Article XV shall prevent the occurrence of any Default
or Event of Default hereunder.

     SECTION 15.02.     Default on Senior Indebtedness.

     In the event and during the continuation of any default by the Company in
the payment of principal, premium, interest or any other payment due on any
Senior Indebtedness, or in the event that the maturity of any Senior
Indebtedness has been accelerated because of a default, then, in either case, no
payment shall be made by the Company with respect to the principal (including
redemption payments) of or premium, if any, or interest on the Securities until
such default shall have been cured or waived in writing or shall have ceased to
exist or such Senior Indebtedness shall have been discharged or paid in full.

     In the event of the acceleration of the maturity of the Securities, then no
payment shall be made by the Company with respect to the principal (including
redemption payments) of or premium, if any, or interest on the Securities until
the holders of all Senior Indebtedness outstanding at the time of such
acceleration shall receive payment in full of such Senior Indebtedness
(including any amounts due upon acceleration).

     In the event that, notwithstanding the foregoing, any payment shall be
received by the Trustee or any Securityholder when such payment is prohibited by
the preceding paragraphs of this Section 15.02, such payment shall be held in
trust for the benefit of, and shall be paid over or delivered to, the holders of
Senior Indebtedness or their respective representatives, or to the trustee 

                                       65
<PAGE>
 
or trustees under any indenture pursuant to which any of such Senior
Indebtedness may have been issued, as their respective interests may appear.

     SECTION 15.03.     Liquidation; Dissolution; Bankruptcy.

     Upon any payment by the Company or distribution of assets of the Company of
any kind or character, whether in cash, property or securities, to creditors
upon any dissolution or winding-up or liquidation or reorganization of the
Company, whether voluntary or involuntary or in bankruptcy, insolvency,
receivership or other proceedings, all Senior Indebtedness of the Company shall
first be paid in full, or payment thereof provided for in money in accordance
with its terms, before any payment is made by the Company on account of the
principal (and premium, if any) or interest (including Compounded Interest and
Additional Sums, if any) and Liquidated Damages, if any, on the Securities; and
upon any such dissolution or winding-up or liquidation or reorganization, any
payment by the Company, or distribution of assets of the Company of any kind or
character, whether in cash, property or securities, to which the Securityholders
or the Trustee would be entitled to receive from the Company, except for the
provisions of this Article XV, shall be paid by the Company or by any receiver,
trustee in bankruptcy, liquidating trustee, agent or other Person making such
payment or distribution, or by the Securityholders or by the Trustee under the
Indenture if received by them or it, directly to the holders of Senior
Indebtedness of the Company (pro rata to such holders on the basis of the
respective amounts of Senior Indebtedness held by such holders, as calculated by
the Company) or their representative or representatives, or to the trustee or
trustees under any indenture pursuant to which any instruments evidencing such
Senior Indebtedness may have been issued, as their respective interests may
appear, to the extent necessary to pay all such Senior Indebtedness in full, in
money in accordance with its terms, after giving effect to any concurrent
payment or distribution to or for the holders of such Senior Indebtedness,
before any payment or distribution is made to the Securityholders or to the
Trustee.

     In the event that, notwithstanding the foregoing, any payment or
distribution of assets of the Company of any kind or character, whether in cash,
property or securities, prohibited by the foregoing, shall be received by the
Trustee or any Securityholder before all Senior Indebtedness is paid in full, or
provision is made for such payment in money in accordance with its terms, such
payment or distribution shall be held in trust for the benefit of and shall be
paid over or delivered to the holders of such Senior Indebtedness or their
representative or representatives, or to the trustee or trustees under any
indenture pursuant to which any instruments evidencing such Senior Indebtedness
may have been issued, as their respective interests may appear, as calculated by
the Company, for application to the payment of all Senior Indebtedness remaining
unpaid to the extent necessary to pay all such Senior Indebtedness in full in
money in accordance with its terms, after giving effect to any concurrent
payment or distribution to or for the benefit of the holders of such Senior
Indebtedness.

     For purposes of this Article XV only, the words "assets of the Company of
any kind or character, whether cash, property or securities" shall not be deemed
to include shares of stock of the 

                                       66
<PAGE>
 
Company as reorganized or readjusted, or securities of the Company or any other
corporation provided for by a plan of reorganization or readjustment, the
payment of which is subordinated at least to the extent provided in this Article
XV with respect to the Securities to the payment of Senior Indebtedness that may
at the time be outstanding; provided that (i) such Senior Indebtedness is
assumed by the new corporation, if any, resulting from any such reorganization
or readjustment, and (ii) the rights of the holders of such Senior Indebtedness
are not, without the consent of such holders, altered by such reorganization or
readjustment. The consolidation of the Company with, or the merger of the
Company into, another Person or the liquidation or dissolution of the Company
following the sale, conveyance, transfer or lease of its property as an
entirety, or substantially as an entirety, to another Person upon the terms and
conditions provided for in Article X of this Indenture shall not be deemed a
dissolution, winding-up, liquidation or reorganization for the purposes of this
Section 15.03 if such other Person shall, as a part of such consolidation,
merger, sale, conveyance, transfer or lease, comply with the conditions stated
in Article X of this Indenture. Nothing in Section 15.02 or in this Section
15.03 shall apply to claims of, or payments to, the Trustee under or pursuant to
Section 6.06 of this Indenture.

     Nothing contained in this Article XV or elsewhere in this Indenture or in
the Securities is in  tended to or shall impair, as between the Company, its
creditors other than the holders of Senior Indebtedness of the Company, and the
holders of the Securities, the obligation of the Company, which is absolute and
unconditional, to pay to the holders of the Securities the principal of (and
premium, if any) and interest (including Compounded Interest and Additional
Sums, if any) and Liquidated Damages, if any, on the Securities as and when the
same shall become due and payable in accordance with their terms, or is intended
to or shall affect the relative rights of the holders of the Securities and
creditors of the Company, as the case may be, other than the holders of Senior
Indebtedness of the Company, as the case may be, nor shall anything herein or
therein prevent the occurrence of an Event of Default under Article V or prevent
the Trustee or the holder of any Security from exercising all remedies otherwise
permitted by applicable law upon the occurrence of a Default or an Event of
Default under the Indenture, subject to the rights, if any, under this Article
XV of the holders of such Senior Indebtedness in respect of cash, property or
securities of the Company, as the case may be, received upon the exercise of any
such remedy.

     SECTION 15.04.     Subrogation.

     Subject to the payment in full of all Senior Indebtedness, the rights of
the Securityholders shall be subrogated to the rights of the holders of such
Senior Indebtedness to receive payments or distributions of cash, property or
securities of the Company, as the case may be, applicable to such Senior
Indebtedness until the principal of (and premium, if any) and interest on the
Securities shall be paid in full; and, for the purposes of such subrogation, no
payments or distributions to the holders of such Senior Indebtedness of any
cash, property or securities to which the Securityholders or the Trustee would
be entitled except for the provisions of this Article XV, and no payment over
pursuant to the provisions of this Article XV to or for the benefit of the
holders of such Senior Indebtedness by Securityholders or the Trustee, shall, as
between the Company, its creditors other than holders 

                                       67
<PAGE>
 
of Senior Indebtedness of the Company, and the holders of the Securities, be
deemed to be a payment by the Company to or on account of such Senior
Indebtedness. It is understood that the provisions of this Article XV are and
are intended solely for the purposes of defining the relative rights of the
holders of the Securities, on the one hand, and the holders of such Senior
Indebtedness on the other hand.

     SECTION 15.05.     Trustee to Effectuate Subordination.

     Each Securityholder by such Securityholder's acceptance thereof authorizes
and directs the Trustee on such Securityholder's behalf to take such action as
may be necessary or appropriate to effectuate the subordination provided in this
Article XV and appoints the Trustee such Securityholder's attorney-in-fact for
any and all such purposes.

     SECTION 15.06.     Notice by the Company.

     The Company shall give prompt written notice to a Responsible Officer of
the Trustee of any fact known to the Company that would prohibit the making of
any payment of monies to or by the Trustee in respect of the Securities pursuant
to the provisions of this Article XV.  Notwithstanding the provisions of this
Article XV or any other provision of this Indenture, the Trustee shall not be
charged with knowledge of the existence of any facts that would prohibit the
making of any payment of monies to or by the Trustee in respect of the
Securities pursuant to the provisions of this Article XV, unless and until a
Responsible Officer of the Trustee shall have received written notice thereof
from the Company or a holder or holders of Senior Indebtedness or from any
trustee therefor; and before the receipt of any such written notice, the
Trustee, subject to the provisions of Article VI of this Indenture, shall be
entitled in all respects to assume that no such facts exist; provided, however,
that if the Trustee shall not have received the notice provided for in this
Section 15.06 at least two Business Days prior to the date upon which by the
terms hereof any money may become payable for any purpose (including, without
limitation, the payment of the principal of (or premium, if any) or interest
(including Compounded Interest and Additional Sums, if any) and Liquidated
Damages, if any, on any Security), then, anything herein contained to the
contrary notwithstanding, the Trustee shall have full power and authority to
receive such money and to apply the same to the purposes for which they were
received, and shall not be affected by any notice to the contrary that may be
received by it within two Business Days prior to such date.

     The Trustee, subject to the provisions of Article VI of this Indenture,
shall be entitled to conclusively rely on the delivery to it of a written notice
by a Person representing himself to be a holder of Senior Indebtedness of the
Company (or a trustee on behalf of such holder), as the case may be, to
establish that such notice has been given by a holder of such Senior
Indebtedness or a trustee on behalf of any such holder or holders.  In the event
that the Trustee determines in good faith that further evidence is required with
respect to the right of any Person as a holder of such Senior Indebtedness to
participate in any payment or distribution pursuant to this Article XV, the
Trustee may request such Person to furnish evidence to the reasonable
satisfaction of the Trustee as to the 

                                       68
<PAGE>
 
amount of such Senior Indebtedness held by such Person, the extent to which such
Person is entitled to participate in such payment or distribution and any other
facts pertinent to the rights of such Person under this Article XV, and, if such
evidence is not furnished, the Trustee may defer any payment to such Person
pending judicial determination as to the right of such Person to receive such
payment.

     Upon any payment or distribution of assets of the Company referred to in
this Article XV, the Trustee and the Securityholders shall be entitled to rely
upon any order or decree entered by any court of competent jurisdiction in which
such insolvency, bankruptcy, receivership, liquidation, reorganization,
dissolution, winding up or similar case or proceeding is pending, or a
certificate of the trustee in bankruptcy, liquidating trustee, custodian,
receiver, assignee for the benefit of creditors, agent or other person making
such payment or distribution, delivered to the Trustee or to the
Securityholders, for the purpose of ascertaining the persons entitled to
participate in such payment or distribution, the holders of Senior Indebtedness
and other indebtedness of the Company, the amount thereof or payable thereon,
the amount or amounts paid or distributed thereon and all other facts pertinent
thereto or to this Article XV.

     SECTION 15.07.     Rights of the Trustee; Holders of Senior Indebtedness.

     The Trustee in its individual capacity shall be entitled to all the rights
set forth in this Article XV in respect of any Senior Indebtedness at any time
held by it, to the same extent as any other holder of Senior Indebtedness, and
nothing in this Indenture shall deprive the Trustee of any of its rights as such
holder.

     With respect to the holders of Senior Indebtedness of the Company, the
Trustee undertakes to perform or to observe only such of its covenants and
obligations as are specifically set forth in this Article XV, and no implied
covenants or obligations with respect to the holders of such Senior Indebtedness
shall be read into this Indenture against the Trustee.  The Trustee shall not be
deemed to owe any fiduciary duty to the holders of such Senior Indebtedness and,
subject to the provisions of Article VI of this Indenture, the Trustee shall not
be liable to any holder of such Senior Indebtedness if it shall pay over or
deliver to Securityholders, the Company or any other Person money or assets to
which any holder of such Senior Indebtedness shall be entitled by virtue of this
Article XV or otherwise.

     Nothing in this Article XV shall apply to claims of, or payments to, the
Trustee under or pursuant to Section 6.06.

     SECTION 15.08.     Subordination May Not Be Impaired.

     No right of any present or future holder of any Senior Indebtedness of the
Company to enforce subordination as herein provided shall at any time in any way
be prejudiced or impaired by any act or failure to act on the part of the
Company, as the case may be, or by any act or failure to 

                                       69
<PAGE>
 
act, in good faith, by any such holder, or by any noncompliance by the Company,
as the case may be, with the terms, provisions and covenants of this Indenture,
regardless of any knowledge thereof that any such holder may have or otherwise
be charged with.

     Without in any way limiting the generality of the foregoing paragraph, the
holders of Senior Indebtedness of the Company may, at any time and from time to
time, without the consent of or notice to the Trustee or the Securityholders,
without incurring responsibility to the Securityholders and without impairing or
releasing the subordination provided in this Article XV or the obligations
hereunder of the holders of the Securities to the holders of such Senior
Indebtedness, do any one or more of the following:  (i) change the manner, place
or terms of payment or extend the time of payment of, or renew or alter, such
Senior Indebtedness, or otherwise amend or supplement in any manner such Senior
Indebtedness or any instrument evidencing the same or any agreement under which
such Senior Indebtedness is outstanding; (ii) sell, exchange, release or
otherwise deal with any property pledged, mortgaged or otherwise securing such
Senior Indebtedness; (iii) release any Person liable in any manner for the
collection of such Senior Indebtedness; and (iv) exercise or refrain from
exercising any rights against the Company, as the case may be, and any other
Person.

     SECTION 15.09.     Article Applicable to Paying Agents.

     In case at any time any paying agent other than the Trustee shall have been
appointed by the Company and be then acting hereunder, the term "Trustee" as
used in this Article shall in such case (unless the context otherwise requires)
be construed as extending to and including such paying agent within its meaning
as fully for all intents and purposes as if such paying agent were named in this
Article in addition to or in place of the Trustee.

     SECTION 15.10.     Trust Moneys Not Subordinated.

     Notwithstanding anything contained herein to the contrary, payments from
money or the proceeds of U.S. Government Obligations held in trust under Section
11.02 of this Indenture by the Trustee for the payment of principal of and
interest on the Securities shall not be subordinated to the prior payment of any
Senior Indebtedness or subject to the restrictions set forth in this Article XV,
and none of the Securityholders shall be obligated to pay over any such amount
to the Company or any holder of Senior Indebtedness of the Company or any other
creditor of the Company.


                                  ARTICLE XVI
                     EXTENSION OF INTEREST PAYMENT PERIOD
                     ------------------------------------

     SECTION 16.01.     Extension of Interest Payment Period.

     So long as no Event of Default has occurred and is continuing, the Company
shall have the right, at any time and from time to time during the term of the
Securities, to defer payments of 

                                       70
<PAGE>
 
interest by extending the interest payment period of such Securities for a
period not exceeding 10 consecutive semi-annual periods, including the first
such semi-annual period during such extension period (the "Extended Interest
Payment Period"), during which Extended Interest Payment Period no interest
shall be due and payable; provided that no Extended Interest Payment Period
shall end on a date other than an Interest Payment Date or extend beyond the
Maturity Date. To the extent permitted by applicable law, interest, the payment
of which has been deferred because of the extension of the interest payment
period pursuant to this Section 16.01, will bear interest thereon at the Coupon
Rate compounded semi-annually for each semi-annual period of the Extended
Interest Payment Period ("Compounded Interest"). At the end of the Extended
Interest Payment Period, the Company shall pay all interest accrued and unpaid
on the Securities, including any Additional Sums and Compounded Interest
(together, "Deferred Interest") that shall be payable to the holders of the
Securities in whose names the Securities are registered in the Security Register
on the first record date preceding the end of the Extended Interest Payment
Period. Before the termination of any Extended Interest Payment Period, the
Company may further defer payments of interest by further extending such period;
provided that such period, together with all such previous and further
extensions within such Extended Interest Payment Period, shall not exceed 10
consecutive semi-annual periods, including the first such semi-annual period
during such Extended Interest Payment Period, end on a date other than an
Interest Payment Date or extend beyond the Maturity Date of the Securities. Upon
the termination of any Extended Interest Payment Period and the payment of all
Deferred Interest then due, the Company may commence a new Extended Interest
Payment Period, subject to the foregoing requirements. No interest shall be due
and payable during an Extended Interest Payment Period, except at the end
thereof, but the Company may prepay at any time all or any portion of the
interest accrued during an Extended Interest Payment Period.

     SECTION 16.02.     Notice of Extension.

          (a) If the Property Trustee is the only registered holder of the
     Securities at the time the Company selects an Extended Interest Payment
     Period, the Company shall give written notice to the Administrative
     Trustees, the Property Trustee and the Trustee of its selection of such
     Extended Interest Payment Period five Business Days before the earlier of
     (i) the next succeeding date on which Distributions on the Trust Securities
     issued by NGC Corporation Capital Trust are payable, or (ii) the date the
     Trust is required to give notice of the record date, or the date such
     Distributions are payable, to any national securities exchange or to
     holders of the Capital Securities issued by the Trust, but in any event at
     least five Business Days before such record date.

          (b) If the Property Trustee is not the only holder of the Securities
     at the time the Company selects an Extended Interest Payment Period, the
     Company shall give the holders of the Securities and the Trustee written
     notice of its selection of such Extended Interest Payment Period at least
     10 Business Days before the earlier of (i) the next succeeding Interest
     Payment Date, or (ii) the date the Company is required to give notice of
     the record or payment date of such interest payment to any national
     securities exchange.

                                       71
<PAGE>
 
          (c) The semi-annual period in which any notice is given pursuant to
     paragraphs (a) or (b) of this Section 16.02 shall be counted as one of the
     10 semi-annual periods permitted in the maximum Extended Interest Payment
     Period permitted under Section 16.01.

     The First National Bank of Chicago hereby accepts the trusts in this
Indenture declared and provided, upon the terms and conditions hereinabove set
forth.

                                       72
<PAGE>
 
          IN WITNESS WHEREOF, the parties hereto have caused this Indenture to
be duly executed by their respective officers thereunto duly authorized, as of
the day and year first above written.

                                    NGC CORPORATION


                                    By:
                                         Name:
                                         Title:



                                    THE FIRST NATIONAL BANK OF
                                    CHICAGO, as Trustee


                                    By:
                                         Name:
                                         Title:

                                       73
<PAGE>
 
                                   EXHIBIT A
                                   ---------

                          (FORM OF FACE OF SECURITY)


     THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "SECURITIES ACT") OR ANY STATE SECURITIES LAWS OR ANY OTHER
APPLICABLE SECURITIES LAW.  NEITHER THIS SECURITY NOR ANY INTEREST OR
PARTICIPATION HEREIN MAY BE REOFFERED, SOLD, ASSIGNED, TRANSFERRED, PLEDGED,
ENCUMBERED OR OTHERWISE DISPOSED OF IN THE ABSENCE OF SUCH REGISTRATION OR
UNLESS SUCH TRANSACTION IS EXEMPT FROM, OR NOT SUBJECT TO, REGISTRATION.  EACH
PURCHASER OF THIS SECURITY IS HEREBY NOTIFIED THAT THE SELLER MAY BE RELYING ON
THE EXEMPTION FROM THE PROVISIONS OF SECTION 5 OF THE SECURITIES ACT PROVIDED BY
RULE 144A THEREUNDER.


     THE HOLDER OF THIS SECURITY BY ITS ACCEPTANCE HEREOF REPRESENTS,
ACKNOWLEDGES AND AGREES FOR THE BENEFIT OF THE COMPANY, THAT: IT HAS ACQUIRED A
"RESTRICTED SECURITY" WHICH HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT AND
AGREES TO OFFER, SELL OR OTHERWISE TRANSFER THIS SECURITY, PRIOR TO THE DATE
(THE "RESALE RESTRICTION TERMINATION DATE") WHICH IS TWO YEARS AFTER THE LATER
OF THE ORIGINAL ISSUANCE DATE HEREOF AND THE LAST DATE ON WHICH THE COMPANY OR
ANY "AFFILIATE" OF THE COMPANY WAS THE OWNER OF THIS SECURITY (OR ANY
PREDECESSOR OF THIS SECURITY) ONLY (A) TO THE COMPANY, (B) PURSUANT TO A
REGISTRATION STATEMENT WHICH HAS BEEN DECLARED EFFECTIVE UNDER THE SECURITIES
ACT, (C) SO LONG AS THIS SECURITY IS ELIGIBLE FOR RESALE PURSUANT TO RULE 144A
UNDER THE SECURITIES ACT ("RULE 144A"), TO A PERSON IT REASONABLY BELIEVES IS A
"QUALIFIED INSTITUTIONAL BUYER" (AS DEFINED IN RULE 144A) THAT PURCHASES FOR ITS
OWN ACCOUNT OR FOR THE ACCOUNT OF A QUALIFIED INSTITUTIONAL BUYER TO WHOM NOTICE
IS GIVEN THAT THE TRANSFER IS BEING MADE IN RELIANCE ON RULE 144A, (D) PURSUANT
TO OFFERS AND SALES TO NON-U.S. PERSONS THAT OCCUR OUTSIDE THE UNITED STATES
WITHIN THE MEANING OF REGULATION S UNDER THE SECURITIES ACT, (E) TO AN
INSTITUTIONAL "ACCREDITED INVESTOR" WITHIN THE MEANING OF SUBPARAGRAPH (A)(1),
(2), (3) OR (7) OF RULE 501 UNDER THE SECURITIES ACT THAT IS ACQUIRING THIS
SECURITY FOR ITS OWN ACCOUNT, OR FOR THE ACCOUNT OF SUCH AN INSTITUTIONAL
ACCREDITED INVESTOR, FOR INVESTMENT PURPOSES AND NOT WITH A VIEW TO, OR FOR
OFFER OR SALE IN CONNECTION WITH, ANY DISTRIBUTION IN VIOLATION OF THE
SECURITIES ACT OR (F) PURSUANT TO ANY OTHER AVAILABLE EXEMPTION FROM THE
REGISTRATION REQUIREMENTS UNDER THE SECURITIES ACT, SUBJECT 


                                      A-1
<PAGE>
 
TO THE RIGHT OF THE COMPANY PRIOR TO ANY SUCH OFFER, SALE OR TRANSFER (i)
PURSUANT TO CLAUSE (D), (E) OR (F) TO REQUIRE THE DELIVERY OF AN OPINION OF
COUNSEL, CERTIFICATIONS AND/OR OTHER INFORMATION SATISFACTORY TO THE COMPANY,
AND (ii) PURSUANT TO CLAUSE (D) OR (E), TO REQUIRE THAT A CERTIFICATE OF
TRANSFER IN THE FORM APPEARING ON THE REVERSE OF THIS SECURITY IS COMPLETED AND
DELIVERED BY THE TRANSFEREE TO THE COMPANY. SUCH HOLDER FURTHER AGREES THAT IT
WILL DELIVER TO EACH PERSON TO WHOM THIS SECURITY IS TRANSFERRED A NOTICE
SUBSTANTIALLY TO THE EFFECT OF THIS LEGEND.

     THE SECURITIES WILL BE ISSUED AND MAY BE TRANSFERRED ONLY IN BLOCKS HAVING
AN AGGREGATE PRINCIPAL AMOUNT OF NOT LESS THAN $100,000 (100 SECURITIES).  ANY
SUCH TRANSFER OF SECURITIES IN A BLOCK HAVING AN AGGREGATE PRINCIPAL AMOUNT OF
LESS THAN $100,000 SHALL BE DEEMED TO BE VOID AND OF NO LEGAL EFFECT WHATSOEVER.
ANY SUCH TRANSFEREE SHALL BE DEEMED NOT TO BE THE HOLDER OF SUCH SECURITIES FOR
ANY PURPOSE, INCLUDING BUT NOT LIMITED TO THE RECEIPT OF PRINCIPAL, PREMIUM (IF
ANY) OR INTEREST OF SUCH SECURITIES, AND SUCH TRANSFEREE SHALL BE DEEMED TO HAVE
NO INTEREST WHATSOEVER IN SUCH SECURITIES.

     THE HOLDER OF THIS SECURITY BY ITS ACCEPTANCE HEREOF ALSO AGREES,
REPRESENTS AND WARRANTS THAT EITHER (i) IT IS NOT AN EMPLOYEE BENEFIT OR OTHER
PLAN SUBJECT TO THE EMPLOYEE RETIREMENT INCOME SECURITY ACT OF 1974, AS AMENDED
("ERISA") OR SECTION 4975 OF THE INTERNAL REVENUE CODE OF 1986, AS AMENDED, OR
AN ENTITY WHOSE UNDERLYING ASSETS INCLUDE "PLAN ASSETS" BY REASON OF ANY SUCH
PLAN'S INVESTMENT IN THE ENTITY AND IS NOT PURCHASING OR HOLDING SUCH SECURITIES
ON BEHALF OF OR WITH "PLAN ASSETS" OR (ii) IT IS ELIGIBLE FOR THE EXEMPTIVE
RELIEF AVAILABLE UNDER PTCE 96-23, 95-60, 91-38, 90-1 OR 84-14 WITH RESPECT TO
SUCH PURCHASE AND HOLDING.



                                      A-2
<PAGE>
 
No._____________

                                NGC CORPORATION

          SERIES A 8.316% SUBORDINATED DEFERRABLE INTEREST DEBENTURE
                               DUE JUNE 1, 2027

     NGC Corporation, a Delaware corporation (the "Company", which term includes
any successor Person under the Indenture hereinafter referred to), for value
received, hereby promises to pay to The First National Bank of Chicago, as
Property Trustee for NGC Corporation Capital Trust I or registered assigns, the
principal sum of TWO HUNDRED SIX MILLION TWO HUNDRED THOUSAND DOLLARS
($206,200,000) on June 1, 2027 (the "Maturity Date"), unless previously
redeemed, and to pay interest on the outstanding principal amount hereof from
May 28, 1997, or from the most recent interest payment date (each such date, an
"Interest Payment Date") to which interest has been paid or duly provided for,
semi-annually (subject to deferral as set forth herein) in arrears on June 1 and
December 1 of each year, commencing December 1, 1997, at the rate of 8.316% per
annum until the principal hereof shall have become due and payable, and on any
overdue principal and premium, if any, and (without duplication and to the
extent that payment of such interest is enforceable under applicable law) on any
overdue installment of interest at the same rate per annum compounded semi-
annually.  The amount of interest payable on any Interest Payment Date shall be
computed on the basis of a 360-day year of twelve 30-day months and, for any
period less than a full calendar month, the number of days elapsed in such
month.  In the event that any date on which the principal of (or premium, if
any) or interest on this Security is payable is not a Business Day, then the
payment payable on such date will be made on the next succeeding day that is a
Business Day (and without any interest or other payment in respect of any such
delay), except that if such next succeeding Business Day falls in the next
calendar year, then such payment shall be made on the immediately preceding
Business Day, in each case with the same force and effect as if made on the date
such payment is otherwise required.  Pursuant to the Indenture, in certain
circumstances the Company will be required to pay Additional Sums and
Compounded Interest (each as defined in the Indenture) with respect to this
Security.  Pursuant to the Registration Rights Agreement, in certain limited
circumstances the Company will be required to pay Liquidated Damages (as defined
in the Registration Rights Agreement) with respect to this Security.

     The interest installment so payable, and punctually paid or duly provided
for, on any Interest Payment Date will, as provided in the Indenture, be paid to
the Person in whose name this Security (or one or more Predecessor Securities,
as defined in said Indenture) is registered at the close of business on the
regular record date for such interest installment, which shall be at the close
of business on the 15th day of the month preceding the month in which the
relevant interest payment date falls.  Any such interest installment not
punctually paid or duly provided for shall forthwith cease to be payable to the
holders on such regular record date and may be paid to the Person in whose name
this Security (or one or more Predecessor Securities) is registered at the close
of business on a special record date to be fixed by the Trustee for the payment
of such defaulted 

                                      A-3
<PAGE>
 
interest, notice whereof shall be given to the holders of Securities not less
than 10 days prior to such special record date, or may be paid at any time in
any other lawful manner not inconsistent with the requirements of any securities
exchange on which the Securities may be listed, and upon such notice as may be
required by such exchange, all as more fully provided in the Indenture.

     The principal of (and premium, if any) and interest (including Compounded
Interest and Additional Sums, if any) and Liquidated Damages, if any, on this
Security shall be payable at the office or agency of the Trustee maintained for
that purpose in any coin or currency of the United States of America that at the
time of payment is legal tender for payment of public and private debts;
provided, however, that, payment of interest may be made at the option of the
Company by (i) check mailed to the holder at such address as shall appear in the
Security Register or (ii) by transfer to an account maintained by the Person
entitled thereto; provided that proper written transfer instructions have been
received by the relevant record date.  Notwithstanding the foregoing, so long as
the Holder of this Security is the Property Trustee, the payment of the
principal of (and premium, if any) and interest (including Compounded Interest
and Additional Sums, if any) and Liquidated Damages, if any, on this Security
will be made at such place and to such account as may be designated by the
Property Trustee.

     The indebtedness evidenced by this Security is, to the extent provided in
the Indenture, subordinate and junior in right of payment to the prior payment
in full of all Senior Indebtedness, and this Security is issued subject to the
provisions of the Indenture with respect thereto.  Each holder of this Security,
by accepting the same, (a) agrees to and shall be bound by such provisions, (b)
authorizes and directs the Trustee on his or her behalf to take such action as
may be necessary or appropriate to acknowledge or effectuate the subordination
so provided and (c) appoints the Trustee his or her attorney-in-fact for any and
all such purposes.  Each holder hereof, by his or her acceptance hereof, hereby
waives all notice of the acceptance of the subordination provisions contained
herein and in the Indenture by each holder of Senior Indebtedness, whether now
outstanding or hereafter incurred, and waives reliance by each such holder upon
said provisions.

     This Security shall not be entitled to any benefit under the Indenture
hereinafter referred to, be valid or become obligatory for any purpose until the
Certificate of Authentication hereon shall have been signed by or on behalf of
the Trustee.

     The provisions of this Security are continued on the reverse side hereof
and such provisions shall for all purposes have the same effect as though fully
set forth at this place.

          IN WITNESS WHEREOF, the Company has caused this instrument to be duly
executed and sealed.


Dated:  May 28, 1997


                                      A-4
<PAGE>
 
                              NGC CORPORATION

                              By: ____________________________
                              Name:
                              Title:


Attest:

By: _______________________
Name:
Title:


                                      A-5
<PAGE>
 
                    (FORM OF CERTIFICATE OF AUTHENTICATION)

                         CERTIFICATE OF AUTHENTICATION

          This is one of the Securities referred to in the within-mentioned
Indenture.



THE FIRST NATIONAL BANK OF CHICAGO,
as Trustee


By____________________
  Authorized Signatory



                                      A-6
<PAGE>
 
                         (FORM OF REVERSE OF SECURITY)

     This Security is one of the Securities of the Company (herein sometimes
referred to as the "Securities"), specified in the Indenture, all issued or to
be issued under and pursuant to an Indenture, dated as of May 28, 1997 (the
"Indenture"), duly executed and delivered between the Company and The First
National Bank of Chicago, as Trustee (the "Trustee"), to which Indenture
reference is hereby made for a description of the rights, limitations of rights,
obligations, duties and immunities thereunder of the Trustee, the Company and
the holders of the Securities.

     Upon the occurrence and continuation of a Special Event at any time, the
Company shall have the right, at any time within 90 days following the
occurrence of such Special Event, to redeem this Security in whole (but not in
part) at the Special Event Redemption Price.  "Special Event Redemption Price"
shall mean, with respect to any redemption of the Securities following a Special
Event, an amount in cash equal to the Make-Whole Amount.  The "Make-Whole
Amount" shall be equal to the greater of (a) 100% of the principal amount of the
Securities or (b) the sum, as determined by the Reference Treasury Dealer, of
the present values of the remaining scheduled payments of principal and interest
on the Securities, discounted to the date of redemption on a semi-annual basis
(assuming a 360-day year consisting of twelve 30-day months) at the Adjusted
Treasury Rate, plus, in each case, accrued and unpaid interest thereon
(including Additional Sums and Liquidated Damages, if any) to the date of
redemption.

     "Adjusted Treasury Rate" means, with respect to any redemption date, the
rate per annum equal to (i) the yield, under the heading which represents the
average for the immediately prior week, appearing in the most recently published
statistical release designated "H.15(519)" or any successor publication which is
published weekly by the Federal Reserve and which establishes yields on actively
traded United States Treasury securities adjusted to constant maturity under the
caption "Treasury Constant Maturities," for the maturity date corresponding to
the Maturity Date (if no maturity date is within three months before or after
the Maturity Date, yields for the two published maturities most closely
corresponding to the Maturity Date shall be interpolated and the Adjusted
Treasury Rate shall be interpolated or extrapolated from such yields on a
straight-line basis, rounding to the nearest month) or (ii) if such release (or
any successor release) is not published during the week preceding the
calculation date or does not contain such yields, the rate per annum equal to
the semiannual equivalent yield to maturity of the Comparable Treasury Issue,
assuming a price for the Comparable Treasury Issue (expressed as a percentage of
its principal amount) equal to the Comparable Treasury Price for such redemption
date plus (i) 1.03% if such redemption date occurs prior to June 1, 1998 and
(ii) 0.50% in all other cases.

     In addition, the Company shall have the right to redeem this Security, in
whole or in part, at any time (an "Optional Redemption"), at a redemption price
(the "Optional Redemption Price") equal to the greater of (i) 100% of the
principal amount of the Securities to be so redeemed plus accrued and unpaid
interest thereon (including Additional Sums and Liquidated Damages, if any) to
the date of redemption and (ii) the sum of the present values of the remaining
scheduled payments of 

                                      A-7
<PAGE>
 
principal of the Securities to be so redeemed and interest thereon (including
Additional Sums and Liquidated Damages, if any) discounted to the date of
redemption, on a semi-annual basis (assuming a 360-day year consisting of twelve
30-day months), at the Treasury Rate (as defined herein) plus 25 basis points
plus accrued interest thereon to the date of redemption.

          "Comparable Treasury Issue" means the United States Treasury security
selected by the Reference Treasury Dealer as having a maturity comparable to the
remaining term of the Securities to be redeemed that would be utilized, at the
time of selection and in accordance with customary financial practice, in
pricing new issues of corporate debt securities of comparable maturity to the
remaining term of such Securities.

          "Treasury Rate" means, with respect to any redemption date, the rate
per annum equal to the semiannual equivalent yield to maturity of the Comparable
Treasury Issue, assuming a price for the Comparable Treasury Issue (expressed as
a percentage of its principal amount) equal to the Comparable Treasury Price for
such redemption date.

          "Comparable Treasury Price" means, with respect to any redemption
date, (i) the average of the bid and asked prices for the Comparable Treasury
Issue (expressed in each case as a percentage of its principal amount) on the
third business day preceding such redemption date, as set forth in the daily
statistical release (or any successor release) published by the Federal Reserve
Bank of New York and designated "Composite 3:30 p.m. Quotations for U.S.
Government Securities" or (ii) if such release (or any successor release) is not
published or does not contain such prices on such business day, the average of
the Reference Treasury Dealer Quotations for such redemption date.

          "Reference Treasury Dealer Quotations" means, with respect to the
Reference Treasury Dealer and any redemption date, the average, as determined by
the Debenture Trustee, of the bid and asked prices for the Comparable Treasury
Issue (expressed in each case as a percentage of its principal amount) quoted in
writing to the Debenture Trustee by the Reference Treasury Dealer at 5:00 p.m.,
New York City time, on the third business day preceding such redemption date.

          "Reference Treasury Dealer" means Lehman Brothers Inc. and its
successors; provided however, that if Lehman Brothers Inc. shall cease to be a
primary U.S. Government securities dealer in New York City (a "Primary Treasury
Dealer"), the Company shall substitute therefor another Primary Treasury Dealer.


     The Optional Redemption Price or the Special Event Redemption Price, as the
case requires, shall be paid prior to 12:00 noon, New York City time, on the
date of such redemption or at such earlier time as the Company determines;
provided, that the Company shall deposit with the Trustee an amount sufficient
to pay the applicable Redemption Price by 10:00 a.m., New York City time, on the
date such Redemption Price is to be paid.  Any redemption pursuant to this
paragraph will be 

                                      A-8
<PAGE>
 
made upon not less than 30 days nor more than 60 days notice. If the Securities
are only partially redeemed by the Company pursuant to an Optional Redemption,
the particular Securities to be redeemed shall be selected on a pro rata basis
not more than 60 days prior to the date fixed for redemption from the
outstanding Securities not previously called for redemption; provided, however,
that with respect to Securityholders that would be required to hold Securities
with an aggregate principal amount of less than $100,000 but more than an
aggregate principal amount of zero as a result of such pro rata redemption, the
Company shall redeem Securities of each such Securityholder so that after such
redemption such Securityholder shall hold Securities either with an aggregate
principal amount of at least $100,000 or such Securityholder no longer holds any
Securities and shall use such method (including, without limitation, by lot) as
the Company shall deem fair and appropriate; provided, further, that any such
proration may be made on the basis of the aggregate principal amount of
Securities held by each Securityholder thereof and may be made by making such
adjustments as the Company deems fair and appropriate in order that only
Securities in denominations of $1,000 or integral multiples thereof shall be
redeemed.

     In the event of redemption of this Security in part only, a new Security or
Securities for the unredeemed portion hereof will be issued in the name of the
holder hereof upon the cancellation hereof.

     In case an Event of Default, as defined in the Indenture, shall have
occurred and be continuing, the principal of all of the Securities may be
declared, and upon such declaration shall become, due and payable, in the
manner, with the effect and subject to the conditions provided in the Indenture.

     The Indenture contains provisions for satisfaction and discharge of the
entire indebtedness of this Security upon compliance by the Company with certain
conditions set forth in the Indenture.

     The Indenture contains provisions permitting the Company and the Trustee,
with the consent of the holders of a majority in aggregate principal amount of
the Securities at the time outstanding, as defined in the Indenture, to execute
supplemental indentures for the purpose of adding any provisions to or
changing in any manner or eliminating any of the provisions of the Indenture or
of modifying in any manner the rights of the holders of the Securities;
provided, however, that no such supplemental indenture shall, without the
consent of each holder of Securities then outstanding and affected thereby, (i)
change the Maturity Date of any Securities, or reduce the principal amount
thereof, or reduce any amount payable on redemption thereof, or reduce the rate
or extend the time of payment of interest thereon (subject to Article XVI of the
Indenture), or make the principal of, or interest or premium on, the Securities
payable in any coin or currency other than U.S. dollars, or impair or affect the
right of any holder of Securities to institute suit for the payment thereof, or
(ii) reduce the aforesaid percentage of Securities, the holders of which are
required to consent to any such supplemental indenture.  The Indenture also
contains provisions permitting the holders of a majority in aggregate principal
amount of the Securities at the time outstanding affected thereby, on behalf of
all of the holders of the Securities, to waive any past default in the
performance of any of 

                                      A-9
<PAGE>
 
the covenants contained in the Indenture, or established pursuant to the
Indenture, and its consequences, except a default in the payment of the
principal of or premium, if any, or interest on any of the Securities or a
default in respect of any covenant or provision under which the Indenture cannot
be modified or amended without the consent of each holder of Securities then
outstanding. Any such consent or waiver by the holder of this Security (unless
revoked as provided in the Indenture) shall be conclusive and binding upon such
Holder and upon all future holders and owners of this Security and of any
Security issued in exchange herefor or in place hereof (whether by registration
of transfer or otherwise), irrespective of whether or not any notation of such
consent or waiver is made upon this Security.

     No reference herein to the Indenture and no provision of this Security or
of the Indenture shall alter or impair the obligation of the Company, which is
absolute and unconditional, to pay the principal of and premium, if any, and
interest (including Compounded Interest and Additional Sums, if any) and
Liquidated Damages, if any, on this Security at the time and place and at the
rate and in the money herein prescribed.

     So long as no Event of Default shall have occurred and be continuing, the
Company shall have the right, at any time and from time to time during the term
of the Securities, to defer payments of interest by extending the interest
payment period of such Securities for a period not exceeding 10 consecutive
semi-annual periods, including the first such semi-annual period during such
extension period, and not extending beyond the Maturity Date of the Securities
(an "Extended Interest Payment Period") or ending on a date other than an
Interest Payment Date, at the end of which period the Company shall pay all
interest then accrued and unpaid (together with interest thereon at the rate
specified for the Securities to the extent that payment of such interest is
enforceable under applicable law).  Before the termination of any such Extended
Interest Payment Period, the Company may further defer payments of interest by
further extending such Extended Interest Payment Period; provided that such
Extended Interest Payment Period, together with all such previous and further
extensions within such Extended Interest Payment Period, (i) shall not exceed 10
consecutive semi-annual periods, including the first semi-annual period during
such Extended Interest Payment Period, (ii) shall not end on any date other than
an Interest Payment Date, and (iii) shall not extend beyond the Maturity Date of
the Securities.  Upon the termination of any such Extended Interest Payment
Period and the payment of all accrued and unpaid interest and any additional
amounts then due, the Company may commence a new Extended Interest Payment
Period, subject to the foregoing requirements.

     The Company has agreed that it will not (i) declare or pay any dividends or
distributions on, or redeem, purchase, acquire, or make a liquidation payment
with respect to, any of the Company's capital stock (which includes common and
preferred stock) (other than (a) dividends or distributions in shares of, or
options, warrants or rights to subscribe for or purchase shares of, Common Stock
of the Company, (b) any declaration of a dividend in connection with the
implementation of a stockholder's rights plan, or the issuance of stock under
any such plan in the future, or the redemption or repurchase of any such
rights pursuant thereto, (c) as a result of a reclassification of the 


                                     A-10
<PAGE>
 
Company's capital stock or the exchange or the conversion of one class or series
of the Company's capital stock, for another class or series of the Company's
capital stock, (d) the purchase of fractional interests in shares of the
Company's capital stock pursuant to the exchange or conversion of such capital
stock or the security being exchanged or converted, and (e) purchases of Common
Stock related to the issuance of Common Stock or rights under any of the
Company's benefit plans for its directors, officers or employees or any of the
Company's dividend reinvestment plans) or (ii) make any payment of principal,
interest or premium, if any, on or repay or repurchase or redeem any debt
securities of the Company that rank pari passu with or junior in right of
payment to the Securities or (iii) make any guarantee payments with respect to
any guarantee (other than payments under the Capital Securities Guarantee) by
the Company of the debt securities of any Subsidiary of the Company if such
guarantee ranks pari passu or junior in right of payment to the Securities if at
such time (1) there shall have occurred any event of which the Company has
actual knowledge that (a) with the giving of notice or the lapse of time, or
both, would be, an Event of Default and (b) in respect of which the Company
shall not have taken reasonable steps to cure, (2) an Event of Default hereunder
shall have occurred and be continuing, (3) if the Securities are held by the
Property Trustee, the Company shall be in default with respect to its payment
obligations under the Capital Securities Guarantee or (4) the Company shall have
given notice of its election of the exercise of its right to extend the interest
payment period and any such extension shall be continuing.

     Subject to the receipt by the Company of an opinion of counsel to the
effect that such distribution will not be a taxable event to holders of Capital
Securities, the Company will have the right at any time to liquidate the NGC
Corporation Capital Trust and cause the Securities to be distributed to the
holders of the Trust Securities in liquidation of the Trust.

     The Securities are issuable only in registered form without coupons in
denominations of $1,000.00 and any integral multiple thereof.  As provided in
the Indenture and subject to the transfer restrictions limitations as may be
contained herein and therein from time to time, this Security is transferable by
the holder hereof on the Security Register of the Company, upon surrender of
this Security for registration of transfer at the office or agency of the
Company in the City and State of New York accompanied by a written instrument or
instruments of transfer in form satisfactory to the Company or the Trustee duly
executed by the holder hereof or his attorney duly authorized in writing, and
thereupon one or more new Securities of authorized denominations and for the
same aggregate principal amount and series will be issued to the designated
transferee or transferees. No service charge will be made for any such
registration of transfer, but the Company may require payment of a sum
sufficient to cover any tax or other governmental charge payable in relation
thereto.

     Prior to due presentment for registration of transfer of this Security, the
Company, the Trustee, any authenticating agent, any paying agent, any transfer
agent and the registrar may deem and treat the holder hereof as the absolute
owner hereof (whether or not this Security shall be overdue and notwithstanding
any notice of ownership or writing hereon made by anyone other than the Security
Registrar) for the purpose of receiving payment of or on account of the
principal hereof 


                                     A-11
<PAGE>
 
and premium, if any, and (subject to the Indenture) interest due hereon and for
all other purposes, and neither the Company nor the Trustee nor any
authenticating agent nor any paying agent nor any transfer agent nor any
registrar shall be affected by any notice to the contrary.

     No recourse shall be had for the payment of the principal of or premium, if
any, or interest on this Security, or for any claim based hereon, or otherwise
in respect hereof, or based on or in respect of the Indenture, against any
incorporator, stockholder, officer or director, past, present or future, as
such, of the Company or of any predecessor or successor Person, whether by
virtue of any constitution, statute or rule of law, or by the enforcement of any
assessment or penalty or otherwise, all such liability being, by the acceptance
hereof and as part of the consideration for the issuance hereof, expressly
waived and released.

     All terms used in this Security that are defined in the Indenture shall
have the meanings assigned to them in the Indenture.

     THE INDENTURE AND THE SECURITIES SHALL BE DEEMED TO BE A CONTRACT MADE
UNDER THE LAWS OF THE STATE OF NEW YORK, AND FOR ALL PURPOSES SHALL BE GOVERNED
BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.



                                     A-12