<PAGE>

                                                                     EXHIBIT 4.3


                            DUQUESNE LIGHT COMPANY

                                      TO

                           THE CHASE MANHATTAN BANK

                                    Trustee

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


                         Supplemental Indenture No. 15

                         Dated as of November 1, 1999




                   Supplemental to the Indenture of Mortgage
                  and Deed of Trust dated as of April 1, 1992








             Establishing thirteen series of Securities designated
                First Mortgage Bonds, Pollution Control Series
                                 K1, K2 and K3
                                 L1, L2 and L3
                            M1, M2, M3, M4 and M5,
                                   N1 and N2
     limited in aggregate principal amount, collectively, to $417,985,000





     SUPPLEMENTAL INDENTURE No. 15, dated as of November 1, 1999, between
DUQUESNE LIGHT COMPANY, a corporation duly organized and existing under the laws
of the Commonwealth of Pennsylvania (hereinafter sometimes called the
"Company"), and THE CHASE MANHATTAN BANK, a New York banking corporation and
successor in trust to Mellon Bank, N.A., trustee (hereinafter sometimes called
the "Trustee"), under the Indenture of Mortgage and Deed of Trust, dated as of
April 1, 1992 (hereinafter called the "Original Indenture"), this Supplemental
Indenture No. 15 being
<PAGE>

supplemental thereto. The Original Indenture and any and all indentures and
instruments supplemental thereto are hereinafter sometimes collectively called
the "Indenture."

                            Recitals of the Company

         The Original Indenture was authorized, executed and delivered by the
Company to provide for the issuance from time to time of its Securities (such
term and all other capitalized terms used herein without definition having the
meanings assigned to them in the Original Indenture), to be issued in one or
more series as contemplated therein, and to provide security for the payment of
the principal of and premium, if any, and interest, if any, on the Securities.

         The Original Indenture has been recorded in the Recorders' Offices of
the various counties of Pennsylvania as follows:

         In Allegheny County in Mortgage Book Vol. 12068, page 8;
         In Beaver County in Mortgage Book Vol. 1208, page 520;
         In Greene County in Mortgage Book Vol. 100, page 174;
         In Washington County in Mortgage Book Vol. 1873, page 1;
         In Westmoreland County in Mortgage Book Vol. 2862, page 221;

and has also been recorded in the Office of the Clerk of County Commission of
Monongahela County, West Virginia, in Deed of Trust Book Vol. 672, page 129, the
Office of the Clerk of County Commission of Hancock County, West Virginia, in
Deed of Trust Book Vol. 293, page 46, the Recorder's Office of Belmont County,
Ohio, in Mortgage Book Vol. 586, page 273, the Recorder's Office of Columbiana
County, Ohio, in Mortgage Book Vol. 318, page 289, the Recorder's Office of
Jefferson County, Ohio, in Mortgage Book Vol. 65, page 675, the Recorder's
Office of Lake County, Ohio, in Mortgage Book Vol. 711, page 217, and the
Recorder's Office of Monroe County, Ohio, in Mortgage Book Vol. 129, page 301.

         The Company has heretofore executed and delivered to the Trustee
Supplemental Indentures for the purposes recited therein and for the purpose of
creating series of Securities as set forth in Schedule A hereto.

         The Company desires to establish thirteen series of Securities, each
series to have the designation, to be limited in aggregate principal amount and
to have the series number as set forth below:

<TABLE>
<CAPTION>
                 Series Designation                                  Principal Amount              Series No.
                 ------------------                                  ----------------              ----------
<S>                                                                  <C>                           <C>
First Mortgage Bonds, Pollution Control Series K1                       $ 49,500,000                   10
First Mortgage Bonds, Pollution Control Series K2                       $ 13,500,000                   11
First Mortgage Bonds, Pollution Control Series K3                       $ 33,955,000                   12
First Mortgage Bonds, Pollution Control Series L1                       $ 21,500,000                   13
First Mortgage Bonds, Pollution Control Series L2                       $ 20,500,000                   14
First Mortgage Bonds, Pollution Control Series L3                       $  4,655,000                   15
First Mortgage Bonds, Pollution Control Series M1                       $ 25,000,000                   16
First Mortgage Bonds, Pollution Control Series M2                       $ 13,700,000                   17
First Mortgage Bonds, Pollution Control Series M3                       $ 18,000,000                   18
First Mortgage Bonds, Pollution Control Series M4                       $ 44,250,000                   19
First Mortgage Bonds, Pollution Control Series M5                       $ 75,500,000                   20
First Mortgage Bonds, Pollution Control Series N1                       $ 50,000,000                   21
First Mortgage Bonds, Pollution Control Series N2                       $ 47,925,000                   22
                                                                        ============
</TABLE>

                                       2
<PAGE>

                                                             Total  $417,985,000


         The Company has duly authorized the execution and delivery of this
Supplemental Indenture No. 15 to establish the Securities of Series Nos. 10
through 22 and has duly authorized the issuance of such Securities; and all acts
necessary to make this Supplemental Indenture No. 15 a valid agreement of the
Company, and to make the Securities of Series Nos. 10 through 22 valid
obligations of the Company, have been performed.

         NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE NO. 15 WITNESSETH, that, in
consideration of the premises and of the purchase of the Securities by the
Holders thereof, and in order to secure the payment of the principal of and
premium, if any, and interest, if any, on all Securities from time to time
Outstanding and the performance of the covenants contained therein and in the
Indenture and to declare the terms and conditions on which such Securities are
secured, the Company hereby grants, bargains, sells, releases, conveys, assigns,
transfers, mortgages, pledges, sets over and confirms to the Trustee, and grants
to the Trustee a security interest in, the following:

                             Granting Clause First

               All right, title and interest of the Company in and to property
         (other than Excepted Property), real, personal and mixed and wherever
         situated, in any case used or to be used in or in connection with the
         generation, purchase, transmission, distribution or sale by the Company
         of electric energy (whether or not such use is the sole use of such
         property), including without limitation (a) all lands, easements,
         servitudes, licenses, permits, rights of way and other rights and
         interests in or relating to real property or the occupancy or use of
         the same; (b) all plants, generators, turbines, engines, boilers, fuel
         handling and transportation facilities, air and water pollution control
         and sewage and solid waste disposal facilities and other machinery and
         facilities for the generation of electric energy; (c) all switchyards,
         lines, towers, substations, transformers and other machinery and
         facilities for the transmission of electric energy; (d) all lines,
         poles, conduits, conductors, meters, regulators and other machinery and
         facilities for the distribution of electric energy; (e) all buildings,
         offices, warehouses and other structures; and (f) all pipes, cables,
         insulators, ducts, tools, computers and other data processing and/or
         storage equipment and other equipment, apparatus and facilities and all
         other property, of whatever kind and nature, ancillary to or otherwise
         used or to be used in conjunction with any or all of the foregoing or
         otherwise, directly or indirectly, in furtherance of the generation,
         purchase, transmission, distribution or sale by the Company of electric
         energy;

                            Granting Clause Second

               Subject to the applicable exceptions permitted by Section 810,
         Section 1303 and Section 1305 of the Original Indenture, all property
         (other than Excepted Property) of the kind and nature described in
         Granting Clause First which may be hereafter acquired by the Company,
         it being the intention of the Company that all such property acquired
         by the Company after the date of the execution and delivery of this
         Supplemental Indenture No. 15 shall be as fully embraced within and
         subjected to the Lien hereof as if such property were owned by the
         Company as of the date of the execution and delivery of this
         Supplemental Indenture No. 15;

                                       3
<PAGE>

                            Granting Clause Fourth

                  All other property of whatever kind and nature subjected or
         intended to be subjected to the Lien of the Indenture by any of the
         terms and provisions thereof;

                                Excepted Property

                  Expressly excepting and excluding, however, from the Lien and
         operation of the Indenture all Excepted Property of the Company,
         whether now owned or hereafter acquired;

         TO HAVE AND TO HOLD all such property, real, personal and mixed, unto
the Trustee forever;

         SUBJECT, HOWEVER, to Permitted Liens and to Liens which have been
granted by the Company to other Persons prior to the date of the execution and
delivery of the Original Indenture and subject also, as to any property acquired
by the Company after the date of execution and delivery of the Original
Indenture, to vendors' Liens, purchase money mortgages and other Liens thereon
at the time of the acquisition thereof (including, but not limited to, the Lien
of any Class "A" Mortgage), it being understood that with respect to any of such
property which was at the date of execution and delivery of the Original
Indenture or thereafter became or hereafter becomes subject to the Lien of any
Class "A" Mortgage, the Lien of the Indenture shall at all times be junior and
subordinate to the Lien of such Class "A" Mortgage;

         IN TRUST, NEVERTHELESS, for the equal and proportionate benefit and
security of the Holders from time to time of all Outstanding Securities without
any priority of any such Security over any other such Security;

         PROVIDED, HOWEVER, that if, after the right, title and interest of the
Trustee in and to the Mortgaged Property shall have ceased, terminated and
become void in accordance with Article Nine of the Original Indenture, the
principal of and premium, if any, and interest, if any, on the Securities shall
have been paid to the Holders thereof, or shall have been paid to the Company
pursuant to Section 603 of the Original Indenture, then and in that case the
Indenture and the estate and rights thereby granted shall cease, terminate and
be void, and the Trustee shall cancel and discharge the Indenture and execute
and deliver to the Company such instruments as the Company shall require to
evidence the discharge thereof; otherwise the Indenture shall be and remain in
full force and effect; and

         THE PARTIES HEREBY FURTHER COVENANT AND AGREE as follows:

                                  ARTICLE ONE

               Tenth through Twenty-Second Series of Securities

         There are hereby created thirteen series of Securities each having the
designation and being limited in aggregate principal amount as set forth below:


               Series Designation                               Principal Amount
               ------------------                               ----------------
First Mortgage Bonds, Pollution Control Series K1                  $  49,500,000
First Mortgage Bonds, Pollution Control Series K2                  $  13,500,000
First Mortgage Bonds, Pollution Control Series K3                  $  33,955,000

                                       4
<PAGE>

First Mortgage Bonds, Pollution Control Series L1                  $  21,500,000
First Mortgage Bonds, Pollution Control Series L2                  $  20,500,000
First Mortgage Bonds, Pollution Control Series L3                  $   4,655,000
First Mortgage Bonds, Pollution Control Series M1                  $  25,000,000
First Mortgage Bonds, Pollution Control Series M2                  $  13,700,000
First Mortgage Bonds, Pollution Control Series M3                  $  18,000,000
First Mortgage Bonds, Pollution Control Series M4                  $  44,250,000
First Mortgage Bonds, Pollution Control Series M5                  $  75,500,000
First Mortgage Bonds, Pollution Control Series N1                  $  50,000,000
First Mortgage Bonds, Pollution Control Series N2                  $  47,925,000
                                                                   =============

                                                       Total       $ 417,985,000

         The form and terms of the Securities of each of Series Nos. 10 through
22 shall be established in or pursuant to an Officer's Certificate.

                                  ARTICLE TWO

                                  Amendments
SECTION 1. Current Amendments.

         (a) Section 301 of the Original Indenture is hereby amended to read as
follows:

         SECTION 301.  Amount Unlimited; Issuable in Series.

                  Subject to the provisions of Article Four, the aggregate
         principal amount of Securities which may be authenticated and delivered
         under this Indenture is unlimited.

                  The Securities may be issued in one or more series. Subject to
         the last paragraph of this Section, prior to the authentication and
         delivery of Securities of any series there shall be established by
         specification in a supplemental indenture or in a Board Resolution, or
         in an Officer's Certificate pursuant to a supplemental indenture or a
         Board Resolution (provided, however, that the information contemplated
         by clauses (a) and (b) below shall in any event be set forth in a
         supplemental indenture):

                         (a) the title of the Securities of such series (which
                  shall distinguish the Securities of such series from
                  Securities of all other series);

                         (b) any limit upon the aggregate principal amount of
                  the Securities of such series which may be authenticated and
                  delivered under this Indenture (except for Securities
                  authenticated and delivered upon registration of transfer of,
                  or in exchange for, or in lieu of, other Securities of such
                  series pursuant to Section 304, 305, 306, 506 or 1406 and
                  except for any Securities which, pursuant to Section 303, are
                  deemed never to have been authenticated and delivered
                  hereunder);

                         (c) the Persons (without specific identification) to
                  whom interest on Securities of such series, or any Tranche
                  thereof, shall be payable on any Interest Payment Date, if
                  other than the Persons in whose names such Securities (or one
                  or

                                       5
<PAGE>

                  more Predecessor Securities) are registered at the close of
                  business on the Regular Record Date for such interest;

                         (d) the date or dates on which the principal of the
                  Securities of such series, or any Tranche thereof, is payable
                  or any formulary or other method or other means by which such
                  date or dates shall be determined, by reference to an index or
                  other fact or event ascertainable outside of this Indenture or
                  otherwise (without regard to any provisions for redemption,
                  prepayment, acceleration, purchase or extension);

                         (e) the rate or rates at which the Securities of such
                  series, or any Tranche thereof, shall bear interest, if any
                  (including the rate or rates at which overdue principal shall
                  bear interest, if different from the rate or rates at which
                  such Securities shall bear interest prior to Maturity, and, if
                  applicable, the rate or rates at which overdue premium or
                  interest shall bear interest, if any), or any formulary or
                  other method or other means by which such rate or rates shall
                  be determined, by reference to an index or other fact or event
                  ascertainable outside of this Indenture or otherwise; the date
                  or dates from which such interest shall accrue; the Interest
                  Payment Dates on which such interest shall be payable and the
                  Regular Record Date, if any, for the interest payable on such
                  Securities on any Interest Payment Date; and the basis of
                  computation of interest, if other than as provided in Section
                  310;

                         (f) the place or places at which and/or the methods (if
                  other than as provided elsewhere in this Indenture) by which
                  (i) the principal of and premium, if any, and interest, if
                  any, on Securities of such series, or any Tranche thereof,
                  shall be payable, (ii) registration of transfer of Securities
                  of such series, or any Tranche thereof, may be effected, (iii)
                  exchanges of Securities of such series, or any Tranche
                  thereof, may be effected and (iv) notices and demands to or
                  upon the Company in respect of the Securities of such series,
                  or any Tranche thereof, and this Indenture may be served; the
                  Security Registrar and any Paying Agent or Agents for such
                  series or Tranche; and, if such is the case, that the
                  principal of such Securities shall be payable without the
                  presentment or surrender thereof;

                         (g) the period or periods within which or the date or
                  dates on which, the price or prices at which and the terms and
                  conditions upon which the Securities of such series, or any
                  Tranche thereof, may be redeemed, in whole or in part, at the
                  option of the Company;

                         (h) the obligation or obligations, if any, of the
                  Company to redeem or purchase the Securities of such series,
                  or any Tranche thereof, pursuant to any sinking fund or other
                  mandatory redemption provisions or at the option of a Holder
                  thereof and the period or periods within which or the date or
                  dates on which, the price or prices at which and the terms and
                  conditions upon which such Securities shall be redeemed or
                  purchased, in whole or in part, pursuant to such obligation,
                  and applicable exceptions to the requirements of Section 504
                  in the case of mandatory redemption or redemption at the
                  option of the Holder;

                                       6
<PAGE>

                         (i) the denominations in which Securities of such
                  series, or any Tranche thereof, shall be issuable if other
                  than denominations of One Thousand Dollars ($1,000) and any
                  integral multiple thereof;

                         (j) the currency or currencies, including composite
                  currencies, in which payment of the principal of and premium,
                  if any, and interest, if any, on the Securities of such
                  series, or any Tranche thereof, shall be payable (if other
                  than in Dollars); it being understood that, for purposes of
                  calculations under this Indenture (including calculations of
                  Annual Interest Requirements contemplated by Section 103 and
                  calculations of principal amount under Article Four), any
                  amounts denominated in a currency other than Dollars or in a
                  composite currency shall be converted to Dollar equivalents by
                  calculating the amount of Dollars which could have been
                  purchased by the amount of such other currency based on such
                  quotations or methods of determination as shall be specified
                  pursuant to this clause (j);

                         (k) if the principal of or premium, if any, or
                  interest, if any, on the Securities of such series, or any
                  Tranche thereof, are to be payable, at the election of the
                  Company or a Holder thereof, in a coin or currency other than
                  that in which the Securities are stated to be payable, the
                  coin or currency in which payment of any amount as to which
                  such election is made will be payable, the period or periods
                  within which, and the terms and conditions upon which, such
                  election may be made; it being understood that, for purposes
                  of calculations under this Indenture (including calculations
                  of Annual Interest Requirements contemplated by Section 103
                  and calculations of principal amount under Article Four), any
                  such election shall be required to be taken into account, in
                  the manner contemplated in clause (j) of this paragraph, only
                  after such election shall have been made.

                         (l) if the principal of or premium, if any, or
                  interest, if any, on the Securities of such series, or any
                  Tranche thereof, are to be payable, or are to be payable at
                  the election of the Company or a Holder thereof, in securities
                  or other property, the type and amount of such securities or
                  other property, or the formulary or other method or other
                  means by which such amount shall be determined, and the period
                  or periods within which, and the terms and conditions upon
                  which, any such election may be made; it being understood that
                  all calculations under this Indenture (including calculations
                  of Annual Interest Requirements contemplated by Section 103
                  and calculations of principal amount under Article Four) shall
                  be made on the basis of the fair market value of such
                  securities or the fair value of such other property, in either
                  case as of the most recent practicable date, except that, in
                  the case of any amount of principal or interest that may be so
                  payable at the election of the Company or a Holder, if such
                  election shall not yet have been made, such calculations shall
                  be made on the basis of the amount of principal or interest,
                  as the case may be, that would be payable if no such election
                  were made;

                             if the amount payable in respect of principal of or
                  premium, if any, or interest, if any, on the Securities of
                  such series, or any Tranche thereof, may be

                                       7
<PAGE>

                  determined with reference to an index or other fact or event
                  ascertainable outside of this Indenture, the manner in which
                  such amounts shall be determined (to the extent not
                  established pursuant to clause (e) of this paragraph); it
                  being understood that all calculations under this Indenture
                  (including calculations of Annual Interest Requirements
                  contemplated by Section 103 (except as otherwise provided in
                  such Section) and calculations of principal amount under
                  Article Four) shall be made on the basis of the amount that
                  would be payable as principal if such principal were due, or
                  on the basis of the interest rates in effect, as the case may
                  be, on the date next preceding the date of such calculation;

                         (m) if other than the principal amount thereof, the
                  portion of the principal amount of Securities of such series,
                  or any Tranche thereof, which shall be payable upon
                  declaration of acceleration of the Maturity thereof pursuant
                  to Section 1002;

                         (n) the terms, if any, pursuant to which the Securities
                  of such series, or any Tranche thereof, may be converted into
                  or exchanged for shares of capital stock or other securities
                  of the Company or any other Person;

                         (o) the obligations or instruments, if any, which shall
                  be considered to be Eligible Obligations in respect of the
                  Securities of such series, or any Tranche thereof, denominated
                  in a currency other than Dollars or in a composite currency,
                  and any additional or alternative provisions for the
                  reinstatement of the Company's indebtedness in respect of such
                  Securities after the satisfaction and discharge thereof as
                  provided in Section 901;

                         (p) if the Securities of such series, or any Tranche
                  thereof, are to be issued in global form, (i) any limitations
                  on the rights of the Holder or Holders of such Securities to
                  transfer or exchange the same or to obtain the registration of
                  transfer thereof, (ii) any limitations on the rights of the
                  Holder or Holders thereof to obtain certificates therefor in
                  definitive form in lieu of temporary form and (iii) any and
                  all other matters incidental to such Securities;

                         (q) if the Securities of such series, or any Tranche
                  thereof, are to be issuable as bearer securities, any and all
                  matters incidental thereto which are not specifically
                  addressed in a supplemental indenture as contemplated by
                  clause (f) of Section 1401;

                         (r) to the extent not established pursuant to clause
                  (q) of this paragraph, any limitations on the rights of the
                  Holders of the Securities of such Series, or any Tranche
                  thereof, to transfer or exchange such Securities or to obtain
                  the registration of transfer thereof; and if a service charge
                  will be made for the registration of transfer or exchange of
                  Securities of such series, or any Tranche thereof, the amount
                  or terms thereof;

                                       8
<PAGE>

                         (s) any exceptions to Section 116, or variation in the
              definition of Business Day, with respect to the Securities of such
              series, or any Tranche thereof; and

                         (t) any other terms of the Securities of such series,
              or any Tranche thereof, not inconsistent with the provisions of
              the Indenture.

              With respect to Securities of a series subject to a Periodic
         Offering, the indenture supplemental hereto or the Board Resolution
         which establishes such series, or the Officer's Certificate pursuant to
         such supplemental indenture or Board Resolution, as the case may be,
         may provide general terms or parameters for Securities of such series
         and provide either that the specific terms of Securities of such
         series, or any Tranche thereof, shall be specified in a Company Order
         or that such terms shall be determined by the Company or its agents in
         accordance with procedures specified in a Company Order as contemplated
         by clause (b) of Section 401.

         (b)  The definition of "Stated Interest Rate" in Section 101 of the
         Original Indenture is hereby amended to read as follows:

                         "Stated Interest Rate" means a rate (whether fixed or
              variable) at which an obligation by its terms is stated to bear
              simple interest. Any calculation or other determination to be made
              under this Indenture by reference to the Stated Interest Rate on a
              Security shall be made without regard to the effective interest
              cost to the Company of such Security and without regard to the
              Stated Interest Rate on, or the effective cost to the Company of,
              any other obligation for which such Security is pledged or
              otherwise delivered as security.

         (c)  (i)    Clause (c)(i) in Section 804 of the Original Indenture is
         hereby amended to delete the phrase "to the Company";

              (ii)   Clause (c)(iii)(B) in Section 804 of the Original Indenture
is hereby amended to delete the phrase "to the Company" the first time it
currently appears; and

              (iii)  Clause (d) in Section 804 of the Original Indenture is
hereby amended to delete the phrase "to the Company".

         (d)  Section 1108 of the Original Indenture is hereby amended to read
as follows:

                                       9
<PAGE>

               SECTION 1108.  Disqualification; Conflicting Interests.

                    If the Trustee shall have or acquire any conflicting
               interest within the meaning of the Trust Indenture Act, it shall
               either eliminate such conflicting interest or resign to the
               extent, in the manner and with the effect, and subject to the
               conditions, provided in the Trust Indenture Act and this
               Indenture. For purposes of Section 310(b)(1) of the Trust
               Indenture Act and to the extent permitted thereby, the Trustee,
               in its capacity as trustee in respect of the Securities of any
               series, shall not be deemed to have a conflicting interest
               arising from its capacity as trustee in respect of the Securities
               of any other series.

          (e)  Section 1113 of the Original Indenture is hereby amended to read
          as follows:

               SECTION 1113.  Preferential Collection of Claims against Company.

                    If the Trustee shall be or become a creditor of the Company
               or any other obligor upon the Securities (other than by reason of
               a relationship described in Section 311(b) of the Trust Indenture
               Act), the Trustee shall be subject to any and all applicable
               provisions of the Trust Indenture Act regarding the collection of
               claims against the Company or such other obligor. For purposes of
               Section 311(b) of the Trust Indenture Act:

                    (a) the term "cash transaction" means any transaction in
               which full payment for goods or securities sold is made within
               seven days after delivery of the goods or securities in currency
               or in checks or other orders drawn upon banks or bankers and
               payable upon demand; and

                    (b) the term "self-liquidating paper" means any draft, bill
               of exchange, acceptance or obligation which is made, drawn,
               negotiated or incurred by the Company or such obligor for the
               purpose of financing the purchase, processing, manufacturing,
               shipment, storage or sale of goods, wares or merchandise and
               which is secured by documents evidencing title to, possession of,
               or a lien upon, the goods, wares or merchandise or the
               receivables or proceeds arising from the sale of the goods, wares
               or merchandise previously constituting the security, provided the
               security is received by the Trustee simultaneously with the
               creation of the creditor relationship with the Company or such
               obligor arising from the making, drawing, negotiating or
               incurring of the draft, bill of exchange, acceptance or
               obligation.

          (f)  Subsection (g) of Section 107 of the Original Indenture is hereby
          amended by adding at the end thereof the following:

               Any such request, demand, authorization, direction, notice,
               consent, waiver or other Act, given or made as aforesaid, shall
               be effective whether or not the Holders which authorized or
               agreed or consented to the same remain Holders after such record
               date and whether or not the Securities held by such Holders
               remain Outstanding after such record date.

          (g)  Clause (ii) of subsection (b) of Section 104 of the Original
          Indenture is hereby amended by:

                                       10
<PAGE>

          (i)    deleting the word "and" at the end of subclause (B) therein;

          (ii)   adding after said subclause (B) a new subclause (C) reading as
follows:

                 "(C) the Cost or fair value to the Company (whichever is less)
                 of any Property Additions not theretofore so added and which
                 the Company then elects so to add, to the extent that the same
                 shall have been made the basis of the release of Funded
                 Property retired (such fair value to be the amount shown in the
                 Engineer's Certificate delivered to the Trustee in connection
                 with such release); and"

                                                                      and

          (iii)  changing old subclause (C) to subclause (D).

     (h)  Subsection (c) of Section 104 of the Original Indenture is hereby
     amended by deleting from the first clause of the first paragraph thereof
     the phrase "made the basis under any of the provisions of this Indenture of
     one or more Authorized Purposes".

SECTION 2. Prospective Amendments.

          The Holders of the Securities of Series Nos. 10 through 22 shall be
deemed to have consented to the execution and delivery of a supplemental
indenture containing one or more, or all, the amendments to the Original
Indenture set forth below:

     (a)  The amendment of the definition of Stated Interest Rate in Section 101
     of the Original Indenture to read as follows:

                 "Stated Interest Rate" means a rate (whether fixed or variable)
     at which an obligation by its terms is stated to bear simple interest. Any
     calculation or other determination to be made under this Indenture by
     reference to the Stated Interest Rate on an obligation shall be made (a) if
     the Company's obligations in respect of any other indebtedness shall be
     evidenced or secured in whole or in part by such obligation, by reference
     to the lower of the Stated Interest Rate on such obligation and the Stated
     Interest Rate on such other indebtedness and (b) without regard to the
     effective interest cost to the Company of such obligation or of any such
     other indebtedness.

     (b)  The amendment of the definition of "Cost" in Section 104 of the
     Original Indenture to read as follows:

                 Except as otherwise provided in Section 803, the term "Cost"
     with respect to Property Additions shall mean the sum of (i) any cash
     delivered in payment therefor or for the acquisition thereof, (ii) an
     amount equivalent to the fair market value in cash (as of the date of
     delivery) of any securities or other property delivered in payment therefor
     or for the acquisition thereof, (iii) the principal amount of any
     obligations secured by prior Lien (other than a Class A Mortgage) upon such
     Property Additions outstanding at the time of the acquisition thereof, (iv)
     the principal amount of any other obligations incurred or assumed in
     connection with the payment for such Property Additions or for

                                       11
<PAGE>

     the acquisition thereof and (v) any other amounts which, in accordance with
     generally accepted accounting principles, are properly charged or
     chargeable to the plant or other property accounts of the Company with
     respect to such Property Additions as part of the cost of construction or
     acquisition thereof, including, but not limited to, any allowance for funds
     used during construction or any similar or analogous amount; provided,
     however, that, notwithstanding any other provision of this Indenture,

               (x) with respect to Property Additions owned by a successor
          corporation immediately prior to the time it shall have become such by
          consolidation or merger or acquired by a successor corporation in or
          as a result of a consolidation or merger (excluding, in any case,
          Property Additions owned by the Company immediately prior to such
          time), Cost shall mean the amount or amounts at which such Property
          Additions are recorded in the plant or other property accounts of such
          successor corporation, or the predecessor corporation from which such
          Property Additions are acquired, as the case may be, immediately prior
          to such consolidation or merger;

               (y) with respect to Property Additions which shall have been
          acquired (otherwise than by construction) by the Company without any
          consideration consisting of cash, securities or other property or the
          incurring or assumption of indebtedness, no determination of Cost
          shall be required, and, wherever in this Indenture provision is made
          for Cost or fair value, Cost with respect to such Property Additions
          shall mean an amount equal to the fair value to the Company thereof
          or, if greater, the aggregate amount reflected in the Company's books
          of account with respect thereto upon the acquisition thereof; and

               (z) in no event shall the Cost of Property Additions be required
          to reflect any depreciation or amortization in respect of such
          Property Additions, or any adjustment to the amount or amounts at
          which such Property Additions are recorded in plant or other property
          accounts due to the non-recoverability of investment or otherwise.

     (c)  The amendment of the proviso to clause (d) in Section 803 of the
     Original Indenture to:

               (1) delete therefrom clause (x) or to provide that clause (x) may
          be disregarded upon specified conditions; and/or

               (2) (a)   to delete therefrom clause (z) or to provide that
               clause (z) may be disregarded upon specified conditions; or

                   (b)   to delete from clause (z) therein the phrase "fifteen
               per centum (15%) of"; or

                   (c) to change the phrase "fifteen per centum (15%)" in clause
               (z) therein to any higher percentage not exceeding one hundred
               per centum (100%).

                                       12
<PAGE>

     (d)  The addition to the Original Indenture of a definition of the term
     "purchase money mortgage" substantially to the following effect:

                    "Purchase money mortgage" means, with respect to any
               property being acquired or disposed of by the Company or being
               released from the Lien of this Indenture, a Lien on such property
               which

                    (a) is taken or retained by the transferor of such property
               to secure all or part of the purchase price thereof;

                    (b) is granted to one or more Persons other than the
               transferor which, by making advances or incurring an obligation,
               give value to enable the grantor of such Lien to acquire rights
               in or the use of such property;

                    (c) is granted to any other Person in connection with the
               release of such property from the Lien of this Indenture on the
               basis of the deposit with the Trustee or the trustee or other
               holder of a Lien prior to the Lien of this Indenture of
               obligations secured by such Lien on such property (as well as any
               other property subject thereto);

                    (d) is held by a trustee or agent for the benefit of one or
               more Persons described in clause (a), (b) and/or (c) above,
               provided that such Lien may be held, in addition, for the benefit
               of one or more other Persons which shall have theretofore given,
               or may thereafter give, value to or for the benefit or account of
               the grantor of such Lien for one or more other purposes; or

                    (e) otherwise constitutes a purchase money mortgage or a
               purchase money security interest under applicable law;

               and, without limiting the generality of the foregoing, for
               purposes of this Indenture, the term shall be deemed to include
               any Lien described above whether or not such Lien (x) shall
               permit the issuance or other incurrence of additional
               indebtedness secured by such Lien on such property, (y) shall
               permit the subjection to such Lien of additional property and the
               issuance or other incurrence of additional indebtedness on the
               basis thereof and/or (z) shall have been granted prior to the
               acquisition, disposition or release of such property, shall
               attach to or otherwise cover property other than the property
               being acquired, disposed of or released and/or shall secure
               obligations issued prior and/or subsequent to the issuance of the
               obligations delivered in connection with such acquisition,
               disposition or release.

     (e)  The addition to the Original Indenture of a definition of the term
     "fair value" substantially to the following effect:

               "Fair value", with respect to property, means the fair value of
     such property as may be determined by reference to (a) the amount which
     would be likely to be obtained in an arm's-length transaction with respect
     to such property between an informed and willing buyer and an informed and
     willing seller, under no compulsion, respectively, to buy or sell, (b) the
     amount of investment with respect to such property which, together

                                       13
<PAGE>

     with a reasonable return thereon, would be likely to be recovered through
     ordinary business operations or otherwise, (c) the Cost, accumulated
     depreciation and replacement cost with respect to such property and/or (d)
     any other relevant factors; provided, however, that (x) the fair value of
     property shall be determined without deduction for any Liens on such
     property prior to the Lien of this Indenture (except as otherwise provided
     in Section 803) and (y) the fair value to the Company of Property Additions
     shall not reflect any reduction relating to the fact that such Property
     Additions may be of less value to a Person which is not the owner or
     operator of the Mortgaged Property or any portion thereof than to a Person
     which is such owner or operator. Fair value may be determined, without
     physical inspection, by the use of accounting and engineering records and
     other data maintained by the Company or otherwise available to the Expert
     certifying the same.

          (f) (i)  The amendment of the proviso to clause (i) of subsection (b)
     of Section 404 of the Original Indenture to read as follows:

                   provided, however, that no Net Earnings Certificate shall be
                   required to be delivered; and

              (ii) The amendment of the proviso to clause (ii) of subsection (b)
     of Section 402 of the Original Indenture to read as follows:

                   provided, however, that no Net Earnings Certificate shall be
                   required to be delivered if there shall be delivered an
                   Officer's Certificate to the effect that such Class "A" Bonds
                   were authenticated and delivered under the related Class "A"
                   Mortgage on the basis of retired Class "A" Bonds; and


                                 ARTICLE THREE

                           Miscellaneous Provisions

     This Supplemental Indenture No. 15 is a supplement to the Indenture. As
supplemented by this Supplemental Indenture No. 15, the Indenture is in all
respects ratified, approved and confirmed, and the Indenture and this
Supplemental Indenture No. 15 shall together constitute one and the same
instrument.

          IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture No. 15 to be duly executed, and their respective corporate seals to be
hereunto affixed and attested, all as of the day and year first above written.


                                        DUQUESNE LIGHT COMPANY


                                        By: FROSINA CORDISCO
                                            ----------------------------------
                                               Treasurer

Attest:


STEVEN DELCOTTO
------------------------
   Assistant Secretary

                                       14
<PAGE>

                                        THE CHASE MANHATTAN BANK, Trustee


                                        By:  KENT CHRISTMAN
                                            ------------------------------------
                                                Vice President

Attest:


KIM SCHELLMAN
---------------------------
   Authorized Officer

                                       15
<PAGE>

COMMONWEALTH OF PENNSYLVANIA)
                                ) ss.:
COUNTY OF ALLEGHENY)


          On the 15th day of November, 1999, before me personally came Frosina
C. Cordisco, to me known, who, being by me duly sworn, did depose and say that
she is the Treasurer of Duquesne Light Company, the corporation described in and
which executed the foregoing instrument; that she knows the seal of said
corporation; that the seal affixed to said instrument is such corporate seal;
that it was so affixed by authority of the Board of Directors of said
corporation, and that she signed her name thereto by like authority.

                                        Kelly M. Golgan
                                        -----------------------------
                                        Notary Public


COMMONWEALTH OF PENNSYLVANIA)
                                ) ss.:
COUNTY OF ALLEGHENY)


          On the 15th day of November, 1999, before me personally came         ,
to me known, who, being by me duly sworn, did depose and say that he is a Vice
President of The Chase Manhattan Bank, the banking corporation described in and
which executed the foregoing instrument; that he knows the seal of said banking
corporation; that the seal affixed to said instrument is the seal of said
banking corporation; that it was so affixed by authority of the Board of
Directors of said banking corporation, and that he signed his name thereto by
like authority.

                                        Robert D. Weising
                                        ----------------------------------------
                                        Notary Public

                                       16
<PAGE>

                       CERTIFICATE OF PRECISE RESIDENCE


          I hereby certify that the precise residence of The Chase Manhattan
Bank is One Oxford Center Suite 1100, Pittsburgh, Allegheny County,
Pennsylvania.

                         KENT CHRISTMAN
                         -------------------------------------------------------
                         Authorized Signatory of The Chase Manhattan Bank

                                                               November 15, 1999

                                       17
<PAGE>

                                                                      Schedule A

<TABLE>
<CAPTION>
                                                                                                  Principal Amount
Supplemental                           Securities of      Series                    Authorized        Issued/1/      Outstanding/1/
                                                                                    ----------        ---------      --------------
Indenture No.          Dated as of      Series No.      Designation
-------------          -----------      ----------      -----------
<S>                <C>                 <C>            <C>                         <C>             <C>                <C>
        1          April 1, 1992             1        Secured Medium-             $400,000,000      $400,000,000     $203,000,000
                                                      Term Notes,
                                                      Series B
        2          October 1, 1992           2        First Collateral            $400,000,000      $400,000,000     $240,000,000
                                                      Trust Bonds,
                                                      Series C
        3          December 1, 1992          3        First Collateral            $47,925,000       $47,925,000       $47,925,000
                                                      Trust Bonds,
                                                      Pollution
                                                      Control Series D
        4          March 30, 1993          None       None                             None             None             None
        5          June 1, 1993              4        First Collateral             $300,000,000     $300,000,000     $300,000,000
                                                      Trust Bonds,
                                                      Series E
        6          June 1, 1993              5        First Collateral             $25,000,000      $25,000,000       $25,000,000
                                                      Trust Bonds,
                                                      Pollution
                                                      Control Series F
        7          August 1, 1993            6        First Collateral             $20,500,000      $20,500,000       $20,500,000
                                                      Trust Bonds,
                                                      Pollution
                                                      Control Series G
        8          March 21, 1994          None       None                             None             None             None
        9          October 1, 1994           7        First Collateral             $75,500,000      $75,500,000       $75,500,000
                                                      Trust Bonds,
                                                      Pollution
                                                      Control Series H
       10          March 22, 1995          None       None                             None             None             None

       11          June 1, 1995              8        First Collateral             $923,000,000     $923,000,000         None
                                                      Trust Bonds,
                                                      Series I
       12          September 1, 1995         9        First Mortgage               $685,000,000     $345,000,000    $280,000,0002
                                                      Bonds, Series J
       13          March 22, 1996          None       None                             None             None             None
       14          March 17, 1997          None       None                             None             None             None
</TABLE>

_______________________________
/1/ As of November 15th, 1999.

/2/ $45,000,000 scheduled to be paid at maturity on November 16, 1999.

                                       18
<PAGE>

                             RECORDING INFORMATION


                     Allegheny County, Pennsylvania
                     Office of Recorder of Deeds
                     Recorded
                     Mortgage Book Volume

                     Beaver County, Pennsylvania
                     Office of Recorder of Deeds
                     Recorded
                     Mortgage Book Volume

                     Greene County, Pennsylvania
                     Office of Recorder of Deeds
                     Recorded
                     Mortgage Book Volume

                     Washington County, Pennsylvania
                     Office of Recorder of Deeds
                     Recorded
                     Mortgage Book Volume

                     Westmoreland County, Pennsylvania
                     Office of Recorder of Deeds
                     Recorded
                     Mortgage Book Volume

                     Belmont County, Ohio
                     Office of Recorder
                     Received
                     Recorded
                     Mortgage Book Volume

                     Columbiana County, Ohio
                     Office of Recorder
                     Recorded
                     Official Records Volume

                     Jefferson County, Ohio
                     Office of Recorder
                     Received
                     Recorded
                     Official Records Volume

                                       19
<PAGE>

                     Lake County, Ohio
                     Office of Recorder
                     Recorded
                     Official Records Volume

                     Monroe County, Ohio
                     Office of Recorder
                     Received
                     Recorded
                     Official Records Volume

                     Hancock County, West Virginia
                     Office of Clerk of County Commission
                     Recorded
                     Deed of Trust Book

                     Monongahela County, West Virginia
                     Office of Clerk of County Commission
                     Recorded
                     Deed of Trust Book

                                       20
<PAGE>

                            DUQUESNE LIGHT COMPANY

                                      TO

                           THE CHASE MANHATTAN BANK

                                    Trustee

                            ______________________


                         Supplemental Indenture No. 16

                         Dated as of December 2, 1999




                   Supplemental to the Indenture of Mortgage
                  and Deed of Trust dated as of April 1, 1992








               Subjecting additional property to the lien of the
                    Indenture of Mortgage and Deed of Trust
                           dated as of April 1, 1992

                                       21
<PAGE>

     SUPPLEMENTAL INDENTURE No. 16, dated as of December 2, 1999, between
DUQUESNE LIGHT COMPANY, a corporation duly organized and existing under the laws
of the Commonwealth of Pennsylvania (hereinafter sometimes called the
"Company"), and THE CHASE MANHATTAN BANK, a New York banking corporation and
successor in trust to Mellon Bank, N.A., trustee (hereinafter sometimes called
the "Trustee"), under the Indenture of Mortgage and Deed of Trust, dated as of
April 1, 1992 (hereinafter called the "Original Indenture"), this Supplemental
Indenture No. 16 being supplemental thereto. The Original Indenture and any and
all indentures and instruments supplemental thereto are hereinafter sometimes
collectively called the "Indenture."

                            Recitals of the Company

     The Original Indenture was authorized, executed and delivered by the
Company to provide for the issuance from time to time of its Securities (such
term and all other capitalized terms used herein without definition having the
meanings assigned to them in the Original Indenture), to be issued in one or
more series as contemplated therein, and to provide security for the payment of
the principal of and premium, if any, and interest, if any, on the Securities.

     The Original Indenture has been recorded in the Recorders' Offices of the
various counties of Pennsylvania as follows:

     In Allegheny County in Mortgage Book Vol. 12068, page 8;
     In Beaver County in Mortgage Book Vol. 1208, page 520;
     In Greene County in Mortgage Book Vol. 100, page 174;
     In Washington County in Mortgage Book Vol. 1873, page 1;
     In Westmoreland County in Mortgage Book Vol. 2862, page 221;

and has also been recorded in the Office of the Clerk of County Commission of
Monongahela County, West Virginia, in Deed of Trust Book Vol. 672, page 129, the
Office of the Clerk of County Commission of Hancock County, West Virginia, in
Deed of Trust Book Vol. 293, page 46, the Recorder's Office of Belmont County,
Ohio, in Mortgage Book Vol. 586, page 273, the Recorder's Office of Columbiana
County, Ohio, in Mortgage Book Vol. 318, page 289, the Recorder's Office of
Jefferson County, Ohio, in Mortgage Book Vol. 65, page 675, the Recorder's
Office of Lake County, Ohio, in Mortgage Book Vol. 711, page 217, and the
Recorder's Office of Monroe County, Ohio, in Mortgage Book Vol. 129, page 301.

     Section 1401 of the Original Indenture provides that the Company and the
Trustee may enter into one or more supplemental indentures for the purpose,
among others, of subjecting additional property to the Lien of the Indenture.

     The Company has acquired additional properties which it desires to
specifically subject to the Lien of the Indenture by this Supplemental Indenture
No. 16.

     NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE NO. 16 WITNESSETH, that, in
consideration of the premises and of the purchase of the Securities by the
Holders thereof, and in order to secure the payment of the principal of and
premium, if any, and interest, if any, on all Securities from time to time
Outstanding and the performance of the covenants contained therein and in the
Indenture and to declare the terms and conditions on which such Securities are
secured, the Company hereby grants, bargains, sells, releases, conveys, assigns,
transfers, mortgages, pledges, sets over and confirms to the Trustee, and grants
to the Trustee a security interest in, the following:

                                      21
<PAGE>

                             Granting Clause First

          All right, title and interest of the Company in and to property (other
     than Excepted Property), real, personal and mixed and wherever situated, in
     any case used or to be used in or in connection with the generation,
     purchase, transmission, distribution or sale by the Company of electric
     energy (whether or not such use is the sole use of such property),
     including without limitation (a) all lands and interest in land described
     in the Appendices hereto which relate, respectively, to (i) the Avon Lake
     Generating Station located in Lorain County, Ohio, (ii) the New Castle
     Generating Station located in Lawrence County, Pennsylvania and (iii) the
     Niles Generating Station located in Trumbull County, Ohio, (b) all other
     lands, easements, servitudes, licenses, permits, rights of way and other
     rights and interests in or relating to real property or the occupancy or
     use of the same; (c) all plants, generators, turbines, engines, boilers,
     fuel handling and transportation facilities, air and water pollution
     control and sewage and solid waste disposal facilities and other machinery
     and facilities for the generation of electric energy; (d) all switchyards,
     lines, towers, substations, transformers and other machinery and facilities
     for the transmission of electric energy; (e) all lines, poles, conduits,
     conductors, meters, regulators and other machinery and facilities for the
     distribution of electric energy; (f) all buildings, offices, warehouses and
     other structures; and (g) all pipes, cables, insulators, ducts, tools,
     computers and other data processing and/or storage equipment and other
     equipment, apparatus and facilities and all other property, of whatever
     kind and nature, ancillary to or otherwise used or to be used in
     conjunction with any or all of the foregoing or otherwise, directly or
     indirectly, in furtherance of the generation, purchase, transmission,
     distribution or sale by the Company of electric energy;

                               Excepted Property

          Expressly excepting and excluding, however, from the Lien and
     operation of the Indenture all Excepted Property of the Company, whether
     now owned or hereafter acquired;

     TO HAVE AND TO HOLD all such property, real, personal and mixed, unto the
Trustee forever;

     SUBJECT, HOWEVER, to Permitted Liens and to Liens which have been granted
by the Company to other Persons prior to the date of the execution and delivery
of the Original Indenture and subject also, as to any property acquired by the
Company after the date of execution and delivery of the Original Indenture, to
vendors' Liens, purchase money mortgages and other Liens thereon at the time of
the acquisition thereof (including, but not limited to, the Lien of any Class
"A" Mortgage), it being understood that with respect to any of such property
which was at the date of execution and delivery of the Original Indenture or
thereafter became or hereafter becomes subject to the Lien of any Class "A"
Mortgage, the Lien of the Indenture shall at all times be junior and subordinate
to the Lien of such Class "A" Mortgage;

     IN TRUST, NEVERTHELESS, for the equal and proportionate benefit and
security of the Holders from time to time of all Outstanding Securities without
any priority of any such Security over any other such Security;

                                      22
<PAGE>

     THE PARTIES HEREBY FURTHER COVENANT AND AGREE this Supplemental Indenture
No. 16 is a supplement to the Indenture. As supplemented by this Supplemental
Indenture No. 16, the Indenture is in all respects ratified, approved and
confirmed, and the Indenture and this Supplemental Indenture No. 16 shall
together constitute one and the same instrument.

     IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture No. 16 to be duly executed, and their respective corporate seals to be
hereunto affixed and attested, all as of the day and year first above written.

                                        DUQUESNE LIGHT COMPANY


                                        By: /s/ Frosina C. Cordisco
                                           -------------------------------------
                                              Treasurer

Attest:


  /s/ Steven J. DelCotto
-------------------------------------
  Asst. Secretary




                                        THE CHASE MANHATTAN BANK, Trustee


                                        By:   /s/ K. M. Christman
                                           -------------------------------------
                                              Vice President


Attest:


   /s/ John J. Scarpiniti
----------------------------------------
   Authorized Officer

                                      23
<PAGE>

COMMONWEALTH OF PENNSYLVANIA)
                              ) ss.:
COUNTY OF ALLEGHENY)


          On the 2nd day of December, 1999, before me personally came Frosina C.
Cordisco, to me known, who, being by me duly sworn, did depose and say that she
is the Treasurer of Duquesne Light Company, the corporation described in and
which executed the foregoing instrument; that she knows the seal of said
corporation; that the seal affixed to said instrument is such corporate seal;
that it was so affixed by authority of the Board of Directors of said
corporation, and that she signed her name thereto by like authority.

                                        /s/ Kathleen T. Pitchford
                                        ----------------------------------------
                                        Notary Public


COMMONWEALTH OF PENNSYLVANIA)
                              ) ss.:
COUNTY OF ALLEGHENY)


          On the 2nd day of December, 1999, before me personally came K. M.
Christman, to me known, who, being by me duly sworn, did depose and say that he
is a Vice President of The Chase Manhattan Bank, the banking corporation
described in and which executed the foregoing instrument; that he knows the seal
of said banking corporation; that the seal affixed to said instrument is the
seal of said banking corporation; that it was so affixed by authority of the
Board of Directors of said banking corporation, and that he signed his name
thereto by like authority.

                                        /s/ Kathleen T. Pitchford
                                        ----------------------------------------
                                        Notary Public

                                      24
<PAGE>

                       CERTIFICATE OF PRECISE RESIDENCE


          I hereby certify that the precise residence of The Chase Manhattan
Bank is One Oxford Center, Suite 1100, Pittsburgh, Allegheny County,
Pennsylvania.

                              /s/ K. M. Christman
                              --------------------------------------------------
                              Authorized Signatory of The Chase Manhattan Bank

                                                                December 2, 1999

                                      25
<PAGE>

                                                                  APPENDIX _____