FIFTH SUPPLEMENTAL INDENTURE

Exhibit 4.2

FIFTH SUPPLEMENTAL INDENTURE

dated as of April 1, 2008

to

INDENTURE

dated as of  November 1, 2000

AMEREN ENERGY GENERATING COMPANY

to

THE BANK OF NEW YORK TRUST COMPANY, N.A., as Trustee

$300,000,000 7.00% Senior Notes, Series G Due 2018

FIFTH SUPPLEMENTAL INDENTURE (this “Fifth Supplemental Indenture”), dated as of April 1, 2008, to the Indenture, dated as of November 1, 2000 (the “Original Indenture”), from AMEREN ENERGY GENERATING COMPANY, an Illinois corporation (together with its successors and assigns, the “Issuer”), its principal office and mailing address being at One Ameren Plaza, 1901 Chouteau Avenue, P.O. Box 66149, St. Louis, Missouri 63166-6149, to THE BANK OF NEW YORK TRUST COMPANY, N.A., a national banking association organized and existing under and by virtue of the laws of the United States, as trustee (the “Trustee”), its office and mailing address being at 911 Washington Avenue, 3rd Floor St. Louis, Missouri 63101.

W I T N E S S E T H:

WHEREAS, the Issuer and the Trustee have heretofore executed and delivered the Original Indenture to provide for the issuance from time to time of the Issuer’s Securities (as defined in the Original Indenture) to be issued in one or more series;

WHEREAS, Sections 2.1 and 7.1(b) of the Original Indenture provide, among other things, that the Issuer and the Trustee may enter into indentures supplemental to the Original Indenture for, among other things, the purpose of establishing the designation, form, terms and provisions of Securities of any series as permitted by Sections 2.1 and 7.1(b) of the Original Indenture;

WHEREAS, the Issuer (i) desires the issuance of a series of Securities to be designated as hereinafter provided and (ii) has requested the Trustee to enter into this Fifth Supplemental Indenture for the purpose of establishing the designation, form, terms and provisions of the Securities of such series;

WHEREAS, all action on the part of the Issuer necessary to authorize the issuance of said Securities under the Original Indenture and this Fifth Supplemental Indenture (the Original Indenture, as supplemented by this Fifth Supplemental Indenture, being hereinafter called the “Indenture”) has been duly taken; and

WHEREAS, all acts and things necessary to make said Securities, when executed by the Issuer and authenticated and delivered by the Trustee as provided in the Original Indenture, the legal, valid and binding obligations of the Issuer, and to constitute these presents a valid and binding supplemental indenture according to its terms, have been done and performed, and the execution of this Fifth Supplemental Indenture and the creation and issuance under the Indenture of said Securities have in all respects been duly authorized, and the Issuer, in the exercise of the legal right and power vested in it, executes this Fifth Supplemental Indenture and proposes to create, execute, issue and deliver said Securities;

NOW, THEREFORE, in order to establish the designation, form, terms and provisions of, and to authorize the authentication and delivery of, said Securities, and in consideration of the acceptance of said Securities by the holders thereof and of other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto agree as follows:

ARTICLE I

DEFINITIONS

Capitalized terms not otherwise defined herein shall have the meanings set forth in the Original Indenture.

ARTICLE II

THE TERMS OF THE SERIES G NOTES

Section 2.1            Terms of 7.00% Senior Notes, Series G due 2018 .  (a)  There is hereby created one (1) series of Securities designated:  7.00% Senior Notes, Series G due 2018, in the initial aggregate principal amount of $300,000,000 (the “Series G Senior Notes”).  Upon delivery of a written order to the Trustee in accordance with the provisions of Section 2.1 of the Original Indenture, the Trustee shall authenticate and deliver the Series G Senior Notes.  Such written order shall specify the amount of the Series G Senior Notes to be authenticated and the date on which such Series G Senior Notes are to be authenticated.

(b)            The Series G Senior Notes shall be substantially in the form of Exhibit A hereto.

Section 2.2            Terms of Series G Senior Notes Issued Hereunder in Global Form .

(a)            So long as DTC or its nominee is the registered owner or Holder of a Global Security, DTC or its nominee, as the case may be, will be considered the sole owner or Holder of the Series G Senior Notes represented by such Global Security for all purposes under the Original Indenture and under the Series G Senior Notes.  No beneficial owner of an interest in a Global Security will be able to transfer that interest except in accordance with DTC’s applicable procedures unless the Issuer shall issue certificates for the Series G Senior Notes in definitive registered form.

(b)            All payments of the principal of, and interest and additional interest and premium, if any, on, a Global Security will be made to DTC or its nominees, as the registered owners thereof.

(c)            Transfers between participants in DTC will be effected in the ordinary way in accordance with DTC rules and will be settled in same-day funds.

(d)            Certificated definitive Series G Senior Notes may be in denominations of less than $100,000 to the extent any redemption has reduced such Holder’s aggregate holding of such Series G Senior Notes to less than $100,000.

(e)            If any redemption affecting the Series G Senior Notes would result in the amount to be paid to a Holder of such affected Senior Note in respect of such redemption not to equal $1,000 or an integral multiple thereof, the Issuer shall instruct the Trustee to round the amount to be paid to such Holder to the nearest $1,000 so that the amount to be paid to such Holder equals $1,000 or an integral multiple thereof.

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(f)             Beneficial interests in a Global Security (and any Series G Senior Notes issued in exchange therefor) will be subject to certain restrictions on transfer set forth therein and in the Original Indenture and as set forth on the form of such Global Security.  Exhibit B, as referred to in Section 2.4 of the Original Indenture, and Exhibits C, D, E and F, as referred to in Section 2.6 of the Original Indenture, are attached hereto as Exhibits B, C, D, E and F, respectively.

(g)            Except in the limited circumstances described under Section 2.2(h) below, beneficial interests in a Global Security will only be recorded by book-entry, and owners of beneficial interests in a Global Security will not be entitled to receive physical delivery of certificates representing Series G Senior Notes.

(h)            If (i) DTC or any successor depository notifies the Issuer that it is unwilling or unable to continue as a depository for a Global Security or ceases to be a “clearing agency” registered under the Exchange Act and a successor depository is not appointed by the Issuer within 90 days of such notice, (ii) an Event of Default under the Series G Senior Notes has occurred and is continuing and payment of principal and interest has been accelerated or (iii) the Issuer decides, at its option, to discontinue use of the book-entry system through DTC, then the Issuer shall issue certificates for the Series G Senior Notes in definitive registered form substantially in the form attached hereto in exchange for the Global Security outstanding.

(i)             The holder of a certificated definitive registered Series G Senior Note may transfer such Series G Senior Note in whole or in part by surrendering it at the Corporate Trust Office of the Trustee in accordance with the terms of the Indenture and such Series G Senior Note.  Upon the transfer, exchange or replacement of definitive Series G Senior Notes, the Issuer will deliver only definitive Series G Senior Notes that bear a restrictive legend unless there is delivered to the Issuer such satisfactory evidence, which may include an opinion of counsel, as may reasonably be required by the Issuer, that neither the legend nor the restrictions on transfer set forth therein are required to ensure compliance with the provisions of the Securities Act.

Section 2.3            Interest, Principal, Maturity Date and Regular Record Date .  The Series G Senior Notes shall bear interest on the unpaid principal amount thereof from time to time outstanding from the date of authentication thereof until such amount is paid in full at the rate of interest set forth in the form of such Series G Senior Note attached hereto.  The principal amount of the Series G Senior Notes shall be due and payable at maturity as set forth in the form of Series G Senior Note attached hereto.

Payment of principal, premium, if any, and interest on the Series G Senior Notes shall be made as provided in Sections 2.4, 2.10, 3.2 and 3.4 of the Original Indenture, except that the final payment of principal of the Series G Senior Notes shall be made on the due date therefor to the account of the Holder as such account shall appear in the Security Register, which amount shall be payable upon presentation and surrender of such Series G Senior Note at the office of the Issuer.

The Series G Senior Notes shall mature on the date and in the amounts set forth thereon.

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The record date applicable to the Series G Senior Notes issued hereunder shall be as set forth in the form of Series G Senior Note attached hereto.

All payments of principal, premium, if any, and interest with respect to certificated Series G Senior Notes will be made by bank check mailed on the interest payment date to the address of such Holder on the Security Register or, for Holders of at least U.S. $1,000,000 in aggregate principal amount of Series G Senior Notes, by wire transfer on the interest payment date of immediately available funds to a dollar account maintained by such Holder with a bank or other financial institution; provided that a written request from such Holder to such effect designating such account is received by the Trustee and the Issuer or the paying agent no later than the record date immediately preceding such Interest Payment Date.  Unless such designation is revoked, any such designation made by such person with respect to such certificated Series G Senior Notes will remain in effect with respect to any future payments with respect to such certificated Senior Note payable to such person.

Section 2.4            Optional Redemption .  The Series G Senior Notes issued hereunder are subject to optional redemption, in whole or in part, at any time at the option of the Issuer at a redemption price equal to 100% of the outstanding principal amount of the Series G Senior Notes being so redeemed plus accrued and unpaid interest thereon to the date fixed for redemption (the “Determination Date”) together with the Applicable Premium applicable thereto.

Section 2.5            Reopen Series .  The Issuer, from time to time, without the consent of the Holders of the Series G Senior Notes, may reopen the Series G Senior Notes and create and issue further senior debt securities under the Indenture having the same terms and conditions (including the same CUSIP number) as the Series G Senior Notes issued hereunder in all respects, except for the date of original issuance, the initial interest payment date and the offering price.  Such additional senior debt securities shall be consolidated with, and form a single series with, the previously outstanding Series G Senior Notes hereunder.

Section 2.6            Applicable Premium .  As used herein, “Applicable Premium” means an amount calculated as follows:

(i)             the average life of the remaining scheduled payments of principal in respect of Outstanding Series G Senior Notes (the “Remaining Average Life”) shall be calculated as of the Determination Date;

(ii)            the yield to maturity calculated as of a date not more than five days prior to the Determination Date for the United States Treasury security having an average life equal to the Remaining Average Life and trading in the secondary market at the price closest to the principal amount thereof (the “Primary Issue”); provided , however , that if no United States Treasury security has an average life equal to the Remaining Average Life, the yields (the “Other Yields”) for the two maturities of United States Treasury securities having average lives most closely corresponding to such Remaining Average Life and trading in the secondary market at the price closest to the principal amount thereof shall be calculated, and the yield to maturity for the Primary Issue shall be the yield interpolated or

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extrapolated from such Other Yields on a straight line basis, rounding in each of such relevant periods to the nearest month;

(iii)           the discounted present value of the then-remaining scheduled payments of principal and interest (but excluding that portion of any scheduled payment of interest that is actually due and paid on the Determination Date) in respect of the Outstanding Series G Senior Notes shall be calculated as of the Determination Date using a discount factor equal to the sum of (x) the yield to maturity for the Primary Issue, plus (y) 50 basis points; and

(iv)           the amount of Applicable Premium in respect of the Series G Senior Notes to be redeemed shall be an amount equal to (x) the discounted present value of such Series G Senior Notes to be redeemed determined in accordance with clause (iii) above, minus (y) the unpaid principal amount of such Series G Senior Notes; provided , however , that the Applicable Premium shall not be less than zero; and

(v)            such calculation shall be made by an Investment Banker.

Section 2.7            Amendments for Benefit of Series G Senior Notes .  The Indenture is hereby amended, pursuant to Section 7.1(d) of the Original Indenture for the benefit of the holders of the Series G Senior Notes and for so long as the Series G Senior Notes are outstanding, as follows:

(a)            Section 1.1 of the Original Indenture is amended by adding to the definitions the following: “‘ Existing Generating Assets ’, when used in respect of the $300,000,000 7.00% Senior Notes, Series G due 2018, means the coal-fired and oil-fired units and gas-fired units owned by the Issuer as of the date of issuance of such Notes.”

(b)            Notwithstanding Section 3.13 of the Original Indenture, the Issuer may not cease to comply with the covenants of Sections 3.11 and 3.12 of the Original Indenture unless, in addition to complying with such Section 3.13, the rating on the Series G Senior Notes from Standard & Poor’s Ratings Services (or any successor thereto) is at least BBB+ after giving effect to such cessation.

(c)            Section 3.9 of the Original Indenture is amended to delete the words “Initial Generation Assets” and insert in lieu thereof the words “Existing Generating Assets”.

(d)            Section 4.1(i) of the Original Indenture is amended to delete the words “Initial Generating Assets” and insert in lieu thereof the words “Existing Generating Assets”.

(e)            In Section 4.1(i) of the Original Indenture, for the avoidance of doubt, the reference to the “Genco-Marketing Co. PPA” shall be deemed to refer to the Amended and Restated Power Supply Agreement dated as of March 28, 2008 between the Issuer and Marketing Co.

(f)             References in Section 2.4 of the Original Indenture to “restricted period” shall be deemed to refer to “distribution compliance period” as defined in Regulation S.

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ARTICLE III

MISCELLANEOUS

Section 3.1            Execution of Supplemental Indenture .  This Fifth Supplemental Indenture is executed and shall be construed as an indenture supplemental to the Original Indenture and, as provided in the Original Indenture, this Fifth Supplemental Indenture forms a part thereof.

Section 3.2            Concerning the Trustee .  The Trustee shall not be responsible in any manner for or with respect to the validity or sufficiency of this Fifth Supplemental Indenture, or the due execution hereof by the Issuer, or for or with respect to the recitals and statements contained herein, all of which recitals and statements are made solely by the Issuer.

Section 3.3            Counterparts .  This Fifth Supplemental Indenture may be executed in any number of counterparts, each of which when so executed shall be deemed to be an original; but all such counterparts shall together constitute but one and the same instrument.

Section 3.4            GOVERNING LAW .  THIS FIFTH SUPPLEMENTAL INDENTURE AND THE SERIES G SENIOR NOTES ISSUED HEREUNDER SHALL, PURSUANT TO SECTION 5-1401 OF THE NEW YORK GENERAL OBLIGATIONS LAW, BE GOVERNED BY THE LAW OF THE STATE OF NEW YORK, WITHOUT REFERENCE TO THE CHOICE OF LAW PROVISIONS THEREOF (OTHER THAN SUCH SECTION 5-1401).

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IN WITNESS WHEREOF, the parties hereto have caused this Fifth Supplemental Indenture to be duly executed as of April 1, 2008.

[FIFTH SUPPLEMENTAL INDENTURE]

EXHIBIT A

FORM OF SECURITY

[INCLUDE IF SECURITY IS A GLOBAL SECURITY DEPOSITED WITH THE U.S. DEPOSITARY – UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY TO THE ISSUER OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND ANY CERTIFICATE ISSUED IN EXCHANGE FOR THIS CERTIFICATE OR ANY PORTION HEREOF IS REGISTERED IN THE NAME OF CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY OR SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY (AND ANY PAYMENT HEREON IS MADE TO CEDE & CO.), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON OTHER THAN THE DEPOSITORY TRUST COMPANY OR A NOMINEE THEREOF IS WRONGFUL INASMUCH AS THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

TRANSFERS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS IN WHOLE, BUT NOT IN PART, TO NOMINEES OF THE DEPOSITORY TRUST COMPANY OR TO A SUCCESSOR THEREOF OR SUCH SUCCESSOR’S NOMINEE AND TRANSFERS OF PORTIONS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS MADE IN ACCORDANCE WITH THE RESTRICTIONS SET FORTH IN SECTION 2.6 OF THE INDENTURE REFERRED TO ON THE REVERSE HEREOF.]

[INCLUDE IF SECURITY IS NOT ISSUED TO FOREIGN PURCHASERS UNDER REGULATION S – THIS NOTE (OR ITS PREDECESSOR) WAS ORIGINALLY ISSUED IN A TRANSACTION EXEMPT FROM REGISTRATION UNDER THE UNITED STATES SECURITIES ACT OF 1933, AS AMENDED (THE “SECURITIES ACT”), AND MAY NOT BE OFFERED, SOLD, PLEDGED OR OTHERWISE TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN APPLICABLE EXEMPTION THEREFROM AND IN ANY EVENT MAY BE SOLD OR OTHERWISE TRANSFERRED ONLY IN ACCORDANCE WITH THE INDENTURE, COPIES OF WHICH ARE AVAILABLE FOR INSPECTION AT THE CORPORATE TRUST OFFICE OF THE TRUSTEE IN NEW YORK.

EACH PURCHASER OF THIS NOTE 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.  EACH HOLDER OF THIS NOTE REPRESENTS TO AMEREN ENERGY GENERATING COMPANY THAT (a) SUCH HOLDER WILL NOT SELL, PLEDGE OR OTHERWISE TRANSFER THIS NOTE (WITHOUT THE CONSENT OF AMEREN ENERGY GENERATING COMPANY) PRIOR TO THE DATE WHICH IS ONE YEAR (OR SIX MONTHS IF ALL THE APPLICABLE CONDITIONS TO SUCH RESALE UNDER RULE 144 UNDER THE SECURITIES ACT (OR ANY SUCCESSOR PROVISION THEREOF) ARE SATISFIED) AFTER THE LATER OF THE ORIGINAL ISSUANCE DATE THEREOF, THE ISSUANCE DATE OF ANY S