Indenture

Indenture dated as of the 1 ^st day of March, 2007 (hereinafter sometimes referred to as this “Supplemental Indenture”), between Central Illinois Light Company, a corporation of the State of Illinois (hereinafter sometimes referred to as the “Company”), party of the first part, and Deutsche Bank Trust Company Americas, a corporation of the State of New York, as successor Trustee (hereinafter sometimes referred to as the “Trustee”), party of the second part, under the Indenture of Mortgage and Deed of Trust between Illinois Power Company and Bankers Trust Company (predecessor of Deutsche Bank Trust Company Americas), as Trustee, dated as of April 1, 1933, as amended and supplemented by Indenture between said Illinois Power Company and said Bankers Trust Company (predecessor of Deutsche Bank Trust Company Americas), dated as of June 30, 1933, and as amended, supplemented and assumed by Indenture between the Company and said Bankers Trust Company (predecessor of Deutsche Bank Trust Company Americas), dated as of July 1, 1933, and as amended and supplemented by various Indentures between the Company and said Bankers Trust Company (predecessor of Deutsche Bank Trust Company Americas) bearing subsequent dates (said Indenture of Mortgage and Deed of Trust as amended, supplemented and assumed being hereinafter sometimes referred to as the “Indenture”).

WHEREAS, the Indenture provides for the issuance of bonds thereunder in one or more series, the form of which series of bonds to be substantially in the form set forth therein with such insertions, omissions and variations as the Board of Directors of the Company may determine; and

WHEREAS, the Company has entered into a Credit Agreement, dated as of February 9, 2007 (as amended or otherwise modified from time to time, the “Credit Agreement”) by and among the Company, Central Illinois Public Service Company, Illinois Power Company, AmerenEnergy Resources Generating Company and CILCORP Inc., as borrowers, the lenders from time to time party thereto (the “Lenders”) and JPMorgan Chase Bank, N.A., as agent (in such capacity, the “Agent”) for the Lenders, providing for the making of certain financial accommodations thereunder to the Company, and pursuant to such Credit Agreement, the Company has agreed to issue to the Agent, as evidence of and security for the Obligations (as such term is defined in the Credit Agreement) of the Company (the “Company Obligations”), a new series of bonds under the Indenture; and

WHEREAS, for such purposes, the Company, by appropriate corporate action in conformity with the terms of the Indenture, has duly determined to create a series of bonds under the Indenture to be designated as “First Mortgage Bonds, 2007 Credit Agreement Series” (hereinafter sometimes referred to as the “bonds of the 2007 Credit Agreement Series”), the bonds of which series are to be issued as registered bonds without coupons and are to bear interest as specified in the form of bond of the 2007 Credit Agreement Series set forth below and are to mature, subject to prior acceleration and redemption, on the Maturity Date (as such term is defined in the Credit Agreement); and

WHEREAS, the bonds of 2007 Credit Agreement Series shall be issued to the Agent as evidence of and security for the Company Obligations under the Credit Agreement; and

WHEREAS, the definitive registered bonds without coupons of the 2007 Credit Agreement Series (certain of the provisions of which may be printed on the reverse side thereof)

and the Trustee’s certificate of authentication to be borne by such bonds are to be substantially in the following forms, respectively:

Notwithstanding any provisions hereof or in the Indenture this Bond is not assignable or transferable except to a successor Agent appointed in accordance with the Credit Agreement, dated as of February 9, 2007, hereinafter referred to.

Central Illinois Light Company, a corporation of the State of Illinois (hereinafter called the “Company”), for value received, hereby promises to pay to JPMorgan Chase Bank, N.A., as agent (in such capacity, the “Agent”) for the Lenders (as defined below) under the Credit Agreement, dated as of February 9, 2007, by and among the Company, Central Illinois Public Service Company, Illinois Power Company, AmerenEnergy Resources Generating Company and CILCORP Inc., as borrowers, the lenders from time to time party thereto (the “Lenders”) and JPMorgan Chase Bank, N.A., as agent (as amended or otherwise modified from time to time, the “Credit Agreement”), or registered assigns, the principal amount specified above or such lesser principal amount as shall be equal to the amount of the Borrower Credit Exposure (as defined in the Credit Agreement) of the Company outstanding on the Maturity Date (having at any time the meaning such term has at such time under the Credit Agreement) of the Company, but not in excess of the principal amount of this bond, and to pay interest thereon at the Interest Rate (as defined below) until the principal hereof is paid or duly made available for payment on the Maturity Date or in the event of redemption of this bond, until the redemption date.

Interest on this bond shall be payable on each Interest Payment Date (as defined below), commencing on the first Interest Payment Date next succeeding the date of this bond. If the Maturity Date falls on a day which is not a Business Day, as defined below, principal and any interest and/or fees payable with respect to the Maturity Date will be paid on the next succeeding Business Day. The interest payable, and punctually paid or duly provided for, on any Interest Payment Date will, subject to certain exceptions provided in the Supplemental Indenture dated as of March 1, 2007, hereinafter referred to, be paid to the person in whose name this bond (or one or more predecessor bonds) is registered at the close of business on the Record Date (as defined below); provided, however, that interest payable on the Maturity Date will be payable to the person to whom the principal hereof shall be payable. Should the Company default in the payment of interest (“Defaulted Interest”), the Defaulted Interest shall be paid to the person in whose name this bond is registered on the Record Date to be established by the Trustee for

payment of such Defaulted Interest. As used herein, (A) “Business Day” shall have the meaning assigned thereto in the Credit Agreement; (B) “Interest Payment Date” shall mean each date on which Company Obligations constituting interest and/or fees are due and payable from time to time pursuant to the Credit Agreement; (C) “Interest Rate” shall mean a rate of interest per annum, adjusted as necessary, to result in an interest payment equal to the aggregate amount of Company Obligations constituting interest and fees of the Company due under the Credit Agreement on the applicable Interest Payment Date; and (D) “Record Date” with respect to any Interest Payment Date shall mean the day (whether or not a Business Day) immediately next preceding such Interest Payment Date.

Both the principal of and the interest on this bond shall be payable, in immediately available funds, at the office of the Trustee hereinafter referred to.

This bond is to be issued and delivered to the Agent in order to evidence and secure the obligations of the Company under the Credit Agreement to make payments to the Lenders under the Credit Agreement and to provide the Lenders the benefit of the lien of the Indenture with respect to the 2007 Credit Agreement Series Bonds.

The obligation of the Company to make payments with respect to principal under the Credit Agreement shall not give rise to an obligation to pay principal of the 2007 Credit Agreement Series Bonds except on the Maturity Date of the Company or upon redemption hereof. If at any time any permanent reduction of the Borrower Sublimit (as defined in the Credit Agreement) of the Company or the Borrower Credit Exposure (as defined in the Credit Agreement) of the Company shall result in the principal of the 2007 Credit Agreement Series Bonds being greater than the greater of the Borrower Sublimit and the Borrower Credit Exposure, a payment obligation with respect to the principal of the 2007 Credit Agreement Series Bonds in the amount of such excess shall be deemed discharged upon the effectiveness of such permanent reduction. No payment of principal under the Credit Agreement shall reduce the principal amount of the   2007 Credit Agreement Series Bonds to an amount less than the greater of the Borrower Sublimit and the Borrower Credit Exposure.

The obligation of the Company to make payments with respect to the interest on this bond shall be fully or partially, as the case may be, satisfied and discharged to the extent that, at the time that any such payment shall be due, the then due interest and/or fees of the Company under the Credit Agreement shall have been fully or partially paid. Satisfaction of any obligation to the extent that payment is made with respect to the interest and/or fees of the Company under the Credit Agreement means that if any payment is made on the interest and/or fees of the Company under the Credit Agreement, a corresponding payment obligation with respect to the interest on this bond shall be deemed discharged in the same amount as such payment made on the interest and/or fees of the Company under the Credit Agreement.

The Trustee may at any time and all times conclusively assume that the obligation of the Company to make payments with respect to the principal of and interest on this bond, so far as such payments at the time have become due, has been fully satisfied and discharged pursuant to the foregoing paragraphs unless and until the Trustee shall have received a written notice from the Agent stating (i) that timely payment of principal of or interest on this bond has not been made, (ii) that the Company is in arrears as to the payments required to be made by it to the

Agent in connection with the Company Obligations pursuant to the Credit Agreement, and (iii) the amount of the arrearage.

This bond is one of an issue of bonds of the Company, issuable in series, and is one of a series known as its First Mortgage Bonds of the series designated in its title, all issued and to be issued under and equally secured (except as to any sinking fund established in accordance with the provisions of the Mortgage (defined below) for the bonds of any particular series) by an Indenture of Mortgage and Deed of Trust dated as of April 1, 1933, executed by Illinois Power Company to Bankers Trust Company (predecessor of Deutsche Bank Trust Company Americas) or its successor (hereinafter sometimes referred to as the “ Trustee ”) as Trustee, as amended by Indenture dated as of June 30, 1933, as assumed by the Company and as amended and supplemented by Indentures between the Company and the Trustee bearing subsequent dates, including the Indenture dated as of March 1, 2007 (all of which indentures are herein collectively called the “ Mortgage ”), to which reference is made for a description of the property mortgaged and pledged, the nature and extent of the security, the rights of the holders of the bonds in respect thereof and the terms and conditions upon which the bonds are secured.

As more fully described in the supplemental indenture establishing the terms and provisions of the bonds of this series, the rights and obligations of the Company and the rights of the bondholders may be modified with the consent of the holders of not less than 60% in principal amount of the bonds adversely affected; provided, however, that no modification shall (1) extend the time, or reduce the amount, of any payment on any bond, without the consent of the holder of each bond so affected, (2) permit the creation of any lien, not otherwise permitted, prior to or on a parity with the lien of the Mortgage, without the consent of the holders of all bonds then outstanding, or (3) reduce the above percentage of the principal amount of bonds the holders of which are required to approve any such modification without the consent of the holders of all bonds then outstanding.

The principal hereof may be declared or may become due on the conditions, with the effect, in the manner and at the time set forth in the Mortgage, upon the occurrence of a completed default as in the Mortgage provided.

This bond is not redeemable except upon written demand of the Agent following the occurrence of a Default by the Company under the Credit Agreement and the acceleration of the Company Obligations under the Credit Agreement.

In the manner and upon payment of the charges prescribed in the Mortgage, registered bonds without coupons of this series may be exchanged for a like aggregate principal amount of fully registered bonds of other authorized denominations of the same series, upon presentation and surrender thereof, for cancellation, to the Trustee at its principal office in the Borough of Manhattan, The City of New York, New York.

This bond shall not be assignable or transferable except to a successor Agent appointed in accordance with the Credit Agreement. Subject to the restriction on transfer of this bond hereinbefore set forth, this bond is transferable as prescribed in the Mortgage by the registered owner hereof in person, or by his duly authorized attorney, at the office or agency of the Company in the Borough of Manhattan, The City of New York, New York, upon surrender and

cancellation of this bond, and, thereupon, a new fully registered bond of the same series for a like principal amount will be issued to the transferee in exchange therefor as provided in the Mortgage, and upon payment, if the Company shall require it, of the charges therein prescribed; provided, that the Company shall not be required to exchange any bonds of this series for a period of ten (10) days next preceding an Interest Payment Date with respect to such bonds.

The Agent shall surrender this bond to the Trustee when each of the Borrower Sublimit and the Borrower Credit Exposure of the Company have been reduced to zero and all fees and other amounts payable by the Company pursuant to the Credit Agreement with respect to the Company Obligations shall have been duly paid.

No recourse shall be had for the payment of the principal of or interest on this bond against any incorporator or any past, present or future subscriber to the capital stock, stockholder, officer or director of the Company or of any predecessor or successor corporation, either directly or through the Company or any predecessor or successor corporation, under any rule of law, statute or constitution or by the enforcement of any assessment or otherwise, all such liability of incorporators, subscribers, stockholders, officers and directors being released by the holder or owner hereof by the acceptance of this bond and being likewise waived and released by the terms of the Mortgage.

This bond shall not become obligatory until Deutsche Bank Trust Company Americas, the Trustee under the Mortgage, or its successor thereunder, shall have signed the form of certificate endorsed hereon.

IN WITNESS WHEREOF, Central Illinois Light Company has caused this bond to be signed in its name by its President or a Vice President by a facsimile of his signature and a facsimile of its corporate seal to be printed hereon, attested by its Secretary or an Assistant Secretary by a facsimile of his signature.

This bond is one of the bonds of the series designated therein, described in the within-mentioned Mortgage.

WHEREAS, all things necessary to make the bonds of the 2007 Credit Agreement Series, when authenticated by the Trustee and issued as in the Indenture provided, the valid, binding and legal obligations of the Company, entitled in all respects to the security of the Indenture, have been done and performed, and the creation, execution and delivery of this Supplemental Indenture have in all respects been duly authorized;

WHEREAS, the Company and the Trustee deem it advisable to enter into this Supplemental Indenture for the purpose of describing the bonds of the 2007 Credit Agreement Series, and of providing the terms and conditions of redemption thereof;

WHEREAS, the Company has reserved the right, without the consent or other action by the holders of the bonds of each series that is outstanding as of the date hereof, to amend the Indenture to add a new Section 115A thereto as provided for herein; and

WHEREAS, the Company by appropriate corporate action in conformity with the terms of the Indenture has elected to exercise such right to so amend the Indenture;

NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE WITNESSETH: That Central Illinois Light Company, in c