SUPPLEMENTAL INDENTURE

Exhibit 4.3

W HEN R ECORDED M AIL TO :

Illinois Power Company

Craig W. Stensland

One Ameren Plaza (MC 1310)

1901 Chouteau Avenue

St. Louis, Missouri 63103

ILLINOIS POWER COMPANY

TO

THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A.

formerly

BNY Midwest Trust Company, As Successor Trustee To Harris Trust And Savings Bank

SUPPLEMENTAL INDENTURE

DATED AS OF JUNE 15, 2009

TO

GENERAL MORTGAGE INDENTURE AND DEED OF TRUST

DATED AS OF NOVEMBER 1, 1992

This instrument was prepared by Steven R. Sullivan, Senior Vice President, General Counsel and Secretary of Illinois Power Company c/o Ameren Corporation, One Ameren Plaza, 1901 Chouteau Avenue, St. Louis, Missouri 63103.

SUPPLEMENTAL INDENTURE dated as of June 15, 2009 (“Supplemental Indenture”), made by and between ILLINOIS POWER COMPANY, a corporation organized and existing under the laws of the State of Illinois (the “Company”), party of the first part, and THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A., a national banking corporation, formerly BNY Midwest Trust Company, a corporation organized and existing under the laws of the State of Illinois, as successor trustee to Harris Trust and Savings Bank, a corporation organized and existing under the laws of the State of Illinois (the “Trustee”), as Trustee under the General Mortgage Indenture and Deed of Trust dated as of November 1, 1992, hereinafter mentioned, party of the second part;

WHEREAS, the Company has heretofore executed and delivered its General Mortgage Indenture and Deed of Trust dated as of November 1, 1992 as from time to time amended and supplemented (the “Indenture”), to the Trustee, for the security of the Bonds of the Company issued and to be issued thereunder (the “Bonds”); and

WHEREAS, pursuant to the terms and provisions of the Indenture there were created and authorized by supplemental indentures thereto bearing the following dates, respectively, the Mortgage Bonds of the series issued thereunder and respectively identified opposite such dates:

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and

WHEREAS, a supplemental indenture with respect to the Bonds of the 2007 Series and the Bonds of the 2012 Series listed above was executed and filed but such Bonds of the 2007 Series and Bonds of the 2012 Series were never issued and a release with respect to such supplemental indenture was subsequently executed and filed; and

WHEREAS, the Company has entered into a 2009 Credit Agreement (as amended or otherwise modified from time to time, the “Credit Agreement”) by and among Ameren Corporation, the Company, Central Illinois Light Company and Central Illinois Public Service Company, 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 desires to create a new series of Bonds to be issued under the Indenture to be known as Mortgage Bonds, 2009 Credit Agreement Series (the “2009 Credit Agreement Series Bonds”); and

WHEREAS, the 2009 Credit Agreement Series Bonds shall be issued to the Agent as evidence of and security for the Company Obligations under the Credit Agreement; and

WHEREAS, the Company, in the exercise of the powers and authority conferred upon and reserved to it under the provisions of the Indenture, and pursuant to appropriate resolutions of the Board of Directors, has duly resolved and determined to make, execute and deliver to the Trustee this Supplemental Indenture in the form hereof for the purposes herein provided; and

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WHEREAS, all conditions and requirements necessary to make this Supplemental Indenture a valid, binding and legal instrument have been done, performed and fulfilled and the execution and delivery hereof have been in all respects duly authorized;

NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE

WITNESSETH:

THAT Illinois Power Company, in consideration of the purchase and ownership from time to time of the Bonds and the service by the Trustee, and its successors, under the Indenture and of One Dollar to it duly paid by the Trustee at or before the ensealing and delivery of these presents, the receipt whereof is hereby acknowledged, hereby covenants and agrees to and with the Trustee and its successors in the trust under the Indenture, for the benefit of those who shall hold the Bonds as follows:

ARTICLE I

DESCRIPTION OF 2009 CREDIT AGREEMENT SERIES BONDS.

SECTION 1. The Company hereby creates a new series of Bonds to be known as “2009 Credit Agreement Series Bonds.” The 2009 Credit Agreement Series Bonds shall be executed, authenticated and delivered in accordance with the provisions of, and shall in all respects be subject to, all of the terms, conditions and covenants of the Indenture, as amended and supplemented. The 2009 Credit Agreement Series Bonds shall be issued only to and in the name of the Agent under the Credit Agreement to evidence and secure any and all Company Obligations under the Credit Agreement.

The 2009 Credit Agreement Series Bonds shall be dated as of the Interest Payment Date (as defined below) thereof to which interest was paid next preceding the date of issue, unless (a) issued on an Interest Payment Date thereof to which interest was paid, in which event it shall be dated as of such issue date, or (b) issued prior to the occurrence of the first Interest Payment Date thereof to which interest was paid, in which event it shall be dated the date of original issuance.

The 2009 Credit Agreement Series Bonds shall be issued in the aggregate principal amount of $350,000,000 and shall mature on the Commitment Termination Date (having at any time the meaning such term has at such time under the Credit Agreement) applicable to the Company.

The 2009 Credit Agreement Series Bonds shall bear interest from their date as set forth in the form thereof hereinafter recited. Interest on the 2009 Credit Agreement Series Bonds shall be payable on each Interest Payment Date (defined below), commencing on the first Interest Payment Date next succeeding the date of the 2009 Credit Agreement Series Bonds. Payment of principal on the 2009 Credit Agreement Series Bonds shall be due on the

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Commitment Termination Date. If the Commitment Termination Date falls on a day which is not a Business Day (as defined below), principal and any interest and/or fees payable by the Company with respect to the Commitment Termination Date will be paid on the next succeeding Business Day.

Both the principal of and the interest on the 2009 Credit Agreement Series Bonds shall be payable at the times and in the manner set forth in the form of bond set out herein and in immediately available funds at the office or agency of the Trustee, in any coin or currency of the United States of America which at the time of payment is legal tender for public and private debts.

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 2009 Credit Agreement Series Bonds except on the Commitment Termination Date of the Company or upon redemption as provided in this Supplemental Indenture. 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 2009 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 2009 Credit Agreement Series Bonds in the amount of