SIXTH SUPPLEMENTAL INDENTURE

Exhibit 4.1

EXECUTION VERSION

PROLOGIS

as Issuer

and

U.S. BANK NATIONAL ASSOCIATION

as Trustee

SIXTH SUPPLEMENTAL INDENTURE

Dated as of May 7, 2008

2.625% Convertible Senior Notes due 2038

TABLE OF CONTENTS

SIXTH SUPPLEMENTAL INDENTURE

2.625% Convertible Senior Notes due 2038

THIS SIXTH SUPPLEMENTAL INDENTURE, dated as of May 7, 2008 (this “ Sixth Supplemental Indenture ”), by and between PROLOGIS (formerly ProLogis Trust and prior thereto Security Capital Industrial Trust), a real estate investment trust organized under the laws of the State of Maryland having its principal office at 4545 Airport Way, Denver, Colorado 80239 (the “ Company ”), and U.S. BANK NATIONAL ASSOCIATION (as successor in interest to State Street Bank and Trust Company), having a corporate trust office at Corporate Trust Services, 100 Wall Street, Suite 1600, New York, New York 10005, as successor Trustee (in such capacity, the “ Trustee ”) under the Base Indenture (defined below).

RECITALS OF THE COMPANY:

          The Company and the Trustee have heretofore entered into an Indenture dated as of March 1, 1995, as amended by a First Supplemental Indenture dated as of February 9, 2005, a Second Supplemental Indenture dated as of November 2, 2005, a Third Supplemental Indenture, dated as of November 2, 2005, a Fourth Supplemental Indenture dated as of March 26, 2007, and a Fifth Supplemental Indenture dated as of November 8, 2007 (as so supplemented hereinafter called the “ Base Indenture ”), between the Company and the Trustee, providing for the issuance by the Company from time to time of its senior debt securities evidencing its unsubordinated indebtedness (the “ Securities ”).

          Section 301 of the Base Indenture provides for various matters with respect to any series of Securities issued under the Base Indenture to be established in an indenture supplemental to the Base Indenture.

          Section 901(7) of the Base Indenture provides for the Company and the Trustee to enter into an indenture supplemental to the Base Indenture to establish the form or terms of Securities of any series as provided by Sections 201 and 301 of the Base Indenture without the consent of the Holders of any Securities.

          WHEREAS, for its lawful corporate purposes, the Company has duly authorized the issue of its 2.625% Convertible Senior Notes due 2038 (hereinafter referred to as the “ Notes ”), initially in an aggregate principal amount not to exceed $500,000,000 (or $550,000,000 if the Underwriters exercise their option to purchase additional Notes in full as set forth in the Underwriting Agreement), and in order to provide the terms and conditions upon which the Notes are to be authenticated, issued and delivered, the Board of Trustees of the Company has duly authorized the execution and delivery of this Sixth Supplemental Indenture; and

          WHEREAS, the Notes, the certificate of authentication to be borne by the Notes, a form of assignment, a form of the Fundamental Change Repurchase Notice, a form of option to elect repayment on a Put Right Repurchase Date, a form of conversion notice and certificate of transfer to be borne by the Notes are to be substantially in the forms hereinafter provided for; and

          All things necessary to make the Base Indenture, as hereby modified, a valid agreement of the Company, in accordance with its terms, have been done.

          NOW THEREFORE, THIS SIXTH SUPPLEMENTAL INDENTURE WITNESSETH:

          For and in consideration of the premises and of the covenants contained herein and in the Base Indenture, the Company and the Trustee covenant and agree, for the equal and proportionate benefit of all Holders of the Notes issued on or after the date of this Sixth Supplemental Indenture, as follows:

ARTICLE I

DEFINITIONS

          Section 1.01 Relation to Base Indenture . This Sixth Supplemental Indenture constitutes an integral part of the Base Indenture.

          Section 1.02 Definitions. For all purposes of this Sixth Supplemental Indenture, except as otherwise expressly provided for or unless the context otherwise requires:

               (a) Capitalized terms used but not defined herein shall have the respective meanings assigned to them in the Base Indenture;

               (b) Terms defined both herein and in the Base Indenture shall have the meanings assigned to them herein;

               (c) All references herein to Articles and Sections, unless otherwise specified, refer to the corresponding Articles and Sections of this Sixth Supplemental Indenture; and

               (d) All other terms used in this Sixth Supplemental Indenture, which are defined in the Trust Indenture Act or which are by reference therein defined in the Securities Act (except as herein otherwise expressly provided or unless the context otherwise requires) shall have the meanings assigned to such terms in said Trust Indenture Act and in said Securities Act as in force at the date of the execution of this Sixth Supplemental Indenture. The words “herein,” “hereof,” “hereunder,” and words of similar import refer to this Sixth Supplemental Indenture as a whole and not to any particular Article, Section or other subdivision. The terms defined in this Article include the plural as well as the singular.

          “ Additional Shares ” shall have the meaning specified in Section 8.01(g).

           “Business Day” means any day, other than a Saturday or Sunday, or legal holidays on which banks in The City of New York or The City of Boston are not required or authorized by law or executive order to be closed.

           “cash percentage” shall have the meaning specified in Section 8.02(a)(4).

           “cash percentage notice” shall have the meaning specified in Section 8.02(a)(4).

          “ close of business ” means 5:00 p.m. (New York City time).

          “ Change of Control ” shall be deemed to occur upon the consummation of any transaction or event (whether by means of an exchange offer, liquidation, tender offer, consolidation, merger, combination, reclassification, recapitalization or otherwise) in connection with which more than 50% of the Common Shares are exchanged for, converted into, acquired for or constitutes solely the right to receive, consideration which is not at least 90% common stock (or American Depositary Shares representing shares of common stock) that is: (a) listed on, or immediately after the consummation of such transaction or event, will be listed on, a United States national securities exchange or (b) approved, or immediately after such transaction or event will be approved, for listing or quotation on any United States system of automated dissemination of quotations of securities prices.

          “ Common Shares ” means, subject to Section 8.06, common shares of beneficial interest of the Company, par value $0.01 per share, at the date of this Sixth Supplemental Indenture or shares of any class or classes resulting from any reclassification or reclassifications thereof and that have no preference in respect of dividends or of amounts payable in the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company and that are not subject to redemption by the Company; provided that if at any time there shall be more than one such resulting class, the shares of each such class then so issuable shall be substantially in the proportion which the total number of shares of such class resulting from all such reclassifications bears to the total number of shares of all such classes resulting from all such reclassifications.

          “ Company ” means ProLogis, a Maryland real estate investment trust, and subject to the provisions of Article VII, shall include its successors and assigns.

          “ Company Put Right Notice ” shall have the meaning specified in Section 9.01(c).

          “ Company Put Right Notice Date ” shall have the meaning specified in Section 9.01(c).

          “ Conversion Agent ” shall mean the Trustee or any successor office or agency where the Notes may be surrendered for exchange.

          “ Conversion Date ” shall have the meaning specified in Section 8.02(c).

          “ Conversion Obligation ” shall have the meaning specified in Section 8.01(a).

          “ Conversion Price ” means as of any date $1,000 divided by the Conversion Rate as of such date.

          “ Conversion Rate ” shall have the meaning specified in Section 8.01(a).

          “ Conversion Trigger Price ” shall have the meaning specified in Section 8.01(c).

          “ Daily Conversion Value ” means, for each of the 20 consecutive Trading Days during the Observation Period, one-twentieth (1/20) of the product of (a) the applicable Conversion Rate and (b) the Daily VWAP of the Common Shares (or the Reference Property, if applicable) on such day.

          “ Daily Settlement Amount ,” for each of the 20 Trading Days during the Observation Period, shall consist of:

          (i) cash equal to the lesser of $50 and the Daily Conversion Value relating to such day; and

          (ii) if such Daily Conversion Value exceeds $50, a number of Common Shares equal to (A) the difference between such Daily Conversion Value and $50, divided by (B) the Daily VWAP of the Common Shares for such day, subject to the Company’s right to deliver cash in lieu of all or a portion of such Common Shares pursuant to Section 8.02(a)(4) hereof.

          “ Daily VWAP ” for the Common Shares means, for each of the 20 consecutive Trading Days during the Observation Period, the per share volume-weighted average price as displayed under the heading “Bloomberg VWAP” on Bloomberg page PLD <equity> AQR in respect of the period from 9:30 a.m. to 4:00 p.m. (New York City time) on such Trading Day (or if such volume-weighted average price is unavailable, the market value of one Common Share on such Trading Day as the Board of Trustees determines in good faith using a volume-weighted method).

          “ Debt Instrument ” means any bond, debenture, note, mortgage, indenture (including the Indenture) or other instrument.

          “ Depositary ” means, with respect to the Notes issuable or issued in whole or in part in global form, the person specified in the Base Indenture as the Depositary with respect to such Notes, until a successor shall have been appointed and become such pursuant to the applicable provisions of this Sixth Supplemental Indenture, and thereafter, “Depositary” shall mean or include such successor.

          “ Distributed Property ” shall have the meaning specified in Section 8.04(c).

          “ Dividend Threshold Amount ” shall have the meaning specified in Section 8.04(d).

          “ Effective Date ” shall have the meaning specified in Section 8.01(g)(1).

          “ Event of Default ” means, with respect to the Notes, any event specified in Section 5.01, continued for the period of time, if any, and after the giving of notice, if any, therein designated.

          “ Ex-Date ” means, with respect to any issuance or distribution on the Common Shares or any other equity security, the first date on which the shares of Common Shares or such

other equity security trade on the applicable exchange or in the applicable market, regular way, without the right to receive such issuance or distribution.

          “ Ex-Dividend Date ” means, with respect to Section 8.01(e), the first date upon which a sale of the Common Shares does not automatically transfer the right to receive the relevant dividend from the seller of the Common Shares to its buyer.

          “ Exchange Act ” means the Securities Exchange Act of 1934, as amended, and the rules and regulations promulgated thereunder.

          “ Fundamental Change ” means a Change of Control or a Termination of Trading.

          “ Fundamental Change Company Notice ” shall have the meaning specified in Section 9.02(b).

          “ Fundamental Change Repurchase Date ” shall have the meaning specified in Section 9.02(a).

          “ Fundamental Change Repurchase Notice ” shall have the meaning specified in Section 9.02(a)(i).

          “ Fundamental Change Repurchase Price ” shall have the meaning specified in Section 9.02(a).

          “ Global Note ” shall have the meaning specified in Section 2.06(e).

          “ Independent Securities Dealer ” shall have the meaning specified in Section 8.01(b).

          “ interest ” means, when used with reference to the Notes, any interest payable under the terms of the Notes.

          “ Interest Payment Date ” means May 15 and November 15 of each year, beginning on November 15, 2008.

          “ Last Reported Sale Price ” means, with respect to the Common Shares or any other security for which a Last Reported Sale Price must be determined, on any date, the closing sale price per share of the Common Shares or unit of such other security (or, if no closing sale price is reported, the average of the last bid and last ask prices or, if more than one in either case, the average of the average last bid and the average last ask prices) on such date as reported in composite transactions for the principal U.S. securities exchange on which the Common Shares or such other security are traded. If the Common Shares or such other security are not listed for trading on a United States national or regional securities exchange on the relevant date, the Last Reported Sale Price shall be the last quoted bid price per share of Common Shares or such other security in the over-the-counter market on the relevant date, as reported by the National Quotation Bureau or similar organization. If the Common Shares or such other security are not so quoted, the Last Reported Sale Price shall be the average of the mid-point of the last bid and

ask prices for the Common Shares or such other security on the relevant date from each of at least three nationally recognized independent investment banking firms selected from time to time by the Board of Trustees of the Company for that purpose. The Last Reported Sale Price shall be determined without reference to extended or after hours trading.

          “ Market Disruption Event ” means the occurrence or existence for more than a one-half hour period in the aggregate on any scheduled Trading Day for the Common Shares of any suspension or limitation imposed on trading (by reason of movements in price exceeding limits permitted by the stock exchange or otherwise) in the Common Shares or in any options, contracts or future contracts relating to the Common Shares, and such suspension or limitation occurs or exists at any time before 1:00 p.m. (New York City time) on such day.

          “ Maturity Date ” means May 15, 2038.

          “ Measurement Period ” shall have the meaning specified in Section 8.01(b).

          “ Merger Event ” shall have the meaning specified in Section 8.06.

          “ Noteholder ” or “ Holder ” or “ holder ,” as applied to any Note, or other similar terms (but excluding the term “beneficial holder”), means any person in whose name at the time a particular Note is registered on the Security Register.

          “ Notice of Conversion ” shall have the meaning specified in Section 8.02(c).

          “ Observation Period ” means the 20 consecutive Trading Day period beginning on and including the second Trading Day after the related Conversion Date in respect of such Note.

          “ Opening of Business ” means 9:00 a.m. (New York City time).

          “ Predecessor Note ” of any particular Note means every previous Note evidencing all or a portion of the same debt as that evidenced by such particular Note; and, for the purposes of this definition, any Note authenticated and delivered under Section 306 of the Base Indenture in lieu of a lost, destroyed or stolen Note shall be deemed to evidence the same debt as the lost, destroyed or stolen Note that it replaces.

          “ Put Right Repurchase Date ” shall have the meaning assigned to it in Section 9.01(b).

          “ Put Right Repurchase Notice ” shall have the meaning assigned to it in Section 9.01(b)(i).

          “ Put Right Repurchase Price ” shall have the meaning assigned to it in Section 9.01(b).

          “ Record Date ,” with respect to the payment of interest on any Interest Payment Date, shall have the meaning specified in Section 2.03.

          “ Reference Property ” shall have the meaning specified in Section 8.06(b).

          “ Securities Act ” means the Securities Act of 1933, as amended, and the rules and regulations promulgated thereunder.

          “ Settlement Amount ” shall have the meaning specified in Section 8.02(a)(1).

          “ Spin-Off ” shall have the meaning specified in Section 8.04(c).

          “ Stock Price ” means the price paid per Common Share in connection with a Fundamental Change pursuant to which Additional Shares shall be added to the Conversion Rate as set forth in Section 8.01(e)(ii) hereof, which shall be equal to (i) if holders of Common Shares receive only cash in such Fundamental Change, the cash amount paid per Common Share and (ii) in all other cases, the average of the Last Reported Sale Prices of the Common Shares over the five consecutive Trading Day period ending on the Trading Day preceding the Effective Date of the Fundamental Change.

          “ Termination of Trading ” shall be deemed to occur if Common Shares, or any Common Shares (or American Depositary Receipts in respect of Common Shares) into which the Notes are convertible pursuant to the terms of this Sixth Supplemental Indenture, are not listed for trading on any of the New York Stock Exchange, the NASDAQ Global Market or the NASDAQ Global Select Market (or any of their respective successors).

          “ Trading Day ” means a day during which (i) trading in Common Shares generally occurs, (ii) there is no Market Disruption Event and (iii) a Last Reported Sale Price for Common Shares (other than a Last Reported Sale Price referred to in the next to last sentence of such definition) is available for such day; provided that if the Common Shares are not admitted for trading or quotation on or by any exchange, bureau or other organization referred to in the definition of Last Reported Sale Price (excluding the next to last sentence of that definition), Trading Date shall mean any Business Day.

          “ Trading Price ” with respect to the Notes, on any date of determination, means the average of the secondary market bid quotations obtained by the Company and delivered to the Trustee for $2.0 million principal amount of Notes at approximately 3:30 p.m., New York City time, on such determination date from three independent nationally recognized securities dealers selected by the Company; provided that if three such bids cannot reasonably be obtained, but two such bids are obtained, then the average of the two bids shall be used, and if only one such bid can reasonably be obtained, that one bid shall be used. If the Company cannot reasonably obtain at least one bid for $2.0 million principal amount of Notes from a nationally recognized securities dealer, then the Trading Price per $1,000 principal amount of Notes will be deemed to be less than 98% of the product of the Last Reported Sale Price of the Common Shares and the Conversion Rate.

          “ Trigger Event ” shall have the meaning specified in Section 8.04(c).

          “ Underwriters ” means Goldman, Sachs & Co., Bank of America Securities LLC and Citigroup Global Markets Inc.

          “ Underwriting Agreement ” means that certain Underwriting Agreement relating to the Notes, dated May 1, 2008, between the Company and the Underwriters.

ARTICLE II

ISSUE, DESCRIPTION, EXECUTION, REGISTRATION AND EXCHANGE OF NOTES

          Section 2.01 Designation and Amount . The Notes shall be designated as the “2.625% Convertible Senior Notes due 2038.” The aggregate principal amount of Notes that may be authenticated and delivered under this Sixth Supplemental Indenture is initially limited to $500,000,000 (or $550,000,000 if the Underwriters exercise their option to purchase additional Notes in full as set forth in the Underwriting Agreement), subject to Section 2.07 and except for Notes authenticated and delivered upon registration or transfer of, or in exchange for, or in lieu of other Notes pursuant to Section 2.06, Section 8.02 and Section 9.02 hereof and Section 306 and Section 906 of to the Base Indenture.

          Section 2.02 Form of Notes . The Notes and the Trustee’s certificate of authentication to be borne by such Notes shall be substantially in the form set forth in Exhibit A.

          Any of the Notes may have such letters, numbers or other marks of identification and such notations, legends or endorsements as the officers executing the same may approve (execution thereof to be conclusive evidence of such approval) and as are not inconsistent with the provisions of this Sixth Supplemental Indenture, or as may be required by the Depositary, as may be required to comply with any law or with any rule or regulation made pursuant thereto or with any rule or regulation of any securities exchange or automated quotation system on which the Notes may be listed or designated for issuance, or to conform to usage or to indicate any special limitations or restrictions to which any particular Notes are subject.

          A Global Note shall represent such principal amount of the Outstanding Notes as shall be specified therein and shall provide that it shall represent the aggregate principal amount of Outstanding Notes from time to time endorsed thereon and that the aggregate principal amount of Outstanding Notes represented thereby may from time to time be increased or reduced to reflect repurchases, conversions, transfers or exchanges permitted hereby. Any endorsement of a Global Note to reflect the amount of any increase or decrease in the amount of Outstanding Notes represented thereby shall be made by the Trustee or the Custodian, at the direction of the Trustee, in such manner and upon instructions given by the Holder of such Notes in accordance with this Sixth Supplemental Indenture. Payment of principal and accrued and unpaid interest on a Global Note shall be made to the Holder of such Note on the date of payment, unless a Record Date or other means of determining Holders eligible to receive payment is provided for herein.

          The terms and provisions contained in the form of Note attached as Exhibit A hereto are incorporated herein and shall constitute, and are hereby expressly made, a part of this Sixth Supplemental Indenture and to the extent applicable, the Company and the Trustee, by their execution and delivery of this Sixth Supplemental Indenture, expressly agree to such terms and provisions and to be bound thereby.

          Section 2.03 Date and Denomination of Notes; Payments of Interest . The Notes shall be issuable in registered form without coupons in denominations of $1,000 principal amount and integral multiples thereof. Each Note shall be dated the date of its authentication

and shall bear interest from the date specified on the face of the form of Note attached as Exhibit A hereto. Interest on the Notes shall be computed on the basis of a 360-day year comprised of twelve 30-day months .

          The Person in whose name any Note (or its Predecessor Note) is registered on the Security Register at the close of business on any Record Date with respect to any Interest Payment Date shall be entitled to receive the interest payable on such Interest Payment Date. Interest shall be payable at the office of the Company maintained by the Company for such purposes in the Borough of Manhattan, City of New York or the City of Boston, which shall initially be an office or agency of the Trustee. The Company shall pay interest (i) on any Notes in certificated form by check mailed to the address of the Person entitled thereto as it appears in the Security Register (or upon written application by such Person to the Security Registrar not later than the relevant record date, by wire transfer in immediately available funds to such Person’s account within the United States, if such Person is entitled to interest on an aggregate principal in excess of $1,000,000) or (ii) on any Global Note by wire transfer of immediately available funds to the account of the Depositary or its nominee. The term “Record Date” with respect to any Interest Payment Date shall mean the May 1 or November 1 preceding the applicable May 15 or November 15 Interest Payment Date, respectively.

          Section 2.04 Intentionally Omitted .

          Section 2.05 Execution, Authentication and Delivery of Notes . Section 303 of the Base Indenture shall be applicable to the Notes.

          Section 2.06 Exchange and Registration of Transfer of Notes; Restrictions on Transfer; Depositary .

               (a) The Company shall provide for the registration of the Notes and of transfers of the Notes in the Security Register. Upon surrender for registration of transfer of any Note to the Security Registrar or any co-registrar, and satisfaction of the requirements for such transfer set forth in this Section 2.06, the Company shall execute, and the Trustee shall authenticate and deliver, in the name of the designated transferee or transferees, one or more new Notes of any authorized denominations and of a like aggregate principal amount.

          Notes may be exchanged for other Notes of any authorized denominations and of a like aggregate principal amount, upon surrender of the Notes to be exchanged at any such office or agency maintained by the Company pursuant to Section 4.02. Whenever any Notes are so surrendered for exchange, the Company shall execute, and the Trustee shall authenticate and deliver, the Notes which the Noteholder making the exchange is entitled to receive, bearing registration numbers not contemporaneously outstanding.

          All Notes presented or surrendered for registration of transfer or for exchange, repurchase or conversion shall (if so required by the Company, the Trustee, the Security Registrar or any co-registrar) be duly endorsed, or be accompanied by a written instrument or instruments of transfer in form satisfactory to the Company and duly executed, by the Noteholder thereof or his attorney-in-fact duly authorized in writing.

          No service charge shall be charged to the Noteholder for any exchange or registration of transfer of Notes, but the Company or the Trustee may require payment of a sum sufficient to cover any tax, assessments or other governmental charges that may be imposed in connection therewith.

          None of the Company, the Trustee, the Security Registrar or any co-registrar shall be required to exchange or register a transfer of (a) any Notes surrendered for conversion or, if a portion of any Note is surrendered for conversion, such portion thereof surrendered for conversion or (b) any Notes, or a portion of any Note, surrendered for repurchase (and not withdrawn) in accordance with Article IX hereof.

          All Notes issued upon any registration of transfer or exchange of Notes in accordance with this Sixth Supplemental Indenture shall be the valid, binding and legal obligations of the Company, evidencing the same debt, and entitled to the same benefits under this Sixth Supplemental Indenture as the Notes surrendered upon such registration of transfer or exchange.

               (b)  Intentionally Omitted .

               (c)  Intentionally Omitted .

               (d) The Trustee shall have no obligation or duty to monitor, determine or inquire as to compliance with any restrictions on transfer imposed under this instrument or under applicable law with respect to any transfer of any interest in any Note other than to require delivery of such certificates and other documentation or evidence as are expressly required by, and to do so if and when expressly required by, the terms of this instrument, and to examine the same to determine substantial compliance as to form with the express requirements hereof.

               (e) So long as the Notes are eligible for book-entry settlement with the Depositary, unless otherwise required by law, all Notes shall be represented by one or more Notes in global form (each, a “ Global Note ”) registered in the name of the Depositary or the nominee of the Depositary. The transfer and exchange of beneficial interests in a Global Note, which does not involve the issuance of a definitive Note, shall be effected through the Depositary (but not the Trustee or the Custodian) in accordance with this Sixth Supplemental Indenture (including the restrictions on transfer set forth herein) and the procedures of the Depositary therefor.

          Section 2.07 Additional Notes; Repurchases . The Company may, without the consent of the Noteholders and notwithstanding Section 2.01, issue additional Notes hereunder with the same terms and with the same CUSIP number as the Notes initially issued hereunder in an unlimited aggregate principal amount, which will form the same series with the Notes initially issued hereunder, provided that no such additional Notes may be issued unless fungible with the Notes initially issued hereunder for U.S. federal income tax purposes. The Company may also from time to time repurchase the Notes in open market purchases by tender at any price or by private agreement without prior notice to Noteholders.

          Section 2.08 No Sinking Fund . The provisions of Article Twelve of the Base Indenture shall not be applicable to the Notes. No sinking fund is provided for the Notes.

          Section 2.09 Ranking . The Notes constitute a senior general obligation of the Company, ranking equally with other existing and future senior and unsubordinated indebtedness of the Company and ranking senior in right of payment to any future indebtedness of the Company that is expressly made subordinate to the Notes by the terms of such indebtedness.

ARTICLE III

REDEMPTION

          Section 3.01 Right to Redeem .

               (a) Notwithstanding any provision of the Base Indenture, as modified by this Sixth Supplemental Indenture, to the contrary, the Company may redeem the Notes prior to May 15, 2038, in whole, in order to preserve the Company’s status as a real estate investment trust under the Code.

               (b) Except as provided in Section 3.01(a), the Company may not redeem the Notes prior to May 20, 2013. On or after May 20, 2013, the Company, at its option, may redeem the Notes from time to time in whole or in part.

               (c) Any redemption of Notes shall be at a Redemption Price equal to 100% of the principal amount of the Notes being redeemed, plus accrued and unpaid interest; provided, however , that the Company may deduct and withhold from such Redemption Price any amount required to be deducted and withheld under applicable law.

          Section 3.02 Selection of Notes to be Redeemed .

               (a) The provisions of Section 1103 of the Base Indenture shall govern the selection of Notes to be redeemed by the Trustee provided, however , that if less than all of the Notes are to be redeemed, the Trustee shall make the selection from the Notes of that series Outstanding and not previously called for redemption, by lot, or in its discretion, on a pro rata basis..

               (b) If any Note selected for partial redemption is converted in part before termination of the conversion right with respect to the portion of the Note so selected, the converted portion of such Note shall be deemed to be part of the portion selected for redemption. Notes which have been converted subsequent to the Trustee commencing selection of Notes to be redeemed but prior to redemption of such Notes shall be treated by the Trustee as Outstanding for the purpose of such selection.

          Section 3.03 Notice of Redemption . The provisions of Section 1104 of the Base Indenture shall govern notices of redemption of the Notes; provided, however, that in addition to the information specified in Section 1104 of the Base Indenture, notices of redemption of the Notes shall also state:

               (a) the then-current Conversion Price;

               (b) the name and address of the Conversion Agent;

               (c) that Holders who wish to convert Notes must surrender such Notes for conversion no later than the close of business on the second Business Day immediately preceding the Redemption Date and must satisfy the other requirements set forth herein; and

               (d) whether the Company will satisfy its Conversion Obligation with respect to any Notes called for redemption that are surrendered for conversion in cash, Common Shares or both as provided herein; provided that the Company may not provide notice of a redemption of Notes at the Company’s option that specifies that the Company will settle conversions of Notes prior to such redemption in cash and Common Shares unless, at the time of such notice, the Company has available to it a sufficient number of authorized Common Shares to satisfy its Conversion Obligation in respect of the Notes to be redeemed.

ARTICLE IV

PARTICULAR COVENANTS OF THE COMPANY

          Section 4.01 Payment of Principal and Interest .

               (a) Sections 307 and 1001 of the Base Indenture shall apply to the Notes; provided, however, that, with respect to any Noteholder with an aggregate principal amount in excess of $1,000,000, at the application of such Holder in writing to the Security Registrar not later than the relevant record date, accrued and unpaid interest on such Holder’s Notes shall be paid by wire transfer in immediately available funds to such Holder’s account in the United States supplied by such Holder from time to time to the Trustee and Paying Agent (if different from Trustee); provided further that payment of accrued and unpaid interest made to the Depositary shall be paid by wire transfer in immediately available funds in accordance with such wire transfer instructions and other procedures provided by the Depositary from time to time.

               (b) Except as otherwise provided in this Section 4.01(b), a Holder of any Notes at the close of business on a Record Date shall be entitled to receive interest on such Notes on the corresponding Interest Payment Date. A Holder of any Notes as of a Record Date that are converted after the close of business on such Record Date and prior to the opening of business on the corresponding Interest Payment Date shall be entitled to receive interest on the principal amount of such Notes, notwithstanding the conversion of such Notes prior to such Interest Payment Date. However, a Holder that surrenders any Notes for conversion between the close of business on a Record Date and the opening of business on the corresponding Interest Payment Date shall be required to pay the Company an amount equal to the interest payable by the Company with respect to such Notes on such Interest Payment Date at the time such Holder surrenders such Notes for conversion, provided, however, that this sentence shall not apply to a Holder that converts Notes:

               (i) in respect of which the Company has given notice of redemption pursuant to Section 3.03 on a Redemption Date that is after the relevant Record Date and on or prior to the relevant Interest Payment Date; or

               (ii) to the extent of any overdue interest, if any overdue interest exists at the time of conversion with respect to such Notes;

               (iii) in connection with a Fundamental Change in which the Company has specified a Fundamental Change Repurchase Date that is after a Record Date and on or prior to the next Interest Payment Date; or

               (iv) after 5:00 p.m., New York City time on the Record Date immediately preceding the Maturity Date.

Accordingly, a Holder that converts Notes under any of the circumstances described in clauses (i), (ii), (iii) or (iv) above will not be required to pay to the Company an amount equal to the interest payable by the Company with respect to such Notes on the relevant Interest Payment Date.

          Section 4.02 Maintenance of Office or Agency for Conversion Agent. If at any time the Conversion Agent is not the Trustee or an office or agency designated or appointed by the Trustee, the Company will give prompt written notice to the Trustee of the location of such Conversion Agent. If at any time the Company shall fail to maintain an office or agency for the Conversion Agent, presentations, surrenders, notices and demands related to conversions of Notes may be made or served at the Corporate Trust Office or the office or agency of the Trustee in the Borough of Manhattan, the City of New York or in the City of Boston.

          Section 4.03 Intentionally Omitted.

          Section 4.04 Intentionally Omitted.

          Section 4.05 Exclusion of Certain Provisions From Base Indenture . Section 1004, Section 1006, Section 1007 and Section 1011 of the Base Indenture shall not apply to the Notes. Section 1002, Section 1003, Section 1005, Section 1008, Section 1009 (as amended by Section 2.2 of the Second Supplemental Indenture to the Base Indenture), Section 1010 and Section 1012 of the Base Indenture shall be applicable to the Notes.

ARTICLE V

DEFAULTS AND REMEDIES

          Section 5.01 Events of Default. The provisions of Section 501(2) and Section 501(3) of the Base Indenture shall not be applicable to the Notes. As contemplated under Section 301 and Section 501(9) of the Base Indenture, the following events, in addition to the events described in clauses (1), (4), (5) (as amended by the Second Supplemental Indenture to the Base Indenture), (6) (as amended by the Second Supplemental Indenture to the Base Indenture), (7) and (8) of the Base Indenture, shall be Events of Default with respect to the Notes:

               (a) default in the payment of principal or premium, if any, of any Note when due and payable on the Maturity Date, upon redemption, repurchase, declaration or otherwise;

               (b) failure by the Company to comply with its obligation to convert the Notes into cash, Common Shares or a combination of cash and Common Shares, as applicable, upon exercise of a Holder’s conversion right, and such failure continues for a period of 10 days; or

               (c) failure by the Company to issue a Fundamental Change Company Notice in accordance with Section 9.02 when due, and such failure continues for a period of two days.

          Section 5.02 Article Five of Base Indenture . Except as amended by Section 5.01 hereof, all of the provisions of Article Five of the Base Indenture shall be applicable to the Notes.

ARTICLE VI

SUPPLEMENTAL INDENTURES

          Section 6.01 Supplemental Indentures Without Consent of Noteholders. The provisions of Section 901 of the Base Indenture shall be applicable to the Notes.

          Section 6.02 Modification and Amendment with Consent of Noteholders. Section 902 of the Base Indenture shall be applicable to the Notes. As contemplated by Sections 301 and 902 of the Base Indenture, no supplemental indenture shall, without the consent of the Holder of each Outstanding Note affected thereby:

               (a) make any change that adversely affects the conversion rights of any Notes;

               (b) reduce the Fundamental Change Repurchase Price, Redemption Price or Put Right Repurchase Price of any Note, or amend or modify in any manner adverse to Noteholders the Company’s obligation to make such payments or Article III or Article IX of this Sixth Supplemental Indenture, whether through an amendment or waiver of provisions in the covenants, definitions or otherwise.

          Section 6.03 Effect of Supplemental Indentures . Upon the execution of any supplemental indenture under this Article, the Base Indenture and this Sixth Supplemental Indenture shall be modified in accordance therewith, and such supplemental indenture shall form a part of this Sixth Supplemental Indenture for all purposes; and every Holder of Notes theretofore or thereafter authenticated and delivered hereunder and of any coupon appertaining thereto shall be bound thereby.

          Section 6.04 Article Nine of Base Indenture . Except as amended by this Article VI, all of the provisions of Article Nine of the Base Indenture shall be applicable to the Notes.

ARTICLE VII

CONSOLIDATION, MERGER, SALE, CONVEYANCE AND LEASE

          Section 7.01 Company May Consolidate, Etc. on Certain Terms . Article Eight of the Base Indenture shall be applicable to the Notes.

ARTICLE VIII

CONVERSION OF NOTES

          Section 8.01 Conversion Privilege .

               (a) Subject to the conditions described in clauses (b) through (f) below and to Section 8.11 hereof, and upon compliance with the provisions of this Article VIII, a Holder of Notes shall have the right, at such Holder’s option, to convert all or any portion (if the portion to be converted is $1,000 principal amount or an integral multiple thereof) of such Note at any time prior to the close of business on the scheduled Trading Day immediately preceding February 15, 2013 at a rate (the “ Conversion Rate ”) of 13.1203 Common Shares (subject to adjustment by the Company as provided in Section 8.04) per $1,000 principal amount of Notes (the “ Conversion Obligation ”) under the circumstances and during the periods set forth below. On and after February 15, 2013, regardless of the conditions described in clause (b) through (f) below, upon compliance with the provisions of this Article VIII and subject to Section 8.11 hereof, a Noteholder shall have the right, at such holder’s option, to convert all or any portion (if the portion to be converted is $1,000 principal amount or an integral multiple thereof) of such Note at any time prior to the close of business on the scheduled Trading Day immediately preceding the Maturity Date at a Conversion Rate of 13.1203 Common Shares (subject to adjustment by the Company as provided in Section 8.04) per $1,000 principal amount of Notes.

               (b) (1) A Holder of Notes shall have the right, at such Holder’s option, to convert its Notes prior to February 15, 2013, during the five Business Day period immediately after any ten consecutive Trading Day period (the “ Measurement Period ”) in which the Trading Price per $1,000 principal amount of Notes for each day of such Measurement Period was less than 98% of the product of the Last Reported Sale Price of the Common Shares on such date and the Conversion Rate on such date, all as determined by the Trustee in the manner described in the immediately succeeding paragraph. The Trustee shall have no obligation to determine the Trading Price of the Notes unless requested by the Company to do so in writing, and the Company shall have no obligation to make such request unless a Noteholder or Noteholders of at least $1,000,000 aggregate principal amount of Notes provides the Company with reasonable evidence that the Trading Price per $1,000 principal amount of the Notes would be less than 98% of the product of the Last Reported Sale Price at such time and the then-

applicable Conversion Rate, at which time the Company shall select three independent nationally recognized securities dealers (each, an “ Independent Securities Dealer ”), request that the Independent Securities Dealers provide a secondary market quotation for the Notes and provide such determination to the Company and the Trustee in writing, and the Company shall instruct the Independent Securities Dealers to provide a secondary market quotation for the Notes beginning on the next Trading Day and on each successive Trading Day until the Trading Price per $1,000 principal amount of the Notes is greater than or equal to 98% of the product of the Last Reported Sale Price on such date and the then-applicable Conversion Rate. If the Trading Price condition set forth above has been met, the Company shall so notify the Noteholders. If at any time after the Trading Price condition set forth above has been met, the Trading Price per $1,000 principal amount of Notes is greater than 98% of the product of the Last Reported Sale Price on such date and the then-applicable Conversion Rate, the Company shall so notify the Noteholders.

               (2) Any request by the Company to the Trustee for a determination of the Trading Price and whether the Trading Price condition set forth in the first sentence of the immediately preceding paragraph has been met shall be accompanied by an Officers’ Certificate setting forth, for each day of determination (as identified in such Certificate), the name of the Independent Securities Dealers providing the secondary market bid quotations, a statement certifying that that such dealers are “Independent Securities Dealers” as required in this Section 8.01, the secondary market bid quotations obtained from such Independent Securities Dealers (a copy of which will be attached to such Officers’ Certificate), the Company’s calculation of the Trading Price for such date. The Trustee shall be entitled to conclusively rely, without independent verification, on the quotations provided by the Company in making its determinations hereunder. On the basis of such quotations, the Trustee shall determine the Trading Price of the Notes, and provide such determination to the Company. Absent manifest error, the Trustee’s determination of the Trading Price will be binding on the Company. Unless and until a Responsible Officer of the Trustee shall have received a request from the Company for determination of the Trading Price for the Notes and the Officers’ Certificate contemplated herein, the Trustee shall have no obligation to make any determination of the Trading Price of the Notes or whether the Trading Price condition has been met.

               (c) A Holder of Notes shall have the right, at such Holder’s option, to convert Notes during any calendar quarter after the quarter ended June 30, 2008, and only during such calendar quarter, if the Last Reported Sale Price for the Common Shares for at least 20 Trading Days during the period of 30 consecutive Trading Days ending on the last Trading Day of the previous calendar quarter exceeds 130% of the Conversion Price (the “Conversion Trigger Price”) on such last Trading Day, which Conversion Price shall be subject to adjustment in accordance with this Article VIII. The Conversion Agent shall, on the Company’s behalf, determine at the beginning of each calendar quarter whether the Notes are convertible as a result of the price of Common Shares as contemplated in the previous sentence and notify the Company and the Trustee.

               (d) In the event that the Company has delivered a notice of redemption in accordance with Section 1104 of the Base Indenture and Section 3.03 of this Sixth Supplemental Indenture to the Holders of Notes, a Holder of Notes may convert Notes at any time prior to the close of business on the second Business Day immediately preceding the

corresponding Redemption Date; provided, however, that a Holder who has already delivered a Fundamental Change Repurchase Notice with respect to a Note may not convert such Note until the Holder has withdrawn the Fundamental Change Repurchase Notice in accordance with the terms of the Note and this Sixth Supplemental Indenture.

               (e) (i) In the event that the Company elects to:

               (A) distribute to all or substantially all holders of Common Shares rights entitling them to purchase, for a period expiring within 60 days, Common Shares at a price less than the Last Reported Sale Price of the Common Shares for the Trading Day immediately preceding the declaration date of such distribution; or

               (B) distribute to all or substantially all holders of Common Shares, assets or debt securities of the Company or rights to purchase the Company’s securities, which distribution has a per share value (as determined by the Board of Trustees) exceeding 15% of the Last Reported Sale Price of the Common Shares on the day immediately preceding the date of declaration of such distribution,

then, in either case, Holders may surrender the Notes for conversion at any time on and after the date that the Company provides notice to Holders referred to in the next sentence until the earlier of the close of business on the Business Day immediately preceding the Ex-Dividend Date for such distribution or the date the Company announces that such distribution will not take place. The Company shall notify Holders of any distribution referred to in either clause (A) or clause (B) above and of the resulting conversion right no later than the 35th Business Day prior to the Ex-Dividend Date for such distribution.

               (ii) If the Company is a party to any transaction or event that constitutes a Fundamental Change, a Holder may surrender Notes for conversion at any time from and after the 30th scheduled Trading Day prior to the anticipated Effective Date of such transaction or event until the related Fundamental Change Repurchase Date and, upon such surrender, the Holder shall be entitled to the increase in the Conversion Rate, if any, specified in Section 8.01(g). The Company shall give notice to all record Noteholders and the Trustee and issue a press release of the Fundamental Change no later than 30 scheduled Trading Days prior to the anticipated effective date of the Fundamental Change.

               (iii) If the Company is a party to a consolidation, merger, binding share exchange or sale or conveyance of all or substantially all of its properties and assets, in each case pursuant to which the Common Shares would be converted into cash, securities and/or other property, then the Holders shall have the right to convert Notes at any time beginning fifteen calendar days prior to the date announced by the Company as the anticipated effective date of the transaction and until and including the date that is fifteen calendar days after the date that is the effective date of such transaction; provided such transaction does not otherwise constitute a Fundamental Change to which the provisions of Section 8.01(e)(ii) shall apply. The Company shall give notice to all record Noteholders and the Trustee and issue a press release at least 20

calendar days prior to the anticipated effective date of such transaction. If the Board of Trustees determines the anticipated effective date of the transaction, such determination shall be conclusive and binding on the Holders.

               (f) The Notes shall be convertible at any time beginning on the first Business Day after any 30 consecutive Trading Day period during which Common Shares are not listed on either a U.S. national securities exchange.

               (g) (1) If a Noteholder elects to convert Notes in connection with a Fundamental Change that occurs prior to May 20, 2013, the Conversion Rate applicable to each $1,000 principal amount of Notes so converted shall be increased by an additional number of Common Shares (the “ Additional Shares ”) as described below. Settlement of Notes tendered for conversion to which Additional Shares shall be added to the Conversion Rate as provided in this subsection shall be settled pursuant to Section 8.02 below, as applicable. For purposes of this Section 8.01(g), a conversion shall be deemed to be “in connection with” a Fundamental Change to the extent that the related conversion notice is delivered during the time period beginning on the 30th Trading Day prior to the anticipated Effective Date of such Fundamental Change and ending on the related Fundamental Change Repurchase Date, inclusive (regardless of whether the provisions of clauses (b), (c), (d), (e) or (f) of this Section 8.01 shall apply to such conversion). Such conversion notice shall indicate that the Holder of Notes has elected to convert Notes in connection with a Fundamental Change; provided, however , that the failure to so indicate shall not in any way affect the Conversion Obligation or the right of such Holder to receive Additional Shares in connection with such conversion.

               (i) The number of Additional Shares by which the Conversion Rate will be increased shall be determined by reference to the table attached as Schedule A hereto, based on the date on which the Fundamental Change occurs or becomes effective (the “ Effective Date ”), and the Stock Price; provided , that if the Stock Price is between two Stock Price amounts in the table or the Effective Date is between two Effective Dates in the table, the number of Additional Shares shall be determined by a straight-line interpolation between the number of Additional Shares set forth for the next higher and next lower Stock Price amounts and the two nearest Effective Dates, as applicable, based on a 365-day year; provided further that if (1) the Stock Price is greater than $120.00 per Common Share (subject to adjustment in the same manner as set forth in Section 8.04), no Additional Shares will be added to the Conversion Rate, and (2) the Stock Price is less than $62.86 per Common Share (subject to adjustment in the same manner as set forth in Section 8.04), no Additional Shares will be added to the Conversion Rate. Notwithstanding the foregoing, in no event will the total number of Common Shares issuable upon conversion exceed 15.9083 per $1,000 principal amount of Notes (subject to adjustment in the same manner as set forth in clauses (a), (b) and (c) of Section 8.04).

               (ii) The Stock Prices set forth in the first row of the table in Schedule A hereto shall be adjusted as of any date on which the Conversion Rate of the Notes is adjusted. The adjusted Stock Prices shall equal the Stock Prices applicable immediately prior to such adjustment, multiplied by a fraction, the numerator of which is the Conversion Rate in effect immediately prior to the adjustment giving rise to the Stock

Price adjustment and the denominator of which is the Conversion Rate as so adjusted. The number of Additional Shares within the table shall be adjusted in the same manner as the Conversion Rate as set forth in Section 8.04 (other than by operation of an adjustment to the Conversion Rate by adding Additional Shares).

          Section 8.02 Conversion Procedures.

               (a) (1) The Company shall settle its Conversion Obligations as described in Section 8.02(a)(3), unless, within the applicable time period specified in this Section 8.02(a)(1), the Company elects to settle its conversion Obligations as described in Section 8.02(a)(2) or Section 8.02(a)(4). The cash and/or Common Shares which the Company is required to deliver in accordance with this Section 8.02 in settlement of its Conversion Obligations is referred to herein as the “ Settlement Amount .” If the Company desires to settle its Conversion Obligations as described in Section 8.02(a)(2) or Section 8.02(a)(4), the Company shall notify each converting Noteholder by notice to the Trustee (for further distribution to Noteholders) of the method the Company will choose to satisfy its Conversion Obligations no later than the second Trading Day immediately following the Company’s receipt of a Notice of Conversion from such Holder, and such notice shall specify the section of this Sixth Supplemental Indenture pursuant to which the Company is electing to satisfy its conversion obligations; provided, however, that the Company shall have the right to irrevocably elect, in its sole discretion and without the consent of Noteholders, by notice to the Trustee (for further distribution to Noteholders), on or prior to February 15, 2013, to settle all of its future Conversion Obligations entirely in Common Shares as described in Section 8.02(a)(2), and provided further, that the Company is required to settle all conversions with a Conversion Date occurring on or after February 15, 2013 in the same manner, and the Company shall notify Noteholders by notice to the Trustee (for further distribution to Noteholders) of the manner of settlement (including specifying the applicable section of this Sixth Supplemental Indenture that describes such manner of settlement) on or before such date. The Company shall treat all Noteholders converting on the same Trading Day in the same manner; however, the Company shall not have any obligation to settle its Conversion Obligations arising on different Trading Days in the same manner, except for conversions with a Conversion Date occurring on or after February 15, 2013, which shall all be satisfied in the same manner.

               (2) If the Company has elected, within the applicable time periods specified in Section 8.02(a)(1), to settle its Conversion Obligations as described in this Section 8.02(a)(2), the Company shall have the right to settle its conversion obligations entirely in Common Shares. If the Company elects to satisfy its Conversion Obligation entirely in Common Shares, the Company shall deliver a number of Common Shares equal to (i) the aggregate principal amount of Notes to be converted divided by $1,000, multiplied by (ii) the applicable Conversion Rate (which shall include any increases to reflect any Additional Shares that such Holder is entitled to receive pursuant to Section 8.01(g) above). The Company shall deliver such Common Shares as soon as practicable after it has notified the converting Holder pursuant to Section 8.02(a)(1) above, that it has elected to satisfy its Conversion Obligation entirely in Common Shares.

               (3) If the Company does not elect, within the applicable time periods specified in Section 8.02(a)(1), to settle its conversion obligations as described in

Section 8.02(a)(2) or 8.02(a)(4), the Company shall settle its Conversion Obligations as described in this Section 8.02(a)(3), subject to Section 8.02(b) hereof. The Company shall deliver in respect of each $1,000 principal amount of Notes being converted a Settlement Amount equal to the sum of the Daily Settlement Amounts for each of the 20 consecutive Trading Days during the Observation Period, on the third Trading Day immediately following the last day of the related Observation Period; provided that the Company will deliver cash in lieu of fractional Common Shares as set forth pursuant to clause (k) below. The Daily Settlement Amounts shall be determined by the Company promptly following the last day of the Observation Period.

               (4) If the Company has elected, within the applicable time periods specified in Section 8.02(a)(1), to settle its Conversion Obligations as described in this Section 8.02(a)(4), the Company shall have the right to settle all or a portion of the amount by which the Daily Conversion Value exceeds $50 in cash in accordance with this Section 8.02(a)(4). In such case, the Company shall specify a percentage of the amount by which the Daily Conversion Value exceeds $50 that will be settled in cash, or the “ cash percentage .” The Company will inform converting Holders by notice to the Trustee (for further distribution to Noteholders) no later than two Trading Days prior to the first day of the applicable Observation Period if it elects to pay cash upon conversion of the Notes and shall specify in such notice (the “ cash percentage notice ”) the applicable cash percentage. If the Company elects to specify a cash percentage, the amount of cash that the Company shall deliver in respect of each Trading Day in the applicable Observation Period shall equal the product of (w) the cash percentage and (x) the amount by which the Daily Conversion Value exceeds $50 for such Trading Day. The number of Common Shares deliverable in respect of each Trading Day in the applicable Observation Period shall equal (i) the product of (y) 100% minus the cash percentage and (z) the amount by which the Daily Conversion Value exceeds $50 for such Trading Day, divided by (ii) the Daily VWAP of the Common Shares for such Trading Day. If the Company does not specify a cash percentage, it must settle the entire amount by which the Daily Conversion Value exceeds $50 with Common Shares pursuant to Section 8.02(a)(3) above; provided, however, that the Company will deliver cash in lieu of fractional Common Shares as set forth pursuant to clause (k) below. If the Company specifies a cash percentage, the Company shall satisfy its Conversion Obligation by delivering, on the third Trading Day immediately following the la