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                                                                     EXHIBIT 4.3

                                 FIRST AMENDMENT

         This First Amendment dated as of April 30, 1997 (the "First
Amendment"), between ENERGY BIOSYSTEMS (the "Company") and KEYCORP SHAREHOLDER
SERVICES, INC., successor in interest to Society National Bank (the "Agent").

                                   WITNESSETH

         WHEREAS, the Company and the Agent entered into that Stockholders
Rights Agreement dated as of March 8, 1995 (the "Agreement"); and

         WHEREAS, the Company and the Agent desire to amend the Agreement to
reflect a change in the requirements regarding the appointment of a successor
Agent.

         NOW, THEREFORE, in consideration of the mutual promises and other good
and valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the parties agree as follows:

         Section 1.01 DEFINED TERMS. Capitalized terms used in this First
Amendment and not defined shall have the meanings assigned to them in the
Agreement.

         Section 1.02 AMENDMENT OF SECTION 21. Section 21 of the Agreement shall
be amended as follows:

                  Section 21. CHANGE OF RIGHTS AGENT. The Rights Agent or any
         successor Rights Agent may resign and be discharged from its duties
         under this Agreement upon thirty (30) days' notice in writing mailed to
         the Company and to each transfer agent of the Preferred Shares and
         Common Shares by registered or certified mail, and to the holders of
         the Rights Certificates by first-class mail. The Company may remove the
         Rights Agent or any successor Rights Agent upon thirty (30) days'
         notice in writing, mailed to the Rights Agent or successor Rights
         Agent, as the case may be, and to each transfer agent of the Preferred
         Shares or Common Shares by registered or certified mail, and to the
         holders of the Rights Certificates by first-class mail. If the Rights
         Agent shall resign or be removed or shall otherwise become incapable of
         acting, the resigning, removed, or incapacitated Rights Agent shall
         remit to the Company, or to any successor Rights Agent designated by
         the Company, all books, records, funds, certificates or other documents
         or instruments of any kind then in its possession which were acquired
         by such resigning, removed or incapacitated Rights Agent in connection
         with its services as Rights Agent hereunder, and shall thereafter be
         discharged from all duties and obligations hereunder. Following notice
         of such removal, resignation or incapacity, the Company shall appoint a
         successor to such Rights Agent. If the Company shall fail to make such
         appointment within a period of thirty (30) days after giving notice of
         such removal or after it has been notified in writing of such
         resignation or incapacity by the resigning or incapacitated Rights
         Agent or by the holder of a Rights Certificate (who shall, with such
         notice, submit his Rights Certificate for inspection by the Company),
         then any registered holder of any Rights Certificate may apply to any
         court of competent jurisdiction for the appointment of a new Rights
         Agent. Any successor Rights Agent, whether appointed by the Company or
         by such a court, shall be either (a) a corporation organized and doing
         business under the laws of the United States, or of the State of New
         York, Texas, or Illinois (or of any other state of the United States so
         long as such corporation is authorized to do business as a banking
         institution in the State of New York or Texas), in good standing,
         having an office or an affiliate with an office in the State of New
         York, which is authorized under such laws to exercise corporate trust
         or stock transfer powers and is subject to supervision or examination
         by federal or state authority and which has at the time of its
         appointment as Rights Agent a combined capital and surplus of at least
         $50 million or (b) an affiliate of a corporation described in clause
         (a) of this sentence which is a corporation organized and doing
         business under the laws of the United States or of the State of New
         York or Texas (or of any other state of the United States so long as
         such corporation is authorized to do business as a banking institution
         in the State of New York or Texas), in good standing, having a
         principal office in the State of New York, which is authorized under
         such laws to exercise corporate trust or stock transfer powers and is
         subject to supervision or examination by federal or state authority.
         After appointment, the successor Rights Agent shall be vested

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         with the same powers, rights, duties and responsibilities as if it had
         been originally named as Rights Agent without further act or deed; but
         the predecessor Rights Agent shall deliver and transfer to the
         successor Rights Agent any property at the time held by it hereunder,
         and execute and deliver any further assurance, conveyance, act or deed
         necessary for the purpose. Not later than the effective date of any
         such appointment, the Company shall file notice thereof in writing with
         the predecessor Rights Agent and each transfer agent of the Common
         Shares or Preferred Shares, and mail a notice thereof in writing to the
         registered holders of the Rights Certificates. Failure to give any
         notice provided for in this Section 21, however, or any defect therein,
         shall not affect the legality or validity of the resignation or removal
         of the Rights Agent or the appointment of the successor Rights Agent,
         as the case may be.

         Section 1.03 AGREEMENT TO REMAIN IN FULL FORCE AND EFFECT. Other than
as specifically amended by this First Amendment, the other terms of the
Agreement shall remain in full force and effect.

         IN WITNESS WHEREOF, the parties hereto have caused this First Amendment
to be duly executed as of the day and year first above written.

                                       ENERGY BIOSYSTEMS CORPORATION

                                       By:  /s/ Paul G. Brown, III
                                          --------------------------------------

                                       KEYCORP SHAREHOLDER SERVICES, INC.

                                       By:  /s/ Barbara Shepherd
                                          --------------------------------------<PAGE>

                                                                     EXHIBIT 4.4

                SECOND AMENDMENT TO STOCKHOLDER RIGHTS AGREEMENT

         THIS SECOND AMENDMENT TO STOCKHOLDER RIGHTS AGREEMENT (this
"Amendment"), is executed by and between Enchira Biotechnology Corporation
(f/k/a Energy BioSystems Corporation), a Delaware corporation (the "Company"),
and Harris Trust and Savings Bank (successor in interest to Society National
Bank) (the "Rights Agent"), at the direction of the Company.

                                    RECITALS

         WHEREAS, the Company and the Rights Agent are parties to a Stockholder
Rights Agreement dated as of March 8, 1995, as amended by that certain First
Amendment dated as of April 30, 1997 (the "Rights Agreement");

         WHEREAS, the Company, pursuant to Section 26 of the Rights Agreement,
desires to amend the Rights Agreement and on June 8, 2000, the Board of
Directors approved resolutions authorizing the amendment of the Rights Plan as
herein provided; and

         WHEREAS, pursuant to Section 26 of the Rights Agreement, the Company
and the Rights Agent, at the direction of the Company, hereby agree to amend the
Rights Agreement as set forth below.

         NOW, THEREFORE, the Rights Agreement is hereby amended as follows:

         1.       AMENDMENT OF SECTION 1.

         Section 1(a) of the Rights Agreement is amended in its entirety to read
as follows:

                           (a)      "Acquiring Person" shall mean any Person (as
                  such term is hereinafter defined) who or which, together with
                  all Affiliates and Associates (as such terms are hereinafter
                  defined) of such Person, shall be the Beneficial Owner (as
                  such term is hereinafter defined) of 20% or more of the Common
                  Shares of the Company then outstanding, but shall not include
                  (i) the Company, (ii) any Subsidiary (as such term is
                  hereinafter defined) of the Company, (iii) any employee
                  benefit plan of the Company or of any Subsidiary of the
                  Company or any Person or entity holding shares of capital
                  stock of the Company for or pursuant to the terms of any such
                  plan, in its capacity as an agent or trustee for any such
                  plan, (iv) Ethyl Corporation, a Delaware corporation
                  ("Ethyl"), (v) Gryphon Ventures II, Limited Partnership, a
                  Massachusetts limited partnership ("Gryphon"), (vi) William M.
                  Haney, III ("Haney"), and (vii) any registered investment
                  company or registered investment adviser acquiring or holding
                  shares in the ordinary course of business for its clients and
                  not for its own benefit for so long as such registered
                  investment company or registered investment adviser is
                  permitted to report its Beneficial Ownership of Common Shares
                  on a Schedule 13G filed under the Exchange Act (as such term
                  is hereinafter defined) (such registered investment company
                  and registered investment adviser being hereinafter referred
                  to as a "Schedule 13G Filer"). Notwithstanding the foregoing,
                  no Person shall become an "Acquiring Person" as the result of
                  an acquisition of Common Shares by the Company which, by
                  reducing the number of shares outstanding, increases the
                  proportionate number of shares beneficially owned by such
                  Person to 20% or more of the Common Shares of the Company then
                  outstanding; provided, however, that if a Person shall become
                  the Beneficial Owner of 20% or more of the Common Shares of
                  the Company then outstanding by reason of share purchases by
                  the Company and shall, after such share purchases by the
                  Company, become the Beneficial Owner of any additional Common
                  Shares of the Company, then such Person shall be deemed to be
                  an "Acquiring Person."

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         2.       AMENDMENT OF SECTION 3.

         Section 3(a) of the Rights Agreement is amended in its entirety to read
as follows:

                           (a)      Until the earlier of (i) the close of
                  business on the 30th day after the Shares Acquisition Date and
                  (ii) the close of business on the 30th day after the date that
                  a tender or exchange offer by any Person (other than the
                  Company, any Subsidiary of the Company, any employee benefit
                  plan of the Company or of any Subsidiary of the Company, or
                  any entity holding shares of capital stock of the Company for
                  or pursuant to the terms of any such plan, in its capacity as
                  an agent or trustee for any such plan, Ethyl, Gryphon, Haney
                  or a Schedule 13G Filer) is first published or sent or given
                  within the meaning of Rule 14d-2(a) (or any successor rule) of
                  the General Rules and Regulations under the Exchange Act, the
                  consummation of which would result in any Person becoming the
                  Beneficial Owner of Common Shares aggregating 20% or more of
                  the then outstanding Common Shares (including any such date
                  which is after the date of this Agreement and prior to the
                  issuance of the Rights; the earlier of such dates being herein
                  referred to as the "Distribution Date"), (x) the Rights will
                  be evidenced (subject to the provisions of paragraph (b) of
                  this Section 3) by the certificates for Common Shares
                  registered in the names of the holders thereof (which
                  certificates for Common Shares shall also be deemed to be
                  certificates for the Rights) and not by separate certificates,
                  and (y) the Rights and interests therein will be transferable
                  only in connection with the transfer of the associated Common
                  Shares. As soon as practicable after the Distribution Date,
                  the Company will prepare and execute, the Rights Agent will
                  countersign, and the Company will send or cause to be sent
                  (and the Rights Agent will, if requested, send) by
                  first-class, insured, postage-prepaid mail, to each record
                  holder of Common Shares as of the close of business on the
                  Distribution Date, at the address of such holder shown on the
                  records of the Company, one or more Rights certificates, in
                  substantially the form of Exhibit B hereto (a "Rights
                  Certificate"), evidencing one Right for each Common Share so
                  held, subject to adjustment as provided herein. In the event
                  that an adjustment in the number of Rights per Common Share
                  has been made pursuant to Section 11(n) hereof, then at the
                  time of distribution of the Rights Certificates, the Company
                  shall make the necessary and appropriate rounding adjustments
                  (in accordance with Section 14(a) hereof) so that Rights
                  Certificates representing only whole numbers of Rights are
                  distributed and cash is paid in lieu of any fractional Rights.
                  As of and after the Distribution Date, the Rights will be
                  evidenced solely by such Rights Certificate.

         3.       AMENDMENT OF SECTION 11.

         Section 11(a)(ii) of the Rights Agreement is amended in its entirety to
read as follows:

                                    (ii)     In the event that any Person (other
                  than (i) the Company, (ii) any Subsidiary of the Company,
                  (iii) any employee benefit plan of the Company or of any
                  Subsidiary of the Company or any Person or entity holding
                  shares of capital stock of the Company for or pursuant to the
                  terms of any such plan, in its capacity as an agent or trustee
                  for any such plan, (iv) Ethyl, (v) Gryphon, (vi) Haney or
                  (vii) a Schedule 13G Filer), alone or together with its
                  Affiliates and Associates, shall, at any time after the Rights
                  Dividend Declaration Date, become an Acquiring Person then,
                  promptly following the first occurrence of such event, proper
                  provision shall be made so that each holder of a Right (except
                  as provided in Section 7(e) hereof) shall thereafter have the
                  right to receive, upon exercise thereof at the then current
                  Purchase Price in accordance with the terms of this Agreement
                  and in lieu of Preferred Share Fractions, such number of
                  Common Shares of the Company as shall equal the result
                  obtained by (x) multiplying the then current Purchase Price by
                  the then

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                  number of Preferred Share Fractions for which a Right is then
                  exercisable and (y) dividing that product (which shall
                  thereafter be referred to as the "Purchase Price" for each
                  Right and for all purposes of this Agreement) by 50% of the
                  then current market price (determined pursuant to Section
                  11(d) hereof) per Common Share on the fifth day after the date
                  on which a Person has become an Acquiring Person, or the fifth
                  day after the Shares Acquisition Date, whichever market price
                  shall be less (such number of shares being hereinafter
                  referred to as the "Adjustment Shares"). In the event that any
                  Person shall become an Acquiring Person and the Rights shall
                  then be outstanding, the Company shall not take any action
                  that would eliminate or diminish the benefits intended to be
                  afforded by the Rights.

         4.       EFFECTIVENESS.

         This Amendment shall be deemed effective as of June 3, 1999. Except as
amended hereby, the Rights Agreement shall remain in full force and effect and
shall be otherwise unaffected hereby.

         5.       MISCELLANEOUS.

         This Amendment shall be deemed to be a contract made under the laws of
the State of Delaware and for all purposes shall be governed by and construed in
accordance with the laws of such state applicable to contracts to be made and
performed entirely within such state. This Amendment may be executed in any
number of counterparts, each of such counterparts shall for all purposes be
deemed to be an original, and all such counterparts shall together constitute
but one and the same instrument. If any term, provision, covenant or restriction
of this Amendment is held by a court of competent jurisdiction or other
authority to be invalid, illegal, or unenforceable, the remainder of the terms,
provisions, covenants and restrictions of this Amendment shall remain in full
force and effect and shall in no way be affected, impaired or invalidated.

                  [Remainder of Page Intentionally Left Blank]

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         IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed on June 23, 2000.

                                   ENCHIRA BIOTECHNOLOGY CORPORATION

                                   By:   /s/ Peter P. Policastro
                                      ------------------------------------------
                                         Peter P. Policastro, Ph.D.
                                         President and Chief Executive Officer

                                   HARRIS TRUST AND SAVINGS BANK,
                                   AS RIGHTS AGENT

                                   By:   /s/ Lorraine Rodewald
                                      ------------------------------------------
                                         Lorraine Rodewald
                                         Vice President

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