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                                                                     EXHIBIT 3.A

                              AMENDED AND RESTATED

                          CERTIFICATE OF INCORPORATION

                                       OF

                         COLORADO INTERSTATE GAS COMPANY


                     Pursuant to Sections 242 and 245 of the
                General Corporation Law of the State of Delaware

         Colorado Interstate Gas Company (the "Corporation"), a corporation
organized and existing under the laws of the State of Delaware, hereby certifies
as follows:

         1. The name of the Corporation is Colorado Interstate Gas Company. The
Corporation was originally incorporated on June 8, 1927, under the name of
Colorado Interstate Gas Company.

         2. This Amended and Restated Certificate of Incorporation restates and
further amends the Certificate of Incorporation of the Corporation and has been
adopted and approved in accordance with Sections 242 and 245 of the General
Corporation Law of the State of Delaware. Stockholder approval of this Restated
Certificate of Incorporation was given by unanimous written consent of the
stockholders of the Corporation in accordance with Section 228 of the General
Corporation Law of the State of Delaware.

         3. The text of the Certificate of Incorporation, as heretofore amended,
is hereby amended and restated to read in its entirety as follows:

                  FIRST: The name of the corporation is Colorado Interstate Gas
         Company.

                  SECOND: The address of the Corporation's registered office in
         the State of Delaware is Corporation Trust Center, 1209 Orange Street,
         City of Wilmington, New Castle County, Delaware 19801, and the name of
         its registered agent at the above address is The Corporation Trust
         Company.

                  THIRD: The purpose of the corporation is to engage in any
         lawful act or activity for which corporations may be organized under
         the General Corporation Law of the State of Delaware.

                  FOURTH: The total number of shares of stock the corporation
         shall have authority to issue is one thousand (1,000) shares of common
         stock with a par value of one dollar ($1.00) per share.


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                  Shares of stock of this corporation whether with or without
         par value, of any class or classes hereby or hereafter authorized, may
         be issued by this corporation from time to time for such consideration
         permitted by law as may be fixed from time to time by the Board of
         Directors.

                  FIFTH: Unless required by the By-laws, the election of the
         Board of Directors need not be by written ballot.

                  SIXTH: The Board of Directors of this corporation is expressly
         authorized to make, alter, or repeal the By-laws of the corporation,
         but the stockholders may make additional By-laws and may alter or
         repeal any By-law whether or not adopted by them.

                  SEVENTH: No director of the Corporation shall be personally
         liable to the corporation or its stockholders for monetary damages for
         breach of fiduciary duty as a director, for any act or omission, except
         that a director may be liable (i) for breach of the director's duty of
         loyalty to the corporation or its stockholders, (ii) for acts or
         omissions not in good faith or that involve intentional misconduct or a
         knowing violation of law, (iii) under Section 174 of the General
         Corporation Law of the State of Delaware, or (iv) for any transaction
         from which the director derived an improper personal benefit. If the
         General Corporation Law of the State of Delaware is amended to
         authorize corporate action further eliminating or limiting the personal
         liability of directors, then the liability of the directors shall be
         eliminated or limited to the fullest extent permitted by the General
         Corporation Law of the State of Delaware, as so amended. The
         elimination and limitation of liability provided herein shall continue
         after a director has ceased to occupy such position as to acts or
         omissions occurring during such director's term or terms of office. Any
         amendment, repeal or modification of this Article Seventh shall not
         adversely affect any right of protection of a director of the
         Corporation existing at the time of such repeal or modification.

         IN WITNESS WHEREOF, Southern Natural Gas Company has caused this
Amended and Restated Certificate of Incorporation to be signed by a duly
authorized officer this 7th day of March 2002.


                                        COLORADO INTERSTATE GAS COMPANY



                                        By:         /s/ Teresa L. McDonald
                                                --------------------------------
                                                      Teresa L. McDonald
                                                        Vice President


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