exhibit 4. 01 acceptance, assignment and assumption agreement by and among public service company of colorado u. s. bank trust national association and u. s. bank national association dated as of november 8, 2010 acceptance, assignment and assumption agreement dated as of november 8, 2010 ( this “ agreement ” ) among public service company of colorado ( the “ company ” ), u. s. bank trust national association ( “ trustee ” ) and u. s. bank national association ( “ successor trustee ” ). whereas, the company and the trustee are parties to an indenture dated as of october 1, 1993 ( as supplemented by a first supplemental indenture dated as of november 1, 1993, by a second supplemental indenture dated as of january 1, 1994, by a third supplemental indenture dated as of september 2, 1994, by a fourth supplemental indenture dated as of may 1, 1996, by a fifth supplemental indenture dated as of november 1, 1996, by a sixth supplemental indenture dated as of february 1, 1997, by a seventh supplemental indenture dated as of april 1, 1998, by an eight supplemental indenture dated as of august 15, 2002, by a ninth supplemental indenture dated as of september 1, 2002, by a tenth supplemental indenture dated as of september 15, 2002, by an eleventh supplemental indenture dated as of april 1, 2003, by a twelfth supplemental indent ure dated as of march 1, 2003, by a thirteenth supplemental indenture dated as of september 15, 2003, by a fourteenth supplemental indenture dated as of may 1, 2003, by a fifteenth supplemental indenture dated as of september 1, 2003, by a sixteenth supplemental indenture dated as of august 1, 2005, by a seventeenth supplemental indenture dated as of august 1, 2007, by an eighteenth supplemental indenture dated as of august 1, 2008, and by a nineteenth supplemental indenture dated as of may 1, 2009 ( the “ indenture ” ), relating to the company ’ s securities ; whereas, the company and the trustee desire that the successor trustee replace the trustee as trustee under the indenture, with effect from the date hereof ; whereas, by notice dated november 8, 2010, the company and the trustee provided written notice of the replacement of the trustee with the successor trustee, in accordance with the indenture to the company