Armbrister was nominated to the SRC in 2007 by Gov. Ed Rendell, and her term does not expire until next year.
But, in a written statement, Armbrister said she believed "there is a need to complete the transition to new leadership in the SRC and to allow the new members to undergo a coordinated orientation, which will allow them to quickly focus on the children for whom they pledge to serve."
Armbrister, a public-school parent, is executive director of the Wells Fargo Regional Foundation and has spent most of her professional life in the financial industry. She rarely spoke out from her seat on the SRC and seemed uncomfortable in the limelight.
Lori Shorr, Mayor Nutter's education secretary, called Armbrister a "levelheaded, steady presence" on the SRC.
Armbrister did not return calls for comment, but said in the statement that she had "come to the conclusion that my stepping aside now will enhance the orderly transition to new SRC members, increase the prospects for stability, and, I hope, encourage the Commonwealth and the City of Philadelphia to move forward with a coordinated and coherent agenda, together with the SRC and the" district.
Gov. Corbett will "soon" nominate Armbrister's replacement, said Tim Eller, a spokesman for the state Department of Education. Eller declined to comment on the speculation that the nominee would be Houstoun.
Houstoun did not return calls for comment.
Acting School Superintendent Leroy Nunery II said Armbrister had served "with great distinction."
Nutter also had warm words for Armbrister, wife of his former chief of staff Clarence "Clay" Armbrister.
"Denise is a true public servant who cares passionately about the well-being of children and their education," Nutter said. "Her service to the School Reform Commission was tremendous, and she's also a great friend. I very much admired her taking on this challenge of public service."
Armbrister's resignation caps a tumultuous period for the SRC, which lost two members in September with the resignations of former chairman Robert L. Archie Jr. and Johnny Irizarry.
Archie and Irizarry, mayoral appointees, have been replaced by Rutgers-Camden Chancellor Wendell Pritchett, who is acting chair, and novelist Lorene Cary.
The SRC's fifth seat has been empty since David Girard-diCarlo resigned in February. Pedro Ramos, a former school board president, has been nominated by Corbett to replace Girard-diCarlo, but awaits Senate confirmation.
If confirmed, Ramos will become SRC chairman.
A spokesman for state Senate Majority Leader Dominic Pileggi (R., Delaware County) could not say when Ramos' confirmation hearing might be scheduled.
But sources said that if Houstoun was identified as Armbrister's successor, her nomination process could move quickly and that hearings for Ramos and Houstoun might be held at the same time.
Joseph Dworetzky, a former city solicitor, is the lone remaining member from the old SRC. Though he has moved to California since his appointment to the panel, Dworetzky commutes to Philadelphia for meetings at his own expense and has indicated he wanted to remain on the SRC.
In recent weeks, the city and state have been paying much more attention to the district - and collaborating much more - than they have in the past.
That collaboration appears to have speeded the SRC transition.
Nutter and state Secretary of Education Ronald Tomalis also recently named two "executive advisers" - Shorr and Ed Williams - to work with the district and the SRC.
The mayor and Tomalis also ordered formation of a working group of business experts to advise the SRC on reforms in matters of operations and administration.
Contact staff writer Kristen Graham at 215-854-5146, email@example.com,
or @newskag on Twitter. Read her blog, "Philly School Files,"
Inquirer staff writer Susan Snyder contributed to this article.