Mayor Bloomberg on Tuesday announced a $25 million contribution from New York City to establish a new home for Off Broadway mainstay Signature Theater Company that’s expected to be completed by 2012.Designed by Frank Gehry, the three-theater Signature Center will be part of Related Companies’ $800 million, 59-story residential building and hotel in the theater district. Signature founding a.d. James Houghton; 2010-11 playwright-in-residence Tony Kushner; and artists-in-residence Bill Irwin, Ruben Santiago-Hudson, Hallie Foote, John Guare and Edward Albee were among those present at the 42nd Street construction site for the announcement. The new theaterplex will allow for three unique programs, more than doubling the audience capacity of Signature’s existing base. The project previously had been announced to be part of the World Trade Center Site. The new mainstage will continue to house Signature’s Master Playwright Residency, exploring the body of work of a different writer each season; second stage will expand the Legacy Program, revisiting artists previously produced by Signature; and the third will introduce the Emerging Playwrights Residency, for early and mid-career scribes. In addition to the City’s $25 million contribution, Signature has raised $16 million from private investors. A capital campaign, co-chaired by Edward Norton, is under way to raise the final $19 million needed to complete the project. Thanks in part to a robust support program from Time Warner that allows for $20 tickets to all performances, Signature has emerged as one of the most consistently successful Off Broadway companies in recent seasons. Signature currently is presenting Horton Foote’s three-part, nine-hour “The Orphan’s Home Cycle,” co-produced with Hartford Stage and playing in rep through March 28. While the deal is not yet finalized, commercial producers Jeffrey Richards and Daryl Roth have been crunching numbers for a possible Broadway transfer before the end of this season.