UPDATE 1-AllianceBernstein-backed REIT to debut next week

* Foursquare Capital to price week of Sept. 21

* Foursquare sees selling 25 mln shares for $20 each

NEW YORK, Sept 14 (Reuters) - Foursquare Capital Corp, a real estate investment trust that will be run and advised by a unit of money manager AllianceBernstein (AB.N), will price its initial public offering and make its trading debut the week of Sept. 21, according to one of deal's underwriters.

Foursquare said in prospectus filed on Monday with the U.S. Securities and Exchange Commission that it plans to sell 25 million shares and estimated they would price for $20 each.

The REIT, which specializes in commercial and residential mortgage-backed securities, aims to capitalize on government programs to buy assets at reduced prices.

AllianceBernstein beat out more than 90 other managers earlier this year to work with the U.S. Treasury in the Public-Private Investment Plan, or PPIP, meant to rid banks' balance sheet of assets whose values have fallen during the financial crisis.

Foursquare plans to use a "significant amount of the proceeds" for AllianceBernstein's participation in PPIP, it said when it filed in July for the IPO.

Foursquare's IPO will be led by Bank of America Merrill Lynch and Morgan Stanley.

Foursquare's IPO is one of seven scheduled to price and start trading next week, including two others by REITs: Apollo Commercial Real Estate, which will be managed by a subsidiary of private equity firm Apollo Global Management [APOLO.UL], and Colony Financial Inc. (Reporting by Phil Wahba; editing by Gunna Dickson)