Maguire Properties Inc. is adding $3 billion worth of office space to its Southern California portfolio.

Los Angeles-based Maguire, already downtown’s largest landlord, has agreed to the purchase of 23 buildings in Los Angeles and Orange counties from Blackstone Group LP, according to a Feb. 17 Bloomberg report.

The 6.8 million-square-foot deal will include downtown’s 52-story Two California Plaza and 555 S. Hope St. The public REIT already owns the tallest building west of the Mississippi River, the 72-story U.S. Bank Tower.

Industry experts say that office building rental rates will likely increase in the aftermath of the transaction as investments are recouped.

Shares of Maguire closed down 10 cents to $41.29 on Friday and the markets were closed Monday in observation of the Presidents’ Day holiday.

Private equity firm Blackstone has been selling property rapidly since its Feb. 9 purchase of Equity Office Properties Trust of Chicago for $39 billion. Among the selloff deals is a $7 billion sale of New York City office buildings to Macklowe Properties Inc.

After Blackstone purchased Equity Office, a REIT analyst told the Business Journal that Blackstone only made its blockbuster purchase of Equity Office because “they had buyers lined up waiting in the wings” to unload some of the newly bought assets.

Last fall Maguire Properties chief executive Robert Maguire put his company up for sale, but by the end of the year the REIT was off the table after offers came in below expectations.