A lawyer who has represented some of the nation’s largest banks — and helped them in their bids to grow even larger — has been tapped to advise the Treasury Department in its unprecedented effort to nationalize some of those same institutions to shore up troubled financial markets.

Lee Meyerson, one of the nation’s top banking industry merger and acquisition lawyers, will be leading a team of lawyers from the New York firm Simpson Thacher & Bartlett in representing the Treasury Department, the firm’s website reports.

Among the numerous big banking deals on Meyerson’s resume: representing J.P. Morgan Chase in its $58 billion merger with Bank One in 2004; Mellon Financial in its $16.8 billion combination with The Bank of New York in 2007; and Wachovia in its $14.7 billion merger with First Union in 2001 (while fending off a hostile takeover by Suntrust Banks).

Now, J.P. Morgan Chase, Bank of New York Mellon, and Wells Fargo (which is moving to purchase Wachovia) are among the nine institutions in which the Treasury Department is poised to invest $250 billion. (The others are Bank of America, Merrill Lynch, Citigroup, Goldman Sachs, Morgan Stanley, and State Street.)

Treasury Secretary Henry Paulson has made clear the plan to purchase equity in banks with taxpayer money will involve complex lawyering. Treasury is “working to develop a standardized program that is open to a broad array of financial institutions,” Paulson said, adding he hoped to design it to encourage the raising of new private capital to complement the federal cash.

Meyerson, 52, who has been with Simpson Thacher for his entire career, certainly has experience in dealing with financial industry emergencies. He represented Washington Mutual earlier this year in its effort to climb out of a deep subprime mortgage hole with a $7.2 billion capital infusion from the private equity firm TPG and other investors. Alas, the lifeline wasn’t long enough, and WaMu, the nation’s largest savings and loan, was placed into government receivership and then sold to J.P. Morgan last month.