TIAA-CREF, the manager of retirement accounts for teachers, agreed to by Nuveen Investments from Madison Dearborn Partners LLC for $6.25 billion, including outstanding debt.

The transaction adds $221 billion in assets, bringingTIAA-CREF’s total to about $800 billion, according to a statement today from the New York-based company. Chicago-based Madison Dearborn purchased Nuveen in 2007, then the largest buyout of a U.S. asset manager, for $5.75 billion.

“This makesTIAA-CREFa real contender in the marketplace,” said Geoff Bobroff, a mutual fund consultant based in East Greenwich, Rhode Island. “They will have an interesting mix of businesses.”

The combined company would rank among the top-20 largest U.S. mutual-fund firms, data from research firm Morningstar Inc. show.TIAA-CREFhad $72 billion in open-ended mutual fund assets as of Feb. 28 and Nuveen had $54 billion. Founded in 1918,TIAA-CREF is led by Roger Ferguson, the former U.S. Federal Reserve vice chairman.

Bank of America Corp., Wells Fargo & Co. and Citigroup Inc. are the bankers for Nuveen Investments. UBS AG, Goldman Sachs Group Inc. and Moelis & Co. advised Nuveen’s management.

Madison Dearborn was advised by Morgan Stanley, Deutsche Bank AG and Royal Bank of Canada.