John Paulson, who amassed a fortune by betting against U.S. mortgage markets, became the biggest stakeholder in SunTrust Banks Inc. while fellow billionaire Warren Buffett was cutting his stake in the Atlanta lender.

Paulson & Co., a hedge fund based in New York, boosted its ownership to 6.1% of SunTrust's shares, federal filings showed this week. That gives his firm about 30.4 million SunTrust shares as of Dec. 31, an increase from 1.5 million on Sept. 30. Buffett's Berkshire Hathaway Inc. reduced its SunTrust holdings by 22%, to 2.4 million shares.

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