JPMorgan and Merrill Lynch are backing Forstmann Little & Co.'s $1.6 billion acquisition of 24 Hour Fitness Worldwide. No terms have been set. The transaction is expected to close in June. Mark Mastrov, who founded 24 Hour Fitness in 1983, will retain a significant stake in the business. The fitness company has annual revenues of more than $1.1 billion with three million members.