Department of Genetics and Microbiology, University of Pavia, Via Abbiategrasso 207, Pavia, 27100 Italy, Departments of Biochemistry and Chemistry, Emory University, Clifton Road 1510, Atlanta, Georgia 30322, and Research and Development Division, Teva Pharmaceutical Industries, P.O. Box 8077, Netanya, Israel (At 2004-02-27)