1. Departments of Biochemistry and Chemistry, Emory University, 1510 Clifton Road, Atlanta, Georgia 30322, Department of Genetics and Microbiology, University of Pavia, Via Abbiategrasso 207, Pavia, 27100 Italy, Research and Development Division, Teva Pharmaceutical Industries, P.O. Box 8077, Netanya, 42504 Israel, and Eve Topf and NPF Centers and Department of Pharmacology, Technion Faculty of Medicine, Efron Street, P.O. Box 9697, Haifa, 31096 Israel