1. Division of Nutritional Sciences, Cornell University, Ithaca, New York 14853 USA, Department of Chemistry and Centre for Biotechnology, Brock University, St. Catharines, Ontario, Canada, L2S 3A1, and Institute of Microbiology, Eidgenössische Technische Hochschule (ETH) Schmelzbergstrasse 7 CH-8092 Zürich, Switzerland