1. Institut de Biologie Moléculaire et Cellulaire, UPR 9002 du CNRS, 15 rue Descartes, F-67084 Strasbourg Cedex, France, Department of Chemistry and Waksman Institute, Rutgers University, New Brunswick, New Jersey 08855, and Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584-CH Utrecht, The Netherlands