1. Unité de Physico-chimie des Macromolécules Biologiques, CNRS:URA 1773, Institut Pasteur, 25, rue du Dr Roux, 75724 Paris Cedex 15, France, and Department of Microbiology and Immunology, Morse Institute for Molecular Genetics, State University of New York Health Science Center at Brooklyn, 450 Clarkson Avenue, Brooklyn, New York 11203-2098