1. Laboratoire de Biophysique Moléculaire, Institut de Biologie Structurale, UMR 5075, CEA-CNRS-UJF, 41 rue Jules Horowitz, 38027 Grenoble Cedex 1, France, and Department of Microbiology, University of Bergen, Jahnebakken 5, P.O. Box 7800, N-5020 Bergen, Norway