1. Institut de Biologie Structurale, CEA-CNRS, 41 Avenue des Martyrs, F-38027 Grenoble Cedex 1, France, Department of Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv 69978, Israel, Université Joseph Fourier, 38000 Grenoble, France, and Institut Laue Langevin, BP 156X, 38042 Grenoble, France