1. Institut de Biochimie et Génétique Cellulaires, CNRS UMR5095, F-33077 Bordeaux Cedex,1 and
2. INSERM U.441, F-33600 Pessac,2 France, and
3. Department of Molecular Genetics, Max-Planck-Institute for Biophysical Chemistry, D-37070 Göttingen,3 Germany