1. S. Marcand, Department of Microbiology, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA, and Laboratoire de Biologie Moléculaire et Cellulaire, Ecole Normale Supérieure de Lyon, 69364 Lyon Cédex 07, France.
2. E. Gilson, Laboratoire de Biologie Moléculaire et Cellulaire, Ecole Normale Supérieure de Lyon, UMR49 CNRS/ENSL, 69364 Lyon Cédex 07, France.
3. D. Shore, Department of Microbiology, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA, and Department of Molecular Biology, University of Geneva, CH-1211 Geneva 4, Switzerland.