1. Laboratoire de Génétique Microbienne, Institut National de la Recherche Agronomique; 78352 Jouy en Josas Cedex France
2. Department of Microbiology and Immunology; Temple University School of Medicine; Philadelphia Pennsylvania 19140-5196
3. Division of Cellular Biology, Department of Molecular and Experimental Medicine; The Scripps Research Institute; La Jolla California 92037-1093