1. Department of Cell Biology and Molecular Genetics, Microbiology Building, University of Maryland, College Park, Maryland 20742
2. Department of Marine and Estuarine Environmental Studies, Microbiology Building, University of Maryland, College Park, Maryland 20742
3. Architecture et Fonction des Macromolecules Biologiques, UMR6908, CNRS, and Universites Aix-Marseille I and II, Case 932, 163 Avenue de Luminy, 13288 Marseille Cedex 9, France