1. Section of Microbial Pathogenesis, Boyer Center for Molecular Medicine, Yale University School of Medicine; 295 Congress Ave. New Haven CT 06536
2. Department of Molecular Microbiology; Washington University School of Medicine; St. Louis MO 63110
3. Institute of Structural Molecular Biology, Birkbeck and University College London; Malet St. London WC1 7HX United Kingdom