1. Department of Microbiology and Cancer Center, University of Virginia Health Sciences Center, Charlottesville, Virginia 229081;
2. Infectious Disease Research, Pharmacia & Upjohn, Inc., Kalamazoo, Michigan 490012; and
3. Laboratory of Structural Biology, National Institute of Arthritis and Musculoskeletal and Skin Diseases,3 and
4. Computational Bioscience and Engineering Laboratory, Center for Information Technology,4 National Institutes of Health, and
5. Biomedical Engineering and Instrumentation Program, National Center for Research Resources,5Bethesda, Maryland 20892