1. Departments of Chemistry and Immunology, The Skaggs Institute for Chemical Biology, and Worm Institute of Research and Medicine (WIRM), The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, The Department of Microbiology and Molecular Genetics, Medical College of Wisconsin, Milwaukee, Wisconsin, and The Division of Infectious Diseases, Department of Biomedical Sciences, Tufts University Cummings School of Veterinary Medicine, North Grafton, Massachusetts