1. Department of Biochemistry and Department of Microbiology and Molecular Genetics, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226, and Departments of Chemistry and Immunology, The Skaggs Institute for Chemical Biology, and Worm Institute for Research and Medicine, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037