1. Department of Biochemistry and Molecular Biology; Michigan State University; East Lansing MI 48824 USA
2. Department of Chemistry & Biochemistry; University of California, San Diego; La Jolla CA 92093 USA
3. Division of Biological Sciences; University of California, San Diego; La Jolla CA 92093 USA
4. Division of Microbiology and Immunology; Department of Pathology; University of Utah School of Medicine; Salt Lake City UT 84112 USA
5. Microbiology and Molecular Genetics; Michigan State University; East Lansing MI 48824 USA