1. Department of Biochemistry; Vanderbilt University; Nashville TN 37235 USA
2. Department of Pathology; Microbiology and Immunology; Vanderbilt University School of Medicine; Nashville TN 37235 USA
3. University of Waterloo; Waterloo ON Canada
4. Department of Biological Sciences; Columbia University; New York NY 10027 USA
5. Department of Biochemistry and Molecular Biophysics; Columbia University; New York 10032 USA
6. Department of Chemistry; Vanderbilt University; Nashville TN 37235 USA