1. Department of Molecular Biophysics and Biochemistry
2. Department of Chemistry, Yale University, New Haven, CT 06520–8114, USA.
3. Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
4. Department of Microbiology, University of Georgia, Athens, GA 30602–2605, USA.
5. Department of Microbiology, The Ohio State University, Columbus, OH 43210–1292, USA.