1. Division of Biological Sciences, University of California at San Diego, La Jolla, CA 92093-0116, USA
2. Scripps Genome Center, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0202, USA
3. Department of Biochemistry, University of California at Berkeley, Berkeley, CA, USA
4. Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI 48109, USA