[1] Univ Calif San Francisco, Dept Microbiol & Immunol, San Francisco, CA 94143 USA
[2] Univ Calif San Francisco, Dept Biochem & Biophys, San Francisco, CA 94143 USA
[3] Johns Hopkins Univ, Sch Med, Dept Mol Biol & Genet, Howard Hughes Med Inst, Baltimore, MD 21205 USA