[1] Univ Texas, Sch Med, Dept Microbiol & Mol Genet, Houston, TX 77030 USA
[2] Salk Inst Biol Studies, La Jolla, CA 92037 USA
[3] CNRS, Inst Pasteur, Unite Toxines & Pathogen Bacterienne, Paris, France
[4] No Arizona Univ, Dept Biol, Flagstaff, AZ 86011 USA