1. Division of Biology; California Institute of Technology; Pasadena CA 91125
2. Department of Biology; Texas A & M University; College Station TX 77843-3258
3. Molecular Microbiology Section, Laboratory of Clinical Infectious Diseases; National Institute of Allergy and Infectious Diseases, National Institutes of Health; Bethesda MD 20892
4. Department of Molecular Genetics and Microbiology; Duke University Medical Center; Durham NC 27710