1. Department of Microbiology, Immunology, and Molecular Genetics, and the Molecular Biology Institute, University of California, and the David Geffen School of Medicine, Los Angeles, California 90095
2. Department of Microbiology, Columbia University Medical Center, New York, New York 10032
3. Department of Biochemistry and Molecular Biophysics, Columbia University Medical Center, New York, New York 10032