[^1]: Current address: Department of Pathology, University of Florida School of Medicine, Gainesville, FL 32610-0275.

[^2]: Current address: Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724.

[^3]: Current address: Department of Microbiology, University of Pennsylvania, Philadelphia, PA 19104.

[^4]: Current address: Department of Microbiology and Immunology, Baylor College of Medicine, Houston, TX 77030.
