[1
]Department of Microbiology, Dartmouth Medical School, Hanover, New Hampshire 03755;
e-mail:
[2
]Department of Microbiology and Molecular Genetics, University of Texas Medical School,
Houston, Texas 77030; e-mail:
[3
]Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston,
Massachusetts 02115; e-mail: