1. Whitehead Institute for Biomedical Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142
2. Current address: Laboratory of Chemical Biology and Microbial Pathogenesis, The Rockefeller University, New York, New York 10021
3. Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115
4. Current address: Department of Molecular Cell Biology, Harvard University, Cambridge, Massachusetts 02142