1. T. Li is in the Department of Biophysics and Biophysical Chemistry and C. Wolberger is in the Department of Biophysics and Biophysical Chemistry and the Howard Hughes Medical Institute, Johns Hopkins University School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205-2185, USA.
2. M. R. Stark and A. D. Johnson are in the Department of Microbiology and Immunology, University of California School of Medicine, San Francisco, CA 94143-0502, USA.