1. S. C. Mande, Department of Biological Structure and Biomolecular Structure Center, University of Washington, Seattle, WA 98195, USA.
2. V. Mehra, Department of Microbiology and Immunology, University of Washington, Seattle, WA 98195, USA.
3. B. R. Bloom, Department of Microbiology and Immunology, University of Washington, Seattle, WA 98195, USA, and Howard Hughes Medical Institute, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
4. W. G. J. Hol, Department of Biological Structure and Biomolecular Structure Center, and Howard Hughes Medical Institute, University of Washington, Box 357742, Seattle, WA 98195, USA.