1. Barnett Institute and Department of Chemistry, Northeastern University, Boston, Massachusetts 02115, Department of Biochemistry and Molecular Biology, and Center for Microbial Structural Biology, 205 South Frear Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802, and Department of Microbiology, Immunology, and Molecular Genetics, and the Molecular Biology Institute, University of California, Los Angeles, California 90095