1. Department of Molecular Biosciences, University of Kansas, Lawrence, Kansas 66045, Department of Physics, Cornell University, Ithaca, New York 14853, Department of Molecular and Cellular Developmental Biology, University of Colorado, Boulder, Colorado 80309, E. A. Doisy Department of Biochemistry and Molecular Biology, St. Louis University Medical School, St. Louis, Missouri 63104, and Department of Genetics, Waksman Institute of Microbiology, The State University of New Jersey, Rutgers, New Jersey 08854