1. Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, St. Louis, Missouri 65211, Department of Biochemistry and Molecular Biology, University of Parma, Parma, Italy, Stanford Synchrotron Radiation Laboratory, Stanford, California 94309, Department of Biology, University of Louisiana, Lafayette, Louisiana 70504, and Department of Biochemistry, University of Mississippi Medical Center, Jackson, Mississippi 39216