1. Bioscience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati, College of Medicine, Cincinnati, Ohio 45267, Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Birmingham, Alabama 35294, and Large Scale Structures Group, Institut Laue-Langevin, Avenue des Martyrs, BP 156 F-38042, Grenoble Cedex 9 France