1. Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, Illinois 60637, Department of Physics, Oklahoma State University, Stillwater, Oklahoma 74078, Faculty of Physics and Astronomy, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands, Wyeth-Ayherst Research, 401 North Middletown Road, Pearl River, New York 10965, and Department of Microbiology, E. C. Slater Institute, University of Amsterdam, 1018 WS Amsterdam, The Netherlands