1. Biophysics Research Division, University of Michigan, Ann Arbor, Michigan, U.S.A. 48109-1055, USA
2. Department of Microbiology and Immunology, Medical College of Ohio, Toledo, Ohio, U.S.A., 43614-5806, USA
3. Department of Biochemistry, Emory University, Atlanta, Georgia, U.S.A. 30322-4218, USA