1. Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
2. Department of Biology and McCollum-Pratt Institute, Johns Hopkins University, Baltimore, Maryland 21218
3. Present address: Laboratory of Molecular Gerontology, NIA, National Institutes of Health, Baltimore, MD 21224.
4. Department of Biology, Loyola University Maryland, Baltimore, Maryland 21210