1. Department of Biochemistry and Molecular Biology, Bloomberg School of Public Health, Johns Hopkins University, 615 North Wolfe Street, Baltimore, Maryland 21205, Department of Biophysics and Biophysical Chemistry, School of Medicine, Johns Hopkins University, 725 North Wolfe Street, Baltimore, Maryland 21205, and Department of Biological Sciences, Vanderbilt University, 2326 Stevenson Center, Nashville, Tennessee 37232