1. Molecular Biology and Biochemistry Department, Molecular Biophysics Program, Wesleyan University, Middletown, Connecticut 06459-0175, Pediatric Branch, National Cancer Institute, Building 10/13N240, 10 Center Drive, MSC 1928, Bethesda, Maryland 20892-1928, and WP 44-B122, Department of Antiviral Research, Merck Research Laboratories, West Point, Pennsylvania 19486