1. Department of Biochemistry, College of Medicine, University of Vermont, Burlington, Vermont 05405, Program in Molecular Biology and Biochemistry, Middlebury College, Middlebury, Vermont 05753, Biochemistry Department, Duke University Medical School, Durham, North Carolina 27710, Chemistry Department and Biological Chemistry Department, University of Michigan, Ann Arbor, Michigan 48109, and Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003