1. Program in Cellular & Molecular Biology, University of Wisconsin, and Department of Biomolecular Chemistry, Medical School, University of Wisconsin, 1300 University Avenue, Madison, Wisconsin 53706, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, and Institute for Enzyme Research and Department of Biochemistry, College of Agriculture and Life Sciences, University of Wisconsin, Madison, Wisconsin 53705