1. Department of Biochemistry and Molecular Biology, Wayne State University School of Medicine, Detroit, Michigan 48201, School of Molecular Biosciences, Washington State University, Pullman, Washington 99164, Department of Medicine, Division of Hematology-Oncology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, and Laboratoire d'Ingenierie des Proteines et Controle Metabolique, Institut Jacques Monod, Paris, France