1. Contribution from the Institut de Génétique et Biologie Moléculaire et Cellulaire (C.N.R.S.), 1 rue Laurent Fries (C.U. de Strasbourg), 67404 Illkirch, France, and Department of Molecular Biology, The Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, California 92037