1. Departments of Molecular Biology and Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Department of Biochemistry and Molecular Biology, University of Bergen, Årstadveien 19, N-5009 Bergen, Norway, and Biotechnology Centre of Oslo, P.O. Box 1125, Blindern, N-0316 Oslo, Norway