1. Department of Biochemistry, Imperial College of Science and Technology, London SW7 2AZ, U.K.
2. Department of Clinical Pharmacology, Royal Postgraduate Medical School, London W12 OHS, U.K.
3. Department of Chemistry, Imperial College of Science and Technology, London SW7 2AY, U.K.
4. Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, U.K.