1. Department of Biochemistry, University of London, Rowland Hill Street, London NW3 2PF, U.K.
2. Department of Protein and Molecular Biology, University of London, Rowland Hill Street, London NW3 2PF, U.K.
3. Division of Communicable Diseases, Royal Free Hospital School of Medicine, University of London, Rowland Hill Street, London NW3 2PF, U.K.