1. Krebs Institute for Biomolecular Research, MBB, University of Sheffield, Sheffield S10 2UH, UK., Health and Safety Laboratory, Broad Lane, Sheffield, U.K., Department of Molecular Microbiology and Department of University Medicine, University of Southampton, Southampton General Hospital, Tremona Road, Southampton, U.K., and Laboratory of Molecular Biology, NIDDK, and Bioengineering and Physical Science Program, OD, National Institutes of Health, Bethesda, Maryland 20892