1. Department of Biochemistry, University of Leicester, The Adrian Building, University Road, Leicester LE1 7RH, U.K., Department of Cell and Organism Biology, Lund University, Sölvegatan 35, SE-223 62 Lund, Sweden, Centre for Chemical Biology, Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, S3 7HK, U.K., and Biotechnology Research and Development, DSM Nutritional Products, P.O. Box 3255, Building 203/20A, CH-4002, Basel, Switzerland