1. 1 Department of Biochemistry, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2AZ, UK
2. 2 Department of Microbiology, University of Reading, PO Box 228, Reading RG6 2AJ, UK
3. 3 Department of Protein Engineering, Institute of Food Research, Reading Laboratory, Earley Gate, Whiteknights Road, Reading RG6 2EF, UK