1. School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
2. School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
3. Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.
4. Department of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield S10 2TN, U.K.