Department of Biochemistry and Molecular Biology, University College, London, Gower Street, London WC1E 6BT.|Department of Biochemistry and Molecular Biology, University College, London, Gower Street, London WC1E 6BT.|Department of Biochemistry and Molecular Biology, University College, London, Gower Street, London WC1E 6BT.|Department of Biochemistry and Molecular Biology, University College, London, Gower Street, London WC1E 6BT.|Department of Biochemistry and Molecular Biology, University College, London, Gower Street, London WC1E 6BT.