1. Laboratory of Molecular Biophysics and Oxford Centre for Molecular Sciences, Department of Biochemistry, Rex Richards Building, South Parks Road, Oxford OX1 3QU, U.K., National Hellenic Research Foundation, Institute for Biological Research and Biotechnology, 48 vas Constantinou Avenue, Athens 116 35, Greece, MRC Laboratory of Molecular Biology, Hills Road, Cambridge, CB2 2QH, U.K., and Dyson Perrins Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QY, U.K.