1. Medical Research Council (MRC) Laboratory of Molecular Biology, Hills Road, Cambridge CB2 0QH, UK.
2. Université de Rennes 1 and Institut Universitaire de France, “Translation and Folding” group, UMR CNRS 6290, Institute of Genetics and Development of Rennes (IGDR), Campus de Beaulieu 35042 Rennes cedex, France.