1. Department of Biochemistry and Microbiology, University of Victoria, P.O. Box 3055 STN CSC, Victoria, British Columbia V8W 3P6, Canada
2. School of Biomedical Sciences, University of Newcastle upon Tyne, Newcastle upon Tyne NE2 4HH, U.K.
3. Laboratoire d’Architecture et de Fonction des macromolécules Biologiques, IBSM, CNRS Marseille and University Aix-Marseille I & II, 31 Chemin Joseph Aiguier, 13402 Marseille Cedex 20, France.