1. Department of Biochemistry and Microbiology, University of Victoria, Petch Building, 258, Victoria, B.C., Canada, V8W 3P6, and Unidad de Biofísica (CSIC-UPV/EHU) and Departamento de Bioquímica y Biología Molecular, Universidad del País Vasco, PO Box 644, 48080 Bilbao, Spain