1. Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden, Department of Clinical Microbiology, Division of Virology, Umeå University, SE-901 87 Umeå, Sweden, and European Molecular Biology Laboratory, EMBL Grenoble, BP 181, 6 Rue Jules Horowitz, 38042 Grenoble Cedex 9, France