1. European Molecular Biology Laboratory, Meyerhofstrasse 1, Postfach 102209, D-69012 Heidelberg, Germany, Department of Medical Chemistry, Institute of Biomedical Sciences, University of Helsinki, P.O. Box 8, FIN-00014 Helsinki, Finland, and Department of Biophysics, University of Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany