1. Martin-Luther-Universität Halle-Wittenberg, Institut für Biochemie, Kurt-Mothes-Strasse 3, D-06099 Halle, Germany, University of California, Berkeley, Department of Molecular and Cellular Biology, Stanley Hall, Berkeley, California 94720, and Universität Tübingen, Physiologisch-chemisches Institut, Hoppe-Seyler-Strasse 4, D-72076 Tübingen, Germany