1. Institut für Biophysik der Johann-Wolfgang-Goethe-Universität, Theodor-Stern-Kai 7, Haus 74, 60590 Frankfurt/M., Germany, Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, Department of Biochemistry, University of Illinois, 600 South Mathews Avenue, Urbana, Illinois, and Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan