1. School of Agricultural and Life Science, The University of Tokyo, Tokyo 113-8657, Japan, College of Life Resources Science, Dongguk University, Seoul 100-715, Korea, Intitut Laue Langevin, BP 156, 38042 Grenoble Cedex 9, France, and Bioscience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545