1. Life Sciences Division, Los Alamos National Laboratory, MS-M888, Los Alamos New Mexico 87545, Graduate School of Agricultural and Life Science, The University of Tokyo, Yayoi, Tokyo 113, 8657 Japan, and Centre de Recherches sur les Macromolécules Végétales, CNRS, affiliated with the Joseph Fourier University of Grenoble, BP 53, 38041 Grenoble Cedex 9, France