1. Department of Cell and Molecular Biology, University of Uppsala, Biomedical Centre, Box 596, SE-751 24 Uppsala, Sweden, Department of Molecular Biology and Genetics, Cornell University, 458 Biotechnology Building, Cornell University, Ithaca, New York 14853-2703, and Centre de Recherches sur les Macromolécules Végétales CNRS, B.P. 53, 38041 Grenoble Cedex 9, France