1. Departments of Biological and Nutritional Sciences and Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K., Department of Biochemistry and Genetics, University of Newcastle upon Tyne, Newcastle upon Tyne NE4 4HH, U.K., Department of Cellular Physiology, The Babraham Institute, Babraham, Cambridge CB2 4AT, U.K., and Centre de Recherches sur les Macromoleculécules Végétales, Centre Nationale des Researches Scientifique, BP 53, F-38041 Grenoble cedex 9, France