1. Contribution from the Plant Physiology, Biochemistry, and Molecular Biology Program, University of Kentucky, Lexington, Kentucky 40546, Structural Biology Laboratory, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, California 92037, and Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093