1. Plant Biology Laboratory (D.M.F., J.L., J.C.),
2. Department of Biology, University of California at San Diego, La Jolla, California 92037 (D.M.F.)
3. Molecular Biology and Virology Laboratory (C.A.P.J., T.H.), and
4. Howard Hughes Medical Institute (J.C.), The Salk Institute for Biological Studies, La Jolla, California 92037; and