1. Basic Sciences Division, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109–1024 (C.M.M., E.A.G., S.H.);
2. Molecular and Cellular Biology Program (C.M.M.) and
3. Department of Botany (L.C.), University of Washington, Seattle, Washington 98195; and
4. Howard Hughes Medical Institute Research Laboratories, Seattle, Washington 98109 (S.H.)