1. Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3200
2. Department of Plant and Microbial Biology, University of California, Berkeley, California 94720-3102
3. Laboratory of Genetics, University of Wisconsin, Madison, Wisconsin 53706
4. Delaware Biotechnology Institute, University of Delaware, Newark, Delaware 19711
5. Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403