1. Whitehead Institute for Biomedical Research,Cambridge, MA 02142.
2. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139.
3. Department of Biochemistry, University of Utah School of Medicine, Salt Lake City, UT 84132.
4. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139.
5. Department of Chemistry, University of Utah, Salt Lake City, UT 84112.