1. Division
of Biology, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States
2. Departments
of Chemistry and Biochemistry, University of Wisconsin—Madison, 433 Babcock Drive, Madison, Wisconsin 53706, United States
3. Howard Hughes Medical Institute, 1200 East California Boulevard, Pasadena, California 91125, United States
4. Department
of Chemistry and Biochemistry and Molecular Biophysics Program, University of Colorado, Boulder, Colorado 80309, United States
5. Department
of Chemistry, University of Massachusetts, 710 North Pleasant Street, Amherst, Massachusetts 01003, United States