1. Biochemistry Option, California Institute of Technology, Mail Code 147-75, Pasadena, California 91125, Division of Chemistry and Chemical Engineering, California Institute of Technology, Mail Code 147-75CH, Pasadena, California 91125, Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697-3900, and Howard Hughes Medical Institute and Division of Chemistry and Chemical Engineering, California Institute of Technology, Mail Code 147-75CH, Pasadena, California 91125