1. Graduate Option in Biochemistry, 147-75CH, California Institute of Technology, Pasadena, California 91125, Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, and Howard Hughes Medical Institute, Division of Chemistry and Chemical Engineering, 147-75CH, California Institute of Technology, Pasadena, California 91125