1. Department of Chemistry, Department of Biological Engineering, and Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Department of Biochemistry and Molecular Pharmacology, and Chemical Biology Program, University of Massachuetts Medical School, Worcester, Massachusetts 01605, and Center for Advanced Research in Biotechnology, University of Maryland Biotechnology Institute, 9600 Gudelsky Drive, Rockville, Maryland 20850