1. Department of Chemistry, Yale University, New Haven, CT 06520, USA.
2. Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY 10032, USA.
3. MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, UK.
4. Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, Stanford University, Stanford, CA 94305, USA.