1. Institute of Molecular Biology and Biophysics, Eidgenössische Technische Hochschule Zurich (ETH Zurich), 8093 Zurich, Switzerland.
2. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
3. Department of Molecular Biology and Genetics, Cornell University, Ithaca, NY 14853, USA.
4. Howard Hughes Medical Institute, Department of Molecular and Cell Biology, Department of Chemistry, University of California Berkeley; Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.