1. Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720–3204, USA.
2. Biophysics Graduate Group, University of California, Berkeley, CA 94720–3204, USA.
3. Department of Physics, University of California, Berkeley, CA 94720–3204, USA.
4. Howard Hughes Medical Institute, University of California, Berkeley, CA 94720–3204, USA.
5. Division of Molecular Genetics, Department of Biochemistry, University of Oxford, OX1 3QU, UK.