1. Biophysics Group, Department of Physics and Astronomy, Faculty of Sciences, VU University, Amsterdam, The Netherlands
2. Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, Illinois 60637
3. Commissariat à l’Énergie Atomique, Institut de Biologie et de Technologie de Saclay, F-91191, Gif-sur-Yvette Cedex, France
4. The Committee on Microbiology, The University of Chicago, Chicago, Illinois 60637