1. Division of Biological Sciences, University of California San Diego, La Jolla, CA 92093, USA.
2. School of Biological Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA.
3. Department of Biology, University of Maryland, College Park, MD 20742, USA.
4. Department of Physics, University of Maryland, College Park, MD 20742, USA.
5. Institut de Biologie, École Normale Supérieure, 75005 Paris, France.