1. Harvard-Smithsonian Center for Astrophysics, Harvard University, 60 Garden Street, Cambridge, MA 02138, USA
2. Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
3. Howard Hughes Medical Institute, Department of Molecular Biology and Center for Computational and Integrative Biology, Massachusetts General Hospital, Boston, Massachusetts 02114, USA
4. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA
5. Department of Genetics, Harvard Medical School, Boston, Massachusetts 02115, USA
6. Howard Hughes Medical Institute, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, USA