1. Department of Chemistry and Chemical Biology, Harvard University , 12 Oxford Street , Cambridge , MA 02138 , USA
2. Department of Molecular Biology and Center for Computational and Integrative Biology, Massachusetts General Hospital , 185 Cambridge Street , Boston , MA 02114 , USA
3. Department of Genetics, Harvard Medical School , 77 Avenue Louis Pasteur , Boston , MA 02115 , USA
4. Howard Hughes Medical Institute, Department of Chemistry, The University of Chicago , Chicago , IL 60637 , USA