1. Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142, United States
2. Department
of Chemistry, Center for Molecular Discovery (BU-CMD), Boston University, Boston, Massachusetts 02215, United States
3. Department
of Biological Sciences, The University of Alabama, Tuscaloosa, Alabama 35487, United States
4. Department
of Chemistry, MIT, Cambridge, Massachusetts 02139, United States
5. Department
of Biology, MIT, Cambridge, Massachusetts 02139, United States
6. Howard Hughes Medical Institute, Cambridge, Massachusetts 02139, United States