1. Department of Molecular, Cellular, and Developmental Biology, Yale University, Box 208103, New Haven, CT 06520–8103, USA.
2. Department of Chemistry, Yale University, Box 208103, New Haven, CT 06520–8103, USA.
3. Howard Hughes Medical Institute, Yale University, Box 208103, New Haven, CT 06520–8103, USA.
4. Department of Molecular Biophysics and Biochemistry, Yale University, Box 208103, New Haven, CT 06520–8103, USA.