About the Authors
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, United States of America
Howard Hughes Medical Institute and the Department of Molecular and Cell Biology, University of California, Berkeley, California, United States of America
The authors have declared that no competing interests exist.
The author(s) have made the following declarations about their contributions: Conceived and designed the experiments: CB CCE DT. Performed the experiments: CB CCE. Analyzed the data: CB PG CCE EN DT. Wrote the paper: CB PG EN DT.