1. S. Liu, J. Widom, J. Clardy, Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853–1301, USA. C. W. Kemp, Kemp Biotechnologies, Incorporated, Frederick, MD 21704, USA. C. M. Crews, Department of Molecular, Cellular, and Developmental Biology, Kline Biology Tower, Yale University, New Haven, CT 06520–8103, USA.