1. H. Schindelin, C. Kisker, and D. C. Rees are in the Division of Chemistry and Chemical Engineering, 147-75CH, California Institute of Technology, Pasadena, California 91125, USA.
2. J. Hilton and K. V. Rajagopalan are in the Department of Biochemistry, Duke University, Medical Center, Durham, North Carolina 27710, USA.