1. X. Zhu, X. Zhao, W. F. Burkholder, A. Gragerov, M. E. Gottesman, and W. A. Hendrickson are in the Department of Biochemistry and Molecular Biophysics, College of Physicians and Surgeons, Columbia University, New York, NY 10032, USA. X. Zhu and W. A. Hendrickson are also with the Howard Hughes Medical Institute, Columbia University. W. F. Burkholder and M. E. Gottesman are also in the Institute of Cancer Research, Columbia University, New York, NY 10032, USA.
2. C. M. Ogata is with the Howard Hughes Medical Institute, National Synchrotron Light Source, Brookhaven National Laboratory, Stony Brook, New York, USA.