1. Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218.
2. Thomas Jenkins Department of Biophysics, Johns Hopkins University, Baltimore, Maryland 21218.
3. Biocrystallography Laboratory, V.A. Medical Center, Pittsburgh, Pennsylvania 15240, and a professor in the Department of Crystallography, University of Pittsburgh, Pittsburgh, Pennsylvania 15260.
4. Department of Physics, University of California at San Diego, La Jolla 92093.