1. Institute of Molecular Biology, University of Oregon, Eugene, OR 97403.
2. Institute of Molecular Biology and Department of Physics University of Oregon, Eugene, OR 97403.
3. Department of Biology and McCollum·Pratt Institute, The Johns Hopkins University, Baltimore, MD 21218.
4. Department 01 Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843.