1. Biochemistry Center of Heidelberg University, D-69120 Heidelberg, Germany
2. Structural and Computational Biology Unit, European Molecular Biology Laboratory, D-69117 Heidelberg, Germany
3. Division of Biophysics of Macromolecules, German Center Research Center, D-69120 Heidelberg, Germany
4. Department of Biochemistry and Biophysics, Oregon State University, Corvallis, OR 97331