1. Interfaculty Institute for Biochemistry, University of Tübingen, D-72076 Tübingen, Germany
2. Institute for Physical and Theoretical Chemistry, University of Tübingen, D-72076 Tübingen, Germany
3. Division of Virology, Department of Clinical Microbiology, University of Umeå, SE-90185 Umeå, Sweden
4. Vanderbilt University School of Medicine, Nashville, Tennessee 37232