1. Department of Molecular Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Goettingen, Germany, Institute of Molecular Genetics, Russian Academy of Sciences, Kurchatov's Square, 123182 Moscow, Russia, and Biophysical Engineering Group, Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands