1. P. Nissen, M. Kjeldgaard, S. Thirup, G. Polekhina, B. F. C. Clark, and J. Nyborg are in the Department of Biostructural Chemistry, Institute of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark.
2. L. Reshetnikova is at the Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, 32 Vavilov str., 117984 Moscow, Russia.