1. Max-Planck-Institut för Molekulare Genetik; Ihnestraße 73 Dahlem D-14195 Berlin Germany
2. Division of Molecular Microbiology; Biozentrum, University of Basel; Klingelbergstraße 50-70 CH-4056 Basel Switzerland
3. Laboratory for Protein Biochemistry, Ghent University; K. L. Ledeganckstraat 35 B-9000 Ghent Belgium
4. Institute of Structural Molecular Biology, Birkbeck and University College London; Malet St. London WC1E 7HX United Kingdom