1. M E Müller Institute for Structural Biology, Biozentrum, University of Basel, Klingelbergstrasse 70 CH‐4056 Basel Switzerland
2. Max‐Planck‐Institute of Molecular Cell Biology and Genetics, Pfotenhauerstrasse 108 D‐01307 Dresden Germany
3. Institute of Microbiology, ETH, Schmelzbergstrasse 7 CH‐8092 Zürich Switzerland