1. Department of Microbiology; Radboud University Nijmegen; Heyendaalseweg 135 6525 AJ Nijmegen The Netherlands
2. Department of Biomolecular Mechanisms; Max-Planck Institute for Medical Research; Jahnstrasse 29 D-69120 Heidelberg Germany
3. Department of General Instruments; Radboud University Nijmegen; Heyendaalseweg 135 6525 AJ Nijmegen The Netherlands