1. Institute for Biology (Genetics), Humboldt-University Berlin, D-10115 Berlin, Germany
2. Max Delbrück Center for Molecular Medicine, D-13092 Berlin, Germany
3. Research Center for Infectious Diseases, University of Würzburg, D-97080 Wuerzburg, Germany
4. Institute for Molecular Infection Biology, University of Würzburg, D-97080 Wuerzburg, Germany