1. Institute of Microbiology and Genetics, Georg August University, D-37077 Göttingen, Germany.
2. Department of Bacteriology and Department of Genetics, University of Wisconsin-Madison, Madison, WI 53706, USA.
3. Department of Plant Pathology and Medical Microbiology and Immunology, University of Wisconsin-Madison, Madison, WI 53706, USA.
4. Deutsche Forschungsgemeinschaft (DFG) Research Center for Molecular Physiology of the Brain (CMPB), D-37077 Göttingen, Germany.