1. Department of Biology, Laboratory for Molecular Microbiology; Philipps-University Marburg; Karl-von-Frisch Str. 8 D-35043 Marburg Germany
2. Department of Microbiology and Immunobiology; Harvard Medical School; 77 Avenue Louis Pasteur Boston MA 02115-5701 USA
3. Department of General Microbiology; Institute of Microbiology and Genetics, Georg August University Göttingen; Griesebachstr, 8 D-37077 Göttingen Germany