1. Department of Microbiology and Genetics, Berlin University of Technology, Seestr. 13, 13353 Berlin, Germany; Laboratory of Molecular Cell Biology, Institute of Botany and Microbiology, Katholieke Universiteit Leuven, Kasteelpark Arenberg 31, bus 2438, B-3001 Heverlee, Flanders, Belgium; and Department of Molecular Microbiology, VIB, Kasteelpark Arenberg 31, bus 2438, B-3001 Heverlee, Flanders, Belgium