Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, 9751 NN Haren, , The Netherlands1
Author for correspondence: Juke S. Lolkema. Tel: +31 50 3632155. Fax: +31 50 3632154. e-mail: j.s.lolkema{at}biol.rug.nl