Laboratoire de Biologie Microbienne, Université de Lausanne, CH-1015 Lausanne , Switzerland
For correspondence: Dieter Haas. Tel: +41 21 692 56 31. Fax: +41 21 692 56 35. e-mail: Dieter.Haas{at}lbm.unil.ch
a Present address: Nestlé Research Center, PO Box 44, CH-1000 Lausanne 26, Switzerland
b Present address: Aventis Pharma Deutschland, Functional Genomics, Industriepark Hoechst, D-65926 Frankfurt, Germany