1. Department of Biochemistry, Physiology and Microbiology, University of Gent, Ledeganckstraat 35, 9000 Gent, Belgium, Department of Biochemistry, University of Arizona, Tucson, Arizona 85721, and Department of Microbiology, E. C. Slater Institute, BioCentrum, University of Amsterdam, Nieuwe Achtergracht 127, 1018 WS Amsterdam, The Netherlands