1. Laboratory for Microbiology, Swammerdam Institute for Life Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, Biophysics Group, Department of Physics, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany, and Plant Physiology, Freie Universität Berlin, Königin-Luise-Strasse 12-16, D-14195 Berlin, Germany