1. Contribution from the Max Planck Institute for Terrestrial Microbiology, Karl-von-Frisch Strasse, D-35043 Marburg, Germany, Max Planck Institute for Bioinorganic Chemistry, Stift Strasse 34-36, D-45470 Mülheim an der Ruhr, Germany, and Biochemistry, Swammerdam Institute for Life Sciences, University of Amsterdam, Plantage Muidergracht 12, NL-1018 TV Amsterdam, The Netherlands