Rhizobium meliloti Genes for Exopolysaccharide Synthesis and Nodule Infection Located on Megaplasmid 2 Are Actively Transcribed during Symbiosis. Mathias Keller. Lehrstuhl für Genetik, Fakultät für Biologie, Universität Bielefeld, Postfach 8640, D-4800 Bielefeld 1, Federal Republic of Germany. Peter Müller, Reinbhard Simon, and Alfred Pühler. Lehrstuhl für Genetik, Fankultät für Biologie, Universität Bielefeld, Postfach 8640, D-4800 Bielefeld 1, Federal Republic of Germany.. MPMI 1:267-274. Accepted 21 October 1988. Copyright 1988 The American Phytopathological Society.

A 7.8-kb DNA fragment of the second megaplasmid of Rhizobium meliloti 2011 known to carry genes for the synthesis of acidic exopolysaccharide (EPS) and for nodule infection (Inf) was mutagenized in Escherichia coli by transposon Tn5-B20. Transfer of the mutagenized fragment to R. meliloti resulted in the construction of merodiploid and homogenotized strains. Of eight homogenotized strains, five turned out to be of the Inf^- EPS^- type. Four of them were mutated in a region characterized by the R. meliloti mutant 0540 (Müller, Hynes, Kapp, Niehaus, and Pühler 1988, Mol. Gen. Genet. 211:17). The fifth Inf^- EPS^- mutant (Rm154), mutated in a different region, could be stained with Congo red and aniline blue, indicating an unusual surface polysaccharide. Two other mutants of this second region exhibited no defects in symbiosis but differed from the wild type in the level of EPS production. The promoterless lacZ gene of transposon Tn5-B20 was used to monitor the transcriptional activity at the insertion sites of merodiploid and homogenotized R. meliloti strains in the free-living as well as in the bacteroid state. Three regions showed a high level of transcription in the free-living state. The transcription of these regions was not switched off during symbiosis.