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Global Changes in Gene Expression in Sinorhizobium meliloti 1021 under Microoxic and Symbiotic Conditions

¹Institut für Genomforschung, Centrum für Biotechnologie, Universität Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany; ²Lehrstuhl für Genetik, Universität Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany; ³International NRW Graduate School in Bioinformatics and Genome Research, Universität Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany; ⁴Laboratoire des Interactions Plantes-Microorganismes, UMR INRA 441-CNRS 2594, BP27-31326 Castanet-Tolosan cedex, France

Sinorhizobium meliloti is an α-proteobacterium that alternates between a free-living phase in bulk soil or in the rhizosphere of plants and a symbiotic phase within the host plant cells, where the bacteria ultimately differentiate into nitrogen-fixing organelle-like cells, called bacteroids. As a step toward understanding the physiology of S. meliloti in its free-living and symbiotic forms and the transition between the two, gene expression profiles were determined under two sets of biological conditions: growth under oxic versus microoxic conditions, and in free-living versus symbiotic state. Data acquisition was based on both macro- and microarrays. Transcriptome profiles highlighted a profound modification of gene expression during bacteroid differentiation, with 16% of genes being altered. The data are consistent with an overall slow down of bacteroid metabolism during adaptation to symbiotic life and acquisition of nitrogen fixation capability. A large number of genes of unknown function, including potential regulators, that may play a role in symbiosis were identified. Transcriptome profiling in response to oxygen limitation indicated that up to 5% of the genes were oxygen regulated. However, the microoxic and bacteroid transcriptomes only partially overlap, implying that oxygen contributes to a limited extent to the control of symbiotic gene expression.

Additional keywords: macroarray, root nodule.