Inositol Catabolism, a Key Pathway in Sinorhizobium meliloti for Competitive Host Nodulation

The nitrogen-fixing symbiont of alfalfa, Sinorhizobium meliloti, is able to use myo-inositol as the sole carbon source. Putative inositol catabolism genes (iolA and iolRCDEB) have been identified in the S. meliloti genome based on their similarities with the Bacillus subtilis iol genes. In this study, functional mutational analysis revealed that the iolA and iolCDEB genes are required for growth not only with the myo-isomer but also for growth with scyllo- and D-chiro-inositol as the sole carbon source. An additional, hypothetical dehydrogenase of the IdhA/MocA/GFO family encoded by the smc01163 gene was found to be essential for growth with scyllo-inositol, whereas the idhA-encoded myo-inositol dehydrogenase was responsible for the oxidation of D-chiro-inositol. The putative regulatory iolR gene, located upstream of iolCDEB, encodes a repressor of the iol genes, negatively regulating the activity of the myo- and the scyllo-inositol dehydrogenases. Mutants with insertions in the iolA, smc01163, and individual iolRCDE genes could not compete against the wild type in a nodule occupancy assay on alfalfa plants. Thus, a functional inositol catabolic pathway and its proper regulation are important nutritional or signaling factors in the S. meliloti-alfalfa symbiosis.