Dicarboxylic Acid Utilization and Nitrogen Fixation Efficiency in Rhizobium-Legume Symbiosis

Abstract

Two separate regions of Rhizobium meliloti DNA which encode genes involved in the transport of dicarboxylic acids have been partially characterised. One region (pMC5) is approximately 27 kb long and contains sequences homologous to the Rhizobium leguminosarum dctA, B and D genes. The other region (pRK290:4:46) is approximately 40 kb long and contains no sequences homologous to the known dct genes. Plasmid pMC5 complements all R. meliloti Dct− strains tested while plasmid pRK290:4:46 complements only specific Dct− strains. Plasmid pRK290:4:46, but not pMC5, has the ability to increase the succinate uptake activity of both Rhizobium and Bradyrhizobium strains into which it has been mobilised. Bradyrhizobium japonicum strains containing plasmid pRK290:4:46 also exhibit an increased nitrogen fixation activity. In addition to characterising the R. meliloti dct regions, the levels of expression of nif-specific genes in R. meliloti Dct− strains have also been investigated using gene fusions. The levels of expression of translational fusions of the R. meliloti symbiotic promoters P1 (nif HDK), P2 (fix ABCX) and PnifA (nifA) in bacteroids formed by Dct− strains are very low compared to the levels found in bacteroids formed by the corresponding wild type strains. These results suggest that a regulatory link may exist between the utilisation of dicarboxylic acids and expression of nitrogen fixation genes.