Analysis of C4-dicarboxylate transport genes in Rhizobium meliloti

A 5.1 kbp DNA fragment was isolated which complemented C4-dicarboxylate transport mutants (dct) of Rhizobium meliloti. Characterization of this fragment by subcloning, transposon mutagenesis, and complementation analysis revealed three loci, designated dctA, dctB, and dctD. TnphoA-generated alkaline phosphatase fusions to dctA suggested that this gene encodes the structural transport protein and allowed the determination of its direction of transcription. Analysis of the fusions in various mutant backgrounds demonstrated that dctB, dctD, and ntrA products are required for dctA expression. The dctA fusion was constitutively expressed in a dctA mutant background, but was not expressed in dctA dctB or dctA dctD double mutants. This suggests that the constitutive expression in a dctA mutant background is mediated through dctB and dctD. Three independent second-site Dct+ revertant mutations in ntrA mutant strains mapped to the dct locus. Succinate transport in these revertant strains was constitutive, whereas in the wild type, succinate transport was inducible. These results are consistent with the direct requirement of the ntrA gene product for dctA expression. Alfalfa plants inoculated with the dctB and dctD mutants showed reduced nitrogen-fixing activity. Nodules induced by dctA mutants failed to fix nitrogen. These symbiotic phenotypes are consistent with previous suggestions that dctA expression in bacteroids can occur independently of dctB and dctD.