Rhizobium meliloti and Rhizobium leguminosarum dctD gene products bind to tandem sites in an activation sequence located upstream to σ⁵⁴-dependent dctA promoters

Free-living rhizobia transport external C₄-dicarboxylates to use as sole carbon sources, and uptake of these compounds is essential for nitrogen fixation by rhizobial bacteroides. In both Rhizobium leguminosarum and Rhizobium meliloti, the genes dctB and dctD are believed to form an ntrB/ntrC-like two-component system which regulates the synthesis of a C₄-dicarboxylate transport protein encoded by dctA. Here we confirm the identity of σ⁵⁴-dependent promoters previously hypothesized for the R. leguminosarum and R. meliloti dctA genes and demonstrate that repeated, partial dyad symmetry elements located about 75 base pairs upstream of each promoter are essential for fully regulated transcription. Furthermore, we show that both repeats bound dctD protein and that together they resulted in succinate-sensitive transcription when placed upstream of another σ⁵⁴ consensus promoter, that of R. meliloti nifH.