Genetic analysis and regulation of the Rhizobium meliloti genes controlling C4-dicarboxylic acid transport.

The genes controlling the transport of C4-dicarboxylic acids from Rhizobium meliloti have been cloned and analysed. The nucleotide sequence of the control region of the structural dctA and the regulatory dctBD genes has been determined. Comparison with the Rhizobium leguminosarum dct genes revealed a high degree of homology. Gene fusions to the enteric lacZY reporter gene were constructed and the expression of the dctA and dctBD genes studied under various physiological conditions. In free-living cells, the regulatory dctBD genes are absolutely required for the expression of the dctA gene. In the root nodule environment, a dctA::lacZY gene fusion was found to be expressed in an R. meliloti strain mutated in both the dctB and dctD genes, but not in a strain mutated in the dctB gene alone. The presence of the conserved upstream NifA-binding sites on the dctA promoter sequence, coupled with the fact that the dctA::lacZY gene fusion is not expressed in root nodules formed by a nifA mutant strain of R. meliloti, supports the suggestion that NifA may be involved in the symbiotic expression of dctA in the absence of the regulatory dctBD genes. Under micro-aerobic conditions, however, NifA induction alone is not sufficient for expression of the dctA promoter, even though the NifA-dependent nifHDK promoter is highly expressed under these conditions.