REGULATION OF DCT GENES IN THE RHIZOBIUM-MELILOTI ALFALFA INTERACTION

In order to support symbiotic N-2 fixation, Rhizobium meliloti must be able to utilize the C-4-dicarboxylic acids provided by its legume hos t, alfalfa. These compounds are taken up via a single transport protei n, DctA. Transcription from the dctA promoter is positively regulated by the DctB/DctD two-component system. In response to dicarboxylic aci ds, the transmembrane sensor DctB, activates the transcriptional activ ator DctD, which together with sigma(54) holoenzyme initiates transcri ption of dctA. In bacteroids an alternative mode of activation has als o been implicated in dctA expression and the exact nature of this syst em remains to be elucidated. Evidence also suggests that expression of the dctA promoter can be influenced negatively by other DNA regulator y proteins.