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