The expression of the Rhizobium meliloti C4-dicarboxylic acid permease gene (dctA) is controlled by the sensor DctB and the transcriptional regulator, DctD. The R. meliloti Dct system has been reconstituted in Escherichia coli. Expression of the dctA promoter is DctBD dependent and is induced in the presence of C4-dicarboxylic acids (dCA). Other carbon sources also influence dctA expression. We demonstrate that the cAMP receptor protein (CRP) has a repressive effect on the dctA promoter. A mutated CRP molecule (CRP-H159L), unable to activate catabolic promoters (but still proficient in DNA binding), gives similar results. This suggests that the CRP-cAMP complex represses the dctA promoter activity by direct interaction with the DNA. Direct binding of the CRP-cAMP complex to the dctA promoter was confirmed in vitro by gel mobility-shift assays. Sequence analysis of the dctA promoter indicates that the most likely binding sites for CRP are the two confirmed DctD-binding sites. It is proposed that the CRP-cAMP complex competes with DctD for occupancy of these sites. Since in the presence of CRP-cAMP complex the uninduced levels of dctA expression are reduced, whereas induced levels are largely unaffected, such competition appears to be an essential regulatory feature of dctA expression.