The gene products of the Rhizobium meliloti dctB and dctD genes, which control the expression of the C-4-dicarboxylic acid transporter DctA, were overproduced in Escherichia coli and purified. The purified sensor protein, DctB, was shown to have autophosphorylation activity in vitro and could subsequently phosphorylate the transcriptional activator, DctD. The presence of C-4-dicarboxylic acids did not affect either reaction. In vitro experiments aimed at investigating 'crosstalk' between cognate components demonstrated that the phospho-transfer activity was specific between DctB and DctD. Studies on truncated versions of the DctB protein in vitro revealed that the cytoplasmic domain of DctB had strong autophosphorylation activity. Data from gel retardation experiments demonstrated that once the activator protein, DctD, was phosphorylated it had increased affinity for binding to the dctA promoter DNA.