Complementation of Rhizobium leguminosarum dct mutants with a cosmid bank yielded Rhizobium meliloti homologs of the dctA, dctB, and dctD genes. The genes dctB and dctD are thought to form a two-component system which responds to the presence of C4-dicarboxylates to regulate expression of a transport protein encoded by dctA. DNA sequence analysis showed that det coding and intergenic regions, including putative binding sites for the dctD protein and r54-RNA polymerase, were highly conserved between these two Rhizobium species. Mutation of R. meliloti dctD showed that it was not essential for symbiotic nitrogen fixation but was needed for growth on dctA promoter depends on an analog of the enteric alternative sigma factor r54 encoded by rpoN (also called ntrA and ginF) (60). Indeed, a DNA sequence very similar to the ur5 consensus promoter (28) was observed in the region between the R. leguminosarum dctA and dctB genes (56). Screening a transposon TnS insertion library for cells that were unable to express a dctA-IacZ fusion gene in the presence of succinate allowed the identification and subsequent characterization of rhizobial rpoN (60). However, this screen entailed a complex heterologous system in which R. leguminosarum dct genes were present in R. meliloti cells. The nature and organization of R. meliloti dct genes remain unknown.We have undertaken the isolation and characterization of R. meliloti dct genes, hoping that conservation between two closely related species would allow us to focus on key regulatory elements. Moreover, in R. meliloti the rhizobial rpoN gene and other two-component regulation systems (ntrB/ntrC [66] and fixLlfixJ [14]) have been characterized; thus, this work provides the basis for future comparative studies of the predicted signal-transducing domains and potential cross-talk interactions between the systems. We found that R. meliloti contains identically arranged homologs of the R. leguminosarum genes dctA, dctB, and dctD. Blocks of conserved DNA sequence that are separated from each other by nonconserved residues in the dctA-dctB intergenic region suggest elements that are probably involved in the regulation of transcription of these genes, among which are two DCTD-binding sites that precede a C54 consensus promoter for dctA (H. Ledebur, B. Gu, and B. T. Nixon, manuscript in preparation). We also found that in R. meliloti, dctD is required for basal and induced expression of dctA and that R. meliloti may contain dozens of genes that fall into the subclass of regulators that need rpoN to stimulate transcription.