ABSTRACT
The gene (dctA) encoding the aerobic C4-dicarboxylate transporter (DctA) of Escherichia coli was previously mapped to the 79-min region of the linkage map. The nucleotide sequence of this region reveals two candidates for the dctA gene: f428 at 79.3 min and theo157a-o424-o328 (or orfQMP) operon at 79.9 min. The f428 gene encodes a homologue of theSinorhizobium meliloti and Rhizobium leguminosarum H+/C4-dicarboxylate symporter, DctA, whereas the orfQMP operon encodes homologues of the aerobic periplasmic-binding protein- dependent C4-dicarboxylate transport system (DctQ, DctM, and DctP) ofRhodobacter capsulatus. To determine which, if either, of these loci specify the E. coli DctA system, the chromosomalf428 and orfM genes were inactivated by inserting Spr or Apr cassettes, respectively. The resulting f428 mutant was unable to grow aerobically with fumarate or malate as the sole carbon source and grew poorly with succinate. Furthermore, fumarate uptake was abolished in thef428 mutant and succinate transport was ∼10-fold lower than that of the wild type. The growth and fumarate transport deficiencies of the f428 mutant were complemented by transformation with an f428-containing plasmid. No growth defect was found for the orfM mutant. In combination, the above findings confirm that f428 corresponds to thedctA gene and indicate that the orfQMP products play no role in C4-dicarboxylate transport. Regulation studies with a dctA-lacZ (f428-lacZ) transcriptional fusion showed that dctA is subject to cyclic AMP receptor protein (CRP)-dependent catabolite repression and ArcA-mediated anaerobic repression and is weakly induced by the DcuS-DcuR system in response to C4-dicarboxylates and citrate. Interestingly, in a dctA mutant, expression ofdctA is constitutive with respect to C4-dicarboxylate induction, suggesting that DctA regulates its own synthesis. Northern blot analysis revealed a single, monocistronic dctA transcript and confirmed thatdctA is subject to regulation by catabolite repression and CRP. Reverse transcriptase-mediated primer extension indicated a single transcriptional start site centered 81 bp downstream of a strongly predicted CRP-binding site.
Inactivation and Regulation of the Aerobic C4-Dicarboxylate Transport (dctA) Gene ofEscherichia coli
