Function and Regulation of the C4-Dicarboxylate Transporters in .

C4-dicarboxylates are important molecules for the human pathogen , as they are used as carbon and electron acceptor molecules, as sugars cannot be utilized by this microaerophilic organism. Based on the genome analysis, may possess five different C4-dicarboxylate transporters: DctA, DcuA, DcuB, and two homologs of DcuC. Here, we investigated the regulation and function of various C4-dicarboxylate transporters in . Transcription of the and homologs is constitutive, while and are both directly regulated by the two-component RacR/RacS system in response to limited oxygen availability and the presence of nitrate. The DctA transporter is the only C4-dicarboxylate transporter to allow to grow on C4-carbon sources such as aspartate, fumarate, and succinate at high oxygen levels (10% O) and is indispensable for the uptake of succinate from the medium under these conditions. Both DcuA and DcuB can sequester aspartate from the medium under low-oxygen conditions (0.3% O). However, under these conditions, DcuB is the only transporter to secrete succinate to the environment. Under low-oxygen conditions, nitrate prevents the secretion of succinate to the environment and was able to overrule the phenotype of the C4-transporter mutants, indicating that the activity of the aspartate-fumarate-succinate pathway in is strongly reduced by the addition of nitrate in the medium.