Asuc_0142 of 130Z is the l-aspartate/C4-dicarboxylate exchanger DcuA.

Anaerobic bacteria often use antiporters DcuB (malate/succinate antiport) or DcuA (l-aspartate/succinate antiport) for the excretion of succinate during fumarate respiration. The rumen bacterium is able to produce large amounts of succinate by fumarate respiration, using the DcuB-type transporter DcuE for l-malate/succinate antiport. Asuc_0142 was annotated as a second DcuB-type transporter. Deletion of Asuc_0142 decreased the uptake rate for l-[C]aspartate into cells. Properties of transport by heterologously expressed Asuc_0142 were investigated in an mutant deficient of anaerobic C4DC transporters. Expression of Asuc_0142 resulted in high uptake activity for l-[C]fumarate or l-[C]aspartate, but the former showed a strong competitive inhibition by l-aspartate. In loaded with l-[C]aspartate, [C]succinate or [C]fumarate, extracellular C4DCs initiated excretion of the intracellular substrates, with a preference for l-aspartate/succinate or l-aspartate/fumarate antiport. These findings indicate that Asuc_0142 represents a DcuA-type transporter for l-aspartate uptake and l-aspartate/C4DC antiport, differentiating it from the DcuB-type transporter DcuE for l-malate/succinate antiport. Sequence analysis and predicted structural characteristics confirm structural similarity of Asuc_0142 to DcuA, and Asuc_0142 was thus re-named as DcuA. The bovine rumen fluid contains l-aspartate (99.6 µM), whereas fumarate and l-malate are absent. Therefore, bovine rumen colonisers depend on l-aspartate as an exogenous substrate for fumarate respiration. encodes HemG (protoporphyrinogen oxidase) and PyrD (dihydroorotate dehydrogenase) for haem and pyrimidine biosynthesis. The enzymes require fumarate as an electron acceptor, suggesting an essential role for l-aspartate, DcuA, and fumarate respiration for growing in the bovine rumen.