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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 Actinobacillus succinogenes 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 -[ 14 C]aspartate into A. succinogenes cells. Properties of transport by heterologously expressed Asuc_0142 were investigated in an Escherichia coli mutant deficient of anaerobic C4DC transporters. Expression of Asuc_0142 resulted in high uptake activity for l -[ 14 C]fumarate or l -[ 14 C]aspartate, but the former showed a strong competitive inhibition by l -aspartate. In E. coli loaded with l -[ 14 C]aspartate, [ 14 C]succinate or [ 14 C]fumarate, extracellular C4DCs initiated excretion of the intracellular substrates, with a preference for l -aspartate ex /succinate in or l -aspartate ex /fumarate in antiport. These findings indicate that Asuc_0142 represents a DcuA-type transporter for l -aspartate uptake and l -aspartate ex /C4DC in antiport, differentiating it from the DcuB-type transporter DcuE for l -malate ex /succinate in antiport. Sequence analysis and predicted structural characteristics confirm structural similarity of Asuc_0142 to DcuA, and Asuc_0142 was thus re-named as DcuA As . 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. A. succinogenes encodes HemG (protoporphyrinogen o