LAUSR

DcuA of Escherichia coli is known as an alternative C4‐dicarboxylate transporter for the main anaerobic C4‐dicarboxylate transporter DcuB. Since dcuA is expressed constitutively under aerobic and anaerobic conditions, DcuA was suggested to serve aerobically as a backup for the aerobic (DctA) transporter, or for the anabolic uptake of C4‐dicarboxylates. In this work, it is shown that DcuA is required for aerobic growth with L‐aspartate as a nitrogen source, whereas for growth with L‐aspartate as a carbon source, DctA was needed. Strains with DcuA catalyzed L‐aspartate and C4‐dicarboxylate uptake (like DctA), or an L‐aspartate/C4‐dicarboxylate antiport (unlike DctA). DcuA preferred L‐aspartate to succinate in transport (KM = 43 and 844 µM, respectively), whereas DctA has higher affinity for C4‐dicarboxylates like succinate compared to L‐aspartate. When L‐aspartate was supplied as the sole nitrogen source together with glycerol as the carbon source, L‐aspartate was taken up by the bacteria and fumarate (or L‐malate) was excreted in equimolar amounts. Both reactions depended on DcuA. L‐Aspartate was taken up in amounts required for nitrogen metabolism but not for carbon metabolism. Therefore, DcuA catalyzes an L‐aspartate/C4‐dicarboxylate antiport serving as a nitrogen shuttle for nitrogen supply without net carbon supply.