DcuA of aerobically grown Escherichia coli serves as a nitrogen shuttle (L-aspartate/fumarate) for nitrogen uptake

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 (K_M = 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.