DcuB of Escherichia coli catalyzes C 4 -dicarboxylate/succinate antiport during growth by fumarate respiration. The expression of genes of fumarate respiration, including the genes for DcuB (dcuB) and fumarate reductase (frdABCD) is transcriptionally activated by C 4 -dicarboxylates via the DcuS-DcuR two-component system, comprising the sensor kinase DcuS, which contains a periplasmic sensing domain for C 4 -dicarboxylates. Deletion or inactivation of dcuB caused constitutive expression of DcuSregulated genes in the absence of C 4 -dicarboxylates. The effect was specific for DcuB and not observed after inactivation of the homologous DcuA or the more distantly related DcuC transporter. Random and site-directed mutation identified three point mutations (T394I, D398N, and K353A) in DcuB that caused a similar derepression as dcuB deletion, whereas the transport activity of the DcuB mutants was retained. Constitutive expression in the dcuB mutants depended on the presence of a functional DcuS-DcuR two-component system. Mutation of residues E79A, R83A, and R127A of DcuB, on the other hand, inactivated growth by fumarate respiration and transport of [ 14 C]succinate, whereas the expression of dcuBlacZ was not affected. Therefore, the antiporter DcuB is a bifunctional protein and has a regulatory function that is independent from transport, and sites for transport and regulation can be differentiated.The fumarate/succinate antiporter DcuB (dicarboxylate uptake) of Escherichia coli catalyzes the uptake of external C 4 -dicarboxylates like fumarate, L-malate, or aspartate (1-3). Fumarate is used as an electron acceptor in fumarate respiration by fumarate reductase, which carries the active site at the cytoplasmic side of the membrane (for reviews, see Refs. 4 -6). L-Malate and aspartate are converted to fumarate by fumarase and aspartase, respectively, and are then metabolized in the same way as fumarate. Fumarate respiration results in the generation of a proton potential and drives ATP synthesis and growth of the bacteria. The product of fumarate respiration, succinate, is not further catabolized in anaerobic growth and is excreted by DcuB. DcuB, therefore, is responsible for the substrate/product antiport of fumarate, L-malate, or aspartate against succinate. DcuB and fumarase B are encoded by the dcuB fumB gene cluster (1, 7).