Escherichia coli catabolizes L-tartrate under anaerobic conditions to oxaloacetate by the use of L-tartrate/succinate antiporter TtdT and L-tartrate dehydratase TtdAB. Subsequently, L-malate is channelled into fumarate respiration and degraded to succinate by the use of fumarase FumB and fumarate reductase FrdABCD. The genes encoding the latter pathway (dcuB, fumB and frdABCD) are transcriptionally activated by the DcuS-DcuR two-component system. Expression of the L-tartrate-specific ttdABT operon encoding TtdAB and TtdT was stimulated by the LysRtype gene regulator TtdR in the presence of L-and meso-tartrate, and repressed by O 2 and nitrate. Anaerobic expression required a functional fnr gene, and nitrate repression depended on NarL and NarP. Expression of ttdR, encoding TtdR, was repressed by O 2 , nitrate and glucose, and positively regulated by TtdR and DcuS. Purified TtdR specifically bound to the ttdR-ttdA promoter region. TtdR was also required for full expression of the DcuS-DcuR-dependent dcuB gene in the presence of tartrate. Overall, expression of the ttdABT genes is subject to L-/meso-tartratedependent induction, and to aerobic and nitrate repression. The control is exerted directly at ttdA and in addition indirectly by regulating TtdR levels. TtdR recognizes a subgroup (L-and mesotartrate) of the stimuli perceived by the sensor DcuS, which responds to all C 4 -dicarboxylates; both systems apparently communicate by mutual regulation of the regulatory genes.
D-malate to pyruvate. Induction of dmlA encoding DmlA required an intact dmlR (formerly yeaT) gene, which encodes DmlR, a LysR-type transcriptional regulator. Induction of dmlA by DmlR required the presence of D-malate or L-or meso-tartrate, but only D-malate supported aerobic growth. The regulator of general C 4 -dicarboxylate metabolism (DcuS-DcuR twocomponent system) had some effect on dmlA expression. The anaerobic L-tartrate regulator TtdR or the oxygen sensors ArcB-ArcA and FNR did not have a major effect on dmlA expression. DmlR has a high level of sequence identity (49%) with TtdR, the L-and meso-tartrate-specific regulator of L-tartrate fermentation in E. coli. dmlA was also expressed at high levels under anaerobic conditions, and the bacteria had D-malate dehydrogenase activity. These bacteria, however, were not able to grow on D-malate since the anaerobic pathway for D-malate degradation has a predicted yield of <0 ATP/mol D-malate. Slow anaerobic growth on D-malate was observed when glycerol was also provided as an electron donor, and D-malate was used in fumarate respiration. The expression of dmlR is subject to negative autoregulation. The network for regulation and coordination of the central and peripheral pathways for C 4 -dicarboxylate metabolism by the regulators DcuS-DcuR, DmlR, and TtdR is discussed.Escherichia coli is able to use a large number of C 4 -dicarboxylates for growth under aerobic and anaerobic conditions. Under aerobic conditions the main C 4 -dicarboxylates used for growth are fumarate, succinate, and L-malate. The metabolism of these substrates involves uptake by the DctA transporter (encoded by the dctA gene), the citric acid cycle, and the malic enzyme/pyruvate dehydrogenase bypass to produce acetyl coenzyme A (acetyl-CoA) for feeding the citric acid cycle (9,33,35,41). Anaerobically, fumarate is metabolized by fumarate respiration using the fumarate reductase FrdABCD (encoded by the frdABCD genes) and the fumarate/succinate antiporter DcuB (encoded by the dcuB gene) (for reviews, see references 22, 47, and 48). The C 4 -dicarboxylates L-malate and aspartate are transported to the cytoplasm by DcuB and then converted by fumarase B (fumB) and aspartase AspA to fumarate, which is used for fumarate respiration. In L-tartrate fermentation, the substrate is taken up by the L-tartrate/succinate antiporter TtdT (encoded by ttdT) and converted by the L-tartrate dehydratase TtdAB (encoded by ttdAB) to oxaloacetate (24, 38). Oxaloacetate is then metabolized by reactions of the central metabolism to fumarate and converted to succinate by fumarate respiration.The pathways are regulated at the transcriptional level in response to O 2 , nitrate, and the C 4 -dicarboxylates. Many genes involved in aerobic catabolism, including dctA, are repressed under anaerobic conditions by the ArcB-ArcA two-component system (9,20). The genes for fumarate respiration and related enzymes (dcuB, frdABCD, fumB, ttdAB, and ttdT) are subject to anaerobic induction by the oxygen sensor FNR and to nitrate repre...
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