Regulation of tartrate metabolism by TtdR and relation to the DcuS–DcuR-regulated C4-dicarboxylate metabolism of Escherichia coli

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 LysR-type gene regulator TtdR in the presence of l- and meso-tartrate, and repressed by O2 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 O2, 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-tartrate-dependent 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 meso-tartrate) of the stimuli perceived by the sensor DcuS, which responds to all C4-dicarboxylates; both systems apparently communicate by mutual regulation of the regulatory genes.

The l-Tartrate/Succinate Antiporter TtdT (YgjE) of l-Tartrate Fermentation in Escherichia coli

ABSTRACT Escherichia coli ferments l-tartrate under anaerobic conditions in the presence of an additional electron donor to succinate. The carrier for l-tartrate uptake and succinate export and its relation to the general C4-dicarboxylate carriers DcuA, DcuB, and DcuC were studied. The secondary carrier TtdT, encoded by the ttdT (previously called ygjE) gene, is required for the uptake of l-tartrate. The ttdT gene is located downstream of the ttdA and ttdB genes, encoding the l-tartrate dehydratase TtdAB. Analysis of mRNA by reverse transcription-PCR showed that ttdA, ttdB, and ttdT are cotranscribed. Deletion of ttdT abolished growth by l-tartrate and degradation of l-tartrate completely. Bacteria containing TtdT catalyze l-tartrate or succinate uptake and specific heterologous l-tartrate/succinate antiporting. d-Tartrate is not a substrate for TtdT. TtdT operates preferentially in the direction of tartrate uptake and succinate excretion. The Dcu carriers do not support anaerobic growth on l-tartrate or l-tartrate transport. TtdT is related in sequence and function to CitT, which catalyzes heterologous citrate/succinate antiporting in citrate fermentation.

Functional identification of ygiP as a positive regulator of the ttdA-ttdB-ygjE operon

Functional characterization of unknown genes is currently a major task in biology. The search for gene function involves a combination of various in silico, in vitro and in vivo approaches. Available knowledge from the study of more than 21 LysR-type regulators in Escherichia coli has facilitated the classification of new members of the family. From sequence similarities and its location on the E. coli chromosome, it is suggested that ygiP encodes a lysR regulator controlling the expression of a neighbouring operon; this operon encodes the two subunits of tartrate dehydratase (TtdA, TtdB) and YgiE, an integral inner-membrane protein possibly involved in tartrate uptake. Expression of tartrate dehydratase, which converts tartrate to oxaloacetate, is required for anaerobic growth on glycerol as carbon source in the presence of tartrate. Here, it has been demonstrated that disruption of ygiP, ttdA or ygjE abolishes tartrate-dependent anaerobic growth on glycerol. It has also been shown that tartrate-dependent induction of the ttdA-ttdB-ygjE operon requires a functional YgiP.