Anaerobic growth of Escherichia coli on D-tartrate depends on the fumarate carrier DcuB and fumarase, rather than the L-tartrate carrier TtdT and L-tartrate dehydratase.

Escherichia coli is able to grow under anaerobic conditions on D: -tartrate when glycerol is supplied as an electron donor (D-tartrate fermentation). D-Tartrate was converted to succinate. Growth was lost in strains deficient for DcuB, the fumarate/succinate antiporter of fumarate respiration. The L-tartrate/succinate antiporter TtdT of L-tartrate fermentation, or the C4-dicarboxylate carriers DcuA and DcuC, were not able to support D-tartrate transport and fermentation. Deletion of fumB demonstrated, that fumarase B is required for growth on D-tartrate. The mutant lost most (about 79%) of D-tartrate dehydratase activity. L-Tartrate dehydratase (TtdAB), and fumarase A or C, showed no or only a small contribution to D-tartrate dehydratase activity. Therefore D-tartrate is metabolised by a sequence of reactions analogous to that from L-tartrate fermentation, including dehydration to oxaloacetate, which is then converted to malate, fumarate and succinate. The stereoisomer specific carrier TtdT and dehydratase TtdAB of L-tartrate fermentation are substituted by enzymes from general anaerobic fumarate metabolism, the antiporter DcuB and fumarase B, which have a broader substrate specificity. No D-tartrate specific carriers and enzymes are involved in the pathway.

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