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 C ₄-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.