The dcuD (former yhcL) gene product of Escherichia coli as a member of theDcuC family of C4-dicarboxylate carriers: lack of evident expression

The dcuD gene (formerly yhcL) of Escherichia coli shows significant sequenc e similarity only to the dcuC gene of E. coli, which encodes a C4-dicarboxy late carrier (DcuC) that functions during anaerobic growth. Inactivation of dcuD had no effect on the growth of E. coli under a large number of condit ions and led to no detectable changes in phenotype. Translational dcuD'-'la cZ gene fusions were not significantly expressed in the presence of dicarbo xylates or monocarboxylates under oxic or anoxic conditions. Other potentia l substrates such as amino sugar derivatives, amino acids, and alpha-aspart yl dipeptides also did not lead to expression of dcuD. Changes in medium co mposition, pH, ionic strength, and temperature had no significant effects o n dcuD expression. A dcuD gene amplified from a natural isolate of E. coli was not expressed in wild-type and E. coli K-12 backgrounds. Cloning of dcu D behind an inducible promoter resulted in the synthesis of a protein of th e expected size (49 kDa), which, however, did not complement for the loss o f DcuC or other C4-dicarboxylate carriers. It is suggested that dcuD encode s a protein of the DcuC family of anaerobic C4-dicarboxylate carriers and t hat dcuD is not significantly expressed or is expressed only under conditio ns not related to carboxylate metabolism. When two adjacent open reading fr ames (y0585 and y0586) from Haemophilus influenzae are fused, the re suitin g hypothetical protein has sequence similarity to DcuC and DcuD.