Transcriptional Activation of the Aldehyde Reductase YqhD by YqhC and Its Implication in Glyoxal Metabolism of Escherichia coli K-12

Abstract

ABSTRACT The reactive α-oxoaldehydes such as glyoxal (GO) and methylglyoxal (MG) are generated in vivo from sugars through oxidative stress. GO and MG are believed to be removed from cells by glutathione-dependent glyoxalases and other aldehyde reductases. We isolated a number of GO-resistant (GO r ) mutants from Escherichia coli strain MG1655 on LB plates containing 10 mM GO. By tagging the mutations with the transposon Tn phoA -132 and determining their cotransductional linkages, we were able to identify a locus to which most of the GO r mutations were mapped. DNA sequencing of the locus revealed that it contains the yqhC gene, which is predicted to encode an AraC-type transcriptional regulator of unknown function. The GO r mutations we identified result in missense changes in yqhC and were concentrated in the predicted regulatory domain of the protein, thereby constitutively activating the product of the adjacent gene yqhD . The transcriptional activation of yqhD by wild-type YqhC and its mutant forms was established by an assay with a β-galactosidase reporter fusion, as well as with real-time quantitative reverse transcription-PCR. We demonstrated that YqhC binds to the promoter region of yqhD and that this binding is abolished by a mutation in the potential target site, which is similar to the consensus sequence of its homolog SoxS. YqhD facilitates the removal of GO through its NADPH-dependent enzymatic reduction activity by converting it to ethadiol via glycolaldehyde, as detected by nuclear magnetic resonance, as well as by spectroscopic measurements. Therefore, we propose that YqhC is a transcriptional activator of YqhD, which acts as an aldehyde reductase with specificity for certain aldehydes, including GO.