The FlxABCD ‐ HdrABC proteins correspond to a novel NADH dehydrogenase/heterodisulfide reductase widespread in anaerobic bacteria and involved in ethanol metabolism in D esulfovibrio vulgaris H ildenborough

Abstract

Summary Flavin‐based electron bifurcation ( FBEB ) is an important mechanism for the energy metabolism of anaerobes. A new family of NADH dehydrogenases, the flavin oxidoreductase ( FlxABCD , previously called FloxABCD ), was proposed to perform FBEB in sulphate‐reducing organisms coupled with heterodisulfide reductase ( HdrABC ). We found that the hdrABC‐flxABCD gene cluster is widespread among anaerobic bacteria, pointing to a general and important role in their bioenergetics. In this work, we studied FlxABCD of D esulfovibrio vulgaris H ildenborough. The hdr‐flx genes are part of the same transcriptional unit and are increased in transcription during growth in ethanol‐sulfate, and to a less extent during pyruvate fermentation. Two mutant strains were generated: one where expression of the hdr‐flx genes was interrupted and another lacking the flxA gene. Both strains were unable to grow with ethanol‐sulfate, whereas growth was restored in a flxA ‐ complemented strain. The mutant strains also produced very reduced amounts of ethanol compared with the wild type during pyruvate fermentation. Our results show that in D . vulgaris , the FlxABCD ‐ HdrABC proteins are essential for NADH oxidation during growth on ethanol, probably involving a FBEB mechanism that leads to reduction of ferredoxin and the small protein DsrC , while in fermentation they operate in reverse, reducing NAD + for ethanol production.

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