The FlxABCD‐HdrABC proteins correspond to a novel NADH dehydrogenase/heterodisulfide reductase widespread in anaerobic bacteria and involved in ethanol metabolism in Desulfovibrio vulgaris Hildenborough

Abstract

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 Desulfovibrio vulgaris Hildenborough. 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.

A Post-Genomic View of the Ecophysiology, Catabolism and Biotechnological Relevance of Sulphate-Reducing Prokaryotes

“…FlxA oxidizes NADH and transfers electrons to HdrABC, via FlxB and FlxCD. Then HdrABC is proposed to bifurcate electrons to reduce both ferredoxin and the cysteine-containing protein DsrC (Ramos et al, 2015). In support of this hypothesis, the dsrC gene is found next to a hdrA/L-flxACBD gene cluster in some SRP, including D. autotrophicum, Desulfosarcina sp.…”

“…The hdr-flx gene cluster was recently shown to form a single transcriptional unit in D. vulgaris Hildenborough (Ramos et al, 2015). The flxA gene codes for a protein having one FAD and one NAD(P)-binding sites and binding one [2Fe-2S] 2+/1+ centre.…”

“…So, sequence analysis indicates that FlxABCD is a new type of NAD(P)H dehydrogenase and suggests that the FlxABCD-HdrABC complex is analogous to the MvhDGA-HdrABC complex of methanogens, where the Mvh and Flx proteins probably constitute parallel pathways for HdrA reduction from H 2 or NAD(P)H, respectively . The hdrCBA-flxDCBA gene cluster is widespread in anaerobic bacteria besides SRP, suggesting a general function in anaerobic energy metabolism (Ramos et al, 2015).…”

“…The flx-hdr genes are often implicated in expression and proteomic studies of this organism energy metabolism (Caffrey et al, 2007;Haveman et al, 2003;Meyer, Kuehl, Deutschbauer, Price, et al, 2013;Meyer et al, 2014;Ramos et al, 2015;Walker, He, et al, 2009;Zhang, Culley, Scholten, et al, 2006). Very recently, the function of the FlxABCD-HdrABC complex was elucidated in D. vulgaris, which contains a single copy of these genes, where it was shown to be essential for growth with ethanol/sulphate and to be involved in the production of ethanol during fermentation of pyruvate (Ramos et al, 2015). A mutant where expression of the hdr-flx gene cluster was suppressed and a ΔflxA mutant lacking the flxA gene were both unable to grow with ethanol/sulphate, whereas growth was restored in a complemented strain.…”

Insights into the Evolution of Host Association through the Isolation and Characterization of a Novel Human Periodontal Pathobiont,Desulfobulbus oralis

“…The complex appears to have originated from Desulfovibrio by horizontal gene transfer (HGT), and is most closely related to that of the human oral pathogen D. fairfieldensis. The FlxABC complex has flavin adenine dinucleotide (FAD), NAD(P)-binding sites, and an iron-sulfur center, representing a distinct type of NAD(P)H dehydrogenase, which was proposed to oxidize NAD(P)H using both ferredoxin and a high-redox disulfide center (DsrC) as electron acceptors (52). The reverse reaction, NAD ϩ recycling to NADH, can be coupled in environmental Desulfovibrio with aldehyde conversion to ethanol during pyruvate fermentation, using electrons coming from reduced ferredoxin through HdrABC (52).…”