Basic Parts: HSU-ETCD
HSU-ETCD, which stands for Hydrogen Synthesis Upregulation-Electron Transport Chain Downregulation, functions to upregulate the formation fermentation pathway to promote anaerobic energy production and downregulate the ETC. It contains a variety of genes that perform different functions. Most notably, it contains FhlA, FNR, ArcA, and hyfR. FhlA, or formate hydrogenlyase transcriptional activator, is required for the induction of expression of the formate dehydrogenase H and hydrogenase-3 structural genes. Thus, it activates transcription of fdhF and the hyc, hyp, and hydN-hypF operons. Through the incorporation of FlhA, therefore, hydrogen (H2) synthesis is enhanced and increased. FNR, or Ferredoxin-NADP+-Reductase, first oxidizes NADPH to NADP+ while reducing the Ferredoxin protein. ArcA, or aerobic respiration control protein, induces the transcription of pyruvate formate-lyase. hyfR, which is an FhlA homologue for Hyd‐4, is a DNA‐binding transcriptional activator and is involved in formate sensing.
Development and Detailed Review
In certain species of bacteria, including Escherichia coli, the anaerobic process of mixed acid fermentation converts formate to hydrogen and carbon dioxide via the formate hydrogen lyase (FHL) complex. The FHL complex is regulated by fhlA, which activates transcription of the fdhF, hyp, hyc, and hydN-hypF operons.hyfR is a homolog of fhlA that transcribes the hyf operon (hydrogenase-4) and has also been found to enhance expression of fdhF [9, 3]. Hydrogenase-4, along with hydrogenase-3 (hyc operon), is the hydrogen-producing unit of the FHL complex . The fnr and arc systems work in concert to regulate anaerobic respiration . In the absence of oxygen, both ArcA and FNR repress aerobic processes (ETC “dampers”) and promote the transcription of anaerobic enzymes , including fhlA. FNR is activated by its oxygen-sensitive [4Fe-4S] clusters and appears to be specific to anaerobic respiration and fermentation pathways. ArcA is phosphorylated by its counterpart ArcB and exerts a broader range of control over various intracellular redox conditions . Both systems are sensitive to different levels of oxygen in the environment, meaning the level of expression of target operons varies depending on the degree of air saturation . In anaerobic conditions, both FNR and ArcA induce the transcription of pyruvate formate-lyase, the enzyme responsible for converting pyruvate into formate [2,6]. Thus, overexpression of the fnr and arc systems may increase formate concentrations during fermentation and consequently increase biohydrogen production via the FHL complex. There is a lack of experimental evidence for the combined effects of FNR and ArcA on fhlA or hyfR expression. However, it is possible that the binding of the FNR enzyme nearby the hyf sequence may minimize the ability of the larger hyfR protein to bind to its upstream target sequence .
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