OhsR acts as an organic peroxide-sensing transcriptional activator using an S-mycothiolation mechanism in Corynebacterium glutamicum

Regulation of oxidative stress response and antioxidant modification in Corynebacterium glutamicum

As an efficient and safe industrial bacterium, Corynebacterium glutamicum has extensive application in amino acid production. However, it often faces oxidative stress induced by reactive oxygen species (ROS), leading to diminished production efficiency. To enhance the robustness of C. glutamicum, numerous studies have focused on elucidating its regulatory mechanisms under various stress conditions such as heat, acid, and sulfur stress. However, a comprehensive review of its defense mechanisms against oxidative stress is needed. This review offers an in-depth overview of the mechanisms C. glutamicum employs to manage oxidative stress. It covers both enzymatic and non-enzymatic systems, including antioxidant enzymes, regulatory protein families, sigma factors involved in transcription, and physiological redox reduction pathways. This review provides insights for advancing research on the antioxidant mechanisms of C. glutamicum and sheds light on its potential applications in industrial production.

Low-molecular-weight thiol transferases in redox regulation and antioxidant defence

Low-molecular-weight (LMW) thiols are produced in all living cells in different forms and concentrations. Glutathione (GSH), coenzyme A (CoA), bacillithiol (BSH), mycothiol (MSH), ergothioneine (ET) and trypanothione T(SH)2 are the main LMW thiols in eukaryotes and prokaryotes. LMW thiols serve as electron donors for thiol-dependent enzymes in redox-mediated metabolic and signaling processes, protect cellular macromolecules from oxidative and xenobiotic stress, and participate in the reduction of oxidative modifications. The level and function of LMW thiols, their oxidized disulfides and mixed disulfide conjugates in cells and tissues is tightly controlled by dedicated oxidoreductases, such as peroxiredoxins, glutaredoxins, disulfide reductases and LMW thiol transferases. This review provides the first summary of the current knowledge of structural and functional diversity of transferases for LMW thiols, including GSH, BSH, MSH and T(SH)2. Their role in maintaining redox homeostasis in single-cell and multicellular organisms is discussed, focusing in particular on the conjugation of specific thiols to exogenous and endogenous electrophiles, or oxidized protein substrates. Advances in the development of new research tools, analytical methodologies, and genetic models for the analysis of known LMW thiol transferases will expand our knowledge and understanding of their function in cell growth and survival under oxidative stress, nutrient deprivation, and during the detoxification of xenobiotics and harmful metabolites. The antioxidant function of CoA has been recently discovered and the breakthrough in defining the identity and functional characteristics of CoA S-transferase(s) is soon expected.

The flavohaemoprotein hmp maintains redox homeostasis in response to reactive oxygen and nitrogen species in Corynebacterium glutamicum

BACKGROUND

During the production of L-arginine through high dissolved oxygen and nitrogen supply fermentation, the industrial workhorse Corynebacterium glutamicum is exposed to oxidative stress. This generates reactive oxygen species (ROS) and reactive nitrogen species (RNS), which are harmful to the bacteria. To address the issue and to maintain redox homeostasis during fermentation, the flavohaemoprotein (Hmp) was employed.

RESULTS

The results showed that the overexpression of Hmp led to a decrease in ROS and RNS content by 9.4% and 22.7%, respectively, and improved the survivability of strains. When the strains were treated with H2O2 and NaNO2, the RT-qPCR analysis indicated an up-regulation of ammonium absorption and transporter genes amtB and glnD. Conversely, the deletion of hmp gives rise to the up-regulation of eight oxidative stress-related genes. These findings suggested that hmp is associated with oxidative stress and intracellular nitrogen metabolism genes. Finally, we released the inhibitory effect of ArnR on hmp. The Cc-ΔarnR-hmp strain produced 48.4 g/L L-arginine during batch-feeding fermentation, 34.3% higher than the original strain.

CONCLUSIONS

This report revealed the influence of dissolved oxygen and nitrogen concentration on reactive species of Corynebacterium glutamicum and the role of the Hmp in coping with oxidative stress. The Hmp first demonstrates related to redox homeostasis and nitrite metabolism, providing a feasible strategy for improving the robustness of strains.

The TetR-type regulator AtsR is involved in multidrug response in Corynebacterium glutamicum

Background

The TetR (tetracycline repressor) family is one of the major transcription factor families that regulate expression of genes involved in bacterial antimicrobial resistance systems. NCgl0886 protein, designated as AtsR, is a member of the TetR family identified in Corynebacterium glutamicum, which is conserved in several species of the genera Corynebacterium, also including the well-known pathogen C. diphtheriae. AtsR is located at no far upstream of the identically oriented ncgl0884 gene, encoding a putative multidrug efflux pump protein, and in the same operon with ncgl0887, encoding a resistance, nodulation and cell division (RND) superfamily drug exporter. However, the role of AtsR is not clearly understood.

Results

Here we showed that dimeric AtsR directly repressed the expression of the ncgl0887-atsR operon, as well as indirectly controlled the ncgl0884 transcription. Antibiotics and toxic compounds induced the expression of ncgl0887-atsR operon. A perfect palindromic motif (5΄-TGCAA-N₂-TTGCA-3΄; 12 bp) was identified in the upstream region of ncgl0887-atsR operon. Electrophoretic mobility shift assays (EMSAs) demonstrated specific binding of AtsR to this motif, and hydrogen peroxide (H₂O₂) blocked binding. H₂O₂ oxidized cysteine residues to form Cys123-Cys187 intermolecular disulfide bonds between two subunits in AtsR dimer, which altered its DNA-binding characteristics and caused its dissociation, thereby leading to derepression of the drug efflux protein. Deletion of ncgl0884 and ncgl0887 increased the susceptibilities of C. glutamicum for several toxic compounds, but overexpression of atsR decreased the drug tolerance of C. glutamicum.

Conclusions

Our study revealed that AtsR was a redox regulator that sensed oxidative stress via thiol modification. The results obtained here will contribute to our understanding of the drug response mechanism not only in C. glutamicum but also in the related bacteria C. diphtheriae.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01850-0.

Ohr - OhrR, a neglected and highly efficient antioxidant system: Structure, catalysis, phylogeny, regulation, and physiological roles

Ohrs (organic hydroperoxide resistance proteins) are antioxidant enzymes that play central roles in the response of microorganisms to organic peroxides. Here, we describe recent advances in the structure, catalysis, phylogeny, regulation, and physiological roles of Ohr proteins and of its transcriptional regulator, OhrR, highlighting their unique features. Ohr is extremely efficient in reducing fatty acid peroxides and peroxynitrite, two oxidants relevant in host-pathogen interactions. The highly reactive Cys residue of Ohr, named peroxidatic Cys (C_p), composes together with an arginine and a glutamate the catalytic triad. The catalytic cycle of Ohrs involves a condensation between a sulfenic acid (C_p-SOH) and the thiol of the second conserved Cys, leading to the formation of an intra-subunit disulfide bond, which is then reduced by dihydrolipoamide or lipoylated proteins. A structural switch takes place during catalysis, with the opening and closure of the active site by the so-called Arg-loop. Ohr is part of the Ohr/OsmC super-family that also comprises OsmC and Ohr-like proteins. Members of the Ohr, OsmC and Ohr-like subgroups present low sequence similarities among themselves, but share a high structural conservation, presenting two Cys residues in their active site. The pattern of gene expression is also distinct among members of the Ohr/OsmC subfamilies. The expression of ohr genes increases upon organic hydroperoxides treatment, whereas the signals for the upregulation of osmC are entry into the stationary phase and/or osmotic stress. For many ohr genes, the upregulation by organic hydroperoxides is mediated by OhrR, a Cys-based transcriptional regulator that only binds to its target DNAs in its reduced state. Since Ohrs and OhrRs are involved in virulence of some microorganisms and are absent in vertebrate and vascular plants, they may represent targets for novel therapeutic approaches based on the disruption of this key bacterial organic peroxide defense system.

Involvement of a mycothiol-dependent reductase NCgl0018 in oxidative stress response of Corynebacterium glutamicum

Corynebacterium glutamicum is an important industrial strain for amino acids and a key model organism for human pathogens. The study of C. glutamicum oxidoreductases, such as mycoredoxin 1 (Mrx1), dithiol-disulfide isomerase DsbA, and DsbA-like Mrx1, is helpful for understanding the survival, pathogenic infection, and stress resistance of its homologous species. However, the action mode and enzymatic function of C. glutamicum NCgl0018 preserving the Cys-Pro-Phe-Cys motif, annotated as a putative DsbA, have remained enigmatic. Here, we report that the NCgl0018-deleted strain increased sensitivity to various oxidative stresses. The ncgl0018 expression was induced in the stress-responsive extracytoplasmic function-sigma (ECF-σ) factor SigH- and organic peroxide- and antibiotic-sensing regulator (OasR)-dependent manner by stress. NCgl0018 reduced S-mycothiolated mixed disulfides and intramolecular disulfides via a monothiol-disulfide mechanism preferentially linking the mycothiol/mycothione reductase/NADPH electron pathway. Site-directed mutagenesis confirmed Cys107 was the resolving Cys residue, while Cys104 was the nucleophilic cysteine that was oxidized to a sulfenic acid and then could form an intramolecular disulfide bond with Cys107 or a mixed disulfide with mycothiol under stress. Biochemical analyses indicated that NCgl0018 lacked oxidase properties like the classical DsbA. Further, enzymatic rates and substrate preferences of NCgl0018 were highly similar to those of DsbA-like Mrx1. Collectively, our study presented the first evidence that NCgl0018 protected against stresses by functioning as a novel DsbA-like Mrx1 but not DsbA and Mrx1.

Corynebacterium glutamicum Mycoredoxin 3 protects against multiple oxidative stresses and displays thioredoxin-like activity

Glutaredoxins (Grxs) and thioredoxins (Trxs) play a critical role in resistance to oxidative conditions. However, physiological and biochemical roles of Mycoredoxin 3 (Mrx3) that shared a high amino acid sequence similarity to Grxs remain unknown in Corynebacterium glutamicum. Here we showed that mrx3 deletion strains of C. glutamicum was involved in the protection against oxidative stress. Recombinant Mrx3 not only catalytically reduced the disulfide bonds in ribonucleotide reductase (RNR), insulin and 5,5'-dithiobis-(2-nitro-benzoicacid) (DTNB), but also reduced the mixed disulphides between mycothiol (MSH) and substrate, which was exclusively linked to the thioredoxin reductase (TrxR) electron transfer pathway by a dithiol mechanism. Site-directed mutagenesis confirmed that the conserved Cys17 and Cys20 in Mrx3 were necessary to maintain its activity. The mrx3 deletion mutant showed decreased resistance to various stress, and these sensitive phenotypes were almost fully restored in the complementary strain. The physiological roles of Mrx3 in resistance to various stress were further supported by the induced expression of mrx3 under various stress conditions, directly under the control of the stress-responsive extracytoplasmic function-sigma (ECF-σ) factor SigH. Thus, we presented the first evidence that Mrx3 protected against various oxidative stresses by acting as a disulfide oxidoreductase behaving like Trx.

OsnR is an autoregulatory negative transcription factor controlling redox-dependent stress responses in Corynebacterium glutamicum

Background

Corynebacterium glutamicum is used in the industrial production of amino acids and nucleotides. During the course of fermentation, C. glutamicum cells face various stresses and employ multiple regulatory genes to cope with the oxidative stress. The osnR gene plays a negative regulatory role in redox-dependent oxidative-stress responses, but the underlying mechanism is not known yet.

Results

Overexpression of the osnR gene in C. glutamicum affected the expression of genes involved in the mycothiol metabolism. ChIP-seq analysis revealed that OsnR binds to the promoter region of multiple genes, including osnR and cg0026, which seems to function in the membrane-associated redox metabolism. Studies on the role of the osnR gene involving in vitro assays employing purified OsnR proteins and in vivo physiological analyses have identified that OsnR inhibits the transcription of its own gene. Further, oxidant diamide stimulates OsnR-binding to the promoter region of the osnR gene. The genes affected by the overexpression of osnR have been found to be under the control of σ^H. In the osnR-overexpressing strain, the transcription of sigH is significantly decreased and the stimulation of sigH transcription by external stress is lost, suggesting that osnR and sigH form an intimate regulatory network.

Conclusions

Our study suggests that OsnR not only functions as a transcriptional repressor of its own gene and of those involved in redox-dependent stress responses but also participates in the global transcriptional regulation by controlling the transcription of other master regulators, such as sigH.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01693-1.

A novel mycothiol-dependent thiol-disulfide reductase in Corynebacterium glutamicum involving oxidative stress resistance

ncgl2478 gene from Corynebacterium glutamicum encodes a thiol–disulfide oxidoreductase enzyme annotated as dithiol–disulfide isomerase DsbA. It preserves a Cys–Pro–Phe–Cys active-site motif, which is presumed to be an exclusive characteristic of the novel DsbA–mycoredoxin 1 (Mrx1) cluster. However, the real mode of action, the nature of the electron donor pathway and biological functions of NCgl2478 in C. glutamicum have remained enigmatic so far. Herein, we report that NCgl2478 plays an important role in stress resistance. Deletion of the ncgl2478 gene increases the size of growth inhibition zones. The ncgl2478 expression is induced in the stress-responsive extra-cytoplasmic function-sigma (ECF-σ) factor SigH-dependent manner by stress. It receives electrons preferentially from the mycothiol (MSH)/mycothione reductase (Mtr)/NADPH pathway. Further, NCgl2478 reduces S-mycothiolated mixed disulfides and intramolecular disulfides via a monothiol–disulfide and a dithiol–disulfide exchange mechanism, respectively. NCgl2478 lacks oxidase activity; kinetic properties of its demycothiolation are different from those of Mrx1. Site-directed mutagenesis confirms Cys24 is the resolving Cys residue, while Cys21 is the nucleophilic cysteine that is oxidized to a sulfenic acid and then forms an intramolecular disulfide bond with Cys24 or a mixed disulfide with MSH under oxidative stress. In conclusion, our study presents the first evidence that NCgl2478 protects against various stresses by acting as an MSH-dependent thiol–disulfide reductase, belonging to a novel DsbA–Mrx1 cluster.

Ammonium Ions Induce Cellulase Synthesis in Trichoderma koningii

Cellulase plays an important role in addressing the issue of the energy crisis. However, the yield and degradation efficiency of cellulase remain a major challenge. In the present study, we aimed to verify whether ammonium ion (NH₄⁺) could induce cellulase synthesis from T. koningii AS3.2774 and to explore new functional genes related to the cellulase production. Our results indicated that NH₄⁺ induces cellulase production in a way different from nitrogen sources. NH₄⁺-mediated mycelia displayed a significant increase in transport vesicles. Under NH₄⁺ mediation, CBHI, CBHII, glycoside hydrolase family 5 proteins, Hap2/3/5 complexes, "ribosome biogenesis", and "heme binding" were significantly up-regulated, and differentially expressed genes (DEGs) were mainly involved in "Metabolism". Collectively, our findings illustrated that NH₄⁺ induced the cellulase production at morphological and gene expression levels, which might be related to the Hap2/3/5 complex, ribosomes, and genes involved in various amino acid metabolism, pyruvate metabolism, and glycolysis/gluconeogenesis. Taken together, our results provided valuable insights into the regulatory network of cellulase gene expression in filamentous fungi.

The cssR gene of Corynebacterium glutamicum plays a negative regulatory role in stress responses

Background

CssR, the product of the Corynebacterium glutamicum ncgl1578 gene cotranscribed with ncgl1579, is a TetR (tetracycline regulator) family repressor. Although many TetR-type regulators in C. glutamicum have been extensively described, members of the TetR family involved in the stress response remain unidentified.

Results

In this study, we found that CssR regulated the transcription of its own gene and the ncgl1576-ncgl1577 operon. The ncgl1576-ncgl1577 operon, which is located upstream of cssR in the orientation opposite that of the cssR operon, encodes an ATP-binding cassette (ABC), some of which are involved in the export of a wide range of antimicrobial compounds. The cssR-deletion (ΔcssR) mutant displayed increased resistance to various stresses. An imperfect palindromic motif (5′-TAA(G)TGN₁₃CA(G)TTA-3′; 25 bp) located at the intergenic region between cssR and ncgl1577 was identified as the sole binding site for CssR. Expression of cssR and ncgl1577 was induced by antibiotics and heavy metals but not H₂O₂ or diamide, and the DNA-binding activity of CssR was impaired by antibiotics and heavy metals but not H₂O₂. Antibiotics and heavy metals caused CssR dissociation from target gene promoters, thus derepressing their transcription. Oxidant treatment neither altered the conformation of CssR nor modified its cysteine residues, indicating that the cysteine residues in CssR have no redox activity. In the ΔcssR mutant strain, genes involved in redox homeostasis also showed increased transcription levels, and the NADPH/NADP⁺ ratio was higher than that of the parental strain.

Conclusion

The stress response mechanism of CssR in C. glutamicum is realized via ligand-induced conformational changes of the protein, not via cysteine oxidation-based thiol modification. Moreover, the crucial role of CssR in the stress response was demonstrated by negatively controlling the expression of the ncgl1576-ncgl1577 operon, its structural gene, and/or redox homeostasis-related genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01600-8.

The thiol oxidation-based sensing and regulation mechanism for the OasR-mediated organic peroxide and antibiotic resistance in C. glutamicum

Corynebacterium glutamicum, an important industrial and model microorganism, inevitably encountered stress environment during fermentative process. Therefore, the ability of C. glutamicum to withstand stress and maintain the cellular redox balance was vital for cell survival and enhancing fermentation efficiency. To robustly survive, C. glutamicum has been equipped with many types of redox sensors. Although cysteine oxidation-based peroxide-sensing regulators have been well described in C. glutamicum, redox sensors involving in multiple environmental stress response remained elusive. Here, we reported an organic peroxide- and antibiotic-sensing MarR (multiple antibiotics resistance regulators)-type regulator, called OasR (organic peroxide- and antibiotic-sensing regulator). The OasR regulator used Cys95 oxidation to sense oxidative stress to form S-mycothiolated monomer or inter-molecular disulfide-containing dimer, resulting in its dissociation from the target DNA promoter. Transcriptomics uncovered the strong up-regulation of many multidrug efflux pump genes and organic peroxide stress-involving genes in oasR mutant, consistent with the phenomenon that oasR mutant showed a reduction in sensitivity to antibiotic and organic peroxide. Importantly, the addition of stress-associated ligands such as cumene hydroperoxide and streptomycin induced oasR and multidrug efflux pump protein NCgl1020 expression in vivo. We speculated that cell resistance to antibiotics and organic peroxide correlated with stress response-induced up-regulation of genes expression. Together, the results revealed that OasR was a key MarR-type redox stress-responsive transcriptional repressor, and sensed oxidative stress generated through hydroxyl radical formation to mediate antibiotic resistance in C. glutamicum.

Molecular mechanisms of Mycoredoxin-1 in resistance to oxidative stress in Corynebacterium glutamicum

Glutaredoxins (Grxs) with Cys-Pro-Phe (Tyr)-Cys motif and a thioredoxin fold structure play an important role in the anti-oxidant system of bacteria by catalyzing a variety of thiol-disulfide exchange reactions with a 2-Cys mechanism or a 1-Cys mechanism. However, the catalytic and physiological mechanism of Corynebacterium glutamicum Mycoredoxin 1 (Mrx1) that shares a high amino acid sequence similarity to Grxs has not been fully elucidated. Here, we report that Mrx1 has a protective function against various adverse conditions, and the decrease of cell viability to various stress conditions by deletion of the Mrx1 in C. glutamicum was confirmed in the mrx1 mutant. The physiological roles of Mrx1 in defence to oxidative stress were corroborated by its induced expression under various stresses, regulated directly by the stress-responsive extracytoplasmic function-sigma (ECF-σ) factor SigH. As well as reducing mycothiol (MSH) mixed disulfide bonds via a 1-Cys mechanism, C. glutamicum Mrx1 catalytically reduced the disulfides in the Ib RNR, insulin and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) by exclusively linking the MSH/Mtr (mycothiol disulfide reductase)/NADPH electron pathway via a 2-Cys mechanism. Thus, we present the first evidence that the Mrx1 is able to protect against the damaging effects of various exogenous stresses by acting as a disulfide oxidoreductase, thereby giving a new insight in how C. glutamicum survives oxidative stressful conditions.

MsrR is a thiol-based oxidation-sensing regulator of the XRE family that modulates C. glutamicum oxidative stress resistance

Background

Corynebacterium glutamicum thrives under oxidative stress caused by the inevitably extreme environment during fermentation as it harbors antioxidative stress genes. Antioxidant genes are controlled by pathway-specific sensors that act in response to growth conditions. Although many families of oxidation-sensing regulators in C. glutamicum have been well described, members of the xenobiotic-response element (XRE) family, involved in oxidative stress, remain elusive.

Results

In this study, we report a novel redox-sensitive member of the XER family, MsrR (multiple stress resistance regulator). MsrR is encoded as part of the msrR-3-mst (3-mercaptopyruvate sulfurtransferase) operon; msrR-3-mst is divergent from multidrug efflux protein MFS. MsrR was demonstrated to bind to the intergenic region between msrR-3-mst and mfs. This binding was prevented by an MsrR oxidation-mediated increase in MsrR dimerization. MsrR was shown to use Cys62 oxidation to sense oxidative stress, resulting in its dissociation from the promoter. Elevated expression of msrR-3-mst and mfs was observed under stress. Furthermore, a ΔmsrR mutant strain displayed significantly enhanced growth, while the growth of strains lacking either 3-mst or mfs was significantly inhibited under stress.

Conclusion

This report is the first to demonstrate the critical role of MsrR-3-MST-MFS in bacterial stress resistance.

Corynebacterium glutamicum CrtR and Its Orthologs in Actinobacteria: Conserved Function and Application as Genetically Encoded Biosensor for Detection of Geranylgeranyl Pyrophosphate

Carotenoid biosynthesis in Corynebacteriumglutamicum is controlled by the MarR-type regulator CrtR, which represses transcription of the promoter of the crt operon (PcrtE) and of its own gene (PcrtR). Geranylgeranyl pyrophosphate (GGPP), and to a lesser extent other isoprenoid pyrophosphates, interfere with the binding of CrtR to its target DNA in vitro, suggesting they act as inducers of carotenoid biosynthesis. CrtR homologs are encoded in the genomes of many other actinobacteria. In order to determine if and to what extent the function of CrtR, as a metabolite-dependent transcriptional repressor of carotenoid biosynthesis genes responding to GGPP, is conserved among actinobacteria, five CrtR orthologs were characterized in more detail. EMSA assays showed that the CrtR orthologs from Corynebacteriumcallunae, Acidipropionibacteriumjensenii, Paenarthrobacternicotinovorans, Micrococcusluteus and Pseudarthrobacterchlorophenolicus bound to the intergenic region between their own gene and the divergently oriented gene, and that GGPP inhibited these interactions. In turn, the CrtR protein from C. glutamicum bound to DNA regions upstream of the orthologous crtR genes that contained a 15 bp DNA sequence motif conserved between the tested bacteria. Moreover, the CrtR orthologs functioned in C. glutamicum in vivo at least partially, as they complemented the defects in the pigmentation and expression of a PcrtE_gfpuv transcriptional fusion that were observed in a crtR deletion mutant to varying degrees. Subsequently, the utility of the PcrtE_gfpuv transcriptional fusion and chromosomally encoded CrtR from C. glutamicum as genetically encoded biosensor for GGPP was studied. Combined FACS and LC-MS analysis demonstrated a correlation between the sensor fluorescent signal and the intracellular GGPP concentration, and allowed us to monitor intracellular GGPP concentrations during growth and differentiate between strains engineered to accumulate GGPP at different concentrations.

CarR, a MarR-family regulator from Corynebacterium glutamicum, modulated antibiotic and aromatic compound resistance

MarR (multiple antibiotic resistance regulator) proteins are a family of transcriptional regulators that is prevalent in Corynebacterium glutamicum. Understanding the physiological and biochemical function of MarR homologs in C. glutamicum has focused on cysteine oxidation-based redox-sensing and substrate metabolism-involving regulators. In this study, we characterized the stress-related ligand-binding functions of the C. glutamicum MarR-type regulator CarR (C. glutamicum antibiotic-responding regulator). We demonstrate that CarR negatively regulates the expression of the carR (ncgl2886)-uspA (ncgl2887) operon and the adjacent, oppositely oriented gene ncgl2885, encoding the hypothetical deacylase DecE. We also show that CarR directly activates transcription of the ncgl2882-ncgl2884 operon, encoding the peptidoglycan synthesis operon (PSO) located upstream of carR in the opposite orientation. The addition of stress-associated ligands such as penicillin and streptomycin induced carR, uspA, decE, and PSO expression in vivo, as well as attenuated binding of CarR to operator DNA in vitro. Importantly, stress response-induced up-regulation of carR, uspA, and PSO gene expression correlated with cell resistance to β-lactam antibiotics and aromatic compounds. Six highly conserved residues in CarR were found to strongly influence its ligand binding and transcriptional regulatory properties. Collectively, the results indicate that the ligand binding of CarR induces its dissociation from the carR-uspA promoter to derepress carR and uspA transcription. Ligand-free CarR also activates PSO expression, which in turn contributes to C. glutamicum stress resistance. The outcomes indicate that the stress response mechanism of CarR in C. glutamicum occurs via ligand-induced conformational changes to the protein, not via cysteine oxidation-based thiol modifications.

Thioredoxin and protein-disulfide isomerase selectivity for redox regulation of proteins in Corynebacterium glutamicum

Thioredoxins (Trxs) and protein-disulfide isomerases (PDIs) are believed to play a pivotal role in ensuring the proper folding of proteins, facilitating appropriate functioning of proteins, and maintaining intracellular redox homeostasis in bacteria. Two thioredoxins (Trxs) and three thiol-disulfide isomerases (PDIs) have been annotated in Corynebacterium glutamicum. However, nothing is known about their functional diversity in the redox regulation of proteins. Thus, we here analyzed the Trx- and PDI-dependent redox shifts of ribonucleotide reductase (RNR), insulin, 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), and several thiol-dependent peroxidases by measuring enzyme activity and thiol status in vitro. We found that the two Trxs and the three PDIs had activities in the cleavage of the disulfidebond, whereas the PDIs had a lower efficiency than the two Trxs. Trx2 could activate thiol-dependent peroxidases with an efficiency comparable with that of Trx1, but the PDIs were inefficient. The redox-active Cys-X-X-Cys motif harbored in both Trxs and PDIs was essential to supply efficiently the donor of reducing equivalents for protein disulfides. In addition, stress-responsive extracytoplasmic function (ECF)-sigma factor H (SigH)-dependent Trxs and PDIs expressions were observed. These results contributed importantly to our overall understanding of reducing functionality of the Trx and PDI systems, and also highlighted the complexity and plasticity of the intracellular redox network.

Characterization of Xi-class mycothiol S-transferase from Corynebacterium glutamicum and its protective effects in oxidative stress

Background

Oxidative stress caused by inevitable hostile conditions during fermentative process was the most serious threat to the survival of the well-known industrial microorganism Corynebacterium glutamicum. To survive, C. glutamicum developed several antioxidant defenses including millimolar concentrations of mycothiol (MSH) and protective enzymes. Glutathione (GSH) S-transferases (GSTs) with essentially defensive role in oxidative stress have been well defined in numerous microorganisms, while their physiological and biochemical functions remained elusive in C. glutamicum thus far.

Results

In the present study, we described protein NCgl1216 belonging to a novel MSH S-transferase Xi class (MstX), considered as the equivalent of GST Xi class (GSTX). MstX had a characteristic conserved catalytic motif (Cys-Pro-Trp-Ala, C-P-W-A). MstX was active as thiol transferase, dehydroascorbate reductase, mycothiolyl-hydroquinone reductase and MSH peroxidase, while it showed null activity toward canonical GSTs substrate as 1-chloro-2,4-dinitrobenzene (CDNB) and GST Omega’s specific substance glutathionyl-acetophenones, indicating MstX had some biochemical characteristics related with mycoredoxin (Mrx). Site-directed mutagenesis showed that, among the two cysteine residues of the molecule, only the residue at position 67 was required for the activity. Moreover, the residues adjacent to the active Cys67 were also important for activity. These results indicated that the thiol transferase of MstX operated through a monothiol mechanism. In addition, we found MstX played important role in various stress resistance. The lack of C. glutamicum mstX gene resulted in significant growth inhibition and increased sensitivity under adverse stress condition. The mstX expression was induced by stress.

Conclusion

Corynebacterium glutamicum MstX might be critically involved in response to oxidative conditions, thereby giving new insight in how C. glutamicum survived oxidative stressful conditions.

OsmC in Corynebacterium glutamicum was a thiol-dependent organic hydroperoxide reductase

Bacterial antioxidants play a vital role in the detoxification of exogenous peroxides. Several antioxidant defenses including low-molecular-weight thiols (LMWTs) and protective enzymes were developed to help the bacterium withstand the adverse stress. Although osmotically induced bacterial protein C (OsmC), classified as the organic hydroperoxide reductase (Ohr)/OsmC superfamily, has been demonstrated in some mycobacterial species, including M. tuberculosis and M. smegmatis, its physiological and biochemical functions in C. glutamicum remained elusive. Here we found the lack of C. glutamicum osmC gene resulted in decreased cell viability and increased intracellular reactive oxygen species accumulation under organic hydroperoxides (OHPs) stress conditions. The osmC expression was induced in the multiple antibiotic resistance regulator MarR-dependent manner by OHPs, and not by other oxidants or osmotic stress. Peroxide reductase activity showed that OsmC had a narrow range of substrates-only degrading OHPs, and detoxified OHPs mainly by linking the alkyl hydroperoxide reductase (AhpD) system (AhpD/dihydrolipoamide dehydrogenase (Lpd)/dihydrolipoamide acyltransferase (SucB)). Site-directed mutagenesis confirmed Cys48 was the peroxidatic cysteine, while Cys114 was the resolving Cys residue that formed an intramolecular disulfide bond with oxidized Cys48. Therefore, C. glutamicum OsmC was a thiol-dependent OHP reductase and played important role of protection against OHPs together with Ohr.