SpiE interacts with Corynebacterium glutamicum WhcE and is involved in heat and oxidative stress responses

WhiB-like proteins: Diversity of structure, function and mechanism

The WhiB-Like (Wbl) proteins are a large family of iron-sulfur (Fe-S) cluster-containing transcription factors exclusively found in the phylum Actinobacteria, including the notable genera like Mycobacteria, Streptomycetes and Corynebacteria. These proteins play pivotal roles in diverse biological processes, such as cell development, redox stress response and antibiotic resistance. Members of the Wbl family exhibit remarkable diversity in their sequences, structures and functions, attracting great attention since their first discovery. This review highlights the most recent breakthroughs in understanding the structural and mechanistic aspects of Wbl-dependent transcriptional regulation.

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.

CdbC: a disulfide bond isomerase involved in the refolding of mycoloyltransferases in Corynebacterium glutamicum cells exposed to oxidative conditions

In Corynebacterium glutamicum cells, cdbC, which encodes a protein containing the CysXXCys motif, is regulated by the global redox-responsive regulator OsnR. In this study, we assessed the role of the periplasmic protein CdbC in disulfide bond formation and its involvement in mycomembrane biosynthesis. Purified CdbC efficiently refolded scrambled RNaseA, exhibiting prominent disulfide bond isomerase activity. The transcription of cdbC was decreased in cells grown in the presence of the reductant dithiothreitol (DTT). Moreover, unlike wild-type and cdbC-deleted cells, cdbC-overexpressing (P180-cdbC) cells grown in the presence of DTT exhibited retarded growth, abnormal cell morphology, increased cell surface hydrophobicity and altered mycolic acid composition. P180-cdbC cells cultured in a reducing environment accumulated trehalose monocorynomycolate, indicating mycomembrane deformation. Similarly, a two-hybrid analysis demonstrated the interaction of CdbC with the mycoloyltransferases MytA and MytB. Collectively, our findings suggest that CdbC, a periplasmic disulfide bond isomerase, refolds misfolded MytA and MytB and thereby assists in mycomembrane biosynthesis in cells exposed to oxidative conditions.

Enhanced L-serine production by Corynebacterium glutamicum based on novel insights into L-serine exporters

The L-serine exporters ThrE and SerE play important roles in L-serine production by Corynebacterium glutamicum. Deletion of both thrE and serE decreased L-serine titer by 60%, suggesting the existence of other L-serine exporters. A comparative transcriptomics identified NCgl0254 and NCgl0255 as novel L-serine exporters. Further analysis of the contributions of ThrE, SerE, NCgl0254, and NCgl0255 found that SerE was the major L-serine exporter in C. glutamicum and these four L-serine exporters were responsible for 79.7% of L-serine export. Deletion of one L-serine exporter upregulated the transcription levels of the other three, which might be coursed by increased intracellular concentrations of L-serine. Overexpression of NCgl0254 and NCgl0255 increased L-serine titer by 20.8% in C. glutamicum A36, while overexpression of the four L-serine exporters increased L-serine production by 31.9% (41.1 g·L-1 ) in C. glutamicum A36. The identification of novel L-serine exporters in C. glutamicum will help to improve industrial production of L-serine.

An intricate regulation of WblA controlling production of silent tylosin analogues and abolishment of expressible nikkomycin

Genome sequencing has revealed that actinomycetes possess the potential to produce many more secondary metabolites than previously thought. The existing challenge is to devise efficient methods to activate these silent biosynthetic gene clusters (BGCs). In Streptomyces ansochromogenes, disruption of wblA, a pleiotropic regulatory gene, activated the expression of cryptic tylosin analogues and abolished nikkomycin production simultaneously. Overexpressing pathway-specific regulatory genes tylR1 and tylR2 can also trigger the biosynthesis of silent tylosin analogues, in which TylR1 exerted its function via enhancing tylR2 expression. Bacterial one-hybrid system experiments unveiled that WblA directly inhibits the transcription of tylR1 and tylR2 to result in the silence of tylosin analogues BGC. Furthermore, WblA can activate the nikkomycin production through up-regulating the transcription of pleiotropic regulatory gene adpA. More interestingly, AdpA can activate sanG (an activator gene in nikkomycin BGC) but repress wblA. Our studies provide a valuable insight into the complex functions of pleiotropic regulators.

Trehalose biosynthetic gene otsB of Corynebacterium glutamicum is regulated by whcE in response to oxidative stress

The gene whcE of Corynebacterium glutamicum plays a positive role in oxidative stress responses and the WhcE protein interacts with SpiE. By utilizing 2D-PAGE analysis, we identified the otsB gene to be under the control of whcE. The transcription of otsB, encoding trehalose 6-phosphatase, was stimulated by oxidative stress, and whcE and spiE were involved in diamide-mediated transcriptional stimulation. The ΔotsB strain was created and found to be sensitive to the thiol-specific oxidant diamide, suggesting a role of the gene in stress responses. Genes located upstream of otsB, such as NCgl2534 and otsA, formed an operon and purified WhcE was able to bind to the promoter region of the operon (P_NCgl2534), but the binding was only possible in the presence of the oxidant diamide. In addition, the transcriptional activation of P_NCgl2534 by WhcE was demonstrated in in vivo assays and the transcription was stimulated in cells exposed to the oxidant diamide. These findings indicate that WhcE is a transcriptional activator, and otsB, which is involved in trehalose biosynthesis, has a role in oxidative stress responses in C. glutamicum.

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

Among the Corynebacterium glutamicum ORFs that have been implicated in stress responses, we chose ORF cg3230, designated osnR, and analyzed it further. Unlike the osnR-deleted strain (ΔosnR), the osnR-overexpressing strain (P180-osnR) developed growth defects and increased sensitivity to various oxidants including H2O2. Transcription in the P180-osnR strain of genes such as sodA (superoxide dismutase), ftn (ferritin biosynthesis), and ahpD (alkyl hydroperoxide reductase; cg2674), which are involved in the detoxification of reactive oxygen species, was only 40% that of the wild type. However, transcription of katA, encoding H2O2-detoxifying catalase, was unchanged in this strain. Genes such as trxB (thioredoxin reductase) and mtr (mycothiol disulfide reductase), which play roles in redox homeostasis, also showed decreased transcription in the strain. 2D-PAGE analysis indicated that genes involved in redox reactions were considerably affected by osnR overexpression. The NADPH/NADP+ ratio of the P180-osnR strain (1.35) was higher than that of the wild-type stain (0.78). Collectively, the phenotypes of the ΔosnR and P180-osnR strains suggest a global regulatory role as well as a negative role for the gene in stress responses, particularly in katA-independent oxidative stress responses.

The actinobacterial WhiB-like (Wbl) family of transcription factors

The WhiB‐like (Wbl) family of proteins are exclusively found in Actinobacteria. Wbls have been shown to play key roles in virulence and antibiotic resistance in Mycobacteria and Corynebacteria, reflecting their importance during infection by the human pathogens Mycobacterium tuberculosis, Mycobacterium leprae and Corynebacterium diphtheriae. In the antibiotic‐producing Streptomyces, several Wbls have important roles in the regulation of morphological differentiation, including WhiB, a protein that controls the initiation of sporulation septation and the founding member of the Wbl family. In recent years, genome sequencing has revealed the prevalence of Wbl paralogues in species throughout the Actinobacteria. Wbl proteins are small (generally ~80–140 residues) and each contains four invariant cysteine residues that bind an O₂‐ and NO‐sensitive [4Fe–4S] cluster, raising the question as to how they can maintain distinct cellular functions within a given species. Despite their discovery over 25 years ago, the Wbl protein family has largely remained enigmatic. Here I summarise recent research in Mycobacteria, Corynebacteria and Streptomyces that sheds light on the biochemical function of Wbls as transcription factors and as potential sensors of O₂ and NO. I suggest that Wbl evolution has created diversity in protein–protein interactions, [4Fe–4S] cluster‐sensitivity and the ability to bind DNA.