Oxidative-stress detoxification and signalling in cyanobacteria: the crucial glutathione synthesis pathway supports the production of ergothioneine and ophthalmate

Current insights into molecular mechanisms of environmental stress tolerance in Cyanobacteria

The photoautotrophic nature of cyanobacteria, coupled with their fast growth and relative ease of genetic manipulation, makes these microorganisms very promising factories for the sustainable production of bio-products from atmospheric carbon dioxide. However, both in nature and in cultivation, cyanobacteria go through different abiotic stresses such as high light (HL) stress, heavy metal stress, nutrient limitation, heat stress, salt stress, oxidative stress, and alcohol stress. In recent years, significant improvement has been made in identifying the stress-responsive genes and the linked pathways in cyanobacteria and developing genome editing tools for their manipulation. Metabolic pathways play an important role in stress tolerance; their modification is also a very promising approach to adapting to stress conditions. Several synthetic as well as systems biology approaches have been developed to identify and manipulate genes regulating cellular responses under different stresses. In this review, we summarize the impact of different stresses on metabolic processes, the small RNAs, genes and heat shock proteins (HSPs) involved, changes in the metabolome and their adaptive mechanisms. The developing knowledge of the adaptive behaviour of cyanobacteria may also be utilised to develop better stress-responsive strains for various applications.

Reactivation and long-term stabilization of the [NiFe] Hox hydrogenase of Synechocystis sp. PCC6803 by glutathione after oxygen exposure

Hydrogenases are key enzymes forming or consuming hydrogen. The inactivation of these transition metal biocatalysts with oxygen limits their biotechnological applications. Oxygen-sensitive hydrogenases are distinguished from oxygen-insensitive (tolerant) ones by their initial hydrogen turnover rates influenced by oxygen. Several hydrogenases, such as the oxygen-sensitive bidirectional [NiFe] Hox hydrogenase (Hox) of the unicellular cyanobacterium Synechocystis sp. PCC6803, are reactivated after oxygen-induced deactivation by redox mechanisms. In cyanobacteria, the glutathione (GSH) redox buffer majorly controls intracellular redox potentials. The relationship between Hox turnover rates and the redox potential in its natural reaction environment is not fully understood. We thus determined hydrogen oxidation rates as activities of Hox in cell-free extracts of Synechocystis using benzyl viologen as artificial electron acceptor. We found that GSH modulates Hox hydrogen oxidation rates under oxygen-free conditions. After oxygen exposure, it influences the maximal turnover rate and aids in the reactivation of Hox. Moreover, GSH stabilizes the long-term Hox activity under anoxic conditions and attenuates oxygen-induced deactivation of Hox in a concentration-dependent manner, probably by fostering reactivation. Conversely, oxidized GSH (GSSG) negatively affects Hox activity and oxygen insensitivity. Using Blue Native PAGE followed by mass spectrometry, we showed that oxygen affects Hox complex integrity. The in silico predicted structure of the Hox complex and complexome analyses reveal the formation of various Hox subcomplexes under different conditions. Our findings refine our current classification of oxygen-hydrogenase interactions beyond sensitive and insensitive, which is particularly important for understanding hydrogenase function under physiological conditions in future.

sll1019 and slr1259 encoding glyoxalase II improve tolerance of Synechocystis sp. PCC 6803 to methylglyoxal- and ethanol- induced oxidative stress by glyoxalase pathway

The glyoxalase pathway is the primary detoxification mechanism for methylglyoxal (MG), a ubiquitous toxic metabolite that disrupts redox homeostasis. In the glyoxalase pathway, glyoxalase II (GlyII) can completely detoxify MG. Increasing the activity of the glyoxalase system can enhance the resistance of plants or organisms to abiotic stress, but the relevant mechanism remains largely unknown. In this study, we investigated the physiological functions of GlyII genes (sll1019 and slr1259) in Synechocystis sp. PCC 6803 under MG or ethanol stress based on transcriptome and metabolome data. High-performance liquid chromatography (HPLC) results showed that proteins Sll1019 and Slr1259 had GlyII activity. Under stress conditions, sll1019 and slr1259 protected the strain against oxidative stress by enhancing the activity of the glyoxalase pathway and raising the contents of antioxidants such as glutathione and superoxide dismutase. In the photosynthetic system, sll1019 and slr1259 indirectly affected the light energy absorption by strains, synthesis of photosynthetic pigments, and activities of photosystem I and photosystem II, which was crucial for the growth of the strain under stress conditions. In addition, sll1019 and slr1259 enhanced the tolerance of strain to oxidative stress by indirectly regulating metabolic networks, including ensuring energy acquisition, NADH and NADPH production, and phosphate and nitrate transport. This study reveals the mechanism by which sll1019 and slr1259 improve oxidative stress tolerance of strains by glyoxalase pathway. Our findings provide theoretical basis for breeding, seedling, and field production of abiotic stress tolerance-enhanced variety.IMPORTANCEThe glyoxalase system is present in most organisms, and it is the primary pathway for eliminating the toxic metabolite methylglyoxal. Increasing the activity of the glyoxalase system can enhance plant resistance to environmental stress, but the relevant mechanism is poorly understood. This study revealed the physiological functions of glyoxalase II genes sll1019 and slr1259 in Synechocystis sp. PCC 6803 under abiotic stress conditions and their regulatory effects on oxidative stress tolerance of strains. Under stress conditions, sll1019 and slr1259 enhanced the activity of the glyoxalase pathway and the antioxidant system, maintained photosynthesis, ensured energy acquisition, NADH and NADPH production, and phosphate and nitrate transport, thereby protecting the strain against oxidative stress. This study lays a foundation for further deciphering the mechanism by which the glyoxalase system enhances the tolerance of cells to abiotic stress, providing important information for breeding, seedling, and selection of plants with strong stress resistance.

Engineering Saccharomyces boulardii for Probiotic Supplementation of l-Ergothioneine

Saccharomyces boulardii, as a probiotic yeast, has shown great potential in regulating gut health and treating gastrointestinal diseases. Due to its unique antimicrobial and immune-regulating functions, it has become a significant subject of research in the field of probiotics. In this study, we aim to enhance the antioxidant properties of S. boulardii by producing l-ergothioneine (EGT). We first constructed a double knockout of ura3 and trp1 gene in S. boulardii to facilitate plasmid-based expressions. To further enable effective genome editing of S. boulardii, we implemented the PiggyBac system to transpose the heterologous gene expression cassettes into the chromosomes of S. boulardii. By using enhanced green fluorescent protein (EGFP) as the reporter gene, we achieved random chromosomal integration of EGFP expression cassette. By using PiggyBac transposon system, a great variety of EGT-producing strains was obtained, which is not possible for the conventional single target genome editing, and one best isolated top producer reached 17.50 mg/L EGT after 120 h cultivation. In summary, we have applied the PiggyBac transposon system to S. boulardii for the first time for genetic engineering. The engineered probiotic yeast S. boulardii has been endowed with new antioxidant properties and produces EGT. It has potential applications in developing novel therapeutics and dietary supplements for the prevention and treatment of gastrointestinal disorders.

Extensive Genomic Rearrangement of Catalase-Less Cyanobloom-Forming Microcystis aeruginosa in Freshwater Ecosystems

Many of the world's freshwater ecosystems suffer from cyanobacteria-mediated blooms and their toxins. However, a mechanistic understanding of why and how Microcystis aeruginosa dominates over other freshwater cyanobacteria during warmer summers is lacking. This paper utilizes comparative genomics with other cyanobacteria and literature reviews to predict the gene functions and genomic architectures of M. aeruginosa based on complete genomes. The primary aim is to understand this species' survival and competitive strategies in warmer freshwater environments. M. aeruginosa strains exhibiting a high proportion of insertion sequences (~ 11%) possess genomic structures with low synteny across different strains. This indicates the occurrence of extensive genomic rearrangements and the presence of many possible diverse genotypes that result in greater population heterogeneities than those in other cyanobacteria in order to increase survivability during rapidly changing and threatening environmental challenges. Catalase-less M. aeruginosa strains are even vulnerable to low light intensity in freshwater environments with strong ultraviolet radiation. However, they can continuously grow with the help of various defense genes (e.g., egtBD, cruA, and mysABCD) and associated bacteria. The strong defense strategies against biological threats (e.g., antagonistic bacteria, protozoa, and cyanophages) are attributed to dense exopolysaccharide (EPS)-mediated aggregate formation with efficient buoyancy and the secondary metabolites of M. aeruginosa cells. Our review with extensive genome analysis suggests that the ecological vulnerability of M. aeruginosa cells can be overcome by diverse genotypes, secondary defense metabolites, reinforced EPS, and associated bacteria.

Ophthalmate is a new regulator of motor functions via CaSR: implications for movement disorders

Dopamine's role as the principal neurotransmitter in motor functions has long been accepted. We broaden this conventional perspective by demonstrating the involvement of non-dopaminergic mechanisms. In mouse models of Parkinson's disease, we observed that L-DOPA elicited a substantial motor response even when its conversion to dopamine was blocked by inhibiting the enzyme aromatic amino acid decarboxylase (AADC). Remarkably, the motor activity response to L-DOPA in the presence of an AADC inhibitor (NSD1015) showed a delayed onset, yet greater intensity and longer duration, peaking at 7 h, compared to when L-DOPA was administered alone. This suggests an alternative pathway or mechanism, independent of dopamine signalling, mediating the motor functions. We sought to determine the metabolites associated with the pronounced hyperactivity observed, using comprehensive metabolomics analysis. Our results revealed that the peak in motor activity induced by NSD1015/L-DOPA in Parkinson's disease mice is associated with a surge (20-fold) in brain levels of the tripeptide ophthalmic acid (also known as ophthalmate in its anionic form). Interestingly, we found that administering ophthalmate directly to the brain rescued motor deficits in Parkinson's disease mice in a dose-dependent manner. We investigated the molecular mechanisms underlying ophthalmate's action and discovered, through radioligand binding and cAMP-luminescence assays, that ophthalmate binds to and activates the calcium-sensing receptor (CaSR). Additionally, our findings demonstrated that a CaSR antagonist inhibits the motor-enhancing effects of ophthalmate, further solidifying the evidence that ophthalmate modulates motor functions through the activation of the CaSR. The discovery of ophthalmate as a novel regulator of motor function presents significant potential to transform our understanding of brain mechanisms of movement control and the therapeutic management of related disorders.

Effects of Acremonium terricola Culture on Lactation Performance, Immune Function, Antioxidant Capacity, and Intestinal Flora of Sows

This study aimed to determine the effects of different doses of Acremonium terricola culture (ATC) on lactation performance, immune function, antioxidant capacity, and intestinal flora of sows. Forty-five Landrace sows (3-6 parity) were randomly assigned to the following three treatments from 85 days of gestation to 21 days after farrowing: a control diet (CON, basal diet), a low-dose Acremonium terricola culture diet (0.2% ATC, basal diet + 0.2% ATC), and a high-dose Acremonium terricola culture diet (0.4% ATC, basal diet + 0.4% ATC). Compared with the CON group, the supplementation of 0.2% ATC increased the average daily milk yield of sows by 4.98%, increased milk fat, total solids, and freezing point depression on day 1 postpartum (p < 0.05), increased serum concentration of Triiodothyronine, Thyroxin, and Estradiol on day 21 postpartum (p < 0.05). Compared with the CON group, the supplementation of 0.4% ATC increased the average daily milk yield of sows by 9.38% (p < 0.05). Furthermore, the supplementation of 0.2% ATC increased serum concentration of IgG, IgM, and IFN-γ, CD4 on day 1 postpartum (p < 0.05) and increased serum concentration of immunoglobulin A ( IgA), immunoglobulin G (IgG), immunoglobulin M ( IgM), complement 3 (C3), cluster of differentiation 4 (CD4), cluster of differentiation 8 (CD8), interferon-γ (IFN-γ) on day 21 postpartum (p < 0.05), while the supplementation of 0.4% ATC reduced serum concentration of IL-2 on day 21 postpartum (p < 0.05). Moreover, the supplementation of 0.4% ATC significantly increased serum concentration of catalase (CAT) (p < 0.05). Additionally, the supplementation of ATC affected the relative abundance of the intestinal flora at different taxonomic levels in sows and increased the abundance of beneficial bacteria such as in the norank_f__Eubacterium_coprostanoligenes group, Eubacterium_coprostanoligenes group, and Lachnospiraceae_XPB1014 group of sows, while reducing the abundance of harmful bacteria such as Phascolarctobacterium and Clostridium_sensu_stricto_1. These data revealed that the supplementation of ATC during late gestation and lactation can improve lactation performance, immune function, antioxidant capacity, and the gut microbiota. Compared with supplementation of 0.4% ATC, 0.2% ATC enhances the levels of thyroid-related hormones, specific antibodies, and cytokines in serum, promotes the diversity of beneficial gut microbiota, beneficial bacteria in the intestine, reduces the population of harmful bacteria, and thereby bolsters the immunity of sows. Hence, 0.2% ATC is deemed a more optimal concentration.

Glutathione-mediated redox regulation in Cryptococcus neoformans impacts virulence

The fungal pathogen Cryptococcus neoformans is well adapted to its host environment. It has several defence mechanisms to evade oxidative and nitrosative agents released by phagocytic host cells during infection. Among them, melanin production is linked to both fungal virulence and defence against harmful free radicals that facilitate host innate immunity. How C. neoformans manipulates its redox environment to facilitate melanin formation and virulence is unclear. Here we show that the antioxidant glutathione is inextricably linked to redox-active processes that facilitate melanin and titan cell production, as well as survival in macrophages and virulence in a murine model of cryptococcosis. Comparative metabolomics revealed that disruption of glutathione biosynthesis leads to accumulation of reducing and acidic compounds in the extracellular environment of mutant cells. Overall, these findings highlight the importance of redox homeostasis and metabolic compensation in pathogen adaptation to the host environment and suggest new avenues for antifungal drug development.

Ophthalmic acid is a glutathione regulating tripeptide

Since its discovery in 1958 in the lens of cows, ophthalmic acid (OPH) has stood in the shadow of its anti-oxidant analog: glutathione (GSH). Lacking the thiol group that gives GSH many of its important properties, ophthalmic acid's function has remained elusive, and it has been widely presumed to be an accidental product of the same enzymes. In this review, we compile evidence demonstrating that OPH is a ubiquitous metabolite found in bacteria, plants, fungi, and animals, produced through several layers of metabolic regulation. We discuss the limitations of the oft-repeated suggestions that aberrations in OPH levels should solely indicate GSH deficiency or oxidative stress. Finally, we discuss the available literature and suggest OPH's role in metabolism as a GSH-regulating tripeptide; controlling both cellular and organelle influx and efflux of GSH, as well as modulating GSH-dependent reactions and signaling. Ultimately, we hope that this review reinvigorates and directs more research into this versatile metabolite.

Natural Products That Contain Higher Homologated Amino Acids

This review focuses on discussing natural products (NPs) that contain higher homologated amino acids (homoAAs) in the structure as well as the proposed and characterized biosynthesis of these non-proteinogenic amino acids. Homologation of amino acids includes the insertion of a methylene group into its side chain. It is not a very common modification found in NP biosynthesis as approximately 450 homoAA-containing NPs have been isolated from four bacterial phyla (Cyanobacteria, Actinomycetota, Myxococcota, and Pseudomonadota), two fungal phyla (Ascomycota and Basidiomycota), and one animal phylum (Porifera), except for a few examples. Amino acids that are found to be homologated and incorporated in the NP structures include the following ten amino acids: alanine, arginine, cysteine, isoleucine, glutamic acid, leucine, phenylalanine, proline, serine, and tyrosine, where isoleucine, leucine, phenylalanine, and tyrosine share the comparable enzymatic pathway. Other amino acids have their individual homologation pathway (arginine, proline, and glutamic acid for bacteria), likely utilize the primary metabolic pathway (alanine and glutamic acid for fungi), or have not been reported (cysteine and serine). Despite its possible high potential in the drug discovery field, the biosynthesis of homologated amino acids has a large room to explore for future combinatorial biosynthesis and metabolic engineering purpose.

Melatonin disturbed rumen microflora structure and metabolic pathways in vitro

IMPORTANCE

In in vitro studies, it has been found that the effects of MLT on rumen microorganisms and metabolites can change the rumen flora structure, significantly inhibit the relative abundance of harmful Acinetobacter, and improve the relative abundance of beneficial bacteria. MLT may regulate the "arginine-glutathione" pathway, "phenylalanine, tyrosine and tryptophan biosynthesis-tryptophan generation" branch, "tryptophan-kynurenine" metabolism, and "tryptophan-tryptamine-serotonin" pathway through microorganisms.

A newly identified small tRNA fragment reveals the regulation of different wool types and oxidative stress in lambs

Novel small RNAs derived from tRNAs are continuously identified, however, their biological functions are rarely reported. Here, we accidentally found the reads peak at 32nt during statistical analysis on the miRNA-seq data of lamb skin tissue, and found that it was related to the wool type of lambs. This 32nt peak was composed of small tRNA fragments. The main component sequence of this peak was a novel small tRNA derived from Glycyl tRNA (tRNAGly), the expression level of tRNAGly-derived tRNA fragments (tRFGly) was 5.77 folds higher in the coarse wool lambs than that in the fine wool lambs. However, in contrast, the expression of tRNAGly in the skin of fine wool lambs is 6.28 folds more than that in coarse wool lambs. tRNAGly promoted the synthesis of high glycine protein including KAP6 in fine wool lamb skin. These proteins were reported as the major genes for fine curly wool. Integrative analysis of target gene prediction, proteomics and metabolomics results revealed that tRFGly reduced the level of reactive oxygen species (ROS) in the skin of coarse wool lambs by targeted inhibition of the Metabolic signal and the corresponding Glutathione metabolic pathway, on the contrary, the level of oxidative stress in the skin of fine wool lambs was significantly higher. This study revealed for the first time the relationship between tRNAGly and its derived tRFGly and animal traits. tRFGly has the function of targeting and regulating protein synthesis. At the same time, tRFGly can reduce the expression of its resource complete tRNA, thereby reducing its ability to transport specific amino acid and affecting the expression of corresponding proteins.

The Glutathione System: A Journey from Cyanobacteria to Higher Eukaryotes

From bacteria to plants and humans, the glutathione system plays a pleiotropic role in cell defense against metabolic, oxidative and metal stresses. Glutathione (GSH), the γ-L-glutamyl-L-cysteinyl-glycine nucleophile tri-peptide, is the central player of this system that acts in redox homeostasis, detoxification and iron metabolism in most living organisms. GSH directly scavenges diverse reactive oxygen species (ROS), such as singlet oxygen, superoxide anion, hydrogen peroxide, hydroxyl radical, nitric oxide and carbon radicals. It also serves as a cofactor for various enzymes, such as glutaredoxins (Grxs), glutathione peroxidases (Gpxs), glutathione reductase (GR) and glutathione-S-transferases (GSTs), which play crucial roles in cell detoxication. This review summarizes what is known concerning the GSH-system (GSH, GSH-derived metabolites and GSH-dependent enzymes) in selected model organisms (Escherichia coli, Saccharomyces cerevisiae, Arabidopsis thaliana and human), emphasizing cyanobacteria for the following reasons. Cyanobacteria are environmentally crucial and biotechnologically important organisms that are regarded as having evolved photosynthesis and the GSH system to protect themselves against the ROS produced by their active photoautotrophic metabolism. Furthermore, cyanobacteria synthesize the GSH-derived metabolites, ergothioneine and phytochelatin, that play crucial roles in cell detoxication in humans and plants, respectively. Cyanobacteria also synthesize the thiol-less GSH homologs ophthalmate and norophthalmate that serve as biomarkers of various diseases in humans. Hence, cyanobacteria are well-suited to thoroughly analyze the role/specificity/redundancy of the players of the GSH-system using a genetic approach (deletion/overproduction) that is hardly feasible with other model organisms (E. coli and S. cerevisiae do not synthesize ergothioneine, while plants and humans acquire it from their soil and their diet, respectively).

Safe and Effective Antioxidant: The Biological Mechanism and Potential Pathways of Ergothioneine in the Skin

Ergothioneine, a sulfur-containing micromolecular histidine derivative, has attracted increasing attention from scholars since it was confirmed in the human body. In the human body, ergothioneine is transported and accumulated specifically through OCTN-1, especially in the mitochondria and nucleus, suggesting that it can target damaged cells and tissues as an antioxidant. It shows excellent antioxidant, anti-inflammatory effects, and anti-aging properties, and inhibits melanin production. It is a mega antioxidant that may participate in the antioxidant network system and promote the reducing glutathione regeneration cycle. This review summarizes studies on the antioxidant effects of ergothioneine on various free radicals in vitro to date and systematically introduces its biological activities and potential mechanisms, mostly in dermatology. Additionally, the application of ergothioneine in cosmetics is briefly summarized. Lastly, we propose some problems that require solutions to understand the mechanism of action of ergothioneine. We believe that ergothioneine has good prospects in the food and cosmetics industries, and can thus meet some needs of the health and beauty industry.

Cysteine reduced the inhibition of CO2 on heterotrophic denitrification: Restoring redox balance, facilitating iron acquisition and carbon metabolism

The direct effect of CO₂ on denitrification has attracted great attention currently. Our previous studies have confirmed that CO₂ inhibited heterotrophic denitrification and caused high nitrite accumulation and nitrous oxide emission. Cysteine is a widely reported bio-accelerator; however, its effect on denitrification under CO₂ exposure remains unknown. In this paper, the effect of cysteine on heterotrophic denitrification and its mechanisms under CO₂ exposure were explored with the model denitrifier, Paracoccus denitrificans. We observed that total nitrogen removal increased from 17.9% to 90.4% as cysteine concentration increased from 0 to 50 μM, probably due to restoration of cell growth and viability. Further study showed that cysteine reduced the inhibition of CO₂ on denitrification due to multiple positive influences: (1) regulating glutathione metabolism to eliminate intracellular reactive nitrogen species (RNS), while reducing extracellular polymeric substances (EPS) levels and altering its composition, ultimately restoring cell membrane integrity (2) facilitating the transport and metabolism of carbon sources to increase NADH production, and (3) increasing intracellular iron and up-regulating the expression of key iron transporters genes (AfuA, AfuB, ExbB and TonB) to restore the transport and consumption of electron. This study suggests that cysteine can be added to recover heterotrophic denitrification performance after inhibition by elevated CO₂.

Ergothioneine as a Natural Antioxidant Against Oxidative Stress-Related Diseases

L-Ergothioneine (EGT) is a natural antioxidant derived from microorganisms, especially in edible mushrooms. EGT is found to be highly accumulated in tissues that are susceptible to oxidative damage, and it has attracted extensive attention due to its powerful antioxidant activity and the tight relationships of this natural product with various oxidative stress-related diseases. Herein, we 1) introduce the biological source and in vivo distribution of EGT; 2) review the currently available evidence concerning the relationships of EGT with diabetes, ischemia-reperfusion injury-related diseases like cardiovascular diseases and liver diseases, neurodegenerative diseases, and other diseases pathogenically associated with oxidative stress; 3) summarize the potential action mechanisms of EGT against these diseases; 4) discuss the advantages of EGT over other antioxidants; and 5) also propose several future research perspectives for EGT. These may help to promote the future application of this attractive natural antioxidant.

C-ferroptosis is an iron-dependent form of regulated cell death in cyanobacteria

Ferroptosis is an oxidative and iron-dependent form of regulated cell death (RCD) recently described in eukaryotic organisms like animals, plants, and parasites. Here, we report that a similar process takes place in the photosynthetic prokaryote Synechocystis sp. PCC 6803 in response to heat stress. After a heat shock, Synechocystis sp. PCC 6803 cells undergo a cell death pathway that can be suppressed by the canonical ferroptosis inhibitors, CPX, vitamin E, Fer-1, liproxstatin-1, glutathione (GSH), or ascorbic acid (AsA). Moreover, as described for eukaryotic ferroptosis, this pathway is characterized by an early depletion of the antioxidants GSH and AsA, and by lipid peroxidation. These results indicate that all of the hallmarks described for eukaryotic ferroptosis are conserved in photosynthetic prokaryotes and suggest that ferroptosis might be an ancient cell death program.

Regulation of antioxidant defense and glyoxalase systems in cyanobacteria

Oxidative stress is common consequence of abiotic stress in plants as well as cyanobacteria caused by generation of reactive oxygen species (ROS), an inevitable product of respiration and photosynthetic electron transport. ROS act as signalling molecule at low concentration however, when its production exceeds the endurance capacity of antioxidative defence system, the organisms suffer oxidative stress. A highly toxic metabolite, methylglyoxal (MG) is also produced in cyanobacteria in response to various abiotic stresses which consequently augment the ensuing oxidative damage. Taking recourse to the common lineage of eukaryotic plants and cyanobacteria, it would be worthwhile to explore the regulatory role of glyoxalase system and antioxidative defense mechanism in combating abiotic stress in cyanobacteria. This review provides comprehensive information on the complete glyoxalase system (GlyI, GlyII and GlyIII) in cyanobacteria. Furthermore, it elucidates the recent understanding regarding the production of ROS and MG, noteworthy link between intracellular MG and ROS and its detoxification via synchronization of antioxidants (enzymatic and non-enzymatic) and glyoxalase systems using glutathione (GSH) as common co-factor.

Determination of the Intracellular Complexation of Inorganic and Methylmercury in Cyanobacterium Synechocystis sp. PCC 6803

Understanding of mercury (Hg) complexation with low molecular weight (LMW) bioligands will help elucidate its speciation. In natural waters, the rate of this complexation is governed by physicochemical, geochemical, and biochemical parameters. However, the role of bioligands involved in Hg intracellular handling by aquatic microorganisms is not well documented. Here, we combine the use of isotopically labeled Hg species (inorganic and monomethylmercury, iHg and MeHg) with gas or liquid chromatography coupling to elemental and molecular mass spectrometry to explore the role of intracellular biogenic ligands involved in iHg and MeHg speciation in cyanobacterium Synechocystis sp. PCC 6803, a representative phytoplankton species. This approach allowed to track resulting metabolic and newly found intracellular Hg biocomplexes (e.g., organic thiols) in Synechocystis sp. PCC 6803 finding different intracellular Hg species binding affinities with both high and low molecular weight (HMW and LMW) bioligands in the exponential and stationary phase. Furthermore, the parallel detection with both elemental and molecular ionization sources allowed the sensitive detection and molecular identification of glutathione (GSH) as the main low molecular weight binding ligand to iHg ((GS)₂-Hg) and MeHg (GS-MeHg) in the cytosolic fraction. Such a novel experimental approach expands our knowledge on the role of biogenic ligands involved in iHg and MeHg intracellular handling in cyanobacteria.

Dissecting Metabolism of Leaf Nodules in Ardisia crenata and Psychotria punctata

Root-microbe interaction and its specialized root nodule structures and functions are well studied. In contrast, leaf nodules harboring microbial endophytes in special glandular leaf structures have only recently gained increased interest as plant-microbe phyllosphere interactions. Here, we applied a comprehensive metabolomics platform in combination with natural product isolation and characterization to dissect leaf and leaf nodule metabolism and functions in Ardisia crenata (Primulaceae) and Psychotria punctata (Rubiaceae). The results indicate that abiotic stress resilience plays an important part within the leaf nodule symbiosis of both species. Both species showed metabolic signatures of enhanced nitrogen assimilation/dissimilation pattern and increased polyamine levels in nodules compared to leaf lamina tissue potentially involved in senescence processes and photosynthesis. Multiple links to cytokinin and REDOX-active pathways were found. Our results further demonstrate that secondary metabolite production by endophytes is a key feature of this symbiotic system. Multiple anhydromuropeptides (AhMP) and their derivatives were identified as highly characteristic biomarkers for nodulation within both species. A novel epicatechin derivative was structurally elucidated with NMR and shown to be enriched within the leaf nodules of A. crenata. This enrichment within nodulated tissues was also observed for catechin and other flavonoids indicating that flavonoid metabolism may play an important role for leaf nodule symbiosis of A. crenata. In contrast, pavettamine was only detected in P. punctata and showed no nodule specific enrichment but a developmental effect. Further natural products were detected, including three putative unknown depsipeptide structures in A. crenata leaf nodules. The analysis presents a first metabolomics reference data set for the intimate interaction of microbes and plants in leaf nodules, reveals novel metabolic processes of plant-microbe interaction as well as the potential of natural product discovery in these systems.

Calm on the surface, dynamic on the inside. Molecular homeostasis of Anabaena sp. PCC 7120 nitrogen metabolism

Nitrogen sources are all converted into ammonium/ia as a first step of assimilation. It is reasonable to expect that molecular components involved in the transport of ammonium/ia across biological membranes connect with the regulation of both nitrogen and central metabolism. We applied both genetic (i.e., Δamt mutation) and environmental treatments to a target biological system, the cyanobacterium Anabaena sp PCC 7120. The aim was to both perturb nitrogen metabolism and induce multiple inner nitrogen states, respectively, followed by targeted quantification of key proteins, metabolites and enzyme activities. The absence of AMT transporters triggered a substantial whole-system response, affecting enzyme activities and quantity of proteins and metabolites, spanning nitrogen and carbon metabolisms. Moreover, the Δamt strain displayed a molecular fingerprint indicating nitrogen deficiency even under nitrogen replete conditions. Contrasting with such dynamic adaptations was the striking near-complete lack of an externally measurable altered phenotype. We conclude that this species evolved a highly robust and adaptable molecular network to maintain homeostasis, resulting in substantial internal but minimal external perturbations. This analysis provides evidence for a potential role of AMT transporters in the regulatory/signalling network of nitrogen metabolism and the existence of a novel fourth regulatory mechanism controlling glutamine synthetase activity.

Facile, Smart, and Degradable Metal-Organic Framework Nanopesticides Gated with FeIII-Tannic Acid Networks in Response to Seven Biological and Environmental Stimuli

Nanopesticides were selected as one of the top 10 emerging technologies in chemistry that will change our world in 2019. Facile, smart, and degradable metal-organic framework MIL-101(Fe^III) nanopesticides gated with Fe^III-tannic acid (TA) networks are created using a universal strategy. The capping of the Fe^III-TA network gatekeepers is instinctively oriented by the coordinatively unsaturated Fe^III sites on the surfaces of the MIL-101(Fe^III) nanocarriers; thus, their combination is perfectly matched. This is the first example that one smart gated nanoparticle is integrated with seven stimuli-responsive performances to meet the diverse controlled release of encapsulated cargos by the disassembly of the gatekeepers and/or the degradation of the nanocarriers. More importantly, each of the seven stimuli (acidic/alkaline pH, H₂O₂, glutathione, phosphates, ethylenediaminetetraacetate, and near-infrared light of sunlight) is closely related to the biological and natural environments of crops, and the biocompatible nanocarriers are eventually degraded against bioaccumulation even if the nanopesticides enter crops. These mechanisms of the stimuli-responsive controlled release are identified and clearly elaborated. It is found that the natural polyphenol can improve the wettability of aqueous droplets of nanopesticides on model hydrophobic foliage for pesticide adhesion and retention. The nanopesticides encapsulated with the fungicide tebuconazole show high fungicidal activities against pathogenic fungi Rhizoctonia solani (rice sheath blight) and Fusarium graminearum (wheat head blight); good safety on seed germination, seedling emergence, and plant height of wheat by seed dressing; and satisfactory control efficacy in wheat powdery mildew caused by Blumeria graminis in the greenhouse. The nanopesticides have potential applications in the field for high quality and yield of agricultural production.

Genetic, Genomics, and Responses to Stresses in Cyanobacteria: Biotechnological Implications

Cyanobacteria are widely-diverse, environmentally crucial photosynthetic prokaryotes of great interests for basic and applied science. Work to date has focused mostly on the three non-nitrogen fixing unicellular species Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002, which have been selected for their genetic and physiological interests summarized in this review. Extensive "omics" data sets have been generated, and genome-scale models (GSM) have been developed for the rational engineering of these cyanobacteria for biotechnological purposes. We presently discuss what should be done to improve our understanding of the genotype-phenotype relationships of these models and generate robust and predictive models of their metabolism. Furthermore, we also emphasize that because Synechocystis PCC 6803, Synechococcus PCC 7942, and Synechococcus PCC 7002 represent only a limited part of the wide biodiversity of cyanobacteria, other species distantly related to these three models, should be studied. Finally, we highlight the need to strengthen the communication between academic researchers, who know well cyanobacteria and can engineer them for biotechnological purposes, but have a limited access to large photobioreactors, and industrial partners who attempt to use natural or engineered cyanobacteria to produce interesting chemicals at reasonable costs, but may lack knowledge on cyanobacterial physiology and metabolism.

Modulating NF-κB, MAPK, and PI3K/AKT signaling by ergothioneine attenuates iron overload-induced hepatocellular injury in rats

The liver is highly susceptible to iron overload-evoked oxidative injury. Ergothioneine is a thio-histidine amino acid that has exhibited strong antioxidant and metal chelating activities. This study aimed at exploring the potential modulating effects of ergothioneine on iron-triggered liver injury. The results showed that ergothioneine inhibited iron-evoked inflammation and apoptosis as demonstrated by a significant reduction in tumor necrosis factor-α and interleukin-6 levels and in caspase-3 activity. Ergothioneine significantly improved liver cell survival as indicated by modulating phosphatidylinositol 3-kinase/protein kinase B signaling. Consistent with reduced necrotic cell death, ergothioneine diminished the iron-evoked histopathological changes and decreased serum activity of the liver enzymes. Mechanistically, ergothioneine reduced nuclear translocation of nuclear factor kappa B p65 and modulated p38 mitogen-activated protein kinase/c-Fos signaling. In addition, it enhanced the liver tissue antioxidant potential and curbed hepatic iron load. Together, these results point out the modulatory effects of ergothioneine on iron-evoked liver cell injury that are possibly mediated via anti-inflammatory, antioxidant, and possible iron chelation capabilities.

The biology of ergothioneine, an antioxidant nutraceutical

Ergothioneine (ERG) is an unusual thio-histidine betaine amino acid that has potent antioxidant activities. It is synthesised by a variety of microbes, especially fungi (including in mushroom fruiting bodies) and actinobacteria, but is not synthesised by plants and animals who acquire it via the soil and their diet, respectively. Animals have evolved a highly selective transporter for it, known as solute carrier family 22, member 4 (SLC22A4) in humans, signifying its importance, and ERG may even have the status of a vitamin. ERG accumulates differentially in various tissues, according to their expression of SLC22A4, favouring those such as erythrocytes that may be subject to oxidative stress. Mushroom or ERG consumption seems to provide significant prevention against oxidative stress in a large variety of systems. ERG seems to have strong cytoprotective status, and its concentration is lowered in a number of chronic inflammatory diseases. It has been passed as safe by regulatory agencies, and may have value as a nutraceutical and antioxidant more generally.

Measurement of Ascorbic Acid and Glutathione Content in Cyanobacterium Synechocystis sp. PCC 6803

Ascorbic acid (AsA) and gluthathione (GSH) are two key components of the antioxidant machinery of eukaryotic and prokaryotic cells. The cyanobacterium Synechocystis sp. PCC 6803 presents both compounds in different concentrations (AsA, 20-100 μM and GSH, 2-5 mM). Therefore, it is important to have precise and sensitive methods to determine the redox status in the cell and to detect variations in this antioxidants. In this protocol, we describe an improved method to estimate the content of both antioxidants (in their reduced and oxidized forms) from the same sample obtained from liquid cultures of Synechocystis sp. PCC 6803.

Methylglyoxal Detoxification Revisited: Role of Glutathione Transferase in Model Cyanobacterium Synechocystis sp. Strain PCC 6803

In most organisms, methylglyoxal (MG), a toxic metabolite by-product that causes diabetes in humans, is predominantly detoxified by the glyoxalase enzymes. This process begins with the so-called “spontaneous” conjugation of MG with the cytoprotectant metabolite glutathione (GSH). In this study, we unravel a logical, but as yet unsuspected, link between MG detoxification and a (prokaryotic) representative of the ubiquitous glutathione transferase (GST) enzymes. We show that a GST of a model cyanobacterium plays a prominent role in the detoxification of MG in catalyzing its conjugation with GSH. This finding is important because this reaction, always regarded as nonenzymatic, could exist in plants and/or human and thus have an impact on agriculture and/or human health.

Methylglyoxal (MG) is a detrimental metabolic by-product that threatens most organisms (in humans MG causes diabetes). MG is predominantly detoxified by the glyoxalase pathway. This process begins with the conjugation of MG with glutathione (GSH), yielding a hemithioacetal product that is subsequently transformed by the glyoxalase enzymes into d-lactate and GSH. MG has been overlooked in photosynthetic organisms, although they inevitably produce it not only by the catabolism of sugars, lipids, and amino acids, as do heterotrophic organisms, but also by their active photoautotrophic metabolism. This is especially true for cyanobacteria that are regarded as having developed photosynthesis and GSH-dependent enzymes to detoxify the reactive oxygen species produced by their photosynthesis (CO₂ assimilation) and respiration (glucose catabolism), which they perform in the same cell compartment. In this study, we used a combination of in vivo and in vitro approaches to characterize a logical, but as yet never described, link between MG detoxification and a (prokaryotic) representative of the evolutionarily conserved glutathione transferase (GST) detoxification enzymes. We show that the Sll0067 GST of the model cyanobacterium Synechocystis sp. strain PCC 6803 plays a prominent role in MG tolerance and detoxification, unlike the other five GSTs of this organism. Sll0067 catalyzes the conjugation of MG with GSH to initiate its elimination driven by glyoxalases. These results are novel because the conjugation of MG with GSH is always described as nonenzymatic. They will certainly stimulate the analysis of Sll0067 orthologs from other organisms with possible impacts on human health (development of biomarkers or drugs) and/or agriculture.

Recent Advances in the Photoautotrophic Metabolism of Cyanobacteria: Biotechnological Implications

Cyanobacteria constitute the only phylum of oxygen-evolving photosynthetic prokaryotes that shaped the oxygenic atmosphere of our planet. Over time, cyanobacteria have evolved as a widely diverse group of organisms that have colonized most aquatic and soil ecosystems of our planet and constitute a large proportion of the biomass that sustains the biosphere. Cyanobacteria synthesize a vast array of biologically active metabolites that are of great interest for human health and industry, and several model cyanobacteria can be genetically manipulated. Hence, cyanobacteria are regarded as promising microbial factories for the production of chemicals from highly abundant natural resources, e.g., solar energy, CO2, minerals, and waters, eventually coupled to wastewater treatment to save costs. In this review, we summarize new important discoveries on the plasticity of the photoautotrophic metabolism of cyanobacteria, emphasizing the coordinated partitioning of carbon and nitrogen towards growth or compound storage, and the importance of these processes for biotechnological perspectives. We also emphasize the importance of redox regulation (including glutathionylation) on these processes, a subject which has often been overlooked.

3-Bromotetrazine: labelling of macromolecules via monosubstituted bifunctional s-tetrazines

We report the synthesis and first characterisation of the novel chemical probe 3-bromotetrazine and establish its reactivity towards nucleophiles. This led to the synthesis of several novel classes of 3-monosubstituted s-tetrazines. A remarkable functional group selectivity is observed and is utilised to site-selectively functionalise different complex molecules. The stability of 3-bromotetrazine under the reaction conditions facilitated the development of a protocol for protein functionalisation, which enabled a "minimal", bifunctional tetrazine unit as a bio-orthogonal handle for inverse electron demand Diels-Alder reactions. Additionally, a novel tetrazine-based chemical probe was developed and its application in the context of thiol-targeted natural product isolation and labelling of mammalian cells is demonstrated.

Regeneration of ergothioneine after reaction with singlet oxygen

Ergothioneine (ET), an imidazole-2-thione derivative of histidine betaine, is generally considered an antioxidant. Important antioxidants are typically regenerated from their oxidized products, to prevent the interceptors from being lost after a single chemical reaction with a reactive oxygen species. However, no mechanism for the complete regeneration of ET has yet been uncovered. Here we define a non-enzymatic multi-step cycle for the regeneration of ET after reaction with singlet oxygen (¹O₂). All reaction steps were verified by density functional theory computations. Four molecules of GSH are used per turn to detoxify ¹O₂ to water. Pure ¹O₂ was generated by thermolysis at 37 °C of the endoperoxide DHPNO₂. Addition of 1 mM ET to 10 mM DHPNO₂ and 10 mM GSH increased the production of oxidized GSH (GSSG), measured by LC-MS/MS, by a factor of 26 (water) and 28 (D₂O), respectively. In the same assay, the ring of ET alone was able to drive the cycle at equal speed; thus, the zwitterionic amino acid backbone was not involved. Our data suggest that ET reacts at least 4-fold faster with ¹O₂ than ascorbic acid. ET must now be viewed as tightly linked with the GSH/GSSG redox couple. The necessary thiol foundation is present in all mammalian and vertebrate cells, and also in all species that generate ET, such as cyanobacteria, mycobacteria, and fungi. Regeneration provides a decisive advantage for ET over other reactive, but non-recoverable, compounds. Our findings substantiate the importance of ET for the eradication of noxious ¹O₂.

Biogas Biocatalysis: Methanotrophic Bacterial Cultivation, Metabolite Profiling, and Bioconversion to Lactic Acid

Anaerobic digestion (AD) of waste substrates, and renewable biomass and crop residues offers a means to generate energy-rich biogas. However, at present, AD-derived biogas is primarily flared or used for combined heat and power (CHP), in part due to inefficient gas-to-liquid conversion technologies. Methanotrophic bacteria are capable of utilizing methane as a sole carbon and energy source, offering promising potential for biological gas-to-liquid conversion of AD-derived biogas. Here, we report cultivation of three phylogenetically diverse methanotrophic bacteria on biogas streams derived from AD of a series of energy crop residues. Strains maintained comparable central metabolic activity and displayed minimal growth inhibition when cultivated under batch configuration on AD biogas streams relative to pure methane, although metabolite analysis suggested biogas streams increase cellular oxidative stress. In contrast to batch cultivation, growth arrest was observed under continuous cultivation configuration, concurrent with increased biosynthesis and excretion of lactate. We examined the potential for enhanced lactate production via the employ of a pyruvate dehydrogenase mutant strain, ultimately achieving 0.027 g lactate/g DCW/h, the highest reported lactate specific productivity from biogas to date.

Production of Ophthalmic Acid Using Engineered Escherichia coli

Ophthalmic acid (OA; l-γ-glutamyl-l-2-aminobutyryl-glycine) is an analog of glutathione (GSH; l-γ-glutamyl-l-cysteinyl-glycine) in which the cysteine moiety is replaced by l-2-aminobutyrate. OA is a useful peptide for the pharmaceutical and/or food industries. Herein, we report a method for the production of OA using engineered Escherichia coli cells. yggS-deficient E. coli, which lacks the highly conserved pyridoxal 5'-phosphate-binding protein YggS and naturally accumulates OA, was selected as the starting strain. To increase the production of OA, we overexpressed the OA biosynthetic enzymes glutamate-cysteine ligase (GshA) and glutathione synthase (GshB), desensitized the product inhibition of GshA, and eliminated the OA catabolic enzyme γ-glutamyltranspeptidase. The production of OA was further enhanced by the deletion of miaA and ridA with the aim of increasing the availability of ATP and attenuating the unwanted degradation of amino acids, respectively. The final strain developed in this study successfully produced 277 μmol/liter of OA in 24 h without the formation of by-products in a minimal synthetic medium containing 1 mM each glutamate, 2-aminobutyrate, and glycine.IMPORTANCE Ophthalmic acid (OA) is a peptide that has the potential for use in the pharmaceutical and/or food industries. An efficient method for the production of OA would allow us to expand our knowledge about its physiological functions and enable the industrial/pharmaceutical application of this compound. We demonstrated the production of OA using Escherichia coli cells in which OA biosynthetic enzymes and degradation enymes were engineered. We also showed that unique approaches, including the use of a ΔyggS mutant as a starting strain, the establishment of an S495F mutation in GshA, and the deletion of ridA or miaA, facilitated the efficient production of OA in E. coli.

Role of Ergothioneine in Microbial Physiology and Pathogenesis

SIGNIFICANCE

L-ergothioneine is synthesized in actinomycetes, cyanobacteria, methylobacteria, and some fungi. In contrast to other low-molecular-weight redox buffers, glutathione and mycothiol, ergothioneine is primarily present as a thione rather than a thiol at physiological pH, which makes it resistant to autoxidation. Ergothioneine regulates microbial physiology and enables the survival of microbes under stressful conditions encountered in their natural environments. In particular, ergothioneine enables pathogenic microbes, such as Mycobacterium tuberculosis (Mtb), to withstand hostile environments within the host to establish infection. Recent Advances: Ergothioneine has been reported to maintain bioenergetic homeostasis in Mtb and protect Mtb against oxidative stresses, thereby enhancing the virulence of Mtb in a mouse model. Furthermore, ergothioneine augments the resistance of Mtb to current frontline anti-TB drugs. Recently, an opportunistic fungus, Aspergillus fumigatus, which infects immunocompromised individuals, has been found to produce ergothioneine, which is important in conidial health and germination, and contributes to the fungal resistance against redox stresses.

CRITICAL ISSUES

The molecular mechanisms of the functions of ergothioneine in microbial physiology and pathogenesis are poorly understood. It is currently not known if ergothioneine is used in detoxification or antioxidant enzymatic pathways. As ergothioneine is involved in bioenergetic and redox homeostasis and antibiotic susceptibility of Mtb, it is of utmost importance to advance our understanding of these mechanisms.

FUTURE DIRECTIONS

A clear understanding of the role of ergothioneine in microbes will advance our knowledge of how this thione enhances microbial virulence and resistance to the host's defense mechanisms to avoid complete eradication. Antioxid. Redox Signal. 28, 431-444.

Distribution and accumulation of dietary ergothioneine and its metabolites in mouse tissues

L-ergothioneine (ET) is a diet-derived amino acid that accumulates at high concentrations in animals and humans. Numerous studies have highlighted its antioxidant abilities in vitro, and possible cytoprotective capabilities in vivo. We investigated the uptake and distribution of ET in various organs by a highly sensitive and specific liquid chromatography coupled tandem mass spectrometry (LC-MS/MS) technique, both before and after oral administration of pure ET (35 and 70 mg/kg/day for 1, 7, and 28 days) to male C57BL6J mice. ET primarily concentrates in the liver and whole blood, and also in spleen, kidney, lung, heart, intestines, eye, and brain tissues. Strong correlations were found between ET and its putative metabolites - hercynine, ET-sulfonate (ET-SO3H), and S-methyl ET. Hercynine accumulates in the brain after prolonged ET administration. This study demonstrates the uptake and distribution of ET and provides a foundation for future studies with ET to target oxidative damage in a range of tissues in human diseases.

Ergothioneine Antioxidant Function: From Chemistry to Cardiovascular Therapeutic Potential

Ergothioneine (ESH), the betaine of 2-mercapto-L-histidine, is a water-soluble naturally occurring amino acid with antioxidant properties. ESH accumulates in several human and animal tissues up to millimolar concentration through its high affinity transporter, namely the organic cation transporter 1 (OCTN1). ESH, first isolated from the ergot fungus (Claviceps purpurea), is synthesized only by Actinomycetales and non-yeast-like fungi. Plants absorb ESH via symbiotic associations between their roots and soil fungi, whereas mammals acquire it solely from dietary sources. Numerous evidence demonstrated the antioxidant and cytoprotective effects of ESH, including protection against cardiovascular diseases, chronic inflammatory conditions, ultraviolet radiation damages, and neuronal injuries. Although more than a century after its discovery has gone by, our understanding on the in vivo ESH mechanism is limited and this compound still intrigues researchers. However, recent evidence about differences in chemical redox behavior between ESH and alkylthiols, such as cysteine and glutathione, has opened new perspectives on the role of ESH during oxidative damage. In this short review, we discuss the role of ESH in the complex machinery of the cellular antioxidant defense focusing on the current knowledge on its chemical mechanism of action in the protection against cardiovascular disease.

Mushrooms: A rich source of the antioxidants ergothioneine and glutathione

While mushrooms are the highest dietary source for the unique sulfur-containing antioxidant ergothioneine, little is known regarding levels of the major biological antioxidant glutathione. Thus, our objectives were to determine and compare levels of glutathione, as well as ergothioneine, in different species of mushrooms. Glutathione levels varied >20-fold (0.11-2.41mg/gdw) with some varieties having higher levels than reported for other foods. Ergothioneine levels also varied widely (0.15-7.27mg/gdw) and were highly correlated with those of glutathione (r=0.62, P<0.001). Both antioxidants were more concentrated in pileus than stipe tissues in selected mushrooms species. Agaricus bisporus harvested during the third cropping flush contained higher levels of ergothioneine and glutathione compared to the first flush, possibly as a response to increased oxidative stress. This study demonstrated that certain mushroom species are high in glutathione and ergothioneine and should be considered an excellent dietary source of these important antioxidants.

Comparative Genomics of DNA Recombination and Repair in Cyanobacteria: Biotechnological Implications

Cyanobacteria are fascinating photosynthetic prokaryotes that are regarded as the ancestors of the plant chloroplast; the purveyors of oxygen and biomass for the food chain; and promising cell factories for an environmentally friendly production of chemicals. In colonizing most waters and soils of our planet, cyanobacteria are inevitably challenged by environmental stresses that generate DNA damages. Furthermore, many strains engineered for biotechnological purposes can use DNA recombination to stop synthesizing the biotechnological product. Hence, it is important to study DNA recombination and repair in cyanobacteria for both basic and applied research. This review reports what is known in a few widely studied model cyanobacteria and what can be inferred by mining the sequenced genomes of morphologically and physiologically diverse strains. We show that cyanobacteria possess many E. coli-like DNA recombination and repair genes, and possibly other genes not yet identified. E. coli-homolog genes are unevenly distributed in cyanobacteria, in agreement with their wide genome diversity. Many genes are extremely well conserved in cyanobacteria (mutMS, radA, recA, recFO, recG, recN, ruvABC, ssb, and uvrABCD), even in small genomes, suggesting that they encode the core DNA repair process. In addition to these core genes, the marine Prochlorococcus and Synechococcus strains harbor recBCD (DNA recombination), umuCD (mutational DNA replication), as well as the key SOS genes lexA (regulation of the SOS system) and sulA (postponing of cell division until completion of DNA reparation). Hence, these strains could possess an E. coli-type SOS system. In contrast, several cyanobacteria endowed with larger genomes lack typical SOS genes. For examples, the two studied Gloeobacter strains lack alkB, lexA, and sulA; and Synechococcus PCC7942 has neither lexA nor recCD. Furthermore, the Synechocystis PCC6803 lexA product does not regulate DNA repair genes. Collectively, these findings indicate that not all cyanobacteria have an E. coli-type SOS system. Also interestingly, several cyanobacteria possess multiple copies of E. coli-like DNA repair genes, such as Acaryochloris marina MBIC11017 (2 alkB, 3 ogt, 7 recA, 3 recD, 2 ssb, 3 umuC, 4 umuD, and 8 xerC), Cyanothece ATCC51142 (2 lexA and 4 ruvC), and Nostoc PCC7120 (2 ssb and 3 xerC).

Ergothioneine Biosynthesis and Functionality in the Opportunistic Fungal Pathogen, Aspergillus fumigatus

Ergothioneine (EGT; 2-mercaptohistidine trimethylbetaine) is a trimethylated and sulphurised histidine derivative which exhibits antioxidant properties. Here we report that deletion of Aspergillus fumigatus egtA (AFUA_2G15650), which encodes a trimodular enzyme, abrogated EGT biosynthesis in this opportunistic pathogen. EGT biosynthetic deficiency in A. fumigatus significantly reduced resistance to elevated H₂O₂ and menadione, respectively, impaired gliotoxin production and resulted in attenuated conidiation. Quantitative proteomic analysis revealed substantial proteomic remodelling in ΔegtA compared to wild-type under both basal and ROS conditions, whereby the abundance of 290 proteins was altered. Specifically, the reciprocal differential abundance of cystathionine γ-synthase and β-lyase, respectively, influenced cystathionine availability to effect EGT biosynthesis. A combined deficiency in EGT biosynthesis and the oxidative stress response regulator Yap1, which led to extreme oxidative stress susceptibility, decreased resistance to heavy metals and production of the extracellular siderophore triacetylfusarinine C and increased accumulation of the intracellular siderophore ferricrocin. EGT dissipated H₂O₂ in vitro, and elevated intracellular GSH levels accompanied abrogation of EGT biosynthesis. EGT deficiency only decreased resistance to high H₂O₂ levels which suggests functionality as an auxiliary antioxidant, required for growth at elevated oxidative stress conditions. Combined, these data reveal new interactions between cellular redox homeostasis, secondary metabolism and metal ion homeostasis.

Diverse fission yeast genes required for responding to oxidative and metal stress: Comparative analysis of glutathione-related and other defense gene deletions

Living organisms have evolved multiple sophisticated mechanisms to deal with reactive oxygen species. We constructed a collection of twelve single-gene deletion strains of the fission yeast Schizosaccharomyces pombe designed for the study of oxidative and heavy metal stress responses. This collection contains deletions of biosynthetic enzymes of glutathione (Δgcs1 and Δgsa1), phytochelatin (Δpcs2), ubiquinone (Δabc1) and ergothioneine (Δegt1), as well as catalase (Δctt1), thioredoxins (Δtrx1 and Δtrx2), Cu/Zn- and Mn- superoxide dismutases (SODs; Δsod1 and Δsod2), sulfiredoxin (Δsrx1) and sulfide-quinone oxidoreductase (Δhmt2). First, we employed metabolomic analysis to examine the mutants of the glutathione biosynthetic pathway. We found that ophthalmic acid was produced by the same enzymes as glutathione in S. pombe. The identical genetic background of the strains allowed us to assess the severity of the individual gene knockouts by treating the deletion strains with oxidative agents. Among other results, we found that glutathione deletion strains were not particularly sensitive to peroxide or superoxide, but highly sensitive to cadmium stress. Our results show the astonishing diversity in cellular adaptation mechanisms to various types of oxidative and metal stress and provide a useful tool for further research into stress responses.