Essential role of glutathione in acclimation to environmental and redox perturbations in the cyanobacterium Synechocystis sp. PCC 6803

Systemic analysis of stress transcriptomics of Synechocystis reveals common stress genes and their universal triggers

Systemic analysis of stress transcriptomics reveals that ROS and redox changes may universally trigger stress responses in Synechocystis (cyanobacteria).

Unraveling the mechanism responsible for the contrasting tolerance of Synechocystis and Synechococcus to Cr(VI): Enzymatic and non-enzymatic antioxidants

Two unicellular cyanobacteria, Synechocystis and Synechococcus, showed contrasting tolerance to Cr(VI); with Synechococcus being 12-fold more tolerant than Synechocystis to potassium dichromate. The mechanism responsible for this differential sensitivity to Cr(VI) was explored in this study. Total content of photosynthetic pigments as well as photosynthetic activity decreased at lower concentration of Cr(VI) in Synechocystis as compared to Synechococcus. Experiments with (51)Cr showed Cr to accumulate intracellularly in both the cyanobacteria. At lower concentrations, Cr(VI) caused excessive ROS generation in Synechocystis as compared to that observed in Synechococcus. Intrinsic levels of enzymatic antioxidants, i.e., superoxide dismutase, catalase and 2-Cys-peroxiredoxin were considerably higher in Synechococcus than Synechocystis. Content of total thiols (both protein as well as non-protein) and reduced glutathione (GSH) was also higher in Synechococcus as compared to Synechocystis. This correlated well with higher content of carbonylated proteins observed in Synechocystis than Synechococcus. Additionally, in contrast to Synechocystis, Synechococcus exhibited better tolerance to other oxidative stresses like high intensity light and H2O2. The data indicate that the disparity in the ability to detoxify ROS could be the primary mechanism responsible for the differential tolerance of these cyanobacteria to Cr(VI).

Adaptive Engineering of Phytochelatin-based Heavy Metal Tolerance

Metabolic engineering approaches are increasingly employed for environmental applications. Because phytochelatins (PC) protect plants from heavy metal toxicity, strategies directed at manipulating the biosynthesis of these peptides hold promise for the remediation of soils and groundwaters contaminated with heavy metals. Directed evolution of Arabidopsis thaliana phytochelatin synthase (AtPCS1) yields mutants that confer levels of cadmium tolerance and accumulation greater than expression of the wild-type enzyme in Saccharomyces cerevisiae, Arabidopsis, or Brassica juncea. Surprisingly, the AtPCS1 mutants that enhance cadmium tolerance and accumulation are catalytically less efficient than wild-type enzyme. Metabolite analyses indicate that transformation with AtPCS1, but not with the mutant variants, decreases the levels of the PC precursors, glutathione and γ-glutamylcysteine, upon exposure to cadmium. Selection of AtPCS1 variants with diminished catalytic activity alleviates depletion of these metabolites, which maintains redox homeostasis while supporting PC synthesis during cadmium exposure. These results emphasize the importance of metabolic context for pathway engineering and broaden the range of tools available for environmental remediation.

Characterization of a Highly pH Stable Chi-Class Glutathione S-Transferase from Synechocystis PCC 6803

Glutathione S-transferases (GSTs) are multifunctional enzymes present in virtually all organisms. Besides having an essential role in cellular detoxification, they also perform various other functions, including responses in stress conditions and signaling. GSTs are highly studied in plants and animals; however, the knowledge regarding GSTs in cyanobacteria seems rudimentary. In this study, we report the characterization of a highly pH stable GST from the model cyanobacterium- Synechocystis PCC 6803. The gene sll0067 was expressed in Escherichia coli (E. coli), and the protein was purified to homogeneity. The expressed protein exists as a homo-dimer, which is composed of about 20 kDa subunit. The results of the steady-state enzyme kinetics displayed protein’s glutathione conjugation activity towards its class specific substrate- isothiocyanate, having the maximal activity with phenethyl isothiocyanate. Contrary to the poor catalytic activity and low specificity towards standard GST substrates such as 1-chloro-2,4-dinitrobenzene by bacterial GSTs, PmGST B1-1 from Proteus mirabilis, and E. coli GST, sll0067 has broad substrate degradation capability like most of the mammalian GST. Moreover, we have shown that cyanobacterial GST sll0067 is catalytically efficient compared to the best mammalian enzymes. The structural stability of GST was studied as a function of pH. The fluorescence and CD spectroscopy in combination with size exclusion chromatography showed a highly stable nature of the protein over a broad pH range from 2.0 to 11.0. To the best of our knowledge, this is the first GST with such a wide range of pH related structural stability. Furthermore, the presence of conserved Proline-53, structural motifs such as N-capping box and hydrophobic staple further aid in the stability and proper folding of cyanobacterial GST- sll0067.

Responses to oxidative and heavy metal stresses in cyanobacteria: recent advances

Cyanobacteria, the only known prokaryotes that perform oxygen-evolving photosynthesis, are receiving strong attention in basic and applied research. In using solar energy, water, CO₂ and mineral salts to produce a large amount of biomass for the food chain, cyanobacteria constitute the first biological barrier against the entry of toxics into the food chain. In addition, cyanobacteria have the potential for the solar-driven carbon-neutral production of biofuels. However, cyanobacteria are often challenged by toxic reactive oxygen species generated under intense illumination, i.e., when their production of photosynthetic electrons exceeds what they need for the assimilation of inorganic nutrients. Furthermore, in requiring high amounts of various metals for growth, cyanobacteria are also frequently affected by drastic changes in metal availabilities. They are often challenged by heavy metals, which are increasingly spread out in the environment through human activities, and constitute persistent pollutants because they cannot be degraded. Consequently, it is important to analyze the protection against oxidative and metal stresses in cyanobacteria because these ancient organisms have developed most of these processes, a large number of which have been conserved during evolution. This review summarizes what is known regarding these mechanisms, emphasizing on their crosstalk.

Functional classification and biochemical characterization of a novel rho class glutathione S-transferase in Synechocystis PCC 6803

•

A novel class of glutathione S-transferase (GST) is reported.

•

This GST catalyzes dichloroacetate (DCA) degradation and hydroperoxide reactions.

•

Functionally this GST is similar to zeta and theta/alpha classes but structurally very different.

•

In contrast to other bacterial GSTs, this GST exists as a monomer in solution.

•

First report of DCA degradation by any bacterial GST and has potential biotechnological applications.

We report a novel class of glutathione S-transferase (GST) from the model cyanobacterium Synechocystis PCC 6803 (sll1545) which catalyzes the detoxification of the water pollutant dichloroacetate and also shows strong glutathione-dependent peroxidase activity representing the classical activities of zeta and theta/alpha class respectively. Interestingly, sll1545 has very low sequence and structural similarity with these classes. This is the first report of dichloroacetate degradation activity by any bacterial GST. Based on these results we classify sll1545 to a novel GST class, rho. The present data also indicate potential biotechnological and industrial applications of cyanobacterial GST in dichloroacetate-polluted areas.

Electron-transfer kinetics in cyanobacterial cells: methyl viologen is a poor inhibitor of linear electron flow

The inhibitor methyl viologen (MV) has been widely used in photosynthesis to study oxidative stress. Its effects on electron transfer kinetics in Synechocystis sp. PCC6803 cells were studied to characterize its electron-accepting properties. For the first hundreds of flashes following MV addition at submillimolar concentrations, the kinetics of NADPH formation were hardly modified (less than 15% decrease in signal amplitude) with a significant signal decrease only observed after more flashes or continuous illumination. The dependence of the P700 photooxidation kinetics on the MV concentration exhibited a saturation effect at 0.3 mM MV, a concentration which inhibits the recombination reactions in photosystem I. The kinetics of NADPH formation and decay under continuous light with MV at 0.3 mM showed that MV induces the oxidation of the NADP pool in darkness and that the yield of linear electron transfer decreased by only 50% after 1.5-2 photosystem-I turnovers. The unexpectedly poor efficiency of MV in inhibiting NADPH formation was corroborated by in vitro flash-induced absorption experiments with purified photosystem-I, ferredoxin and ferredoxin-NADP(+)-oxidoreductase. These experiments showed that the second-order rate constants of MV reduction are 20 to 40-fold smaller than the competing rate constants involved in reduction of ferredoxin and ferredoxin-NADP(+)-oxidoreductase. The present study shows that MV, which accepts electrons in vivo both at the level of photosystem-I and ferredoxin, can be used at submillimolar concentrations to inhibit recombination reactions in photosystem-I with only a moderate decrease in the efficiency of fast reactions involved in linear electron transfer and possibly cyclic electron transfer.

Advances in the function and regulation of hydrogenase in the cyanobacterium Synechocystis PCC6803

In order to use cyanobacteria for the biological production of hydrogen, it is important to thoroughly study the function and the regulation of the hydrogen-production machine in order to better understand its role in the global cell metabolism and identify bottlenecks limiting H2 production. Most of the recent advances in our understanding of the bidirectional [Ni-Fe] hydrogenase (Hox) came from investigations performed in the widely-used model cyanobacterium Synechocystis PCC6803 where Hox is the sole enzyme capable of combining electrons with protons to produce H2 under specific conditions. Recent findings suggested that the Hox enzyme can receive electrons from not only NAD(P)H as usually shown, but also, or even preferentially, from ferredoxin. Furthermore, plasmid-encoded functions and glutathionylation (the formation of a mixed-disulfide between the cysteines residues of a protein and the cysteine residue of glutathione) are proposed as possible new players in the function and regulation of hydrogen production.

Proteome-wide light/dark modulation of thiol oxidation in cyanobacteria revealed by quantitative site-specific redox proteomics

Reversible protein thiol oxidation is an essential regulatory mechanism of photosynthesis, metabolism, and gene expression in photosynthetic organisms. Herein, we present proteome-wide quantitative and site-specific profiling of in vivo thiol oxidation modulated by light/dark in the cyanobacterium Synechocystis sp. PCC 6803, an oxygenic photosynthetic prokaryote, using a resin-assisted thiol enrichment approach. Our proteomic approach integrates resin-assisted enrichment with isobaric tandem mass tag labeling to enable site-specific and quantitative measurements of reversibly oxidized thiols. The redox dynamics of ∼2,100 Cys-sites from 1,060 proteins under light, dark, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (a photosystem II inhibitor) conditions were quantified. In addition to relative quantification, the stoichiometry or percentage of oxidation (reversibly oxidized/total thiols) for ∼1,350 Cys-sites was also quantified. The overall results revealed broad changes in thiol oxidation in many key biological processes, including photosynthetic electron transport, carbon fixation, and glycolysis. Moreover, the redox sensitivity along with the stoichiometric data enabled prediction of potential functional Cys-sites for proteins of interest. The functional significance of redox-sensitive Cys-sites in NADP-dependent glyceraldehyde-3-phosphate dehydrogenase, peroxiredoxin (AhpC/TSA family protein Sll1621), and glucose 6-phosphate dehydrogenase was further confirmed with site-specific mutagenesis and biochemical studies. Together, our findings provide significant insights into the broad redox regulation of photosynthetic organisms.

Genomic responses to arsenic in the cyanobacterium Synechocystis sp. PCC 6803

Arsenic is a ubiquitous contaminant and a toxic metalloid which presents two main redox states in nature: arsenite [As^III] and arsenate [As^V]. Arsenic resistance in Synechocystis sp. strain PCC 6803 is mediated by the arsBHC operon and two additional arsenate reductases encoded by the arsI1 and arsI2 genes. Here we describe the genome-wide responses to the presence of arsenate and arsenite in wild type and mutants in the arsenic resistance system. Both forms of arsenic produced similar responses in the wild type strain, including induction of several stress related genes and repression of energy generation processes. These responses were transient in the wild type strain but maintained in time in an arsB mutant strain, which lacks the arsenite transporter. In contrast, the responses observed in a strain lacking all arsenate reductases were somewhat different and included lower induction of genes involved in metal homeostasis and Fe-S cluster biogenesis, suggesting that these two processes are targeted by arsenite in the wild type strain. Finally, analysis of the arsR mutant strain revealed that ArsR seems to only control 5 genes in the genome. Furthermore, the arsR mutant strain exhibited hypersentivity to nickel, copper and cadmium and this phenotype was suppressed by mutation in arsB but not in arsC gene suggesting that overexpression of arsB is detrimental in the presence of these metals in the media.

Unraveling the redox properties of the global regulator FurA from Anabaena sp. PCC 7120: disulfide reductase activity based on its CXXC motifs

Cyanobacterial FurA works as a global regulator linking iron homeostasis to photosynthetic metabolism and the responses to different environmental stresses. Additionally, FurA modulates several genes involved in redox homeostasis and fulfills the characteristics of a heme-sensor protein whose interaction with this cofactor negatively affects its DNA binding ability. FurA from Anabaena PCC 7120 contains five cysteine residues, four of them arranged in two redox CXXC motifs.

AIMS

Our goals were to analyze in depth the putative contribution of these CXXC motifs in the redox properties of FurA and to identify potential interacting partners of this regulator.

RESULTS

Insulin reduction assays unravel that FurA exhibits disulfide reductase activity. Simultaneous presence of both CXXC signatures greatly enhances the reduction rate, although the redox motif containing Cys(101) and Cys(104) seems a major contributor to this activity. Disulfide reductase activity was not detected in other ferric uptake regulator (Fur) proteins isolated from heterotrophic bacteria. In vivo, FurA presents different redox states involving intramolecular disulfide bonds when is partially oxidized. Redox potential values for CXXC motifs, -235 and -238 mV, are consistent with those reported for other proteins displaying disulfide reductase activity. Pull-down and two-hybrid assays unveil potential FurA interacting partners, namely phosphoribulokinase Alr4123, the hypothetical amidase-containing domain All1140 and the DNA-binding protein HU.

INNOVATION

A novel biochemical activity of cyanobacterial FurA based on its cysteine arrangements and the identification of novel interacting partners are reported.

CONCLUSION

The present study discloses a putative connection of FurA with the cyanobacterial redox-signaling pathway.

Comparative proteomics reveals that a saxitoxin-producing and a nontoxic strain of Anabaena circinalis are two different ecotypes

In Australia, saxitoxin production is restricted to the cyanobacterial species Anabaena circinalis and is strain-dependent. We aimed to characterize a saxitoxin-producing and nontoxic strain of A. circinalis at the proteomic level using iTRAQ. Seven proteins putatively involved in saxitoxin biosynthesis were identified within our iTRAQ experiment for the first time. The proteomic profile of the toxic A. circinalis was significantly different from the nontoxic strain, indicating that each is likely to inhabit a unique ecological niche. Under control growth conditions, the saxitoxin-producing A. circinalis displayed a higher abundance of photosynthetic, carbon fixation and nitrogen metabolic proteins. Differential abundance of these proteins suggests a higher intracellular C:N ratio and a higher concentration of intracellular 2-oxoglutarate in our toxic strain compared with the nontoxic strain. This may be a novel site for posttranslational regulation because saxitoxin biosynthesis putatively requires a 2-oxoglutarate-dependent dioxygenase. The nontoxic A. circinalis was more abundant in proteins, indicating cellular stress. Overall, our study has provided the first insight into fundamental differences between a toxic and nontoxic strain of A. circinalis, indicating that they are distinct ecotypes.

BMAA inhibits nitrogen fixation in the cyanobacterium Nostoc sp. PCC 7120

Cyanobacteria produce a range of secondary metabolites, one being the neurotoxic non-protein amino acid β-N-methylamino-L-alanine (BMAA), proposed to be a causative agent of human neurodegeneration. As for most cyanotoxins, the function of BMAA in cyanobacteria is unknown. Here, we examined the effects of BMAA on the physiology of the filamentous nitrogen-fixing cyanobacterium Nostoc sp. PCC 7120. Our data show that exogenously applied BMAA rapidly inhibits nitrogenase activity (acetylene reduction assay), even at micromolar concentrations, and that the inhibition was considerably more severe than that induced by combined nitrogen sources and most other amino acids. BMAA also caused growth arrest and massive cellular glycogen accumulation, as observed by electron microscopy. With nitrogen fixation being a process highly sensitive to oxygen species we propose that the BMAA effects found here may be related to the production of reactive oxygen species, as reported for other organisms.

Probing the origins of glutathione biosynthesis through biochemical analysis of glutamate-cysteine ligase and glutathione synthetase from a model photosynthetic prokaryote

Glutathione biosynthesis catalysed by GCL (glutamate-cysteine ligase) and GS (glutathione synthetase) is essential for maintaining redox homoeostasis and protection against oxidative damage in diverse eukaroytes and bacteria. This biosynthetic pathway probably evolved in cyanobacteria with the advent of oxygenic photosynthesis, but the biochemical characteristics of progenitor GCLs and GSs in these organisms are largely unexplored. In the present study we examined SynGCL and SynGS from Synechocystis sp. PCC 6803 using steady-state kinetics. Although SynGCL shares ~15% sequence identity with the enzyme from plants and α-proteobacteria, sequence comparison suggests that these enzymes share similar active site residues. Biochemically, SynGCL lacks the redox regulation associated with the plant enzymes and functions as a monomeric protein, indicating that evolution of redox regulation occurred later in the green lineage. Site-directed mutagenesis of SynGCL establishes this enzyme as part of the plant-like GCL family and identifies a catalytically essential arginine residue, which is structurally conserved across all forms of GCLs, including those from non-plant eukaryotes and γ-proteobacteria. A reaction mechanism for the synthesis of γ-glutamylcysteine by GCLs is proposed. Biochemical and kinetic analysis of SynGS reveals that this enzyme shares properties with other prokaryotic GSs. Initial velocity and product inhibition studies used to examine the kinetic mechanism of SynGS suggest that it and other prokaryotic GSs uses a random ter-reactant mechanism for the synthesis of glutathione. The present study provides new insight on the molecular mechanisms and evolution of glutathione biosynthesis; a key process required for enhancing bioenergy production in photosynthetic organisms.

Low-molecular-weight thiol-dependent antioxidant and antinitrosative defences in Salmonella pathogenesis

We found herein that the intracytoplasmic pool of the low-molecular-weight (LMW) thiol glutathione (GSH) is readily oxidized in Salmonella exposed to nitric oxide (NO). The hypersusceptibility of gshA and gshB mutants lacking γ-glutamylcysteine and glutathione synthetases to NO and S-nitrosoglutathione indicates that GSH antagonizes the bacteriostatic activity of reactive nitrogen species. Metabolites of the GSH biosynthetic pathway do not affect the enzymatic activity of classical NO targets such as quinol oxidases. In contrast, LMW thiols diminish the nitrosative stress experienced by enzymes, such as glutamine oxoglutarate amidotransferase, that contain redox active cysteines. LMW thiols also preserve the transcription of Salmonella pathogenicity island 2 gene targets from the inhibitory activity of nitrogen oxides. These findings are consistent with the idea that GSH scavenges reactive nitrogen species (RNS) other than NO. Compared with the adaptive response afforded by inducible systems such as the hmp-encoded flavohaemoprotein, gshA, encoding the first step of GSH biosynthesis, is constitutively expressed in Salmonella. An acute model of salmonellosis has revealed that the antioxidant and antinitrosative properties associated with the GSH biosynthetic pathway represent a first line of Salmonella resistance against reactive oxygen and nitrogen species engendered in the context of a functional NRAMP1(R) divalent metal transporter.

Analysis of the grasspea proteome and identification of stress-responsive proteins upon exposure to high salinity, low temperature, and abscisic acid treatment

Abiotic stress causes diverse biochemical and physiological changes in plants and limits crop productivity. Plants respond and adapt to such stress by altering their cellular metabolism and activating various defense machineries. To understand the molecular basis of stress tolerance in plants, we have developed differential proteomes in a hardy legume, grasspea (Lathyrus sativus L.). Five-week-old grasspea seedlings were subjected independently to high salinity, low temperature and abscisic acid treatment for duration of 36h. The physiological changes of stressed seedlings were monitored, and correlated with the temporal changes of proteome using two-dimensional gel electrophoresis. Approximately, 400 protein spots were detected in each of the stress proteome with one-fourth showing more than 2-fold differences in expression values. Eighty such proteins were subjected to LC-tandem MS/MS analyses that led to the identification of 48 stress-responsive proteins (SRPs) presumably involved in a variety of functions, including metabolism, signal transduction, protein biogenesis and degradation, and cell defense and rescue. While 33 proteins were responsive to all three treatments, 15 proteins were expressed in stress-specific manner. Further, we explored the possible role of ROS in triggering the stress-induced degradation of large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase (Rubisco). These results might help in understanding the spectrum of stress-regulated proteins and the biological processes they control as well as having implications for strategies to improve stress adaptation in plants.

Glutathione facilitates antibiotic resistance and photosystem I stability during exposure to gentamicin in cyanobacteria

Understanding mechanisms of antibiotic resistance is important to the fields of biology and medicine. We find that glutathione contributes to antibiotic resistance in the cyanobacterium Synechocystis sp. PCC 6803. Our results also suggest that glutathione protects photosystem I from oxidative damage resulting from growth in the presence of gentamicin.

Glutathione in Synechocystis 6803: a closer look into the physiology of a ∆gshB mutant

Glutathione (GSH) is a low molecular weight thiol compound that plays many roles in photosynthetic organisms. We utilized a ∆gshB (glutathione synthetase) mutant strain as a tool to evaluate the role of GSH in the cyanobacterium Synechocystis sp. PCC 6803 (hereafter Synechocystis 6803), a model photosynthetic organism. The ∆gshB mutant does not synthesize glutathione, but instead accumulates the GSH precursor, gamma-glutamylcysteine (gamma-EC), to millimolar levels. We found that gamma-EC was sufficient to permit cellular proliferation during optimal conditions, but not when cells were exposed to conditions promoting oxidative stress. Furthermore, we found that many factors affecting growth rate and photosynthetic activities strongly influenced cellular thiol content. Here, we are providing some additional insights into the role of GSH and gamma-EC in Synechocystis 6803 during conditions promoting oxidative stress.