Redox-dependent chaperone/peroxidase function of 2-Cys-Prx from the cyanobacterium Anabaena PCC7120: role in oxidative stress tolerance

Modulation of oxidative stress machinery determines the contrasting ability of cyanobacteria to adapt to Se(VI) or Se(IV)

Excess of selenium (Se) in aquatic ecosystems has necessitated thorough investigations into the effects/consequences of this metalloid on the autochthonous organisms exposed to it. The molecular details of Se-mediated adaptive response remain unknown in cyanobacteria. This study aims to uncover the molecular mechanisms driving the divergent physiological responses of cyanobacteria on exposure to selenate [Se(VI)] or selenite [Se(IV)], the two major water-soluble oxyanions of Se. The cyanobacterium, Anabaena PCC 7120, withstood 0.4 mM of Se(VI), whereas even 0.1 mM of Se(IV) was detrimental, affecting photosynthesis and enhancing endogenous ROS. Surprisingly, Anabaena pre-treated with Se(VI), but not Se(IV), showed increased tolerance to oxidative stress mediated by H2O2/methyl viologen. RNA-Seq analysis showed Se(VI) to elevate transcription of genes encoding anti-oxidant proteins and Fe-S cluster biogenesis, whereas the photosynthesis-associated genes, which were mainly downregulated by Se(IV), remained unaffected. Specifically, the content of typical 2-Cys-Prx (Alr4641), a redox-maintaining protein in Anabaena, was elevated with Se(VI). In comparison to the wild-type, the Anabaena strain over-expressing the Alr4641 protein (An4641+) showed enhanced tolerance to Se(VI) stress, whereas the corresponding knockdown-strain (KD4641) was sensitive to this stressor. Incidentally, among these strains, only An4641+ was better protected from the ROS-mediated damage caused by high dose of Se(VI). These results suggest that altering the content of the antioxidant protein 2-Cys-Prx, could be a potential strategy for modulating resistance to selenate. Thus, involvement of oxidative stress machinery appears to be the major determinant, responsible for the contrasting physiological differences observed in response to selenate/selenite in cyanobacteria.

Fluorometric Protocol for Estimating Peroxiredoxin Activity in Biological Tissues

This protocol describes a detailed fluorometric method for measuring peroxiredoxin (Prx) enzyme activity in vitro. Peroxide dissociation is the rate-limiting step in the Prx-controlled enzymatic reaction. To prevent interference by the catalase enzyme, we developed a peroxiredoxin assay that measures Prx activity using the substrate tert-Butyl hydroperoxide (t-BOOH). Prx enzyme activity is measured by incubating the enzymatic substrates 1,4-dithio-DL-threitol (DTT) and t-BOOH in a suitable buffer at 37 °C for 10 min in the presence of the desired volume of Prx enzyme. Next, the reagent N-(9-Acridinyl)maleimide (NAM) is used to stop the enzymatic reaction and form a fluorescent end product. Finally, Prx activity is measured by thiol fluorometry using a Box-Behnken design to optimize reaction conditions. This novel protocol was validated by evaluating Prx activity in matched samples against a reference assay. The correlation coefficient between our protocol and the reference assay was 0.9933, demonstrating its precision compared with existing methods. The NAM-Prx protocol instead uses t-BOOH as a substrate to measure Prx activity. Because catalase does not participate in the dissociation of t-BOOH, this approach does not require sodium azide. Furthermore, the method eliminates the need for concentrated acids to terminate the Prx enzymatic reaction since the NAM reagent can inhibit the enzymatic reaction regulated by the Prx enzyme.

The atypical thioredoxin 'Alr2205', a newly identified partner of the typical 2-Cys-Peroxiredoxin, safeguards the cyanobacterium Anabaena from oxidative stress

Thioredoxins (Trxs) are ubiquitous proteins that play vital roles in several physiological processes. Alr2205, a thioredoxin-like protein from Anabaena PCC 7120, was found to be evolutionarily closer to the Trx-domain of the NADPH-Thioredoxin Reductase C than the other thioredoxins. The Alr2205 protein showed disulfide reductase activity despite the presence a non-canonical active site motif 'CPSC'. Alr2205 not only physically interacted with, but also acted as a physiological reductant of Alr4641 (the typical 2-Cys-Peroxiredoxin from Anabaena), supporting its peroxidase function. Structurally, Alr2205 was a monomeric protein that formed an intramolecular disulfide bond between the two active site cysteines (Cys-38 and Cys-41). However, the Alr2205C41S protein, wherein the resolving cysteine was mutated to serine, was capable of forming intermolecular disulfide bond and exist as a dimer when treated with H2O2. Overproduction of Alr2205 in E. coli protected cells from heavy metals, but not oxidative stress. To delve into its physiological role, Alr2205/Alr2205C41S was overexpressed in Anabaena, and the ability of the corresponding strains (An2205+ or An2205C41S+) to withstand environmental stresses was assessed. An2205+ showed higher resistance to H2O2 than An2205C41S+, indicating that the disulfide reductase function of this protein was critical to protect cells from this peroxide. Although, An2205+ did not show increased capability to withstand cadmium stress, An2205C41S+ was more susceptible to this heavy metal. This is the first study that provides a vital understanding into the function of atypical thioredoxins in countering the toxic effects of heavy metals/H2O2 in prokaryotes.

Loss of 2-Cys-Prx affects cellular ultrastructure, disturbs redox poise and impairs photosynthesis in cyanobacteria

In a striking similarity to plant chloroplasts, the cyanobacterium Anabaena displays very low catalase activity, but expresses several peroxiredoxins (Prxs), including the typical 2-Cys-Prx (annotated as Alr4641), that detoxify H₂ O₂ . Due to the presence of multiple Prxs, the precise contribution of Alr4641 to the oxidative stress response of Anabaena is not well-defined. To unambiguously assess its in vivo function, the Alr4641 protein was knocked down using the CRISPRi approach in Anabaena PCC 7120. The knockdown strain (An-KD4641), which showed over 85% decrease in the content of Alr4641, was viable, but grew slower than the control strain (An-dCas9). An-KD4641 showed elevated levels of reactive oxygen species and the expression of several redox-responsive genes was analogous to that of An-dCas9 subjected to oxidative stress. The knockdown strain displayed reduced filament size, altered thylakoid ultrastructure, a marked drop in the ratio of phycocyanin to chlorophyll a and decreased photosynthetic parameters compared to An-dCas9. In comparison to the control strain, exposure to H₂ O₂ had a more severe effect on the photosynthetic parameters or survival of An-KD4641. Thus, in the absence of adequate catalase activity, 2-Cys-Prx appears to be the principal Prx responsible for maintaining redox homoeostasis in diverse photosynthetic systems ranging from chloroplasts to cyanobacteria.

Exploring the Diversity of the Thioredoxin Systems in Cyanobacteria

Cyanobacteria evolved the ability to perform oxygenic photosynthesis using light energy to reduce CO₂ from electrons extracted from water and form nutrients. These organisms also developed light-dependent redox regulation through the Trx system, formed by thioredoxins (Trxs) and thioredoxin reductases (TRs). Trxs are thiol-disulfide oxidoreductases that serve as reducing substrates for target enzymes involved in numerous processes such as photosynthetic CO₂ fixation and stress responses. We focus on the evolutionary diversity of Trx systems in cyanobacteria and discuss their phylogenetic relationships. The study shows that most cyanobacteria contain at least one copy of each identified Trx, and TrxA is the only one present in all genomes analyzed. Ferredoxin thioredoxin reductase (FTR) is present in all groups except Gloeobacter and Prochlorococcus, where there is a ferredoxin flavin-thioredoxin reductase (FFTR). Our data suggest that both TRs may have coexisted in ancestral cyanobacteria together with other evolutionarily related proteins such as NTRC or DDOR, probably used against oxidative stress. Phylogenetic studies indicate that they have different evolutionary histories. As cyanobacteria diversified to occupy new habitats, some of these proteins were gradually lost in some groups. Finally, we also review the physiological relevance of redox regulation in cyanobacteria through the study of target enzymes.

Unique functional insights into the antioxidant response of the cyanobacterial Mn-catalase (KatB)

KatB, a hexameric Mn-catalase, plays a vital role in overcoming oxidative and salinity stress in the ecologically important, N₂-fixing cyanobacterium, Anabaena. The 5 N-terminal residues of KatB, which show a high degree of conservation in cyanobacteria, form an antiparallel β-strand at the subunit interface of the KatB hexamer. In this study, the contribution of these N-terminal non-active site residues, towards the maintenance of the structure, biochemical properties, and redox balance was evaluated. Each N-terminal amino acid residue from the 2nd to the 7th position of KatB was individually mutated to Ala (to express KatBF2A/KatBF3A/KatBH4A/KatBK5E/KatBK6A/KatBE7A) or this entire 6 amino acid stretch was deleted (to yield KatBTrunc). All the above-mentioned KatB variants, along with the wild-type KatB protein (KatBWT), were overproduced in E. coli and purified. In comparison to KatBWT, the KatBF2A/KatBH4A/KatBTrunc proteins were less compact, more prone to chemical/thermal denaturation, and were unexpectedly inactive. KatBF3A/KatBK5E/KatBK6A showed biophysical/biochemical properties that were in between that of KatBWT and KatBF2A/KatBH4A/KatBTrunc. Surprisingly, KatBE7A was more thermostable with higher activity than KatBWT. On exposure to H₂O₂, E. coli expressing KatBWT/KatBE7A showed considerably reduced formation of ROS and increased survival than the other KatB variants. Utilizing the KatB structure, the molecular basis responsible for the altered stability/activity of the KatB mutants was delineated. This study demonstrates the physiological importance of the N-terminal β-strand of Mn-catalases in combating H₂O₂ stress and shows that the non-active site residues can be used for rational protein engineering to develop Mn-catalases with improved characteristics.

Depletion of m-type thioredoxin impairs photosynthesis, carbon fixation, and oxidative stress in cyanobacteria

Thioredoxins (Trxs) are disulfide oxidoreductases that regulate many biological processes. The m-type thioredoxin (TrxA) is the only Trx present in all oxygenic photosynthetic organisms. Extensive biochemical and proteomic analyses have identified many TrxA target proteins in different photosynthetic organisms. However, the precise function of this essential protein in vivo is still poorly known. In this study, we generated a conditional Synechocystis sp. PCC 6803 mutant strain (STXA2) using an on-off promoter that is able to survive with only 2% of the TrxA level of the wild-type (WT) strain. STXA2 characterization revealed that TrxA depletion results in growth arrest and pronounced impairment of photosynthesis and the Calvin-Benson-Bassham (CBB) cycle. Analysis of the in vivo redox state of the bifunctional enzyme fructose-1,6-bisphosphatase/sedoheptulose-1,7-bisphosphatase showed higher levels of oxidation that affected enzyme activity in STXA2. This result implies that TrxA-mediated redox regulation of the CBB cycle is conserved in both cyanobacteria and chloroplasts, although the targets have different evolutionary origins. The STXA2 strain also accumulated more reactive oxygen species and was more sensitive to oxidative stress than the WT. Analysis of the in vivo redox state of 2-Cys peroxiredoxin revealed full oxidation, corresponding with TrxA depletion. Overall, these results indicate that depletion of TrxA in STXA2 greatly alters the cellular redox state, interfering with essential processes such as photosynthetic machinery operativity, carbon assimilation, and oxidative stress response. The TrxA regulatory role appears to be conserved along the evolution of oxygenic photosynthetic organisms.

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.

Functional and mechanistic insights into the differential effect of the toxicant 'Se(IV)' in the cyanobacterium Anabaena PCC 7120

Selenium, an essential trace element for animals, poses a threat to all forms of life above a threshold concentration. The ubiquitously present cyanobacteria, a major photosynthetic biotic component of aquatic and other ecosystems, are excellent systems to study the effects of environmental toxicants. The molecular changes that led to beneficial or detrimental effects in response to different doses of selenium oxyanion Se(IV) were analyzed in the filamentous cyanobacterium Anabaena PCC 7120. This organism showed no inhibition in growth up to 15 mg/L sodium selenite, but above this dose i.e. 20-100 mg/L of Se(IV), both growth and photosynthesis were substantially inhibited. Along with the increased accumulation of non-protein thiols, a consistent reduction in levels of ROS was observed at 10 mg/mL dose of Se(IV). High dose of Se(IV) (above 20 mg/L) enhanced endogenous reactive oxygen species (ROS)/lipid peroxidation, and decreased photosynthetic capability. Treatment with 100 mg/L Se(IV) downregulated transcription of several photosynthesis pathways-related genes such as those encoding photosystem I and II proteins, phycobilisome rod-core linker protein, phycocyanobilin, phycoerythrocyanin-associated proteins etc. Interestingly, at a dose range of 10-15 mg/L Se(IV), Anabaena showed an increase in PSII photosynthetic yield and electron transport rate (at PSII), suggesting improved photosynthesis. Se was incorporated into the Anabaena cells, and Se-enriched thylakoid membranes showed higher redox conductivity than the thylakoid membranes from untreated cells. Overall, the data supports that modulation of photosynthetic machinery is one of the crucial mechanisms responsible for the dose-dependent contrasting effect of Se(IV) observed in Anabaena.

Gazing into the remarkable world of non-heme catalases through the window of the cyanobacterial Mn-catalase 'KatB'

Catalases, enzymes that decompose H₂O₂, are broadly categorized as heme catalases or non-heme catalases. The non-heme catalases are also known as Mn-catalases as they have Mn atoms in their active sites. However, unlike the well characterized heme-catalases, the study of Mn-catalases has gained importance only in the last few years. The filamentous, heterocystous, N₂-fixing cyanobacterium Anabaena PCC 7120, shows the presence of two Mn-catalases, KatA and KatB, but lacks heme catalases. Of the two Mn-catalases, KatB, which is induced by salt/desiccation, plays a major role in overcoming salinity/oxidative stress. In this mini review, we have summarized the recent advances made in the field of Mn-catalases, particularly KatB, and have interpreted these results in the larger context of stress physiology. These aspects bring to the fore the distinctive biochemical/structural properties of Mn-catalases and furthermore highlight the in vivo importance of these enzymes in adapting to oxidative stresses.

A 2-Cys peroxiredoxin gene from Tamarix hispida improved salt stress tolerance in plants

BACKGROUND

Peroxiredoxins (Prxs) are a large family of antioxidant enzymes that respond to biotic and abiotic stress by decomposing reactive oxygen species (ROS). In this study, the stress tolerance function of the Th2CysPrx gene was further analysed. It lays a foundation for further studies on the salt tolerance molecular mechanism of T. hispida and improved salt tolerance via transgenic plants.

RESULTS

In this study, the stress tolerance function of the Th2CysPrx gene was further analysed. The results of transgenic tobacco showed higher seed germination rates, root lengths, and fresh weight under salt stress than wild-type tobacco. Simultaneously, physiological indicators of transgenic tobacco and T. hispida showed that Th2CysPrx improved the activities of antioxidant enzymes and enhanced ROS removal ability to decrease cellular damage under salt stress. Moreover, Th2CysPrx improved the expression levels of four antioxidant genes (ThGSTZ1, ThGPX, ThSOD and ThPOD).

CONCLUSIONS

Overall, these results suggested that Th2CysPrx enhanced the salt tolerance of the transgenic plants. These findings lay a foundation for further studies on the salt tolerance molecular mechanism of T. hispida and improved salt tolerance via transgenic plants.

Expression of Heterologous OsDHAR Gene Improves Glutathione (GSH)-Dependent Antioxidant System and Maintenance of Cellular Redox Status in Synechococcus elongatus PCC 7942

An excess of reactive oxygen species (ROS) can cause severe oxidative damage to cellular components in photosynthetic cells. Antioxidant systems, such as the glutathione (GSH) pools, regulate redox status in cells to guard against such damage. Dehydroascorbate reductase (DHAR, EC 1.8.5.1) catalyzes the glutathione-dependent reduction of oxidized ascorbate (dehydroascorbate) and contains a redox active site and glutathione binding-site. The DHAR gene is important in biological and abiotic stress responses involving reduction of the oxidative damage caused by ROS. In this study, transgenic Synechococcus elongatus PCC 7942 (TA) was constructed by cloning the Oryza sativa L. japonica DHAR (OsDHAR) gene controlled by an isopropyl β-D-1-thiogalactopyranoside (IPTG)-inducible promoter (Ptrc) into the cyanobacterium to study the functional activities of OsDHAR under oxidative stress caused by hydrogen peroxide exposure. OsDHAR expression increased the growth of S. elongatus PCC 7942 under oxidative stress by reducing the levels of hydroperoxides and malondialdehyde (MDA) and mitigating the loss of chlorophyll. DHAR and glutathione S-transferase activity were higher than in the wild-type S. elongatus PCC 7942 (WT). Additionally, overexpression of OsDHAR in S. elongatus PCC 7942 greatly increased the glutathione (GSH)/glutathione disulfide (GSSG) ratio in the presence or absence of hydrogen peroxide. These results strongly suggest that DHAR attenuates deleterious oxidative effects via the glutathione (GSH)-dependent antioxidant system in cyanobacterial cells. The expression of heterologous OsDHAR in S. elongatus PCC 7942 protected cells from oxidative damage through a GSH-dependent antioxidant system via GSH-dependent reactions at the redox active site and GSH binding site residues during oxidative stress.

A Novel Thioredoxin-Dependent Peroxiredoxin (TPx-Q) Plays an Important Role in Defense Against Oxidative Stress and Is a Possible Drug Target in Babesia microti

Thioredoxin peroxidases (TPxs) are ubiquitous cysteine-based peroxidases that reduce peroxides as part of antioxidant defenses and redox signaling and are essential for Babesia microti protection against adverse environment agents like reactive oxygen species (ROS) and reactive nitrogen species (RNS). To better systematically understand TPxs, we identified a novel 2-Cys peroxiredoxin-Q (BmTPx-Q) of B. microti. The full-length BmTPx-Q gene is 653 bp that consists of an intact open reading frame of 594 bp that encodes a 197-amino acid protein. The predicted protein has a molecular weight of 22.3 kDa and an isoelectric point of 9.18. Moreover, BmTPx-Q showed low identity at the amino acid level to other peroxiredoxins (Prxs) among the currently known subfamilies. The recombinant BmTPx-Q protein (rBmTPx-Q) was expressed in Escherichia coli and purified with beads. The native protein BmTPx-Q was detected using mouse anti-BmTPx-Q polyclonal serum with western blotting and indirect immunofluorescence assay (IFA). In addition, enzyme activity was observed using nicotinamide adenine dinucleotide phosphate (NADPH) as substrate and triggered the NADPH-dependent reduction of the Trx/TrxR system. It was also discovered that BmTPx-Q mainly exists as a monomer whether under its native or functional states. In addition, when incubated with Chloroquine diphosphate salt for 24 h in vitro, the expression of BmTPx-Q showed a marked downward trend with the increase of drug concentration. These results suggest that B. microti uses BmTPx-Q to reduce and detoxify hydrogen peroxides to survive and proliferate inside the host. Furthermore, BmTPx-Q showed the lowest identity with host enzymes and could be a potential drug target for the development of novel strategies to control B. microti infection.

Peroxiredoxin (2-cys-prx) and catalase (katA) cyanobacterial-based bioluminescent bioreporters to detect oxidative stress in the aquatic environment

The detection of oxidative stress caused by emerging pollutants in aquatic systems is essential to carry out toxicological analysis since they can bring us information about the mechanisms of toxic action of the pollutants, which might be useful to address this contamination. To achieve this goal, two self-bioluminescent strains that respond to oxidative stress based on the filamentous cyanobacterium Nostoc sp. PCC7120, which has a high ecological relevance in aquatic continental systems, have been constructed. Nostoc sp. PCC7120 pBG2172 harbours the promoter region of the 2-cys-prx gene (P2-cys-prx), encoding a cytoplasmic peroxiredoxin, fused to luxCDABE genes of the bacterium Photorhabdus luminescens. Nostoc sp. PCC7120 pBG2173 harbours the promoter region of the KatA gene (PkatA), a cytoplasmic catalase, also fused to luxCDABE genes. Both strains have been characterized by exposing them to H₂O₂: Nostoc sp. PCC7120 pBG2172 responded while Nostoc sp. PCC7120 pBG2173 did not respond to this pollutant. In order to know their specificity, they were exposed to methyl viologen (MV), an herbicide that produces superoxide anion (O₂^-) and a bioluminescence response was observed in both strains. Besides, the utility of these strains for the detection of H₂O₂ and MV in natural water samples, both pristine and wastewater samples has been tested by spiking experiments. Finally, the possible application of these strains for the detection of the emerging pollutant triclosan has also been tested showing to be suitable bioreporters to study oxidative stress in aquatic environments.

Facile generation of a biotechnologically-relevant catalase showcases the efficacy of a blue-green algal biomass as a suitable bioresource for protein overproduction

Here, we show the utility of a cyanobacterial biomass for overproduction and easy downstream processing of the thermostable protein KatB (a Mn-catalase). The nitrogen-fixing blue-green alga, Anabaena, was bioengineered to overexpress the KatB protein (An-KatB). Interestingly, pure An-KatB could be isolated from Anabaena by a simple physical process, obviating the need of expensive resins or chromatographic steps. An-KatB was an efficient H₂O₂-detoxifying protein that retained all the properties of Mn-catalases. Surprisingly, the purified An-KatB showed improved characteristics than the corresponding KatB (Ec-KatB) protein purified after over-expression in E. coli. An-KatB was unaffected by exposure to high temperature (85 °C), whereas a commercially procured heme-catalase showed an appreciable drop in activity beyond 50 °C. These data convincingly demonstrate the utility of Anabaena as a competent microbial bioresource for overproduction of proteins and further highlight the advantage of An-KatB over heme-catalases in bioprocesses where H₂O₂ is to be decomposed at elevated temperatures.

Novel molecular insights into the anti-oxidative stress response and structure-function of a salt-inducible cyanobacterial Mn-catalase

KatB, a salt-inducible Mn-catalase, protects the cyanobacterium Anabaena from salinity/oxidative stress. In this report, we provide distinctive insights into the biological-biochemical function of KatB at the molecular level. Anabaena overexpressing the wild-type KatB protein (KatBWT) detoxified H₂ O₂ efficiently, showing reduced burden of reactive oxygen species compared with the strain overproducing KatBF2V (wherein F-2 is replaced by V). Correspondingly, the KatBWT protein also displayed several folds more activity than KatBF2V. Interestingly, the KatB variants with large hydrophobic amino acids (F/W/Y) were more compact, showed enhanced activity, and were resistant to thermal/chemical denaturation than variants with smaller residues (G/A/V) at the second position. X-ray crystallography-based analysis showed that F-2 was required for appropriate interactions between two subunits. These contacts provided stability to the hexamer, making it more compact. F-2, through its interaction with F-66 and W-43, formed the proper hydrophobic pocket that held the active site together. Consequently, only residues that supported activity (i.e., F/Y/W) were selected at the second position in Mn-catalases during evolution. This study (a) demonstrates that modification of nonactive site residues can alter the response of catalases to environmental stress and (b) has expanded the scope of amino acids that can be targeted for rational protein engineering in plants.

Novel functions of peroxiredoxin Q from Deinococcus radiodurans R1 as a peroxidase and a molecular chaperone

Deinococcus radiodurans R1 is extremely resistant to ionizing radiation and oxidative stress. In this study, we characterized DR0846, a candidate peroxiredoxin in D. radiodurans. DR0846 is a peroxiredoxin Q containing two conserved cysteine residues. DR0846 exists mainly in monomeric form with an intramolecular disulfide bond between the two cysteine residues. We found that DR0846 functions as a molecular chaperone as well as a peroxidase. A mutational analysis indicates that the two cysteine residues are essential for enzymatic activity. A double‐deletion mutant lacking DR0846 and catalase DR1998 exhibits decreased oxidative and heat shock stress tolerance with respect to the single mutants or the wild‐type cells. These results suggest that DR0846 contributes to resistance against oxidative and heat stresses in D. radiodurans.

Physiological Significance of Plant Peroxiredoxins and the Structure-Related and Multifunctional Biochemistry of Peroxiredoxin 1

SIGNIFICANCE

Sessile plants respond to oxidative stress caused by internal and external stimuli by producing diverse forms of enzymatic and nonenzymatic antioxidant molecules. Peroxiredoxins (Prxs) in plants, including the Prx1, Prx5, Prx6, and PrxQ isoforms, constitute a family of antioxidant enzymes and play important functions in cells. Each Prx localizes to a specific subcellular compartment and has a distinct function in the control of plant growth, development, cellular metabolism, and various aspects of defense signaling. Recent Advances: Prx1, a typical Prx in plant chloroplasts, has redox-dependent multiple functions. It acts as a hydrogen peroxide (H₂O₂)-catalyzing peroxidase, a molecular chaperone, and a biological circadian marker. Prx1 undergoes a functional switching from a peroxidase to a molecular chaperone in response to oxidative stress, concomitant with the structural changes from a low-molecular-weight species to high-molecular-weight complexes mediated by the post-translational modification of its active site Cys residues. The redox status of the protein oscillates diurnally between hyperoxidation and reduction, showing a circadian rhythmic output. These dynamic structural and functional transformations mediate the effect of plant Prx1 on protecting plants from a myriad of harsh environmental stresses.

CRITICAL ISSUES

The multifunctional diversity of plant Prxs and their roles in cellular defense signaling depends on their specific interaction partners, which remain largely unidentified. Therefore, the identification of Prx-interacting proteins is necessary to clarify their physiological significance.

FUTURE DIRECTIONS

Since the functional specificity of the four plant Prx isoforms remains unclear, future studies should focus on investigating the physiological importance of each Prx isotype. Antioxid. Redox Signal. 28, 625-639.

Formation of supramolecular protein structures on gold surfaces

Recent research has highlighted the exciting possibilities enabled by the use of protein structures as nanocomponents to form functional nanodevices. To this end, control over protein-protein and protein-surface interactions is essential. In this study, the authors probe the interaction of human peroxiredoxin 3 with gold surfaces, a protein that has been previously identified as having potential use in nanotechnology. Analytical ultracentrifugation and transmission electron microscopy revealed the pH mediated assembly of protein toroids into tubular structures across a small pH range. Quartz crystal microbalance with dissipation measurements showed differences in absorbed protein mass when pH is switched from pH 8.0 to 7.2, in line with the formation of supramolecular structures observed in solution studies. Scanning tunneling microscopy under ambient conditions showed that these protein tubes form on surfaces in a concentration dependent manner, with a tendency for protein adsorption and supramolecular assembly at the edges of Au(111) terraces. Finally, self-assembled monolayer modification of Au surfaces was explored as a means to control the adsorption and orientation of pH triggered protein structures.

OsPRX2 contributes to stomatal closure and improves potassium deficiency tolerance in rice

Peroxiredoxins (Prxs) which are thiol-based peroxidases have been implicated in the toxic reduction and intracellular concentration regulation of hydrogen peroxide. In Arabidopsis thaliana At2-CysPrxB (At5g06290) has been demonstrated to be essential in maintaining the water-water cycle for proper H₂O₂ scavenging. Although the mechanisms of 2-Cys Prxs have been extensively studied in Arabidopsis thaliana, the function of 2-Cys Prxs in rice is unclear. In this study, a rice homologue gene of At2-CysPrxB, OsPRX2 was investigated aiming to characterize the effect of 2-Cys Prxs on the K⁺-deficiency tolerance in rice. We found that OsPRX2 was localized in the chloroplast. Overexpressed OsPRX2 causes the stomatal closing and K⁺-deficiency tolerance increasing, while knockout of OsPRX2 lead to serious defects in leaves phenotype and the stomatal opening under the K⁺-deficiency tolerance. Detection of K⁺ accumulation, antioxidant activity of transgenic plants under the starvation of potassium, further confirmed that OsPRX2 is a potential target for engineering plants with improved potassium deficiency tolerance.

Molecular cloning, expression and antioxidative activity of 2-cys-peroxiredoxin from freshwater mussel Cristaria plicata

Peroxiredoxins (Prxs) play an important role against various oxidative stresses by catalyzing the reduction of hydrogen peroxide (H₂O₂) and organic hydroperoxides to less harmful form. A 2-cys peroxiredoxin, designated as CpPrx, was cloned from hemocytes of freshwater mussel Cristaria plicata. The full length cDNA of CpPrx is 1247 bp, which includes an open reading frame (ORF) of 591bp, encoding 196 amino acids. CpPrx possesses two conserved cysteine residues (Cys49, Cys170). The deduced amino acid sequence of CpPrx showed a high level (67-74%) of sequence similarity to 2-Cys Prxs from other species. The results of real-time quantitative PCR revealed that CpPrx mRNA was constitutively expressed in tissues, and the highest expression levels were in hepatopancreas and gills. After peptidoglycan (PGN) and Aeromonas hydrophila challenge, the expression levels of CpPrx mRNA were up-regulated in hemocytes and hepatopancreas. The cDNA of CpPrx was cloned into the plasmid pET-32, and the recombinant protein was expressed in Escherichia coli BL21(DE3). Comparison with DE3-pET-32 and DE3 strain, the cells of DE3-pET-32-CpPrx exhibited resistance to the concentration of 0.4, 0.8 and 1.2 mmoL/L H₂O₂ in vivo.

Novel molecular insights into the function and the antioxidative stress response of a Peroxiredoxin Q protein from cyanobacteria

The Peroxiredoxin Q (PrxQ) proteins are thiol-based peroxidases that are important for maintaining redox homeostasis in several organisms. Activity of PrxQs is mediated by two cysteines, peroxidatic (C_p) and resolving (C_r), in association with a reducing partner. A PrxQ, Alr3183, from the cyanobacterium, Anabaena PCC 7120, was characterized in this study. Alr3183, which required thioredoxin A (TrxA) for peroxidase activity, was an intramolecular disulfide bond-containing monomeric protein. However, Alr3183 lacking C_p (Alr3183C46S) or C_r (Alr3183C51S) formed intermolecular disulfide linkages and was dimeric. Alr3183C46S was completely inactive, while Alr3183C51S required higher concentration of TrxA for peroxidase activity. Surface plasmon resonance analysis showed that unlike Alr3183 or Alr3183C46S, Alr3183C51S bound rather poorly to TrxA. Also, compared to the oxidized protein, the DTT-treated (reduced) Alr3183 displayed decreased interaction with TrxA. In vivo, Alr3183 was found to be induced in response to γ-radiation. On exposure to H₂O₂, Anabaena strain over-expressing Alr3183 showed reduced formation of ROS, intact photosynthetic pigments and consequently better survival than the wild-type, whereas overproduction of Alr3183C46S did not provide any protection. Significantly, this study (1) reveals the importance of C_r for interaction with thioredoxins and (2) demonstrates that over-expression of PrxQs can protect cyanobacteria from oxidative stresses.

Down-Regulation of the Alternative Sigma Factor SigJ Confers a Photoprotective Phenotype to Anabaena PCC 7120

Alternative sigma factors belonging to Group 3 are thought to play an important role in the adaptation of cyanobacteria to environmental challenges by altering expression of genes needed for coping with such stresses. In this study, the role of an alternative sigma factor, SigJ, was analyzed in the filamentous nitrogen-fixing cyanobacterium, Anabaena sp. PCC 7120 by knocking down the expression of the sigJ gene (alr0277) employing an antisense RNA-mediated approach. In the absence of any stress, the knock-down (KD0277) or the wild-type strain both grew similarly. Upon exposure to high-intensity light, KD0277 showed substantially reduced bleaching of its pigments, higher photosynthetic activity and consequently better survival than the wild type. KD0277 also showed an enhanced accumulation of two carotenoids, which were identified as myxoxanthophyll and keto-myxoxanthophyll. Further, KD0277 was more tolerant to ammonium-triggered photodamage than the wild type. Moreover, PSII was better protected against photodamage in KD0277 than in the wild type. Down-regulation of sigJ in Anabaena PCC 7120, however, reduced its ability to cope with desiccation. This study demonstrates that down-regulation of the sigJ gene in Anabaena PCC 7120 differentially affects its ability to tolerate two environmentally relevant stresses, i.e. high-intensity light and desiccation.

Functional Significance of NADPH-Thioredoxin Reductase C in the Antioxidant Defense System of Cyanobacterium Anabaena sp. PCC 7120

The redox regulation system is widely accepted as a crucial mechanism for controlling the activities of various metabolic enzymes. In addition to thioredoxin reductase/thioredoxin cascades, NADPH-thioredoxin reductase C (NTRC), a hybrid protein formed by an NADPH-thioredoxin reductase domain and a thioredoxin (Trx) domain, is present in chloroplasts and in most cyanobacteria species. Although several target proteins and physiological functions of NTRC in chloroplasts have been characterized, little is known about NTRC functions in cyanobacteria. Therefore, we investigated the molecular basis and physiological significance of NTRC-dependent redox regulation in the filamentous heterocyst-forming nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120 (Anabaena 7120). Initially, we identified six candidate NTRC targets in Anabaena 7120 using NTRC affinity chromatography. Subsequently, we compared the efficiency of reducing-equivalent transfer from NTRC and Trx-m1 to the NTRC target protein 2-Cys peroxiredoxin. Biochemical analyses revealed that compared with Trx-m1, NTRC more efficiently transfers reducing equivalents to 2-Cys peroxiredoxin. Subsequently, we constructed and analyzed an ntrC knockout strain in Anabaena 7120. The mutant showed impaired growth under oxidative stress conditions and lower concentrations of reduced 2-Cys peroxiredoxin in cells. Taken together, the present in vitro and in vivo results indicate that NTRC is a significant electron donor for 2-Cys peroxiredoxin and plays a pivotal role in antioxidant defense systems in Anabaena 7120 cells.

Cyanobacterial Mn-catalase 'KatB': Molecular link between salinity and oxidative stress resistance

Catalases are ubiquitous enzymes that detoxify H₂O₂ in virtually all organisms exposed to oxygen. The filamentous, nitrogen-fixing cyanobacterium, Anabaena PCC 7120, shows the presence of 2 genes (katA and katB) that encode Mn-catalases. We have recently shown that pre-treatment of Anabaena with NaCl causes substantial induction of the KatB protein, which consequently leads to increased oxidative stress resistance in that cyanobacterium. Interestingly, when compared to the wild-type, the katB mutant shows decreased growth and impaired photosynthetic activity in the presence of NaCl. Furthermore, the NaCl-treated katB mutant is extremely sensitive to H₂O₂. In this study, the ultrastructural changes occurring in the katB mutant and the wild-type Anabaena cells are analyzed to understand the cellular basis of the above-mentioned protective phenomena. Other data show that a wide variety of osmolytes induce katB expression in Anabaena, indicating that katB is a genuine osmo-inducible gene. These results have important biotechnological implications for the development of novel cyanobacterial biofertilzers and transgenic plants with improved resistance to salinity.

Molecular and biochemical characterisation of a novel type II peroxiredoxin (XvPrx2) from the resurrection plant Xerophyta viscosa

A type II peroxiredoxin gene (XvPrx2) was isolated from a Xerophyta viscosa (Baker) cDNA cold-stress library. The polypeptide displayed significant similarity to other plant type II peroxiredoxins, with the conserved amino acid motif (PGAFTPTCS) proposed to constitute the active site of the enzyme. Northern blot analyses showed that XvPrx2 gene was stress-inducible in response to abiotic stresses while gel analyses revealed that XvPrx2 homologues exist within the X. viscosa proteome. Using a yellow fluorescent reporter protein, the XvPrx2 protein localised to the cytosol. A mutated protein (XvV7) was generated by converting the valine at position 76 to a cysteine and an in vitro DNA protection assay showed that, in the presence of either XvPrx2 or XvV7, DNA protection occurred. In addition, an in vivo assay showed that increased protection was conferred to Escherichia coli cells overexpressing either XvPrx2 or XvV7. The XvPrx2 activity was maximal with DTT as electron donor and H2O2 as substrate. Using E. coli thioredoxin, a 2-15-fold lower enzyme activity was observed. The XvPrx2 activity with glutathione was significantly lower and glutaredoxin had no measurable effect on this reaction. The XvV7 protein displayed significantly lower activity compared with XvPrx2 for all substrates assessed.

A Salt-Inducible Mn-Catalase (KatB) Protects Cyanobacterium from Oxidative Stress

Catalases, enzymes that detoxify H2O2, are widely distributed in all phyla, including cyanobacteria. Unlike the heme-containing catalases, the physiological roles of Mn-catalases remain inadequately characterized. In the cyanobacterium Anabaena, pretreatment of cells with NaCl resulted in unusually enhanced tolerance to oxidative stress. On exposure to H2O2, the NaCl-treated Anabaena showed reduced formation of reactive oxygen species, peroxides, and oxidized proteins than the control cells (i.e. not treated with NaCl) exposed to H2O2. This protective effect correlated well with the substantial increase in production of KatB, a Mn-catalase. Addition of NaCl did not safeguard the katB mutant from H2O2, suggesting that KatB was indeed responsible for detoxifying the externally added H2O2. Moreover, Anabaena deficient in KatB was susceptible to oxidative effects of salinity stress. The katB gene was strongly induced in response to osmotic stress or desiccation. Promoter-gfp analysis showed katB to be expressed only in the vegetative cells but not in heterocysts. Biochemically, KatB was an efficient, robust catalase that remained active in the presence of high concentrations of NaCl. Our findings unravel the role of Mn-catalase in acclimatization to salt/oxidative stress and demonstrate that the oxidative stress resistance of an organism can be enhanced by a simple compound such as NaCl.

Microbial 2-Cys Peroxiredoxins: Insights into Their Complex Physiological Roles

The peroxiredoxins (Prxs) constitute a very large and highly conserved family of thiol-based peroxidases that has been discovered only very recently. We consider here these enzymes through the angle of their discovery, and of some features of their molecular and physiological functions, focusing on complex phenotypes of the gene mutations of the 2-Cys Prxs subtype in yeast. As scavengers of the low levels of H2O2 and as H2O2 receptors and transducers, 2-Cys Prxs have been highly instrumental to understand the biological impact of H2O2, and in particular its signaling function. 2-Cys Prxs can also become potent chaperone holdases, and unveiling the in vivo relevance of this function, which is still not established, should further increase our knowledge of the biological impact and toxicity of H2O2. The diverse molecular functions of 2-Cys Prx explain the often-hard task of relating them to peroxiredoxin genes phenotypes, which underscores the pleiotropic physiological role of these enzymes and complex biologic impact of H2O2.

In vivo parameters influencing 2-Cys Prx oligomerization: The role of enzyme sulfinylation

2-Cys Prxs are H₂O₂-specific antioxidants that become inactivated by enzyme hyperoxidation at elevated H₂O₂ levels. Although hyperoxidation restricts the antioxidant physiological role of these enzymes, it also allows the enzyme to become an efficient chaperone holdase. The critical molecular event allowing the peroxidase to chaperone switch is thought to be the enzyme assembly into high molecular weight (HMW) structures brought about by enzyme hyperoxidation. How hyperoxidation promotes HMW assembly is not well understood and Prx mutants allowing disentangling its peroxidase and chaperone functions are lacking. To begin addressing the link between enzyme hyperoxidation and HMW structures formation, we have evaluated the in vivo 2-Cys Prxs quaternary structure changes induced by H₂O₂ by size exclusion chromatography (SEC) on crude lysates, using wild type (Wt) untagged and Myc-tagged S. cerevisiae 2-Cys Prx Tsa1 and derivative Tsa1 mutants or genetic conditions known to inactivate peroxidase or chaperone activity or altering the enzyme sensitivity to hyperoxidation. Our data confirm the strict causative link between H₂O₂-induced hyperoxidation and HMW formation/stabilization, also raising the question of whether C_P hyperoxidation triggers the assembly of HMW structures by the stacking of decamers, which is the prevalent view of the literature, or rather, the stabilization of preassembled stacked decamers.

Protein plasticity underlines activation and function of ATP-independent chaperones

One of the key issues in biology is to understand how cells cope with protein unfolding caused by changes in their environment. Self-protection is the natural immediate response to any sudden threat and for cells the critical issue is to prevent aggregation of existing proteins. Cellular response to stress is therefore indistinguishably linked to molecular chaperones, which are the first line of defense and function to efficiently recognize misfolded proteins and prevent their aggregation. One of the major protein families that act as cellular guards includes a group of ATP-independent chaperones, which facilitate protein folding without the consumption of ATP. This review will present fascinating insights into the diversity of ATP-independent chaperones, and the variety of mechanisms by which structural plasticity is utilized in the fine-tuning of chaperone activity, as well as in crosstalk within the proteostasis network. Research into this intriguing class of chaperones has introduced new concepts of stress response to a changing cellular environment, and paved the way to uncover how this environment affects protein folding.

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).