A high constitutive catalase activity confers resistance to methyl viologen-promoted oxidative stress in a mutant of the cyanobacterium Nostoc punctiforme ATCC 29133

Differential catalase activity and tolerance to hydrogen peroxide in the filamentous cyanobacteria Nostoc punctiforme ATCC 29133 and Anabaena sp. PCC 7120

Photoautotrophic cyanobacteria often confront hydrogen peroxide (H₂O₂), a reactive oxygen species potentially toxic to cells when present in sufficiently high concentrations. In this study, H₂O₂ tolerance ability of filamentous cyanobacteria Nostoc punctiforme ATCC 29133 (Nostoc 29133) and Anabaena sp. PCC 7120 (Anabaena 7120) was investigated at increasing concentrations of H₂O₂ (0-0.5 mM). In Nostoc 29133, 0.25 and 0.5 mM H₂O₂ caused a reduction in chlorophyll a content by 12 and 20%, respectively, whereas with similar treatments, a total loss of chlorophyll a was detected in Anabaena 7120. Further, Nostoc 29133 was able to maintain its photosystem II performance in the presence of H₂O₂ up to a concentration of 0.5 mM, whereas in Anabaena 7120, 0.25 mM H₂O₂ caused a complete reduction of photosystem II performance. The intracellular hydroperoxide level (indicator of oxidative status) did not increase to the same high level in Nostoc 29133, as compared to in Anabaena 7120 after H₂O₂ treatment. This might be explained by that Nostoc 29133 showed a 20-fold higher intrinsic constitutive catalase activity than Anabaena 7120, thus indicating that the superior tolerance of Nostoc 29133 to H₂O₂ stems from its higher ability to decompose H₂O₂. It is suggested that difference in H₂O₂ tolerance between closely related filamentous cyanobacteria, as revealed in this study, may be taken into account for judicious selection and effective use of strains in biotechnological applications.

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.

Tolerance and sorption of Bromacil and Paraquat by thermophilic cyanobacteria Leptolyngbya 7M from Costa Rican thermal springs

We studied the adsorption ability and tolerance of the thermophilic filamentous cyanobacteria Letolyngbya 7M towards Paraquat and Bromacil. Adsorption isotherms at pH = 7.0 showed an adsorption capacity of 24.4 mg/g and 66.8 mg/g, respectively, and a good fit to the Langmuir model (R² = 0.97 and 0.99, respectively). To evaluate the effect of both herbicides on photosynthetic pigments and viability of cyanobacteria, cell autoflorescence and esterase activity was determined using flow cytometry. Autofluorescence was less sensitive to changes in cell viability, as it was only slightly reduced at high Paraquat and Bromacil concentrations. Herbicide effect on esterase activity is dose-dependent. Bromacil did not cause a significant effect on either chlorophyll a content or cell viability. This study demonstrates the potential of Leptolyngbya 7M to remove Paraquat and Bromacil herbicides from aqueous solution under laboratory conditions.

First in vivo Evidence That Glutathione-S-Transferase Operates in Photo-Oxidative Stress in Cyanobacteria

Although glutathione (GSH) and GSH-dependent enzymes, such as glutathione transferases (GSTs), are thought to have been developed by cyanobacteria to cope with the reactive oxygen species (ROS) that they massively produced by their active photosynthesis, there had been no in vivo analysis of the role of GSTs in cyanobacteria so far. Consequently, we have analyzed two of the six GSTs of the model cyanobacterium Synechocystis PCC 6803, namely Sll1545 (to extend its in vitro study) and Slr0236 (because it is the best homolog to Sll1545). We report that Sll1545 is essential to cell growth in standard photo-autotrophic conditions, whereas Slr0236 is dispensable. Furthermore, both Sll1545 and Slr0236 operate in the protection against stresses triggered by high light, H₂O₂, menadione and methylene blue. The absence of Slr0236 and the depletion of Sll1545 decrease the tolerance to methylene blue in a cumulative way. Similarly, the combined absence of Slr0236 and depletion of Sll1545 decrease the resistance to high light. Attesting their sensitivity to high-light or methylene blue, these Δslr0236-sll1545 cells transiently accumulate ROS, and then reduced and oxidized glutathione in that order. In contrast, the absence of Slr0236 and the depletion of Sll1545 increase the tolerance to menadione in a cumulative way. This increased menadione resistance is due, at least in part, to the higher level of catalase and/or peroxidase activity of these mutants. Similarly, the increased H₂O₂ resistance of the Δslr0236-sll1545 cells is due, at least in part, to its higher level of peroxidase activity.

Cyanobacterial Catalase Activity Prevents Oxidative Stress Induced by Pseudomonas fluorescens DUS1-27 from Inhibiting Brassica napus L. (canola) Growth

Plant growth-promoting bacteria (PGPB) inhabit the rhizosphere of plants and are capable of enhancing plant growth through a number of mechanisms. A strain of Pseudomonas fluorescens DUS1-27 was identified as a potential PGPB candidate based on its ability to increase the growth of Brassica napus L. (canola) over that of uninoculated control plants in a soil-based system. The same P. fluorescens isolate was found to reduce plant growth in a hydroponic growth system, with plants showing the symptoms of a microbe-associated molecular pattern (MAMP) response to the bacteria. The amperometric quantification of H2O2, fluorescence-based total peroxidase assays, and quantification of catalase gene expression levels using qRT-PCR revealed that oxidative stress reduced plant growth in the hydroponic system. The addition of the cyanobacterium Nostoc punctiforme (known to have high catalase activity levels) in the hydroponic system as a co-inoculant reduced oxidative stress (49.7% decrease in H2O2 concentrations) triggered by the addition of P. fluorescens DUS1-27, thereby enabling plants to grow larger than uninoculated control plants. These results show the advantage of inoculating with multiple bacteria to promote plant growth and, for the first time, demonstrate that N. punctiforme beneficially assists plants under oxidative stress through its catalase activity in planta.

Activation of catalase activity by a peroxisome-localized small heat shock protein Hsp17.6CII

Plant catalases are important antioxidant enzymes and are indispensable for plant to cope with adverse environmental stresses. However, little is known how catalase activity is regulated especially at an organelle level. In this study, we identified that small heat shock protein Hsp17.6CII (AT5G12020) interacts with and activates catalases in the peroxisome of Arabidopsis thaliana. Although Hsp17.6CII is classified into the cytosol-located small heat shock protein subfamily, we found that Hsp17.6CII is located in the peroxisome. Moreover, Hsp17.6CII contains a novel non-canonical peroxisome targeting signal 1 (PTS1), QKL, 16 amino acids upstream from the C-terminus. The QKL signal peptide can partially locate GFP to peroxisome, and mutations in the tripeptide lead to the abolishment of this activity. In vitro catalase activity assay and holdase activity assay showed that Hsp17.6CII increases CAT2 activity and prevents it from thermal aggregation. These results indicate that Hsp17.6CII is a peroxisome-localized catalase chaperone. Overexpression of Hsp17.6CII conferred enhanced catalase activity and tolerance to abiotic stresses in Arabidopsis. Interestingly, overexpression of Hsp17.6CII in catalase-deficient mutants, nca1-3 and cat2 cat3, failed to rescue their stress-sensitive phenotypes and catalase activity, suggesting that Hsp17.6CII-mediated stress response is dependent on NCA1 and catalase activity. Overall, we identified a novel peroxisome-located catalase chaperone that is involved in plant abiotic stress resistance by activating catalase activity.

The two Dps proteins, NpDps2 and NpDps5, are involved in light-induced oxidative stress tolerance in the N2-fixing cyanobacterium Nostoc punctiforme

Cyanobacteria are photosynthetic prokaryotes that are considered biotechnologically prominent organisms for production of high-value compounds. Cyanobacteria are subject to high-light intensities, which is a challenge that needs to be addressed in design of efficient bio-engineered photosynthetic organisms. Dps proteins are members of the ferritin superfamily and are omnipresent in prokaryotes. They play a major role in oxidative stress protection and iron homeostasis. The filamentous, heterocyst-forming Nostoc punctiforme, has five Dps proteins. In this study we elucidated the role of these Dps proteins in acclimation to high light intensity, the gene loci organization and the transcriptional regulation of all five dps genes in N. punctiforme was revealed, and dps-deletion mutant strains were used in physiological characterization. Two mutants defective in Dps2 and Dps5 activity displayed a reduced fitness under increased illumination, as well as a differential Photosystem (PS) stoichiometry, with an elevated Photosystem II to Photosystem I ratio in the dps5 deletion strain. This work establishes a Dps-mediated link between light tolerance, H₂O₂ detoxification, and iron homeostasis, and provides further evidence on the non-redundant role of multiple Dps proteins in this multicellular cyanobacterium.

Molecular Cloning and Biochemical Characterization of the Iron Superoxide Dismutase from the Cyanobacterium Nostoc punctiforme ATCC 29133 and Its Response to Methyl Viologen-Induced Oxidative Stress

Superoxide dismutase (SOD) detoxifies cell-toxic superoxide radicals and constitutes an important component of antioxidant machinery in aerobic organisms, including cyanobacteria. The iron-containing SOD (SodB) is one of the most abundant soluble proteins in the cytosol of the nitrogen-fixing cyanobacterium Nostoc punctiforme ATCC 29133, and therefore, we investigated its biochemical properties and response to oxidative stress. The putative SodB-encoding open reading frame Npun_R6491 was cloned and overexpressed in Escherichia coli as a C-terminally hexahistidine-tagged protein. The purified recombinant protein had a SodB specific activity of 2560 ± 48 U/mg protein at pH 7.8 and was highly thermostable. The presence of a characteristic iron absorption peak at 350 nm, and its sensitivity to H2O2 and azide, confirmed that the SodB is an iron-containing SOD. Transcript level of SodB in nitrogen-fixing cultures of N. punctiforme decreased considerably (threefold) after exposure to an oxidative stress-generating herbicide methyl viologen for 4 h. Furthermore, in-gel SOD activity analysis of such cultures grown at increasing concentrations of methyl viologen also showed a loss of SodB activity. These results suggest that SodB is not the primary scavenger of superoxide radicals induced by methyl viologen in N. punctiforme.

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.