Comparison of salt- and heat-induced alterations of protein synthesis in the cyanobacterium Synechocystis sp. PCC 6803

Ameliorative effects of endogenous and exogenous indole-3-acetic acid on atrazine stressed paddy field cyanobacterial biofertilizer Cylindrospermum stagnale

Across the world, paddy fields naturally harbour cyanobacteria that function as biofertilizers and secrete various compounds like Indole-3-acetic acid (IAA) that help organisms in regulating their growth. Also, paddy field farming utilizes large amounts of pesticides (e.g. atrazine); but their continued application in the agricultural field causes toxicity in non-target cyanobacterial species that hinder their performance as a biofertilizer. Hence, the current study is an attempt to ameliorate the atrazine stress in cyanobacterium Cylindrospermum stagnale by addition of IAA (1 mM each) under different atrazine levels (0, 60, 80, 100, 120, 140 µg/l). Atrazine toxicity affected C. stagnale in a dose-dependent manner further experiments revealed that both the exogenous and endogenous IAA mitigated the detrimental effects of atrazine. It reduced MDA content and simultaneously increased chlorophyll content, total protein content, and multiple antioxidant enzyme activities [superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX)] at 140 µg/l. A molecular docking study revealed that the pesticide binds to the D1 protein of the photoelectric chain in photosynthesis. Hence, the application of IAA or cyanobacterial biofertilizer that secretes a sufficient amount of IAA may assist sustainable agriculture in counteracting the atrazine toxicity.

Characterizing metabolic stress-induced phenotypes of Synechocystis PCC6803 with Raman spectroscopy

BACKGROUND

During their long evolution, Synechocystis sp. PCC6803 developed a remarkable capacity to acclimate to diverse environmental conditions. In this study, Raman spectroscopy and Raman chemometrics tools (RametrixTM) were employed to investigate the phenotypic changes in response to external stressors and correlate specific Raman bands with their corresponding biomolecules determined with widely used analytical methods.

METHODS

Synechocystis cells were grown in the presence of (i) acetate (7.5-30 mM), (ii) NaCl (50-150 mM) and (iii) limiting levels of MgSO4 (0-62.5 mM) in BG-11 media. Principal component analysis (PCA) and discriminant analysis of PCs (DAPC) were performed with the RametrixTM LITE Toolbox for MATLABⓇ. Next, validation of these models was realized via RametrixTM PRO Toolbox where prediction of accuracy, sensitivity, and specificity for an unknown Raman spectrum was calculated. These analyses were coupled with statistical tests (ANOVA and pairwise comparison) to determine statistically significant changes in the phenotypic responses. Finally, amino acid and fatty acid levels were measured with well-established analytical methods. The obtained data were correlated with previously established Raman bands assigned to these biomolecules.

RESULTS

Distinguishable clusters representative of phenotypic responses were observed based on the external stimuli (i.e., acetate, NaCl, MgSO4, and controls grown on BG-11 medium) or its concentration when analyzing separately. For all these cases, RametrixTM PRO was able to predict efficiently the corresponding concentration in the culture media for an unknown Raman spectra with accuracy, sensitivity and specificity exceeding random chance. Finally, correlations (R > 0.7) were observed for all amino acids and fatty acids between well-established analytical methods and Raman bands.

Probing Synechocystis-Arsenic Interactions through Extracellular Nanowires

Microbial nanowires (MNWs) can play an important role in the transformation and mobility of toxic metals/metalloids in environment. The potential role of MNWs in cell-arsenic (As) interactions has not been reported in microorganisms and thus we explored this interaction using Synechocystis PCC 6803 as a model system. The effect of half maximal inhibitory concentration (IC50) [~300 mM As (V) and ~4 mM As (III)] and non-inhibitory [4X lower than IC50, i.e., 75 mM As (V) and 1 mM As (III)] of As was studied on Synechocystis cells in relation to its effect on Chlorophyll (Chl) a, type IV pili (TFP)-As interaction and intracellular/extracellular presence of As. In silico analysis showed that subunit PilA1 of electrically conductive TFP, i.e., microbial nanowires of Synechocystis have putative binding sites for As. In agreement with in silico analysis, transmission electron microscopy analysis showed that As was deposited on Synechocystis nanowires at all tested concentrations. The potential of Synechocystis nanowires to immobilize As can be further enhanced and evaluated on a large scale and thus can be applied for bioremediation studies.

Identification of common motifs in the regulation of light harvesting: The case of cyanobacteria IsiA

When cyanobacteria are grown under iron-limited or other oxidative stress conditions the iron stress inducible pigment-protein IsiA is synthesized in variable amounts. IsiA accumulates in aggregates inside the photosynthetic membrane that strongly dissipate chlorophyll excited state energy. In this paper we applied Stark fluorescence (SF) spectroscopy at 77K to IsiA aggregates to gain insight into the nature of the emitting and energy dissipating state(s). Our study shows that two emitting states are present in the system, one emitting at 684 nm and the other emitting at about 730 nm. The new 730 nm state exhibits strongly reduced fluorescence (F) together with a large charge transfer character. We discuss these findings in the light of the energy dissipation mechanisms involved in the regulation of photosynthesis in plants, cyanobacteria and diatoms. Our results suggest that photosynthetic organisms have adopted common mechanisms to cope with the deleterious effects of excess light under unfavorable growth conditions.

Salt and UV-B induced changes in Anabaena PCC 7120: physiological, proteomic and bioinformatic perspectives

This study examines response of Anabaena sp. PCC 7120 to salt and UV-B stress by combining physiological, biochemical, proteomics and bioinformatics approaches. Sixty five significantly altered protein spots corresponding to 51 protein genes identified using MALDI-TOF MS/MS were divided into nine functional categories. Based on relative abundance, these proteins were grouped into four major sets. Of these, 27 and 5 proteins were up- and downregulated, respectively, both under salt and UV-B while 8 and 11 proteins showed accumulation in salt and UV-B applied singly. Some responses common to salt and UV-B included (i) enhanced expression of FeSOD, alr3090 and accumulation of MDA indicating oxidative stress, (ii) accumulation of PDH, G6P isomerase, FBPaldolase, TK, GAPDH and PGK suggesting enhanced glycolysis, (iii) upregulation of 6-PGD, 6PGL and NADPH levels signifying operation of pentose phosphate pathway, (iv) upregulation of Dps, NDK and alr3199 indicating DNA damage, and (v) accumulation of proteins of ribosome assembly, transcriptional and translational processing. In contrast, enhanced expression of RUBISCO, increased glycolate oxidase activity and ammonium content under salt signify the difference. Salt was found to be more damaging than UV-B probably due to a cumulative effect of ionic, osmotic and oxidative damage. A group of proteins having common expression represent decreased toxicity of salt and UV-B when applied in combination.

Salt stress inhibits photosystems II and I in cyanobacteria

Recent studies of responses of cyanobacterial cells to salt stress have revealed that the NaCl-induced decline in the photosynthetic activities of photosystems II and I involves rapid and slow changes. The rapid decreases in the activities of both photosystems, which occur within a few minutes, are reversible and are associated with osmotic effects, which induce the efflux of water from the cytosol through water channels and rapidly increase intracellular concentrations of salts. Slower decreases in activity, which occur within hours, are irreversible and are associated with ionic effects that are due to the influx of Na(+) and Cl(-) ions through K(+)(Na(+)) channels and, probably, Cl(-) channels, with resultant dissociation of extrinsic proteins from photosystems. In combination with light stress, salt stress significantly stimulates photoinhibition by inhibiting repair of photodamaged photosystem II. Tolerance of photosystems to salt stress can be enhanced by genetically engineered increases in the unsaturation of fatty acids in membrane lipids and by intracellular synthesis of compatible solutes, such as glucosylglycerol and glycinebetaine. In this review, we summarize recent progress in research on the effects of salt stress on photosynthesis in cyanobacteria.

Proteomics with a pinch of salt: a cyanobacterial perspective

Cyanobacteria are ancient life forms and have adapted to a variety of extreme environments, including high salinity. Biochemical, physiological and genetic studies have contributed to uncovering their underlying survival mechanisms, and as recent studies demonstrate, proteomics has the potential to increase our overall understanding further. To date, most salt-related cyanobacterial proteomic studies have utilised gel electrophoresis with the model organism Synechocystis sp. PCC6803. Moreover, focus has been on 2-4% w/v NaCl concentrations within different cellular compartments. Under these conditions, Synechocystis sp. PCC6803 was found to respond and adapt to salt stress through synthesis of general and specific stress proteins, altering the protein composition of extracellular layers, and re-directing control of complex central intermediary pathways. Post-transcriptional control was also predicted through non-correlating transcript level data and identification of protein isoforms.In this paper, we also review technical developments with emphasis on improving the quality and quantity of proteomic data and overcoming the detrimental effects of salt on sample preparation and analysis. Developments in gel-free methods include protein and peptide fractionation workflows, which can increase coverage of the proteome (20% in Synechocystis sp. PCC6803). Quantitative techniques have also improved in accuracy, resulting in confidence in quantitation approaching or even surpassing that seen in transcriptomic techniques (better than 1.5-fold in differential expression). Furthermore, in vivo metabolic labelling and de novo protein sequencing software have improved the ability to apply proteomics to unsequenced environmental isolates. The example used in this review is a cyanobacterium isolated from a Saharan salt lake.

Proteome analysis of salt stress response in the cyanobacteriumSynechocystis sp. strain PCC 6803

In the present study, changes in protein synthesis patterns after salt shock visualized by 35S-methionine labeling and the changed protein composition in salt-acclimated cells of the cyanobacterium Synechocystis sp. strain PCC 6803 were analyzed by a combination of 2-DE for protein separation and PMF for protein identification. As a basis for the differential analysis, a proteome map with 500 identified protein spots comprising 337 different protein species was established. Fifty-five proteins were found, which are induced by salt shock or accumulated after long-term salt acclimation. Some of the proteins are salt stress-specific, such as enzymes involved in the synthesis of the compatible solute glucosylglycerol, while most of them are involved in general stress acclimation. Particularly, heat-shock proteins and proteins acting against lesions by reactive oxygen species were found. Moreover, changes in enzymes involved in basic carbohydrate metabolism were detected. The dynamic of the proteome of salt-stressed Synechocystis cells was compared to previous data concerning transcriptome analysis revealing that 89% of the proteins induced shortly after salt shock were also found to be induced at the RNA level. However, 42% of the stably up-regulated proteins in salt-acclimated cells were not detected previously using DNA microarrays. The comparison of transcriptomic and proteomic analyses shows the significance of post-transcriptional regulatory mechanisms in acclimation of Synechocystis to high salt concentrations.

Proteomic analysis of the heat shock response in Synechocystis PCC6803 and a thermally tolerant knockout strain lacking the histidine kinase 34 gene

Proteomic analysis of the heat shock response of wild type and a mutant of the histidine kinase 34 gene (Deltahik34), which shows increased thermal tolerance, has been performed in the cyanobacterium Synechocystis sp. PCC6803. In vivo radioactive labelling demonstrates that major proteomic changes occur within 1 h of heat shock. 2-D DIGE and MS have been used to quantify changes in specific proteins following heat shock in the wild type and the mutant. Over 100 spots, corresponding to 65 different proteins alter following heat shock. Changes occur not only in the classical heat shock proteins but also in the protein biosynthetic machinery, amino acid biosynthetic enzymes, components of the light and dark acts of photosynthesis and energy metabolism. The Deltahik34 cells have elevated levels of heat shock proteins under both non-heat shock and heat shock conditions, in comparison to the wild type, consistent with Hik34, or a down stream component, being a negative regulator of heat shock-responsive genes.

Proteomic screening of salt-stress-induced changes in plasma membranes of Synechocystis sp. strain PCC 6803

The plasma membrane of a cyanobacterial cell is crucial as barrier against the outer medium. It is also an energy-transducing membrane as well as essential for biogenesis of cyanobacterial photosystems and the endo-membrane system. Previously we have identified 57 different proteins in the plasma membrane of control cells from Synechocystis sp. strain PCC6803. In the present work, proteomic screening of salt-stress proteins in the plasma membrane resulted in identification of 109 proteins corresponding to 66 different gene products. Differential and quantitative analyses of 2-DE profiles of plasma membranes isolated from both control and salt-acclimated cells revealed that twenty proteins were enhanced/induced and five reduced during salt stress. More than half of the enhanced/induced proteins were periplasmic binding proteins of ABC-transporters or hypothetical proteins. Proteins that exhibited the highest enhancement during salt stress include FutA1 (Slr1295) and Vipp1 (Sll0617), which have been suggested to be involved in protection of photosystem II under iron deficiency and in thylakoid membrane formation, respectively. Other salt-stress proteins were regulatory proteins such as PII protein, LrtA, and a protein that belongs to CheY subfamily. The physiological significance of the identified salt-stress proteins in the plasma membrane is discussed integrating our current knowledge on cyanobacterial stress physiology.

SspA, an outer membrane protein, is highly induced under salt-stressed conditions and is essential for growth under salt-stressed aerobic conditions in Rhodobacter sphaeroides f. sp. denitrificans

We have previously shown that an outer membrane protein, SspA, is prominently induced by salt stress in a photosynthetic bacterium, Rhodobacter sphaeroides f. sp. denitrificans IL106 (R. sphaeroides). In this study, we investigated the physiological role of SspA under various stress conditions. Using recombinant SspA expressed in Escherichia coli as an antigen, the polyclonal antiserum of SspA was prepared. Western blot analysis demonstrated that SspA was highly induced by salt stress under both anaerobic and aerobic conditions. SspA was also induced, but to a lesser extent, by osmotic and acid stress. It is reduced under heat and cold compared to non-stressed conditions. While sspA-disrupted R. sphaeroides grew normally under anaerobic conditions in either the presence or absence of stress, it displayed significantly retarded growth under aerobic conditions in the dark, especially when osmotic or salt stress were imposed. In addition, the sspA disruptant, but not the wild type, formed cell aggregates when grown under both anaerobic and aerobic conditions, and this phenotype was significantly enhanced under salt-stressed aerobic conditions. Together, our findings suggest that SspA is critical under salt-stressed, aerobic growth conditions.

Comparative analysis of the hspA mutant and wild-type Synechocystis sp. strain PCC 6803 under salt stress: evaluation of the role of hspA in salt-stress management

DNA microarray analysis has previously revealed that hspA, which encodes a small heat-shock protein, is the second most highly expressed gene under salt stress in Synechocystis sp. strain PCC 6803. Consequently, an hspA deletion mutant was studied under various salt stresses in order to identify a potential role of HspA in salt stress management. The mutant had a growth disadvantage under moderate salt stress. It lost the ability to develop tolerance to a lethal salt treatment by a moderate salt pre-treatment when the tolerance was evaluated by cell survival and the level of major soluble proteins, phycocyanins, while the wild-type acquired tolerance. Under various salt stresses, the mutant failed to undergo the ultrastructural changes characteristic of wild-type cells. The mutant, which showed higher survival than the wild-type after a direct shift to lethal salt conditions, accumulated higher levels of groESL1 and groEL2 transcripts and the corresponding proteins, GroES, GroEL1, and GroEL2, suggesting a role for these heat-shock proteins in conferring basal salt tolerance. Under salt stress, heat-shock genes, such as hspA, groEL2, and dnaK2, were transcriptionally induced and greatly stabilized, indicating a transcriptional and post-transcriptional mechanism of acclimation to salt stress involving these heat-shock genes.

Unsaturated fatty acids in membrane lipids protect the photosynthetic machinery against salt-induced damage in Synechococcus

In this study, the tolerance to salt stress of the photosynthetic machinery was examined in relation to the effects of the genetic enhancement of the unsaturation of fatty acids in membrane lipids in wild-type and desA+ cells of Synechococcus sp. PCC 7942. Wild-type cells synthesized saturated and mono-unsaturated fatty acids, whereas desA+ cells, which had been transformed with the desA gene for the Delta12 acyl-lipid desaturase of Synechocystis sp. PCC 6803, also synthesized di-unsaturated fatty acids. Incubation of wild-type and desA+ cells with 0.5 M NaCl resulted in the rapid loss of the activities of photosystem I, photosystem II, and the Na+/H+ antiport system both in light and in darkness. However, desA+ cells were more tolerant to salt stress and osmotic stress than the wild-type cells. The extent of the recovery of the various photosynthetic activities from the effects of 0.5 M NaCl was much greater in desA+ cells than in wild-type cells. The photosystem II activity of thylakoid membranes from desA+ cells was more resistant to 0.5 M NaCl than that of membranes from wild-type cells. These results demonstrated that the genetically engineered increase in unsaturation of fatty acids in membrane lipids significantly enhanced the tolerance of the photosynthetic machinery to salt stress. The enhanced tolerance was due both to the increased resistance of the photosynthetic machinery to the salt-induced damage and to the increased ability of desA+ cells to repair the photosynthetic and Na+/H+ antiport systems.

Stress responses of Synechocystis sp. strain PCC 6803 mutants impaired in genes encoding putative alternative sigma factors

In the complete genome sequence of the cyanobacterium SYNECHOCYSTIS: sp. strain PCC 6803 [Kaneko et al. (1996 ). DNA Res 3, 109-136] genes were identified encoding putative group 3 sigma-factors SigH (Sll-0856), SigG (Slr-1545) and SigF (Slr-1564) and the regulatory protein RsbU (Slr-2031). Mutations in these genes were generated by interposon mutagenesis to study their importance in stress acclimation. For the genes sigH, sigF and rsbU, the loci segregated completely. However, attempts to mutagenize the sigG locus resulted in merodiploids. Under standard growth conditions only minor differences were detected between the mutants and wild-type. However, cells of the RsbU mutant showed a clear defect in regenerating growth after a nitrogen- and sulphur-starvation-induced stationary phase. After applying salt, heat and high-light shocks, stress protein synthesis was analysed by means of one- and two-dimensional electrophoresis. Cells of the SigF mutant showed a severe defect in the induction of salt stress proteins. Although the acclimation to moderate salt stress up to 684 mM NaCl was not significantly changed in this mutant, its ability to acclimate to higher concentrations of NaCl was reduced. Northern blot experiments showed a constitutive expression of the rsbU and sigF genes. The expression of the sigH gene was found to be stress-stimulated, particularly in heat-shocked cells, whilst that of sigG was transiently decreased under stress conditions. Possible functions of these regulatory proteins in stress acclimation of Synechocystis cells are discussed.

Inactivation of photosystems I and II in response to osmotic stress in Synechococcus. Contribution of water channels

The effects of osmotic stress due to sorbitol on the photosynthetic machinery were investigated in the cyanobacterium Synechococcus R-2. Incubation of cells in 1.0 M sorbitol inactivated photosystems I and II and decreased the intracellular solute space by 50%. These effects of sorbitol were reversible: Photosynthetic activity and cytoplasmic volume returned to the original values after removal of the osmotic stress. A blocker of water channels prevented the osmotic-stress-induced inactivation and shrinkage of the intracellular space. It also prevented the recovery of photosynthetic activity and cytoplasmic volume when applied just before release from osmotic stress. Inhibition of protein synthesis by lincomycin had no significant effects on the inactivation and recovery processes, an observation that suggests that protein synthesis was not involved in these processes. Our results suggest that osmotic stress decreased the amount of water in the cytoplasm via the efflux of water through water channels (aquaporins), with resultant increases in intracellular concentrations of ions and a decrease in photosynthetic activity.

Genetic engineering of the unsaturation of fatty acids in membrane lipids alters the tolerance of Synechocystis to salt stress

The role of unsaturated fatty acids in membrane lipids in the tolerance of the photosynthetic machinery to salt stress was studied by comparing the desA-/desD- mutant of Synechocystis sp. PCC 6803, which contained monounsaturated fatty acids, with the wild-type strain, which contained a full complement of polyunsaturated fatty acids. In darkness, the loss of oxygen-evolving photosystem II activity in the presence of 0.5 M NaCl or 0.5 M LiCl was much more rapid in desA-/desD- cells than in wild-type cells. Oxygen-evolving activity that had been lost during incubation with 0.5 M NaCl in darkness returned when cells were transferred to conditions that allowed photosynthesis or respiration. Recovery was much greater in wild-type than in desA-/desD- cells, and it was prevented by lincomycin. Thus, the unsaturation of fatty acids is important in the tolerance of the photosynthetic machinery to salt stress. It appears also that the activity and synthesis of the Na+/H+ antiporter system might be suppressed under high-salt conditions and that this effect can be reversed, in part, by the unsaturation of fatty acids in membrane lipids.