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In photosynthetic organisms, excess light is a stress that induces production of reactive oxygen species inside the chloroplasts. As a response, the capacity of antioxidative defence mechanisms increases. However, when cells of Chlamydomonas reinhardtii were shifted from dark to high light, a reversible partial inactivation of catalase activity was observed, which correlated with a transient increase in the level of H2O2 in the 10 μ m range. This concentration range seems to be necessary to activate H2O2-dependent signalling pathways stimulating the expression of H2O2 responsive genes, such as the heat shock protein HSP22C. Catalase knock-down mutants had lost the transient accumulation of H2O2, suggesting that a decrease in catalase activity was the key element for establishing a transient H2O2 burst. Catalase was inactivated by a one-electron event consistent with the reduction of a single cysteine. We propose that under high light intensity, the redox state of the photosynthetic electron transport chain is sensed and transmitted to the cytosol to regulate the catalase activity. This allows a transient accumulation of H2O2, inducing a signalling event that is transmitted to the nucleus to modulate the expression of chloroplast-directed protection enzymes. [ABSTRACT FROM AUTHOR]

The article focuses on the targets and the potential physiological functions of protein glutathionylation to photosynthetic organisms. It provides an overview on the underlying molecular mechanisms and functions of glutaredoxins (GRXs). It says that the GRXs appear specifically involved in the control of glutathionylation while thioredoxins (TRXs) were suggested to catalyze denitrosylation reactions. Both TRXs and GRXs were reported to undergo nitrosylation and glutathionylation.

Protein S-glutathionylation, the reversible formation of a mixed-disulfide between glutathione and protein thiols, is involved in protection of protein cysteines from irreversible oxidation, but also in protein redox regulation. Recent studies have implicated S-glutathionylation as a cellular response to oxidative/nitrosative stress, likely playing an important role in signaling. Considering the potential importance of glutathionylation, a number of methods have been developed for identifying proteins undergoing glutathionylation. These methods, ranging from analysis of purified proteins in vitro to large-scale proteomic analyses in vivo, allowed identification of nearly 200 targets in mammals. By contrast, the number of known glutathionylated proteins is more limited in photosynthetic organisms, although they are severely exposed to oxidative stress. The aim of this review is to detail the methods available for identification and analysis of glutathionylated proteins in vivo and in vitro. The advantages and drawbacks of each technique will be discussed as well as their application to photosynthetic organisms. Furthermore, an overview of known glutathionylated proteins in photosynthetic organisms is provided and the physiological importance of this post-translational modification is discussed. [ABSTRACT FROM PUBLISHER]

In a continuing effort to analyze the selectivity/redundancy of the three glutaredoxin (Grx) enzymes of the model cyanobacte- rium Synechocystis PCC6803, we have characterized an enzyme system that plays a crucial role in protection against two toxic metal pollutants, mercury and uranium. The present data show that Grxl (Sir1562 in CyanoBase) selectively interacts with the presumptive mercuric reductase protein (S1r1849). This MerA enzyme plays a crucial role in cell defense against both mercuric and uranyl ions, in catalyzing their NADPH-driven reduction. Like MerA, Grxl operates in cell protection against both mercury and uranium. The Grxl-MerA interaction requires cysteine 86 (C86) of Grxl and C78 of MerA, which is critical for its reductase activity. MerA can be inhibited by glutathionylation and subsequently reactivated by Grxl, likely through deglutathionylation. The two Grxl residues C31, which belongs to the redox active site (CX2C), and C86, which operates in MerA interactions, are both required for reactivation of MerA. These novel findings emphasize the role of glutaredoxins in tolerance to metal stress as well as the evolutionary conservation of the glutathionylation process, so far described mostly for eukaryotes [ABSTRACT FROM AUTHOR]

Algae belonging to the Microchloropsis genus are promising organisms for biotech purposes, being able to accumulate large amounts of lipid reserves. These organisms adapt to different trophic conditions, thriving in strict photoautotrophic conditions, as well as in the concomitant presence of light plus reduced external carbon as energy sources (mixotrophy). In this work, we investigated the mixotrophic responses of Microchloropsis gaditana (formerly Nannochloropsis gaditana ). Using the Biolog growth test, in which cells are loaded into multiwell plates coated with different organic compounds, we could not find a suitable substrate for Microchloropsis mixotrophy. By contrast, addition of the Lysogeny broth (LB) to the inorganic growth medium had a benefit on growth, enhancing respiratory activity at the expense of photosynthetic performances. To further dissect the role of respiration in Microchloropsis mixotrophy, we focused on the mitochondrial alternative oxidase (AOX), a protein involved in energy management in other algae prospering in mixotrophy. Knocking-out the AOX1 gene by transcription activator-like effector nuclease (TALE-N) led to the loss of capacity to implement growth upon addition of LB supporting the hypothesis that the effect of this medium was related to a provision of reduced carbon. We conclude that mixotrophic growth in Microchloropsis is dominated by respiratory rather than by photosynthetic energetic metabolism and discuss the possible reasons for this behavior in relationship with fatty acid breakdown via β-oxidation in this oleaginous alga., Competing Interests: EB, FL, LF, and SC are employed by the company Total Refining Chemicals. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Bo, Magneschi, Bedhomme, Billey, Deragon, Storti, Menneteau, Richard, Rak, Lapeyre, Lembrouk, Conte, Gros, Tourcier, Giustini, Falconet, Curien, Allorent, Petroutsos, Laeuffer, Fourage, Jouhet, Maréchal, Finazzi and Collin.)