To circumvent ROS-linked damage, photosynthetic organisms contain elaborate defense mechanisms for detoxification. ROS react with metal ions and perpetuate their own formation, and cause oxidative damage to protein, DNA, and lipid biomolecules found in cells 1, 2. As with all organisms, the production of ROS in photosynthetic organisms can have detrimental effects. ROS are generally referred to as the inescapable cost of aerobic metabolism and are known to arise in photosynthetic organisms from the continual liberation of molecular oxygen (O 2), the electron transport chains of aerobic respiration and photosynthesis, and via reaction of excited state photosynthetic pigments with O 2 2. −), singlet oxygen ( 1O 2), and hydrogen peroxide (H 2O 2) 1, 2.−), peroxide (O 2 2−), hydroxyl radicals (OH.To thrive in these environments, photosynthetic organisms must have strategies to perceive and defend against the production of reactive oxygen species (ROS), a class of O 2-derived species that includes superoxide (O 2 These organisms inhabit environments that contain different amounts of molecular oxygen (O 2) and experience different wavelengths and intensities of light. The diversity of environments that photosynthetic microorganisms are equipped to survive in is unparalleled. Finally, bioinformatics analysis of the ArsR-SmtB family indicates that the vicinal disulfide “redox switch” is a unique feature of cyanobacteria in the Nostocales order, presenting an interesting case where an ArsR-SmtB protein scaffold has been evolved to showcase peroxidatic activity and facilitate redox-based regulation. Furthermore, an entrance channel for H 2O 2 was identified and key residues implicated in H 2O 2 activation were pinpointed. The importance of the disulfide-forming Cys residues was corroborated using site-directed mutagenesis, mass spectrometry, and H 2O 2-consumption assays. These structures reveal that RexT showcases the ArsR-SmtB winged-helix-turn-helix fold and forms a vicinal disulfide bond to orchestrate a response to H 2O 2. Here, we present high-resolution crystal structures of RexT in the reduced and H 2O 2-treated states. Different from many other members of the ArsR-SmtB family which bind metal ions, RexT has been proposed to use disulfide bond formation as a trigger to bind and release DNA. PCC 7120, one such strategy is the use of an ArsR-SmtB transcriptional regulator RexT that senses H 2O 2 and upregulates expression of thioredoxin to maintain cellular redox homeostasis. In the heterocystous cyanobacterium Nostoc sp. Organisms have a myriad of strategies for sensing, responding to, and combating reactive oxygen species, which are unavoidable consequences of aerobic life.
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