Membrane targeting of MnSOD is essential for oxidative stress tolerance of nitrogen-fixing cultures of Anabaena sp. strain PCC7120

Timing the evolution of antioxidant enzymes in cyanobacteria

The ancestors of cyanobacteria generated Earth's first biogenic molecular oxygen, but how they dealt with oxidative stress remains unconstrained. Here we investigate when superoxide dismutase enzymes (SODs) capable of removing superoxide free radicals evolved and estimate when Cyanobacteria originated. Our Bayesian molecular clocks, calibrated with microfossils, predict that stem Cyanobacteria arose 3300-3600 million years ago. Shortly afterwards, we find phylogenetic evidence that ancestral cyanobacteria used SODs with copper and zinc cofactors (CuZnSOD) during the Archaean. By the Paleoproterozoic, they became genetically capable of using iron, nickel, and manganese as cofactors (FeSOD, NiSOD, and MnSOD respectively). The evolution of NiSOD is particularly intriguing because it corresponds with cyanobacteria's invasion of the open ocean. Our analyses of metalloenzymes dealing with reactive oxygen species (ROS) now demonstrate that marine geochemical records alone may not predict patterns of metal usage by phototrophs from freshwater and terrestrial habitats.

Metalloproteins in the Biology of Heterocysts

Cyanobacteria are photoautotrophic microorganisms present in almost all ecologically niches on Earth. They exist as single-cell or filamentous forms and the latter often contain specialized cells for N₂ fixation known as heterocysts. Heterocysts arise from photosynthetic active vegetative cells by multiple morphological and physiological rearrangements including the absence of O₂ evolution and CO₂ fixation. The key function of this cell type is carried out by the metalloprotein complex known as nitrogenase. Additionally, many other important processes in heterocysts also depend on metalloproteins. This leads to a high metal demand exceeding the one of other bacteria in content and concentration during heterocyst development and in mature heterocysts. This review provides an overview on the current knowledge of the transition metals and metalloproteins required by heterocysts in heterocyst-forming cyanobacteria. It discusses the molecular, physiological, and physicochemical properties of metalloproteins involved in N₂ fixation, H₂ metabolism, electron transport chains, oxidative stress management, storage, energy metabolism, and metabolic networks in the diazotrophic filament. This provides a detailed and comprehensive picture on the heterocyst demands for Fe, Cu, Mo, Ni, Mn, V, and Zn as cofactors for metalloproteins and highlights the importance of such metalloproteins for the biology of cyanobacterial heterocysts.

Regulation of multiple abiotic stress tolerance by LexA in the cyanobacterium Anabaena sp. strain PCC7120

The paradigm of involvement of LexA in regulation of only SOS-response in bacteria through the down-regulation of DNA repair genes was challenged in the unicellular cyanobacterium, Synechocystis PCC6803, wherein it was originally shown not to be associated with DNA repair and later also involved in management of carbon-starvation through up-regulation of C-metabolism genes. In the filamentous cyanobacterium, Anabaena sp. strain PCC7120, global stress management role for LexA and a consensus LexA-binding box (AnLexA-box) has been established using a LexA-overexpressing recombinant strain, AnlexA+. High levels of LexA rendered Anabaena cells sensitive to different DNA damage and oxidative stress-inducing agents, through the transcriptional down-regulation of the genes involved in DNA repair and alleviation of oxidative stress. LexA overexpression enhanced the ability of Anabaena to tolerate C-depletion, induced by inhibiting photosynthesis, by up-regulating genes involved in C-fixation and down-regulating those involved in C-breakdown, while maintaining the overall photosynthetic efficiency. A consensus LexA-binding box, AnLexA-box [AGT-N4-11-ACT] was identified upstream of both up- and down-regulated genes using a subset of Anabaena genes identified on the basis of proteomic analysis of AnlexA+ strain along with a few DNA repair genes. A short genome search revealed the presence of AnLexA box in at least 40 more genes, with functional roles in fatty acid biosynthesis, toxin-antitoxin systems in addition to DNA repair, oxidative stress, metal tolerance and C-metabolism. Thus, Anabaena LexA modulates the tolerance to multitude of stresses through transcriptional up/down-regulation of their functional genes directly by binding to the AnLexA Box present in their promoter region.