RbrA, a cyanobacterial rubrerythrin, functions as a FNR-dependent peroxidase in heterocysts in protection of nitrogenase from damage by hydrogen peroxide in Anabaena sp. PCC 7120

• Published in: Molecular microbiology Vol. 66; no. 5; pp. 1219 - 1230
• Main Authors: Zhao, Weixing, Ye, Zi, Zhao, Jindong
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 01.12.2007; Blackwell Publishing Ltd; Blackwell Science
• Subjects: Aerobiosis; Anabaena; Anabaena - enzymology; Anabaena - genetics; Anabaena - growth & development; Anaerobiosis; Bacteria; Bacterial Proteins - biosynthesis; Bacterial Proteins - genetics; Bacterial Proteins - physiology; Bacteriology; Biological and medical sciences; Cyanobacteria; Enzymes; Ferredoxin-NADP Reductase - metabolism; Ferredoxins - biosynthesis; Ferredoxins - genetics; Ferredoxins - physiology; Fundamental and applied biological sciences. Psychology; Gene Expression Regulation, Bacterial; Hemerythrin; Hydrogen; Hydrogen bonds; Hydrogen Peroxide - metabolism; Hydrogen Peroxide - toxicity; Microbiology; Miscellaneous; NADP - metabolism; Nitrogen; Nitrogen Fixation - physiology; Nitrogenase - drug effects; Organelles - enzymology; Oxidoreductases - genetics; Oxidoreductases - physiology; Peroxidases - biosynthesis; Peroxidases - genetics; Peroxidases - physiology; Promoter Regions, Genetic; Rubredoxins; Peroxidases; Enzyme; Nitrogenase; Anabaena; Cyanobacteria; Bacteria; Peroxidase; Oxidoreductases; Hydrogen Peroxides
• ISSN: 0950-382X; 1365-2958
• DOI: 10.1111/j.1365-2958.2007.05994.x

The heterocyst is a specialized cell for nitrogen fixation in some filamentous cyanobacteria. Here we report that a rubrerythrin (RbrA) from Anabaena sp. PCC 7120 functions as a peroxidase in heterocysts and plays an important role in protection of nitrogenase. The electron donor for RbrA in H₂O₂ reduction is NADPH and the electron transfer from NADPH to RbrA depends on ferredoxin:NADP⁺ oxidoreductase. A rbrA mutant (r27) grew much more slowly than the wild type under diazotrophic conditions. Its nitrogenase activity measured in air was only 8% of that measured under anoxic conditions. Staining r27 filaments with 2',7'-dichlorodihydrofluorescein diacetate indicated that heterocysts had a higher H₂O₂ concentration than the vegetative cells. The expression of rbrA was controlled by two promoters and the promoter for the smaller transcript was regulated by HetR. Spatial expression of rbrA was studied and the results showed that the transcription is localized predominantly in heterocysts. In a mutant lacking nifH and rbrA, the H₂O₂ concentration in heterocysts was lower than that in the vegetative cells, suggesting that NifH is involved in H₂O₂ generation. Our results demonstrate that RbrA is a critical enzyme for H₂O₂ decomposition and provide evidence that nitrogenase autoprotection is important in heterocysts.