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

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 H ₂O₂ concentration in heterocysts was lower than that in the vegetative cells, suggesting that NifH is involved in H₂O ₂ generation. Our results demonstrate that RbrA is a critical enzyme for H₂O₂ decomposition and provide evidence that nitrogenase autoprotection is important in heterocysts.