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.
ASJC Scopus subject areas
- Molecular Biology