{'fraction_non_alphanumeric': 0.020242914979757085, 'fraction_numerical': 0.01349527665317139, 'mean_word_length': 5.619047619047619, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Heterocyst-specific flavodiiron protein Flv3B enables oxic diazotrophic growth of the filamentous cyanobacterium Anabaena sp. PCC 7120

Significance Cyanobacterial flavodiiron proteins (FDPs) comprise a protein family with unique modular structure and photoprotective functions in an oxygenic environment. It is conceivable that FDPs have made the development of oxygenic photosynthesis possible in cyanobacteria. Here, we report the ability of specific FDPs to reduce O2 directly to water in heterocyst-forming filamentous cyanobacteria, not only to support the photosynthetic machinery, but also to prevent oxidative damage of the N2-fixing enzyme nitrogenase. Whilst in the ancient environment, N2 fixation was secured from O2 inhibition, the later increase of atmospheric O2 may have initiated an important role for FDP-mediated protection of nitrogenase in maintaining the N2-fixing activity of cyanobacteria. Flavodiiron proteins are known to have crucial and specific roles in photoprotection of photosystems I and II in cyanobacteria. The filamentous, heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 contains, besides the four flavodiiron proteins Flv1A, Flv2, Flv3A, and Flv4 present in vegetative cells, two heterocyst-specific flavodiiron proteins, Flv1B and Flv3B. Here, we demonstrate that Flv3B is responsible for light-induced O2 uptake in heterocysts, and that the absence of the Flv3B protein severely compromises the growth of filaments in oxic, but not in microoxic, conditions. It is further demonstrated that Flv3B-mediated photosynthetic O2 uptake has a distinct role in heterocysts which cannot be substituted by respiratory O2 uptake in the protection of nitrogenase from oxidative damage and, thus, in an efficient provision of nitrogen to filaments. In line with this conclusion, the Δflv3B strain has reduced amounts of nitrogenase NifHDK subunits and shows multiple symptoms of nitrogen deficiency in the filaments. The apparent imbalance of cytosolic redox state in Δflv3B heterocysts also has a pronounced influence on the amounts of different transcripts and proteins. Therefore, an O2-related mechanism for control of gene expression is suggested to take place in heterocysts.