Distinct and differently regulated Mo‐dependent nitrogen‐fixing systems evolved for heterocysts and vegetative cells of Anabaena variabilis ATCC 29413: characterization of the fdxH1/2 gene regions as part of the nif1/2 gene clusters
Two different fdxH genes (fdxH1, fdxH2) have been isolated from the nitrogen‐fixing, heterocyst‐forming cyanobacterium Anabaena variabilis ATCC 29413. They are part of two different nif gene clusters, nif1 and nif2. fdxH1 encodes the [2Fe—2S] ferredoxin that is known as the direct electron donor to nitrogenase in heterocysts, and is very similar to FdxH from Anabaena sp. PCC 7120. FdxH2 has more residues in common and shares its oxygen sensitivity with the single FdxH from the non‐heterocystous, filamentous cyanobacterium Plectonema boryanum PCC 73110. The latter expresses nitrogenase early ( 3–4 h) after nitrogen depletion in vegetative cells and exclusively under anaerobic conditions. fdxH2 and the nif2 genes of Anabaena 29413 are also transcribed 4 h after onset of nitrogen‐stepdown, exclusively under anaerobic growth conditions and long before functional heterocysts appear. At this time, no fdxH1 and nif1 gene transcription was observed. It occurred later and was associated with nitrogen fixation under aerobic conditions, i.e. within heterocysts. fdxH2 and nifHDK2 were not transcribed during aerobic, nitrogen‐fixing growth. In addition, neither was an fdxH2‐type gene found nor an anaerobically and early inducible Nif2 system detectable in Anabaena 7120. These data reveal that in filamentous cyanobacteria two different Nif systems have evolved based on molybdenum nitrogenases. It is concluded that a Nif2‐type system operates in vegetative cells of non‐heterocystous and some, but not all, heterocyst‐forming filamentous cyanobacteria. It is environmentally regulated by the levels of both oxygen and combined nitrogen in the habitat. To simultaneously allow for oxygen‐evolving photosynthesis and oxygen‐sensitive nitrogen fixation, the Nif1‐type system probably branched from an ancestral Nif2‐type system and has evolved for an exclusive operation within heterocysts. Accordingly, its expression has become an obligate late event in the developmental programme of heterocyst differentiation, irrespective of aerobic or anaerobic growth conditions.