Distinct and differently regulated Mo‐dependent nitrogen‐fixing systems evolved for heterocysts and vegetative cells ofAnabaena variabilisATCC 29413: characterization of thefdxH1/2gene regions as part of thenif1/2gene clusters

摘要

Two differentfdxHgenes (fdxH1, fdxH2) have been isolated from the nitrogen‐fixing, heterocyst‐forming cyanobacteriumAnabaena variabilisATCC 29413. They are part of two differentnifgene clusters,nif1andnif2. fdxH1encodes the 2Fe—2S ferredoxin that is known as the direct electron donor to nitrogenase in heterocysts, and is very similar to FdxH fromAnabaenasp. PCC 7120. FdxH2 has more residues in common and shares its oxygen sensitivity with the single FdxH from the non‐heterocystous, filamentous cyanobacteriumPlectonema boryanumPCC 73110. The latter expresses nitrogenase early ( 3–4 h) after nitrogen depletion in vegetative cells and exclusively under anaerobic conditions.fdxH2and thenif2genes ofAnabaena29413 are also transcribed 4 h after onset of nitrogen‐stepdown, exclusively under anaerobic growth conditions and long before functional heterocysts appear. At this time, nofdxH1andnif1gene transcription was observed. It occurred later and was associated with nitrogen fixation under aerobic conditions, i.e. within heterocysts.fdxH2andnifHDK2were not transcribed during aerobic, nitrogen‐fixing growth. In addition, neither was anfdxH2‐type gene found nor an anaerobically and early inducible Nif2 system detectable inAnabaena7120. 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 o