DISTINCT AND DIFFERENTLY REGULATED MO-DEPENDENT NITROGEN-FIXING SYSTEMS EVOLVED FOR HETEROCYSTS AND VEGETATIVE CELLS OF ANABAENA-VARIABILISATCC-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 ni trogen-fixing, heterocyst-forming cyanobacterium Anabaena variabilis A TCC 29413, They are part of two different nif gene clusters, nif1 and nif2. fdxH1 encodes the [2Fe-2S] ferredoxin that is known as the direc t electron donor to nitrogenase in heterocysts, and is very similar to FdxH from Anabaena sp, PCC 7120, FdxH2 has more residues in common an d shares its oxygen sensitivity with the single FdxH from the non-hete rocystous, filamentous cyanobacterium Plectonema boryanum PCC 73110. T he latter expresses nitrogenase early (less than or equal to 3-4 h) af ter nitrogen depletion in vegetative cells and exclusively under anaer obic conditions, fdxH2 and the nif2 genes of Anabaena 29413 are also t ranscribed less than or equal to 4 h after onset of nitrogen-stepdown, exclusively under anaerobic growth conditions and long before functio nal heterocysts appear. At this time, no fdxH1 and nif1 gene transcrip tion 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, I n addition, neither was an fdxH2-type gene found nor an anaerobically and early inducible Nif2 system detectable in Anabaena 7120. These dat a reveal that in filamentous cyanobacteria two different Nif systems h ave evolved based on molybdenum nitrogenases. It is concluded that a N if2-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 n itrogen in the habitat, To simultaneously allow for oxygen-evolving ph otosynthesis and oxygen-sensitive nitrogen fixation, the Nif1-type sys tem probably branched from an ancestral Nif2-type system and has evolv ed for an exclusive operation within heterocysts. Accordingly, its exp ression has become an obligate late event in the developmental program me of heterocyst differentiation, irrespective of aerobic or anaerobic growth conditions.