Different organization of nif genes in nonheterocystous and heterocystous cyanobacteria.

Labeled probes carrying the Anabaena PCC 7120 nitrogenase (nifK and nifD) and nitrogenase reductase (nifH) genes were hybridized to Southern blots of DNA from diverse N2-fixing cyanobacteria in order to test a previous observation of different nif gene organization in nonheterocystous and heterocystous strains. The nif probes showed no significant hybridization to DNA from a unicellular cyanobacterium incapable of N2 fixation. All nonheterocystous cyanobacteria examined (unicellular and filamentous) had a contiguous nifKDH gene cluster whereas all of the heterocystous strains showed separation of nifK from contiguous nifDH genes. These findings suggest that nonheterocystous and heterocystous cyanobacteria have characteristic and fundamentally different nif gene arrangements. The noncontiguous nif gene pattern, as shown with two Het(-) mutants, is independent of phenotypic expression of heterocyst differentiation and aerobic N2-fixation. Thus nif arrangement could be a useful taxonomic marker to distinguish between phenotypically Het(-) heterocystous cyanobacteria and phylogenetically unrelated nonheterocystous strains.