Abstract

The nonheterocystous, filamentous cyanobacterium, Plectonema boryanum fixes nitrogen only under microaerophilic conditions. The organization of nitrogen fixation genes (nifH, D, K) in Plectonema was determined by using cloned fragments from the Anabaena nif genes as probes in Southern hybridizations. Regions of Plectonema DNA were homologous to Anabaena nifH, nifD, and nifK genes, and the resulting pattern of hybridization was used to construct a map of nifH, D, K DNA isolated from Plectonema cells grown under non-nitrogen fixing conditions (combined nitrogen and O2 present). The nifH and nifD genes are on the same 3 kbp Hin dIII fragment, and nifK is on a 1 kbp Hin dIII fragment. All three nif fragments are adjacent to one another on a 12 kbp Cla I fragment.

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