sp. strain Mac is capable of microaerobic, but not aerobic, nitrogen fixation (Fox). Mac grows as long, relatively straight, filaments that are well dispersed in the culture medium. However, spontaneously-arising revertant strains selected for aerobic nitrogen fixation (Fox) all grow as coiled filaments that associate in macroscopic clumps or balls of varying dimensions. DNA restriction fragment length polymorphism, using nitrogenase () structural genes as probes, established identity between revertants and the parental culture. Mapping of the fragments and lack of hybridization to specific probes indicated the absence of a DNA sequence interrupting the gene in one Fox revertant. Such a element is assumed to be present in essentially all heterocyst-forming cyanobacteria. Only one clone out of 223 Fox and Fox Mac clones surveyed lacked the element, indicating that loss of the element is a rare event. The element is present in the same location in the genome of Mac as it is in all other heterocyst-forming cyanobacteria analysed. No phenotypic differences could be detected between two Fox clones containing or lacking the element, including repression and derepression of nitrogen fixation in response to the presence or absence of combined nitrogen. We suspect that retention of the element in vegetative cells of heterocyst-forming cyanobacteria is a consequence of selective pressure, although such selective conditions in laboratory cultures have not been identified.


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