Comparison of plasmids from the cyanobacterium Nostoc PCC 7524 with two mutant strains unable to form heterocysts

Abstract

Cyanobacteria (bluegreen bacteria) are O₂-evolving photosynthetic prokaryotes some species of which fix N₂ in air because the nitrogenase is protected from O₂ inactivation by being localized in differentiated cells called heterocysts. Recently much attention has been paid to the possible role of plasmids as vehicles for the transfer of nif genes and other genetic determinants associated with N₂-fixation in prokaryotes. This paper characterizes the extrachromosomal DNA of the N₂-fixing heterocystous cyanobacterium Nostoc 7524 and two nonheterocystous deletion mutants of this organism using gel electrophoresis, electron microscopy and restriction enzyme analysis. Three species of covalently closed circular (CCC) DNA of different molecular weight were found. It is also concluded that the failure of the nonketerocystous mutants to differentiate heterocysts cannot be readily ascribed to a loss of plasmid-borne genetic material.