Six strains of unicellular cyanobacteria were examined for the presence of plasmids. Analysis of lysates of these strains by CsCl-ethidium bromide density centrifugation yielded a major chromosomal DNA band and a minor band containing covalently closed circular plasmid DNA, as shown by electron microscopy and agarose gel electrophoresis. The sizes of the various plasmid species were determined; in each of the Synechococcus strains 6301, 6707, and 6908 two plasmid species were found with molecular weights of 5.3 × 106 and 32.7 × 106. Synechococcus strain 7425 had two plasmids of molecular weight 5.4 × 106 and 24 × 106. Synechococcus strain 6312 and Synechocystis strain 7005 each contained one plasmid species with molecular weight of 15.9 × 106 and 2.0 × 106, respectively. Restriction enzyme analysis revealed identical cleavage patterns for the plasmids of identical molecular weight
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