SUMMARY: Eight strains of thermophilic bacteria were examined for the presence of covalently closed circular deoxyribonucleic acid molecules by caesium chloride-ethidium bromide density gradient centrifugation. Four of the eight strains tested, Bs1, AT61, AT62 and HB8 carried covalently closed circular DNA molecules. BS1 harboured two species of plasmids with molecular weights of 6-1 x 10 and 17-0 x 10 as determined by electron microscopy. HB8, AT61 and AT62 carried plasmids with molecular weights of 6-2 x 10, 6-6 x 10 and 6-6 x 10, respectively. Plasmids from AT61 and AT62 were indistinguishable in their electrophoretic patterns in agarose or acrylamide gel after digestion with various restriction endonucleases. This is the first evidence for the presence of plasmids in extremely thermophilic bacteria, though their functions are unknown.


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