Isolation of Extrachromosomal Deoxyribonucleic Acids from Extremely Thermophilic Bacteria

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Abstract
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, Thermus flavus Bs1, AT61, AT62 and Thermus thermophilus HB8 carried covalently closed circular DNA molecules. Thermus flavus BS1 harboured two species of plasmids with molecular weights of 6-1 x 106 and 17-0 x 106 as determined by electron microscopy. Thermus thermophilus HB8, T. flavus AT61 and T. flavus AT62 carried plasmids with molecular weights of 6-2 x 106, 6-6 x 106 and 6-6 x 106, respectively. Plasmids from T. flavus 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.