Summary: Twenty-nine antibiotic-resistant isolates of thermophilic bacilli were examined for the presence of covalently closed circular duplex DNA molecules by agarose-gel electrophoresis and caesium chloride-ethidium bromide density gradient centrifugation. Five of the 29 strains tested contained covalently closed circular molecules. Two of the streptomycin-resistant strains contained the same two plasmids: pAB118A of molecular weight 4.9 × 10 (7.0 kilobases) and pAB118B of molecular weight 3.0 × 10 (4.3 kilobases). Two of the tetracycline-resistant strains each contained a plasmid (pAB124) of molecular weight 2.9 × 10 (4.14 kilobases), while a third harboured a small plasmid (pAB128) of molecular weight 2.5 × 10 (3.57 kilobases). These plasmids were digested with 19 different restriction endonucleases and the numbers of cleavage sites were determined. Transformation of 168 (Trp) with purified plasmid DNA indicated that pAB124 conferred tetracycline resistance on the host.


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