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Abstract
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 × 106 (7·0 kilobases) and pAB118B of molecular weight 3·0 × 106 (4·3 kilobases). Two of the tetracycline-resistant strains each contained a plasmid (pAB124) of molecular weight 2·9 × 106 (4·14 kilobases), while a third harboured a small plasmid (pAB128) of molecular weight 2·5 × 106 (3·57 kilobases). These plasmids were digested with 19 different restriction endonucleases and the numbers of cleavage sites were determined. Transformation of Bacillus subtilis 168 (Trp−) with purified plasmid DNA indicated that pAB124 conferred tetracycline resistance on the host.
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