[Isolation and characteristics of plasmids determining multiple antibiotic resistance in strains of group A...

DNA of the streptococcal plasmid ERLI, determining resistance to erythromycin and lincomycin has been studied. The presence of satellite DNA component in streptococcal DNA has been demonstrated by dye-CsCl and CsCl density centrifugation, by chromatography of denatured-renatured DNA on the nitrocellulose and by electron microscopy. By all these methods the presence of covalently closed circular DNA molecules has been shown. Plasmid DNA has buoyant density in CsCl equal to 1.698 g/cm3 (GC content--38.8%), molecular weight (by electron microscopy) -- 19.8 Mdal; number of copies chromosomal genome equivalent is equal to about 4--4.5. Plasmid ERLI DNA was extracted from an original strain-carrier of plasmid ERLI, from a transduced strain which received plasmid ERLI as a result of transduction and lysogenization by phage "mo" and from the two antibiotic sensitive EMS mutants of resistant strain. Satellite DNA could not be isolated from a sensitive strain which served as a recipient in transduction experiments. The results obtained are in agreement with the literature on the molecular weight of the plasmid measured by the sedimentation analysis and with the previous genetic data on the antibiotic resistance within group A streptococci.