Of seven strains of examined, four contained covalently closed circular DNA molecules. The bacteriocinogenic strain NCIB 7423 carried two plasmids: pCB101 of molecular mass 3.9 megadaltons (5.6 x 10 bases) and pCB102 of molecular mass 5.2 megadaltons (7.4 x 10 bases). Plasmid pCB101 was not restricted by RI which cleaved pCB102 at two sites; both plasmids were restricted by dIII. The three other plasmid-bearing strains of (SA1, SA11 and NCTC 6084) each carried a single plasmid of molecular mass 4.3 megadaltons (6.2 x 10 bases) with a single cleavage site for RI and four sites for dIII, the fragments so produced from the three plasmids being indistinguishable on agarose gel electrophoretograms. It was concluded that the three strains harboured a common plasmid - pCB103.


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