Evidence for a disseminated plasmid in Streptococcus mutans

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Abstract
Based on a survey of 86 isolates, approximately 5% of all naturally occurring strains of S. mutans contains a 3.6 .times. 106-dalton (3.6-megadalton) multicopy plasmid of unknown function. The amount of plasmid DNA/chromosome varies from 2-6% depending on the host strain. About 13% of the total covalently closed circular DNA in each of the 4 plasmid-containing strains consists of dimeric molecules, with interlocked circular forms predominating. Site-specific restriction endonucleases were identified that cleave this 3.6-megadalton plasmid at single and at multiple sites. Each of the 4 plasmids was cleaved once by the HindIII and BamHI restriction enzymes. The HpaI, TaqI and HhaI enzymes generate 2, 5 and 6 components, respectively, and the digestion products of each of the 4 plasmids are identical. Because the 4 plasmid-containing S. mutans strains are physiologically unique with respect to one another, this plasmid may be a disseminated extrachromosomal element in S. mutans.