Extrachromosomal Gene Systems in Streptococcus Mutans

Bacterial extrachromosomal elements (plasmids) are autonomously replicating molecules of covalently closed circular (CCC) deoxyribonucleic acid (DNA). The occurrence of plasmids in some species of streptococci has been well documented, and such plasmid-conferred phenotypes include multiple antibiotic resistance (1), bacteriocin and hemolysin production (2), conjugal donor ability (3), and the catabolism of protein and carbohydrates (4). Moreover, certain streptococcal plasmids are known to contain directly repeated DNA sequences which are capable of mediating selective gene amplification, presumably through a series of site-specific recombinational events (5). The rationale for study of bacterial plasmids has centered largely on their clinical and ecological significance. However, much recent effort in plasmid research has revolved around their use as genetic tools for the cloning and amplification of both homologous and heterologous genetic information (6).