Genetic basis for mutacin N and of its relationship to mutacin I.

BACKGROUND & OBJECTIVES The mutans streptococci (MS) are a group of 7 species of dental cariesassociated bacteria of which Streptococcus mutans and Streptococcus sobrinus are the most important in humans. Many MS produce bacteriocin-like inhibitory substances (BLIS), some of which have been characterised as small peptides capable of inhibiting the growth of closely-related species. These peptides have most commonly been referred to as mutacins. S. mutans strains N and UA140 appear to have closely similar BLIS activities. Both produce mutacins that seem to target the same species of bacteria. On closer analysis however, these two strains have been shown to produce distinctly different mutacins, known as mutacin N and mutacin I respectively. In the present study the mutacin N structural gene (mutN) was cloned and compared with the mutacin I structural gene (mutA). METHODS Cloning and sequencing of S. mutans N was done. The distribution of mutN using DNA from 216 streptococcal strains was determined by dot blotting. RESULTS Mut N was cloned and sequenced from an 1800 bp Bam HI/Eco RI fragment. PCR with the mutN primers mutNF and mutNR on the four mutN-positive strains identified identical bands to S. mutans N. The location of mutN differs significantly from that of mutA in that it is directly upstream of comC, a gene encoding a putative competence stimulating factor. INTERPRETATION & CONCLUSION The close upstream proximity of mutN to comC suggests a link between mutacin N production and competence development. Further studies need to be done to detect competence-related genes in S. mutans strain N.