dbl gene repertoire variation among S. sobrinus strains

Objectives: Streptococcus sobrinus exhibits marked dextran-dependent aggregation (ddag) mediated by glucan-binding proteins. In contrast to Streptococcus mutans in which the gbpC gene is solely responsible for ddag, four gbpC gene homologues (gbpC1, gbpC2, dblA, and dblB) have been recently identified in S. sobrinus strain 6715. The dblA gene contains repeating regions and variation in the number of repeating units was reported. We also analyzed the dblB gene sequences among strains of this organism.

Methods: Fifteen S. sobrinus strains previously isolated from our institution college were employed. Chromosomal DNA was isolated and fragments downstream from the dblA gene were obtained by PCR amplification or by PCR-based chromosomal walking.

Results: When we attempted to amplify the dblB gene fragments, the fragments were unexpectedly not amplified from 5 strains, although the other 3 gbpC homologues were amplified from all 15 strains. A dblB-negative strain, OM55d was further analyzed. Another dbl homologous sequence was detected downstream from the dblA gene in this strain, instead of the dblB gene. The new homologuos sequence was distinct from both the dblA and dblB sequences and was tentatively designated as dblC. Genetic distance between the dblA and dblC genes was almost the same as that between the dblB and dblC genes according to phylogenetic analysis with the S. mutans gbpC gene employed as an outgroup. The nucleotide sequences downstream from the dblB gene in strain 6715 and downstream from the dblC gene in strain OM55d were very conserved.

Conclusions: 1) these results suggest that the dblB/dblC locus may be a potential site for a past event involving horizontal transfer. 2) these results further suggest that S. sobrinus strains may be classified into dblA+dblB and dblA+dblC groups.

Grants-in-Aid for Scientific Research (17591925) from the Japan Society for the Promotion of Science.