Four gbpC Gene Homologues in Streptococcus sobrinus

We previously identified the glucan-binding protein C gene, gbpC, solely involved in dextran-dependent aggregation (ddag) of Streptococcus mutans. Recently, we identified two gbpC gene homologues, gbpC and dbl, in Streptococcus sobrinus, and suggested that the dbl gene was very likely responsible for ddag of this species. However, homology searches with the gbpC or dbl genes against the incomplete TIGR S. sobrinus 6715 database suggested that this strain may have other gbpC homologues. PCR-based chromosomal walking from the gbpC and dbl genes revealed two additional homologous genes designated as gbpC2 and dblB. Proteins encoded by these genes exhibited alpha-1,6 glucan-binding activities. Therefore, gbpC2 and dblB are also logical candidates responsible for the ddag phenotype.