Genes responsible for dextran-dependent aggregation of Streptococcus sobrinus strain 6715

Area of Science:

• Microbiology
• Molecular Biology

Background:

• Streptococcus sobrinus shows greater dextran-dependent aggregation than Streptococcus mutans, mediated by glucan-binding proteins.
• Four glucan-binding protein C (gbpC) homologues (gbpC1, gbpC2, dblA, dblB) were identified in S. sobrinus, unlike the single gbpC gene in S. mutans.

Purpose of the Study:

• To identify which of the four gbpC homologous genes in S. sobrinus is most responsible for dextran-dependent aggregation.

Main Methods:

• Chemical mutagenesis of S. sobrinus strain 6715 using 1-methyl-3-nitro-1-nitrosoguanidine.
• Sequencing of four gbpC homologous genes in the parental strain and an aggregation-negative mutant (NUM-Ssg99).
• Western blot analysis to examine protein localization in the mutant.

Main Results:

• gbpC1, gbpC2, and dblA genes showed identical sequences in the mutant and parental strains.
• A truncated mutation was found in the dblB gene of the mutant.
• The mutant dblB protein lacked the LPXTG motif and was released extracellularly.

Conclusions:

• The dblB gene is the primary contributor to dextran-dependent aggregation in Streptococcus sobrinus strain 6715.
• Extracellular release of a non-functional dblB protein due to mutation leads to aggregation deficiency.