SppR in Streptococcus mutans Plays an Important Role in Stress Tolerance Response and BrpA Expression

Hydrogen peroxide challenge assay at 58 mM also showed the sppR deficiency led to reduction of survival rate of >3-logs after 60 minutes (P<0.001). However, no significant differences in biofilm formation were measured between the sppR mutant and its parent strain. Luciferase reporter fusion assays showed that sppR deficiency led to reduction of reporter activity by >2-fold (P<0.05) when the reporter was under the intact brpA promoter and by >7-fold (P<0.001) when a putative SppR-binding site was deleted or mutated.

EMSA assay showed that inclusion of recombinant SppR (rSppR) resulted in electrophoretic shift of the brpA promoter probes. In vitro transcription assay also showed that rSppR enhanced the transcription of brpA. Transcriptomics (RNA-seq) analysis of an allelic exchange mutant revealed that deficiency of SppR led to altered expression of >136 genes by >2-fold (P<0.05), including 68 genes up-expressed and 68 genes reduced.

Among the up-regulated genes were those acid and oxidative stress tolerance responses, which include those for malolactic enzyme, DNA process protein DprA, class I SAM-dependent methyltransferase SMT, and mutanobactin biosynthesis and transport. These results suggest that SppR in S. mutans plays an important role in S. mutans physiology including acid and oxidative stress tolerance response and brpA expression.