CovR-Controlled Global Regulation of Gene Expression in Streptococcus mutans

Abstract

CovR/S is a two-component signal transduction system (TCS) that controls the expression of various virulence related genes in many streptococci. However, in the dental pathogen Streptococcus mutans, the response regulator CovR appears to be an orphan since the cognate sensor kinase CovS is absent. In this study, we explored the global transcriptional regulation by CovR in S. mutans. Comparison of the transcriptome profiles of the wild-type strain UA159 with its isogenic covR deleted strain IBS10 indicated that at least 128 genes (∼6.5% of the genome) were differentially regulated. Among these genes, 69 were down regulated, while 59 were up regulated in the IBS10 strain. The S. mutans CovR regulon included competence genes, virulence related genes, and genes encoded within two genomic islands (GI). Genes encoded by the GI TnSmu2 were found to be dramatically reduced in IBS10, while genes encoded by the GI TnSmu1 were up regulated in the mutant. The microarray data were further confirmed by real-time RT-PCR analyses. Furthermore, direct regulation of some of the differentially expressed genes was demonstrated by electrophoretic mobility shift assays using purified CovR protein. A proteomic study was also carried out that showed a general perturbation of protein expression in the mutant strain. Our results indicate that CovR truly plays a significant role in the regulation of several virulence related traits in this pathogenic streptococcus.

Two-component signal transduction systems in oral bacteria

“…The overlap includes genes for cell-wall division and biofilm formation ( gbpB , lysM, gtfB , gtfC ), with VicR positively regulating these genes and CovR acting as a repressor [107,141,142]. In EMSA assays, it was observed that VicR Sm VicR and CovR co-bind the promoter regions of lysM , gbpB , and gtfC [107].…”

“…In S. mutans , the system was originally designated GcrR (for glucan-binding protein C regulator) following identification in a screen for genes that regulate aggregation between GbpC and dextran [161]. Later studies revealed that CovR Sm negatively or positively regulates about 6.5% of the S. mutans genome [142]. More importantly, CovR Sm directly represses a panel of genes required for the synthesis of water-soluble and water-insoluble glucan ( gtfB , gtfC ) and other exopolysaccharides derived or not from sucrose ( epsC and ftf ), as well as surface proteins GbpC and GbpB that bind to glucan [107,141,142].…”

“…Later studies revealed that CovR Sm negatively or positively regulates about 6.5% of the S. mutans genome [142]. More importantly, CovR Sm directly represses a panel of genes required for the synthesis of water-soluble and water-insoluble glucan ( gtfB , gtfC ) and other exopolysaccharides derived or not from sucrose ( epsC and ftf ), as well as surface proteins GbpC and GbpB that bind to glucan [107,141,142]. Deletion of covR Sm reduced S. mutans cariogenicity in rat models by mechanisms not completely understood, but likely by affecting biofilm structure [162].…”

“…CovR Sm would be always active and/or likely phosphorylated by small phosphodonors such as AcP [141,172]. The absence of CovS protein in S. mutans and amino acid differences between CovR Spy and CovR Smu [142] (Figure 4) account for this proposal. This hypothesis is compatible with our findings that S. mutans strains isolated from systemic infections that have increased resistance to opsonophagocytic killing by PMN express low levels of covR Sm and upregulated CovR Sm target genes compared with oral strains [95].…”

CovR Alleviates Transcriptional Silencing by a Nucleoid-Associated Histone-Like Protein in Streptococcus mutans

“…In a previous study in our lab using whole-genome microarray analysis, we have shown that in S. mutans, CovR differentially regulates approximately 6.5% of the genes (25), where it acts both as an activator and a repressor. CovR represses half of these genes, including gtfB (glucosyltransferase B), gtfC (glucosyltransferase C), and gbpC (glucan-binding protein C).…”