CovRst acts as a negative regulator in the response of Streptococcus thermophilus to oxidative stress

CovRS two-component regulatory system involved in stress adaptation in Group A Streptococcus (GAS). This system has also been identified in the genome of Streptococcus thermophilus. To investigate its roles, covR (covRst) was knocked out, and transcriptome analysis was performed in S. thermophilus ST222. The deletion of the covRst resulted in the upregulation of the antioxidative enzymes, which possessed the conserved sequence WAAAAAGGAGV in their promoter regions. The transcriptional levels of the antioxidative enzyme genes of the peroxidase efeB and glutathione peroxidase gshF in the covRst defective mutant were increased by approximately 3.3-fold and 1.6-fold, respectively, as determined by qPCR analysis. EMSA analysis also confirmed the binding abilities of the CovRst protein to the promoter regions of gshF and efeB. Moreover, the glutathione (GSH) content and cell viability of the deficient mutant ST222 ΔcovR were higher than those of the wild-type ST222. Furthermore, the intestinal epithelium cells NCM 460 was used as a model to verify that the deficient mutant ST222 ΔcovR with strong cytoprotective roles against H2O2. Therefore, all experimental data demonstrated that CovRst served as a negative regulator to mediate the environmental adaptation of oxidative stress in S. thermophilus.