OrtR regulates the oxidative stress response of Streptococcus mutans

Dental caries is the most common chronic infectious diseases around the world and disproportionately affects the marginalized socioeconomic group. Streptococcus mutans, considered a primary etiological agent of caries, must depend on the coordinated physiological response to tolerate the oxidative stress generated by commensal species within dental plaque, which is a critical aspect of its pathogenicity. Here, we identified and characterized a novel TetR family regulator SMU_1361c, encoded by the TnSmu2 operon, which appears to be acquired by the bacteria via horizontal genes transfer. Surprisingly, smu_1361c functions as a transcriptional repressor to regulate gene expression outside its operon, involved in the oxidative stress response of S. mutans. The smu_1361c overexpression strain UA159/pDL278-1361c was more susceptible to oxidative stress and less competitive against hydrogen peroxide generated by commensal species Streptococcus gordonii and Streptococcus sanguinis. Transcriptomics analysis revealed that smu_1361c overexpression resulted in the significant downregulation of 22 genes mainly belonging to three gene clusters responsible for the oxidative stress response. The conversed DNA binding motif of SMU_1361c was determined by electrophoretic mobility shift and DNase I footprinting assay with purified SMU_1361c protein, therefore, smu_1361c is directly involved in gene transcription related to the oxidative stress response. Crucially, our finding provides a new understanding of how S. mutans deals with the oxidative stress that is required for pathogenesis and will facilitate the development of new and improved therapeutic approaches for dental caries.