RESUMO
The ability of Streptococcus mutans to survive and cause dental caries is dependent on its ability to metabolize various carbohydrates, accompanied by extracellular polysaccharide synthesis and biofilm formation. Here, the role of an rel competence-related regulator (RcrR) in the regulation of multiple sugar transportation and biofilm formation is reported. The deletion of the rcrR gene in S. mutans caused delayed growth, decreased biofilm formation ability, and affected the expression level of its multiple sugar transportation-related genes. Transcriptional profiling revealed 17 differentially expressed genes in the rcrR mutant. Five were downregulated and clustered with the sugar phosphotransferase (PTS) systems (mannitol- and trehalose-specific PTS systems). The conserved sites bound by the rcrR promoter were then determined by electrophoretic mobility shift assays (EMSAs) and DNase I footprinting assays. Furthermore, a potential binding motif in the promoters of the two PTS operons was predicted using MEME Suite 5.1.1. RcrR could bind to the promoter regions of the two operons in vitro, and the sugar transporter-related genes of the two operons were upregulated in an rcrR-overexpressing strain. In addition, when RcrR-binding sites were deleted, the growth rates and final yield of S. mutans were significantly decreased in tryptone-vitamin (TV) medium supplemented with different sugars, but not in absolute TV medium. These results revealed that RcrR acted as a transcription activator to regulate the two PTS systems, accompanied by multiple sugar transportation and biofilm formation. Collectively, these results indicate that RcrR is a critical transcription factor in S. mutans that regulates bacterial growth, biofilm formation, and multiple sugar transportation. IMPORTANCE The human oral cavity is a constantly changing environment. Tooth decay is a commonly prevalent chronic disease mainly caused by the cariogenic bacterium Streptococcus mutans. S. mutans is an oral pathogen that metabolizes various carbohydrates into extracellular polysaccharides (EPSs), biofilm, and tooth-destroying lactic acid. The host diet strongly influences the availability of multiple carbohydrates. Here, we showed that the RcrR transcription regulator plays a significant role in the regulation of biofilm formation and multiple sugar transportation. Further systematic evaluation of how RcrR regulates the transportation of various sugars and biofilm formation was also conducted. Notably, this study decrypts the physiological functions of RcrR as a potential target for the better prevention of dental caries.
RESUMO
Zinc (Zn2+ ) is an essential divalent trace metal for living cells. Intracellular zinc homeostasis is critical to the survival and virulence of bacteria. Thus, the frequent fluctuations of salivary zinc, caused by the low physiological level and the frequent exogenous zinc introduction, present a serious challenge for bacteria colonizing the oral cavity. However, the regulation strategies to keep intracellular Zn2+ homeostasis in Streptococcus mutans, an important causative pathogen of dental caries, are unknown. Because zinc uptake is primarily mediated by an ATP-binding ABC transporter AdcABC in Streptococcus strains, we examined the function of AdcABC and transcription factor AdcR in S. mutans in this study. The results demonstrated that deletion of either adcA or adcCB gene impaired the growth but enhanced the extracellular polymeric matrix production in S. mutans, both of which could be relieved after excessive Zn2+ supplementation. Using RNA sequencing analysis, quantitative reverse transcription polymerase chain reaction examination, LacZ-reporter studies, and electrophoretic mobility shift assay, we showed that a MarR (multiple antibiotic resistance regulator) family transcription factor, AdcR, negatively regulates the expression of the genes adcR, adcC, adcB, and adcA by acting on the adcRCB and adcA promoters in response to Zn2+ concentration in their environmental niches. The deletion of adcR increases the sensitivity of S. mutans to excessive Zn2+ supply. Taken together, our findings suggest that Adc regulon, which consists of a Zn2+ uptake transporter AdcCBA and a Zn2+ -responsive repressor AdcR, plays a prominent role in the maintenance of intracellular zinc homeostasis of S. mutans.