Autoregulation of theS. mutansSloR metalloregulator is constitutive and driven by an independent promoter

Abstract

Streptococcus mutans, one of ∼600 bacterial species in the human oral cavity, is among the most acidogenic constituents of the plaque biofilm. Considered to be the primary causative agent of dental caries, S. mutans harbors a 25kDa SloR metalloregulatory protein which controls metal ion transport across the bacterial cell membrane to maintain essential metal ion homeostasis. The expression of SloR derives, in part, from transcriptional readthrough of the sloABC operon which encodes a Mn2+/Fe2+ ABC transport system. Herein, we describe the details of the sloABC promoter that drives this transcription, as well as a novel independent promoter in an intergenic region (IGR) that contributes to downstream sloR expression. RT-PCR studies support sloR transcription that is independent of sloABC expression, and the results of 5′ RACE revealed a sloR transcription start site in the IGR from which the −10 and −35 promoter regions were predicted. The results of gel mobility shift assays support direct SloR binding to the IGR, albeit with lower affinity than SloR binding to the sloABCR promoter. Function of the sloR promoter was validated in qRT-PCR experiments. Interestingly, sloR expression was not significantly impacted when grown in the presence of high manganese, whereas expression of the sloABC operon was repressed under these conditions. The results of in vitro transcription studies support SloR-mediated transcriptional-activation of sloR and -repression of sloABC. Taken together, these findings implicate SloR as a bifunctional regulator that represses sloABC promoter activity and encourages sloR transcription from an independent promoter.ImportanceTooth decay is a ubiquitous infectious disease that is especially pervasive in underserved communities worldwide. S. mutans-induced carious lesions cause functional, physical, and/or aesthetic impairment in the vast majority of adults, and in 60-90% of schoolchildren in industrialized countries. Billions of dollars are spent annually on caries treatment, and productivity losses due to absenteeism from the workplace are significant. Research aimed at alleviating S. mutans-induced tooth decay is important because it can address the socioeconomic disparity that is associated with dental cavities and improve overall general health which is inextricably linked to oral health. Research focused on the S. mutans SloR metalloregulatory protein can guide the development of novel therapeutics and so alleviate the burden of dental cavities.