Characterization of a Peptide Signaling System Important to Streptococcus mutans Virulence

Dental caries, commonly known as tooth decay, remains an enormous public health concern because of its prevalence, impact on individuals and society, and the expense of treatment. As the global burden of this disease increases, an improved understanding of the pathogenesis of dental caries, thereby managing caries and improving the quality of life, has become crucial. Dental caries is a multifactorial disease, of which Streptococcus mutans is the principle etiologic agent. The pathogenicity of S. mutans involves a diverse array of virulence traits that are coordinately expressed through mechanisms that remain unclear. Many bacteria use intercellular chemical signaling, known as quorum sensing (QS), to coordinate gene expression, and consequently their behavior, across a population. Some streptococci use Rgg-type proteins together with short hydrophobic peptides (SHPs) to mediate quorum sensing. Most species contain multiple Rgg paralogs within their genomes, and studies thus far indicate that each paralog controls a different genetic regulon. The S. mutans genome contains four Rgg paralogs- three of whose regulatory networks are unknown. Some Rgg pathways have been implicated in virulence-related behaviors, including biofilm development and toxin regulation. With this evidence, we hypothesized that virulence gene expression by S. mutans is modulated through Rgg QS pathways. To test this hypothesis, we characterized the regulon of Rgg1509 in S. mutans strain UA159. Using transcriptional luxAB fusions and quantitative reverse transcription-PCR, we demonstrate Rgg1509 comprises a functional quorum sensing system involving an adjacently encoded SHP pheromone that is secreted by an ABC-type transporter (PptAB) and reimported to the cell by an oligopeptide permease (Opp). Under this mechanism, Rgg1509 directly activates the transcription of a proximal biosynthetic gene cluster, encoding a novel natural product of currently unknown function. RNA-seq analysis revealed that Rgg1509 not only regulates the transcription of its proximal target genes but controls the expression of a number of genes throughout the genome, especially those encoding for essential virulence determinants, such as cell surface adhesions. Experimental evidence demonstrated that Rgg1509 plays a key role in adherence, as deletion of rgg1509 increased the aggregative and biofilm forming capabilities of S. mutans under stress. This newly characterized Rgg1509 QS pathway may therefore represent an important mechanism by which S. mutans colonizes the tooth surface, leading to the onset of dental caries.