Intracellular signaling through thecomRSsystem inStreptococcus mutansgenetic competence
Authors: Underhill Simon A.M. , Shields Robert C. , Kaspar Justin R. , Haider Momin , Burne Robert A. , Hagen Stephen J. 
Published: March 2018

Abstract

Entry into genetic competence in streptococci is controlled by ComX, an alternative sigma factor for genes that enable the import of exogenous DNA. InStreptococcus mutans, the immediate activator ofcomXis the ComRS signaling system, which consists of the cytosolic receptor ComR and the 7-residue signal peptide XIP, which is derived from ComS. Extracellular XIP imported by an oligopeptide permease interacts with ComR to form a transcriptional activator for bothcomXandcomS. Therefore, extracellular XIP can function as an exogenous signal to triggerS. mutanscompetence. However, the mechanisms that process ComS and export it as XIP are not fully known inS. mutans. The observation thatcomXis expressed bimodally under some environmental conditions suggests that ComR may also interact with endogenously produced XIP or ComS, creating an intracellular positive feedback loop incomStranscription. Here we use single cell and microfluidic methods to compare the effects of the nativecomSgene and extracellular XIP oncomXexpression. We find that deletion ofcomSreduces the response ofcomXto extracellular XIP. We also find thatcomS-overexpressing cells autoactivate theircomXeven when their growth medium is rapidly exchanged, although this autoactivation requires an intact copy ofcomSunder control of its own promoter. HowevercomS-overexpressing cells do not activatecomS-deficient mutants growing in coculture. These data show that individual cells can activatecomXwithout exporting or importing the XIP or ComS signal, and that endogenously and exogenously produced ComS/XIP have inequivalent effects oncomXbehavior. These data are fully consistent with a model in which intracellular positive feedback incomStranscription plays a role in ComRS signaling, and is responsible for the bimodal expression ofcomX.

Author Summary

Heterogeneous gene expression in genetically identical populations plays an important role in bacterial persistence and survival under changing environmental conditions. In the oral pathogenStreptococcus mutans,the physiological state of genetic competence can exhibit bimodality, with only some cells becoming competent.S. mutanscontrols its entry into competence by using the ComRS signaling system to activatecomX,a gene encoding the master competence regulator ComX. The ComRS system is understood as a quorum sensing system, in which the extracellular accumulation of the small signal peptide XIP, derived from ComS, inducescomXexpression. We coupled observation of bacteria that fluoresce whencomXis active with mathematical analysis and chemical binding assays to show that activation ofcomXdoes not necessarily require extracellular XIP or ComS, and thatcomX-active cells do not necessarily export XIP. Our experiments and mathematical modeling indicate that a positive feedback loop incomStranscription allows a cell to activatecomXin response to its own XIP or ComS in the absence of extracellular XIP, or to amplify itscomXresponse to extracellular XIP if present. Such positive feedback loops are often the cause of bimodal gene expression like that seen inS. mutanscompetence.