Abstract
Entry into genetic competence in streptococci is controlled by ComX, an alternative sigma factor for genes that enable the import of exogenous DNA. In Streptococcus mutans, the immediate activator of comX is 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 both comX and comS. Therefore, extracellular XIP can function as an exogenous signal to trigger S. mutans competence. However, the mechanisms that process ComS and export it as XIP are not fully known in S. mutans. The observation that comX is expressed bimodally under some environmental conditions suggests that ComR may also interact with endogenously produced XIP or ComS, creating an intracellular positive feedback loop in comS transcription. Here we use single cell and microfluidic methods to compare the effects of the native comS gene and extracellular XIP on comX expression. We find that deletion of comS reduces the response of comX to extracellular XIP. We also find that comS-overexpressing cells autoactivate their comX even when their growth medium is rapidly exchanged, although this autoactivation requires an intact copy of comS under control of its own promoter. However comS-overexpressing cells do not activate comS-deficient mutants growing in coculture. These data show that individual cells can activate comX without exporting or importing the XIP or ComS signal, and that endogenously and exogenously produced ComS/XIP have inequivalent effects on comX behavior. These data are fully consistent with a model in which intracellular positive feedback in comS transcription plays a role in ComRS signaling, and is responsible for the bimodal expression of comX.
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 pathogen Streptococcus mutans, the physiological state of genetic competence can exhibit bimodality, with only some cells becoming competent. S. mutans controls its entry into competence by using the ComRS signaling system to activate comX, 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, induces comX expression. We coupled observation of bacteria that fluoresce when comX is active with mathematical analysis and chemical binding assays to show that activation of comX does not necessarily require extracellular XIP or ComS, and that comX-active cells do not necessarily export XIP. Our experiments and mathematical modeling indicate that a positive feedback loop in comS transcription allows a cell to activate comX in response to its own XIP or ComS in the absence of extracellular XIP, or to amplify its comX response to extracellular XIP if present. Such positive feedback loops are often the cause of bimodal gene expression like that seen in S. mutans competence.
