A novel regulatory system promotes group A Streptococcus survival in human blood

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PROJECT SUMMARY The group A Streptococcus (GAS; S. pyogenes) causes significant human morbidity (>700 million infections annually) and mortality (>550,000 deaths annually), with a spectrum of infections that range from mild and self- limiting (e.g. pharyngitis) to severely invasive (e.g. necrotizing fasciitis). We have identified that FasX, the sRNA component of a novel four-component regulatory system (FasBCAX), post-transcriptionally regulates the production of key GAS virulence factors. The regulation afforded by the FasBCAX system influences GAS virulence, enhancing GAS resistance to the bactericidal properties of human blood (see preliminary data) and lethality in a mouse invasive infection model. However, there remains significant gaps in our knowledge regarding the functioning of this model regulatory system, including how FasX reduces GAS killing in human blood by ~30-fold, and how the FasBCA proteins function to enhance fasX expression ~100-fold. We will fill the regulatory, mechanistic, and virulence gaps in our knowledge by pursuing the following aims: Aim 1: Determine the mechanism by which FasX enhances GAS resistance to human blood. In this aim, we will verify our preliminary data that supports the FasX-regulon being twice the size of that currently appreciated, identify which FasX-regulated virulence factor/s are responsible for the resistance phenotype and whether their regulation by FasX modifies the binding of host molecules to the GAS cell surface, and, given our preliminary data that the resistance phenotype occurs via the inhibition of phagocytic cells, test whether neutrophil activation, phagocytosis, and/or oxidative burst is inhibited. Aim 2: Determine the mechanism by which the FasBCA proteins enhance FasX sRNA abundance. The Fas locus consists of the co-transcribed fasBCA and the separately transcribed fasX. FasB and FasC are homologous to sensor kinases while FasA is homologous to response regulators. Preliminary data are consistent with FasB and FasC forming heterodimers, rather than homodimers as classically occurs for sensor kinases. In this aim, we will unravel how the FasBCA proteins interact to enhance the transcription of fasX and other regulatory targets, and will initiate delineation of host factors capable of modulating Fas system activity. Completion of this research will greatly expand what is known about sRNA function in GAS, an organism, along with fellow Lactobacillales pathogens, in which sRNA mechanistic data is limited. We will generate basic science insights by delineating a never-before-described regulatory system in which two sensor kinases heterodimerize to activate activity. We will also generate clinical insights by delineating the molecular basis behind the ability of FasX to inhibit GAS killing in blood, a phenotype that is critical to the ability of this prevalent human pathogen to cause severe invasive disease.