The SaeRS Two-Component System Is a Direct and Dominant Transcriptional Activator of Toxic Shock Syndrome Toxin 1 in Staphylococcus aureus

Toxic shock syndrome toxin 1 (TSST-1) is aStaphylococcus aureus superantigen that has been implicated in both menstrual and nonmenstrual toxic shock syndrome (TSS). Despite the important role of TSST-1 in severe human disease, a comprehensive understanding of staphylococcal regulatory factors that control TSST-1 expression remains incomplete. TheS. aureus exotoxin expression (Sae) operon contains a well-characterized two-component system that regulates a number of important exotoxins inS. aureus , although regulation of TSST-1 by the Sae system has not been investigated. We generated a defined deletion mutant of the Sae histidine kinase sensor (saeS ) in the prototypic menstrual TSS strainS. aureus MN8. Mutation ofsaeS resulted in a complete loss of TSST-1 expression. Using both luciferase reporter experiments and quantitative real-time PCR, we demonstrate that the Sae system is an important transcriptional activator of TSST-1 expression. Recombinant SaeR was able to bind directly to thetst promoter to a region containing two SaeR consensus binding sites. Although the stand-alone SarA transcriptional regulator has been shown to be both a positive and a negative regulator of TSST-1, deletion ofsarA inS. aureus MN8 resulted in a dramatic overexpression of TSST-1. As expected, mutation ofagr also reduced TSST-1 expression, but this phenotype appeared to be independent of Sae. A double mutation ofsaeS andsarA resulted in the loss of TSST-1 expression. This work indicates that the Sae system is a dominant and direct transcriptional activator that is required for expression of TSST-1.IMPORTANCE The TSST-1 superantigen is an exotoxin, produced by some strains ofS. aureus , that has a clear role in both menstrual and nonmenstrual TSS. Although the well-characterizedagr quorum sensing system is a known positive regulator of TSST-1, the molecular mechanisms that directly control TSST-1 expression are only partially understood. Our studies demonstrate that the Sae two-component regulatory system is a positive transcriptional regulator that binds directly to the TSST-1 promoter, and furthermore, our data suggest that Sae is required for expression of TSST-1. This work highlights how major regulatory circuits can converge to fine-tune exotoxin expression and suggests that the Sae regulatory system may be an important target for antivirulence strategies.