Phosphate sensing by PhoPR regulates the cytotoxicity of Staphylococcus aureus
Staphylococcus aureus has evolved a complex regulatory network to coordinate expression of virulence factors, including cytolytic toxins, with host environmental signals. Central to this network are two-component systems, in which a histidine kinase senses an external signal and activates a response regulator via phosphorylation, leading to changes in gene expression. Using a comprehensive screen of transposon mutants in each of the non-essential histidine kinase and response regulatory genes in S. aureus, we demonstrate that 11 of these 16 systems regulate cytotoxicity. Further characterisation of a phoP mutant revealed that its impact on cytotoxicity is mediated through the Agr quorum-sensing system. Notably, we found that unphosphorylated PhoP is an activator of Agr activity, while phosphorylated PhoP also acts as a weak activator of Agr activity in high phosphate environments but as a repressor in low phosphate environments. Overall, we have demonstrated that phosphate sensing through PhoPR is a novel regulator of cytotoxicity in S. aureus. Moreover, our study challenges the canonical model of TCSs as simple on/off systems and highlights the importance of unphosphorylated response regulators in gene regulation. ImportanceThe production of cytolytic toxins is the major means by which bacterial pathogens damage host tissue and cause disease. Understanding the activity and regulation of these toxins is critical for the identification of means to block them and prevent the development of disease. In this study we focused on a specific regulatory mechanism, the two-component systems (TCSs), that enable bacteria to sense their environment and adapt accordingly. In the traditional model of a TCS, a response regulator (RR) is phosphorylated by a histidine kinase (HK), which enables it to activate or repress expression of target genes, which may include toxins or regulators of toxins. We found that 11 of S. aureus 16 TCSs affect toxin production, highlighting that S. aureus integrates a broad range of environmental cues to regulate toxicity. We focused on one of these TCSs, the PhoPR system and found that sensing of inorganic phosphate is a novel regulator of cytotoxicity in S. aureus. Furthermore, we found that the RR of this system acts as a strong activator of toxicity in its unphosphorylated form, challenging the traditional model of a TCS as only active upon signal activation.