Zinc-binding to the cytoplasmic PAS domain regulates the essential WalK histidine kinase of Staphylococcus aureus

WalKR (YycFG) is the only essential two-component regulator in the human pathogen Staphylococcus aureus. WalKR regulates peptidoglycan synthesis, but this function alone does not explain its essentiality. Here, to further understand WalKR function, we investigate a suppressor mutant that arose when WalKR activity was impaired; a histidine to tyrosine substitution (H271Y) in the cytoplasmic Per-Arnt-Sim (PAS^CYT) domain of the histidine kinase WalK. Introducing the WalK^H271Y mutation into wild-type S. aureus activates the WalKR regulon. Structural analyses of the WalK PAS^CYT domain reveal a metal-binding site, in which a zinc ion (Zn²⁺) is tetrahedrally-coordinated by four amino acids including H271. The WalK^H271Y mutation abrogates metal binding, increasing WalK kinase activity and WalR phosphorylation. Thus, Zn²⁺-binding negatively regulates WalKR. Promoter-reporter experiments using S. aureus confirm Zn²⁺ sensing by this system. Identification of a metal ligand recognized by the WalKR system broadens our understanding of this critical S. aureus regulon.