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

ABSTRACT WalKR (YycFG) is the only essential two-component regulator in the human pathogen Staphylococcus aureus. WalKR regulates peptidoglycan synthesis, but this function alone appears not to explain its essentiality. To understand WalKR function we investigated a suppressor mutant that arose when WalKR activity was impaired; a single histidine to tryptophan substitution (H271Y) in the cytoplasmic Per-Arnt-Sim (PAS CYT ) domain of the histidine kinase WalK. Introduction of the WalK H271Y mutation into wild-type S . aureus activated the WalKR regulon. Structural analyses of the WalK PAS CYT domain revealed a hitherto unknown metal binding site, in which a zinc ion (Zn 2+ ) was tetrahedrally-coordinated by four amino acid residues including H271. The Wallk H271Y mutation abrogated metal binding, increasing WalK kinase activity and WalR phosphorylation. Thus, Zn 2+ -binding negatively regulates WalKR activity. Identification of a metal ligand sensed by the WalKR system substantially expands our understanding of this critical S . aureus regulon.