TRANSMEMBRANE TOPOLOGY AND HISTIDINE PROTEIN-KINASE ACTIVITY OF AGRC,THE AGR SIGNAL RECEPTOR IN STAPHYLOCOCCUS-AUREUS

The agr P2 operon in Staphylococcus aureus codes for the elements of a density-sensing cassette made up of a typical two-component signallin g system and its corresponding inducer. It is postulated that the auto inducer, a post-translationally modified octapeptide generated from th e AgrD peptide, interacts with a receptor protein, coded by agrC, to t ransmit a signal via AgrA regulating expression of staphylococcal viru lence genes through expression of agr RNA III. We show by analysis of PhoA fusions that AgrC is a transmembrane protein, and confirm using W estern blotting that a 46 kDa protein corresponding to AgrC is present in the bacterial membrane. This protein is autophosphorylated on a hi stidine residue only in response to supernatants from an agr(+) strain , and can also respond to the purified native octapeptide. A recombina nt fusion protein where most of the N-terminal region of AgrC is repla ced by the Escherichia coli maltose-binding protein is also autophosph orylated in response to stimulation by agr(+) supernatants or purified octapeptide. We conclude that AgrC is the sensor molecule of a typica l two-component signal system in S. aureus, and that the ligand-bindin g site of AgrC is probably located in the third extracellular loop of the protein.