The Staphylococcus aureus agr quorum-sensing system plays a major role in the transition from the persistent to the virulent phenotype. S. aureus agr type I to IV strains are characterized by mutations in the sensor domain of the histidine kinase AgrC and differences in the sequences of the secreted autoinducing peptides (AIP). Here we demonstrate that interactions between the cytosolic domain of AgrC (AgrC Cyto ) and the response regulator domain of AgrA (AgrA RR ) dictate the spontaneity of the cellular response to AIP stimuli. The crystal structure of AgrC Cyto provided a basis for a mechanistic model of AgrC-AgrA interactions. This model enabled an analysis of the biochemical and biophysical parameters of AgrC-AgrA interactions in the context of the conformational features of the AgrC-AgrA complex. This analysis revealed distinct sequence and conformational features that determine the affinity, specificity, and kinetics of the phosphotransfer reaction. This step, which governs the response time for transcriptional reengineering triggered by an AIP stimulus, is independent of the agr type and similar for agonist and antagonist stimuli. These experimental data could serve as a basis on which to validate simulations of the quorum-sensing response and for strategies that employ the agr quorum-sensing system to combat biofilm formation in S. aureus infections. Multiple pore-forming toxins, immune evasion factors, and adhesins contribute to acute and chronic Staphylococcus aureus infections in humans (1). The expression of most of these virulence factors is controlled by the accessory gene regulator (agr), a two-component regulatory system in S. aureus (2). The sequence composition of the autoinducing peptide (AIP) with a five-member thiolactone ring provides a basis on which to distinguish between different S. aureus strains, also referred to as agr types I to IV. The interaction of the AIP with AgrC, a membranebound histidine kinase, regulates the catalytic activity of AgrC. Mutations in AgrC across S. aureus strains of different agr types are confined to the ectodomain. This observation is consistent with experimental data that suggest that the signal recognition mechanism is confined to the extracellular and membrane-associated components (3). The subsequent steps of signal transduction involve phosphorylation of AgrA by AgrC, followed by the interaction of phosphorylated AgrA with cognate promoters to trigger agr-dependent reengineering of the transcriptional profile. This step incorporates signal amplification due to AgrA-induced expression from the P2 promoter and expression of RNAIII, a downstream effector that regulates the expression of virulence factors and other transcriptional regulators (4). Prominent changes in the transcriptional profile induced by AgrA include the expression of the characterized immune response suppressors ␣ and ␤ phenol-soluble modulins (5). The production and secretion of the AIPs involve the action of the permease AgrB on the AgrD propeptide (6).The sensor module of AgrC (resid...

The intracellular trigger for the quorum sensing response mechanism in Staphylococcus aureus involves the phosphorylation of the response regulator AgrA by the membrane anchored histidine kinase AgrC. AgrA activates transcription from three promoter sequences (P1–P3). The promoter strength, conditional association of AgrA with these promoter elements and temporal delay in AgrA-mediated changes in gene expression contribute to the diversity of the quorum sensing response in different S. aureus strains. AgrA promoters comprise of imperfect direct repeats of DNA with a consensus sequence- [TA][AC][CA]GTTN[AG][TG]. Here we describe crystal structures of the DNA-binding (LytTR) domain of AgrA with different cognate DNA sequences that reveal a hitherto unanticipated feature of AgrA-DNA interactions. AgrA promoter interactions are asymmetric with fewer interactions at the binding site proximal to the −35 promoter element. Biochemical assays to evaluate AgrA-promoter interactions suggests that phosphorylation induced dimerization of AgrA can compensate for the asymmetry in AgrA-DNA interactions. The structures also provide a basis to rationalize mutations that were noted to alter AgrA activity without affecting protein-DNA interactions. Put together, the structural data, gene expression and mutational analysis reveal that promoter strength and AgrA phosphorylation enable quorum-sensing triggered transcriptional changes leading to a transition from the persistent to virulent phenotype.

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