Figure 1.

Schematic representation of agr control in staphylococci. As the bacteria multiply, RAP reaches a concentration threshold, inducing the phosphorylation of TRAP that triggers a poorly known mechanism for the synthesis of QSII, which consists of the products of the agr system. The agr system is composed of two divergent transcripts, RNAII and RNAIII. RNAII encodes AgrA, AgrC, AgrD, and AgrB. AgrD produces autoinducing peptide (AIP) that is processed by AgrB maturation and export. At a certain threshold concentration, AIP induces the phosphorylation of its receptor AgrC, and phosphorylated AgrA activates transcription from the P3 promoter, leading to the production of RNAIII. In addition to the presence of the RAP-TRAP system, there is also an antagonist, RIP, that can inhibit RNAIII activity. RIP competes with RAP to block TRAP phosphorylation, shutting down QS1 and agr expression.