Schematic representation of the PlcR-PapR QS system, its regulation and its activity. The product of the papR gene, belonging to the PlcR regulon, is a 48 amino-acid peptide. PapR is secreted, probably via the Sec machinery, processed by the NprB protease and presumably other peptidases. PapR is found in the extracellular compartment and in the bacterial cytoplasm mainly as a seven amino-acid peptide. It is imported in the cell via the oligopeptide permease Opp. Once inside the cell PapR binds to PlcR allowing it to activate the expression of 45 genes composing the PlcR regulon. plcR is autoregulated and under the negative control of Spo0A-P. CodY positively controls the expression of the PlcR-dependent genes via a yet unknown mechanism.