Fig. 1. The quorum sensing network of P. aeruginosa.

P. aeruginosa employs three QS circuits in a hierarchical, intertwined manner. The las system utilizes the homoserine lactone 3-oxo-C12-HSL as autoinducer for LasR, whereas the rhl and pqs systems use C4-HSL and 2-heptyl-4-quinolone (HHQ) or 2-heptyl-3-hydroxy-4-quinolone (PQS) to activate RhlR and PqsR (also termed MvfR), respectively. RhlR requires C4-HSL for up- or downregulation of target genes. The major part of the RhlR regulon is further influenced by association with PqsE, which is either required or at least enhances regulation by RhlR, leading to the indicated three sub-regulons class I-III. The number of up- or downregulated genes according to reference 16 is shown in parentheses¹⁶. LasR, RhlR, PqsR and PqsE are homodimeric proteins. Thick black arrows indicate upregulation (dashed lines: surrogate regulation), capped lines indicate antagonistic activity. Thin black arrows denote association; thin gray arrows represent biosynthetic processes.