Fig. 4.

Model for the effects of farnesol on PQS production in P. aeruginosa. Anthranilic acid (AA) is converted into HHQ by genes encoded by pqsA–D. HHQ is converted into PQS by PqsH. HHQ and PQS contribute to the positive feedback regulation of the pqsA–D genes via PqsR. Farnesol inhibits PqsR activity when PQS is at low levels. Inhibition of PqsR activity by farnesol is not complete, and as HHQ or PQS levels increase, farnesol no longer competes with natural PqsR ligands. Expression of pqsH is controlled by LasR when it is complexed with 3OC12HSL, and the RhlR–C4HSL complex is not needed for induction of pqsH in wild-type cells. In lasR or lasI mutants, RhlR–C4HSL can activate pqsH. While LasR–3OC12HSL is normally required for induction of rhlR and rhlI, farnesol or hydrogen peroxide can stimulate RhlR–C4HSL activity in the absence of LasR–3OC12HSL.