The alternative sigma factor RpoS regulates Pseudomonas aeruginosa quorum sensing response by repressing the pqsABCDE...

Pseudomonas aeruginosa is an important opportunistic pathogen. Several of its virulence-related processes, including the synthesis of pyocyanin (PYO) and biofilm formation, are controlled by quorum sensing (QS). It has been shown that the alternative sigma factor RpoS regulates QS through the reduction of lasR and rhlR transcription (encoding QS regulators). However, paradoxically, the absence of RpoS increases PYO production and biofilm development (that are RhlR dependent) by unknown mechanisms. Here, we show that RpoS represses pqsE transcription, which impacts the stability and activity of RhlR. In the absence of RpoS, rhlR transcript levels are reduced but not the RhlR protein concentration, presumably by its stabilization by PqsE, whose expression is increased. We also report that PYO synthesis and the expression of pqsE and phzA1B1C1D1E1F1G1 operon exhibit the same pattern at different RpoS concentrations, suggesting that the RpoS-dependent PYO production is due to its ability to modify PqsE concentration, which in turn modulates the activation of the phzA1 promoter by RhlR. Finally, we demonstrate that RpoS favors the expression of Vfr, which activates the transcription of lasR and rhlR. Our study contributes to the understanding of how RpoS modulates the QS response in P. aeruginosa, exerting both negative and positive regulation.