Loss of LasR function leads to decreased repression of Pseudomonas aeruginosa PhoB activity at physiological phosphate concentrations.

While Pseudomonas aeruginosa LasR plays a role in quorum sensing (QS) across all phylogenetically-distinct lineages, isolates with loss-of-function mutations in lasR (LasR- strains) are commonly found in diverse settings including infections where they are associated with worse clinical outcomes. In LasR- strains, another QS transcription factor RhlR can be reactivated in low inorganic phosphate (Pi) concentrations via the two- component system PhoR-PhoB, restoring expression of some of the genes in the QS network. Here, we demonstrate a new link between LasR and PhoB in which the absence of LasR increases PhoB activity at physiological Pi concentrations and raises the Pi concentration necessary for PhoB inhibition. PhoB activity was also less repressed by Pi in mutants lacking different QS regulators (RhlR, and PqsR) and in mutants lacking genes required for the production of QS-regulated phenazines suggesting that decreased phenazine production was one reason for decreased PhoB repression by Pi in LasR-. In addition, the CbrA-CbrB two-component system, which is elevated in LasR- strains, was necessary for reduced PhoB repression by Pi and a Δ crc mutant, which lacks the CbrA- CbrB-controlled translational repressor, activated PhoB at higher Pi concentrations than the in the wild type. The Δ lasR mutant had a PhoB-dependent growth advantage in a medium with no added Pi and increased virulence-determinant gene expression in a medium with 0.7 mM Pi. Reanalysis of published RNA-seq data found evidence for PhoB activity in P. aeruginosa in cystic fibrosis sputum indicating that potential strain differences in PhoB activation may impact disease state.