The extracytoplasmic function sigma factor σ(VreI) is active during infection and contributes to phosphate starvation-induced virulence of Pseudomonas aeruginosa

The extracytoplasmic function sigma factor σ(VreI) of the human pathogen Pseudomonas aeruginosa promotes transcription of potential virulence determinants, including secretion systems and secreted proteins. Its activity is modulated by the VreR anti-σ factor that inhibits the binding of σ(VreI) to the RNA polymerase in the absence of a (still unknown) inducing signal. The vreI-vreR genes are expressed under inorganic phosphate (Pi) starvation, a physiological condition often encountered in the host that increases P. aeruginosa pathogenicity. However, whether or not σ(VreI) is active in vivo during infection and contributes to the Pi starvation-induced virulence of this pathogen has not been analyzed yet. Using zebrafish embryos and a human alveolar basal epithelial cell line as P. aeruginosa hosts, we demonstrate in this work that σ(VreI) is active during infection and that lack of σ(VreI) considerably reduces the Pi starvation-induced virulence of this pathogen. Surprisingly, lack of the σ(VreI) inhibitor, the VreR anti-σ factor, also diminishes the virulence of P. aeruginosa. By transcriptomic analyses we show that VreR modulates gene expression not only in a σ(VreI)-dependent but also in a σ(VreI)-independent manner. This includes potential virulence determinants and transcriptional regulators that could be responsible for the reduced virulence of the ΔvreR mutant.