RpoN Promotes Survival in the Presence of Tobramycin.

has developed diverse strategies to respond and adapt to antibiotic stress. Among the factors that modulate survival in the presence of antibiotics, alternative sigma factors play an important role. Here, we demonstrate that the alternative sigma factor RpoN (σ) promotes survival in the presence of tobramycin. The tobramycin-sensitive phenotype of logarithmic phase Δ mutant cells is suppressed by the loss of the alternative sigma factor RpoS. Transcriptional analysis indicated that RpoN positively regulates the expression of RsmA, an RNA-binding protein, in the stationary growth phase in a nutrient-rich medium. The loss of RpoS led to the upregulation of expression in the nutrient-limited medium-grown stationary phase cells. Conversely, in the logarithmic growth phase, the Δ mutant demonstrated lower expression of , underscoring a regulatory role of RpoS for GacA. Supplementation of tobramycin to stationary phase Δ mutant cells grown in nutrient-rich medium resulted in decreased expression of , and without altering the expression of relative to wild-type PAO1. The observed downregulation of and in the Δ mutant in the presence of tobramycin could be reversed through the mutation of in the Δ mutant background. The tobramycin-tolerant phenotype of the ΔΔ mutant logarithmic phase cells may be associated with the expression of , which remained unresponsive upon addition of tobramycin. The logarithmic phase Δ and ΔΔ mutant cells demonstrated increased expression of in response to tobramycin. Together, these results suggest that a complex regulatory interaction between RpoN, RpoS, the Gac/Rsm pathway, and RelA modulates the response to tobramycin.