Integrated activities of two alternative sigma factors coordinate iron acquisition and uptake by Pseudomonas aeruginosa

Summary Alternative sigma (σ) factors govern expression of bacterial genes in response to diverse environmental signals. In Pseudomonas aeruginosa σ PvdS directs expression of genes for production of a siderophore, pyoverdine, as well as a toxin and a protease. σ FpvI directs expression of a receptor for ferripyoverdine import. Expression of the genes encoding σ PvdS and σ FpvI is iron‐regulated and an antisigma protein, FpvR 20 , post‐translationally controls the activities of the sigma factors in response to the amount of ferripyoverdine present. Here we show that iron represses synthesis of σ PvdS to a far greater extent than σ FpvI . In contrast ferripyoverdine exerts similar effects on the activities of both sigma factors. Using a combination of in vivo and in vitro assays we show that σ FpvI and σ PvdS have comparable affinities for, and are equally inhibited by, FpvR 20 . Importantly, in the absence of ferripyoverdine the amount of FpvR 20 per cell is lower than the amount of σ FpvI and σ PvdS , allowing basal expression of target genes that is required to activate the signalling pathway when ferripyoverdine is present. This complex interplay of transcriptional and post‐translational regulation enables a co‐ordinated response to ferripyoverdine but distinct responses to iron.