GacS/GacA activates pyoluteorin biosynthesis through Gac/Rsm-RsmE cascade and RsmA/RsmE-driven feedback loop in Pseudomonas protegens H78

The Gac/Rsm regulatory pathway in Pseudomonas spp. activates the production of various secondary metabolites, such as antibiotics, siderophores and exoenzymes. However, the biosynthesis of antifungal compound pyoluteorin (Plt) in Pseudomonas protegens H78 is almost entirely inhibited by double deletion of two csrA/rsmA family genes, namely, rsmA and rsmE. Here, we investigated the complicated regulatory mechanism of RsmA and RsmE in Plt biosynthesis in P. protegens H78. RsmE negatively regulated Plt biosynthesis and pltLABCDEFG expression by directly interacting with the mRNA leaders of pltR and pltAB. Conversely, the transcription of pltL-G and pltR was positively influenced by RsmA through an uncharacterized mechanism. Further analyses demonstrated that pltL-G expression was diminished in the rsmA/E mutant. The deficiency of pltL-G expression in the gacA mutant was not reversed by any of the rsmA/E single or double mutations. The double deletion of rsmA/E reduced gacA expression by approximately 50% and almost completely inhibited the promoter activities of rsmXYZ sRNAs. The rsmY mutation significantly inhibited Plt biosynthesis. Taken together, GacS/GacA modulates Plt biosynthesis through two distinct pathways: Gac/Rsm-RsmE traditional positive regulatory cascade and RsmA-mediated positive transcriptional regulation. Moreover, a new positive feedback loop between RsmA/E and GacS/A-RsmXYZ is essential for activating Plt biosynthesis.