Discovery and characterization of the PpqI/R quorum sensing system activated by GacS/A and Hfq in Pseudomonas protegens H78

Pseudomonas protegens can generally produce multiple antibiotics including pyoluteorin (Plt), 2,4-diacetylphloroglucinol (DAPG), and pyrrolnitrin (Prn). In this study, we discovered and characterized a quorum sensing (QS) system, PpqI/R, in P. protegens H78. PpqI/R, encoded by two open reading frames (ORFs) (H78_01960/01961) in P. protegens H78 genome, is a LuxI/R-type QS system. Four long-chain acyl homoserine lactone (AHL) signaling molecules, 3-OH-C₁₀-HSL, 3-OH-C₁₂-HSL, C₁₂-HSL, and 3-OH-C₁₄-HSL, are produced by H78. Biosynthesis of these AHLs is catalyzed by PpqI synthase and activated by the PpqR regulator in H78 and in Escherichia coli when heterologously expressed. PpqR activates ppqI expression by targeting the lux box upstream of the ppqI promoter in cooperation with corresponding AHLs. The four aforementioned AHLs exhibited different capabilities to induce ppqI promoter expression, with 3-OH-C₁₂-HSL showing the highest induction activity. In H78 cells, ppqI/R expression is activated by the two-component system GacS/A and the RNA chaperone Hfq. Differential regulation of the PpqI/R system in secondary metabolism has a negative effect on DAPG biosynthesis and ped operon (involved in volatile organic compound biosynthesis) expression. In contrast, Plt biosynthesis and prn operon expression were positively regulated by PpqI/R. In summary, PpqI/R, the first characterized QS system in P. protegens, is activated by GacS/A and Hfq and controls the expression of secondary metabolites, including antibiotics.