A Single Mutation in the oprF mRNA Leader Confers Strict Translational Control by the Gac/Rsm System in Pseudomonas fluorescens CHA0

The rhizobacterium Pseudomonas fluorescens CHA0 is able to antagonize fungal phytopathogens on a variety of crop plants, mainly due to the production of secondary metabolites that are coordinately upregulated by the global regulatory Gac/Rsm cascade. The two-component system GacS/GacA activates transcription of the three small regulatory RNAs RsmX, RsmY, and RsmZ, which counteract translational repression of target mRNAs by RsmA and RsmE proteins. In a search for novel Gac/Rsm targets based on the minimal sequence on mRNA leaders required for RsmA/RsmE control, the leader region of the major porin OprF emerged as a candidate. Although an isogenic CHA0 oprF mutant showed a reduced ability to attach to cucumber and tomato roots, suggesting a role for OprF in root colonization as a requisite for pathogen antagonism, a translational oprF'-'lacZ fusion was weakly regulated by Gac/Rsm despite its high sequence similarity to the hcnA leader. A single base substitution put the modified oprF 5'-UTR under strict control by Gac/Rsm. The results highlight the subtle sequence requirements of Gac/Rsm targets.