Pseudomonas fuscovaginae UPB0736 Transcriptome

Pseudomonas fuscovaginae, a major rice pathogen, possesses two archetypical quorum sensing systems, PfsI/R and PfvI/R, both of which are inactive under in vitro conditions, but likely active in planta. Previously, a random Tn5 mutant with transposon insertion between the pfsI and pfsR genes was found to have a hyperactive PfsI/R system. This intergenic region contains coding sequences for RsaM, a putative protein that has since been hypothesized to play a central role in imposing repression on the PfsI/R system. In this model, RsaM represents a switch element likely to respond to an unknown signal that unlocks the PfsI/R system. In the present study, we aimed to confirm that RsaM is indeed expressed and determine its place within the PfsI/R regulatory circuit. We show that RsaM is not produced in the wild type P. fuscovaginae, and does not contribute to keeping the PfsI/R system shut down. On the other hand, the expression of RsaM is triggered upon targeted introduction of a large insertion in the pfsR/rsaM intergenic region, and accompanies increased levels of both the pfsI and pfsR transcription. Moreover, we demonstrate that RsaM acts as a negative modulator of the PfsI/R circuit in this genetic background, although its molecular mechanism of action remains unknown. Taken together, our results evidence that RsaM acts as a built-in modulator that prevents overactivation of an already derepressed PfsI/R system, rather than a switch.