Abstract
<jats:p> <jats:italic>Pseudomonas fuscovaginae</jats:italic> , a wide-host-range plant pathogen of several cereal and grass species, possesses two canonical <jats:italic>N</jats:italic> -acyl homoserine (AHL)-based quorum sensing (QS) systems called PfsI/R and PfvI/R, both of which are inactive under laboratory conditions but active in planta. The <jats:italic>pfsI</jats:italic> - <jats:italic>pfsR</jats:italic> intergenic region encodes for RsaM, a putative protein that has since been hypothesized to act as a repressor switch of this QS circuit. In the present study, we demonstrate that the stringent repression exerted on the PfsI/R system depends entirely on the divergent promoter/intergenic <jats:italic>pfsR-rsaM</jats:italic> region rather than RsaM itself. Remarkably, this regulatory switch element stringently represses the expression of both the <jats:italic>pfsR</jats:italic> and <jats:italic>rsaM</jats:italic> genes. We further show for the first time that RsaM is endogenously expressed and functions as a negative regulator modulating the PfsI/R circuit instead of preventing its activation. Taken together, our results evidence a unique two-tiered/hierarchical repression of a QS system, provided by a master repressor switch and a repressor modulator. </jats:p>