Characterization of the Pseudomonas sp DF41 quorum sensing locus and its role in fungal antagonism

Pseudomonas sp. DF41 is able to suppress the fungal pathogen Sclerotinia sclerotiorum through production of a lipopeptide called sclerosin. The aim of this study was to isolate the DF41 QS locus and characterize its role in fungal antagonism. Through screening of a fosmid library, one clone was selected that tested positive for AHL production. Sequence analysis revealed the presence of two QS genes: pdfR and pdfI, encoding a LuxR transcriptional activator and an AHL synthase, respectively. Downstream of pdfI lays a gene encoding a transcriptional activator called RfiA followed by pdfC, comprising part of an efflux locus. Characterization of an AHL-deficient strain revealed it to be phenotypically identical to the wild type. Conversely rfiA, which is co-transcribed with pdfI, is essential for both AF activity and sclerosin production. Using a pdfI-lacZ fusion analysis, we discovered that pdfI is positively autoregulated. Additionally, pdfI expression was markedly increased in the rfiA mutant and quantification of AHL levels revealed elevated intracellular signal accumulation. We hypothesize that RfiA is a positive activator of the downstream efflux pump which serves to export both sclerosin and AHL signals. In a gacS mutant, pdfI-lacZ activity was decreased; however, plasmid-borne rsmZ was able to restore expression. Collectively, our findings indicate that: (i) QS indirectly controls DF41 suppression of Sclerotinia through RfiA; and (ii) pdfI expression and AHL signal production are positively regulated by the Gac-Rsm system. Identification of the PdfRI QS system, RfiA and RsmZ add to the increasingly complex network overseeing expression of DF41 biocontrol factors. (C) 2013 Elsevier Inc. All rights reserved.