Characterization of quorum sensing-controlled transcriptional regulator MarR and Rieske (2Fe-2S) cluster-containing protein (Orf5), which are involved in resistance to environmental stresses in Pseudomonas syringae pv. tabaci 6605

Role of Two Sets of RND-Type Multidrug Efflux Pump Transporter Genes, mexAB-oprM and mexEF-oprN, in Virulence of Pseudomonas syringae pv. tabaci 6605

Pseudomonas syringae pv. tabaci 6605 has two multidrug resistance (MDR) efflux pump transporters, MexAB-OprM and MexEF-OprN. To understand the role of these MDR efflux pumps in virulence, we generated deletion mutants, ∆mexB, ∆mexF, and ∆mexB∆mexF, and investigated their sensitivity to plant-derived antimicrobial compounds, antibiotics, and virulence. Growth inhibition assays with KB soft agar plate showed that growth of the wild-type (WT) was inhibited by 5 µl of 1 M catechol and 1 M coumarin but not by other plant-derived potential antimicrobial compounds tested including phytoalexins. The sensitivity to these compounds tended to increase in ∆mexB and ∆mexB∆mexF mutants. The ∆mexB∆mexF mutant was also sensitive to 2 M acetovanillone. The mexAB-oprM was constitutively expressed, and activated in the ∆mexF and ∆mexB∆mexF mutant strains. The swarming and swimming motilities were impaired in ∆mexF and ∆mexB∆mexF mutants. The flood inoculation test indicated that bacterial populations in all mutant strains were significantly lower than that of WT, although all mutants and WT caused similar disease symptoms. These results indicate that MexAB-OprM extrudes plant-derived catechol, acetovanillone, or coumarin, and contributes to bacterial virulence. Furthermore, MexAB-OprM and MexEF-OprN complemented each other's functions to some extent.

MexEF-OprN multidrug efflux pump transporter negatively controls N-acyl-homoserine lactone accumulation in pseudomonas syringae pv. Tabaci 6605

Our previous studies revealed that flagellar-motility-defective mutants such as ∆fliC of Pseudomonas syringae pv. tabaci 6605 (Pta6605) have remarkably reduced production of N-acyl-homoserine lactones (AHL), quorum-sensing molecules. To investigate the reason of loss of AHL production in ∆fliC mutant, we carried out transposon mutagenesis. Among approximately 14,000 transconjugants, we found 11 AHL production-recovered (APR) strains. In these APR strains, a transposon was inserted into either mexE or mexF, genes encoding for the multidrug efflux pump transporter MexEF-OprN, and mexT, a gene encoding a putative transcriptional activator for mexEF-oprN. These results suggest that MexEF-OprN is a negative regulator of AHL production. To confirm the negative effect of MexEF-OprN on AHL production, loss- and gain-of-function experiments for mexEF-oprN were carried out. The ∆fliC∆mexF and ∆fliC∆mexT double mutant strains recovered AHL production, whereas the mexT overexpressing strain abolished AHL production, although the psyI, a gene encoding AHL synthase, is transcribed as wild type. Introduction of a mexF or mexT mutation into another flagellar-motility- and AHL production-defective mutant strain, ∆motCD, also recovered the ability to produce AHL. Furthermore, introduction of the mexF mutation into other AHL production-defective mutant strains such as ∆gacA and ∆aefR also recovered AHL production but not to the ∆psyI mutant. These results indicate that MexEF-OprN is a decisive negative determinant of AHL production and accumulation.