Construction of Pseudomonas sp. M18 qscR- mutant and its regulation on biosynthesis of PCA and Plt

Abstract:

Gram-negative bacteria, global regulator QscR controls the expression of many virulence determinants, secondary metabolites, stationary phase genes and genes involved in biofilm formation through quorum sensing (QS) systems. QscR binds the promoter region of target genes and regulates the gene expression at the transcriptional level. Using homologous recombination technique a chromosomal qscR inactivated mutant strain M-18Q was constructed in Pseudomonas sp. M-18, a strain of plant-growth-promoting rhizobacteria, which could inhibit several soilborn phytopathogens by producing secondary metabolites including phenazine-1-carboxylic acid（PCA）and pyoluteorin (Plt) in one single strain. To further study the effect of QscR on the synthesis of Plt and PCA in the wild type strain M-18, the dynamic curves of Plt and PCA produced respectively by M-18 and M-18Q strains were measured in both KMB and PPM mediums .The synthesis of PCA was much more activated in the mutant than in the wild type both in KMB and PPM mediums. The PCA production in the mutant strain is four-to-six fold over that in the wild type in the PPM medium, reaching 480μg/mL, and three-to-five fold in the KMB medium, reaching 140μg/mL. The synthesis of Plt, however, was not detected in PPM medium and was nearly not influenced by the QscR protein in KMB medium. PCA production was inhibited but Plt biosynthesis was not altered after complementation with qscR gene in trans in the strain of M-18Q. The regulation of qscR gene on PCA production was further confirmed by the analysis of β-galactosidase activities from the translational phzA‘-'lacZ fusion, in which phzA is the first enzyme gene of the phenazine biosynthesis pathway. These results indicate that QscR can control PCA production negatively but not Plt production in M-18，and show that QscR functions as a global regulator to differently regulate the synthesis of PCA and Plt on the gene expression level.