Identification, synthesis and regulatory function of the N-acylated homoserine lactone signals produced by Pseudomonas chlororaphis HT66

Background: Pseudomonas chlororaphis HT66 isolated from the rice rhizosphere is an important plant growth-promoting rhizobacteria that produce phenazine-1-carboxamide (PCN) in high yield. Phenazine production is regulated by a quorum sensing (QS) system that involves the N-acylated homoserine lactones (AHLs)-a prevalent type of QS molecule. Results: Three QS signals were detected by thin layer chromatography (TLC) and high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS), which identified to be N-(3-hydroxy hexanoyl)-l-homoserine lactone (3-OH-C6-HSL), N-(3-hydroxy octanoyl)-l-homoserine lactone (3-OH-C8-HSL) and N-(3-hydroxy decanoyl)-l-homoserine lactone (3-OH-C10-HSL). The signal types and methods of synthesis were different from that in other phenazine-producing Pseudomonas strains. By non-scar deletion and heterologous expression techniques, the biosynthesis of the AHL-signals was confirmed to be only catalyzed by PhzI, while other AHLs synthases i.e., CsaI and HdtS were not involved in strain HT66. In comparison to wild-type HT66, PCN production was 2.3-folds improved by over-expression of phzI, however, phzI or phzR mutant did not produce PCN. The cell growth of HT66δphzI mutant was significantly decreased, and the biofilm formation in phzI or phzR inactivated strains of HT66 decreased to various extents. Conclusion: In conclusion, the results demonstrate that PhzI-PhzR system plays a critical role in numerous biological processes including phenazine production.