[Cloning of phzIR from the endophytic Pseudomonas sp. G5 and its expression in Escherichia coli]

We isolated a new strain of endophytic Pseudomonas G5 from the stems of Chinese parsley (Coriandrum sativum L.), and it is tentatively identified as Pseudomonas aurantiaca according to analysis of the entire substrate utilization profiles using BIOLOG Microstation system (BIOLOG, Inc, Hayward CA). An array of evidence established that many Gram-negative bacteria employ Quorum sensing (QS) system to regulate gene expression in response to cell density using small diffusible signal molecules, N-acyl homoserine lactones (AHLs), and control diverse phenotypic traits in plant-associated bacteria. In this study, we showed that Pseudomonas sp. strain G5 can produce several types of AHLs at a detectable level using Thin Layer Chromatography (TLC) analysis combined with bioreporter Chromobacterium violaceum CV026 bioassay, and N-hexanoyl-homoserine lactone (HHL, C6-HSL) with Rf value 0.4 is the major signal molecule. Furthermore, we have identified its quorum sensing system composed of PhzI and PhzR by cloning and sequencing of phzI-phzR. PhzI is responsible for synthesis of AHLs signal molecules, and PhzR is a transcriptional regulator. Finally, we heterologously expressed the recombinant plasmid pMD-phzIR in Escherichia coli JM109 and verified it using C. violaceum CV026 bioassay. The phylogenetic analysis using MEGA4 revealed highly similarities exist among the phzIR homologs, suggesting it is evolutionary well conserved in the genus Pseudomonas.