Identification and characterization of LuxR solo homolog PplR in pathogenic Pseudomonas plecoglossicida NB2011

<i>Pseudomonas plecoglossicida</i> is a causative agent of visceral granulomas in large yellow croaker (<i>Larimichthys crocea</i>). Quorum sensing (QS) is widely involved in imparting virulence to pathogenic bacteria; however, it has not been studied in <i>P. plecoglossicida</i>. In this study, we annotated a LuxR family transcriptional regulator in <i>P. plecoglossicida</i> NB2011 and designated as PplR. We aligned the protein sequence by BlastP and Clustal X2, monitored the N-acyl-homoserine lactone (AHL) signal production through cross-feeding bioassay and HC-MS/MS; investigated exogenous AHL signal binding by recombinant expression and thin layer chromatography; constructed a deletion mutant of the target gene by method of double homologous recombination; sequenced the transcript RNA and analyzed the data; additionally, characterized phenotypes of wild type and mutant strain. The LuxR homolog PplR was found to share high similarity with PpoR-the LuxR solo of <i>Pseudomonas putida</i>-without a cognate LuxI. The wild-type strain did not produce any AHL signals and the recombinant LuxR protein was found to bind C6-L-homoserine lactone (C6-HSL), C8-HSL, 3-oxo-C10-HSL, and 3-oxo-C12-HSL. RNA-seq analysis indicated 84 differentially expressed genes-5 upregulated and 79 downregulated-mainly enriched in gene ontology terms, such as flagella-dependent motility, integral component of membrane, DNA binding and transcription, and metal ion binding, suggesting that PplR is a master transcription regulator. The mutant strain showed attenuated biofilm-forming ability and stress resistance, and the data support a role for PplR in the regulation of these traits in <i>P. plecoglossicida</i> NB2011 independent of the presence of AHL signals. This is the first study to provide QS-related information on <i>P. plecoglossicida</i>.