Lon protease downregulates phenazine-1-carboxamide biosynthesis by degrading the quorum sensing signal synthase PhzI and exhibits negative feedback regulation of Lon itself in Pseudomonas chlororaphis HT66.

Pseudomonas chlororaphis HT66 exhibits strong antagonistic activity against various phytopathogenic fungi due to its main antibiotic phenazine-1-carboxamide (PCN). PCN gene cluster consists of phzABCDEFG, phzH, phzI, and phzR operons. phzABCDEFG transcription is activated by the PhzI/R quorum sensing system. Deletion of the lon gene encoding an ATP-dependent protease resulted in significant enhancement of PCN production in strain HT66. However, the regulatory pathway and mechanism of Lon on PCN biosynthesis remain unknown. Here, lon mutation was shown to significantly improve antimicrobial activity of strain HT66. The N-acyl-homoserine lactone synthase PhzI mediates the negative regulation of PCN biosynthesis and phzABCDEFG transcription by Lon. Western blot showed that PhzI protein abundance and stability were significantly enhanced by lon deletion. The in vitro degradation assay suggested that Lon could directly degrade PhzI protein. However, Lon with an amino acid replacement (S -A) could not degrade PhzI protein. Lon-recognized region was located within the first 50 amino acids of PhzI. In addition, Lon formed a new autoregulatory feedback circuit to modulate its own degradation by other potential proteases. In summary, we elucidated the Lon-regulated pathway mediated by PhzI during PCN biosynthesis and the molecular mechanism underlying the degradation of PhzI by Lon in P. chlororaphis HT66.