Peptide chain release factor methyltransferase gene hemK regulates multiple bacterial phenotypes and is essential for the pathogenicity of Pseudomonas plecoglossicida

Pseudomonas plecoglossicida is a major pathogen that causes visceral white nodules disease (VWND), a highly devastating disease, in cultured large yellow croaker (Larimichthys crocea), leading to severe economic losses. Peptide chain release factor methyltransferase hemK in bacteria plays a role in pathogenicity, but its exact function in P. plecoglossicida remains unclear. In this study, a P. plecoglossicida mutant strain (Delta hemK) was constructed by deleting hemK, and the roles of hemK in regulating P. plecoglossicida virulence were thereafter investigated. Compared to the wildtype strain, cell motility and biofilm formation of Delta hemK were weaken, and extracellular protein profile was altered. In vitro study showed that the hemK deletion significantly decreased the survival rate of P. plecoglossicida in large yellow croaker macrophage cell line (LYC-FM) by impairing oxidative stress tolerance. Furthermore, large yellow croakers infected with Delta hemK showed a significant reduction in mortality rate, bacterial load, and noticeable nodules within the spleen, when compared to the wild-type strain. These findings indicated that hemK plays an essential role in the pathogenicity of P. plecoglossicida, thus providing a valuable insight into the pathogenic mechanisms of P. plecoglossicida and offering a potential target for its attenuated-vaccine development.