Fig. 10. A model describing S-nitrosylation of VbrK mediates virulence of V. parahaemolyticus.

During the early stage of infection, T3SS1 (which is upregulated in vivo compared to in vitro) stimulates the production of proinflammatory cytokines in the intestinal epithelial cells. These proinflammatory cytokines or bacterial infection itself trigger the expression of iNOS in intestinal epithelial cells, which catalyzes the production of NO from l-arginine. NO is rapidly converted to nitrate through the intermediate product peroxynitrite. Nitrate is reduced to nitrite by nitrate reductase in V. parahaemolyticus. Nitrite subsequently induces S-nitrosylation of VbrK, leading to its phosphorylation and phosphate transfer to VbrR. VbrR can bind exsC promoter and such binding results in the decreased expression of exsC, which ultimately leads to the decreased expression of T3SS1. Reduction in T3SS1 expression could reduce the production of proinflammatory cytokine, leading to robust colonization and virulence. Mutation of the S-nitrosylation site C86 of VbrK leads to the inability of nitrite to reduce T3SS1 gene expression and thus stronger proinflammatory response was induced. Strong proinflammatory response could reduce bacterial colonization and virulence at the later stage of infection.