NrtR Regulates the Type III Secretion System Through cAMP/Vfr Pathway in .

The type III secretion system (T3SS) plays an important role in the pathogenesis of . Expression of the T3SS is controlled under a complicate regulatory network. In this study, we demonstrate that NrtR (PA4916) is involved in the T3SS expression and pathogenesis of in a mouse acute pneumonia model. Overexpression of the T3SS central activator ExsA or exogenous supplementation of cAMP restored the expression of T3SS in the Δ mutant, suggesting that NrtR might regulate T3SS through the cAMP-Vfr signaling pathway. Further experiments demonstrated that the decrease of cAMP content is not due to the expression change of adenylate cyclases or phosphodiesterase in the Δ mutant. As it has been shown that is upregulated in the Δ mutant, we overexpressed in wild type PAK, which reduced the intracellular cAMP level and the expression of the T3SS genes. Meanwhile, deletion of in the Δ mutant restored the expression and secretion of the T3SS. Co-immunoprecipitation assay revealed an interaction between NadD2 and the catalytic domain of the adenylate cyclase CyaB. Further assay indicated that NadD2 repressed the enzymatic activity of CyaB. Therefore, we have identified a novel regulatory mechanism of T3SS in .