ExsE Is a Negative Regulator for T3SS Gene Expression in .

Type III secretion systems (T3SSs) contribute to microbial pathogenesis of species, but the regulatory mechanisms are complex. We determined if the classic ExsACDE protein-protein regulatory model from applies to . Deletion mutants in demonstrated that, as expected, the T3SS is positively regulated by ExsA and ExsC and negatively regulated by ExsD and ExsE. Interestingly, deletion of enhanced the ability of to induce host-cell death while cytotoxicity was inhibited by complementation of this gene in a wild-type strain, a result that differs from a similar experiment with ExsE. We further showed that ExsE is a secreted protein that does not contribute to adhesion to Fathead minnow epithelial cells. An co-immunoprecipitation assay confirmed that ExsE binds to ExsC to exert negative regulatory effect on T3SS genes. T3SS in can be activated in the absence of physical contact with host cells and a separate regulatory pathway appears to contribute to the regulation of ExsA. Consequently, like ExsE from , ExsE is a negative regulator for T3SS gene expression in . Unlike the orthologue, however, deletion of from enhanced cytotoxicity.