The transcriptional regulator HlyU positively regulates expression of exsA, leading to type III secretion system 1 activation in Vibrio parahaemolyticus

Vibrio parahaemolyticus is a marine bacterium that is globally recognized as the leading cause of seafood-borne gastroenteritis. V. parahaemolyticus uses various toxins and two type 3 secretion systems (T3SS-1 and T3SS-2) to subvert host cells during infection. We previously determined that V. parahaemolyticus T3SS-1 activity is upregulated by increasing the expression level of the master regulator ExsA under specific growth conditions. In this study, we set out to identify V. parahaemolyticus genes responsible for linking environmental and growth signals to exsA gene expression. Using transposon mutagenesis in combination with a sensitive and quantitative luminescence screen, we identify HlyU and H-NS as two antagonistic regulatory proteins controlling the expression of exsA and, hence, T3SS-1 in V. parahaemolyticus. Disruption of hns leads to constitutive unregulated exsA gene expression, consistent with its known role in repressing exsA transcription. In contrast, genetic disruption of hlyU completely abrogated exsA expression and T3SS-1 activity. A V. parahaemolyticus hlyU null mutant was significantly deficient for T3SS-1-mediated host cell death during in vitro infection. DNA footprinting studies with purified HlyU revealed a 56-bp protected DNA region within the exsA promoter that contains an inverted repeat sequence. Genetic evidence suggests that HlyU acts as a derepressor, likely by displacing H-NS from the exsA promoter, leading to exsA gene expression and appropriately regulated T3SS-1 activity. Overall, the data implicate HlyU as a critical positive regulator of V. parahaemolyticus T3SS-1-mediated pathogenesis.