AraC/XylS family members, HilC and HilD, directly bind and derepress the Salmonella typhimurium hilA promoter

During infection, Salmonella enterica serovar Typhimurium colonizes the sma ll intestine of its hosts. This process requires a type III secretion syste m encoded by several genes on Salmonella pathogenicity island 1 (SPI1), a 4 0 kb region of DNA near centisome 63 of the Salmonella chromosome. SPI1 gen e expression is controlled by a complex regulatory cascade. HilA, a member of the OmpR/ToxR family of transcriptional regulators, directly activates t he expression of two SPI1 operons encoding type III apparatus components. h ilA transcription is repressed by many environmental conditions and regulat ory mutations. This repression requires an upstream repressing sequence (UR S) located between -314 and -68 relative to the hilA transcription start si te. The repressing activity of the URS is counteracted by two AraC/XylS fam ily members named HilC and HilD. We show that HilC and HilD bind directly t o the hilA promoter region in vitro. We also provide evidence that HilC and HilD bind to the same or overlapping sites within the URS. Our data are co nsistent with a model in which HilC and HilD derepress hilA expression by b inding directly to the URS and counteracting its repressing effect in vivo.