Hha is a negative modulator of transcription of hilA, the Salmonella enterica serovar Typhimurium invasion gene transcriptional activator

An early step in the establishment of Salmonella enterica serovar Typhimuri um murine infection is the penetration of the intestinal mucosa of the smal l intestine. The majority of the genes responsible for the Salmonella invas ive phenotype are encoded on Salmonella pathogenicity island 1, and their t ranscription is controlled by the hilA transcriptional activator. The expre ssion of hilA is regulated by environmental signals including oxygen, osmol arity, pH, and growth phase such that the presence of any one suboptimal co ndition results in repression of hilA expression and the invasive phenotype . We have conducted a search for negative regulators of hilA by introductio n of a Salmonella enterica serovar Typhimurium chromosomal DNA gene bank in to a Salmonella enterica serovar Typhimurium hil4::Tn5lacZY reporter strain . This screen has identified the hha gene as a regulator that exerts a nega tive influence on hil4 expression. Plasmid-encoded hha significantly reduce s hilA::Tn5lacZY chromosomal expression, as well as expression of the invas ion genes invF, prgH, and sipC. An hha null mutation results in substantial derepression of both chromosomally encoded and plasmid-encoded hilA::Tn5la cZY expression. Introduction of plasmid-encoded hha into strain SL1344 resu lts in attenuation of invasion using in vitro and in vivo assays. Important ly, purified Hha protein was found to bind to a hilA DNA promoter fragment, suggesting that the regulatory activity of the Hha protein occurs at the h ilA promoter. These data add detail to the developing model of the regulati on of Salmonella invasion genes.