Integration host factor alleviates H-NS silencing of the Salmonella enterica serovar Typhimurium master regulator of SPI1, hilA

Abstract

Coordination of the expression ofSalmonella entericainvasion genes onSalmonellapathogenicity island 1 (SPI1) depends on a complex circuit involving several regulators that converge on expression of thehilAgene, which encodes a transcriptional activator (HilA) that modulates expression of the SPI1 virulence genes. Two of the global regulators that influencehilAexpression are the nucleoid-associated proteins Hha and H-NS. They interact and form a complex that modulates gene expression. A chromosomal transcriptional fusion was constructed to assess the effects of these modulators onhilAtranscription under several environmental conditions as well as at different stages of growth. The results obtained showed that these proteins play a role in silencinghilAexpression at both low temperature and low osmolarity, irrespective of the growth phase. H-NS accounts for the main repressor activity. At high temperature and osmolarity, H-NS-mediated silencing completely ceases when cells enter the stationary phase, andhilAexpression is induced. Mutants lacking IHF did not inducehilAin cells entering the stationary phase, and this lack of induction was dependent on the presence of H-NS. Band-shift assays andin vitrotranscription data showed that forhilAinduction under certain growth conditions, IHF is required to alleviate H-NS-mediated silencing.