Long-Distance Effects of H-NS Binding in the Control of Expression in the Salmonella SPI1 Locus.

Salmonella enterica serovar Typhimurium utilizes a type three secretion system (T3SS) carried on the Salmonella pathogenicity island 1 (SPI1) to invade intestinal epithelial cells and induce inflammatory diarrhea. HilA activates expression of the T3SS structural genes. Expression of yper nvasion ocus A () is controlled by the transcription factors HilD, HilC, and RtsA, which act in a complex feed-forward regulatory loop. The nucleoid-associated protein H-NS is a xenogeneic silencer that has a major effect on SPI1 expression. In this work, we use genetic techniques to show that disruptions of the chromosomal region surrounding have a effect on H-NS-mediated repression of the promoter; this effect occurs asymmetrically over ∼4 kb spanning the intergenic region. CAT cassettes inserted at various positions in this region are also silenced in relation to the proximity to the promoter. We identify a putative H-NS nucleation site, and its mutation results in derepression of the locus. Furthermore, we genetically show that HilD abrogates H-NS-mediated silencing to activate the promoter. In contrast, H-NS-mediated repression of the promoter, downstream of , is through its control of HilD, which directly activates transcription. Likewise, activation of the promoter, although in a region silenced by H-NS, is strictly dependent on HilA. In summary, we propose a model in which H-NS nucleates within the promoter region to polymerize and exert its repressive effect. Thus, H-NS-mediated repression of SPI1 is primarily through the control of expression, with HilD capable of overcoming H-NS to autoactivate. Members of the foodborne pathogen Salmonella rely on a type III secretion system to invade intestinal epithelial cells and initiate infection. This system was acquired through horizontal gene transfer, essentially creating the Salmonella genus. Expression of this critical virulence factor is controlled by a complex regulatory network. The nucleoid protein H-NS is a global repressor of horizontally acquired genomic loci. Here, we identify the critical site of H-NS regulation in this system and show that alterations to the DNA over a surprisingly large region affect this regulation, providing important information regarding the mechanism of H-NS action.