Salmonella requires genes on the Salmonella pathogenicity island 1 (SPI1) for the intestinal phase of infection in several models of pathogenesis. In Salmonella enterica serovar Typhimurium, most SPI1 genes are arranged in operons that are coordinately regulated by the SPI1-encoded protein HilA. In the past, it has been shown that HilA directly activates two promoters on SPI1, PinvF-1 and PprgH. PinvF-1 contains a HilA binding site, termed a HilA box, that is necessary and sufficient for activation by HilA. The HilA box is 17 nucleotides long and contains a direct repeat comprised of two hexamers separated by 5 nucleotides, centered at −45 relative to the start site of transcription. PprgH also contains a HilA box, and here we investigate its role at PprgH. We have found that the HilA box is necessary, but not sufficient, for HilA-dependent activation of PprgH. Instead, half-site-like hexamers outside the HilA box appear to be required for HilA-dependent activation of PprgH, even though HilA binds to the HilA box in the absence of these hexamers. Thus, although HilA-dependent activation of PinvF-1 and PprgH coordinates the expression of the structural genes for a type III secretion apparatus and the effectors secreted by that apparatus, it is also possible that mechanisms not apparent under in vitro inducing conditions could separate the expression of invFGEABC-spaMNOPQRS-sicA-sipBCDA-iacP-sicP-sptP and prgHIJK-orgABC
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