The HilA, box and sequences outside it determine the magnitude of HilA-dependent activation of PprgH from Salmonella pathogenicity island 1

Salmonella requires genes on the Salmonella pathogenicity island I (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 P-prgH* 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. P-prgH also contains a HilA box, and here we investigate its role at P-prgH*. We have found that the HilA box is necessary, but not sufficient, for HilA-dependent activation of P-prgH*. Instead, half-site-like hexamers outside the HilA box appear to be required for HilA-dependent activation of P-prgH, even though HilA binds to the HilA box in the absence of these hexamers. Thus, although HilA-dependent activation of PinvF-1 and P-prgH 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.