The Salmonella Spi1 Virulence Regulatory Protein HilD Directly Activates Transcription of the Flagellar Master OperonflhDC

Abstract

cInfection of intestinal epithelial cells is dependent on the Salmonella enterica serovar Typhimurium pathogenicity island 1 (Spi1)-encoded type III injectisome system and flagellar motility. Thus, the expression of virulence and flagellar genes is subject to tight regulatory control mechanisms in order to ensure the correct spatiotemporal production of the respective gene products. In this work, we reveal a new level of cross-regulation between the Spi1 and flagellar regulatory systems. Transposon mutagenesis identified a class of mutants that prevented flhDC autorepression by overexpressing HilD. HilD, HilC, RtsA, and HilA comprise a positive regulatory circuit for the expression of the Spi1 genes. Here, we report a novel transcriptional cross talk between the Spi1 and flagellar regulons where HilD transcriptionally activates flhDC gene expression by binding to nucleotides ؊68 to ؊24 upstream from the P5 transcriptional start site. We additionally show that, in contrast to the results of a previous report, HilA does not affect flagellar gene expression. Finally, we discuss a model of the cross-regulation network between Spi1 and the flagellar system and propose a regulatory mechanism via the Spi1 master regulator HilD that would prime flagellar genes for rapid reactivation during host infection.

InvS Coordinates Expression of PrgH and FimZ and Is Required for Invasion of Epithelial Cells by Salmonella enterica serovar Typhimurium

“…The deletion of invS impairs Salmonella motility, suggesting that InvS might function to promote motility to facilitate bacterial invasion. Consistent with this notion, previous reports found that HilD activates the transcription of flagellar genes while HilA does not (31). Our data show that the overexpression of HilD rescued the ΔinvS invasion defect while HilA did not alter the invasion levels.…”

“…This may indicate that InvS is able to regulate invasion in a HilD-dependent but HilA-independent pathway. Singer et al have demonstrated that HilD directly activates the expression of flagellar genes, while HilA does not affect flagellar gene expression (31); this is consist with our data showing that the deletion of InvS results in a decrease of flagellar expression. Furthermore, it was reported HilD is able to activate the transcription of invF from a promoter that is far upstream of its HilA-dependent promoter.…”

HilD Induces Expression of Salmonella Pathogenicity Island 2 Genes by Displacing the Global Negative Regulator H-NS from ssrAB

“…HilD-binding sites have been determined upstream of the promoters of hilA, hilC, hilD, rtsA, and flhDC; these sites present a high AT content and show a high degree of degeneracy between them (24,26,27,31). However, HilD regulates invF by binding to regions located both upstream and downstream of the HilC/Ddependent invF promoter (28,60).…”

“…HilD also directly controls the expression of the SPI-1 genes hilD, hilC, and invF, as well as other acquired and ancestral genes located outside SPI-1, such as rtsA, flhDC, siiA, lpxR, ytfK, STM14_1282, and STM14_2342 (2,(25)(26)(27)(28)(29)(30)(31). Previously, we demonstrated that HilD directly induces the expression of the ssrAB operon, which is located in SPI-2 and codes for the SsrA/B twocomponent system, the central positive regulator of SPI-2, thus establishing a transcriptional cross talk between SPI-1 and SPI-2 (21).…”

A type 6 secretion system (T6SS) encoded gene within Salmonella enterica serovar Enteritidis contributes to virulence

“…Moreover, SPI-1 genes exhibit reduced expression, most notably the master regulator of SPI-1, hilD. HilD also activates flhDC, whose gene products are required for activation of the flagellar class 2 promoters [22,23]. Flagellin is a pathogen-associated molecular patterns (PAMPs) that is recognized by the toll-like receptor 5 (TLR5).…”