Multiple histidines in the periplasmic domain of the Salmonella enterica sensor kinase SsrA enhance signaling in response to extracellular acidification

Abstract

SummaryThe two-component regulatory system SsrA-SsrB in Salmonella enterica controls expression of a virulence gene program required for intracellular survival in host cells. SsrA signaling is induced within the acidic host vacuole in which the bacteria reside; however, the mechanism by which SsrA senses this intracellular environment is unknown. Here, we show that the periplasmic sensor domain of SsrA is enriched in histidine residues that increase SsrA signaling below external pH of 6. While no single histidine accounted for the full acid-responsiveness of SsrA, we localized the acid-responsiveness principally to five histidines in the C-terminal end of the periplasmic sensor domain, with input from additional histidines in the N-terminal end of the senor. A sensor mutant lacking critical pH-responsive histidines was defective for acid-promoted activity, yet retained basal activity similar to wild type at neutral pH, indicating that the role of these histidines is to enhance signaling in response to acidification. In support of this, a pH-blind mutant was insensitive to the vacuole acidification blocking activity of bafilomycin, and was attenuated for competitive fitness during infection of mice. Our data demonstrate that SsrA contains a histidine-rich periplasmic sensor that enhances signaling in response to the innate host defense of vacuolar acidification.

Bacterial Stress Responses during Host Infection

“…Several of these regulators, in particular PhoP, OmpR and σ S , also control the expression of genes required for Salmonella virulence (table 1) (Fang et al, 1992; Belden and Miller, 1994; Lee et al, 2000). Additionally, ionizable imidazole and carboxylic groups in His, Asp and Glu residues endow some transcriptional regulators such as ArsS, CadC, EvgS, PhoQ, PmrB and SsrA with the capacity to sense acid pH (Haneburger et al, 2011; Mulder et al, 2015; Muller et al, 2009; Perez and Groisman, 2007; Prost et al, 2007) (Figure 2A). Some of the adaptive responses to acid pH initiated by the PhoQ/PhoP and EvgS/EvgA TCS work via transcriptional activation of the iraP and iraM genes encoding anti-adaptor proteins, which outcompete σ S for binding to the response regulatory RssB, thus preventing RssB-triggered σ S degradation by the ClpXP protease (Bearson et al, 1996; Eguchi et al, 2011; Tu et al, 2006).…”

A macrophage-based screen identifies antibacterial compounds selective for intracellularSalmonellaTyphimurium

“…Low pH in the intracellular environment is a critical signal for multiple two-component regulatory systems in S . Tm (PhoPQ, PmrAB, SsrAB) 49-51 , such that its detection acts as a cue for virulence gene expression. Given the importance of low pH as a regulatory input governing S .…”

Regulatory Evolution Drives Evasion of Host Inflammasomes by Salmonella Typhimurium

“…Detailed methods are provided in the online version of this paper and include the following: STM DssrAB was generated in Mulder et al, 2015. pWSK129-PssrA-ssrB D56E was generated in Pé rez-Morales et al, 2017 and transformed into STM DssrAB and SBG to generate SsrB + strains.…”

“…Co-inherited with SPI-2 was SsrA-SsrB, a two-component regulatory system (TCS) that controls the expression of SPI-2 genes following intracellular invasion. Under acidic conditions, the SsrA sensor kinase phosphorylates the SsrB response regulator, which then binds to the regulatory region of target genes to regulate their expression directly, or through counter-silencing of H-NS (Mulder et al, 2015;Tomljenovic-Berube et al, 2010;Walthers et al, 2007Walthers et al, , 2011. We and others have shown that genes outside SPI-2 have also been captured into the SsrB regulon through mutations in regulatory DNA (Osborne et al, 2009;Pé rez-Morales et al, 2017;Tomljenovic-Berube et al, 2010;Worley et al, 2000).…”