The Multifaceted Activity of the VirF Regulatory Protein in the Shigella Lifestyle

Abstract

Shigella is a highly adapted human pathogen, mainly found in the developing world and causing a severe enteric syndrome. The highly sophisticated infectious strategy of Shigella banks on the capacity to invade the intestinal epithelial barrier and cause its inflammatory destruction. The cellular pathogenesis and clinical presentation of shigellosis are the sum of the complex action of a large number of bacterial virulence factors mainly located on a large virulence plasmid (pINV). The expression of pINV genes is controlled by multiple environmental stimuli through a regulatory cascade involving proteins and sRNAs encoded by both the pINV and the chromosome. The primary regulator of the virulence phenotype is VirF, a DNA-binding protein belonging to the AraC family of transcriptional regulators. The virF gene, located on the pINV, is expressed only within the host, mainly in response to the temperature transition occurring when the bacterium transits from the outer environment to the intestinal milieu. VirF then acts as anti-H-NS protein and directly activates the icsA and virB genes, triggering the full expression of the invasion program of Shigella. In this review we will focus on the structure of VirF, on its sophisticated regulation, and on its role as major player in the path leading from the non-invasive to the invasive phenotype of Shigella. We will address also the involvement of VirF in mechanisms aimed at withstanding adverse conditions inside the host, indicating that this protein is emerging as a global regulator whose action is not limited to virulence systems. Finally, we will discuss recent observations conferring VirF the potential of a novel antibacterial target for shigellosis.

One Gene and Two Proteins: a Leaderless mRNA Supports the Translation of a Shorter Form of the Shigella VirF Regulator

“…The complex regulatory cascade for activation of the Shigella virulence genes depends on the VirF protein (7). VirF is at the heart of the switch from the noninvasive to the invasive phenotype.…”

“…In a cascade model, VirF triggers activation of the virB and icsA genes. IcsA affects bacterial intracellular spreading, and VirB promotes expression of several virulence genes, including those encoding a type III secretion system (T3SS), its effectors, and the last regulator of the cascade, MxiE (6, 7). Interestingly, MxiE, another AraC-like TF, appears to rely on high-level transcriptional slippage to generate its reading frame from two separate open reading frames (ORFs) (8).…”

“…By binding upstream of the virF promoter between two H-NS sites, VirB also counteracts H-NS-dependent repression of virF transcription (16). Expression of virF is further modulated by integration host factor (IHF) (17) and other environmental factors, such as pH and osmolarity (7), and is affected by tRNA modifications (18). …”

The Intriguing Evolutionary Journey of Enteroinvasive E. coli (EIEC) toward Pathogenicity

“…Other genes encoding proteins crucial for the invasive process cover the pINV plasmid including the OspG and OspF proteins which interfere with the host innate immune response ( Kim et al, 2005 ; Arbibe et al, 2007 ), the PhoN2 protein required for IcsA localization ( Scribano et al, 2014 ), and the IpaH proteins which interfere with the host protein degradation ( Ashida and Sasakawa, 2015 ; Figure 1 ). Moreover, in contrast to the other two virulence regulatory genes ( virB and mxiE ), the virF gene, coding for the primary virulence regulator, is located on a “desert island” surrounded by several IS sequences and far away from all other virulence genes, including those under its direct control, virB and icsA ( Di Martino et al, 2016a ). While the CG content of virF is only slightly lower as compared to that of the entry region ( Buchrieser et al, 2000 ), its position suggests that it has been acquired independently to promote the expression of the virulence genes.…”

VirF Relieves the Transcriptional Attenuation of the Virulence Gene icsA of Shigella flexneri Affecting the icsA mRNA–RnaG Complex Formation

“…The VirF protein (30 kDa) of Shigella , encoded on the primary pathogenicity island carried by the large virulence plasmid, pINV, is the master activator leading to the invasivity phenotype (Di Martino et al, 2016a). Transcription of virF is thermoregulated and occurs only above the critical temperature of 32–34°C to prevent the expression of the virulence genes outside the host (Falconi et al, 1998).…”