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.

Keywords: AraC proteins; DNA binding proteins; Shigella; VirF; antivirulence therapy; bacterial virulence; pathogenic E. coli; transcriptional regulators.

Figure 1

Centrality of VirF in the pINV regulatory cascade of Shigella. The control of VirF occurs at transcriptional and translation levels. The major proteins involved in the regulation of VirF in response to environmental stimuli and nutrient conditions are indicated. Once synthesized, VirF activates the icsA and virB genes and represses the synthesis of the sRNA RnaG. VirB, the second regulator, then activates several operons, including those for a T3SS system and for the early effectors. VirB activates also the last regulator, MxiE, which in association with IpgC activates the late effectors. TCS, two component system.

Figure 2

Temperature-dependent regulation of the promoter of the virF gene. The virF promoter contains two H-NS (indicated in green) and four FIS (indicated in orange) binding sites (Falconi et al., 1998, 2001). The arrow on the top of the figure indicates the transition from the outer environment (colder) to the intestinal host milieu (warmer). Left panel: At lower temperature a compact structure exists where H-NS represses virF by interacting with H-NS binding sites I and II which are separated by a DNA region whose sequence-mediated curvature is favored by the lower temperature. Middle panel: The intrinsic bend located between the two H-NS binding sites melts abruptly around 32°C and the bending center slides downstream unmasking a binding site for FIS. Right panel: By increasing the temperature the DNA bend relaxes further, H-NS interactions to binding sites I and II weaken and FIS gains easier access to its binding boxes, thus counteracting H-NS repression. Altogether these events lead to the formation of an active transcription complex.