Site-specific modification of Shigella flexneri virF mRNA by tRNA-guanine transglycosylase in vitro

Shigella flexneri is an enteropathogen responsible for severe dysentery in humans. VirF is a key transcriptional regulator that activates the expression of the downstream virulence factors required for cellular invasion and cell-to-cell spread of this pathogen. There are several environmental factors that induce the translation of VirF including temperature, pH, osmolarity and post-transcriptional RNA modification. Durand and colleagues (vacC, a virulence-associated chromosomal locus of Shigella flexneri, is homologous to tgt, a gene encoding tRNA-guanine transglycosylase of Escherichia coli K-12. J. Bacteriol., 176, 4627-4634) have demonstrated a correlation between VirF and tRNA-guanine transglycosylase (TGT), which catalyzes the exchange of the hypermodified base queuine for the guanine in the wobble position of certain tRNAs. They characterized tgt-mutant S. flexneri strains in which the translation of VirF is markedly reduced and the bacteria are unable to invade host cells. Although the function of TGT is to modify tRNA, we report that the virF mRNA is recognized by the Escherichia coli TGT (99% identity to the S. flexneri TGT) in vitro. Further, we show that this recognition results in the site-specific modification of a single base in the virF mRNA. In the context of previous reports that small molecule binding motifs ('riboswitches') in mRNAs modulate mRNA conformation and translation, our observations suggest that TGT may modulate the translation of VirF by base modification of the VirF encoding mRNA.