Involvement of RNA-binding protein Hfq in the post-transcriptional regulation of invE gene expression in Shigella sonnei.

The temperature-dependent regulation of Shigella virulence genes is believed to be accomplished at the transcriptional stage by the regulators VirF and InvE. Several lines of evidence herein described indicate that post-transcriptional regulation of InvE expression plays a key role in the temperature-dependent regulation of virulence gene expression: (i) a considerable amount of invE mRNA continues to be transcribed under low temperature conditions, where the production of InvE protein is tightly repressed; (ii) the stability of invE mRNA markedly decreases, because its decay rate is significantly increased under the repressing conditions. Strikingly, in the hfq mutant of Shigella sonnei, a considerable amount of InvE protein was produced even at low temperature. This increase in the InvE level was found to be associated with the improved stability of invE mRNA, in agreement with the finding that the RNA chaperon Hfq influences post-transcriptional regulations of various genes. Consistently, overexpression of the Hfq protein decreased the production of InvE protein even under the expressing condition at 37 degrees C. The binding in vitro of purified Hfq protein to invE RNA was shown to be stronger at 30 degrees C than at 37 degrees C in two experiments, gel shift analysis and surface plasmon resonance (Biacore) analysis. These results altogether suggest that Hfq plays an important role in the temperature-dependent regulation of invE expression at the post-transcriptional step.