The Escherichia coli histone-like protein HU regulates rpoS translation

Escherichia coli HU protein is a major component of the bacterial nucleoid. HU stabilizes higher order nucleoprotein complexes and belongs to a family of DNA architectural proteins. Here, we report that HU is required for eff icient expression of the sigma S subunit of RNA polymerase. This rpoS-encod ed alternative sigmaS factor induces a number of genes implicated in cell s urvival in stationary phase and in multiple stress resistance. By analysis of rpoS-lacZ fusions and by pulse-chase experiments, we show that the effic iency of rpoS translation is reduced in cells lacking HU, whereas neither r poS transcription nor protein stability is affected by HU. Gel mobility shi ft assays show that HU is able to bind specifically an RNA fragment contain ing the translational initiation region of rpoS mRNA 1000-fold more strongl y than double-stranded DNA. Together with the in vivo data, this finding st rongly suggests that, by binding to rpoS mRNA, HU directly stimulates rpoS translation. We demonstrate here that HU, an abundant DNA-binding, histone- like protein, is able specifically to recognize an RNA molecule and therefo re play a role in post-transcriptional regulation.