scientific report 284The Escherichia coli Sm-like host factor I (Hfq) protein is thought to function in post-transcriptional regulation by modulating the function of small regulatory RNAs. Hfq also interferes with ribosome binding on E. coli ompA messenger RNA, indicating that Hfq also interacts with mRNAs. In this study, we have used stimulation of group I intron splicing in vivo and a modified in vitro toeprinting assay to determine whether Hfq acts as an RNA chaperone. Hfq was able to rescue an RNA 'folding trap' in a splicing defective T4 bacteriophage td gene in vivo. Enzymatic analysis showed that Hfq affects the accessibility of the ompA start codon, as well as other bases within the ribosome-binding site, explaining its negative effect on ribosome binding. We also show that the Hfq-induced structural changes in ompA mRNA are maintained after proteolytic digestion of the protein, which classifies Hfq as an RNA chaperone.