Regulation of hepatitis C virus by microRNA-122

Most metazoan miRNAs (microRNAs) bind to sites in the 3′-UTRs (untranslated regions) of mRNA targets and negatively regulate protein synthesis. The liver-specific miR-122, however, exerts a positive effect on HCV (hepatitis C virus) RNA levels by binding directly to a site in the 5′-UTR of the viral RNA. HCV translation and RNA stability are unaffected, and therefore miR-122 is likely to act at the level of viral replication. The miR-122-binding site in HCV RNA was examined to determine whether the nature of the site is responsible for the unusual mode of action for a miRNA. When the site was placed in the 3′-UTR of a reporter mRNA, miR-122 repressed translation, and therefore the location of the miR-122-binding site dictates its effect on gene expression. Additionally, a second binding site for miR-122 was identified in the HCV 5′-UTR, and miR-122 binding to both sites in the same viral RNA was found to be necessary for viral replication. The two sites are adjacent and are separated by a short spacer, which is largely conserved between HCV genotypes. The binding site requirements for miR-122 to positively regulate HCV replication provide an insight into this unusual mode of miRNA action.