Marburg and ebola virus mRNA 39 untranslated regions contain negative regulators of translation that are modulated by ADAR1 editing

The filovirus family includes deadly pathogens such as Ebola virus (EBOV) and Marburg virus (MARV). A substantial portion of filovirus genomes encode 59 and 39 untranslated regions (UTRs) of viral mRNAs. Select viral genomic RNA sequences corresponding to 39 UTRs are prone to editing by adenosine deaminase acting on RNA 1 (ADAR1). A reporter mRNA approach, in which different 59 or 39 UTRs were inserted into luciferase-encoding mRNAs, demonstrates that MARV 39 UTRs yield different levels of reporter gene expression, suggesting modulation of translation. The modulation occurs in cells unable to produce microRNAs (miRNAs) and can be recapitulated in a MARV minigenome assay. Deletion mutants identified negative regulatory regions at the ends of the MARV nucleoprotein (NP) and large protein (L) 39 UTRs. Apparent ADAR1 editing mutants were previously identified within the MARV NP 39 UTR. Introduction of these changes into the MARV nucleoprotein (NP) 39 UTR or deletion of the region targeted for editing enhances translation, as indicated by reporter assays and polysome analysis. In addition, the parental NP 39 UTR, but not the edited or deletion mutant NP 39 UTRs, induces a type I interferon (IFN) response upon transfection into cells. Because some EBOV isolates from the West Africa outbreak exhibited ADAR1 editing of the viral protein of 40 kDa (VP40) 39 UTR, VP40 39 UTRs with parental and edited sequences were similarly assayed. The EBOV VP40 39 UTR edits also enhanced translation, but neither the wild-type nor the edited 39 UTRs induced IFN. These findings implicate filoviral mRNA 39 UTRs as negative regulators of translation that can be inactivated by innate immune responses that induce ADAR1.