3’ untranslated regions of Marburg and Ebola virus mRNAs possess 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 5’ and 3’ untranslated regions (UTRs) of viral mRNAs. Select viral genomic RNA sequences corresponding to 3’UTRs are prone to editing by ADAR1. A reporter mRNA approach, in which different 5’ or 3’UTRs were inserted into luciferase encoding mRNAs, demonstrates that MARV 3’UTRs yield different levels of reporter gene expression suggesting modulation of translation. The modulation occurs in cells unable to produce miRNAs and can be recapitulated in a minigenome assay. Deletion mutants identified negative regulatory regions at end of the MARV NP and L 3’UTRs. Apparent ADAR1 editing mutants were previously identified within the MARV NP 3’UTR. Introduction of these changes into the MARV nucleoprotein (NP) 3’UTR or deletion of the region targeted for editing enhances translation, as indicated by reporter assays and polysome analysis. In addition, the parental NP 3’UTR but not the edited or deletion mutant NP 3’UTRs induce a type I interferon (IFN) response upon transfection into cells. Because some EBOV isolates from the West Africa outbreak exhibited ADAR1 editing of the VP40 3’UTR, VP40 3’UTRs with parental and edited sequences were similarly assayed. The EBOV VP40 3’UTR edits also enhanced translation but neither the wildtype nor the edited 3’UTRs induced IFN. These findings implicate filoviral mRNA 3’UTRs as negative regulators of translation that can be inactivated by innate immune responses that induce ADAR1.