Marburg and Ebola virus mRNA 3 untranslated regions contain negative regulators of translation that are modulated by ADAR1 editing

Abstract

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 3UTRs are prone to editing by ADAR1. A reporter mRNA approach, in which different 5 or 3UTRs were inserted into luciferase encoding mRNAs, demonstrates that MARV 3UTRs 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 3UTRs. Apparent ADAR1 editing mutants were previously identified within the MARV NP 3UTR. Introduction of these changes into the MARV nucleoprotein (NP) 3UTR or deletion of the region targeted for editing enhances translation, as indicated by reporter assays and polysome analysis. In addition, the parental NP 3UTR, but not the edited or deletion mutant NP 3UTRs, 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 3UTR, VP40 3UTRs with parental and edited sequences were similarly assayed. The EBOV VP40 3UTR edits also enhanced translation but neither the wildtype nor the edited 3UTRs induced IFN. These findings implicate filoviral mRNA 3UTRs as negative regulators of translation that can be inactivated by innate immune responses that induce ADAR1. ImportanceUTRs comprise a large percentage of filovirus genomes and are apparent targets of editing by ADAR1, an enzyme with pro- and antiviral activities. However, the functional significance of the UTRs and of ADAR1 editing have been uncertain. This study demonstrates that MARV and EBOV 3UTRs can modulate translation, in some cases negatively. ADAR1 editing or deletion of select regions within the translation suppressing 3UTRs, relieves the negative effects of the UTRs. These data indicate that filovirus 3UTRs contain translation regulatory elements that are modulated by activation of ADAR1, suggesting a complex interplay between filovirus gene expression and innate immunity.