Dengue and Zika virus 5'-UTRs harbor IRES functions

Members of Flavivirus, a genus of Flaviviridae, encompass numerous enveloped plus strand RNA viruses, of which globally dengue virus (DENV) is the leading cause of serious arthropod-borne disease. The genomes of DENV, just as those of yellow fever virus (YFV), West Nile fever virus (WNV), or Zika virus (ZIKV), control their translation by a 5′-terminal capping group. Three other genera of Flaviviridae are remarkable because their viruses use internal ribosomal entry sites (IRESs) to control translation and they are not arthropod transmitted. In 2006 E. Harris9 group published work suggesting that DENV RNA does not stringently need a cap for translation. They proposed that instead DENV translation is controlled by an interplay between 5′ and 3′ termini. Here we present evidence that the DENV or ZIKV 5′-untranslated regions (5′-UTRs) alone have IRES competence. This conclusion is based, first, on the observation that uncapped mono-cistronic mRNAs 5′ terminated with the DENV or ZIKV 5′-UTRs can efficiently direct translation of a reporter gene in BHK and C6/36 cells; second, that either 5′-UTR placed between two reporter genes can efficiently induce expression of the downstream gene in BHK but not in C6/36 cells. These experiments followed observations that uncapped DENV/ZIKV genomic transcripts, 5′ terminated with pppAN... or GpppAN..., can initiate infections of mammalian (BHK) or mosquito (C6/36) cells. IRES competence of the 5′-UTRs of DENV/ZIKV raises many open questions regarding the biology and control, as well as the evolution, of insect-borne flaviviruses.