The 5 ' untranslated region of Rhopalosiphum padi virus contains an internal ribosome entry site which functions efficiently in mammalian, plant, andinsect translation systems

Rhopalosiphum padi virus (RhPV) is one of several picorna-like viruses that infect insects; sequence analysis has revealed distinct differences betwee n these agents and mammalian picornaviruses. RhPV has a single-stranded! po sitive-sense RNA genome of about 10 kb; unlike the genomes of Picornavirida e, however, this genome contains two long open reading frames (ORFs). ORF1 encodes the virus nonstructural proteins, while the downstream ORF, ORF2, s pecifies the structural proteins. Both ORFs are preceded by long untranslat ed regions (UTRs). The intergenic UTR is known to contain an internal ribos ome entry site (IRES) which directs non-AUG-initiated translation of ORF2. We have examined the 5' UTR of RhPV for IRES activity by translating synthe tic dicistronic mRNAs containing this sequence in a variety of systems. We now report that the 5' UTR contains an element which directs internal initi ation of protein synthesis from an AUG codon in mammalian, plant, and Droso phila in vitro translation systems. In contrast, the encephalomyocarditis v irus IRES functions only in the mammalian, system. The RhPV 5' IRES element has features in common with picornavirus IRES elements, in that no coding sequence is required for IRES function, but also with cellular IRES element s, as deletion analysis indicates that this IRES element does not have shar ply defined boundaries.