Abstract
Rhopalosiphum padi viras (RhPV) is one of several picorna-like viruses that infect insects; sequence analysis has revealed distinct differences between these agents and mammalian picornaviruses. RhPV has a single-stranded positive sense RNA genome of about 10kb; unlike the genomes of the Picornaviridae, however, this genome contains two long open reading frames (ORFs). ORF1 encodes the virus non- structural proteins, while ORF2 specifies the structural proteins. Both ORFs are preceeded by long untranslated regions (UTRs). The intergenic UTR is known to contain an internal ribosome entry site (IRES) that directs non-AUG-initiated, translation of ORF2. The 5' UTR of RhPV was assayed for IRES activity by translating synthetic bicistronic mRNAs containing this sequence in a variety of systems. The 5' UTR contains an IRES element that directs internal initiation of protein synthesis from an AUG codon in mammalian. Drosophila and plant in vitro systems and in vivo in an insect non-aphid cell line. In contrast, the encephalomyocarditis virus IRES only functions in mammalian in vitro and in vivo systems. 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 elements, as deletion analysis indicates that this IRES element does not have sharply defined boundaries. IRES activities of the 5' UTRs of other insect picorna-like viruses Acyrthosiphon pisum virus (APV) and the type 2 insect picorna-like virus Sacbrood viras (SBV) were also examined. Both viruses were shown to contain IRES elements that functioned in mammalian and plant in vitro systems and in vivo in an insect non-aphid cell line. Comparison of the IRES activities of RhPV with APV and SBV showed the RhPV 5' IRES element to be more efficient than both the APV and SBV IRES elements in mediating internal initiation of protein synthesis.