Naturally occurring dicistronic cricket paralysis virus RNA is regulated by two internal ribosome entry sites

Cricket paralysis virus is a member of a group of insect picorna-like virus es. Cloning and sequencing of the single plus strand RNA genome revealed th e presence of two nonoverlapping open reading frames, ORF1 and ORF2, that e ncode the nonstructural and structural proteins, respectively. We show that each ORF is preceded by one internal ribosome entry site (IRES). The inter genic IRES is located 6,024 nucleotides from the 5' end of the viral RNA an d is more active than the IRES located at the 5' end of the RNA, providing a mechanistic explanation for the increased abundance of structural protein s relative to nonstructural proteins in infected cells. Mutational analysis of this intergenic-region IRES revealed that ORF2 begins with a noncognate CCU triplet. Complementarity of this CCU triplet with sequences in the IRE S is important for IRES function, pointing to an involvement of RNA-RNA int eractions in translation initiation. Thus, the cricket paralysis virus geno me is an example of a naturally occurring, functionally dicistronic eukaryo tic mRNA whose translation is controlled by two IRES elements located at th e 5' end and in the middle of the mRNA. This finding argues that eukaryotic mRNAs can express multiple proteins not only by polyprotein processing, re initiation and frameshifting but also by using multiple IRES elements.