CARDIOVIRAL INTERNAL RIBOSOMAL ENTRY SITE IS FUNCTIONAL IN A GENETICALLY ENGINEERED DICISTRONIC POLIOVIRUS

HIGH mutation rates have driven RNA viruses to shorten their genomes to the minimum possible size 1. Mammalian (+)-strand RNA viruses and retroviruses have responded by reducing the number of cis-acting regulatory elements, a constraint that has led to the emergence of the polyprotein 2. Poliovirus is a (+)-stranded picornavirus whose polyprotein, encoded by an open reading frame spanning most of the viral RNA 3, is processed by virus-encoded proteinases 4,5. Despite their genetic austerity, picornaviruses have retained long 5' untranslated regions 6-8, which harbour cis-acting elements that promote initiation of translation independently of the uncapped 5' end of the viral messenger RNA 9-12. These elements are termed 'internal ribosomal entry sites' 10 and are formed from highly structured RNA segments 13-15 of at least 400 nucleotides 16. How these elements function-is not known, but special RNA-binding proteins may be involved 17. The ribosome or its 40S subunit probably binds at or near a Y(n)X(m)AUG motif (where Y is a pyrimidine and X is a purine) at the 3' border of the internal ribosomal entry site 17, which either provides the initiating codon 16,18 or enables the ribosome to translocate to one downstream (E.W. et al., submitted). Initiation from most eukaryotic messenger RNAs usually occurs by ribosomal recognition of the 5' and subsequent scanning to the AUG codon 19. Here we describe a genetic strategy for the dissection of polyproteins which proves that an internal ribosomal entry site element can initiate translation independently of the 5' end.