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 size1. 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 polyprotein2. Poliovirus is a (+)-stranded picornavirus whose polyprotein, encoded by an open reading frame spanning most of the viral RNA3, is processed by virus-encoded proteinases4,5. Despite their genetic austerity, picor-naviruses have retained long 5' untranslated regions6–8, which harbour cis-acting elements that promote initiation of translation independently of the uncapped 5' end of the viral messenger RNA9–12. These elements are termed 'internal ribosomal entry sites'10 and are formed from highly structured RNA segments13–15 of at least 400 nucleotides16. How these elements function is not known, but special RNA-binding proteins may be involved17. The ribosome or its 40S subunit probably binds at or near a YnXm AUG motif (where Y is a pyrimidine and X is a purine) at the 3' border of the internal ribosomal entry site17, which either provides the initiating codon16,18 or enables the ribosome to translocate to one downstream (E.W. et al., submitted). Initiation from most eukary-otic messenger RNAs usually occurs by ribosomal recognition of the 5' and subsequent scanning to the AUG codon19. 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.