REVERTANT ANALYSIS OF J-K MUTATIONS IN THE ENCEPHALOMYOCARDITIS VIRUSINTERNAL RIBOSOMAL ENTRY SITE DETECTS AN ALTERED LEADER PROTEIN

The internal ribosomal entry site (IRES) of picornaviruses consists of various sequence and structural elements that collectively impart tra nslational function to the genome. By engineering substitution and del etion mutations into the J-K elements of the encephalomyocarditis viru s IRES, translationally defective viruses with small-plaque phenotypes were generated. From these, 60 larger-plaque revertant viruses were i solated and characterized, and their sequences were compared with a st ructural model of the IRES. The data provide confirming evidence for t he existence of helix J3 within stem J but suggest that helix J1 is 3 bp longer than previously estimated. They also suggest that previously modeled stems L and M should be replaced by an alternative structure. One reversion mutation was mapped to the leader protein coding region . This change of leader amino acid 20 from Pro to Ser increased the vi ral plaque size dramatically but did not alter the cell-free translati onal activity of the mutated, parental IRES.