TRANSLATION EUKARYOTIC INITIATION-FACTOR 4G RECOGNIZES A SPECIFIC STRUCTURAL ELEMENT WITHIN THE INTERNAL RIBOSOME ENTRY SITE OF ENCEPHALOMYOCARDITIS VIRUS-RNA

A complex of eukaryotic initiation factors (eIFs) 4A, 4E, and 4G (coll ectively termed eIF4F) plays a key role in recruiting mRNAs to ribosom es during translation initiation. The site of ribosomal entry onto mos t mRNAs is determined by interaction of the 5'-terminal cap with eIF4E ; eIFs 4A and 4G may facilitate ribosomal entry by modifying mRNA stru cture near the cap and by interacting with ribosome-associated factors . eIF4G recruits uncapped encephalomyocarditis virus (EMCV) mRNA to ri bosomes without the involvement of eIF4E by binding directly to the si milar to 450-nucleotide long EMCV internal ribosome entry site (IRES). We have used chemical and enzymatic probing to map the eIF4G binding site to a structural element within the J-K domain of the EMCV IRES th at consists of an oligo(A) loop at the junction of three helices. The oligo(A) loop itself is not sufficient to form stable complexes with e IF4G since alteration of its structural context abolished its interact ion with eIF4G. Addition of wild type or trans-dominant mutant forms o f eIF4A to binary IRES.eIF4G complexes did not further alter the patte rn of chemical/enzymatic modification of the IRES.