Recruitment of the ribosomal 40S subunit to the 3\u27untranslated region of a viral mRNA, via the eIF4 complex, facilitates cap-independent translation

Translation of uncapped plant viral RNAs can be facilitated by either an internal ribosomal entry site (IRES) in the 5\u27 untranslated region (UTR) or a cap-independent translation element (CITE) in the 3\u27 UTR. Barley yellow dwarf virus (BYDV) mRNA, which lacks both cap and poly(A) tail, has a translation element (3\u27BTE) in its 3\u27 UTR that is essential for efficient translation initiation at the 5\u27-proximal AUG. This mechanism requires binding of the eukaryotic initiation factor 4G (eIF4G) subunit of the heterodimer eIF4F to the 3\u27BTE and base pairing between the 3\u27BTE and the 5\u27 UTR. Here we investigate how this interaction recruits the ribosome to the 5\u27 end of the mRNA. Using fluorescence anisotropy, SHAPE analysis and toe printing, we found that (i) 40S ribosomal subunits bind to the 3\u27BTE, (ii) the helicase complex eIF4F-eIF4A-eIF4B-ATP increases affinity of 40S subunit binding to the conserved SL-I of the 3\u27 BTE by exposing more unpaired bases of the 3\u27BTE and (iii) long-distance base pairing transfers this complex to the 5\u27 end of the mRNA where translation initiates. These results reveal an utterly novel mechanism of ribosome recruitment to an mRNA