Résumé
Eukaryotic mRNAs in which a poly(A) sequence precedes the initiation codon are known to exhibit a significantly enhanced cap-independent translation, both in vivo and in cell-free translation systems. Consistent with high expression levels of poxviral mRNAs, they contain poly(A) sequences at their 5′ ends, immediately before the initiation AUG codon. Here we show that poly(A) as a leader sequence in mRNA constructs promotes the recruitment of the 40S ribosomal subunits and the efficient formation of initiation complexes at cognate AUG initiation codons in the absence of two essential translation initiation factors, eIF3 and eIF4F. These factors are known to be indispensable for the cap-dependent (and ATP-dependent) mechanism of translation initiation but are shown here to be not required if an mRNA contains a 5′-proximal poly(A). Thus, the presence of a pre-AUG poly(A) sequence results in an alternative mechanism of translation initiation. It involves the binding of initiating 40S ribosomal subunits within the 5′ UTR and their phaseless, ATP-independent, diffusional movement (“phaseless wandering”) along the leader sequence, with subsequent recognition of the initiation (AUG) codon.