Abstract
The accuracy of start codon selection is determined by the translation initiation process. In prokaryotes the initiation step on most mRNAs relies on recruitment of the small ribosomal subunit onto the initiation codon by base pairing between the mRNA and the 16S rRNA. Eukaryotes have evolved a complex molecular machinery involving at least 11 initiation factors, and mRNAs do not directly recruit the small ribosomal subunit. Instead the initiation complex is recruited to the 5' end of the mRNA through a complex protein network including eIF4E that interacts with the 5' cap structure and poly-A binding protein that interacts with the 3'end. However, some viral and cellular mRNAs are able to escape this pathway by internal recruitment of one or several components of the translation machinery. Here we review those eukaryotic mRNAs that have been reported to directly recruit the 40S ribosomal subunit internally. In the well characterized cases of viral IRESes, a specific RNA structure is involved in this process, and in addition to recruitment of the ribosome, the mRNA also manipulates the ribosome structure to stimulate the first translocation step. We also review recently described IRES/ribosome interactions in cases where the molecular mechanism leading to translation initiation has yet to be described. Finally we evaluate the possibility that mRNA may recruit the 40S ribosomal subunit through base pairing with the 18S rRNA.