An atypical IRES within the 5' UTR of a dicistrovirus genome.

Translation initiation on the majority of cellular mRNAs is mediated by a cap structure on the 5' end of the mRNA and involves the binding of initiation factors to the cap, followed by recruitment of the 40S ribosomal subunit. However, a number of viral mRNAs are translated using an alternative mechanism, termed internal initiation. In this case, initiation factors and the 40S subunit bind to an internal ribosome entry site (IRES) structure within the 5' untranslated region (UTR) of the mRNA. Although there is no common feature amongst all viral IRES elements, most are believed to contain extensive secondary structure and some of these structures are important for their function. However, an IRES element from the 5' UTR of the genome of Rhopalosiphum padi virus, a Dicistrovirus, challenges this paradigm. This IRES has been shown to function in a number of different translation systems. Intriguingly, the functional region of this IRES element is largely unstructured, and multiple regions from within the 5' UTR function as efficiently as the full-length sequence. This review compares and contrasts the features of this atypical IRES element with other IRES elements and discusses its possible mechanism of action.