Insights into the molecular events underlying internal entry of ribosomesĪ chimeric protein formed from a domain of the translation initiation factor eIF4G and the iron responsive element binding protein-1 (IRP-1) is sufficient to promote translation of the downstream cistron of a bicistronic mRNA bearing the iron responsive element (IRE) RNA sequence in the intercistronic space ( De Gregorio et al., 1999). Since then, and particularly over the last three years, IRES activities have been detected in a restricted but increasing number of cellular mRNAs from yeast, Drosophila, birds and mammals, showing that the internal ribosome entry process is far more extensive than previously thought (see Supplementary data). Indeed, the first cellular IRES was identified in the 220-nucleotide-long 5′ UTR of the immunoglobulin heavy chain-binding protein (BiP) mRNA, whose translation is maintained in poliovirus-infected cells at a time when cap-dependent translation is severely inhibited ( Macejak and Sarnow, 1991). These discoveries argued for an alternative mechanism such as the internal entry of ribosomes. Moreover, a few cellular mRNAs are translated preferentially when cap-dependent initiation of translation is impaired. Their presence in viruses as diverse as flaviviruses, retroviruses and even DNA viruses such as the Kaposi’s sarcoma-associated herpesvirus reveals the widespread nature of these RNA elements (see Supplementary data, available at EMBO reports Online).Īs is the case for many viral mRNAs, a number of cellular mRNAs possess structural features in their 5′ UTRs that make them unlikely to be translated by a 5′ cap-dependent ribosome-scanning mechanism. Using this assay as the basis for defining IRESs, these elements have been found in all picornavirus genera. This strategy can be considered the ‘gold standard’ for characterizing IRESs ( Sachs, 2000) if one considers the presence of cryptic RNA processing signals or promoter sequences in the intercistronic space as having been ruled out ( Kozak, 2001). This was first shown for poliovirus, where inserting a segment of the 5′ UTR of a poliovirus genome between the two ORFs allows translation of the downstream cistron, independent of the cap-mediated translation of the first cistron. Bicistronic RNAs with two non-overlapping open reading frames (ORFs) were shown to be good models to test cap-independent translation initiation. The poliovirus and encephalomyocarditis virus (EMCV) 5′ UTRs were the first to be described to ‘break the rule’ of translation initiation ( Jackson, 1988 Jang et al., 1988 Pelletier and Sonenberg, 1988). Nevertheless, these 5′ UTRs confer efficient 40S joining. Their 5′ UTRs also have complex features predicted to impair ribosome recruitment and linear scanning: (i) a long leader sequence (ii) stable secondary structures and (iii) potential upstream initiation codons. Unlike their cellular counterparts, picornaviral mRNAs are naturally uncapped at their 5′ end. Studies on viral gene translation were essential for the initial discovery of internal entry of ribosomes. How widespread is internal ribosome entry? (An IRES database website is available at. This process is called internal entry of ribosomes and is mediated by internal sequences called internal ribosome entry sites (IRESs). In addition, studies on the translation of viral and eukaryotic mRNAs bearing uncommon features in their 5′ untranslated regions (UTRs) have shed light on an alternative mode of 40S recruitment to the mRNA. The conventional mode of attachment of the 40S ribosomal subunit to the mRNA requires the eIF4F protein adaptor complex, which bridges the 7methyl guanosine capped structure located at the 5′ end of the mRNA and the 40S subunit carrying the eIF3 and eIF2-GTP–Met-tRNA complexes (see Figure 1). In eukaryotes, the situation has proved to be more complicated. This RNA–RNA interaction brings the prokaryotic 30S subunit to an internal position on the mRNA, in the vicinity of an AUG initiation codon. In prokaryotes, the process of ribosome recruitment to the mRNA results from direct base-pairing between the 18S ribosomal RNA and the mRNA Shine–Dalgarno sequence.
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