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Control of translation is one of the major regulatory events in eukaryotic gene expression. Internal ribosome entry sites (IRES) were first discovered in picornavirus RNAs but it is now clear that IRESs are also present in the 5\' untranslated region of many eukaryotic genes including those encoding growth factors (e.g., VEGF, FGF2 and PDGF), genes whose protein products are associated with apoptosis (e.g., Apaf-1, IXAP), transcription factors (e.g., c- myc), and the potassium channel Kv1.4. IRES allows the ribosomes to be recruited to an initiator AUG, which is some distance from the 5\' end of the message RNA to bypass the Kozak scanning mechanism. In the Kozak scanning model, the 40S ribosomal subunit bearing Met-tRNAmet and initiation factors, binds near the capped 5\' end of the mRNA and travels along the mRNA until it comes to the first AUG. IRES-dependent initiation of protein synthesis occurs during apoptosis or some viral infection (e.g., picornavirus). A component of the cap-binding complex eIF-4F, translation initiation factor eIF-4G, is cleaved (by caspases or viral protein 2A) to give a modified form of cap-binding complex that is cap-independent. (This definition may be outdated - see the DesignNote.)
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