Atp

2' structure unwinding, scanning, and start site recognition

(AAA)n 3' 60S subunit-eIF6, eIF5^GTP

80S ribosome

80S ribosome of a GTP associated with it (step 4). Coupling the joining reaction to GTP hydrolysis makes this an irreversible step, so that the ribosomal subunits do not dissociate until the entire mRNA is translated and protein synthesis is terminated. As discussed later, during chain elongation, the growing polypeptide remains attached to the tRNA at this P site in the ribosome.

The eukaryotic protein-synthesizing machinery begins translation of most cellular mRNAs within about 100 nucleotides of the 5' capped end as just described. However, some cellular mRNAs contain an internal ribosome entry site (IRES) located far downstream of the 5' end. In addition, translation of some viral mRNAs, which lack a 5' cap, is initiated at IRESs by the host-cell machinery of infected eu-karyotic cells. Some of the same translation initiation factors that assist in ribosome scanning from a 5' cap are required for locating an internal AUG start codon, but exactly how an IRES is recognized is less clear. Recent results indicate that some IRESs fold into an RNA structure that binds to a third site on the ribosome, the E site, thereby positioning a nearby internal AUG start codon in the P site.

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