During Chain Elongation Each Incoming AminoacyltRNA Moves Through Three Ribosomal Sites

The correctly positioned eukaryotic 80S ribosome-Met-tRNAMet complex is now ready to begin the task of stepwise addition of amino acids by the in-frame translation of the mRNA. As is the case with initiation, a set of special proteins, termed elongation factors (EFs), are required to carry out this process of chain elongation. The key steps in elongation are entry of each succeeding aminoacyl-tRNA, formation of a peptide bond, and the movement, or translocation, of the ribosome one codon at a time along the mRNA.

▲ FIGURE 4-26 Cycle of peptidyl chain elongation during translation in eukaryotes. Once the 80S ribosome with Met-tRNA;Met in the ribosome P site is assembled (top), a ternary complex bearing the second amino acid (aa2) coded by the mRNA binds to the A site (step H). Following a conformational change in the ribosome induced by hydrolysis of GTP in EF1a-GTP (step H), the large rRNA catalyzes peptide bond formation between Met and aa2 (step H). Hydrolysis of GTP in EF2-GTP causes another conformational change in the ribosome that results in its translocation one codon along the mRNA and shifts the unacylated tRNAiMet to the E site and the tRNA with the bound peptide to the P site (step □). The cycle can begin again with binding of a ternary complex bearing aa3 to the now-open A site. In the second and subsequent elongation cycles, the tRNA at the E site is ejected during step Has a result of the conformational change induced by hydrolysis of GTP in EF1a-GTP See the text for details. [Adapted from K. H. Nierhaus et al., 2000, in R. A. Garrett et al., eds., The Ribosome: Structure, Function, Antibiotics, and Cellular Interactions, ASM Press, p. 319.]

At the completion of translation initiation, as noted already, Met-tRNAiMet is bound to the P site on the assembled 80S ribosome (Figure 4-26, top). This region of the ribosome is called the P site because the tRNA chemically linked to the growing polypeptide chain is located here. The second

Entry of next aa-tRNA at A site

Entry of next aa-tRNA at A site

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