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

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment