The next stage in protein synthesis is elongation, in which amino acids are joined to create a polypeptide chain. Elongation requires (1) the 70S initiation complex just described; (2) tRNAs charged with their amino acids; (3) several elongation factors; and (4) GTP.

A ribosome has three sites that can be occupied by tRNAs: the aminoacyl (A) site, the peptidyl (P) site, and the exit (E) site (Figure 11.11a). The initiator tRNA immediately occupies the P site (the only site to which the fMet-tRNA^fMet is able to bind), but all other tRNAs first enter the A site. At the end of initiation, the ribosome is attached to the mRNA, and fMet-tRNA^fMet is positioned over the AUG start codon in the P site; the adjacent A site is unoccupied (see Figure 11.11a).

Elongation takes place in three steps. In the first step (Figure 11.11b), a charged tRNA binds to the A site. This binding takes place when elongation factor Tu (EF-Tu) joins with GTP and then with a charged tRNA to form a three-part complex. This complex enters the A site of the ribosome, where the anticodon on the tRNA pairs with the codon on the mRNA. Once the charged tRNA is in the A site, GTP is cleaved to form GDP, and the EF-Tu–GDP complex is released (Figure 11.11c). Elongation factor Ts (EF-Ts) regenerates EF-Tu–GTP from EF-Tu–GDP. In eukaryotic cells, a similar set of reactions delivers a charged tRNA to the A site.

The second step of elongation is the formation of a peptide bond between the amino acids that are attached to tRNAs in the P and A sites (Figure 11.11d). The formation of this peptide bond releases the amino acid in the P site from its tRNA. Peptide-bond formation occurs within the large subunit of the ribosome. Evidence indicates that the catalytic activity is a property of ribosomal RNA in the large subunit (the 23S rRNA in bacteria, the 28S RNA in eukaryotes); this rRNA acts as a ribozyme (Chapter 10).

The third step in elongation is translocation (Figure 11.11e), the movement of the ribosome down the mRNA in the 5′ → 3′ direction. This step positions the ribosome over the next codon and requires elongation factor G (EF-G) and the hydrolysis of GTP to GDP. Because the tRNAs in the P and A sites are still attached to the mRNA by codon–anticodon pairing, they do not move with the ribosome as it translocates. Consequently, the ribosome shifts so that the tRNA that previously occupied the P site now occupies the E site, from which it moves into the cytoplasm, where it can be recharged with another amino acid. Translocation also causes the tRNA that occupied the A site (which is attached to the growing polypeptide chain) to be in the P site, leaving the A site open. Thus, the progress of each tRNA through the ribosome in the course of elongation can be summarized as follows: cytoplasm → A site → P site → E site → cytoplasm. As stated earlier, the initiator tRNA is an exception: it attaches directly to the P site and never occupies the A site.

After translocation, the A site of the ribosome is empty and ready to receive the tRNA specified by the next codon. The elongation cycle (see Figure 11.11b through e) repeats itself: a charged tRNA and its amino acid occupy the A site, a peptide bond is formed between the amino acids in the A and P sites, and the ribosome translocates to the next codon. Throughout the cycle, the polypeptide chain remains attached to the tRNA in the P site.

Elongation in eukaryotic cells takes place in a similar manner. Eukaryotes possess at least three elongation factors, one of which also acts in initiation and termination. Another of these elongation factors, called eukaryotic elongation factor 2 (eEF2), is the target of a toxin produced by the bacteria that cause diphtheria, a disease that until recently was a leading killer of children. The diphtheria toxin inhibits eEF2, preventing the translocation of the ribosome along the mRNA, and protein synthesis ceases.

CONCEPTS