Elongation

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

A ribosome has three sites that can be occupied by tRNAs; the aminoacyl, or A, site, the peptidyl, or P, site, and the exit, or E, site (Figure 15.22a). The initiator tRNA immediately occupies the P site (the only site to which the fMet-tRNAfMet is capable of binding), but all other tRNAs first enter the A site. After initiation, the ribo-some is attached to the mRNA, and fMet-tRNAfMet is positioned over the AUG start codon in the P site; the adjacent A site is unoccupied (see Figure 15.22a).

Elongation occurs in three steps. The first step (Figure 15.22b) is the delivery of a charged tRNA (tRNA with its amino acid attached) to the A site. This requires the presence of elongation factor Tu (EF-Tu), elongation factor Ts (EF-Ts), and GTP. EF-Tu first joins with GTP and then binds to a charged tRNA to form a three-part complex. This three-part complex enters the A site of the ribosome, where the anticodon on the tRNA pairs with the codon on the mRNA. After the charged tRNA is in the A site, GTP is cleaved to GDP, and the EF-Tu - GDP complex is released (< Figure 15.22c). Factor EF-Ts regenerates EF-Tu - GDP to EF-Tu - GTP. In eukaryotic cells, a similar set of reactions delivers the charged tRNA to the A site.

The second step of elongation is the creation of a pep-tide bond between the amino acids that are attached to tRNAs in the P and A sites (< Figure 15.22d). The formation of this peptide bond releases the amino acid in the P site from its tRNA. The activity responsible for peptide-bond formation in the ribosome is referred to as peptidyl transferase. For many years, the assumption was that this activity is carried out by one of the proteins in the large subunit of the ribosome. Evidence, however, now indicates that the catalytic activity is a property of the rRNA in the large subunit of the ribosome; this rRNA acts as a ribozyme (see pp. 000 in Chapter 14).

The third step in elongation is translocation, (Figure 15.22e), 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 site are still attached to the mRNA through 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 may 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 during elongation can be summarized as follows: cytoplasm : A site : P site : E site : cytoplasm. As discussed earlier, the initiator tRNA is an exception: it attaches directly to the P site and never occupies the A site.

| fMET-tRNAfMet occupies the P site of the ribosome.

| EF-Tu, EF-Ts, GTP, and charged tRNA form a complex.

| fMET-tRNAfMet occupies the P site of the ribosome.

| EF-Tu, EF-Ts, GTP, and charged tRNA form a complex.

EF-Ts regenerates the EF-Tu-GTP complex, which is then ready to combine with another charged tRNA.

415.22 The elongation of translation comprises three steps.

EF-Ts regenerates the EF-Tu-GTP complex, which is then ready to combine with another charged tRNA.

415.22 The elongation of translation comprises three steps.

After translocation, the A site of the ribosome is empty and ready to receive the tRNA specified by the next codon. The elongation cycle (Figure 15.22a through d) 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. The ribosome moves down the mRNA in the 5': 3' direction, adding amino acids one at a time according to the order specified by the mRNA's codon sequence. Elongation in eukaryotic cells takes place in a similar manner.

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