Elongation of Translation

Translation occurs in three distinct stages--initiation, elongation, and termination. Each step requires specific proteins that interact with the tRNAs, mRNA, and/or ribosomes. Some of these are listed in Table 27.4.

Three tRNA binding sites on the 70S ribosome are involved in the process of elongation. Figure 27.22 shows a single round of this cycle, which is repeated until a signal for termination is encountered. The cycle goes as follows:

1. The nascent polypeptide chain is attached to a tRNA in the P site and the A and E sites are empty. Aligned with the A site is the mRNA codon corresponding to the next amino acid to be incorporated.

2. A charged tRNA is escorted to the A site in a complex with the protein elongation factor EF-Tu, which also carries a molecule of GTP.

3. When the appropriate charged tRNA is depositied into the A site, the GTP is hydrolyzed and the EF-Tu-GDP is released. EF-Tu-GTP is regenerated, as shown in Figure 27.20. Proofreading occurs at this step after the charged tRNA is in place. The charged tRNA is checked both before and after the GTP hydrolysis and is rejected if incorrect.

4. Peptide bond formation - The polypeptide chain that was attached to the tRNA in the P site is transferred to the amino group of the amino acid carried by the A-site tRNA. This step is called peptidyl transer and is catalyzed by an enzyme complex called peptidyltransferase, which is an integral part of the 50S subunit. Peptidyltransferase consists of some ribosomal proteins and the ribosomal RNA, which acts as a ribozyme.

5. Translocation - The uncharged tRNA remaining in the P site is transferred to the E site and the tRNA in a hybrid P/A state (that has the nascent polypeptide chain attached to it) is moved to the P site. At the same time, the ribosome moves the mRNA by three nucleotides in the 3' direction, placing a new codon adjacent to the now empty A site. This step requires the protein factor EF-G bound to GTP; GTP is hydrolyzed in the process. EF-G-GTP has a remarkable molecular similarity to the tRNAaminoacid-EF-Tu-GTP complex and may act to displace the bound tRNA.

6. As the old tRNA is released from the E site, the empty A site accepts the aminoacyl tRNA corresponding to the next codon.

The net result of one turn of this cycle is that the polypeptide has grown by one amino acid residue and the ribosome has moved along the mRNA by three nucleotide residues. The process is repeated until a termination signal is reached. Figure 27.25 shows that as the polypeptide chain elongates, it passes through a tunnel in the 50S subunit and emerges from a hole near the bottom.

See also: Initiation of Translation, Structure of tRNAs, Structure of Prokaryotic mRNAs, Termination of Translation, Eukaryotic vs Prokaryotic Translation (from Chapter 28)

INTERNET LINK: Translation Initiation and Elongation Factors