Initiation
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.
Initiation of prokaryotic translation is depicted in Figure 27.20. Steps include:
1. Two initiation factors (IF1 and IF3) bind to a 70S ribosome. IF3 and IF1 appear to promote the dissociation of 70S ribosomes into free 30S and 50S subunits. mRNA and a third initiation factor (IF2), which carries a molecule of GTP and the charged initiator tRNA bind to a free 30S subunit. IF2 is one of a class of G proteins (see here). After these have all bound, the 30S initiation complex is complete.
The initiator tRNA carries an N-formylmethionine. The formyl group is added after the methionine is linked to the tRNA by an enzyme called transformylase. It transfers a formyl group from N10-formyltetrahydrofolate.
Only tRNAfMet is accepted to form the initiation complex. All further charged tRNAs require fully assembled (i.e., 70S) ribosomes. All prokaryotic proteins are synthesized with the same N-terminal residue, N-formylmethionine.
Near the 3' end of the 16S rRNA is a sequence (3' - UCCUCC -5') that can base pair with a sequence near the 5' end of each mRNA. The sequence in the mRNA is called the Shine-Dalgarno sequence (see here). This pairing aligns the message correctly for the start of translation.
2. The 50S subunit binds to the 30S initiation complex. It contains three sites for tRNA binding, called the P site (peptidyl), the A site (aminoacyl), and the E site (exit). When the two ribosomal subunits join, the AUG initiator codon with its bound tRNAfMet aligns with the P site.
3. The GTP carried by IF2 is hydrolyzed, and IF2-GDP, Pi, and IF1 are all released. The 70S initiation complex is ready to accept a second charged tRNA and begin elongation, the next phase of translation.
Anticodon ends of the tRNA molecules contact the 30S subunit and the acceptor ends interact with the 50S subunit (Figure 27.21).
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