Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 188

The currently accepted view of the steps in the initiation of translation at a TIR is outlined in Figure 2.28. Initiation requires three different initiation factors, IF1, IF2, and IF3, in addition to fMet-tRNA^fMet. These initiation factors interact mostly with the initiator tRNA and the P site of the 30S ribosomal subunit.

For initiation to occur, the 70S ribosome must first be separated (or dissociated) into its smaller 30S and 50S subunits. This dissociation occurs after the termination step of translation (see below). The IF3 initiation factor binds to the 30S subunit and helps to keep the subunits dissociated. Therefore, ribosomes are continuously cycling between the 70S ribosome and the 30S and 50S subunits, depending on whether they are active in translation. This is called the ribosome cycle.

Figure 2.28 Initiation of translation. (1) The IF3 factor binds the 30S subunit to keep it dissociated from the 50S subunit during initiation. (2) IF1 binds to the A site to block the site and prevent tRNA binding. (3) A complex is formed between formyl-methionine tRNA (fMet-tRNA^fMet), IF2, and guanosine triphosphate (GTP). (4) The fMet-tRNA^fMet-IF2-GTP complex binds to the P site of the 30S subunit and the mRNA translational initiation region (TIR) site. (5) IF1 and IF3 are released, the cleavage of GTP on IF2 correctly positions the fMet-tRNA^fMet on the P site initiation codon, and the 50S subunit binds. (6) The 70S initiation complex is ready to accept an aminoacyl-tRNA at the A site.

Once the subunits are dissociated, IF1 binds to the A site on the 30S subunit to prevent the fMet-tRNA^fMet from inadvertently binding to this site. IF2, in conjunction with GTP, binds to fMet-tRNA^fMet to form a ternary (three-member) complex, which binds to the mRNA and P site of the 30S ribosomal subunit. The initial binding of fMet-tRNA^fMet does not depend on an initiator codon in the P site. However, IF2, with the help of IF3, adjusts the fMet-tRNA^fMet and the mRNA initiator codon so that the binding becomes codon-specific. IF1 and IF3 are then released, and IF2 promotes the association of this initiation complex with the 50S subunit of the ribosome. IF2 is then released, with the cleavage of GTP to guanosine diphosphate (GDP). The newly formed 70S ribosome is now ready for translation, and another aa-tRNA can enter the A site.