Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transmission
Protein Synthesis and Its Regulation
Polypeptide chain initiation occurs in several stages

Initiation of the polypeptide chain in Prokaryotic Cells requires: 1) the 30S subunit containing 16S rRNA; 2) mRNA encoding the synthesized polypeptide; 3) initiating N-formylmethionyl-tRNA^fMet; 4) three Proteins known as initiation factors (IF-1, IF-2, and IF-3); and 5) GTP (Table 29-1).

The formation of the initiation complex proceeds in three stages. In The First stage, the 30S ribosomal subunit binds initiation factor 3 (IF-3), which prevents the association of the 30S and 50S subunits. Next, mRNA attaches to the 30S subunit such that the initiating mRNA codon (5') AUG(3') pairs with a specific site on the 30S subunit (Fig. 29-14). Proper positioning of the initiating AUG codon on the 30S subunit is ensured by a special initiation signal, which is a region of mRNA located on the 5' side of the AUG codon. This signal consists predominantly of A and G residues, typically comprising 6 to 8 such residues. It is recognized by the complementary sequence of 16S rRNA within the 30S subunit, thereby anchoring the mRNA in the correct position for Translation initiation. Because there is only a single codon for both initiating and internal Methionine residues, the initiation signal 5' to AUG designates the precise site where fMet-tRNA^fMet must bind. Internal AUG codons are specific for Met-tRNA^Met and are incapable of binding fMet-tRNA^fMet.

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Fig. 29-14. The three stages of initiation complex formation, driven by the energy of GTP Hydrolysis to GDP and P_i. IF-1, IF-2, and IF-3 are initiation factors. The letters P and A denote the peptidyl and aminoacyl sites of the ribosome, respectively. The exact role of IF-1 remains to be fully elucidated.

In the second stage of initiation (Fig. 29-14), the complex consisting of the 30S subunit, IF-3, and mRNA increases in size upon joining with initiation factor IF-2, which is already bound to GTP and the initiating N-formylmethionyl-tRNA^fMet, positioning the latter precisely over the initiation codon.

In the Third Stage of initiation, this large complex interacts with the 50S ribosomal subunit; simultaneously, the GTP molecule bound to IF-2 is hydrolyzed to GDP and phosphate, both of which are released from the complex. Initiation factors IF-3 and IF-2 also depart from the ribosome. We now have a functionally active 70S ribosome, termed the initiation complex, which contains the mRNA and the initiating N-formylmethionyl-tRNA^fMet. The correct positioning of N-formylmethionyl-tRNA^fMet in the complete 70S initiation complex is ensured by two recognition and binding points. First, the anticodon triplet of the initiating aminoacyl-tRNA forms complementary Base Pairs with the antiparallel codon triplet AUG in the mRNA. Second, the initiating aminoacyl-tRNA binds to the peptidyl (P) site of the ribosome. The ribosome features two aminoacyl-tRNA binding sites: the aminoacyl (A) site and the peptidyl (P) site. Both are formed by a specific combination of regions from the 30S and 50S subunits. The initiating fMet-tRNA can bind exclusively to the P site (Fig. 29-14), but this is an exception: all subsequent incoming aminoacyl-tRNAs bind to the A site, whereas the P site serves as the ribosomal Location from which "empty" (amino acid-free) tRNAs depart and to which the growing peptidyl-tRNA remains attached.

The initiation complex is now ready for the elongation phase.