The initiation phase of protein synthesis involves the formation of a complex between the ribosomal subunits, an mRNA template and tRNAfmet (Fig. 8.5). A 30S subunit attaches to the ribosome-binding site as described above. tRNAfmet then interacts with the AUG initiation codon, and finally the 50S ribosomal subunit attaches. The ribosome is now complete, and the first tRNA and its amino acid are in place in the P site of the ribosome. A 70S initiation complex has been formed, and protein synthesis can begin. The ribosome is orientated so that it will move along the mRNA in the 5' to 3' direction, the direction in which the information encoded in the mRNA molecule is read.
Three proteins called initiation factors 1, 2, and 3, together with the nucleotide guano-sine triphosphate, are needed to help the 70S initiation complex form. Initiation factors 1 and 3 are attached to the 30S subunit. Initiation factor 3 helps in the recognition of the ribosome-binding site on the mRNA. Initiation factor 2 specifically recognizes tRNAfmet and binds it to the ribosome. When the 50S subunit attaches, the three initiation factors are released and the guanosine triphosphate is hydrolyzed, losing its y phosphate to become guanosine diphosphate.
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