Hydrogen bond mRNA-tRNA complex

The first amino acid for reaction is jV-formylmethionine (fMet) which has a specific tRNA (tRNA 1 1) (as opposed to the Met-specific tRNA tRNAmMl 1). Using GTP hydrolysis as an energy source, the 30S subunit complexes with initiation factors IF1, IF2 and IF3. This complex binds the mRNA with the anticodon (3 -UAC-5 ) of the jV-formylmethionyl-tRNA M t (fMet-tRNAf 11 1) hydrogen bonding to the start codon (5 -AUG — 3 ) of the mRNA, the fMet-tRNA -Ml 1 binding at the so-called P site with release of IF3. 

Chain initiation Initiation requires the smaller ribosomal subunit, an initiation tRNA (with a 5 -CAU-3 anticodon), mRNA with its initiator codon (S -AUG-3 ), and several initiation factors, all of which form the ribosomal initiation complex. The initiation complex is near completion when a tRNA carrying the amino acid methionine hydrogen bonds to the AUG codon on the mRNA. When these components are in place, the larger ribosomal subunit joins the complex in such a way that the initiator met-tRNAjjj j is localized in the P- or peptidyl site (Figure 9-1). The chain initiation phase ends and a second amino acid can be inserted. 

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