Initiation factors in the early events of mRNA translation in bacteria

Initiation of mRNA translation in bacteria proceeds through several steps. The early events entail the selection of the mRNA initiation codon by the 30S ribosomal subunit with the help of initiator fMet-tRNA and initiation factors IF1, IF2 and IF3, which all together form a "30S initiation complex." Subsequently, the choice of the start site is made irreversible by the binding of the 50S ribosomal subunit to the 30S initiation complex, which expels IF1 and IF3 and yields a "70S initiation complex." Among the late initiation events are the adjustment of fMet-tRNA in the ribosomal P site, the A-site binding of the EF-Tu ternary complex carrying the elongator aminoacyl-tRNA encoded by the second mRNA codon, and the subsequent formation of the first peptide bond yielding the initiation dipeptide. At this stage, with the first translocation, the ribosome enters the elongation phase. Before discussing in more detail the mechanistic aspects of these steps, we describe the molecular and structural bases underlying the specific interactions among initiation components, their mutual selective recognition, and their mechanism of action.