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26 Control of Translation Initiation in Prokaryotes

Harry O. Voorma


The initiation phase of translation involves the binding of the initiator tRNA, formyl-methionyl-tRNAf (fMet-tRNAf), and messenger RNA to the 70S ribosome, recognition of the initiator codon, and the precise phasing of the reading frame of the mRNA. The process not only results in the selection of a specific mRNA for translation, but also determines the rate at which the encoded protein is synthesized since the initiation phase is thought to be rate-limiting in most circumstances. The pathway of initiation (Fig. 1) involves the dissociation of 70S ribosomes into 30S and 50S subunits, the binding of fMet-tRNAf and mRNA to the 30S subunit, the junction of the 50S subunit with the 30S preinitiation complex, and the ejection of initiation factors.

The pool of free 30S particles is controlled by two initiation factors, IF1 and IF3, which shift the equilibrium of the 70S particle and its subunits toward dissociation. The 30S subunit, carrying IF3, IF1, and probably IF2 as well, interacts with mRNA, fMet-tRNAf, and GTP through a series of intermediates giving rise to the 30S initiation complex. IF2 is implicated in recognizing the acylated aminoacyl end of fMet-tRNAf, thereby assuring that fMet-tRNAf, not some other aminoacyl-tRNA, is bound to the 30S subunit. IF3 recognizes the anticodon stem of fMet-tRNAf and monitors the anticodon interaction with the initiation codon. The resulting complex places the fMet-tRNAf in the P-site of the 30S subunit (de Smit and van Duin 1990a; McCarthy and Brimacombe 1994). The junction with the 50S ribosomal subunit then takes place,...

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