Translation Initiation in Bacteria

This chapter summarizes the most important advances concerning structural and mechanistic aspects of translation initiation in bacteria which have occurred since the appearance of the last reviews on this subject. The small ribosomal subunit (30S) interacts with mRNA and fMet-tRNA in stochastic order to yield a bona fide 30S initiation complex through the rearrangement, kinetically controlled by the three initiation factors, of an unstable kinetic intermediate called the pre-ternary complex. Systematic variations of the translation initiation region (TIR) elements in the in vivo translation of reporter genes in E. coli and Bacillus subtilis have produced similar results in two similar analyses. The differences observed in a study between heterologous and homologous proteins and the sometimes contradictory conclusions reached by similar studies concerning the relevance of specific TIR elements and the consequent criteria for optimization of translation remind us once again that each gene (mRNA) might be endowed with particular, not easily predictable properties. The dispensable nature of the Shine-Dalgarno (SD) sequence is evidenced by the existence of leaderless mRNAs which begin directly with an AUG initiation triplet. The recognition and binding of fMet-tRNA by IF2 play crucial roles in the translation initiation pathway of bacteria. The understanding of the translation initiation pathway cannot be considered satisfactory without full elucidation of the role played in this process by GTP and by the IF2-dependent GTPase.