Modulation of ribosomal recruitment to 5 '-terminal start codons by translation initiation factors IF2 and IF3

Sequence determinants and structural features of the RNA govern mRNA-riboso me interaction in bacteria. However, ribosomal recruitment to leaderless mR NAs, which start directly with the AUG start codon and do not bear a Shine- Dalgarno sequence like canonical mRNAs, does not appear to rely on 16S rRNA -mRNA interactions. Here, me have studied the effects of translation initia tion factors IF2 and IF3 on 30S initiation at a 5'-terminal AUG and at a co mpeting downstream canonical ribosome binding site. We show that IF2 affect s the forward kinetics of 30S initiation complex formation at the 5'-termin al AUG as well as the stability of these complexes, Moreover, the IF2:IF3 m olar ratio,vas found to play a decisive role in translation initiation of a leaderless mRNA both in vitro and in vivo indicating that the translationa l efficiency of an mRNA is not only intrinsically determined but can be alt ered depending on the availability of components of the translational machi nery, (C) 2001 Published by Elsevier Science B.V. on behalf of the Federati on of European Biochemical Societies.