Selective stimulation of translation of leaderless mRNA by initiation factor 2: evolutionary implications for translation

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Abstract
Translation initiation in bacteria involves a stochastic binding mechanism in which the 30S ribosomal subunit first binds either to mRNA or to initiator tRNA, fMet‐tRNAfMet. Leaderless λ c I mRNA did not form a binary complex with 30S ribosomes, which argues against the view that ribosomal recruitment signals other than a 5′‐terminal start codon are essential for translation initiation of these mRNAs. We show that, in Escherichia coli , translation initiation factor 2 (IF2) selectively stimulates translation of λ c I mRNA in vivo and in vitro . These experiments suggest that the start codon of leaderless mRNAs is recognized by a 30S–fMet‐tRNAfMet–IF2 complex, an intermediate equivalent to that obligatorily formed during translation initiation in eukaryotes. We further show that leaderless λ c I mRNA is faithfully translated in vitro in both archaebacterial and eukaryotic translation systems. This suggests that translation of leaderless mRNAs reflects a fundamental capability of the translational apparatus of all three domains of life and lends support to the hypothesis that the translation initiation pathway is universally conserved.