Preferential Selection of the 5′-Terminal Start Codon on Leaderless mRNAs by Mammalian Mitochondrial Ribosomes

Mammalian mitochondrial mRNAs are basically leaderless, having few or no untranslated nucleotides prior to the 5'-start codon. We demonstrate here that mammalian mitochondrial 55 S ribosomes preferentially form initiation complexes at a 5'-terminal AUG codon over an internal AUG. The preferential use of the 5'-start codon is also seen on mitochondrial 28 S small subunits, which suggests that mitochondrial translation initiation on leaderless mRNAs does not require the large ribosomal subunit. The selection of the 5'-AUG is dependent on the presence of fMet-tRNA and is enhanced by the presence of the mitochondrial initiation factor IF2(mt). In prokaryotes, IF3 is believed to antagonize initiation on leaderless mRNAs. However, IF3(mt) stimulates initiation complex formation on leaderless mRNAs when tested with 55 S ribosomes. The addition of even a few nucleotides 5' to the AUG codon significantly reduces the efficiency of initiation, highlighting the importance of the leaderless or nearly leaderless nature of mitochondrial mRNAs. In addition, very few initiation complexes could form on a hybrid mRNA construct consisting of tRNA(Met) attached at the 5'-end of a mitochondrial protein-coding sequence. This observation demonstrates that post-transcriptional processing