Transplantation of Saccharomyces cerevisiae Rmd9p peptide into mammalian mitochondrial IF2 substitutes for the IF1 function in Escherichia coli

Mitochondrial translation machinery exhibits similarities with the bacterial translation apparatus. Of the three bacterial translation initiation factors (IF1, IF2 and IF3), two (IF2 and IF3) have homologues in mitochondria (mtIF2 and mtIF3). A high conservation of decoding nucleotides in the ribosomal A-site suggests relevance of IF1-like proteins in mitochondria. The mitochondrial translation machineries have evolved with different solutions for the IF1 function. However, in <i>Saccharomyces cerevisiae</i>, the identity of such a protein remains unknown. Here, based on sequence alignment with human mtIF2, we deduced that Rmd9p may contribute to an IF1-like function in <i>S. cerevisiae</i>. Our genetic analyses show that Rmd9p is required for mitochondrial translation. In addition, we show that a sequence from Rmd9p, pivotal for its mitochondrial function, when inserted into mtIF2, substitutes for both the IF2 and IF1 functions in an established model of <i>Escherichia coli</i>. Interestingly, while the mutations at the critical residues in the Rmd9p peptide compromise the IF1 function, the mutant peptide is still able to support <i>E. coli</i> growth, suggesting that the structure (rather than the precise sequence) of the IF1-like insert domain in mitochondrial IF2 plays a major role in the recognition of the decoding nucleotides in the ribosomal A-site.