Mitochondrial translation termination, recycling, and rescue in in-frame and out-of-frame contexts

However, the molecular mechanisms of how mitochondrial ribosomes terminate the translation and are recycled remain obscure. Harnessing the high-resolution mitochondrial Ribo-Seq and Disome-Seq, here we showed that mtRF1L is a universal release factor for all stop codons, whereas mtRF1 only recognizes AGA/AGG noncanonical stop codons. mtRF1 also terminates mitoribosomes translating out-of-frame ORFs ended with AGA/AGG, explaining the presence of this protein in some vertebrates that do not possess the noncanonical stops in the main ORFs.

We also found that mtRRF and mtIF3 require the mitoribosome recycling on stop codons and also reinitiation of internal ORF translation; mitoribosomes stalled at start codons are major substrates of rescue factors of ICT1, mtRF-R, and mtRES1; HEMK1-methylation of release factors enhances the termination reaction on the stop codons. Our results provide an overview of the dynamics of mitoribosomes ending protein synthesis.