LAUSR

The genetic code that specifies the identity of amino acids incorporated into proteins during protein synthesis is almost universally conserved. Mitochondrial genomes feature deviations from the standard genetic code, including the reassignment of two arginine codons to stop codons. The protein required for translation termination at these noncanonical stop codons to release the newly synthesized polypeptides is not currently known. In this study, we used gene editing and ribosomal profiling in combination with cryo–electron microscopy to establish that mitochondrial release factor 1 (mtRF1) detects noncanonical stop codons in human mitochondria by a previously unknown mechanism of codon recognition. We discovered that binding of mtRF1 to the decoding center of the ribosome stabilizes a highly unusual conformation in the messenger RNA in which the ribosomal RNA participates in specific recognition of the noncanonical stop codons. Description Editor’s summary Termination of translation in human mitochondria involves recognition of noncanonical stop codons that in other genomes code for amino acids. To reveal which factor is responsible for this process and to understand the molecular mechanism of stop codon recognition, Saurer et al. used a combination of gene editing, ribosomal profiling and cryo–electron microscopy to investigate the termination process in vivo and in vitro. The specific recognition of the noncanonical stop codon by the identified release factor results in a distortion of the messenger RNA and requires participation of the ribosomal RNA, a recognition mechanism that was not observed for canonical termination complexes. —Di Jiang Noncanonical stop codons AGA and AGG in human mitochondrial translation are recognized by mitochondrial release factor 1.