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Summary To study the function of Rli1/ABCE1 in vivo, we used ribosome profiling and biochemistry to characterize its contribution to ribosome recycling. When Rli1 levels were diminished, 80S ribosomes accumulated both at stop codons and in the adjoining 3′UTRs of most mRNAs. Frequently, these ribosomes reinitiated translation without the need for a canonical start codon, as small peptide products predicted by 3′UTR ribosome occupancy in all three reading frames were confirmed by western analysis and mass spectrometry. Eliminating the ribosome-rescue factor Dom34 dramatically increased 3′UTR ribosome occupancy in Rli1 depleted cells, indicating that Dom34 clears the bulk of unrecycled ribosomes. Thus, Rli1 is crucial for ribosome recycling in vivo and controls ribosome homeostasis. 3′UTR translation occurs in wild-type cells as well, and observations of elevated 3′UTR ribosomes during stress suggest that modulating recycling and reinitiation is involved in responding to environmental changes. [ABSTRACT FROM AUTHOR]

The mitochondrial translation system originates from a bacterial ancestor but has substantially diverged in the course of evolution. Here, we use single-particle cryo-electron microscopy (cryo-EM) as a screening tool to identify mitochondrial translation termination mechanisms and to describe them in molecular detail. We show how mitochondrial release factor 1a releases the nascent chain from the ribosome when it encounters the canonical stop codons UAA and UAG. Furthermore, we define how the peptidyl-tRNA hydrolase ICT1 acts as a rescue factor on mitoribosomes that have stalled on truncated messages to recover them for protein synthesis. Finally, we present structural models detailing the process of mitochondrial ribosome recycling to explain how a dedicated elongation factor, mitochondrial EFG2 (mtEFG2), has specialized for cooperation with the mitochondrial ribosome recycling factor to dissociate the mitoribosomal subunits at the end of the translation process. [Display omitted] • mtRF1a triggers polypeptide release at canonical stop codons UAA and UAG • ICT1 rescues the mitoribosome when stalled on aberrant, truncated mRNAs • mtRRF and mtEFG2 promote ribosome recycling by dissolution of intersubunit contacts • mtEFG2 is electrostatically and sterically optimized for ribosome recycling The mitochondrial translation system is very different from the machinery of the eukaryotic cytosolic protein synthesis. Using a combination of biochemical reconstitution and cryoelectron microscopy, Kummer et al. uncover how mitoribosomes terminate polypeptide synthesis on different mRNA substrates and how the ribosome prepares for the production of the next protein. [ABSTRACT FROM AUTHOR]