The Chlamydomonas mitochondrial ribosome: how to build a ribosome from RNA fragments

Mitochondria are the powerhouse of eukaryotic cells. They possess their own gene expression machineries where highly divergent and specialized ribosomes, named hereafter mitoribosomes, translate the few essential messenger RNAs still encoded by mitochondrial genomes. Here, we present a biochemical and structural characterization of the mitoribosome in the model green alga Chlamydomonas reinhardtii, as well as a functional study of some of its specific components. Single particle cryo-electron microscopy resolves how the Chlamydomonas mitoribosome is assembled from 13 rRNA fragments encoded by separate non-contiguous gene pieces. Novel proteins, mainly helical repeat proteins, including OPR, PPR and mTERF proteins are found in Chlamydomonas mitoribosome, revealing the first structure of an OPR protein in complex with its RNA target. Targeted amiRNA silencing indicated that the novel ribosomal proteins are required for mitoribosome integrity. Finally, we use cryo-electron tomography to show that Chlamydomonas mitoribosomes are attached to the mitochondrial inner membrane via two contact points mediated by Chlamydomonas-specific proteins. Our study expands our understanding of the mitoribosome diversity and the various strategies they adopt for membrane tethering. Highlights* Structure of the Chlamydomonas reinhardtii mitoribosome * Fragmented ribosomal RNAs are stabilized by highly intertwined interactions with Chlamydomonas-specific proteins * Specific r-proteins are essential for rRNA homeostasis and respiratory fitness * Cryo-ET reveals the mitoribosome association to the inner mitochondrial membrane