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