Cryo-EM structure of the RNA-rich plant mitochondrial ribosome.

The vast majority of eukaryotic cells contain mitochondria, essential powerhouses and metabolic hubs. These organelles have a bacterial origin and were acquired during an early endosymbiosis event. Mitochondria possess specialized gene expression systems composed of various molecular machines, including the mitochondrial ribosomes (mitoribosomes). Mitoribosomes are in charge of translating the few essential mRNAs still encoded by mitochondrial genomes. While chloroplast ribosomes strongly resemble those of bacteria, mitoribosomes have diverged significantly during evolution and present strikingly different structures across eukaryotic species. In contrast to animals and trypanosomatids, plant mitoribosomes have unusually expanded ribosomal RNAs and have conserved the short 5S rRNA, which is usually missing in mitoribosomes. We have previously characterized the composition of the plant mitoribosome, revealing a dozen plant-specific proteins in addition to the common conserved mitoribosomal proteins. In spite of the tremendous recent advances in the field, plant mitoribosomes remained elusive to high-resolution structural investigations and the plant-specific ribosomal features of unknown structures. Here, we present a cryo-electron microscopy study of the plant 78S mitoribosome from cauliflower at near-atomic resolution. We show that most of the plant-specific ribosomal proteins are pentatricopeptide repeat proteins (PPRs) that deeply interact with the plant-specific rRNA expansion segments. These additional rRNA segments and proteins reshape the overall structure of the plant mitochondrial ribosome, and we discuss their involvement in the membrane association and mRNA recruitment prior to translation initiation. Finally, our structure unveils an rRNA-constructive phase of mitoribosome evolution across eukaryotes.