GSE119655
Small is big in Arabidopsis mitochondrial ribosome
Mitochondria are responsible for energy production through aerobic respiration and represent  the powerhouse of eukaryotic cells. Their metabolism and gene expression processes  combine bacterial-like features and traits that evolved in eukaryotes. Among mitochondrial  gene expression processes, translation remains the most elusive. In plants, while numerous  pentatricopeptide repeat (PPR) proteins are involved in all steps of gene expression, their  function in translation remains unclear. Here we present the biochemical characterisation of  Arabidopsis mitochondrial ribosomes and identify their protein subunit composition.  Complementary biochemical approaches identify 19 plant specific mitoribosome proteins,  among which 10 are PPR proteins. The knock out mutations of ribosomal PPR (rPPR) genes  result in distinct macroscopic phenotypes including lethality or severe growth delays. The  molecular analysis of rppr1 mutants using ribosome profiling as well as the analysis of  mitochondrial protein levels reveal that rPPR1 is a generic translation factor, which is a novel  function for PPR proteins. Finally, single particle cryo-electron microscopy reveals the unique  structural architecture of Arabidopsis mitoribosomes, characterised by a very large small  ribosomal subunit, larger than the large subunit, bearing an additional RNA domain grafted  onto the head. Overall, our results show that Arabidopsis mitoribosomes are substantially  divergent from bacterial and other eukaryote mitoribosomes, both in terms of structure and of  protein content. Overall design: mitochondrial ribosome footprints from flowers of wildtype or PPR336 (At1g61870) mutant.