Structural insights into maturation and translation of a plant mitoribosome

Ribosomes are key molecular machines that translate mRNA into proteins. Mitoribosomes are specific ribosomes found in mitochondria, which have been shown to be remarkably diverse across eukaryotic lineages. In plants, they possess unique features, including additional rRNA domains stabilized by numerous pentatricopeptide repeat proteins. However, the molecular mechanisms of translation by plant mitoribosomes remain largely unknown. Here, we use cryo-electron microscopy to provide a high-resolution structural characterization of the flowering plant mitoribosome, in translating and maturation states. The structure reveals the mitoribosome bound to tRNA in the peptidyl site, along with a segment of mRNA and a nascent polypeptide. Moreover, we identify an extensive set of ribosomal RNA modifications that we confirmed by nanopore sequencing. Additionally, we observe a late assembly intermediate of the small ribosomal subunit, in complex with the RsgA assembly factor. This reveals how a plant-specific extension of RsgA blocks the mRNA channel to prevent premature mRNA association before complete small subunit maturation. Our findings elucidate key aspects of translation in flowering plant mitochondria, revealing its distinct features compared to other eukaryotic lineages.