Structural insights into cauliflower mitoribosome in translation state and in association with a late assembly factor

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 plant-specific proteins. However, the structural specificities of plant mitoribosomes in translation state remained unknown. We used cryo-electron microscopy to provide a high-resolution structural characterization of the cauliflower mitoribosome, in translating and maturation states. The structure reveals the mitoribosome bound with a tRNA in the peptidyl site, along with a segment of mRNA and a nascent polypeptide. Moreover, using structural data, nanopore sequencing and mass spectrometry, we identify a set of 19 ribosomal RNA modifications. 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 angiosperm plant mitochondria, revealing its distinct features compared to other eukaryotic lineages.