Structure of a mitochondrial ribosome with fragmented rRNA in complexwith membrane-targeting elements

Mitoibosomes of green alga belong to a diverged type with extreme fragmentation in rRNA andprotein material. We present a 2.9-Å resolution structure of an algal mitoribosome with reducedrRNA that is split in to 13 fragments. The architecture features a fragmentation pattern that hassimilarities with apicomplexan parasites. The unique phenomena of the rRNA include permutationand incorporation of a non-canonical and reduced mt-5S rRNA. On the protein level, elevenperipherally associated HEAT-repeat proteins involved in rRNA binding, and a specific trimer ofmL116 binds 3’ termini of three rRNA fragments. In the exit tunnel, mL128 constricts the path,and mL105 a homolog of a membrane targeting component mediates contacts with an innermembrane-bound insertase. Although overall protein content at the tunnel exit site of themitoribosome varies among eukaryotes, we find that the mechanisms of the constriction andmembrane tethering are shared between the algal and mammalian lineages. Therefore, our datareveals characteristics of fragmented rRNA an unexpected convergent evolution in the regulationof protein synthesis in mitochondria