The ribosomal subunit composition of mitochondria is similar to:
Endosymbiosis is supported by mitochondrial translation apparatus resembling bacteria. Mitochondrial ribosomes, mitoribosomes, in mammals sediment as fifty five S composed of twenty eight S small and thirty nine S large subunits, total mass two point seven megadaltons, containing two rRNAs and about eighty two proteins where only thirty percent homologous to bacterial proteins and rest mitochondria-specific adaptations for membrane protein synthesis. They retain bacterial features such as leaderless initiation, formylmethionine start, lack of five prime cap, and sensitivity to tetracycline, chloramphenicol, erythromycin that block bacterial seventy S but leave cytosolic eighty S unaffected, explaining antibiotic side effects like mitochondrial toxicity. They translate thirteen highly hydrophobic subunits of respiratory chain encoded by mtDNA remaining from endosymbiont genome. Evolution retained minimal genome but imported most ribosomal proteins from nuclear genome via TOM/TIM translocases. No similarity to nuclear envelope constituents or cytosolic RNA granules which contain different RNA processing complexes. Structural cryo-EM reveals bacterial core decorated with extra protein shell accommodating insertion into inner membrane via Oxa1 insertase.
Ref: Gray Annu Rev Genet; mitochondria endosymbiont mitoribosome 55S homologous to bacterial 70S.