Mitochondria generate most of a eukaryotic cell’s energy (ATP) through oxidative phosphorylation. Ribosomes are the cell’s protein-making machinery, translating genetic instructions into functional proteins. Given these roles, do mitochondria also possess their own protein-synthesizing ribosomes?
Mitochondria’s Unique Ribosomes
Mitochondria do contain their own ribosomes, distinct from those in the cell’s cytoplasm. These are commonly referred to as mitoribosomes. Their primary function is to translate specific messenger RNAs (mRNAs) encoded by the organelle’s own genetic material. Mitoribosomes are composed of two subunits that perform protein synthesis.
How Mitochondrial Proteins Are Made
Mitoribosomes synthesize a small, important set of proteins directly within the mitochondria. In humans, for instance, mitoribosomes translate about 13 proteins, all components of the electron transport chain. These proteins are encoded by the mitochondrial DNA (mtDNA), a circular genome residing within the organelle. Conversely, the vast majority of proteins necessary for mitochondrial function, estimated to be over a thousand in humans, are encoded by the cell’s nuclear DNA. These nuclear-encoded proteins are synthesized by cytoplasmic ribosomes and subsequently imported into the mitochondria.
Distinct Features of Mitochondrial Ribosomes
Mitoribosomes exhibit several characteristics that set them apart from cytoplasmic ribosomes. Mammalian mitoribosomes, for example, are smaller, having a sedimentation coefficient of 55S, compared to 80S ribosomes in the cytoplasm. This size difference is partly due to their distinct composition; human mitoribosomes are approximately 70% protein and 30% ribosomal RNA (rRNA), a higher protein-to-RNA ratio than cytoplasmic ribosomes. They also feature minimized rRNA components, lacking certain rRNAs such as the 5S rRNA found in cytoplasmic and bacterial ribosomes.
The protein component of mitoribosomes includes many unique mitochondrial-specific proteins. This structure also impacts their sensitivity to certain compounds. Mitoribosomes can be inhibited by some antibiotics, such as chloramphenicol and oxazolidinones, which target bacterial protein synthesis but typically do not affect eukaryotic cytoplasmic ribosomes. This susceptibility can lead to adverse side effects in humans if these antibiotics interfere with mitochondrial protein production.
Mitochondria’s Ancient Partnership
The unique features of mitoribosomes, particularly their similarities to bacterial ribosomes, provide strong support for the endosymbiotic theory. This theory proposes that mitochondria originated from ancient bacteria that were engulfed by early eukaryotic cells. The bacteria and the host cell then formed a mutually beneficial, or symbiotic, relationship.
Key evidence supporting this evolutionary partnership includes their own circular DNA, similar in structure to bacterial chromosomes, and their ability to replicate independently through a process resembling bacterial binary fission. The bacterial-like characteristics of mitoribosomes, such as their size and sensitivity to certain antibiotics, reinforce that mitochondria maintain remnants of their ancient bacterial ancestors. This evolutionary history explains why these cellular components possess their own specialized protein-synthesizing machinery.