Mammalian ribosomes exist in two forms: cytoplasmic ribosomes and mitochondrial ribosomes (mitoribosomes)—which produce proteins critical for mitochondrial function. Since mitochondria are believed to have arisen from endosymbiosis of a eubacterium, it had been assumed that the mitoribosome would be structurally more closely related to bacterial ribosomes, but they were observed to have a protein-to-RNA ratio of 69% protein:31% RNA, almost a complete reversal of the 33% protein:67% RNA in bacterial ribosomes. This increase in protein content and/or size is thought to be compensation for truncated rRNA segments found in mitoribosomes, but the evidence for this assumption has been elusive. In the October 3 Cell, Manjuli R. Sharma and colleagues at the New York State Department of Health report a cryoelectron microscopic (cryo-EM) map of a mitoribosome that suggests that instead of compensating, many of the proteins occupy novel positions in the mitoribosome (Cell, 115:97-108, October 3, 2003)....

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