Ribosomal subunits were isolated from rat liver mitochondria and examined by electron microscopy. Despite their small sedimentation coefficients (28S and 39S, respectively) both ribosomal subunits were of approximately the same size as the 30S and the 50S ribosomal subunits from Escherichia coli . In their gross features mitochondrial ribosomal subunits were similar to ribosomal subunits from other organisms. However, they revealed some unusual fine structural details. Thus on the head of the small ribosomal subunit two characteristic incisions were frequently seen, and in the crown views of the large subunit characteristic incisions below the L1-protuberance and at the base of the particle were observed. Some of these structural features have so far not been observed on ribosomes of other origin, and they are therefore discussed in terms of the molecular composition of mammalian mitochondrial ribosomes.
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