The General Occurrence of 55 S Ribosomes in Mammalian Liver Mitochondria

Ribosomes isolated from highly purified rat, cow, pig, or rabbit liver mitochondria occur predominantly as 55 to 56 S particles under conditions that stabilize 70 S Escherichia coli monosomes. These ribosomes are of mitochondrial origin since they incorporate 3H-leucine when mitochondria are incubated in vitro, under conditions in which extramitochondrial ribosomes are inactive. They are actively involved in mitochondrial protein synthesis, since puromycin strips 55 S ribosomes of their nascent peptide chains while inhibiting mitochondrial protein synthesis. Dissociation studies show that the 55 S ribosome is a mitochondrial monosome, composed of a 28 S small subunit and a 39 S subunit. These mammalian 55 S mitochondrial ribosomes are the smallest monosomes known, and they differ in several respects from extramitochondrial ribosomes, and even from the mitochondrial ribosomes of Neurospora, yeast, and Tetrahymena. They appear to constitute a unique class of miniature ribosomes of the prokaryotic type that are peculiar to animal mitochondria.