EVIDENCE FOR THE PRESENCE OF 5S RIBOSOMAL-RNA IN MAMMALIAN MITOCHONDRIA

Mammalian mitochondrial ribosomes contain two prokaryotic-like rRNAs, 12S and 16S, both encoded by mitochondrial DNA. As opposed to cytosoli c ribosomes, however, these ribosomes are not thought to contain 5S rR NA. For this reason, it has been unclear whether 5S rRNA, which can be detected in mitochondrial preparations, is an authentic organellar sp ecies imported from the cytosol or is merely a copurifying cytosol-der ived contaminant. We now show that 5S rRNA is tightly associated with highly purified mitochondrial fractions of human and rat cells and tha t 5S rRNA transcripts derived from a synthetic gene transfected transi ently into human cells are both expressed in vivo and present in highl y purified mitochondria and mitoplasts. We conclude that 5S rRNA is im ported into mammalian mitochondria, but its function there still remai ns to be clarified.