Mammalian mitochondrial ribosomes contain two prokaryotic-like rRNAs, 12S and 16S, both encoded by mitochondrial DNA. As opposed to cytosolic ribosomes, however, these ribosomes are not thought to contain 5S rRNA. For this reason, it has been unclear whether 5S rRNA, which can be detected in mitochondrial preparations, is an authentic organellar species imported from the cytosol or is merely a copurifying cytosol-derived contaminant. We now show that 5S rRNA is tightly associated with highly purified mitochondrial fractions of human and rat cells and that 5S rRNA transcripts derived from a synthetic gene transfected transiently into human cells are both expressed in vivo and present in highly purified mitochondria and mitoplasts. We conclude that 5S rRNA is imported into mammalian mitochondria, but its function there still remains to be clarified. INTRODUCTIONMitochondria are organelles present in virtually all eukaryotic cells, responsible for most of the energy production required for normal cellular homeostasis. Human mitochondria possess their own DNA (mtDNA), which encodes the two RNA species present in mitochondrial ribosomes (12S and 16S rRNAs), a full set of transfer RNAs (tRNAs) (22 genes) required for protein synthesis (O'Brien et al., 1990), and 13 polypeptides, all constituents of respiratory chain complexes (Anderson et al., 1981).Because mitochondria possess a fully functional genetic apparatus capable of replication, transcription, and translation, they are often considered to be intracellular organelles endowed with a partial autonomy. This autonomy, however, is more apparent than real; in addition to the components encoded by mtDNA, all of the remaining enzymes required for proper functioning of the mitochondrion's genetic machinery (such as DNA and RNA polymerases, ribosomal proteins, aminoacyl tRNA synthetases, etc.) are encoded by nuclear DNA (nDNA), synthesized in the cytosol, and imported into the organelle (Schatz and Dobberstein, 1996;Neupert, 1997). The same is true for all enzymes involved in the myriad metabolic pathways that take place in the mitochondrial environment.Interestingly, at least two mitochondrial enzymes, RNase MRP (a site-specific endoribonuclease involved in primer RNA metabolism in mammalian mitochondria [Chang and Clayton, 1987;Topper and Clayton, 1990;Li et al., 1994]) and RNase P (an endoribonuclease involved in tRNA processing [Doerson et al., 1985]), are ribonucleoproteins that contain an RNA moiety that is encoded by nDNA and is imported into the organelle. However, unlike the mechanisms for protein import into mitochondria, the mechanisms of RNA import into mitochondria are poorly understood.The importation of RNA into mitochondria was first postulated over 30 years ago, as a corollary to mitochondrial protein synthesis and the lack of a full set of tRNA genes in the mitochondrial genome of Tetrahymena (Suyama and Eyer, 1967). This postulate was proven recently (Rusconi and Cech, 1996), and the import of tRNAs into mitochondria has now been (Schneider, 1994;Kaz...

The mitochondrial DNA A3243G mutation causes neuromuscular disease. To investigate the muscle-specific pathophysiology of mitochondrial disease, rhabdomyosarcoma transmitochondrial hybrid cells (cybrids) were generated that retain the capacity to differentiate to myotubes. In some cases, striated muscle-like fibres were formed after innervation with rat embryonic spinal cord. Myotubes carrying A3243G mtDNA produced more reactive oxygen species than controls, and had altered glutathione homeostasis. Moreover, A3243G mutant myotubes showed evidence of abnormal mitochondrial distribution, which was associated with down-regulation of three genes involved in mitochondrial morphology, Mfn1, Mfn2 and DRP1. Electron microscopy revealed mitochondria with ultrastructural abnormalities and paracrystalline inclusions. All these features were ameliorated by anti-oxidant treatment, with the exception of the paracrystalline inclusions. These data suggest that rhabdomyosarcoma cybrids are a valid cellular model for studying muscle-specific features of mitochondrial disease and that excess reactive oxygen species production is a significant contributor to mitochondrial dysfunction, which is amenable to anti-oxidant therapy.