High-resolution electrophoretic fractionation and partial characterization of the mitochondrial translation products from HeLa cells

Mitochondrial genetic control of assembly and function of complex I in mammalian cells

Sixteen years ago, we demonstrated, by immunological and biochemical approaches, that seven subunits of complex I are encoded in mitochondrial DNA (mtDNA) and synthesized on mitochondrial ribosomes in mammalian cells. More recently, we carried out a biochemical, molecular, and cellular analysis of a mutation in the gene for one of these subunits, ND4, that causes Leber's hereditary optic neuropathy (LHON). We demonstrated that, in cells carrying this mutation, the mtDNA-encoded subunits of complex I are assembled into a complex, but the rate of complex I-dependent respiration is decreased. Subsequently, we isolated several mutants affected in one or another of the mtDNA-encoded subunits of complex I by exposing established cell lines to high concentrations of rotenone. Our analyses of these mtDNA mutations affecting subunits of complex I have shown that at least two of these subunits, ND4 and ND6, are essential for the assembly of the enzyme. ND5 appears to be located at the periphery of the enzyme and, while it is not essential for assembly of the other mtDNA-encoded subunits into a complex, it is essential for complex I activity. In fact, the synthesis of the ND5 polypeptide is rate limiting for the activity of the enzyme.

Phenotypic expression of mtDNA heteroplasmy in the skeletal muscle of patients with oculomyopathy: defect in mitochondrial protein synthesis

The biochemical consequences of mtDNA heteroplasmy, observed in patients with a range of diseases associated with the mitochondrial respiratory enzymes deficiency is of particular interest, as they might provide information with regard to the regulatory interactions which govern the expression of the human mitochondrial genome. Three patients with chronic progressive external ophthalmoplegia (CPEO) were investigated to study the consequences of mtDNA heteroplasmy on mitochondrial protein synthesis. All 3 patients exhibited partially deleted mtDNA species (varying in size from 10.5 to 14 kb) in their skeletal muscle, which co-existed with the normal 16.5 kb mtDNA. The examination of mitochondrial translation products following the incorporation of [35S]methionine by isolated mitochondria, showed grossly abnormal patterns of mitochondrial translation products, suggesting a major disturbance in the regulation of mitochondrial protein synthesis.

Isolation and characterization of mitochondria from human B lymphoblastoid cell lines

Mitochondria were isolated from detergent-treated Epstein-Barr virus-transformed human lymphocytes to examine their potential use in the study of the functional expression of genetic disorders of the respiratory chain. The increase of cytochrome c oxidase activity in the mitochondrial fraction indicated a 6-fold purification of intact mitochondria. Polarographic and spectrophotometric studies revealed that the isolated mitochondria were functionally well preserved. Furthermore, the isolated mitochondria supported an active in organello protein synthesis, which was dependent on the presence of a respiratory substrate generating ATP and was essentially abolished by chloramphenicol or by a specific respiratory chain inhibitor, such as antimycin. Thus, B lymphoblastoid cell lines constitute a valuable source of mitochondria to investigate mitochondrial functions in patients affected by respiratory chain disorders.

Evidence for non-uniform translation of individual polypeptides in rat liver mitochondria

[35S]Methionine labelled mitochondrial translation products were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) in the presence of 8 M urea. Following fluorography and scanning, the synthesis time of major polypeptides was determined. It was found that: (1) from 9 to 16 min are required to synthesize various complete polypeptides whose size differs by a factor of more than 5. (2) The specific radioactivity of complete polypeptides with electrophoretic mobilities corresponding to 6 kDa, 16 kDa and 31.5 kDa remains constant during the first 10 min of labelling and attains its final value only after a certain lag-period. (3) In the electrophoretic pattern at least two bands (6.3 kDa and 7.2 kDa) among other bands are nascent, and they accumulate in amounts comparable with that of the complete ones. These data are in agreement with the model of non-uniform elongation rate with a sharp delay (pause) during translation of internal mRNA regions.

In vitro genetic transfer of protein synthesis and respiration defects to mitochondrial DNA-less cells with myopathy-patient mitochondria

A severe mitochondrial protein synthesis defect in myoblasts from a patient with mitochondrial myopathy was transferred with myoblast mitochondria into two genetically unrelated mitochondrial DNA (mtDNA)-less human cell lines, pointing to an mtDNA alteration as being responsible and sufficient for causing the disease. The transfer of the defect correlated with marked deficiencies in respiration and cytochrome c oxidase activity of the transformants and the presence in their mitochondria of mtDNA carrying a tRNA(Lys) mutation. Furthermore, apparently complete segregation of the defective genotype and phenotype was observed in the transformants derived from the heterogeneous proband myoblast population, suggesting that the mtDNA heteroplasmy in this population was to a large extent intercellular. The present work thus establishes a direct link between mtDNA alteration and a biochemical defect.

In vitro genetic transfer of protein synthesis and respiration defects to mitochondrial DNA-less cells with myopathy-patient mitochondria

A severe mitochondrial protein synthesis defect in myoblasts from a patient with mitochondrial myopathy was transferred with myoblast mitochondria into two genetically unrelated mitochondrial DNA (mtDNA)-less human cell lines, pointing to an mtDNA alteration as being responsible and sufficient for causing the disease. The transfer of the defect correlated with marked deficiencies in respiration and cytochrome c oxidase activity of the transformants and the presence in their mitochondria of mtDNA carrying a tRNA(Lys) mutation. Furthermore, apparently complete segregation of the defective genotype and phenotype was observed in the transformants derived from the heterogeneous proband myoblast population, suggesting that the mtDNA heteroplasmy in this population was to a large extent intercellular. The present work thus establishes a direct link between mtDNA alteration and a biochemical defect.

Antimitochondrial autoantibodies of primary biliary cirrhosis as a novel probe in the study of 2-oxo acid dehydrogenases in patients with mitochondrial myopathies

Autoantibodies present in the autoimmune disease primary biliary cirrhosis react by immunoblotting with four human skeletal muscle mitochondrial antigens of 70 kDa, 52 kDa, 50 kDa and 45 kDa, identified as the lipoate acetyl transferases (E2) of the pyruvate dehydrogenase, component X of E2 pyruvate dehydrogenase, E2 of 2-oxo glutarate dehydrogenase and E2 of branched-chain 2-oxo acid dehydrogenase complexes respectively. These autoantibodies have been employed as a novel probe to study whether there is a defect in the synthesis of the 2-oxo acid dehydrogenase complexes in patients with mitochondrial respiratory chain disorders. The reactive antigens are present normally in four patients with oculomyopathy in whom partial deletions of the mtDNA have been detected, and in two patients with MERRF and MELAS encephalomyopathy. Thus, unlike in the yeast Saccharomyces cerevisiae, there appear to be no regulatory interactions which coordinate the assembly of the mitochondrial respiratory chain with the development of the pyruvate dehydrogenase complex, which plays an important role in regulating the flow of metabolic intermediates to oxidative energy metabolism.

Isolation and incubation conditions to study heart mitochondrial protein synthesis

Although much is now known with regard to the processes of mammalian mitochondrial gene expression, relatively little is known concerning the quantitative regulation of this pathway in response to hormones or other physiological stimuli. This has been caused, in large part, by the lack of adequate assay systems in which such processes can be meaningfully measured. The purpose of this and the companion paper [E. E. McKee, B. L. Grier, G. S. Thompson, A. C. F. Leung, and J. D. McCourt. Am. J. Physiol. 258 [Endocrinol. Metab. 21):E503-E510, 1990] is to describe a system in which the quantitative regulation of mitochondrial protein synthesis in rat heart can be investigated. In this report the conditions for mitochondrial isolation and labeling are described, and the importance of isolating intact, tightly coupled mitochondria in obtaining high and reliable rates of protein synthesis is demonstrated. The highest levels of protein synthesis are obtained in mitochondria isolated from hearts perfused and homogenized in the presence of subtilisin, conditions in which the fastest rates of state 3 respiration and the highest respiratory control ratios are also observed. Analysis of the free amino acid pools indicates that isolated heart mitochondria have a negligible level of endogenous methionine as well as other amino acids. As a result, the concentration and specific radioactivity of the [35S]methionine pool serving protein synthesis could be easily determined. Optimal translation occurred at 30 degrees C at a pH of 7.0-7.2 and required the addition of methionine (20 microM), the other 19 amino acids (0.1 mM each), K+ (60-90 mM), Cl- (30-90 mM), Mg2+ (0.5-5 mM), and bovine serum albumin (1 mg/ml). As shown in the companion paper, adenine nucleotide (0.5-4.0 mM) and oxidizable substrate (10-20 mM glutamate) are also required for isolated heart mitochondrial protein synthesis. Analysis of labeled mitochondrial translation products demonstrated that bona fide mitochondrial peptides were synthesized.

Internal frameshifts within the mitochondrial genes for cytochrome oxidase subunit II and maxicircle unidentified reading frame 3 of Leishmania tarentolae are corrected by RNA editing: evidence for translation of the edited cytochrome oxidase subunit II mRNA

The Leishmania tarentolae cytochrome oxidase (EC 1.9.3.1) subunit II (COII) and maxicircle unidentified reading frame 3 (MURF3) mRNAs are edited internally by the addition of four and five uridine residues, respectively, which eliminate -1 and +1 reading frameshifts in the gene sequences. The editing events in COII are conserved in three kinetoplastid species, and those in MURF3 are conserved in two species. A primer extension assay showed that the ratio of edited to unedited RNA differed for each gene: 89% of the COII and 36% of the MURF3 transcripts are edited. Preliminary evidence was obtained for translation of the edited COII transcript into protein: antibodies generated against a synthetic peptide with the predicted carboxyl-terminal amino acid sequence reacted with a polypeptide of the correct molecular weight in immunoblots of a mitochondrial lysate.

The role of mitochondrial DNA in Huntington's disease

Huntington's disease is generally considered to be a late-onset neurodegenerative disorder, which follows a protracted course of deteriorating motor control and cognitive impairment. However, in a minority of cases, the onset of symptoms occurs early in life. A preponderance of the juvenile-onset HD victims have inherited the genetic defect from their fathers. This variation in age of onset, based on the sex of the affected parent, has suggested that maternally inherited genes may influence expression of the disorder. We describe a portion of a large Venezuelan HD pedigree in which both the mother and father of three juvenile-onset HD patients share a common maternal lineage. Scanning of mtDNA from members of this family with 43 restriction endonucleases failed to reveal any differences in the mitochondrial genotype that could account for the difference in age of onset between the affected father and his progeny. Members of a related family with an affected father but no juvenile-onset progeny also appeared to share the same mitochondrial genotype. In addition, the mitochondrial gene products from lymphoblast cell lines of these family members were analyzed on polyacrylamide gels after incubation of cells with [35S]methionine, but no detectable alterations were seen. Taken together, these data suggest that the maternally inherited mitochondrial genome does not play a crucial role in determining in age of onset in HD.

Characterization of a protein fraction containing cytochromes b and c1 from mitochondria of Leishmania tarentolae

A soluble red band fraction was obtained from Leishmania tarentolae cells by sucrose gradient sedimentation of a Triton X-100 lysate. Spectral analysis indicated that cytochrome b was present in the red band: the reduced minus oxidized difference spectra revealed absorption maxima at 562,527, and 431 nm at room temperature and 562, 530, and 422 nm at 77K. In addition, a 28-kDa protein was identified in this fraction which retained heme-associated peroxidase activity even after denaturation on SDS-polyacrylamide gels. The amino acid composition of this protein showed a strong similarity to cytochrome c1 of both bovine and yeast.

Protein synthesis in mitochondria isolated from human skeletal muscle. Detection of polymorphism in mitochondrial translation products

The importance of the mitochondrial protein synthesizing system in the development of functional mitochondria, and thus presumably in the pathogenesis of mitochondrial cytopathies has become apparent in recent years. A procedure has been developed to allow the measurement of the protein synthetic activity in mitochondria isolated from human skeletal muscle biopsy materials. The examination of the mitochondrial protein synthesis products revealed two polymorphic variants with the electrophoretic mobilities in SDS-polyacrylamide gel of a 20/22 kDa and a 45/47 kDa protein. Since functional consideration suggests that these variants are most likely to be associated with conservative amino acid substitutions, the observation indicates that it might be possible to electrophoretically detect certain alterations in the mitochondrial translation products in mitochondrial cytopathies due to mutations in the mitochondrial genome.

Transcription and translation of mitochondrial DNA in interspecific somatic cell hybrids

We examined the mitochondrial transcription and translation products of somatic cell hybrids constructed by the fusion of Chinese hamster and mouse cells. The hybrid cell lines OAC-k, OAC-l, and OAC-m contain approximately equal amounts of hamster and mouse mitochondrial DNA and produced mitochondrial rRNA from both parental species in the same ratio. Cell lines OAC-k, OAC-l, and OAC-m also produced poly(A)+ mouse mitochondrial RNA transcripts comparable in complexity and quantity to poly(A)+ RNA from the mouse parent. However, the overall level of poly(A)+ hamster mitochondrial RNA from these hybrids was significantly reduced compared with that from the hamster parent. The hybrid cells also lacked several poly(A)+ RNA species found in the hamster parent, but contained additional minor transcripts. The mitochondrially coded proteins of the OAC-k, OAC-l, and OAC-m cells were predominantly encoded by the mouse mitochondrial DNA.

URF6, last unidentified reading frame of human mtDNA, codes for an NADH dehydrogenase subunit

The polypeptide encoded in URF6, the last unassigned reading frame of human mitochondrial DNA, has been identified with antibodies to peptides predicted from the DNA sequence. Antibodies prepared against highly purified respiratory chain NADH dehydrogenase from beef heart or against the cytoplasmically synthesized 49-kilodalton iron-sulfur subunit isolated from this enzyme complex, when added to a deoxycholate or a Triton X-100 mitochondrial lysate of HeLa cells, specifically precipitated the URF6 product together with the six other URF products previously identified as subunits of NADH dehydrogenase. These results strongly point to the URF6 product as being another subunit of this enzyme complex. Thus, almost 60% of the protein coding capacity of mammalian mitochondrial DNA is utilized for the assembly of the first enzyme complex of the respiratory chain. The absence of such information in yeast mitochondrial DNA dramatizes the variability in gene content of different mitochondrial genomes.

Mitochondrial protein synthesis in interspecific somatic cell hybrids

The fusion of an oligomycin (OLI)-resistant mutant of mouse LM(TK-) cells to a chloramphenicol (CAP)-resistant mutant of AK412 Chinese hamster cells resulted in a series of interspecific somatic cell hybrids. Hybrids selected in HAT medium retained only mouse mitochondrial genomes while hybrids selected in HAT plus CAP and OLI retained both hamster and mouse mitochondrial genomes in approximately equal amounts. Nuclear-coded mitochondrial proteins from both parental species were incorporated into mitochondria in all of the hybrids. However, the mitochondrially coded proteins of three individually isolated hybrid cell lines were predominantly mouse-specific, with only trace amounts of hamster protein detected.

Transcription and translation of mitochondrial DNA in interspecific somatic cell hybrids

We examined the mitochondrial transcription and translation products of somatic cell hybrids constructed by the fusion of Chinese hamster and mouse cells. The hybrid cell lines OAC-k, OAC-l, and OAC-m contain approximately equal amounts of hamster and mouse mitochondrial DNA and produced mitochondrial rRNA from both parental species in the same ratio. Cell lines OAC-k, OAC-l, and OAC-m also produced poly(A)+ mouse mitochondrial RNA transcripts comparable in complexity and quantity to poly(A)+ RNA from the mouse parent. However, the overall level of poly(A)+ hamster mitochondrial RNA from these hybrids was significantly reduced compared with that from the hamster parent. The hybrid cells also lacked several poly(A)+ RNA species found in the hamster parent, but contained additional minor transcripts. The mitochondrially coded proteins of the OAC-k, OAC-l, and OAC-m cells were predominantly encoded by the mouse mitochondrial DNA.

Identification of the polypeptides encoded in the unassigned reading frames 2, 4, 4L, and 5 of human mitochondrial DNA

In previous work, antibodies prepared against chemically synthesized peptides predicted from the DNA sequence were used to identify the polypeptides encoded in three of the eight unassigned reading frames (URFs) of human mitochondrial DNA (mtDNA). In the present study, this approach has been extended to other human mtDNA URFs. In particular, antibodies directed against the NH2-terminal octapeptide of the putative URF2 product specifically precipitated component 11 of the HeLa cell mitochondrial translation products, the reaction being inhibited by the specific peptide. Similarly, antibodies directed against the COOH-terminal nonapeptide of the putative URF4 product reacted specifically with components 4 and 5, and antibodies against a COOH-terminal heptapeptide of the presumptive URF4L product reacted specifically with component 26. Antibodies against the NH2-terminal heptapeptide of the putative product of URF5 reacted with component 1, but only to a marginal extent; however, the results of a trypsin fingerprinting analysis of component 1 point strongly to this component as being the authentic product of URF5. The polypeptide assignments to the mtDNA URFs analyzed here are supported by the relative electrophoretic mobilities of proteins 11, 4-5, 26, and 1, which are those expected for the molecular weights predicted from the DNA sequence for the products of URF2, URF4, URF4L, and URF5, respectively. With the present assignment, seven of the eight human mtDNA URFs have been shown to be expressed in HeLa cells.

Mitochondrial DNA expression in Drosophila melanogaster: neosynthesized polypeptides in isolated mitochondria

The expression of mitochondrial genome of D. melanogaster in isolated mitochondria was followed by incorporation of 35S methionine in neosynthesized polypeptides. A high level of protein synthesis was obtained after optimization of all the incubation parameters. Two kinds of energy-generating systems were used: an endogenous system where an oxidizable substrate were added for ATP synthesis; an exogenous system with an energy-rich compound for ATP regeneration, the latter proved to be the most effective. The effect of the oxidative phosphorylation uncoupler (Clccp), and an ATPase inhibitor (oligomycine) allow us to postulate the role of the electrochemical potential in the expression of the mitochondrial genome. Electrophoresis and autoradiography of neosynthesized mitochondrial proteins exhibits 18 to 24 protein bands, ranging from 6.5 to 65 Kd; incubation of KC 0% drosophila cells with 35S methionine and cycloheximide gave similar results. Both our results and those published elsewhere suggest that the expression of mitochondrial genome in higher organisms could be more complex than simple translation of the 13 genes presents on these genomes.

Six unidentified reading frames of human mitochondrial DNA encode components of the respiratory-chain NADH dehydrogenase

The products of six unidentified reading frames of human mitochondrial DNA are precipitated from a mitochondrial lysate by antibodies against highly purified native beef heart NADH-ubiquinone oxidoreductase (complex I). These products are enriched greatly in a human submitochondrial fraction enriched in NADH-Q1 and NADH-K3Fe(CN)6 oxidoreductase activities. We conclude that the six reading frames encode components of the respiratory-chain NADH dehydrogenase.

Comparison of protein synthesis in mitochondria, synaptosomes, and intact brain cells

Qualitative aspects of protein synthesis in organelles and intact cultured cells of brain origin were compared to clarify the distinction between synaptosomal and mitochondrial protein synthesis. Brain mitochondria and synaptosomes were isolated either on a traditional Ficoll-sucrose gradient or by a new Percoll gradient procedure, and were incubated in an amino acid incorporation system containing [35S]methionine, then electrophoresed on gradient slab gels. Autoradiography of the gels revealed that in the presence of cycloheximide both mitochondria and synaptosomes synthesized at least 17 proteins in the 6,000-50,000 MW range, and that incubation with chloramphenicol reduced or eliminated these bands. With minor variation these patterns in the low-molecular-weight region also resembled patterns obtained from cycloheximide-inhibited rat liver mitochondria and intact brain cells (cultured glia, glioma, and neuroblastoma). In the higher molecular weight region of the gels (greater than 50,000) banding patterns were more complex and tended to differ between organelles and intact cells. These polypeptides probably reflect nonmitochondrial protein synthesis, and their variable response to inhibitors may account for confusion in the literature with regard to the effects of inhibitors of protein synthesis in brain mitochondria and synaptosomes.

Optimization of in vitro protein synthesis by isolated mouse adrenal mitochondria

The requirements for in vitro mitochondrial protein synthesis have been studied using isolated mitochondria from cultured adrenal Y-1 tumor cells from mice. By reducing the reaction volume to 50 microliter we were able to assay in replicate the requirements for various reaction components using trichloroacetic acid (TCA)-precipitable counts for a quantitative evaluation with time of incubation. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis followed by autoradiography was also used for a qualitative and quantitative evaluation of the translation products. With the optimized system, 1 to 3% of added [35S]methionine was incorporated. The products of mitochondrial protein synthesis range from 70,000 to 5000 molecular weight. Major autoradiographic bands were observed at 38,000, 31,000, 23,000, 20,000, and 5600 molecular weight as separated on 10 to 20% gradient SDS-polyacrylamide gels; however, 20 to 30 protein products of various molecular weights were discernible. Mitochondrial concentrations of 0.8 to 1.4 mg/ml of incubation gave the better incorporation of [35S]methionine per milligram of protein. Total [35S]methionine incorporated into mitochondrial protein was greatest at 25 degrees C after 90 min. Chloramphenicol at 10 micrograms/ml inhibited mitochondrial protein synthesis by more than 50% and at 100 micrograms/ml inhibited incorporation by more than 95%. Cycloheximide had no effect on incorporation at less than 1.0 mg/ml. Magnesium and ATP in a molar ratio of one to one at 5 mM gave optimal incorporation. Other energy generating systems using oxidative phosphorylation to supply ATP for protein synthesis were not as effective as ATP and 5 mM phosphoenol pyruvate, 20 micrograms/ml pyruvate kinase and 5 mM a-ketoglutarate. In contrast to in vitro yeast mitochondrial protein synthesis, no enhancement of in vitro adrenal cell mitochondrial protein synthesis was found with GTP or its analogs. The buffers N,N-bis(2-hydroxyethyl)glycine, N-(tris(hydroxymethyl)methyl)glycine, and N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid were superior to Tris-HCl for mitochondrial protein synthesis. Optimal pH for [35S]methionine incorporation into mitochondrial proteins was pH 7.0 to 7.6. Potassium at 50 to 90 mM gave the best incorporation of [35S]methionine, and the higher molecular weight products of translation were enhanced at these concentrations. Sodium at 10 to 40 mM had no effect; however, 100 mM sodium inhibited label incorporation by 30%. Calcium at 100 microM inhibited mitochondrial protein synthesis by approximately 50%, and at 1.0 mM little if any incorporation occurred.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of the polypeptides encoded in the ATPase 6 gene and in the unassigned reading frames 1 and 3 of human mtDNA

Antibodies prepared against chemically synthesized peptides predicted from the DNA sequence have been used to identify the polypeptides encoded in the ATPase 6 gene and in unidentified reading frames (URFs) 1 and 3 of human mtDNA. In particular, antibodies directed against the COOH-terminal nonapeptide of the putative polypeptide encoded in the ATPase 6 reading frame immunoprecipitated specifically component 17 of the HeLa cell mitochondrial translation products, the reaction being inhibited by the specific peptide. Similarly, antibodies directed against the COOH-terminal undecapeptide of the putative URF1 product or against the COOH-terminal heptapeptide of the presumptive URF3 product were effective in immunoprecipitating specifically component 12 or, respectively, component 24 of the mitochondrial translation products. The sizes of proteins 17, 12, and 24, as estimated from their electrophoretic mobilities, are compatible with their being the products of the ATPase 6 gene, URF1, and URF3, respectively.

Antibodies against synthetic peptides reveal that the unidentified reading frame A6L, overlapping the ATPase 6 gene, is expressed in human mitochondria

Antibodies prepared against chemically synthesized peptides predicted from the DNA sequence have been used to detect human mitochondrial gene products. In particular, antibodies directed against either the NH2-terminal decapeptide or the COOH-terminal undecapeptide of cytochrome c oxidase subunit II (COII) were both very effective in immunoprecipitating the previously identified COII polypeptide from an SDS lysate of mitochondria from HeLa cells. Similarly, antibodies directed against the COOH-terminal nonapeptide of the putative polypeptide encoded in the unidentified reading frame A6L, which overlaps the ATPase 6 gene, immunoprecipitated specifically a component (#25) of the HeLa cell mitochondrial translation products; antibodies directed against the NH2-terminal octapeptide also precipitated protein 25, although less efficiently. The size of protein 25, as estimated from its electrophoretic mobility, is compatible with its being the unidentified reading frame A6L product. Furthermore, a fingerprinting analysis of this protein after trypsin digestion has given results consistent with this identification.

Rearranged mitochondrial genes in the yeast nuclear genome

We have found a contiguous DNA sequence in the yeast nuclear genome with extensive homology to non-contiguous yeast mitochondrial DNA sequences. Closely linked to this nuclear sequence in some, but not all, yeast strains is a tandem pair of transposable (Ty) elements. Certain features of the content and organization of this nuclear DNA sequence suggest that it may have originated from petite mitochondrial DNA which integrated into the nuclear genome.