The role of var1 in the assembly of yeast mitochondrial ribosomes

Distinct types of intramitochondrial protein aggregates protect mitochondria against proteotoxic stress

Mitochondria consist of hundreds of proteins, most of which are inaccessible to the proteasomal quality control system of the cytosol. How cells stabilize the mitochondrial proteome during challenging conditions remains poorly understood. Here, we show that mitochondria form spatially defined protein aggregates as a stress-protecting mechanism. Two different types of intramitochondrial protein aggregates can be distinguished. The mitoribosomal protein Var1 (uS3m) undergoes a stress-induced transition from a soluble, chaperone-stabilized protein that is prevalent under benign conditions to an insoluble, aggregated form upon acute stress. The formation of Var1 bodies stabilizes mitochondrial proteostasis, presumably by sequestration of aggregation-prone proteins. The AAA chaperone Hsp78 is part of a second type of intramitochondrial aggregate that transiently sequesters proteins and promotes their folding or Pim1-mediated degradation. Thus, mitochondrial proteins actively control the formation of distinct types of intramitochondrial protein aggregates, which cooperate to stabilize the mitochondrial proteome during proteotoxic stress conditions.

Biolistic transformation of the yeast Saccharomyces cerevisiae mitochondrial DNA

The insertion of genes into mitochondria by biolistic transformation is currently only possible in the yeast Saccharomyces cerevisiae and the algae Chlamydomonas reinhardtii. The fact that S. cerevisiae mitochondria can exist with partial (ρ- mutants) or complete deletions (ρ0 mutants) of mitochondrial DNA (mtDNA), without requiring a specific origin of replication, enables the propagation of exogenous sequences. Additionally, mtDNA in this organism undergoes efficient homologous recombination, making it well-suited for genetic manipulation. In this review, we present a summarized historical overview of the development of biolistic transformation and discuss iconic applications of the technique. We also provide a detailed example on how to obtain transformants with recombined foreign DNA in their mitochondrial genome.

The translational activator Sov1 coordinates mitochondrial gene expression with mitoribosome biogenesis

Mitoribosome biogenesis is an expensive metabolic process that is essential to maintain cellular respiratory capacity and requires the stoichiometric accumulation of rRNAs and proteins encoded in two distinct genomes. In yeast, the ribosomal protein Var1, alias uS3m, is mitochondrion-encoded. uS3m is a protein universally present in all ribosomes, where it forms part of the small subunit (SSU) mRNA entry channel and plays a pivotal role in ribosome loading onto the mRNA. However, despite its critical functional role, very little is known concerning VAR1 gene expression. Here, we demonstrate that the protein Sov1 is an in bona fide VAR1 mRNA translational activator and additionally interacts with newly synthesized Var1 polypeptide. Moreover, we show that Sov1 assists the late steps of mtSSU biogenesis involving the incorporation of Var1, an event necessary for uS14 and mS46 assembly. Notably, we have uncovered a translational regulatory mechanism by which Sov1 fine-tunes Var1 synthesis with its assembly into the mitoribosome.

rps3 as a Candidate Mitochondrial Gene for the Molecular Identification of Species from the Colletotrichum acutatum Species Complex

Colletotrichum species form one of the most economically significant groups of pathogenic fungi and lead to significant losses in the production of major crops-in particular, fruits, vegetables, ornamental plants, shrubs, and trees. Members of the genus Colletotrichum cause anthracnose disease in many plants. Due to their considerable variation, these fungi have been widely investigated in genetic studies as model organisms. Here, we report the complete mitochondrial genome sequences of four Colletotrichum species (C. fioriniae, C. lupini, C. salicis, and C. tamarilloi). The reported circular mitogenomes range from 30,020 (C. fioriniae) to 36,554 bp (C. lupini) in size and have identical sets of genes, including 15 protein-coding genes, two ribosomal RNA genes, and 29 tRNA genes. All four mitogenomes are characterized by a rather poor repetitive sequence content with only forward repeat representatives and a low number of microsatellites. The topology of the phylogenetic tree reflects the systematic positions of the studied species, with representatives of each Colletotrichum species complex gathered in one clade. A comparative analysis reveals consistency in the gene composition and order of Colletotrichum mitogenomes, although some highly divergent regions are also identified, like the rps3 gene which appears as a source of potential diagnostic markers for all studied Colletotrichum species.

Mt-rps3 is an ancient gene which provides insight into the evolution of fungal mitochondrial genomes

The nuclear ribosomal protein S3 (Rps3) is implicated in the assembly of the ribosomal small subunit. Fungi and plants present a gene copy in their mitochondrial (mt) genomes. An analysis of 303 complete fungal mt genomes showed that, when rps3 is found, it is either a free-standing gene or an anchored gene within the omega intron of the rnl gene. Early divergent fungi, Basidiomycota and all yeasts but the CTG group belong to the first case, and Pezizomycotina to the second. Its position, size and genetic code employed are conserved within species of the same Order. Size variability is attributed to different number of repeats. These repeats consist of AT-rich sequences. MtRps3 proteins lack the KH domain, necessary for binding to rRNA, in their N-terminal region. Their C-terminal region is conserved in all Domains of life. Phylogenetic analysis showed that nuclear and mtRps3 proteins are descendants of archaeal and a-proteobacterial homologues, respectively. Thus, fungal mt-rps3 gene is an ancient gene which evolved within the endosymbiotic model and presents different evolutionary routes: (a) coming from a-proteobacteria, it was relocated to another region of the mt genome, (b) via its insertion to the omega intron, it was transferred to the nucleus and/or got lost, and (c) it was re-routed to the mt genome again. Today, Basidiomycota and Saccharomycetales seem to follow the first evolutionary route and almost all Pezizomycotina support the second scenario with their exceptions being the result of the third scenario, i.e., the gene's re-entry to the mt genome.

Characterization of Gtf1p, the connector subunit of yeast mitochondrial tRNA-dependent amidotransferase

The bacterial GatCAB operon for tRNA-dependent amidotransferase (AdT) catalyzes the transamidation of mischarged glutamyl-tRNA(Gln) to glutaminyl-tRNA(Gln). Here we describe the phenotype of temperature-sensitive (ts) mutants of GTF1, a gene proposed to code for subunit F of mitochondrial AdT in Saccharomyces cerevisiae. The ts gtf1 mutants accumulate an electrophoretic variant of the mitochondrially encoded Cox2p subunit of cytochrome oxidase and an unstable form of the Atp8p subunit of the F(1)-F(0) ATP synthase that is degraded, thereby preventing assembly of the F(0) sector. Allotopic expression of recoded ATP8 and COX2 did not significantly improve growth of gtf1 mutants on respiratory substrates. However, ts gft1 mutants are partially rescued by overexpression of PET112 and HER2 that code for the yeast homologues of the catalytic subunits of bacterial AdT. Additionally, B66, a her2 point mutant has a phenotype similar to that of gtf1 mutants. These results provide genetic support for the essentiality, in vivo, of the GatF subunit of the heterotrimeric AdT that catalyzes formation of glutaminyl-tRNA(Gln) (Frechin, M., Senger, B., Brayé, M., Kern, D., Martin, R. P., and Becker, H. D. (2009) Genes Dev. 23, 1119-1130).

Mammalian mitochondrial initiation factor 2 supports yeast mitochondrial translation without formylated initiator tRNA

Initiation of protein synthesis in mitochondria and chloroplasts is widely believed to require a formylated initiator methionyl-tRNA (fMet-tRNAfMet) in a process involving initiation factor 2 (IF2). However, yeast strains disrupted at the FMT1 locus, encoding mitochondrial methionyl-tRNA formyltransferase, lack detectable fMet-tRNAfMet but exhibit normal mitochondrial function as evidenced by normal growth on non-fermentable carbon sources. Here we show that mitochondrial translation products in Saccharomyces cerevisiae were synthesized in the absence of formylated initiator tRNA. ifm1 mutants, lacking the mitochondrial initiation factor 2 (mIF2), are unable to respire, indicative of defective mitochondrial protein synthesis, but their respiratory defect could be complemented by plasmid-borne copies of either the yeast IFM1 gene or a cDNA encoding bovine mIF2. Moreover, the bovine mIF2 sustained normal respiration in ifm1 fmt1 double mutants. Bovine mIF2 supported the same pattern of mitochondrial translation products as yeast mIF2, and the pattern did not change in cells lacking formylated Met-tRNAfMet. Mutant yeast lacking any mIF2 retained the ability to synthesize low levels of a subset of mitochondrially encoded proteins. The ifm1 null mutant was used to analyze the domain structure of yeast mIF2. Contrary to a previous report, the C terminus of yeast mIF2 is required for its function in vivo, whereas the N-terminal domain could be deleted. Our results indicate that formylation of initiator methionyl-tRNA is not required for mitochondrial protein synthesis. The ability of bovine mIF2 to support mitochondrial translation in the yeast fmt1 mutant suggests that this phenomenon may extend to mammalian mitochondria as well.

Evidence that synthesis of the Saccharomyces cerevisiae mitochondrially encoded ribosomal protein Var1p may be membrane localized

The 5'-untranslated leaders of mitochondrial mRNAs appear to localize translation within the organelle. VAR1 is the only yeast mitochondrial gene encoding a major soluble protein. A chimeric mRNA bearing the VAR1 untranslated regions and the coding sequence for pre-Cox2p appears to be translated at the inner membrane surface. We propose that translation of the ribosomal protein Var1p is also likely to occur in close proximity to the inner membrane.

Rpm2, the protein subunit of mitochondrial RNase P in Saccharomyces cerevisiae, also has a role in the translation of mitochondrially encoded subunits of cytochrome c oxidase

RPM2 is a Saccharomyces cerevisiae nuclear gene that encodes the protein subunit of mitochondrial RNase P and has an unknown function essential for fermentative growth. Cells lacking mitochondrial RNase P cannot respire and accumulate lesions in their mitochondrial DNA. The effects of a new RPM2 allele, rpm2-100, reveal a novel function of RPM2 in mitochondrial biogenesis. Cells with rpm2-100 as their only source of Rpm2p have correctly processed mitochondrial tRNAs but are still respiratory deficient. Mitochondrial mRNA and rRNA levels are reduced in rpm2-100 cells compared to wild type. The general reduction in mRNA is not reflected in a similar reduction in mitochondrial protein synthesis. Incorporation of labeled precursors into mitochondrially encoded Atp6, Atp8, Atp9, and Cytb protein was enhanced in the mutant relative to wild type, while incorporation into Cox1p, Cox2p, Cox3p, and Var1p was reduced. Pulse-chase analysis of mitochondrial translation revealed decreased rates of translation of COX1, COX2, and COX3 mRNAs. This decrease leads to low steady-state levels of Cox1p, Cox2p, and Cox3p, loss of visible spectra of aa(3) cytochromes, and low cytochrome c oxidase activity in mutant mitochondria. Thus, RPM2 has a previously unrecognized role in mitochondrial biogenesis, in addition to its role as a subunit of mitochondrial RNase P. Moreover, there is a synthetic lethal interaction between the disruption of this novel respiratory function and the loss of wild-type mtDNA. This synthetic interaction explains why a complete deletion of RPM2 is lethal.

Molecular genetics of the peptidyl transferase center and the unusual Var1 protein in yeast mitochondrial ribosomes

Mitochondria possess their own ribosomes responsible for the synthesis of a small number of proteins encoded by the mitochondrial genome. In yeast, Saccharomyces cerevisiae, the two ribosomal RNAs and a single ribosomal protein, Var1, are products of mitochondrial genes, and the remaining approximately 80 ribosomal proteins are encoded in the nucleus. The mitochondrial translation system is dispensable in yeast, providing an excellent experimental model for the molecular genetic analysis of the fundamental properties of ribosomes in general as well as adaptations required for the specialized role of ribosomes in mitochondria. Recent studies of the peptidyl transferase center, one of the most highly conserved functional centers of the ribosome, and the Var1 protein, an unusual yet essential protein in the small ribosomal subunit, have provided new insight into conserved and divergent features of the mitochondrial ribosome.

Incorporation of the yeast mitochondrial ribosomal protein Mrp2 into ribosomal subunits requires the mitochondrially encoded Var1 protein

Mrp2 is a protein component of the small subunit of mitochondrial ribosomes in the yeast Saccharomyces cerevisiae. We have examined the expression of Mrp2 in yeast mutants lacking mitochondrial DNA and found that the steady-state level of Mrp2 is dramatically decreased relative to wild type. These data suggest that the accumulation of Mrp2 depends on the expression of one or more mitochondrial gene products. The mitochondrial genome of S. cerevisiae encodes two components of the small ribosomal subunit, 15S rRNA and the Var1 protein, both of which are necessary for the formation of mature 37S subunits. Several studies have shown that in the absence of Var1 incomplete subunits accumulate, which lack a limited number of ribosomal proteins. Here, we show that Mrp2 is one of the proteins absent from subunits lacking Var1, indicating that Var1 plays an important role in the incorporation of Mrp2 into mitochondrial ribosomal subunits.

Effect of chronic ethanol consumption on hepatic mitochondrial transcription and translation

Liver mitochondria from ethanol-fed rats display an impaired ability for protein synthesis in vitro. Studies were conducted to explore the possible mechanisms which might account for this impaired capacity of ethanol mitochondria for protein synthesis. The present studies did not demonstrate any significant ethanol-induced lesion in mitochondrial nucleic acid metabolism in organelles isolated from ethanol-fed rats for any of the parameters investigated (mtDNA content, steady-state mtRNA concentration, mtRNA polymerase activity, concentration of specific mRNAs and rRNAs, mtRNA processing). An investigation of ribosome function in isolated mitochondria demonstrated significant decreases in the number of active ribosomes (55% fewer) in mitochondria from ethanol-fed rats. Initiation of protein synthesis was also significantly depressed (46%) in ethanol mitochondria. In addition, the yield of ribosomal particles from ethanol mitochondria was decreased 32% as compared to the yield of ribosomal particles from control mitochondria. However, isolated ribosomes from ethanol mitochondria were determined to be fully functional in a poly(U)-directed phenylalanine polymerization system. Soluble translation factors from ethanol mitochondria were also found to support full activity of control ribosomes in a poly(U)-directed phenylalanine polymerization system. These results suggest strongly that the ethanol-induced depression of mitochondrial protein synthesis is due to a decrease in the number of competent ribosomes in hepatic mitochondria from chronically ethanol-fed rats.

Neurospora crassa pyruvate dehydrogenase complex: component characterization, catalytic properties and location of translation

We propose a simplified procedure for the purification of the Neurospora crassa pyruvate dehydrogenase complex. The purified complex showed four protein bands with apparent Mr values of 53,400, 52,900, 49,000 and 36,900 upon SDS-polyacrylamide gel electrophoresis. Components, E2 and E3, of N. crassa pyruvate dehydrogenase complex were identified, respectively, as polypeptides 49,000 and 53,400. It can be deduced that component E1 is constituted of two subunits with Mr values of 52,900 and 36,900. The Km values towards different substrates and the optimal pH and temperature were determined. The protein kinase activity associated with the core enzyme was present in our most highly purified preparations. It was demonstrated that all the protein components of the complex are synthesized under the control of the nuclear genome.

Assembly of the mitochondrial ribosomes in a temperature-conditional mutant of Saccharomyces cerevisiae defective in the synthesis of the var1 protein

An investigation of the role of the var1 protein in the assembly of the yeast mitochondrial ribosomes was carried out in a temperature conditional mutant, strain h56, which contains a mutation (tsv1) just upstream of the structural gene for the var1 protein. The mutation results in a marked decrease in the synthesis of the var1 protein at the permissive temperature of 28 degrees C and an apparently complete absence of var1 synthesis at the restrictive temperature of 36 degrees C. Long-term growth of strain h56 at the non-permissive temperature was found to result in the loss of the small (37 S) ribosomal subunit and the appearance of a novel 30 S ribonucleoparticle. Both the small (37 S) and the large (54 S) mitochondrial ribosomal subunits were found to be assembled in strain h56 for at least 3 h after transfer to the non-permissive temperature.

Are all mitochondrial translation products synthesized on membrane-bound ribosomes?

The Arrhenius kinetics for the synthesis of var1, which is a hydrophilic protein of the mitochondrial ribosomes, have been compared to that of other mitochondrial translation products, which are hydrophobic subunits of the respiratory enzyme complexes. Our results indicate that, in the yeast Saccharomyces cerevisiae, the hydrophilic var1 protein is synthesized on membrane-associated mitochondrial ribosomes which cannot be distinguished from those responsible for the synthesis of the hydrophobic mitochondrial translation products.

var1 Gene on the mitochondrial genome of Torulopsis glabrata

We have cloned and sequenced a region of the Torulopsis glabrata mitochondrial genome homologous to the Saccharomyces cerevisiae var1 gene (var1Sc). An open reading frame that could encode a protein of 339 amino acids was found with 72.7% amino acid and 85.3% nucleotide sequence homology to the S. cerevisiae var1 gene. The T. glabrata gene (var1Tg) is transcribed yielding two stable RNAs, a more abundant 13.5 S RNA and a less abundant 18 S species. We have also identified a candidate for a T. glabrata var1 protein among mitochondrial translation products labeled in isolated mitochondria. The var1Tg gene is even more A + T-rich (93%) than var1Sc (89.6%) and has conserved the strong codon bias of var1Sc. Major differences between the two sequences were found. Significant among these are that no GC clusters are found in var1Tg and the sequences surrounding each of the sites where known polymorphisms exist in var1Sc have deletions at the corresponding sites in var1Tg. These data are discussed with respect to possible origins of these var1 genes and translocation of GC clusters in S. cerevisiae mitochondrial DNA.

Defective assembly of the mitochondrial ribosomes in yeast cells grown in the presence of mitochondrial protein synthesis inhibitors

The involvement of mitochondrial protein synthesis in the assembly of the mitochondrial ribosomes was investigated by studying the extent to which the assembly process can proceed in the presence of mitochondrial protein synthesis inhibitors erythromycin and chloramphenicol. Yeast cells grown in the presence of erythromycin (2 mg/ml) do not appear to contain any detectable amounts of the mitochondrial small (37 S) ribosomal subunit. Instead, a ribonucleoparticle with a sedimentation coefficient of 30 S was observed; this particle could be shown to be related to the mitochondrial small ribosomal subunit by two-dimensional gel electrophoretic analysis of its protein components. Since the var1 protein is the only mitochondrial translation product known to be associated with the mitochondrial ribosome, our results suggest that this protein is essential for the assembly of the mature small subunit, and that the var1 protein enters the pathway for the assembly of the small subunit at a late step. In at least one strain of yeast the accumulation of the 30-S particle appears to be very sensitive to catabolite repression. When yeast cells are grown in the presence of chloramphenicol instead of erythromycin, assembly of the small subunit appears to be only partially inhibited, and the presence of the 30-S particle could not be clearly demonstrated. This observation is consistent with the fact that in yeast, chloramphenicol inhibits mitochondrial protein synthesis by about 95% only and that the synthesis of the var1 protein appears to be the least sensitive to this inhibition.

Identification and characterization of mitochondrial translation products in various yeasts and in Prototheca zopfii

Mitochondrial genomes of different eucaryotes are not all alike. We have examined mitochondrial translation products in a number of yeasts (Candida krusei, Hansenula saturnus, Rhodotorula glutinis, Rhodotorula rubra, Torulopsis glabrata and Saccharomyces cerevisiae) and in Prototheca zopfii, a colorless alga, in order to determine whether certain proteins are invariably synthesized within mitochondria, how different these proteins are, and what additional proteins, if any, might be synthesized by diverse mitochondria. Using a variety of techniques and criteria, including immunological analysis and peptide mapping, we show that all the yeasts studied, and probably P. zopfii as well, make versions of the 3 large subunits of cytochrome c oxidase. Not all of these oxidase subunits are equally closely related to their counterparts in S. cerevisiae, however. Mitochondria of some of the yeasts studied do not make, or make only small amounts of, a counterpart to Varl, a major mitochondrially made protein in S. cerevisiae. Mitochondria of P. zopfii possibly do not make an apocytochrome b. T. glabrata, H. saturnus and the two Rhodotorula species each make one or more proteins whose relationship, if any, to mitochondrial translation products of S. cerevisiae is not apparent. These results provide new information about mitochondrial diversity. Whereas mitochondria of all the organisms that we have studied devote the major part of their synthetic effort to making the three large subunits of cytochrome c oxidase, and probably make certain other proteins in common, they do not all synthesize a completely identical set of proteins.

The formation of a defective small subunit of the mitochondrial ribosomes in petite mutants of Saccharomyces cerevisiae

The involvement of mitochondrial protein synthesis in the assembly of the mitochondrial ribosomes was investigated by studying the extent to which the assembly process can proceed in petite mutants of Saccharomyces cerevisiae which lack mitochondrial protein synthetic activity due to the deletion of some tRNA genes and/or one of the rRNA genes on the mtDNA. Petite strains which retain the 15-S rRNA gene can synthesize this rRNA species, but do not contain any detectable amounts of the small mitochondrial ribosomal subunit. Instead, a ribonucleoparticle with a sedimentation coefficient of 30 S (instead of 37 S) was observed. This ribonucleoparticle contained all the small ribosomal subunit proteins with the exception of the var1 and three to five other proteins, which indicates that the 30-S ribonucleoparticle is related to the small mitochondrial ribosomal subunit (37 S). Reconstitution experiments using the 30-S particle and the large mitochondrial ribosomal subunit from a wild-type yeast strain indicate that the 30-S particle is not active in translating the artificial message poly(U). The large mitochondrial ribosomal subunit was present in petite strains retaining the 21-S rRNA gene. The petite 54-S subunit is biologically active in the translation of poly(U) when reconstituted with the small subunit (37 S) from a wild-type strain. The above results indicate that mitochondrial protein synthetic activity is essential for the assembly of the mature small ribosomal subunit, but not for the large subunit. Since the var1 protein is the only mitochondrial translation product known to date to be associated with the mitochondrial ribosomes, the results suggest that this protein is essential for the assembly of the mature small subunit.

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.

Characterization of a 45-kDa polypeptide as the precursor of subunit 1 of cytochrome c oxidase in Neurospora crassa

In a previous paper (Van 't Sant, P., Mak, J.F.C. and Kroon, A.M. (1981) Eur. J. Biochem. 121, 21-26) we showed the existence of three elongated precursor proteins (45, 36 and 25 kDa) of mitochondrial translation products in Neurospora crassa. We presented some indications that the largest precursor could be related to subunit 1 of cytochrome c oxidase. Here we present conclusive evidence that the 45-kDa polypeptide is indeed this precursor by demonstrating that an immunodetectable 45-kDa polypeptide displays the same behaviour as the labeled 45-kDa precursor; both accumulate after long incubation with cycloheximide or by decreasing the temperature and both are not tightly membrane bound. Moreover the antibody against subunit 1 of cytochrome c oxidase also recognizes, in immunoadsorption experiments, besides subunit 1, the 45-kDa polypeptide accumulated by cycloheximide incubation. Furthermore, we developed a small scale purification of antibodies against subunit 1 of cytochrome c oxidase. By means of these purified antibodies it is demonstrated that the 45-kDa polypeptide and subunit 1 have corresponding antigenic determinants. Under the various conditions tested, all three precursors are less firmly membrane-bound than the mature subunits. Finally, it is observed that in short incubations in vivo, chloramphenicol inhibits the processing of the mitochondrially synthesized precursors, under conditions where mitochondrial translation is only partially inhibited.

Structural variations and optional introns in the mitochondrial DNAs of Neurospora strains isolated from nature

Mitochondrial DNAs from ten wild-type Neurospora crassa, Neurospora intermedia, and Neurospora sitophila strains collected from different geographical areas were screened for structural variations by restriction enzyme analysis. The different mtDNAs show much greater structural diversity, both within and among species, than had been apparent from previous studies of mtDNA from laboratory N. crassa strains. The mtDNAs range in size from 60 to 73 kb, and both the smallest and largest mtDNAs are found in N. crassa strains. In addition, four strains contain intramitochondrial plasmid DNAs that do not hybridize with the standard mtDNA. All of the mtDNA species have a basically similar organization. A 25-kb region that includes the rRNA genes and most tRNA genes shows very strong conservation of restriction sites in all strains. The 2.3-kb intron found in the large rRNA gene in standard N. crassa mtDNAs is present in all strains examined, including N. intermedia and N. sitophila strains. The size differences between the different mtDNAs are due to insertions or deletions that occur outside of the rRNA-tRNA region. Restriction enzyme and heteroduplex mapping suggest that four of these insertions are optional introns in the gene encoding cytochrome oxidase subunit I. Mitochondrial DNAs from different wild-type strains contain zero, one, three, or four of these introns.

Identity problems concerning subunits of the membrane factor of the mitochondrial ATPase of Saccharomyces cerevisiae

As confirmed by fingerprint analysis, high-resolution acrylamide gel electrophoresis identifies subunits of the mitochondrial ATPase complex directly at the level of Coomassie-blue-stained yeast mitochondria. Using this technique, the following results were obtained. 1. Three classes of subunits have been found in the ATPase complex. (a) The classical F1 peptides of cytoribosomal origin (57, 53 and 30.5 kDa). (b) Two peptides of mitoribosomal origin with high cycloheximide-resistant label: one of 7.8 kDa (the OLII gene product), the second of 20 kDa (the OLI2 gene product). Neither of these peptides is readily stained by Coomassie blue in the purified ATPase complex, (c) Four Coomassie-blue-stained peptides (27.5, 25, 23.5 and 22.5 kDa), the last one being subunit 6, which (like class b) functionally qualify as subunits of the membrane factor of the ATPase complex. As shown by labeling with 35SO24- and 3H-labeled amino acids, their biosynthesis is only very slightly cycloheximide-resistant, although it is submitted to coordinate regulation distinct from that of the class F1 peptides. 2. Consequently it was shown that the OLI2 gene product, a 20-kDa peptide with high cycloheximide-resistant label, which was generally taken to be 'subunit 6' of the ATPase, is not in fact identical to this peptide.

Larger precursors of mitochondrial translation products in Neurospora crassa. Indications for a precursor of subunit 1 of cytochrome c oxidase

Specific labeling in vivo of the formylated N termini of mitochondrial translation products revealed that some mitochondrially synthesized proteins were not labeled this way. As a consequence, it was worthwhile considering that larger precursor proteins of mitochondrial translation products exist. Although we used a rapid isolation procedure, only after 2-h of labeling in the presence of cycloheximide, could three additional mitochondrial translation products (molecular mass 45, 36, and 25 kilodaltons) be detected. Preincubation with cycloheximide indicated that these proteins might be larger precursors which were no longer processed due to the prolonged presence of cycloheximide. To prevent processing of the precursors during isolation, cells of the slime mutant were directly lysed in boiling sodium dodecyl sulphate solution. In this way, the same three additional mitochondrial translation products were detected after a pulse-labeling of 1 min. These proteins behave in a precursor-like fashion. Labeling at 9 degrees C resulted in a partial accumulation of the three additional proteins. Finally protein blots treated with antibodies and 125I-labeled protein A, support the idea that the 45-kDa protein is a precursor of subunit 1 of cytochrome c oxidase; 50-80% of this precursor could be detected in the post-mitochondrial supernatant, indicating that this polypeptide is not tightly bound to the membrane.

mit-Mutations in the structural gene of subunit III of cytochrome oxidase in Saccharomyces cerevisiae

Two-dimensional electrophoretic analysis of the mitochondrial translation products of four mit-mutants indicate that subunit III of cytochrome oxidase is the only mitochondrial translation product affected by mutations in the oxi2 region of the mtDNA. Mitochondria of two of these mutants synthesize new products which coprecipitate with an anticytochrome oxidase antiserum and produce proteolytic digests similar to those of subunit III of the enzyme complex. These data strongly support the suggestion that the oxi2 region of the yeast mtDNA contains the structural gene of subunit III of cytochrome oxidase.