Localization in yeast mitochondrial DNA of mutations expressed in a deficiency of cytochrome oxidase and/or coenzyme QH2-cytochrome c reductase

Decreased amounts of core proteins I and II and the iron-sulfur protein in mitochondria from yeast lacking cytochrome b but containing cytochrome c1

The effect of cytochrome b on the assembly of the subunits of complex III into the inner mitochondrial membrane has been studied in four mutants of yeast that lack a spectrally detectable cytochrome b and do not synthesize apocytochrome b. Quantitative analysis of intact mitochondria by immunoprecipitation or immunoblotting techniques with specific antisera revealed that the core proteins and the iron-sulfur protein were decreased 50% or more in the mitochondria from the mutants as compared to the wild type. Sonication of wild-type mitochondria did not result in any decrease in any of these proteins from the membrane; however, sonication of mitochondria from the four mutants resulted in a further decrease in the amount of these proteins suggesting that they are not as tightly bound to the mitochondrial membrane in the absence of cytochrome b. By contrast, the amounts of cytochrome c1 in the mitochondria, as determined both spectroscopically and immunologically, were not significantly affected by the absence of cytochrome b. In addition, no loss of cytochrome c1 was observed after sonication of the mitochondria suggesting that this protein is tightly bound to the membrane. These results suggest that the processing and/or assembly of these subunits of complex III into the mitochondrial membrane is affected by the absence of cytochrome b.

Nucleotide sequence of the mitochondrial cytochrome oxidase subunit II gene in the yeast Hansenula saturnus

The gene for subunit II of cytochrome oxidase in the yeast Hansenula saturnus was previously shown to be located on a 1.7 kb HindIII-BamHI fragment of mitochondrial DNA (Lawson and Deters, accompanying paper). In this paper, we report the nucleotide sequence of a large part of this fragment, covering the coding region of the subunit II gene, designated coxII, and its 5' and 3' flanking regions. The coding region of the coxII gene consists of a continuous open reading frame, 744 nucleotides long, containing 6 in frame TGA codons. Examination of the sequence and alignment with known homologous gene sequences of other organisms indicates that TGA codes for tryptophan in H. saturnus mitochondria as it does in several other mitochondria. Despite considerable homology to subunit II of Saccharomyces cerevisiae, there are 9 codons used in coxII that are not used in the corresponding S. cerevisiae gene. CTT, which is believed to code for threonine in S. cerevisiae mitochondria, appears 3 times in coxII and probably codes for leucine. While the CGN family is rarely, if ever, used in S. cerevisiae mitochondria, CGT appears 4 times in coxII and probably codes for arginine. The deduced amino acid sequence, excluding the first ten amino acids at the N-terminus, is 81% homologous to the amino acid sequence of the S. cerevisiae subunit II protein. The first ten amino acids at the N-terminus are not homologous to the N-terminus of the S. cerevisiae protein but are highly homologous to the first ten amino acids of the deduced amino acid sequence of subunit II of Neurospora crassa. Minor variations of a transcription initiation signal and an end of message or processing signal reported in S. cerevisiae are found in the regions flanking the H. saturnus coxII gene. The subunit II gene contains numerous symmetrical elements, i.e. palindromes, inverted repeats, and direct repeats. Some of these have conserved counterparts in the S. cerevisiae subunit II gene, suggesting that they may be functionally or structurally important.

Deletions in the cob gene of yeast mtDNA and their phenotypic effect

Two cob- deletion mutants are characterized. One of them, M9410, is deleted for 911 bp of the noncoding sequences only which separate tRNAGlu and cob exon 1; it thus lacks most of the sequence encoding the 957 bp long cob leader (Bonitz et al. 1982) and some 20 bp 5' to it. The end points of this deletion coincide with 31 bp long direct repeats in wild type mtDNA. The other mutant, M9391, is deleted for all cob coding sequences and most of the cob leader sequence but it retains the 5' terminal 261 bp of this leader. Northern analysis revealed that M9410 totally lacks cob mRNA or pre-mRNA. The large deletion M9391 in contrast accumulates a 13S RNA which probably results from transcription through the junction, which ligates sequences of the cob leader to sequences of the cob-oli1 intergenic spacer.

Nucleotide substitutions in a yeast mitochondria cis-acting mutant located in the last intron of the apocytochrome b gene

The region of mitochondrial DNA corresponding to the intron mutant M6-200 in Saccharomyces cerevisiae D273-10B has been isolated, and the nucleotide sequence of a 519 bp RsaI fragment has been determined. Three nucleotide substitutions were found at nucleotides +2650 (G----T), +2668 (G----A) and +2798 (A----G), all within the genetically defined location in the gene. Particular significance can be attributed to the first two changes (+2650 and +2668), that can be genetically isolated from the third substitution and, in addition, alter conserved sequence features detected in a study [(1982) Biochimie 64, 867-881] of fungal mitochondrial introns.

Genetics of oxidative phosphorylation: petite deletion mapping of the Oli 2 region of the mitochondrial genome of Saccharomyces cerevisiae

Petite deletion mapping has been carried out for the Oli 2 region of the mitochondrial genome of Saccharomyces cerevisiae to produce a fine structure genetic map. Previously unlocated mit- mutants together with the drug resistant loci Oli 2 and Oss 1 have been ordered between the cytochrome oxidase and apocytochrome b genes. As a result of this study a series of isogenic p- clones have been isolated spanning the Oli 2 region.

Mitochondrial membrane biogenesis: characterization and use of pet mutants to clone the nuclear gene coding for subunit V of yeast cytochrome c oxidase

A nuclear pet mutant of Saccharomyces cerevisiae that is defective in the structural gene for subunit V of cytochrome c oxidase has been identified and used to clone the subunit V gene (COX5) by complementation. This mutant, E4-238 [24], and its revertant, JM110, produce variant forms of subunit V. In comparison to the wild-type polypeptide (Mr = 12,500), the polypeptides from E4-238 and JM110 have apparent molecular weights of 9,500 and 13,500, respectively. These mutations directly alter the subunit V structural gene rather than a gene required for posttranslational processing or modification of subunit V because they are cis-acting in diploid cells; that is, both parental forms of subunit V are produced in heteroallelic diploids formed from crosses between the mutant, revertant, and wild type. Several plasmids containing the COX5 gene were isolated by transformation of JM28, a derivative of E4-238, with DNA from a yeast nuclear DNA library in the vector YEp13. One plasmid, YEp13-511, with a DNA insert of 4.8 kilobases, was characterized in detail. It restores respiratory competency and cytochrome oxidase activity in JM28, encodes a new form of subunit V that is functionally assembled into mitochondria, and is capable of selecting mRNA for subunit V. The availability of mutants altered in the structural gene for subunit V (COX5) and of the COX5 gene on a plasmid, together with the demonstration that plasmid-encoded subunit V is able to assemble into a functional holocytochrome c oxidase, enables molecular genetic studies of subunit V assembly into mitochondria and holocytochrome c oxidase.

An mRNA maturase is encoded by the first intron of the mitochondrial gene for the subunit I of cytochrome oxidase in S. cerevisiae

We have localized ten oxi3- mutations in the first, al1, intron of the coxl gene. All are splicing deficient, being unable to excise the intron. Complementation experiments disclose several domains in the intron al1: the 5'-proximal and 3'-proximal domains harbor cis-dominant mutations, while trans-recessive ones are located in the intron's open reading frame. Comprehensive analyses of allele-specific polypeptides accumulating in mutants show that they result from the translation of the intron's ORF. We conclude that a specific mRNA maturase involved in splicing of oxidase mRNA is encoded by the intron al1 in a manner similar to the cytochrome b mRNA maturase.

Biosynthesis of the ubiquinol-cytochrome c reductase complex in yeast. Characterization of precursor forms of the 44-kDa, 40-kDa and 17-kDa subunits and identification of individual messenger RNAs for these and other imported subunits of the complex

The mitochondrial ubiquinol--cytochrome c reductase complex (complex III or cytochrome bc1 complex) is thought to consist of eight subunits, seven of which are specified by nuclear genes and synthesized in the cytoplasm. We have studied the synthesis of five of the nuclear-encoded subunits both in vivo and in vitro and show that of these the 44-kDa, 40-kDa and 17-kDa subunits are synthesized with cleavable extensions, while the 14-kDa and 11-kDa proteins are synthesized without detectable extra sequences. The sizes of the pre-sequences, as determined by the relative mobility of the precursor proteins in sodium dodecyl sulphate/polyacrylamide gels, range from 0.5-kDa for the 44-kDa and 40-kDa subunits to 9-kDa for the 17-kDa subunit. The existence in vivo of precursor forms to the 44-kDa, 40-kDa and 17-kDa subunits implies that import is at least partially a post-translational process. The precursor of the 44-kDa subunit can be processed post-translationally in vitro by isolated mitochondria. The messenger RNAs for subunits of the complex have been studied. Those coding for the 44-kDa, 40-kDa, 14-kDa and 11-kDa proteins and cytochrome c1 are of different sizes, indicating that each of these subunits is synthesized as a separate protein, rather than as part of a polyprotein precursor.

pif mutation blocks recombination between mitochondrial rho+ and rho- genomes having tandemly arrayed repeat units in Saccharomyces cerevisiae

Three allelic nuclear mutants affected in the recombination of mtDNA have been characterized in Saccharomyces cerevisiae and assigned to the PIF locus. In the mutants, the general recombination measured by the recombination frequency between linked or unlinked alleles is normal. However, the pif mutations prevent the integration into the rho+ genome of the markers (oli1, oli2, diu1, ery, oxi1, oxi2) of those rho- genomes that have tandemly arrayed repeat units. Therefore, these rho- genomes characterize a PIF-dependent recombination system. The pif mutations have also revealed the existence of a PIF-independent recombination system used by those rho- genomes that have an inverted organization of their repeat units. The markers of such palindromic rho- genomes exhibit high integration frequency into the rho+ genome even in the presence of the pif mutation. In addition, the pif mutations greatly increase suppressiveness in crosses between pif rho+ strains and PIF-dependent as well as PIF-independent rho- clones. We conclude that the recombination between rho+ and rho- genomes involves at least two distinct systems that depend on the organization of the rho- genome.

Variation, transcription and circular RNAs of the mitochondrial gene for subunit I of cytochrome c oxidase

The gene for subunit I of cytochrome c oxidase, contained within the OX13 region of yeast mitochondrial DNA, is split and shows a remarkable variation in structure, which is strain-dependent. The most complex form so far characterized is that of the Saccharomyces cerevisiae strain KL14-4A, in which nine or possibly ten exons are separated by eight to nine introns. At least four of these are facultative, two being absent from S. cerevisiae strain D273-10B (sequenced by Bonitz et al., 1980) and a further two lacking from the gene in Saccharomyces carlsbergensis. The complexity of the gene in KL14-4A is also reflected in its transcript pattern. RNA blot hybridization with isolated and cloned DNA fragments of the OX13 region permits visualization of more than 60 RNAs, which show overlapping and discontinuous hybridization behaviour. In the less complex strains D273-10B and S. carlsbergensis, this number is 20 and 11, respectively. These RNAs are most likely intermediates in processing events leading to the appearance of the mature messenger RNA for cytochrome c oxidase subunit I, which we identify as a 2100-nucleotide transcript (18SE). Most of the processing events are dependent on mitochondrial protein synthesis and do not constitute a single obligatory processing pathway. Like other yeast mitochondrial mRNAs, the 18 S RNA contains a long, untranslated 5' flanking sequence (approximately 400 nucleotides). One unusual aspect of splicing events involving OX13 transcripts is the accumulation of three of the excised introns as single-stranded RNA circles. These abundant and stable transcripts appear to be covalently closed. The simplest assumption is that they arise as (by)-products of splicing, but secondary ligation events have not been excluded. Their function is as yet unknown.

Genetics and biogenesis of cytochrome b

We have reviewed here the genetic methods used for isolating and manipulating nuclear and mitochondrial mutants of bakers' yeast that affect the function and biogenesis of complex III of the mitochondrial respiratory chain. All the methods have been used with success in the past, and it is hoped that this compilation will aid biochemists in using these techniques to study electron transfer.

Ionophores and intact cells. II. Oleficin acts on mitochondria and induces disintegration of the mitochondrial genome in yeast Saccharomyces cerevisiae

The non-macrolid polyene antibiotic oleficin, which has been shown to function as an ionophore of Mg2+ in isolated rat liver mitochondria, preferentially inhibited growth of the yeast Saccharomyces cerevisiae on non-fermentable substrates. It uncoupled and inhibited respiration of intact cells and converted both growing and resting cells into respiration-deficient mutants. The mutants arose as a result of fragmentation of the mitochondrial genome. Another antibiotic known to be an ionophore of divalent cations, A23187, also selectively inhibited growth of the yeast on non-fermentable substrates, but did not produce the respiration-deficient mutants, neither antibiotic inhibited the energy-dependent uptake of divalent cations by yeast cells nor opened the plasma membrane for these cations. The results indicate that in Saccharomyces cerevisiae both oleficin and A23187 preferentially affected the mitochondrial membrane without acting as ionophores in the plasma membrane.

The cytochrome oxidase subunit I split gene in Saccharomyces cerevisiae: genetic and physical studies of the mtDNA segment encompassing the 'cytochrome b-homologous' intron

We have constructed a refined genetic and physical map of 38 oxi3 mutations. With the help of the rho- clones derived from 'short' and 'long' genes, pairwise crosses between mutants, estimations of their reversion frequencies and analyses of mitochondrially synthesized proteins, we have characterized and localized several mutants in the exon A4 and in the intron aI4. We present genetic and physical evidence that in the 'long' gene the exon A5 is split into at least three quite distinct exons, A5-1, A5-2 and A5-3 where numerous mutations are localized. We suggest that a novel 56 Kd polypeptide, which accumulates in some cis-dominant oxi3- mutants results from the translation of the upstream exons and the downstream aI4 intron.

Biochemical characterization of the OXI mutants of the yeast Saccharomyces cerevisiae

OXI mutants in Saccharomyces cerevisiae lack a functional cytochrome c oxidase. Wild type and OXI mutants were grown in the presence of radioactive delta-amino[14C]levulinic acid, a precursor of porphyrin and heme, and [3H]mevalonic acid, a precursor of the alkyl side-chain of heme a. SDS polyacrylamide gel electrophoresis of the delipidated mitochondria showed that delta-amino[14C]levulinic acid was distributed into three bands migrating in the regions of Mr 28 000, 13 500, and 10 000, while [3H]mevalonic acid was found in a single band with apparent Mr of 10 000. The immunoprecipitates obtained by incubating the solubilized mitochondria of any OXI mutant with antibodies against cytochrome c oxidase, showed, after delipidation, a high specific radioactivity due to delta-amino[14C]levulinic acid and [3H]mevalonic acid. This suggested that a prophyrin a was present in all these OXI mutants. HCl fractionation confirmed the presence of porphyrin a in the apooxidase of these mutants. Atomic absorption spectra of the immunoprecipitate of cytochrome c oxidase showed that copper was not detectable in the mutant OXI IIIa which lacked subunit 1, but was present in the mutant OXI IIIb, which exhibited a minor alteration in the electrophoretic mobility of subunit 1. In OXI I and II mutants there was a 50% reduction in the amount of copper in the immunoprecipitated cytochrome c oxidase. These observations may be interpretable as follows: (1) alterations in polypeptide biosynthesis due to the OXI mutations lead to an improper configuration of cytochrome c oxidase, so that ferrochelatase cannot transfer iron into porphyrin a; (2) subunit I is the binding site for copper, but the mutations in subunits II and III alter the binding site of one of the two copper atoms in subunit I.

Mitochondrial and nuclear myxothiazol resistance in Saccharomyces cerevisiae

Mitochondrial and nuclear mutants resistant to myxothiazol were isolated and characterized. The mitochondrial mutants could be assigned to two loci, myx1 and myx2, by allelism tests. The two loci map in the box region, the split gene coding for apocytochrome b. Locus myx1 maps in the first exon (box4/5) whereas myx2 maps in the last exon (box6). The nuclear mutants could be divided into three groups: two groups of recessive mutations and one of dominant mutations. Respiration of isolated mitochondria from mitochondrial mutants is resistant to myxothiazol. These studies support the conclusion that myxothiazol is an inhibitor of the respiratory chain of yeast mitochondria. The site of action of myxothiazol is mitochondrial cytochrome b.

Kinetic and structural differences between cytochrome c oxidases from beef liver and heart

1. The cytochrome content of beef liver mitochondria differs from that of beef heart mitochondria by an eightfold lower cytochrome aa3 and a twofold lower cytochrome b and c + c1 content. 2. The kinetic properties of cytochrome c oxidases from beef liver and heart were measured with intact cytochrome c-depleted membranes, deoxycholate-dissolved membranes, and with the isolated enzymes at various cytochrome c concentrations with an oxygen electrode. Under all conditions a higher V was found for the liver enzyme, both for the low-affinity and for the high-affinity binding site for cytochrome c. Differences were also found for the Km of the two enzymes. 3. Isolated beef heart mitochondria contained about twice as much cardiolipin than beef liver mitochondria. The isolated enzymes contained one mole cardiolipin per mole of the heart enzyme, but 2 moles cardiolipin per mole of the liver enzyme. 4. By application of a high performance sodium dodecylsulfate gel electrophoretic system the two isolated enzymes could be separated into 13 different protein components, three of which (polypeptides VIa, VIIa and VIII) were found to differ in their apparent molecular weights. The functional meaning of cytochrome c oxidase isoenzymes in liver and heart is discussed.

Critical sequences within mitochondrial introns: cis-dominant mutations of the "cytochrome-b-like" intron of the oxidase gene

We have established the DNA sequence of two cis-dominant mutations located in the fourth intron, a14, of the yeast mitochondrial gene oxi3. These mutations prevent the synthesis of subunit I of cytochrome oxidase. Both mutations affect a very short DNA sequence located several hundred base pairs from the intron-exon junctions. An identical sequence is found in the cob-box gene; and this sequence is critical for the excision of the cytochrome b intron. Our interpretation is that this short sequence represents a common signal that must be recognized by the box7-encoded mRNA maturase, in conjunction with the mitochondrial ribosome, to splice out the introns in the two nonhomologous genes, cob-box and oxi3.

Long range control circuits within mitochondria and between nucleus and mitochondria. II. Genetic and biochemical analyses of suppressors which selectively alleviate the mitochondrial intron mutations

In the preceding paper of this series (Dujardin et al. 1980 a) we described general methods of selecting and genetically characterizing suppressor mutations that restore the respiratory capacity of mit- mitochondrial mutations. Two dominant nuclear (NAM1-1 and NAM2-1) and one mitochondrial (mim2-1) suppressors are more extensively studied in this paper. We have analysed the action spectrum of these suppressors on 433 mit- mutations located in various mitochondrial genes and found that they preferentially alleviate the effects of mutations located within intron open reading frames of the cob-box gene. We conclude that these suppressors permit the maturation of cytochrome b mRNA by restoring the synthesis of intron encoded protein(s) catalytically involved in splicing i.e. mRNA-maturase(s) (cf. Lazowska et al. 1980). NAM1-1 is allele specific and gene non-specific; it suppresses mutations located within different introns. NAM2-1 and mim2-1 are intron-specific: they suppress mutations all located in the same (box7) intron of the cob-box gene. Analyses of cytochrome absorption spectra and mitochondrial translation products of cells in which the suppressors are associated with various other mit- mutations show that the suppressors restore cytochrome b and/or cytochrome oxidase (cox I) synthesis, as expected from their growth phenotype. This suppression is, however, only partial: some new polypeptides characteristic of the mit- mutations can be still detected in the presence of suppressor. Interestingly enough when box7 specific suppressors NAM2-1 and mim2-1 are associated with a complete cob-box deletion (leading to a total deficiency of cytochrome b and oxidase) partial restoration of cox I synthesis is observed while cytochrome b is still totally absent. These results show that in strains carrying NAM2-1 or mim2-1 the presence of cytochrome b gene is no longer required for the expression of the oxi3 gene pointing out to the possibility of a mutational switch-on of silent genes, whether mitochondrial, mim2-1, or nuclear, NAM2-1. This switch-on would permit the synthesis of an active maturase acting as a substitute for the box7 maturase in order to splice the cytochrome b and oxidase mRNAs.

Mitochondrial mutagenesis in Saccharomyces cerevisiae: the origin of mit- mutants

Yeast cells contain many copies of mitochondrial (mit) genomes. The question we tried to answer was howmit−mutations occurring in one genome as a result of mutagenic treatment might yield homoplasmic mutant cells. Three processes were considered. First, that these cells originate by segregation of mutant and standard alleles during cell division. Secondly, that they originate through intracellular selection, for which cell division is not required. Thirdly, that recombination involving the mutant and standard alleles is non-reciprocal and unidirectionalmit+→mit−so that the mutant allele is spread into the entire population of mitochondrial genomes within a cell, thus making it homoplasmicmit−. The results indicate that the first process, although efficiently producing homoplasmic cells from heteroplasmic zygotes (for review see Birky, 1978), seems to play only a minor, if any, role in producing homoplasmic mutant progenies from mutagenized cells. The most important is the second process, that is, intracellular selection occurring in cells which have one or a few genomes carryingmit−mutations, while the remaining genomes are irreversibly damaged. The third process, unidirectionalmit+→mit−conversion, does not seem to play any part.

Membrane potentials in respiring and respiration-deficient yeasts monitored by a fluorescent dye

Changes in fluorescence of 3,3'-dipropylthiodicarbocyanine iodide which had been equilibrated with suspensions of the wild-type yeast Saccharomyces cerevisiae and of respiration-deficient mutants were followed. The changes have been attributed to changes of yeast membrane potentials, since the fluorescence with wild-type yeast could be affected in a predictable manner by uncouplers and the pore-forming agent nystatin. As in other systems, a rise of steady-state fluorescence was ascribed to depolarization and a drop of the fluorescence to hyperpolarization. (1) A considerable rise in steady-state fluorescence was brought about by addition of antimycin A or some other mitochondrial inhibitors to respiring cells. A major part of the composite membrane potential monitored in intact yeast cells appeared to be represented by the membrane potential of mitochondria. (2) Addition of D-glucose and of other substrates of hexokinase, including non-metabolizable 2-deoxy-D-glucose, induced a two-phase response of fluorescence, indicating transient depolarization followed by repolarization. Such a response was not elicited by other sugars which had been reported to be transported into the cells by a glucose carrier or by D-galactose in galactose-adapted cells. The depolarization was explained by electrogenic ATP exit from mitochondria to replenish the ATP consumed in the Hexokinase reaction and the repolarization by subsequent activation of respiration. (3) In non-respiring cells only a drop in fluorescence was induced by glucose and this was ascribed to an ATP-dependent polarization of the plasma membrane. (4) Steady-state fluorescence in suspensions of respiration-deficient mutants, lacking cytochrome a, cytochrome b, or both, was high an remained unaffected by uncouplers and nystatin. This indicates that membranes of the mutants may have been entirely depolarized. A partial polarization, apparently restricted to the plasma membrane, could be achieved by glucose addition.

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.

Mitochondrially synthesized protein subunits of the yeast mitochondrial adenosine triphosphatase. A reassessment

Evidence is presented that a mitochondrial translation product (Mr, 32,000) previously thought to be a subunit of the membrane sector of the yeast mitochondrial ATPase is a contaminant, consisting of subunit II of the cytochrome oxidase complex and cytochrome b apoprotein. Our data suggest that only two subunits (Mr, 7600 and 20,000) of the mitochondrial ATPase are synthesized in the mitochondria.

A novel extranuclear mutant of Neurospora with a temperature-sensitive defect in mitochondrial protein synthesis and mitochondrial ATPase

[C93] is a novel, extranuclear mutant of Neurospora crassa which has a normal mitochondrial phenotype when grown at 25 degrees, but which is deficient in cytochromes b and aa3 when grown at 37 degrees (Pittenger and West 1979). In the present work, the phenotype of [C93] was characterized in greater detail. When [C93] is grown at 37 degrees, the rate of mitochondrial protein synthesis is decreased to approximately 25% that of wild type; the ratio of mitochondrial small to large ribosomal subunits is decreased to 1:4 and mitochondrial small subunits are deficient in the mitochondrially-synthesized protein, S-5. The mitochondrial ribosome assembly defects in 37 degrees-grown [C93] resemble those in chloramphenicol-treated wild-type cells and could merely be a consequence of the decreased rate of mitochondrial protein synthesis. Analysis of mitochondrial translation products by SDS gel electrophoresis suggests that 37 degrees-grown [C93] is grossly deficient in the 19,000 Mr subunit of the oligomycin-sensitive ATPase relative to other mitochondrially-synthesized proteins. The ATPase defect was not found in other extranuclear or nuclear mutants deficient in mitochondrial protein synthesis. These data and additional evidence suggest that the primary defect in [C93] may be in the assembly of the ATPase complex. The possible connection between the ATPase defect and the deficiency of mitochondrial protein synthesis is discussed.

Transfer RNA genes in the cap-oxil region of yeast mitochondrial DNA

A cytoplasmic "petite" (rho-) clone of Saccharomyces cerevisiae has been isolated and found through DNA sequencing to contain the genes for cysteine, histidine, leucine, glutamine, lysine, arginine, and glycine tRNAs. This clone, designated DS502, has a tandemly repeated 3.5 kb segment of the wild type genome from 0.7 to 5.6 units. All the tRNA genes are transcribed from the same strand of DNA in the direction cap to oxil. The mitochondrial DNA segment of DS502 fills a sequence gap that existed between the histidine and lysine tRNAs. The new sequence data has made it possible to assign accurate map positions to all the tRNA genes in the cap-oxil span of the yeast mitochondrial genome. A detailed restriction map of the region from 0 to 17 map units along with the locations of 16 tRNA genes have been determined. The secondary structures of the leucine and glutamine tRNAs have been deduced from their gene sequences. The leucine tRNA exhibits 64% sequence homology to an E. coli leucine tRNA.

Leaky +1 and -1 frameshift mutations at the same site in a yeast mitochondrial gene

Two mutations in a mitochondrial structural gene, which cause leaky premature polypeptide chain termination and leaky growth, are +1 and -1 frameshifts in the same run of five T residues. The partial restoration of reading frame is probably due to ribosomal frameshifting at this site, and may be promoted by the unique structure of the yeast mitochondrial t RNAPhe.

Sequence of introns and flanking exons in wild-type and box3 mutants of cytochrome b reveals an interlaced splicing protein coded by an intron

We have determined the DNA sequence of the wild type and mutated introns as well as their flanking exons in the yeast mitochondrial gene specifying cytochrome b. The second intron (box3) encodes a trans-acting protein "mRNA maturase" responsible for splicing and maturation of cytochrome b mRNA. This protein is interlaced with cytochrome b exon sequences. Its biosynthesis is subject to a negative feedback which may constitute a regulatory mechanism for the expression of split genes.

Cytochrome b-565 in Saccharomyces cerevisiae: use of mutants in the cob-box region of the mitochondrial DNA to study the functional role of this spectral species of cytochrome b. 1. Measurements of cytochromes b-562 and b-565 and selection of revertants devoid of cytochrome b-565

In order to study the functional role of the spectral species of cytochrome b-565 observed in mitochondria, genetically manipulated strains of Saccharomyces cerevisiae have been used. Strains have been found which are devoid of cytochrome b-565 under certain conditions and which nevertheless are able to grow on a respirable substrate. Two different methods have been used to determine the cytochrome b-565 content: anaerobic titrations and antimycin-A-induced reduction of cytochrome b-565. Both yield the same results.