Localization of the gene coding for the mitochondrial 16 S ribosomal RNA using rho- mutants of Saccharomyces cerevisiae

New antibiotic resistance Loci in the ribosomal region of yeast mitochondrial DNA

A large number of mitochondrial antibiotic-resistant mutants have been isolated following mutagenesis with manganese. These include several different phenotypic classes of mutants, as distinguished by cross-resistance patterns, that have been found to be allelic at cap1 or ery1; some have been found to be heteroallelic.--Seven chloramphenicol-resistant mutants have been identified that are nonallelic by recombination tests with the three loci (cap1, spi1 and ery1) previously identified in the ribosomal region. Four of these are allelic with each other and define a new locus, cap3; two others are allelic and define another new locus, cap2; the seventh maps at yet a different locus, cap4. One new spiramycin-resistant mutant has been identified that defines still another new locus, spi2. A variety of genetic techniques have been used to map these loci within the ribosomal region of the mitochondrial genome.-Manganese has been shown to be effective in inducing the mutation from omega(-) to omega(n) in many mutants that experience a simultaneous mutation at the closely linked cap1 locus. The omega(n) mutation has also been described in the cap4 mutant, and this locus has been shown to be more closely linked to omega than cap1 is to omega.

Repeated sequences with unusual properties from the divergent region of Crithidia oncopelti maxicircle kinetoplast DNA

The occurrence of bacterial promoter-like sequences was shown in the divergent region of Crithidia oncopelti maxicircular kinetoplast DNA. A promoter-cloning vector based on the neomycin phosphotransferase II (NPT II) gene was used to localize promoters in three homologous blocks of repeated sequences. The elements found displayed greater promoter strength in Escherichia coli than the normal promoter of the NPT II gene. Sequences of three promoter-containing inserts from different blocks were determined and typical prokaryotic promoter sequences were localized.

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.

A mitochondrial ribosomal RNA mutation and its nuclear suppressors

We have isolated cold-resistant revertants from a mitochondrial cold-sensitive mutation, cs909, localized in the 21S ribosomal RNA gene. Two types of revertants have been isolated: (1) strong revertants which were shown to be due to a single, nuclear, dominant suppressor; (2) weak revertants which are all due to the presence of a single, nuclear, recessive suppressor. The recessive suppressor, when separated from the mitochondrial mutation, itself confers a cold-sensitive phenotype, that is, there is a mutual suppression between the mitochondrial cold-sensitive mutation and the nuclear cold-sensitive mutation. The suppressor by itself produces modified ribosomes and therefore probably codes for an element of the mitochondrial ribosome such as a ribosomal protein.

Synthesis and processing of ribosomal RNA in isolated yeast mitochondria

The synthesis and processing of the 15S and 21S rRNAs have been studied in isolated yeast mitochondria. When mitochondrial transcripts were labeled with [alpha-32p]UTP in an incubation mixture containing 50 microM ATP, the transcripts from the genes for the large and small ribosomal RNAs accumulated in the form of putative precursor molecules. The labeled pre-21S rRNA was converted to mature 21S rRNA during a chase period in the presence of 1 mM ATP. Thus, the maturation of 21S rRNA, a process which includes trimming at the 3' end and, in omega+ strains, the excision of a 1.1 kb intervening sequence, can occur in isolated mitochondria and appears to be dependent on ATP. In contrast, the maturation of 15S rRNA by the removal of approximately 80 nucleotides from the 5' end of a 15.5S transcript is severely restricted in isolated mitochondria, even in the presence of 2.5 mM ATP.

A mitochondrial locus is necessary for the synthesis of mitochondrial tRNA in the yeast Saccharomyces cerevisiae

We have used a cloned yeast mitochondrial tRNAUCNSer gene as a probe to detect RNA species that are transcripts from this gene in wild-type Saccharomyces cerevisiae and in petite deletion mutants. In RNA from wild-type cells, the tRNA is the most prominent transcript of the gene. In RNA from deletion mutants that retain this gene but have lost other regions of mtDNA, high molecular weight transcripts containing the tRNAUCNSer sequences accumulate but tRNAUCNSer is not made. tRNAUCNSer synthesis can be restored in these mutants when they are mated to other deletion mutants that retain a different portion of the mitochondrial genome. Protein synthesis is not necessary for the restoration, and the restoration is not due to a nuclear effect or to an effect of mating alone, because strains without mtDNA are not able to restore tRNA synthesis. These results definitively demonstrate the existence of a yeast mitochondrial locus that is necessary for tRNA synthesis and, because the restoration of tRNAUCNSer synthesis appears to result from intergenic complementation, not recombination, indicate that this locus acts in trans.

Sequence of the intron and flanking exons of the mitochondrial 21S rRNA gene of yeast strains having different alleles at the omega and rib-1 loci

The complete nucleotide sequence has been determined for the intron, its junctions and the flanking exon regions of the 21S rRNA gene in three genetically characterized strains differing by their omega alleles (omega+, omega- and omega n) and by their chloramphenicol-resistant mutations at the rib-1 locus. Comparison of these DNA sequences shows that: --omega+ differs from omega- and omega n by the presence of the intron (1143 bp), as well as by a second and unexpected mini-insert (66 bp) located 156 bp upstream within the exon, whose nature and functions are still unknown but whose striking palindromic structure may suggest a mitochondrial transposable element. --The two mutations C321R and C323R correspond to two different monosubstitutions, 56 bp apart in the omega- and omega n strains but separated by the intron in the omega+ strains. In relation to previous genetic results, a model is discussed assuming that the interactions of two different regions or genetic loci determine the chloramphenicol resistance, one of which contains the omega n mutations. --A long uninterrupted coding sequence able to specify a 235 amino acid polypeptide exists within the intron. This remarkable observation gives new insight into the origin of the mitochondrial introns and raises the question of the possible functions of intron-encoded polypeptides. Finally, sequence comparisons with evolutionarily distant organisms, showing that different rRNA introns are inserted at different positions of an otherwise highly conserved region of the gene, suggest a recent insertion of these introns and a mechanism for splicing after the assembly of the large ribosomal subunit.

Mitochondrial and nuclear mutations that affect the biogenesis of the mitochondrial ribosomes of yeast. I. Genetics

We have isolated about five hundred temperature-sensitive mutants specific for the mitochondrial functions. Their growth on glycerol is defective at 36 degrees C and/or 20 degrees C. While most of the mutations were nuclearly inherited, about thirty were found to be of mitochondrial origin. 1) Four mitochondrial mutations (three cryosensitive, one thermosensitive) were localized close to chloramphenicol and erythromycin resistance loci of the mitochondrial DNA, that is in the region coding for the 23 S ribosomal RNA. One of the mutation interfered with the expression of the chloramphenicol resistance gene. 2) A dozen nuclear mutations were isolated from a strain which is labelled with mitochondrial drug resistance markers (chloramphenicol, erythromycin, and paromomycin). Among the temperature sensitive respiratory deficient mutants, we have selected the mutations that supress the resistant phenotypes. We describe two non allelic such mutations, one being cryosensitive, the other thermosensitive. Both supress the expression of the mitochondrial chloramphenicol resistance gene. The temperature sensitive growth on glycerol and the modified antibiotic phenotype segregated together as a single recessive mutation. A biochemical study of these mutants is presented in a joint paper, confirming their presumed ribosomal nature.

Mitochondrial and nuclear mutations that affect the biogenesis of the mitochondrial ribosomes of yeast. II. Biochemistry

1. Several nuclear mutants have been isolated which showed thermo- or cryo-sensitive growth on non-fermentable media. Although the original strain carried mitochondrial drug resistance mutations (CR, ER, OR and PR), the resistance to one or several drugs was suppressed in these mutants. Two of them showed a much reduced amount of the mitochondrial small ribosomal subunit (37S) and of the corresponding 16S ribosomal RNA. Two dimensional electrophoretic analysis did not reveal any change in the position of any of the mitochondrial ribosomal proteins. However one of the mitochondrial ribosomal proteins. However one of the mutants showed a striking decrease in the amounts of three ribosomal proteins S3, S4 and S15. 2. Four temperature-sensitive mitochondrial mutations have been localized in the region of the gene coding for the large mitochondrial ribosomal RNA (23S). These mutants all showed a marked anomaly in the mitochondrial large ribosomal subunit (50S) and/or the corresponding 23S ribosomal RNA.

The synthesis of chloroplast high-molecular-weight ribosomal ribonucleic acid in spinach

Illuminated suspensions of chloroplasts isolated from young spinach leaves show incorporation of [3H]uridine into several species of RNA. One such RNA species of Mr 2.7 x 10(6) shows sequence homology with both the chloroplast 23-S rRNA (Mr = 1.05 x 10(6)) and 16-S rRNA (Mr = 0.56 x 10(6)), as judged by DNA/RNA competition hybridization. Leaves labelled in vivo with [32P]orthophosphate in the presence of chloramphenicol accumulate labelled RNAs of Mr 1.28 x 10(6), 0.71/0.75 x 10(6) and 0.47 x 10(6). The 1.28 x 10(6)-Mr RNA shows 80.5% sequence homology with the 1.05 x 10(6)-Mr rRNA and the 0.71/0.75 x 10(6)-Mr RNA mixture shows 76% sequence homology with the 0.56 x 10(6)-Mr rRNA. We conclude that the pathway of rRNA maturation in spinach chloroplasts is similar to that of Escherichia coli.

The genetic map of transfer RNA genes of yeast mitochondria: correction and extension

Ninety five rho- mitochondrial DNA's of Saccharomyces cerevisiae were compared for their deletion structure by means of 15 genetic markers and 22 tRNA genes. The patterns of co-deletion and co-retention of different tRNA genes allowed us to determine their positions with respect to each other. The deduced order of tRNA genes was consistent with the order of the genetic markers established by independent genetic approaches. Our previously proposed mitochondrial tRNA gene map has been revised and extended. Transfer RNA genes, corresponding to all 20 aminoacids, and two isoacceptor tRNA genes were localized. The possible position of each tRNA gene has been indicated on the physical map of mitochondrial DNA. Seventeen tRNA genes are carried by a narrow region representing less than 20% of the wild type genome.

Fine structure of the 21S ribosomal RNA region on yeast mitochondrial DNA. II. The organization of sequences in petite mitochondrial DNAs carrying genetic markers from the 21S region

We have investigated the organization of sequences in ten rho- petite mtDNAs by restriction enzyme analysis and electron microscopy. From the comparison of the physical maps of the petite mtDNAs with the physical map of the mtDNA of the parental rho+ strain we conclude that there are at least three different classes of petite mtDNAs: I. Head-to-tail repeats of an (almost) continuous segment of the rho+ mtDNA. II. Head-to-tail repeats of an (almost) continuous segment of the rho+ mtDNA with a terminal inverted duplication. III. Mixed repeats of an (almost) continuous rho+ mtDNA segment. In out petite mtDNAs of the second type, the inverted duplications do not cover the entire conserved rho+ mtDNA segment. We have found that the petite mtDNAs of the third type contain a local inverted duplication at the site where repeating units can insert in two orientations. At least in one case this local inverted duplication must have arisen by mutation. The rearrangements that we have found in the petite mtDNAs do not cluster at specific sites on the rho+ mtDNA map. Large rearrangements or deletions within the conserved rho+ mtDNA segment seem to contribute to the suppressiveness of a petite strain. There is also a positive correlation between the retention of certain segments of the rho+ mtDNA and the suppressiveness of a petite strain. We found no correlation between the suppressiveness of a petite strain and its genetic complexity. The relevance of these findings for the mechanism of petite induction and the usefulness of petite strains for the physical mapping of mitochondrial genetic markers and for DNA sequence analysis are discussed.

Inserted sequence in the mitochondrial 23S ribosomal RNA gene of the yeast Saccharomyces cerevisiae

The sequence organization of the yeast mit-DNA region carrying the large ribosomal RNA gene and the polar locus omega was examined. Hybridization studies using rho- deletion mutants and electron microscopy of the heteroduplexes formed between 23S rRNA and the appropriate restriction fragments, lead to the conclusion that the 23S rRNA1 gene of the omega+ strains is split by an insertion sequence of 1,000-1,100 bp. In contrast, no detactable insertion was found in the 23S rRNA gene of the omega- strains. The size and the location of the insert found in the 23S rRNA gene of the omega+ strains appear to be identical to those of the sequence delta which had previously been found to characterize the difference (at the omega locus) between the mitDNA of the wild type strains carrying the omega+ or omega- alleles (Jacq et al., 1977).

Suppression of mitochondrially-determined resistance to chloramphenicol and paromomycin by nuclear genes in Saccharomyces cerevisiae

Phenotypic "revertants" of a drug resistant strain of Saccharomyces cerevisiae were induced by mutgenesis with manganese. Several of these drug sensitive mutants have been shown to result from mutations in the nuclear genome that cause phenotypic modification (suppression) of the mitochondrially-determined drug resistant genotype. Four mutants carrying a single recessive nuclear gene capable of modifying mitochondrial chloramphenicol resistance are described; these may be assigned to three complementation groups. Chloramphenicol resistant mutants mapping at five separate mitochondrial loci are described. At least two of the nuclear genes cause modification of mitochondrial chloramphenicol resistance determined by mutations at three of these loci, but the other two loci are apparently non-suppressible by these nuclear alleles. This indicates that these modifiers do not act by causing a general decrease in cellular or mitochondrial permeability to the drug. A single dominant nuclear modifier of mitochondrial paromomycin resistance has been identified. It is non-allelic to and does not interact with the genes modifying mitochondrial chloramphenicol resistance.

Physical mapping of genes on yeast mitochondrial DNA: localization of antibiotic resistance loci, and rRNA and tRNA genes

We have physically mapped the loci conferring resistance to antibiotics that inhibit mitochondrial protein synthesis (erythromycin, chloramphenicol and paromomycin) or respiration (oligomycin I and II), as well as the 21s and 14s rRNA and tRNA genes on the restriction map of the mitochondrial genome of the yeast Saccharomyces cerevisiae. The mitochondrial genes were localized by hybridization of labeled RNA probes to restriction fragments of grande (strain MH41-7B) mitochondrial DNA (mtDNA) generated by endonucleases EcoRI, HpaI, BamHI, HindIII, SalI, PstI and HhaI. We have derived the HhaI restriction fragment map of MH41-7B mit DNA, to be added to our previously reported maps for the six other endonucleases. The antibiotic resistance loci (antR) were mapped by hybridization of 3H-cRNA transcribed from single marker petite mtDNA's of low kinetic complexity to grande restriction fragments. We have chosen the single Sal I site as the origin of the circular physical map and have positioned the antibiotic loci as follows: C (99.5-1.Ou)--P (27-36.Ou)--OII (58.3-62u--OI (80-84u)--E (94.4-98.4u). The 21s rRNA is localized at 94.4-99.2u, and the 14s rRNA is positioned between 36.2-39.8u. The two rRNA species are separated by 36% of the genome. Total mitochondrial tRNA labeled with 125I hybridized primarily to two regions of the genome, at 99.5-11.5u and 34-44u. A third region of hybridization was occasionally detected at 70--76u, which probably corresponds to seryl and glutamyl tRNA genes, previously located to this region by petite deletion mapping.

Restriction enzyme analysis of mitochondrial DNAs of petite mutants of yeast: classification of petites, and deletion mapping of mitochondrial genes

We have analyzed the restriction digest patterns of the mitochondrial DNA from 41 cytoplasmic petite strains of Saccharomyces cerevisiae, that have been extensively characterized with respect to genetic markers. Each mitochondrial DNA was digested with seven restriction endonucleases (EcoRI, HPaI, HindIII, BamHI, HhaI, SalI, and PstI) which together make 41 cuts in grande mitochondrial DNA and for which we have derived fragment maps. The petite mitochondrial DNAs were also analyzed with HpaII, HaeIII, and AluI, each of which makes more than 80 cleavages in grande mitochondrial DNA. On the basis of the restriction patterns observed (i.e., only one fragment migrating differently from grande for a single deletion, and more than one for multiple deletions) and by comparing petite and grande mitochondrial DNA restriction maps, the petite clones could be classified into two main groups: (1) petites representing a single deletion of grande mitochondrial DNA and (2) petites containing multiple deletions of the grande mitochondrial DNA resulting in rearranged sequences. Single deletion petites may retain a large portion of the grande mitochondrial genome or may be of low kinetic cimplexity. Many petites which are scored as single continuous deletions by genetic criteria were later demonstrated to be internally deleted by restriction endonuclease analysis. Heterogeneous sequences, manifested by the presence of sub-stoichiometric amounts of some restriction fragments, may accompany the single or multiple deletions. Single deletions with heterogeneous sequences remain useful for mapping if the low concentration sequences represent a subset of the stoichiometric bands. Using a group of petites which retain single continuous regions of the grande mitochondrial DNA, we have physically mapped antibiotic resistance and mit- markers to regions of the grande restriction map as follows: C (99.3--1.4 map units)--OXI-1 (2.5--15.7)--OXI-2 (18.5--25)--P (28.1--34.2)--OXI-3 (32.2--61.2--OII (60--62)--COB (64.6--80.8--0I (80.4--85.7)--E (95--98.9).

The ribosomal RNA genes on Neurospora crassa mitochondrial DNA are adjacent

Hybridization of separated 24 S and 17 S ribosomal RNA from Neurospora crassa mitochondrial ribosomes to restriction fragments of mitochondrial DNA leads to the conclusion that the large and small ribosomal RNA are adjacent on the restriction endonuclease cleavage map of the DNA. The distance between the two genes is estimated at 900 basepairs. This result is consistent with the existence of a ribosomal precursor RNA in N. crassa mitochondria and is in contrast to the situation in yeast, where the ribosomal genes are far apart on the mitochondrial DNA. The position of the ribosomal RNA genes on the cleavage map of N. crassa mtDNA provides a start for ordering the Hind III restriction fragments.

Modified recombination and transmission of mitochondrial genetic markers in rho minus mutants of Saccharomyces cerevisiae

A large number of primary petite (rho-) clones were isolated after ethidium bromide mutagenesis of various grande (rho+) strains of S. cerevisiae that contained the mitochondrial genetic markers, CR, ER, OIR (or OIIR), and PR. From the frequency of coretention of markers in the petites, we have deduced a probable circular order of the markers in the grande mitochondrial genome. From these primary clones several series of pure and stable petite clones were obtained and analyzed genetically. (a) In general, the omega allele is retained or lost together with the region carrying both CR and ER markers. (b) The petites that have retained only the CR marker fall into two classes: some have kept the omega allele of the grande strain they issued from; others exhibit a new omega expression. (c) The proportion of diploid petites in petite X grande crosses is independent of the presence of the omega allele. (d) In most cases, the coordinated transmission of markers observed so far in all grande X grande nonpolar corsses does not exist anymore in petites.

Mapping of mitochondrial genes in Saccharomyces cerevisiae. Populations and pedigree analysis of retention or loss of four genetic markers in Rho-cells

1. Retention or loss of mitochondrial markers CR321, OR1, PR454, TR (gene loci RIB1, OLI1, PAR1, TSM1 respectively has been analysed in a large number of ethidium bromide induced primary rho-clones. Retention of one or more of the four markers with a single clone was observed frequently, only 20 to 25% of clones were found to be (TOCOOOPO). Primary clones retaining two or more of the four markers were found to be mixed, i.e. the primary rho- cell contained a heterogeneous population of variously deleted mitDNA molecules which segregated into different cell lines in the corresponding primary clone. 2. A representative sample of the population of ethidium bromide induced rho- mutants has been analysed by a first subcloning performed after some 30 cell generations of vegetative multiplication in the abscence of the drug. At this level the heterogeneous population of mitDNA molecules, generated by the mutagenic treatment in the primary cell, has been sorted out. The cells forming secondary clones are thus essentially homoplasmic. In contrast to primary clones, genotypes of secondary clones therefore could be determined unambiguously, and the frequency of cell types can be regarded as a faithful representation of the frequency of mitDNA molecules. Retention of markers was low, in less than 2% of secondary clones one or several markers have been found. This observation has been interpreted as indicating that induction of rho-mutants by ethidium bromide is accompanied by deletion of very large sequences of mitDNA in a very large fraction of mitDNA molecules. 3. Five individual rho-clones retaining the four markers TRCRORPR have been isolated and analysed for spontaneous deletion of one or several of these markers during successive subclonings (pedigree analysis). High genetic stability (98-99.5% per cell generation) has been observed in these clones. 4. A method has been developed allowing an unambiguous determination of the order of the four markers on a circular map. It is based on the concomitant loss of two markers and retention of the other two markers (double loss/double retention analysis). The results of four out of five pedigrees of individual rho-clones analysed (spontaneous deletion) and the results of the analysis of populations of secondary rho-clones (ethidium bromide induced deletion) were in full agreement and the order of genes has been determined as being P-T-C-O-P. In the fifth pedigree results suggest an inversion of the T and C markers. 5. Relative distances between pairs of markers have been derived from the frequencies of separation of markers by deletion and were found to be C-T less than C-O less than T-O less than T-P less than C-P less than O-P. Linkage of the four markers could be established, and distances calculated are additive. 6. The general relevance of this approach of mapping by deletion and the methods used for the determination of order and distances of mitochondrial genes has been discussed. (ABSTRACT TRUNCATED)

Electron microscopy of analysis of circular repetitive mitochondrial DNA molecules from genetically characterized rho- mutants of Saccharomyces cerevisiae

1. We have studied mtDNA purified from nine p- petite mutants in which most of the wild type sequence has been deleted but the genetic markers conferring resistance to erythromycin of oligomycin or paromomycin have been retained. 2. All mtDNA contained numerous circular molecules. The size distribution of the circles conformed to a multimeric series which was characteristic for each mutant. We conclude that any one region of the wild type mtDNA molecule, when maintained in a p- clone, while other regions are deleted, can give rise to a multimeric series of circles. 3. In tandem straight repetitive mtDNAs the circles contain odd and even number of unit sequence repeats. In palindrome repetitive mtDNAs the circles contain mostly even number of unit sequence repeats. Thus, one straight or two inverted repeats constitute the monomeric unit of circularization. 4. We found that the frequency distribution of circles follows on a number basis a simple rule: frequency of numeric circles = 1/n frequency of monomeric circles, for n = 2, 3 and 4. Thus, on a mass basis each class represents the same fraction of total mtDNA and the mitochondrial genome has the same probability to constitute one monomeric circle or to be a part of n-meric circle. We interpret this finding that in vivo all molecules are circular. 5. Four mutants displayed a single multimeric series of circles ranging from 0.3 mum to 2.4 mum monomer circle length. Five mutants displayed multiple different multimeric series. In the latter case, the longest unit sequence repeat length was equal to the sum of the two shorter unit sequence repeat lengths. Sorting out, recombination and internal deletions of circular repetitive p- mtDNA molecules are discussed.

Deletion mapping of mitochondrial transfer RNA genes in Saccharomyces cerevisiae by means of cytoplasmic petite mutants

Mitochondrial transfer RNA genes have been ordered relative to the position of five mitochondrial drug resistance markers, namely, chloramphenicol (C),1 erythromycin (E), oligomycin I and II (OI, OII), and paromomycin (P). Forty-six petite yeast clones that were genetically characterized with respect to these markers were used for a study of these relationships. Different regions of the mitochondrial genome are deleted in these individual mutants, resulting in variable loss of genetic markers. Mitochondrial DNA was isolated from each mutant strain and hybridized with eleven individual mitochondrial transfer RNAs. The following results were obtained: i) Of the seven petite clones that retained C, E, and P resistance markers (but not O1 or O11), four carried all eleven transfer RNA genes examined; the other three clones lost several transfer RNA genes, probably by secondary internal deletion; ii) Prolyl and valyl transfer RNA genes were located close to the P marker, whereas the histidyl transfer RNA gene was close to the C marker; iii) Except for a glutamyl transfer RNA gene that was loosely associated with the O1 region, no other transfer RNA genes were found in petite clones retaining only the O1 and/or the OII markers; and iv) Two distinct mitochondrial genes were found for glutamyl transfer RNA, they were not homologous in DNA sequence and were located at two separate loci. The data indicate that the petite mitochondrial genome is the result of a primary deletion followed by successive additional deletions. Thus an unequivocal gene arrangement cannot be readily established by deletion mapping with petite mutants alone. Nevertheless, we have derived a tentative circular map of the yeast mitochondrial genome from the data; the map indicates that all but one of the transfer RNA genes are found between the C and P markers without forming a tight cluster. The following arrangement is suggested: -P-pro-val-ile-(phe, ala, tyr, asp)-glu2- (lys-leu)-his-C-E-O1-glu1-OII-P-.

Genetic analysis of mitochondrial biogenesis and function in Saccharomyces cerevisiae

Different mitochondrial mutants have been isolated that affect mitochondrial ribosome function. These mutants were used to establish most of the known methods and principles of mitochondrial genetics in yeast. Another class of mitochondrial mutants have been shown to affect mitochondrial ATPase and, more specifically, the "membrane factor" of mitochondrial ATPase. These mutants might be very useful in studying the energy-conserving function, and the interaction between the hydrophobic and hydrophylic parts, of the ATPase complex. New types of mitochondrial point mutations, concerning cytochrome a-a3 or b, will soon open up new fields of investigation. The biochemical and genetic analysis of numerous mutants belonging to that category and recently obtained [31] is being currently pursued in Tzagoloff's and Slonimski's laboratories.