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

Genome Editing for Sustainable Crop Improvement and Mitigation of Biotic and Abiotic Stresses

Climate change poses a serious threat to global agricultural activity and food production. Plant genome editing technologies have been widely used to develop crop varieties with superior qualities or can tolerate adverse environmental conditions. Unlike conventional breeding techniques (e.g., selective breeding and mutation breeding), modern genome editing tools offer more targeted and specific alterations of the plant genome and could significantly speed up the progress of developing crops with desired traits, such as higher yield and/or stronger resilience to the changing environment. In this review, we discuss the current development and future applications of genome editing technologies in mitigating the impacts of biotic and abiotic stresses on agriculture. We focus specifically on the CRISPR/Cas system, which has been the center of attention in the last few years as a revolutionary genome-editing tool in various species. We also conducted a bibliographic analysis on CRISPR-related papers published from 2012 to 2021 (10 years) to identify trends and potential in the CRISPR/Cas-related plant research. In addition, this review article outlines the current shortcomings and challenges of employing genome editing technologies in agriculture with notes on future prospective. We believe combining conventional and more innovative technologies in agriculture would be the key to optimizing crop improvement beyond the limitations of traditional agricultural practices.

Genome Editing for Sustainable Crop Improvement and Mitigation of Biotic and Abiotic Stresses

Climate change poses a serious threat to global agricultural activity and food production. Plant genome editing technologies have been widely used to develop crop varieties with superior qualities or can tolerate adverse environmental conditions. Unlike conventional breeding techniques (e.g., selective breeding and mutation breeding), modern genome editing tools offer more targeted and specific alterations of the plant genome and could significantly speed up the progress of developing crops with desired traits, such as higher yield and/or stronger resilience to the changing environment. In this review, we discuss the current development and future applications of genome editing technologies in mitigating the impacts of biotic and abiotic stresses on agriculture. We focus specifically on the CRISPR/Cas system, which has been the center of attention in the last few years as a revolutionary genome-editing tool in various species. We also conducted a bibliographic analysis on CRISPR-related papers published from 2012 to 2021 (10 years) to identify trends and potential in the CRISPR/Cas-related plant research. In addition, this review article outlines the current shortcomings and challenges of employing genome editing technologies in agriculture with notes on future prospective. We believe combining conventional and more innovative technologies in agriculture would be the key to optimizing crop improvement beyond the limitations of traditional agricultural practices.

Application of CRISPR-Mediated Gene Editing for Crop Improvement

Plant gene editing has become an important molecular tool to revolutionize modern breeding of crops. Over the past years, remarkable advancement has been made in developing robust and efficient editing methods for plants. Despite a variety of available genome editing methods, the discovery of most recent system of clustered regularly interspaced short palindromic repeats-CRISPR-associated proteins (CRISPR-Cas) has been one of the biggest advancement in this path, with being the most efficient approach for genome manipulation. Until recently, genetic manipulations were confined to methods, like Agrobacterium-mediated transformations, zinc-finger nucleases, and TAL effector nucleases. However this technology supersedes all other methods for genetic modification. This RNA-guided CRISPR-Cas system is being rapidly developed with enhanced functionalities for better use and greater possibilities in biological research. In this review, we discuss and sum up the application of this simple yet powerful tool of CRISPR-Cas system for crop improvement with recent advancement in this technology.

Homing endonucleases from mobile group I introns: discovery to genome engineering

Homing endonucleases are highly specific DNA cleaving enzymes that are encoded within genomes of all forms of microbial life including phage and eukaryotic organelles. These proteins drive the mobility and persistence of their own reading frames. The genes that encode homing endonucleases are often embedded within self-splicing elements such as group I introns, group II introns and inteins. This combination of molecular functions is mutually advantageous: the endonuclease activity allows surrounding introns and inteins to act as invasive DNA elements, while the splicing activity allows the endonuclease gene to invade a coding sequence without disrupting its product. Crystallographic analyses of representatives from all known homing endonuclease families have illustrated both their mechanisms of action and their evolutionary relationships to a wide range of host proteins. Several homing endonucleases have been completely redesigned and used for a variety of genome engineering applications. Recent efforts to augment homing endonucleases with auxiliary DNA recognition elements and/or nucleic acid processing factors has further accelerated their use for applications that demand exceptionally high specificity and activity.

Homing endonucleases: from genetic anomalies to programmable genomic clippers

Homing endonucleases are strong drivers of genetic exchange and horizontal transfer of both their own genes and their local genetic environment. The mechanisms that govern the function and evolution of these genetic oddities have been well documented over the past few decades at the genetic, biochemical, and structural levels. This wealth of information has led to the manipulation and reprogramming of the endonucleases and to their exploitation in genome editing for use as therapeutic agents, for insect vector control and in agriculture. In this chapter we summarize the molecular properties of homing endonucleases and discuss their strengths and weaknesses in genome editing as compared to other site-specific nucleases such as zinc finger endonucleases, TALEN, and CRISPR-derived endonucleases.

Persistent heteroplasmic cells for mitochondrial genes in Saccharomyces cerevisiae

Genes in mitochondria and chloroplasts segregate rapidly during vegetative reproduction. Models to explain this vegetative segregation invoke either random segregation of organelle DNA molecules, or nonrandom segregation with random recombination events. All such models are basically stochastic. To look at vegetative segregation we took heteroplasmic (HET) cells containing mitochondrial mutations at the cap1, eryl and olil loci from several crosses. HETs were repeatedly selected and subcloned. Even after three to five successive subclonings (approximately 60-100 generations) some cells remained heteroplasmic. This confirms and extends previous observations of persistent HETs by Rank and Bech-Hansen (1972) and Forster and Kleese (1975), and by Bolen et al. (1980) for chloroplast genes in Chlamydomonas.

Homing endonucleases: from microbial genetic invaders to reagents for targeted DNA modification

Homing endonucleases are microbial DNA-cleaving enzymes that mobilize their own reading frames by generating double strand breaks at specific genomic invasion sites. These proteins display an economy of size, and yet recognize long DNA sequences (typically 20 to 30 base pairs). They exhibit a wide range of fidelity at individual nucleotide positions in a manner that is strongly influenced by host constraints on the coding sequence of the targeted gene. The activity of these proteins leads to site-specific recombination events that can result in the insertion, deletion, mutation, or correction of DNA sequences. Over the past fifteen years, the crystal structures of representatives from several homing endonuclease families have been solved, and methods have been described to create variants of these enzymes that cleave novel DNA targets. Engineered homing endonucleases proteins are now being used to generate targeted genomic modifications for a variety of biotech and medical applications.

Nucleo-mitochondrial interactions in mitochondrial gene expression

All proteins encoded by mitochondrial DNA (mtDNA) are dependent on proteins encoded by nuclear genes for their synthesis and function. Recent developments in the identification of these genes and the elucidation of the roles their products play at various stages of mitochondrial gene expression are covered in this review, which focuses mainly on work with the yeast Saccharomyces cerevisiae. The high degree of evolutionary conservation of many cellular processes between this yeast and higher eukaryotes, the ease with which mitochondrial biogenesis can be manipulated both genetically and physiologically, and the fact that it will be the first organism for which a complete genomic sequence will be available within the next 2 to 3 years makes it the organism of choice for drawing up an inventory of all nuclear genes involved in mitochondrial biogenesis and for the identification of their counterparts in other organisms.

Expression of a site-specific endonuclease stimulates homologous recombination in mammalian cells

Double-strand breaks introduced into DNA in vivo have been shown to enhance homologous recombination in a variety of chromosomal and extrachromosomal loci in Saccharomyces cerevisiae. To introduce double-strand breaks in DNA at defined locations in mammalian cells, we have constructed a mammalian expression vector for a modified form of I-Sce I, a yeast mitochondrial intron-encoded endonuclease with an 18-bp recognition sequence. Expression of the modified I-Sce I endonuclease in COS1 cells results in cleavage of model recombination substrates and enhanced extrachromosomal recombination, as assayed by chloramphenicol acetyltransferase activity and Southern blot analysis. Constitutive expression of the endonuclease in mouse 3T3 cells is not lethal, possibly due to either the lack of I-Sce I sites in the genome or sufficient repair of them. Expression of an endonuclease with such a long recognition sequence will provide a powerful approach to studying a number of molecular processes in mammalian cells, including homologous recombination.

Group I introns as mobile genetic elements: facts and mechanistic speculations--a review

Group I introns form a structural and functional group of introns with widespread but irregular distribution among very diverse organisms and genetic systems. Evidence is now accumulating that several group I introns are mobile genetic elements with properties similar to those originally described for the omega system of Saccharomyces cerevisiae: mobile group I introns encode sequence-specific double-strand (ds) endoDNases, which recognize and cleave intronless genes to insert a copy of the intron by a ds-break repair mechanism. This mechanism results in: the efficient propagation of group I introns into their cognate sites; their maintenance at the site against spontaneous loss; and, perhaps, their transposition to different sites. The spontaneous loss of group I introns occurs with low frequency by an RNA-mediated mechanism. This mechanism eliminates introns defective for mobility and/or for RNA splicing. Mechanisms of intron acquisition and intron loss must create an equilibrium, which explains the irregular distribution of group I introns in various genetic systems. Furthermore, the observed distribution also predicts that horizontal transfer of intron sequences must occur between unrelated species, using vectors yet to be discovered.

Site-specific DNA endonuclease and RNA maturase activities of two homologous intron-encoded proteins from yeast mitochondria

Two introns of the mitochondrial genome 777-3A of S. cerevisiae, bl4 in cob and al4 in coxl genes, contain ORFs that can be translated into two homologous proteins. We changed the UGA, AUA, and CUN codons of these ORFs to the universal genetic code, in order to study the functions of their translated products in E. coli and in yeast, by retargeting the nuclear encoded protein into mitochondria. The p27bl4 protein has been shown to be required for the splicing of both introns bl4 and al4. The homologous p28al4 protein is highly toxic to E. coli. It can specifically cleave double-stranded DNA at a sequence representing the junction of the two fused flanking exons. We present evidence that this system is a good model for studying the role of mitochondrial intron-encoded proteins in the rearrangement of genetic information at both the RNA (RNA splicing-bl4 maturase) and DNA levels (intron transposition-al4 transposase).

Molecular archaeology of the mitochondrial genome

We develop a stepwise model for the net transfer of nucleic acid sequences between non-homologous genomes. This model is then used to explain the two major patterns in the evolutionary history of mitochondrial genomes: the gross reduction of the number of genes, and the subsequent acquisition of introns.

Expandable var1 gene of yeast mitochondrial DNA: in-frame insertions can explain the strain-specific protein size polymorphisms

The var1 locus of yeast mitochondrial DNA encodes a protein of the small mitochondrial ribosome subunit, denoted var1 protein. The size of var1 protein was previously shown to exhibit a strain-dependent polymorphism, determined by various combinations of at least three genetic elements. We report here that the var1 gene is itself polymorphic and that the six forms of this gene examined here differ by various combinations of three in-frame insertions into the coding region of the smallest allele. These insertions, which appear to be the molecular basis for the genetic elements, could increase the size of var1 protein by 8, 10, 16, 24, or 26 amino acid residues, accounting for the observed protein polymorphisms. Furthermore, we have characterized three additional sources of sequence variation located outside of the coding region but within major transcripts of the var1 gene.

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.

Analysis of a yeast nuclear gene involved in the maturation of mitochondrial pre-messenger RNA of the cytochrome oxidase subunit I

We have analyzed the mitochondrial RNA of a yeast nuclear pet mutant with no cytochrome oxidase activity. The product of the gene affected in this mutant appears to be necessary for the correct maturation of the mitochondrial pre-mRNA of the cytochrome oxidase subunit I. It does not affect, however, the overall splicing of cytochrome b pre-mRNA or the intron excision of the 21S ribosomal RNA precursor. This gene has been isolated by genetic complementation in yeast, and its DNA sequence has been determined. It is transcribed, as detected by S1 mapping experiments, and could encode a protein of 436 amino acids.

RNA splicing in Neurospora mitochondria. Characterization of new nuclear mutants with defects in splicing the mitochondrial large rRNA

In Neurospora, the gene encoding the mitochondrial large (25S) ribosomal RNA contains an intervening sequence of 2.3 kb. We have identified eight nuclear mutants that are defective in splicing the mitochondrial large ribosomal RNA and that accumulate unspliced precursor RNA. These mutants identify three different nuclear genes required for the same mitochondrial RNA splicing reaction. Some of the mutants have unique phenotypic characteristics (for example, accumulation of an unusual intron RNA) that may provide insight into specific aspects of mitochondrial RNA splicing. Mutations at one locus, cyt4, are subject to partial phenotypic suppression by the electron-transport inhibitor antimycin. This phenomenon suggests that at least one component required for mitochondrial RNA splicing is regulated such that its synthesis or activity is increased in response to impairment of electron transport.

Nature of an inserted sequence in the mitochondrial gene coding for the 15S ribosomal RNA of yeast

The small ribosomal RNA, or 15S RNA, or yeast mitochondria is coded by a mitochondrial gene. In the central part of the gene, there is a guanine-cytosine (GC) rich sequence of 40 base-pairs, flanked by adenine-thymine sequences. The GC-rich sequence is (5') TAGTTCCGGGGCCCGGCCACGGAGCCGAACCCGAAAGGAG (3'). We have found that this sequence is absent in the 15S rRNA gene of some strains of yeast. When present, it is transcribed into the mature 15S rRNA to produce a longer variant of the RNA. Sequences identical or closely related to this GC-rich sequence are present in many regions of the mitochondrial genome of Saccharomyces cerevisiae. The 5' and 3' terminal structures of all these sequences are highly constant.

Two distinct intervening sequences in different ribosomal DNA repeat units of Sciara coprophila

We have prepared a partial gene library of sheared DNA from the fungus fly, Sciara coprophila, by dA-T tailing and insertion into pBR322. Two ribosomal DNA clones which differ from the usual ribosomal DNA organization in this organism were studied in detail. Clone pBc 1L-1 has an intervening sequence of 1.4 kb, and clone pBc 6D-6 has an intervening sequence of 0.9 kb. These intervening sequences occur in about the same position in 28S rDNA, but do not appear to share sequence homology with one another. Previously we found that 90% of Sciara ribosomal DNA is homogenous and lacks an intervening sequence, and our present data explains the size heterogeneity found in most of the remaining 10%. We have found no evidence of size heterogeneity in the nontranscribed spacer.

Only one of two closely related yeast suppressor tRNA genes contains an intervening sequence

The yeast genes that code for the serine-inserting SUP-RL1 amber and SUQ5 ochre suppressors have been cloned and sequenced. These two unlinked genes differ by only three base pairs in their coding regions yet they encode tRNAs of different translational specificities, and while the SUP-RL1 gene has a 19-base pair intervening sequence, the SUQ5 gene has none.

Sequence arrangement of the rRNA genes of the dipteran Sarcophaga bullata

Velocity sedimentation studies of RNA of Sarcophaga bullata show that the major rRNA species have sedimentation values of 26S and 18S. Analysis of the rRNA under denaturing conditions indicates that there is a hidden break centrally located in the 26S rRNA species. Saturation hybridization studies using total genomic DNA and rRNA show that 0.08% of the nuclear DNA is occupied by rRNA coding sequences and that the average repetition frequency of these coding sequences is approximately 144. The arrangement of the rRNA genes and their spacer sequences on long strands of purified rDNA was determined by the examination of the structure of rRNa:DNA hybrids in the electron microscope. Long DNA strands contain several gene sets (18S + 26S) with one repeat unit containing the following sequences in order given: (a) An 18S gene of length 2.12 kb, (b) an internal transcribed spacer of length 2.01 kb, which contains a short sequence that may code for a 5.8S rRNA, (c) A 26S gene of length 4.06 kb which, in 20% of the cases, contains an intron with an average length of 5.62 kb, and (d) an external spacer of average length of 9.23 kb.

Fine structure of ribosomal RNA. IV. Extraordinary evolutionary conservation in sequences that flank introns in rDNA

By hybridization and DNA sequencing, we have defined a specific region in Xenopus rDNA that is extremely conserved between Tetrahymena, a protozoan, and Xenopus, a vertebrate. This highly conserved region is found at the site where an intron has been shown to interrupt Tetrahymena rDNA [1,2], although we have not detected introns in genomic or cloned Xenopus rDNA. We have noted that the sequences corresponding to nuclear rDNA interon-flanking regions show an intriguing complementarity to tRNAiMet. This suggests possible models for tRNA-rRNA interactions in protein synthesis and/or rRNA splicing.

Splice point sequence and transcripts of the intervening sequence in the mitochondrial 21S ribosomal RNA gene of yeast

By S1 nuclease mapping we have located the intervening sequence in the large ribosomal RNA gene of Saccharomyces cerevisiae omega+ strains 570 bp from the 3' end of the rRNA gene. No intervening sequence was detected at this position in S. carlsbergensis, but the sequences of the mature 21S rRNAs of these two strains appear to be identical in this region. By comparing the DNA sequence of the region of the intervening sequence in an omega+ strain with the corresponding sequence in S. carlsbergensis, we have determined the splice points of the 21S rRNA gene. These sequences show no homology with splice points in nuclear and viral genes or with the splice points in the chloroplast 23S rRNA gene of Chlamydomonas. The external borders of the splice points have a complementary sequence in the intervening sequence. The largest transcript hybridizing with the probe of the intervening sequence has a size corresponding to that expected for an rRNA precursor still containing the intervening sequence; the smallest transcript corresponds in size to the intervening sequence itself.

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.

Processing of precursors of 21S ribosomal RNA from yeast mitochondria

The transcription and processing of mitochondrial 21S rRNA in a petite strain of Saccharomyces cerevisiae has been examined by electron microscopic analysis of R-loop hybrids and by hybridization of labeled mitochondrial DNA probes to RNA transferred to diazobenzyloxymethyl paper. We have shown the presence of a large [5.1- to 5.4-kilobase (kb)] transcript that appears to be a precursor of mitochondrial 21S rRNA. This transcript contains sequences homologous to those of the mature 21S rRNA, to the intervening sequence present in the gene, and to additional sequences at the 3' end of the molecule. Our data suggest that this precursor of 21S rRNA is processed in two steps. The intron sequence is usually excised first, followed by removal of the extra 3' sequences. In some cases, however, the 3' extension is first removed and the intron sequence is then excised. Both pathways appear to lead to formation of the 3.1-kb mature 21S rRNA and a stable 1.2-kb intron transcript. Similar results were obtained with grande MH41-7B mitochondrial RNA by RNA transfer hybridization. We have also observed a number of additional transcripts that may be normal processing intermediates or may result from faulty cleavage-ligation during excision of the intervening sequence.

Genetic and physical characterization of a segment of yeast mitochondrial DNA involved in the control of genetic recombination

Genetic recombination between the 3 RIB (ribosomal) loci of yeast mitochondrial DNA is under the control of a mitochondrial locus named omega (with alleles omega+ and omega-) which is tightly linked to the RIBI locus. We have attempted to elucidate the molecular mechanisms(s) involved by using rho- mutants with similar (RIBI+ RIB2+ RIB3(0) genotype but different recombination properties in rho- x rho+ crosses. These were obtained through pedigree analysis and their mitochondrial DNAs were mapped on a high resolution physical map of the RIB section that had been built by analysis of thermal denaturation profiles and electron microscopy of partially denatured molecules. By comparison of physical and genetic data it can be shown that possession of the omega+ allele by the rho- cell is not sufficient for its expression in crosses, some additional DNA segments(s) in the ribosomal region being needed. This result and several features of the rho+ x rho- crosses are discussed in the light of current concepts in mitochondrial genetics of yeast and the recently discovered fact that omega+ and omega- strains differ by the presence of a 1000 base pairs insertion in the former.

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.

Pleiotropic mutations within two yeast mitochondrial cytochrome genes block mRNA processing

The mRNAs from two yeast mitochondrial genes cob-box (cytochrome b) and oxi-3 (cytochrome oxidase 40,000 dalton subunit) are processed from large (7-10 kb) precursors. Certain mutations in each gene block the maturation of the RNAs from both genes at a variety of specific steps. The pleiotropic cytochrome b mutants seem to lack a functional trans-acting RNA required for the processing of both messengers. In contrast, the oxi-3 mutants may act by producing an activity that inhibits specific steps.

Electron microscopic analysis of the yeast mitochondrial DNA segment conferring chloramphenicol resistance

Mitochondrial DNAs from six p- mutants carrying the genetic locus Rib1 and deleted for the rest of the genome were analyzed. Distribution of circular molecules from one mutant followed exactly the frequency rule, l/n, for multimers with discreet classes n, 2n, 3n, etc. Another, genetically unstable mutant displayed a continuous spectrum of circular molecules of various lengths. Four other mutants contained multiple series of circular molecules. Partial denaturation maps show that the mutants analyzed show a common segment ca. 1.0 micron long and differ by characteristic deletions of extremites of this segment. Short terminal deletions of the right i.e. pointing towards the Rib3 locus, terminus of this segment are correlated with modifications of the recombination properties related to the omega locus.