Genetic mapping of mitochondrial markers by recombinational analysis in Chlamydomonas reinhardtii

The mitochondrial genome of Chlamydomonas reinhardtii is a 15.8 kb linear DNA molecule present in multiple copies. In crosses, the meiotic products only inherit the mitochondrial genome of the mating type minus (paternal) parent. In contrast mitotic zygotes transmit maternal and paternal mitochondrial DNA copies to their diploid progeny and recombinational events between molecules of both origins frequently occur. Six mitochondrial mutants unable to grow in the dark (dk^– mutants) were crossed in various combinations and the percentages of wild-type dk⁺ recombinants were determined in mitotic zygotes when all progeny cells had become homoplasmic for the mitochondrial genome. In crosses between strains mutated in the COB (apocytochrome ) gene and strains mutated in the COX1 (subunit 1 of cytochrome oxidase) gene, the frequency of recombination was 13.7% (± 3.2%). The corresponding physical distance between the mutation sites was 4.3 kb. In crosses between strains carrying mutations separated by about 20 bp, a recombinational frequency of 0.04% (± 0.02%) was found. Two other mutants not yet characterized at the molecular level were also used for recombinational studies. From these data, a linear genetic map of the mitochondrial genome could be drawn. This map is consistent with the positions of the mutation sites on the mitochondrial DNA molecule and thereby validates the method used to generate the map. The frequency of recombination per physical distance unit (3.2% ± 0.7% per kilobase) is compared with those obtained for other organellar genomes in yeasts and Chlamydomonas.     

Mutations affecting the mitochondrial genes encoding the cytochrome oxidase subunit I and apocytochrome b of Chlamydomonas reinhardtii

Mitochondrial mutants of the green alga Chlamydomonas reinhardtii that are inactivated in the cytochrome pathway of respiration have previously been isolated. Despite the fact that the alternative oxidase pathway is still active the mutants have lost the capacity to grow heterotrophically (dark + acetate) and display reduced growth under mixotrophic conditions (light + acetate). In crosses between wild-type and mutant cells, the meiotic progeny only inherit the character transmitted by the mt ^– parent, which indicates that the mutations are located in the 15.8 kb linear mitochondrial genome. Two new mutants (dum-18 and dum-19) have now been isolated and characterized genetically, biochemically and at the molecular level. In addition, two previously isolated mutants (dum-11 and dum-15) were characterized in more detail. dum-11 contains two types of deleted mitochondrial DNA molecules: 15.1 kb monomers lacking the subterminal part of the genome, downstream of codon 147 of the apocytochrome b (COB) gene, and dimers resulting from head-to-head fusion of asymmetrically deleted monomers (15.1 and 9.5 kb DNA molecules, respectively). As in the wild type, the three other mutants contain only 15.8 kb mitochondrial DNA molecules. dum-15 is mutated at codon 140 of the COB gene, a serine (TCT) being changed into a tyrosine (TAC). dum-18 and dum-19 both inactivate cytochrome c oxidase, as a result of frameshift mutations (addition or deletion of 1 bp) at codons 145 and 152, respectively, of the COX1 gene encoding subunit I of cytochrome c oxidase. In a total of ten respiratory deficient mitochondrial mutants characterized thus far, only mutations located in COB or COXI have been isolated. The possibility that the inactivation of the other mitochondrial genes is lethal for the cells is discussed.     

The apocytochrome b gene of Chlamydomonas smithii contains a mobile intron related to both Saccharomyces and Neurospora introns.

Summary The mitochondrial DNA of the two interfertile algal species Chlamydomonas smithii and Chlamydomonas reinhardtii are co-linear with the exception of ca. 1 kb insertion (the a insert) present in C. smithii DNA only. In vegetative diploids resulting from interspecific crosses, mitochondrial genomes are transmitted biparentally except for the a insert which is transmitted to all C. reinhardtii molecules in a manner reminiscent of the intron-mediated conversion event that occurs at the omega locus in yeast mitochondria, under the action of the I-SceI endonuclease. Here we report that the insert corresponds to a typical group I intron of 1075 bp, inserted within the gene for apocytochrome b and containing a 237 codon open reading frame (ORF). We also report the complete sequence of the apocytochrome b gene of C. smithii. Comparison with the sequence of the same gene in C. reinhardtii reveals the precise intron insertion site. These data, together with the previous genetic data provide the first example of intron mobility in mitochondria of the plant kingdom. The product of the intronic ORF shows 36% amino acid identity with the I-SceI endonuclease whereas the intron ribozyme shows a 60% identity at the nucleotide level with the Neurospora crassa cob · 1 intron. The possibility of a recent horizontal transfer of introns between fungi and algae is discussed.     

Shared molecular characteristics of successfully transformed mitochondrial genomes in <Emphasis Type="Italic">Chlamydomonas</Emphasis><Emphasis Type="Italic">reinhardtii</Emphasis>

Three types of respiratory deficient mitochondrial strains have been reported in Chlamydomonas reinhardtii: a deficiency due to (i) two base substitutions causing an amino acid change in the apocytochrome b (COB) gene (i.e., strain named dum-15), (ii) one base deletion in the COXI gene (dum-19), or (iii) a large deletion extending from the left terminus of the genome to somewhere in the COB gene (dum-1, -14, and -16). We found that these respiratory deficient strains of C. reinhardtii can be divided into two groups: strains that are constantly transformable and those could not be transformed in our experiments. All transformable mitochondrial strains were limited to the type that has a large deletion in the left arm of the genome. For these mitochondria, transformation was successful not only with purified intact mitochondrial genomes but also with DNA-constructs containing the compensating regions. In comparison, mitochondria of all the non-transformable strains have both of their genome termini intact, leading us to speculate that mitochondria lacking their left genome terminus have unstable genomes and might have a higher potential for recombination. Analysis of mitochondrial gene organization in the resulting respiratory active transformants was performed by DNA sequencing and restriction enzyme digestion. Such analysis showed that homologous recombination occurred at various regions between the mitochondrial genome and the artificial DNA-constructs. Further analysis by Southern hybridization showed that the wild-type genome rapidly replaces the respiratory deficient monomer and dimer mitochondrial genomes, while the E. coli vector region of the artificial DNA-construct likely does not remain in the mitochondria.     

Binding of calmodulin to Nuf1p is required for karyogamy in<Emphasis Type="Italic"> Saccharomyces cerevisiae</Emphasis>

The role of calmodulin (CaM) during mating in Saccharomyces cerevisiae was examined by using a set of Phe-to-Ala substitutions. We identified ten CaM mutants that exhibited significantly reduced mating efficiencies when crossed to a strain of the opposite mating type harboring the same CaM mutation. Most of the mating-defective CaM mutants were bilateral, i.e., they also exhibited mating defects, albeit minor ones, when crossed to the wild type. When strains carrying different bilateral CaM mutations were mated, the mating efficiencies recovered dramatically. We termed this phenomenon "intragenic mating complementation", and classified the mating-defective CaM mutations into two intragenic mating complementation groups. Two mutant alleles belonging to different groups showed minor defects in cell adhesion and cell fusion, but exhibited severe defects in karyogamy. CaM is known to bind to the essential spindle pole body component Nuf1p. This binding appears to be important for karyogamy because the nuf1 ^C911R mutation, which impairs CaM-Nuf1p binding, resulted in a severe defect in karyogamy. Indeed, the two mating-defective CaM mutations were found to compromise formation of the CaM/Nuf1p complex, and the mating defects of these two CaM mutants were suppressible by a dominant, CaM-independent, mutation in NUF1. Taken together, these results suggest that loss of CaM binding to Nuf1p causes a defect in karyogamy, thereby inhibiting productive mating.Communicated by C. P. Hollenberg     

Forward and Reverse Genetic Analysis of Microtubule Motors in Chlamydomonas

The ability to integrate biochemical, cell biological, and genetic approaches makes Chlamydomonas reinhardtii the premier model organism for studies of the eukaryotic flagellum and its associated molecular motors. Hundreds of motility mutations have been identified in Chlamydomonas, including many that affect dyneins and kinesins. These mutations have yielded much information on the structure and function of the motors as well as the roles of individual subunits within the motors. The development of insertional mutagenesis has opened the door to powerful new approaches for genetic analysis in Chlamydomonas. Insertional mutants are created by transforming cells with DNA-containing selectable markers. The DNA is randomly integrated throughout the genome and usually deletes part of the chromosome at the site of insertion, thereby creating mutations that are marked by the integrated DNA. These mutations can be used for forward genetic approaches where one characterizes a mutant phenotype and then clones the relevant gene using the integrated DNA as a tag. The insertional mutants also may be used in a reverse genetic approach in which mutants lacking a gene of interest are identified by DNA hybridization. We describe methods to generate and characterize insertional mutants, using mutations that affect the outer dynein arm as examples.     

Genetic mapping of mitochondrial markers by recombinational analysis in Chlamydomonas reinhardtii

The mitochondrial genome of Chlamydomonas reinhardtii is a 15.8 kb linear DNA molecule present in multiple copies. In crosses, the meiotic products only inherit the mitochondrial genome of the mating type minus (paternal) parent. In contrast mitotic zygotes transmit maternal and paternal mitochondrial DNA copies to their diploid progeny and recombinational events between molecules of both origins frequently occur. Six mitochondrial mutants unable to grow in the dark (dk^? mutants) were crossed in various combinations and the percentages of wild-type dk⁺ recombinants were determined in mitotic zygotes when all progeny cells had become homoplasmic for the mitochondrial genome. In crosses between strains mutated in the COB (apocytochrome ) gene and strains mutated in the COX1 (subunit 1 of cytochrome oxidase) gene, the frequency of recombination was 13.7% (± 3.2%). The corresponding physical distance between the mutation sites was 4.3 kb. In crosses between strains carrying mutations separated by about 20 bp, a recombinational frequency of 0.04% (± 0.02%) was found. Two other mutants not yet characterized at the molecular level were also used for recombinational studies. From these data, a linear genetic map of the mitochondrial genome could be drawn. This map is consistent with the positions of the mutation sites on the mitochondrial DNA molecule and thereby validates the method used to generate the map. The frequency of recombination per physical distance unit (3.2% ± 0.7% per kilobase) is compared with those obtained for other organellar genomes in yeasts and Chlamydomonas.     

Biochemical,genetic and molecular characterization of new respiratory-deficient mutants inChlamydomonas reinhardtii

Eight respiratory-deficient mutants ofChlamydomonas reinhardtii have been isolated after mutagenic treatment with acriflavine or ethidium bromide. They are characterized by their inability to grow or their very reduced growth under heterotrophic conditions. One mutation (Class III) is of nuclear origin whereas the seven remaining mutants (Classes I and II) display a predominantly paternalmt ^- inheritance, typical of mutations residing in the mitochondrial DNA. Biochemical analysis has shown that all mutants are deficient in the cyanide-sensitive cytochrome pathway of the respiration whereas the alternative pathway is still functional. Measurements of complexes II + III (antimycin-sensitive succinate-cytochromec oxido-reductase) and complex IV (cytochromec oxidase) activities allowed to conclude that six mutations have to be localized in the mitochondrial apocytochromeb (COB) gene, one in the mitochondrial cytochrome oxidase subunit I (COI) gene and one in a nuclear gene encoding a component of the cytochrome oxidase complex. By using specific probes, we have moreover demonstrated that five mutants (Class II mutants) contain mitochondrial DNA molecules deleted in the terminal end containing the COB gene and the telomeric region; they also possess dimeric molecules resulting from end-to-end junctions of deleted monomers. The two other mitochondrial mutants (Class I) have no detectable gross alteration. Class I and Class II mutants can also be distinguished by the pattern of transmission of the mutation in crosses.Anin vivo staining test has been developed to identify rapidly the mutants impaired in cyanide-sensitive respiration.     

Toxicity of and mutagenesis by chlorate are independent of nitrate reductase activity in Chlamydomonas reinhardtii

Summary Spontaneous chlorate-resistant (CR) mutants have been isolated from Chlamydomonas reinhardtii wildtype strains. Most of them, 244, were able to grow on nitrate minimal medium, but 23 were not. Genetic and in vivo complementation analyses of this latter group of mutants indicated that they were defective either at the regulatory locus nit-2, or at the nitrate reductase (NR) locus nit-1, or at very closely linked loci. Some of these nit-1 or nit-2 mutants were also defective in pathways not directly related to nitrate assimilation, such as those of amino acids and purines. Chlorate treatment of wild-type cells resulted in both a decrease in cell survival and an increase in mutant cells resistant to a number of different chemicals (chlorate, methylammonium, sulphanilamide, arsenate, and streptomycin). The toxic and mutagenic effects of chlorate in minimal medium were not found when cells were grown either in darkness or in the presence of ammonium, conditions under which nitrate uptake is drastically inhibited. Chlorate was also able to induce reversion of nit ^– mutants of C. reinhardtii, but failed to produce His ⁺ revertants or Ara^r mutants in the BA-13 strain of Salmonella typhimurium. In contrast, chlorate treatment induced mutagenesis in strain E1F1 of the phototrophic bacterium Rhodobacter capsulatus. Genetic analyses of nitrate reductase-deficient CR mutants of C. reinhardtii revealed two types of CR, to low (1.5 mM) and high (15 mM) chlorate concentrations. These two traits were recessive in heterozygous diploids and segregated in genetic crosses independently of each other and of the nit-1 and nit-2 loci. Three her loci and four lcr loci mediating resistance to high (HC) and low (LC) concentrations of chlorate were identified. Mutations at the nit-2 locus, and deletions of a putative locus for nitrate transport were always epistatic to mutations responsible for resistance to either LC or HC. In both nit ⁺ and nit ^– chlorate-sensitive (CS) strains, nitrate and nitrite gave protection from the toxic effect of chlorate. Our data indicate that in C. reinhardtii chlorate toxicity is primarily dependent on the nitrate transport system and independent of the existence of an active NR enzyme. At least seven loci unrelated to the nitrate assimilation pathway and mediating CR are thought to control indirectly the efficiency of the nitrate transporter for chlorate transport. In addition, chlorate appears to be a mutagen capable of inducing a wide range of mutations unrelated to the nitrate assimilation pathway.     

Recombination and mating-type switching in a ligase-defective mutant of Schizosaccharomyces pombe

Summary The ligase-defective cdc17-L16 mutant of Schizosaccharomyces pombe var. pombe was tested for genetic recombination and mating-type switching. Mitotic recombination was studied in both haploid and heteroallelic diploid cells. Cells carrying a heteroallelic ade6 duplication constructed by Schuchert and Kohli were tested for ectopic genetic recombination. We have found that cdc17-L16 is a mitotic hyper-rec mutant, as it increases the instability of the duplication by a factor of about 6 even at the permissive temperature of 23° C. In diploid cells, the enhancement of recombination rates detected was to that of cdc17 ⁺ cells. The temperature-sensitive cell cycle defect is also associated with a reduced level of mating and sporulation but does not significantly affect mating-type switching and intragenic meiotic recombination. It is supposed that the mitotic hyper-rec phenotype is a secondary result of insufficient repair of DNA breaks, while the lack of influence of the reduced ligase activity on the latter two processes might be attributed to their peculiar initiation mechanisms.     

Occurrence of genetic segregation in a putative haploid strain of Endomyces fibuliger met by spontaneous sectoring of protoplast fusants

Detailed genetic analysis of Endomyces fibuliger, an amylolytic yeast which is homothallic and exists predominantly in the diploid state, has not been performed. From a naturally occurring strain, E. fibuliger 8014 met, a morphological mutant, 193 met, was obtained by u.v. mutagenesis. To obtain a haploid strain suitable for genetic analysis, an intergeneric hybrid between E. fibuliger 193 met and a strain of a closely related dimorphic heterothallic lipolytic yeast, Yarrowia lipolytica, A his1, was produced by mass mating. The intergeneric hybrid was highly unstable in vegetative culture on yeast extract/phosphate/soluble starch/agar media and produced numerous mitotic sectors. Most of the sectors were mitotically unstable. However, one mitotically stable sector, N14i60 met, was obtained which also differed from the strain 193 as gauged by the appearance of DNA bands on pulsed-field gel electrophoresis. The putative haploid strain, N14i60 met, had six bands whilst the mutant 193 met had seven. Ultra-violet treatment of cells of N14i60 met produced 19 auxotrophic mutants. Protoplast fusion between pairs of different mutants showed complementation and the fusants were unstable mitotically and gave unstable aneuploid and stable haploid sectors of parental and non-parental combinations of markers. It is postulated that complementary diploid fusants, which were obtained by protoplast fusion, produced sectors by mitotic non-disjunction. Such a mechanism provides a means to establish a genetic analysis system for E. fibuliger via the parasexual cycle.B.H. Nga, L.L. Chiu, S.I. Koh and C.W. Yip are with the Department of Microbiology, National University of Singapore, Lower Kent Ridge Road, Singapore 0511. S. Harashima and Y. Oshima are with the Department of Biotechnology, Faculty of Engineering, Osaka University, Yamada-kami, Suita shi, Osaka 565, Japan.     

Temperature-sensitive mutants of hsp82 of the budding yeast Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has two HSP90-related genes per haploid genome, HSP82 and HSC82. Random mutations were induced in vitro in the HSP82 gene by treatment of the plasmid with hydroxylamine. Four temperature-sensitive (ts) mutants and one simultaneously is and cold-sensitivie (cs) mutant were then selected in a yeast strain in which HSC82 had previously been disrupted. The mutants were found to have single base changes in the coding region, which caused single amino acid substitutions in the HSP82 protein. All of these mutations occurred in amino acid residues that are well conserved among HSP90-related proteins of various species from Escherichia coli to human. Various properties including cell morphology, macromolecular syntheses and thermosensitivity were examined in each mutant at both the permissive and nonpermissive temperatures. The mutations in HSP82 caused pleiotropic effects on these properties although the phenotypes exhibited at the nonpermissive temperature varied among the mutants.     

Cloning and characterization of the Chlamydomonas moewusii mitochondrial genome.

Summary We report that the mitochondrial genome of Chlamydomonas moewusii has a 22 kb circular map and thus contrasts with the mitochondrial genome of Chlamydomonas reinhardtii, which is linear and about 6 kb shorter. Overlapping restriction fragments spanning over 90% of the C. moewusii mitochondrial DNA (mtDNA) were identified in a clone bank constructed using a Sau3AI partial digest of a C. moewusii DNA fraction enriched for mtDNA by preparative CsCI density gradient centrifugation. Overlapping Sau3AI clones were identified by a chromosome walk initiated with a clone of C. moewusii mtDNA. The mtDNA map was completed by Southern blot analysis of the C. moewusii mtDNA fraction using isolated mtDNA clones. Regions that hybridized to C. reinhardtii or wheat mitochondrial gene probes for subunit I of cytochrome oxidase (cox1), apocytochrome b (cob), three subunits of NADH dehydrogenase (nadl, nad2 and nad5) and the small and the large ribosomal RNAs (rrnS and rrnL, respectively) were localized on the C. moewusii mtDNA map by Southern blot analysis. The results show that the order of genes in the mitochondrial genome of C. moewusii is completely rearranged relative to that of C. reinhardtii.     

Chloroplast DNA base substitutions: an experimental assessment

An experimental assessment was carried out to determine directly the frequency and types of spontaneous base substitutions that occur in chloroplast DNA. A target site within the chloroplast 16S rRNA gene of the green alga Chlamydomonas reinhardtii was chosen for the assay. Mutations at this site were known to confer spectinomycin resistance and simultaneously result in the loss of an AatII cleavage site. In the experiments reported here, base substitutions at any individual base occurred at a frequency in the range of 0.9–11 per 10⁹ viable cells plated. Four new mutations that confer resistance to spectinomycin were identified at the target site in the Chlamydomonas chloroplast 16S rRNA gene. When the relative rates of transition and transversion mutations were quantified, a bias toward transversions was observed. The prominence of A/T C/G transversions in the observed mutation spectrum suggests that oxidative damage may be the major cause of base substitution mutations within the chloroplast.     

Mutations affecting the mitochondrial genes encoding the cytochrome oxidase subunit I and apocytochrome b of Chlamydomonas reinhardtii

Mitochondrial mutants of the green alga Chlamydomonas reinhardtii that are inactivated in the cytochrome pathway of respiration have previously been isolated. Despite the fact that the alternative oxidase pathway is still active the mutants have lost the capacity to grow heterotrophically (dark + acetate) and display reduced growth under mixotrophic conditions (light + acetate). In crosses between wild-type and mutant cells, the meiotic progeny only inherit the character transmitted by the mt ^? parent, which indicates that the mutations are located in the 15.8 kb linear mitochondrial genome. Two new mutants (dum-18 and dum-19) have now been isolated and characterized genetically, biochemically and at the molecular level. In addition, two previously isolated mutants (dum-11 and dum-15) were characterized in more detail. dum-11 contains two types of deleted mitochondrial DNA molecules: 15.1 kb monomers lacking the subterminal part of the genome, downstream of codon 147 of the apocytochrome b (COB) gene, and dimers resulting from head-to-head fusion of asymmetrically deleted monomers (15.1 and 9.5 kb DNA molecules, respectively). As in the wild type, the three other mutants contain only 15.8 kb mitochondrial DNA molecules. dum-15 is mutated at codon 140 of the COB gene, a serine (TCT) being changed into a tyrosine (TAC). dum-18 and dum-19 both inactivate cytochrome c oxidase, as a result of frameshift mutations (addition or deletion of 1 bp) at codons 145 and 152, respectively, of the COX1 gene encoding subunit I of cytochrome c oxidase. In a total of ten respiratory deficient mitochondrial mutants characterized thus far, only mutations located in COB or COXI have been isolated. The possibility that the inactivation of the other mitochondrial genes is lethal for the cells is discussed.     

Nuclear genes required for post-translational steps in the biogenesis of the chloroplast cytochrome b6f complex in maize

Nuclear genes essential for the biogenesis of the chloroplast cytochrome b ₆ f complex were identified by mutations that cause the specific loss of the complex. We describe four transposon-induced maize mutants that lack cytochrome b ₆ f proteins but contain normal levels of other photosynthetic complexes. The four mutations define two nuclear genes. To identify the step at which each mutation blocks protein accumulation, mRNAs encoding each subunit were examined by Northern hybridization analysis and the rates of subunit synthesis were examined in pulse-labeling experiments. In each mutant the mRNAs encoding the known subunits of the complex were normal in size and abundance and the major subunits were synthesized at normal rates. Thus, these mutations block the biogenesis of the cytochrome b ₆ f complex at a post-translational step. The two nuclear genes identified by these mutations may encode previously unknown subunits, be involved in prosthetic group synthesis or attachment, or facilitate assembly of the complex. These mutations were also used to provide evidence for the authenticity of a proposed fifth subunit of the complex and to demonstrate a role for the cytochrome b ₆ f complex in protecting photosystem 11 from light-induced degradation.     

The paradox of viable <Emphasis Type="Italic">sup45</Emphasis> STOP mutations: a necessary equilibrium between translational readthrough,activity and stability of the protein

The mechanisms leading to non-lethality of nonsense mutations in essential genes are poorly understood. Here, we focus on the factors influencing viability of yeast cells bearing premature termination codons (PTCs) in the essential gene SUP45 encoding translation termination factor eRF1. Using a dual reporter system we compared readthrough efficiency of the natural termination codon of SUP45 gene, spontaneous sup45-n (nonsense) mutations, nonsense mutations obtained by site-directed mutagenesis (76Q → TAA, 242R → TGA, 317L → TAG). The nonsense mutations in SUP45 gene were shown to be situated in moderate contexts for readthrough efficiency. We showed that readthrough efficiency of some of the mutations present in the sup45 mutants is not correlated with full-length Sup45 protein amount. This resulted from modification of both sup45 mRNA stability which varies 3-fold among sup45-n mutants and degradation rate of mutant Sup45 proteins. Our results demonstrate that some substitutions in the place of PTCs decrease Sup45 stability. The viability of sup45 nonsense mutants is therefore supported by diverse mechanisms that control the final amount of functional Sup45 in cells.     

Further characterization of the respiratory deficient dum-1 mutation of Chlamydomonas reinhardtii and its use as a recipient for mitochondrial transformation

Summary The respiratory deficient dum-1 mutant of Chlamydomonas reinhardtii fails to grow in the dark because of a terminal 1.5 kb deletion in the linear 15.8 kb mitochondrial genome, which affects the apocytochrome b (CYB) gene. In contrast to the wild type where only mitochondrial genomes of monomer length are observed, the dum-1 genomes are present as a mixture of monomer and dimer length molecules. The mutant dimers appear to result from head-to-head fusions of two deleted molecules. Furthermore, mitochondrial genomes of dum-1 were also found to be unstable, with the extent of the deletion varying among single cell clones from the original mutant population. The dum-1 mutant also segregates, at a frequency of ca. 4% per generation, lethal minute colonies in which the original deletion now extends at least into the adjacent gene encoding subunit four of NAD dehydrogenase (ND4). We have used the dum-1 mutant as a recipient to demonstrate stable mitochondrial transformation in C. reinhardtii employing the biolistic method. After 4 to 8 weeks dark incubation, a total of 22 respiratory competent colonies were isolated from plates of dum-1 cells bombarded with C. reinhardtii mitochondrial DNA (frequency 7.3 × 10^–7) and a single colony was isolated from plates bombarded with C. smithii mitochondrial DNA (frequency 0.8 × 10^–7). No colonies were seen on control plates (frequency < 0.96 × 10^–9). All transformants grew normally in the dark on acetate media; 22 transformants were homoplasmic for the wild-type mitochondrial genome typical of the C. reinhardtii donor. The single transformant obtained from the C. smithii donor had a recombinant mitochondrial genome containing the donor CYB gene and the diagnostic HpaI and XbaI restriction sites in the gene encoding subunit I of cytochrome oxidase (COI) from the C. reinhardtii recipient. The characteristic deletion fragments of the dum-1 recipient were not detected in any of the transformants.     

Introduction of cytochrome b mutations in Saccharomyces cerevisiae by a method that allows selection for both functional and non-functional cytochrome b proteins

We have previously used inhibitors interacting with the Qn site of the yeast cytochrome bc₁ complex to obtain yeast strains with resistance-conferring mutations in cytochrome b as a means to investigate the effects of amino acid substitutions on Qn site enzymatic activity [M.G. Ding, J.-P. di Rago, B.L. Trumpower, Investigating the Qn site of the cytochrome bc1 complex in Saccharomyces cerevisiae with mutants resistant to ilicicolin H, a novel Qn site inhibitor, J. Biol. Chem. 281 (2006) 36036-36043.]. Although the screening produced various interesting cytochrome b mutations, it depends on the availability of inhibitors and can only reveal a very limited number of mutations. Furthermore, mutations leading to a respiratory deficient phenotype remain undetected. We therefore devised an approach where any type of mutation can be efficiently introduced in the cytochrome b gene. In this method ARG8, a gene that is normally encoded by nuclear DNA, replaces the naturally occurring mitochondrial cytochrome b gene, resulting in ARG8 expressed from the mitochondrial genome (ARG8^m). Subsequently replacing ARG8^m with mutated versions of cytochrome b results in arginine auxotrophy. Respiratory competent cytochrome b mutants can be selected directly by virtue of their ability to restore growth on non-fermentable substrates. If the mutated cytochrome b is non-functional, the presence of the COX2 respiratory gene marker on the mitochondrial transforming plasmid enables screening for cytochrome b mutants with a stringent respiratory deficiency (mit⁻). With this system, we created eight different yeast strains containing point mutations at three different codons in cytochrome b affecting center N. In addition, we created three point mutations affecting arginine 79 in center P. This is the first time mutations have been created for three of the loci presented here, and nine of the resulting mutants have never been described before.