A nuclear mutation affecting mitochondrial transcription in Saccharomyces cerevisiae

A point mutation in the core subunit gene of yeast mitochondrial RNA polymerase is suppressed by a high level of specificity factor MTF1

The temperature-sensitive yeast mutant pet-ts798 is characterized by an altered mitochondrial transcription apparatus. The mutation has previously been shown to map in the RPO41 gene encoding the core enzyme of mitochondrial RNA polymerase. In the present study the rpo41/pet-ts798 allele was cloned and sequenced, demonstrating that the mutant phenotype is caused by a single amino acid change in a conserved region of the core polymerase. The nuclear gene MTF1, previously isolated as a high copy suppressor of mutant rpo41/pet-ts798, and its gene product were characterized in more detail. Import of a MTF1-COXIV fusion protein in vivo and also import studies with in vitro synthesized MTF1 precursors indicate that MTF1 is a mitochondrial protein and that no apparent cleavage occurs during its import into mitochondria. DNA-binding assays demonstrate that the MTF1 protein alone interacts with DNA in a non-specific manner. An antibody directed against specificity factor MTF1 was raised and used for immunological quantification experiments. The results indicate that suppression is mediated by an increased level of MTF1 protein in mitochondria of the rpo41/pet-ts798 mutant. Possible implications of this finding for the mechanism of suppression are discussed.

Purification and characterization of ribonucleoproteins from pea chloroplasts

RNA-binding proteins are known to mediate the post-transcriptional regulation of genes in many organisms. Recently they have been found to be important in the expression of plastid genes. We have purified a group of three single-stranded nucleic-acid-specific acidic proteins (33, 30 and 28 kDa) from chloroplast extracts of pea (Pisum sativum L.), using single-stranded DNA affinity chromatography. All of them have acidic amino termini but the amino acid sequences are unique to each polypeptide, with partial similarities to the recently reported ribonucleoproteins from tobacco chloroplasts. The pea proteins are also antigenically distinct, as shown by Western blot analysis using polyclonal antisera for purified proteins. Further, from their large nucleic-acid-binding domains and the polynucleotide substrate affinities, they are predicted to belong to a family of pea plastid ribonucleoproteins. In vivo radiolabeling of proteins in the presence of translational inhibitors as well as in vitro translation of leaf tissue RNA suggest that these proteins are encoded in the nucleus. Antibody cross-reactivity experiments reveal that their genes are conserved during plastid evolution.

An yeast nuclear mutation conferring temperature-sensitivity to the mitochondrial tryptophanyl-tRNA synthetase

The conditional respiratory-deficient Saccharomyces cerevisiae mutant pet-ts2281 was complemented by an yeast genomic DNA library. The gene thus isolated was sequenced and proved to be identical to the known MSW1 sequence encoding mitochondrial tryptophanyl-tRNA synthetase (Myers and Tzagoloff 1985). Compared to the wild-type, the ts2281 mutant allele of MSW1 contained a single T----C transition leading to a Leu----Ser replacement at position 294 of the protein sequence. In addition to this mutational alteration, our sequence data for the wild-type gene differ from the originally published MSW1 sequence at five other DNA positions which affect two locally restricted regions of the polypeptide chain. As expected, at the non-permissive temperature ts2281 cells are specifically defective in mitochondrial trp-tRNA formation and, thus, in overall mitochondrial protein synthesis. In addition, the patterns of cytochrome b mRNA maturation intermediates were distinctly different in ts2281 and wild-type yeast cells. The mutational effect of the observed amino-acid substitution in ts2281 is discussed in terms of weakened hydrogen bonding in the C-terminal half of the MSW1-encoded protein.

Dual function of a new nuclear gene for oxidative phosphorylation and vegetative growth in yeast

A new gene essential for cell viability and indispensable for the biogenesis of a functional respiratory chain in Saccharomyces cerevisiae was isolated by complementing a temperature-sensitive mutant. This conditional nuclear mutation selectively affects oxidative phosphorylation at restrictive temperatures. At the molecular level a severe and complex defect inside mitochondria is observed, with drastically reduced levels of mitochondrial transcripts. Surprisingly a null mutation in this nuclear gene in a haploid yeast strain leads to cell death. Spores containing a disrupted copy of the gene exhibit a severe growth defect and cell division stops irreversibly after 3 to 4 days. It is shown that the null and conditional mutants are indeed allelic. This finding demonstrates a dual function of the gene product in oxidative phosphorylation and vegetative growth. The putative protein product, as deduced from the sequence of the relevant reading frame is characterized by a low molecular weight of approximately 14 kDa, a high content of charged amino acids and a very low codon bias index. A transcript of low abundance and with a length of about 600 nucleotides can be assigned to this gene.

Molecular analysis of the mitochondrial transcription factor mtf2 of Saccharomyces cerevisiae

The nuclear gene for a new mitochondrial transcription factor (mtf2) was isolated by transformation of mutant pet-ts3504. It was localized on a 6.4 kb fragment of yeast genomic DNA by subcloning and complementation tests. Sequencing of a 1.7 kb DNA fragment revealed an open reading frame of 1320 bp. A transcript of 1400 nucleotides can be assigned to this region. Gene disruption of this reading frame in a wild-type yeast strain created a stable pet phenotype. Further analysis of this insertion mutation showed that it is allelic to the mutated gene of pet-ts3504. Comparison of the 5' upstream regions of MTF2 and a previously characterized mitochondrial transcription factor (MTF1) revealed common sequence motifs which may be important for coordinated regulation of gene expression.

Mutations in the genes for mitochondrial RNA polymerase and a second mitochondrial transcription factor of Saccharomyces cerevisiae

In our previous work (Lisowsky et al. 1987; Lisowsky and Michaelis 1988) we have identified two nuclear pet genes of yeast that are required for mitochondrial transcription. In this report we show that one of these pet mutations, pet-ts798, maps in the RP041 gene encoding mitochondrial RNA polymerase. The temperature-sensitive lesion of mutant pet-ts798 can be suppressed by a second nuclear gene RF1023 (mtf1) when inserted into a high copy number plasmid. Our assumption that mtf1 codes for a 40 kDa mitochondrial transciription factor is supported by the fact that the cloned gene acts as an intergenic suppressor of a temperature-sensitive RNA polymerase mutant. A third nuclear gene (mtf2) for mitochondrial transcription was identified by analysing mutant pet-ts3504. The in vitro transcriptional activity of isolated mutant mitochondria is temperature sensitive suggesting the presence of an altered component of transcription inside mitochondria. The defect was confirmed by studies with a transcriptionally active DNA-protein complex and by testing the DNA-binding ability of mitochondrial proteins.

A nuclear gene essential for mitochondrial replication suppresses a defect of mitochondrial transcription in Saccharomyces cerevisiae

A genomic DNA fragment from yeast was isolated by transforming a temperature sensitive pet mutant. This mutant, pet-ts 798, has previously been characterized by its altered mitochondrial transcription apparatus. Subcloning and DNA sequencing of the genomic DNA fragment identified a reading frame responsible for the restoration of the pet-ts phenotype. The reading frame of 1023 bp is transcribed as an RNA of about 1100 nucleotides. The putative protein of 40 kDa possesses a hydrophobic amino-terminus and acidic and basic domains characteristic of recently described transcriptional activators. The inactivation of the functional gene by the introduction of an insertion fragment into the reading frame, leads to a stable pet phenotype. Further analysis of this mutant created by gene disruption makes clear that the respiratory defect is caused by the complete loss of mitochondrial DNA. Experimental evidence is given that the cloned gene acts as an intergenic suppressor of the mutant pet-ts 798. Therefore, the isolated gene represents a new factor involved in the regulation of mitochondrial replication and transcription.

Dwarfism and male sterility in interspecific hybrids of Epilobium : 2. Expression of mitochondrial genes and structure of the mitochondrial DNA

Mitochondrial DNA (mtDNA) and transcriptional patterns of mitochondrial genes have been examined in dwarf, normal, fertile and male sterile Epilobium hybrids. No alterations or rearrangements of mitochondrial DNA could be detected in the developmentally disturbed hybrids. They exhibit restriction patterns of mtDNA that correspond exactly to those of their female parents. However, the transcription of at least one mitochondrial gene is significantly altered in the male sterile hybrid E. hirsutum x montanum. In normal plants, one mRNA of 1.6 kb hybridizes to the cytochrome c oxidase subunit II gene, while in male sterile plants a transcript of this size is lacking and instead a major transcript of 2.0 kb and two smaller ones occur. The transcript pattern of the F1 ATPase alpha subunit (atpA) gene exhibits slight alterations in sterile plants also. Since these hybrids have the same cytoplasm as normal plants, an incompatibility between the nuclear and the mitochondrial genotype may be responsible for the altered mitochondrial gene expression. No alteration of the transcripts of the mitochondrial genes tested could be detected in dwarf hybrids. The coincidence of male sterility with an altered transcription pattern of mitochondrial genes suggests that the mitochondria are involved in the occurrence of this phenotype.