Mutational studies of the major tRNA region of the S. cerevisiae mitochondrial genome

Strain-specific nuclear genetic background differentially affects mitochondria-related phenotypes in Saccharomyces cerevisiae

In the course of our studies on mitochondrial defects, we have observed important phenotypic variations in Saccharomyces cerevisiae strains suggesting that a better characterization of the genetic variability will be essential to define the relationship between the mitochondrial efficiency and the presence of different nuclear backgrounds. In this manuscript, we have extended the study of such relations by comparing phenotypic assays related to mitochondrial functions of three wild-type laboratory strains. In addition to the phenotypic variability among the wild-type strains, important differences have been observed among strains bearing identical mitochondrial tRNA mutations that could be related only to the different nuclear background of the cells. Results showed that strains exhibited an intrinsic variability in the severity of the effects of the mitochondrial mutations and that specific strains might be used preferentially to evaluate the phenotypic effect of mitochondrial mutations on carbon metabolism, stress responses, and mitochondrial DNA stability. In particular, while W303-1B and MCC123 strains should be used to study the effect of severe mitochondrial tRNA mutations, D273-10B/A1 strain is rather suitable for studying the effects of milder mutations.

Reintroduction of a characterized Mit tRNA glycine mutation into yeast mitochondria provides a new tool for the study of human neurodegenerative diseases

We report the identification and characterization of a new mutation (ts9) in the Saccharomyces cerevisiae mitochondrial genome, which was first genetically mapped in the tRNAgly region and further identified by means of sequencing as consisting of a G to A transition at position 30 in the tRNA. The mutation causes an almost complete disappearance of mature tRNAgly, while a second mitochondrial mutation with a compensatory C to T change restores it in normal quantities; this points to the importance of the strong bond between bases 30 and 40 of the anticodon stem in the stabilization of the tRNA. In addition to resulting in a clear-cut heat-sensitive phenotype, the ts9 mutation creates a new EcoRV restriction site. Both properties were used as markers to monitor the successful (re) introduction of the mutated allele into a wild-type mitochondrial genome through biolistic transformation. The mutant frequency in the progeny as well as the correct integration of the mutated allele at its proper site demonstrate the feasibility of this method for creating and investigating specific mitochondrial tRNA mutations. The method will provide important applications for the use of yeast as a model system of human mitochondrial pathologies.

Suppression of a mitochondrial point mutation in a tRNA gene can cast light on the mechanisms of 3' end-processing

We used a genetic approach to study the nuclear factors involved in the biogenesis of mitochondrial tRNAs. A point mutation in the mitochondrial tRNA(Asp) gene of Saccharomyces cerevisiae had previously been shown to result in a temperature-sensitive respiratory-deficient phenotype as a result of the absence of 3' end-processing of the tRNA(Asp). Analysis of mitochondrial revertants has shown that all revertants sequenced have a G-A compensatory change at position 53, which restores the hydrogen-bond with the mutated nucleotide. We then searched for nuclear suppressors to identify the nuclear gene(s) involved in mitochondrial tRNA 3' end-processing. One such suppressor mutation was further characterized: it restores tRNA(Asp) maturation and growth at 36 degrees C on glycerol medium in heterozygous diploids, but leads to a defective growth phenotype in haploids.

Genetic approaches to the study of mitochondrial biogenesis in yeast

In contrast to most other organisms, the yeast Saccharomyces cerevisiae can survive without functional mitochondria. This ability has been exploited in genetic approaches to the study of mitochondrial biogenesis. In the last two decades, mitochondrial genetics have made major contributions to the identification of genes on the mitochondrial genome, the mapping of these genes and the establishment of structure-function relationships in the products they encode. In parallel, more than 200 complementation groups, corresponding to as many nuclear genes necessary for mitochondrial function or biogenesis have been described. Many of the latter are required for post-transcriptional events in mitochondrial gene expression, including the processing of mitochondrial pre-RNAs, the translation of mitochondrial mRNAs, or the assembly of mitochondrial translation products into the membrane. The aim of this review is to describe the genetic approaches used to unravel the intricacies of mitochondrial biogenesis and to summarize recent insights gained from their application.

Identification of nuclear genes which participate to mitochondrial translation in Saccharomyces cerevisiae

The mitochondrial protein synthesis presents specific features and uses specific components different from their cytoplasmic counterparts. Since most genes which code for these components are localized in the chromosomes and only a small number are encoded by the mitochondrial DNA, it is important to identify and characterize the nuclear genes involved in this process. In order to do this, we have used a genetic screening which implies the selection and study of nuclear suppressors of mitochondrial mutations (or the reverse situation) which affect the mitochondrial protein synthesis. Three mutations have been used for this purpose. Two of them (ts 1398, cs 909) impair the mitochondrial ribosome; they were used to characterize new interacting components as well as two genes, MBR1 and MBR2, which control the assembly or the regulation of other genes involved in mitochondrial protein synthesis. The third mutation (ts 932), blocks the 3'-end maturation of the mitochondrial aspartyl tRNA. A nuclear suppressor has been obtained which presents all the characteristics of a mutation in the gene encoding the enzyme responsible for this process.