Mechanism of mitochondrial mutation in yeast

From Genome Variation to Molecular Mechanisms: What we Have Learned From Yeast Mitochondrial Genomes?

Analysis of genome variation provides insights into mechanisms in genome evolution. This is increasingly appreciated with the rapid growth of genomic data. Mitochondrial genomes (mitogenomes) are well known to vary substantially in many genomic aspects, such as genome size, sequence context, nucleotide base composition and substitution rate. Such substantial variation makes mitogenomes an excellent model system to study the mechanisms dictating mitogenome variation. Recent sequencing efforts have not only covered a rich number of yeast species but also generated genomes from abundant strains within the same species. The rich yeast genomic data have enabled detailed investigation from genome variation into molecular mechanisms in genome evolution. This mini-review highlights some recent progresses in yeast mitogenome studies.

The in vivo effects of ethidium bromide on mitochondrial and ribosomal DNA in Candida parapsilosis

The ability of Candida parapsilosis to grow in the presence of high levels of ethidium bromide (EB) has been explored to study the effects of this intercalating dye on DNA in vivo. By employing confocal microscopy we have determined that EB penetrates the cellular membranes and binds rapidly to the nucleolus, whereas mitochondrial DNA becomes stained after a longer exposure to this dye. No detectable staining of the nucleus has been detected under these conditions. Electrophoretic studies of both undigested and restricted DNAs confirm that the nuclear DNA is unaffected by high levels of EB, with the exception of the rDNA-bearing chromosome that undergoes significant structural alterations in the presence of EB. Moreover, the hybridization signal with the rDNA probe is proportionally reduced in samples obtained from cultures grown in the presence of EB, suggesting that the average copy number of rRNA genes in these cultures may be affected. In striking contrast to other fungal species, the linear organelle genome in C. parapsilosis retains its structural and functional integrity in the presence of high concentrations of EB.

Extrachromosomal inheritance of nalidixic acid resistance in the petite negative yeast Schizosaccharomyces pombe

Spontaneous mutants resistant to nalidixic acid (NAL) were isolated from the petite negative yeast Schizosaccharomyces pombe (S. pombe). One of these mutants, resistant to 200 micrograms/ml NAL, nalr-Y13, was characterized both genetically and biochemically. The extrachromosomal inheritance of this mutation was demonstrated both by mitotic segregation and by mitotic haploidization analysis. In the wild-type, NAL at a concentration of 100 micrograms/ml almost completely inhibits incorporation of [14C]adenine in total DNA as well as in mitochondrial DNA. In the NAL-resistant mutant both total DNA synthesis and mitochondrial DNA synthesis were resistant to the drug. These results are discussed in view of previously published findings on the close interaction between the two DNA synthesizing systems in S. pombe.

Population changes induced in Candida albicans by nalidixic acid

Cells of Candida albicans plated on media containing nalidixic acid (Nal) either die, adapt physiologically to Nal-tolerance or mutate ot Nal-resistance. The fraction of a population exhibiting each response depends on the growth phase of cells when plated and their nitrogen and carbon nutrition and growth temperatures before and after plating. Nal induces Nal-resistant mutants in very high frequency but only at 37 C on plates containing i) glucose as primary carbon source and ii) adenine, a sulfur amino acid or a representative of the glutamic acid family of amino acids. Nal does not affect either forward mutation to caffeine-resistance or reverse mutation from histidine auxotrophy to prototrophy. Nal-resistant mutants produce minute colonies on N-ALfree medium, respire oxidatively and are unusually sensitive to inhibitors of oxidative phosphorylation. They revert spontaneously to wild type at very high rates but can be propagated indefinitely in the absence of Nal by serial selection and replating of minute colonies. Cellular inactivation and induction of Nal-resistant mutants are greatly affected by specific inhibitors of mitochondrial macromolecular syntheses. The presence of chloramphenicol or erythromycin during exposure to Nal prevents cell death and mutation but has no effect on adaptation to Nal-tolerance. Growth on acriflavin or ethidium bromide enhances resistance of cells to inactivation when subsequently plated on Nal containing media. It is concluded that Nal-induced cellular inactivation and mutation to Nal-resistance, but not adaptation to Nal-tolerance, result from damages to the mitochondrion which are fixed or promoted by macromolecular syntheses within the mitochondrion. Implications of these findings for the therapeutic use of Nal are discussed.

Differential effects of nalidixate on the cell growth of respiratory competent strains and cytoplasmic petite mutants of Saccharomyces cerevisiae

Sodium nalidixate inhibited the cell growth and division of several respiratory competent strains of Saccharomyces cerevisiae. A number of cytoplasmic petite strains (both spontaneous and induced by ethidium bromide) were shown to be more resistant to sodium nalidixate than the wild-type strains from which they were derived. There was considerable variation in sensitivity of different petites derived from the same wild-type. Usually petite strains which were induced by ethidium bromide were more resistant than spontaneously arising petites. The susceptibility of a wild-type strain to nalidixate was found to be least when the mitochondrial respiratory system was maximally repressed. It was also noted that sodium nalidixate (100 mug/ml) induced petite mutants.

Biogenesis of mitochondria. 43. A comparative study of petite induction and inhibition of mitochondrial DNA replication in yeast by ethidium bromide and berenil

The action of ethidium bromide and berenil on the mitochondrial genome of Saccharomyces cerevisiae has been compared in three types of study: (i) early kinetics (up to 4 h) of petite induction by the drugs in the presence or absence of sodium dodecyl sulphate; (ii) genetic consequences of long-term (8 cell generations) exposure to the drugs; (iii) inhibition of mitochondrial DNA replication, both in whole cells and in isolated mitochondria. The results have been interpreted as follows. Firstly, the early events in petite induction differ markedly for the two drugs, as indicated by differences in the short-term kinetics. After some stage a common pathway is apparently followed because the composition of the population of petite cells induced after long-term exposure are very similar for both ethidium bromide and berenil. Secondly, both drugs probably act at the same site to inhibit mitochondrial DNA replication, in view of the fact that a petite strain known to be resistant to ethidium bromide inhibition of mitochondrial DNA replication was found to have simultaneously acquired resistance to berenil. From consideration of the drug concentrations needed to inhibit mitochondrial DNA replication in vivo and in vitro it is suggested that in vivo permeability barriers impede the access of ethidium bromide to the site of inhibition of mitochondrial DNA replication, whilst access of berenil to this site is facilitated. The site at which the drugs act to inhibit mitochondrial DNA replication may be different from the site(s) involved in early petite induction. Binding of the drugs at the latter site(s) is considered to initiate a series of events leading to the fragmentation of yeast mitochondrial DNA and petite induction.

Manganese mutagenesis in yeast. II. Conditions of induction and characteristics of mitochondrial respiratory deficient Saccharomyces cerevisiae mutants induced with manganese and cobalt

Manganese and cobalt are capable of inducing ρ−mutations* in non-growing cells ofSaccharomyces cerevisiae, but their mutagenic action is much stronger in growing cells. At a given concentration cobalt and manganese can be either strongly mutagenic or non-mutagenic, depending on the cell density.

Most of the ρ−mutants induced with manganese and a considerable proportion of those induced with cobalt are suppressive and/or transmit drug resistance markers, so they must still carry mitochondrial DNA. Cobalt can decrease suppressiveness with low efficiency and eliminate drug resistance markers from established ρ−clones.

Mitochondrial genetics. VI. The petite mutation in Saccharomyces cerevisiae: interrelations between the loss of the p+ factor and the loss of the drug resistance mitochondrial genetic markers

The survival of the rho(+) factor and of Drug(R) mitochondrial genetic markers after exposure to ethidium bromide has been studied. A technique allowing the determination of Drug(R) genetic markers among a great number of both grande and petite colonies has been developed. The results have been analyzed by the target theory. The survival of the rho(+) factor is always less than the survival of any Drug(R) genetic marker. The survivals of C(R) and E(R) are similar to each other, while that of O(R) is greater than that of the other two Drug(R) markers. All possible combinations of Drug(R) markers have been found among the rho(-) petite cells induced, while the only type found among the grande colonies is the preexisting one. The loss of the C(R) and E(R) genetic markers was found to be the most frequently concomitant, while the correlation between the loss of the O(R) marker and the other two Drug(R) markers is less strong. Similar results have been obtained after U.V. irradiation. Interpretations concerning the structure of the yeast mitochondrial genome are given and hypotheses on the mechanism of petite mutation discussed.