Nuclear mutants of yeast with reduced spontaneous mutability of the mitochondrial genome

NET1 and HFI1 genes of yeast mediate both chromosome maintenance and mitochondrial rho(-) mutagenesis

An increase in the mitochondrial rho(-) mutagenesis is a well-known response of yeast cells to mutations in numerous nuclear genes as well as to various kinds of stress. Despite extensive studies for several decades, the biological significance of this response is still not fully understood. The genetic approach to solving this enigma includes a study of genes that are required for the high incidence of spontaneous rho(-) mutants. We have obtained mutations of a few nuclear genes of that sort and found that mutations in certain genes, including CDC28, the central cell-cycle regulation gene, result in a decrease in spontaneous rho(-) mutability and simultaneously affect the maintenance of the yeast chromosomes and plasmids. Two more genes resembling CDC28 in this respect are identified in the present work as a result of the characterization of four new mutants. These two genes are NET1 and HFI1 which mediate important regulatory protein-protein interactions in the yeast cell. The effects of four mutations, including net1-srm and hfi1-srm, on the maintenance of the yeast mitochondrial genome, chromosomes and plasmids, as well as on the cell's sensitivity to ionizing radiation, are also described. The data presented suggest that the pleiotropic srm mutations determining coordinate changes in the fidelity of mitotic transmission of chromosomes, plasmids and mtDNA molecules identify genes that most probably operate high up in the hierarchy of the general genetic regulation of yeast.

Maintenance and integrity of the mitochondrial genome: a plethora of nuclear genes in the budding yeast

Instability of the mitochondrial genome (mtDNA) is a general problem from yeasts to humans. However, its genetic control is not well documented except in the yeast Saccharomyces cerevisiae. From the discovery, 50 years ago, of the petite mutants by Ephrussi and his coworkers, it has been shown that more than 100 nuclear genes directly or indirectly influence the fate of the rho(+) mtDNA. It is not surprising that mutations in genes involved in mtDNA metabolism (replication, repair, and recombination) can cause a complete loss of mtDNA (rho(0) petites) and/or lead to truncated forms (rho(-)) of this genome. However, most loss-of-function mutations which increase yeast mtDNA instability act indirectly: they lie in genes controlling functions as diverse as mitochondrial translation, ATP synthase, iron homeostasis, fatty acid metabolism, mitochondrial morphology, and so on. In a few cases it has been shown that gene overexpression increases the levels of petite mutants. Mutations in other genes are lethal in the absence of a functional mtDNA and thus convert this petite-positive yeast into a petite-negative form: petite cells cannot be recovered in these genetic contexts. Most of the data are explained if one assumes that the maintenance of the rho(+) genome depends on a centromere-like structure dispensable for the maintenance of rho(-) mtDNA and/or the function of mitochondrially encoded ATP synthase subunits, especially ATP6. In fact, the real challenge for the next 50 years will be to assemble the pieces of this puzzle by using yeast and to use complementary models, especially in strict aerobes.

Mutations of the CDC28 gene and the radiation sensitivity of Saccharomyces cerevisiae

cdc28-srm, a non-temperature-sensitive (ts) mutation in the CDC28 gene of Saccharomyces cerevisiae that affects fidelity of mitotic transmission of both mitochondrial and nuclear genetic structures (Devin et al., 1990), also affected cell growth and sensitivity to lethal effects of ionizing radiation. At 30 degrees C cdc28-13, a ts mutation, was without appreciable effects on spontaneous mitochondrial rho(-)-mutagenesis, cell growth and radiation sensitivity, whereas all three cell characteristics mentioned were affected (although to a lesser degree than by cdc28-srm) by cdc28-1, another ts mutation. cdc28-srm was without any significant effect on the rates of spontaneous nuclear gene mutations and gamma-ray-induced mitotic recombination. An analysis of double mutants as regards their radiation sensitivity has revealed additive or even synergistic interactions between the cdc28-srm mutation and every one of the rad6-1 and rad52-1 mutations. The rad9 delta allele was found to be epistatic to cdc28-srm. These data suggest that the p34CDC28 protein is involved in the RAD9-dependent feedback control of DNA integrity operating at the cell cycle checkpoints.

The start gene CDC28 and the genetic stability of yeast

The cdc28-srm mutation in Saccharomyces cerevisiae decreases spontaneous and induced mitochondrial rho- mutability and the mitotic stability of native chromosomes and recombinant circular minichromosomes. The effects of cdc28-srm on the genetic stability of cells support the hypothesis that links cell cycle regulation in yeast to changes in chromatin organization dependent on the start gene CDC28 (Hayles and Nurse, 1986).

Spontaneous mutagenesis: the roles of DNA repair, replication, and recombination

There appears to be no dearth of mechanisms to explain spontaneous mutagenesis. In the case of base substitutions, data for bacteriophage T4 and especially for E. coli and S. cerevisiae suggest important roles in spontaneous mutagenesis for the error-prone repair of DNA damage (to produce mutations) and for error-free repair of DNA damage (to avoid mutagenesis). Data from the very limited number of studies on the subject suggest that about 50% of the spontaneous base substitutions in E. coli, and perhaps 90% in S. cerevisiae are due to error-prone DNA repair. On the other hand, spontaneous frameshifts and deletions seem to result from mechanisms involving recombination and replication. Spontaneous insertions have been shown to be important in the strongly polar inactivation of certain loci, but it is less important at other loci. Perhaps with continued study, the term "spontaneous mutagenesis" will be replaced by more specific terms such as 5-methylcytosine deamination mutagenesis, fatty acid oxidation mutagenesis, phenylalanine mutagenesis, and imprecise-recombination mutagenesis. While most studies have concentrated on mutator mutations, the most conclusive data for the actual source of spontaneous mutations have come from the study of antimutator mutations. Further study in this area, perhaps along with an understanding of chemical antimutagens, should be invaluable in clarifying the bases of spontaneous mutagenesis.

A nuclear mutation decreasing rho- production modifies the unstable nitrous-acid sensitivity of yeast

The nuclear mmgl mutation, which reduces rho- mutability in Saccharomyces cerevisiae, renders the rho+ cells less sensitive to inactivation by nitrous acid (NA) but has little or no effect on the NA sensitivity of the rho0 cells devoid of mitochondrial (mt) DNA. Therefore the cells' NA sensitivity seems to be influenced by an interaction of the mmgl mutation and the mt genome rather than the mmgl mutation itself. The clonal variation of NA sensitivity is high in MMG+ yeast and significantly reduced in rho0 mutants and mmgl cells. The results presented suggest that frequent spontaneous heritable changes of the mt genome occur in MMG+ cells, which, (i) unlike rho- mutations, do not damage the respiratory capacity, and (ii) manifest themselves in a high clonal variation of NA sensitivity.

Antimutators of mitochondrial and nuclear DNA in Saccharomyces cerevisiae. Relationship with gamma-ray sensitivity

In Saccharomyces cerevisiae ten antimutator mutants have been isolated. The spontaneous occurrence of mitochondrial mutants resistant to erythromycin, oligomycin, and diuron is decreased 2-60-fold in these strains. The rate of forward and reverse spontaneous mutations of the nuclear genome is also reduced. The meiotic progenies arising from the crosses of seven mutants (LB1, LB2, LB4, LB5, LB6, LB7, LB10) with an isogenic parental strain exhibit 2 : 2 segregations and therefore are the result of mutations in a single nuclear gene. The six mutants LB1, LB2, LB4, LB6, LB7, LB10 are semidominant and determine six complementation groups. The mutant LB5 is dominant and therefore cannot be assigned to any complementation group. The mutants, LB1, LB4 and LB10 are gamma-ray sensitive and, by tetrad analysis, it has been shown that gamma-ray sensitivity and spontaneous antimutability are the result of a single nuclear gene mutation. The other three mutants LB3, LB8, and LB9 exhibit complex tetrad segregations, typical of cytoplasmic inheritance and do not complement each other. However, although the mutations are semidominant, it has not been possible to detect any antimutator cytoductant among some 500 cytoductants carrying the karl-1 nucleus. These results suggest that either several nuclear genes are involved in the expression of the antimutator phenotype or that the antimutator gene is located on nonchromosomal elements of the nucleus. The present study leads to the conclusion that a large number of nuclear genes are able to control simultaneously the spontaneous mutation rate of nuclear and mitochondrial genes. Since out of the ten antimutator mutants, three are also deficient in the repair of gamma-ray damage, it is also concluded that spontaneous and gamma-ray-induced lesions of DNA can be repaired by the same error-free process.