A novel class of Saccharomyces cerevisiae mutants specifically UV-sensitive to "petite" induction

Repair properties in yeast mitochondrial DNA mutators

After ethylmethanesulfonate mutagenesis of the strain Saccharomyces cerevisiae D273-10B, out of 100,000 survivors, 1,000 were selected for their high production of petite mutants at 36 degrees C. Among these 1,000 mutators, 5 also showed an increased frequency of spontaneous point mutations measured at 25 degrees C. Further analysis revealed that in all mutators, except 2, petite accumulation proceeded at 25 degrees C as well as 36 degrees C. In these 2 mutants, the production of petite mutants was much higher at 36 degrees C than at 35 degrees C. In one of them, however, the mutator and the thermosensitive petite phenotypes were due to mutations in two unlinked nuclear genes. In the other mutants, both traits were the result of a mutation in a single nuclear gene. The mutators fell into three complementation groups (tpm1, tpm2, mup1). No complementation was observed between tpm1 mutants and the gam4 mutant previously described by Foury and Goffeau (1979). From the latter and the present works, only four complementation groups (gam1, gam2, gam4 or tpm1, mup1) have been identified and it is likely that the number of genes controlling specifically the spontaneous mutability of the mtDNA is low. The mutators exhibited a variety of responses to damaging agents such as UV light and ethidium bromide; especially in a representative mutant from the complementation group tpm1, the induction of rho- mutants was sensitive to UV light and resistant to ethidium bromide.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Direct selection of mutants influencing gene conversion in the yeast Schizosaccharomyces pombe

In Schizosaccharomyces pombe, a suppressor-active mutation at the anticodon site of the tRNASerUCA gene sup3 leads to opal (UGA)-specific suppression. Second-site mutations (rX) in sup3 inactivate the suppressor. The sup3-UGA, rX double mutants are genetically unstable in meiotic selfings, due to the intergenic transfer of information between sup3 and the unlinked genes sup9 and sup12 (Hofer et al. 1979; Munz and Leupold 1981; Munz et al. 1982). These three genes have considerable sequence homology over about 200 base pairs (Hottinger et al. 1982). Mutants showing a decrease or an increase of the meiotic instability at sup3 have been selected. One mutation (rec3-8) increases both the genetic instability and the frequency of intragenic recombination in sup3 by one order of magnitude. It has no effect on the stability of the nonsense alleles arg1-230 (UAA), ade6-704 and ural1-61 (UGA) or on the frequency of crossing-over between sup3 and the closely linked gene cdc8. The existence of a common genetic control over intragenic recombination and genetic instability at sup3 provides a direct way of selecting for rec mutants in homothallic haploid strains of S. pombe carrying a suppressor-inactive allele of sup3. It also supports the hypothesis that the instability of mutant alleles of this gene is due to chromosome mispairing at meiosis allowing sup3 to pair with sup9 or sup12 and then to undergo recombination by gene conversion restoring the suppressor-active allele sup3-UGA from the suppressor-inactive allele sup3-UGA, rX.(ABSTRACT TRUNCATED AT 250 WORDS)

pif mutation blocks recombination between mitochondrial rho+ and rho- genomes having tandemly arrayed repeat units in Saccharomyces cerevisiae

Three allelic nuclear mutants affected in the recombination of mtDNA have been characterized in Saccharomyces cerevisiae and assigned to the PIF locus. In the mutants, the general recombination measured by the recombination frequency between linked or unlinked alleles is normal. However, the pif mutations prevent the integration into the rho+ genome of the markers (oli1, oli2, diu1, ery, oxi1, oxi2) of those rho- genomes that have tandemly arrayed repeat units. Therefore, these rho- genomes characterize a PIF-dependent recombination system. The pif mutations have also revealed the existence of a PIF-independent recombination system used by those rho- genomes that have an inverted organization of their repeat units. The markers of such palindromic rho- genomes exhibit high integration frequency into the rho+ genome even in the presence of the pif mutation. In addition, the pif mutations greatly increase suppressiveness in crosses between pif rho+ strains and PIF-dependent as well as PIF-independent rho- clones. We conclude that the recombination between rho+ and rho- genomes involves at least two distinct systems that depend on the organization of the rho- genome.

The role of the nuclear gene "mitochondrial mutability control" (MMC1) in the process of mutability of the mitochondrial genome by different mutagens in Saccharomyces cerevisiae

The accumulation of respiratory deficient (RD) mutants in Saccharomyces cerevisiae depended upon the mutagens used and upon the presence of the nuclear gene previously identified as MMC1 (one) which we showed to control the spontaneous and the erythromycin-induced RD mutability. In this paper data are reported about the accumulation of RD mutants in the presence of manganous ions (Mn++) and UV which was higher in the mmc1 (one) than in MMC1 strains. We found that the characters 'low spontaneous' and 'low induced' RD mutability by erythromycin, manganous ions and UV, were controlled by the same genetic determinant. In the presence of manganous ions, also the frequency of antibiotic resistant mutants capR and eryR was higher in the mmc1 strains. Moreover, the accumulation of RD mutants in the presence of berenil, 5-fluorouracil and basic fuchsin was higher in the mmc1 than in MMC1 strains. In contrast, RD mutants accumulation by acriflavine and ethidium bromide treatments did not appear affected by the MMC1 genetic constitution.

Yeast mutants affecting the spontaneous mutation frequency on both the mitochondrial and nuclear genomes

We have previously described two complementation groups of mutants affecting the spontaneous mutation frequency on the mitochondrial genome (Johnston, 1979). In a further search for such mutants the majority isolated fell into one of these groups, 3 into group I and 14 into group II. There are now a total of 12 alleles in the first group and 19 in the second complementation group, suggesting that these are the major classes of mutants affecting spontaneous mutation frequency in the mitochondrion. However, this search also identified a third complementation group, consisting of 2 mutants, which, like the existing groups, is recessive and is coded on the nuclear genome. In contrast to complementation groups I and II, these new mutants have no effect on spontaneous petite mutagenesis and they also increase the spontaneous mutation frequency on the nuclear genome. This nuclear mutation activity may be novel, as it complemented the existing nuclear mutators mut1-mut10. None of the three complementation groups has any detectable phenotype, other than the mutator activity.

Yeast mitochondrial DNA characterization after ultraviolet irradiation

Yeast mitochondrial (mtDNA) 3H-labelled was isolated from exponential phase cells after ultraviolet light irradiation. Both the size and amount of mtDNA were found to be reduced during a 40-h liquid-holding (LH) period in non-growth medium following irradiation as compared to the mtDNA recovered from nonirradiated cells under similar conditions. After the LH period, previously irradiated cells were resuspended in growth medium containing [14C]adenine. Double labelled mtDNA (3H and 14C) was isolated from cell samples removed during new growth. A recovery in the amount and size of mtDNA was observed in irradiated cells during new growth. These biochemical studies agree with the observed loss and recovery of mtDNA genetic markers in UV-irradiated exponential phase yeast after a period of LH and new growth resp.

Genetic effects of formaldehyde in yeast. III. Nuclear and cytoplasmic mutagenic effects

Low concentrations of formaldehyde induce nuclear mutations when yeast cells are allowed to grow in the presence of this compound. The induction of reversions is a linear function of the concentration and depends upon the repair capacities of the treated cells. A strain defective in excision-repair (rad3-12) is more mutable by formaldehyde than the isogenic wild-type whereas a strain blocked in the mutagenic pathway (rad6-1) is not mutable after the same treatment. Allele specificities were found. In particular the lys1-1 mutation is not reversible by formaldehyde. Higher concentrations of formaldehyde induce efficiently the cytoplasmic "petite" mutation in non-growing conditions when a lethal effect is noticeable. The growth phase as well as the physiological state influence this mutagenic effect. The mutagenic effect of formaldehyde in yeast is discussed in relation with the repair processes involved.

Studies on the induction of petite mutants in yeast by analogues of berenil. Characterization of three mutants resistant to the compound Hoe 15,030

Compound Hoe 15,030 is an analogue of berenil which is as effective as berenil in inducing petite mutants in Saccharomyces cerevisiae. Hoe 15,030 has greater stability than berenil in aqueous solution, and is less toxic to yeast at high drug concentrations. Mutants of S. cerevisiae strain J69-1B have been isolated which are resistant to the petite inducing effects of Hoe 15,030. Three mutant strains (HR7, HR8 and HR10) were characterized and each was shown to carry a recessive nuclear mutation determining resistance to Hoe 15,030. The degree of resistance to Hoe 15,030 is different for each mutant, and each was found to be co-ordinately cross-resistant both to berenil and to another analogue of berenil, Hoe 13,548. However, the three mutants show no cross-resistance to other unrelated petite inducing drugs, including ethidium bromide, euflavine and 1-methyl phenyl neutral red. Further studies on the mutants revealed that each strain exhibits characteristic new properties indicative of changes in mitochondrial membrane functions concerned with the replication (and probably also repair) of mitochondrial DNA. Thus, mutant HR7 is hypersensitive to petite induction by the detergent sodium dodecyl sulphate under conditions where the parent J69-1B is unaffected by this agent. Mutant HR8 is even more sensitive to sodium dodecyl sulphate than is HR7, and additionally shows a markedly elevated spontaneous petite frequency. Isolated mitochondria from strains HR8 and HR10 (but not HR7) show resistance to the inhibitory effects of Hoe 15,030 on the replication of mitochondrial DNA in vitro.