Variations in UV-induced lethality and "petite" mutagenesis in synchronous culture of Saccharomyces cerevisiae. II. Responses of radiosensitive mutants to lethal damage

Influence of DNA repair deficiencies on the UV sensitivity of yeast cells in different cell cycle stages

Synchronously dividing haploid yeast cells were UV-irradiated in various stages of the cell cycle after release from alpha-factor arrest. In confirmation of earlier results (Chanet et al., 1973), in wild-type strains G1/S phase cells were found to be the most sensitive and late S/G2 cells the most resistant. Stationary-phase (G0) cells were significantly more UV resistant than G1 cells. Strains defective in nucleotide excision repair lost enhanced resistance in the G2 phase and were most UV-sensitive in the G0 state. Reduced G2 resistance was also observed in rad6 mutants but not in rad9 mutants. After UV-irradiation in G1 phase rad9 mutant cells showed a reduced G1/S phase arrest.

Genetic and physiological factors affecting repair and mutagenesis in yeast

Current views of DNA repair and mutagenesis in the yeast Saccharomyces cerevisiae are discussed in the light of recent data and with emphasis on the isolation and characterization of genetically well-defined mutations that affect DNA metabolism in general (including replication and recombination). Various "pathways" of repair are described, particularly in relation to their involvement in mutagenic mechanisms. In addition to genetic control, certain physiological factors such as "cell age," DNA replication, and the regulatory state of the mating-type locus are shown to also play a role in repair and mutagenesis.

Abolition of the cyclic variations in radiosensitivity during meiosis in a sporulation mutant blocked in premeiotic DNA synthesis

The response to ultraviolet light (254 nm) of two sporulation mutants during the meiotic process was compared to that of a wild type diploid strain of Saccharomyces cerevisiae. The cyclic pattern for cell killing and rho- induction characteristic of diploid wild type cells persists in a strain able to perform the premeiotic DNA synthesis but which is blocked in the further steps of meiosis (spo8 DMS1). On the contrary, these fluctations are abolished in a derived mutant (spo8 dsm1) which is blocked in the premeiotic DNA synthesis. Under these conditions, the response to cell killing can be dissociated from the observed for rho- induction.

Cell-cycle variation in the induction of lethality and mitotic recombination after treatment with UV and nitrous acid in the yeast, Saccharomyces cerevisiae

Exponentially growing yeast cultures separated into discrete periods of the cell cycle by zonal rotor centrifugation show cyclic variation in both UV and nitrous acid induced cell lethality, mitotic gene conversion and mitotic crossing-over. Maximum cell survival after UV treatment was observed in the S and G2 phases of the cell cycle at a time when UV induction of both types of mitotic recombination was at a minimum. In contrast, cell inactivation by the chemical mutagen nitrous acid showed a single discrete period of sensitivity which occurred in S phase cells which are undergoing DNA synthesis. Mitotic gene conversion and mitotic crossing-over were induced by nitrous acid in cells at all stages of the cell cycle with a peak of induction of both events occurring at the time of maximum cell lethality. The lack of correlation observed between maximum cell and the maximum induction of mitotic intragenic recombination suggest that other DNA-repair mechanisms besides DNA-recombination repair are involved in the recovery of inactivated yeast cells during the cell cycle.

Mutagenic action of hydrostatic pressure on yeast: a cell cycle analysis

Pressure-treated log growth cultures (14,000 psi equivalent to 966 x 10(5) N/m2 for 4 h) of Saccharomyces cerevisiae were fractionated across a linear Ficoll gradient by zonal rotor centrifugation. This procedure separated the yeast cells on the basis of size and volume into a continuum of cell cycle ages. Cell survival and petite mutation frequency were determined for several zonal fractions. Survival of yeast cells after pressure treatment was maximal in zonal fractions obtained from either the top (single cells in G1) or the botton ("doublets") of the gradient. Intermediate zonal fractions showed more lethality, with minimal survival occurring in zonal fractions containing a large proportion of yeast cells in which buds were just beginning to emerge (initiation of S phase). The petite mutation frequency was minimal in zonal fractions from the top (single cells in G1) and bottom ("doublets") of the gradient. Induction increased to a maximum in those fractions containing cells in S phase.

Genetic effects of formaldehyde in yeast. II. Influence of ploidly and of mutations affecting radiosensitivity on its lethal effect

Haploid and diploid cells of Saccharomyces cerevisiae have the same sensitivity to formaldehyde, exponentially growing cells being more sensitive than stationary phase cells for both degrees of ploidy. Strains defective (rad 1-3) or with a reduced capacity (p-, cytoplasmic respiratory deficient mutants) in excision repair of ultraviolet-induced pyrimidine dimers show a greater sensitivity to formaldehyde than the corresponding wild type. A mutant defective in radiation-induced gene conversion (rec5) shows the same sensitivity as the wild-type strain. It appears that the excision-repair system plays an important role, especially in stationary phase cells, in repairing a fraction of formaldehyde-induced lesions.

Genetic effects of formaldehyde in yeast. I. Influence of the growth stages on killing and recombination

In random cultures, stationary phase cells of Saccharomyces cerevisiae are more resistant to killing induced by formaldehyde than are exponentially growing cells. It is shown that this compound induces intra- and intergenic recombination in this eucaryotic organism. In synchronized populations the lag and G1 phases demonstrate the higher resistance to both killing and induction of recombinants by formaldehyde whereas maximal sensitivity occurs during the end of G2 and/or the mitotic division. This pattern in contrast with that found after treatments by ionizing or ultraviolet radiations.