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Inducible DNA-repair systems in yeast: competition for lesions
Abstract:
DNA lesions may be recognized and repaired by more than one DNA-repair process. If two repair systems with different error frequencies have overlapping lesion specificity and one or both is inducible, the resulting variable competition for the lesions can change the biological consequences of these lesions. This concept was demonstrated by observing mutation in yeast cells (Saccharomyces cerevisiae) exposed to combinations of mutagens under conditions which influenced the induction of error-free recombinational repair or error-prone repair. Total mutation frequency was reduced in a manner proportional to the dose of 60Co-gamma- or 254 nm UV radiation delivered prior to or subsequent to an MNNG exposure. Suppression was greater per unit radiation dose in cells gamma-irradiated in O2 as compared to N2. A rad3 (excision-repair) mutant gave results similar to wild-type but mutation in a rad52 (rec-) mutant exposed to MNNG was not suppressed by radiation. Protein-synthesis inhibition with heat shock or cycloheximide indicated that it was the mutation due to MNNG and not that due to radiation which had changed. These results indicate that MNNG lesions are recognized by both the recombinational repair system and the inducible error-prone system, but that gamma-radiation induction of error-free recombinational repair resulted in increased competition for the lesions, thereby reducing mutation. Similarly, gamma-radiation exposure resulted in a radiation dose-dependent reduction in mutation due to MNU, EMS, ENU and 8-MOP + UVA, but no reduction in mutation due to MMS. These results suggest that the number of mutational MMS lesions recognizable by the recombinational repair system must be very small relative to those produced by the other agents. MNNG induction of the inducible error-prone systems however, did not alter mutation frequencies due to ENU or MMS exposure but, in contrast to radiation, increased the mutagenic effectiveness of EMS. These experiments demonstrate that in this lower eukaryote, mutagen exposure does not necessarily result in a fixed risk of mutation, but that the risk can be markedly influenced by a variety of external stimuli including heat shock or exposure to other mutagens.
Insights
DNA repair competition influences mutation risk. Yeast cells showed that radiation-induced repair reduced mutations from some mutagens by altering competition for DNA lesions, demonstrating variable mutation outcomes.
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- DNA lesions can be processed by multiple repair pathways.
- Overlapping lesion specificity and inducible repair systems create variable competition.
- This competition can alter the biological consequences of DNA damage.
Purpose of the Study:
- To investigate how competition between DNA repair systems affects mutation frequency.
- To demonstrate the concept using yeast cells exposed to combinations of mutagens.
- To explore the influence of inducible repair pathways on mutation outcomes.
Main Methods:
- Exposing yeast cells (Saccharomyces cerevisiae) to combinations of mutagens (MNNG, gamma-radiation, UV, MNU, EMS, ENU, 8-MOP + UVA, MMS).
- Manipulating conditions to influence error-free recombinational repair and error-prone repair.
- Utilizing wild-type and mutant yeast strains (rad3, rad52) to assess repair pathway involvement.
- Employing protein-synthesis inhibition (heat shock, cycloheximide) to differentiate mutation sources.
Main Results:
- Gamma-radiation or UV exposure prior to MNNG reduced total mutation frequency, with greater suppression in oxygen.
- Mutation in rad52 (rec-) mutants exposed to MNNG was not suppressed by radiation, indicating recombinational repair's role.
- Radiation reduced mutations from MNU, EMS, ENU, and 8-MOP + UVA, but not MMS, suggesting fewer MMS lesions recognized by recombinational repair.
- MNNG induction of error-prone systems did not alter ENU or MMS mutations but increased EMS mutagenicity.
Conclusions:
- Competition between DNA repair systems, particularly inducible ones, significantly influences mutation risk.
- Gamma-radiation induction of error-free recombinational repair increases competition, reducing MNNG-induced mutations.
- Mutagen exposure does not guarantee a fixed mutation risk; external stimuli can markedly alter outcomes in yeast.