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Site specificity and variability in the mutator and antimutator effects of phage T4 gene 43 mutants
Abstract:
Spontaneous, 2-aminopurine- and 5-bromouracil-induced mutations at six rII nonsense codons were studied in phage T4 strains possessing wild-type and mutant gene 43 alleles. The mutation pathways studied included interconversions and reversions of nonsense codons. The tsCB87 allele, which specifies an antimutator DNA polymerase, reduced base-analogue-induced mutation frequencies along all pathways. However, GC base pairs were less affected than AT base pairs. The frequency of spontaneous UAA leads to UAG conversions was also reduced by tsCB87, but that of spontaneous UAA leads to UAG UGA conversions was often increased. Mutation in the presence of the mutator allele tsL56 was increased along all pathways, with no preference for either AT or GC base pairs. Mutation frequencies in the presence of the two mutant DNA polymerases were highly variable. A strong correlation was found between 2-aminopurine-induced mutation frequencies in ts+ tsCB87 phage along the reversion and UAA changed to UAG (but not UAA changed to UGA) pathways.
Insights
Antimutator DNA polymerase reduced base-analogue mutations, while mutator polymerase increased them. Specific mutation pathways showed varied responses to these DNA polymerase mutations in phage T4.
Area of Science:
- Molecular Biology
- Genetics
- Virology
Background:
- DNA polymerase fidelity is crucial for preventing mutations.
- Mutations in DNA polymerase genes can alter mutation rates and spectra.
- Nonsense codons are key sites for studying mutation pathways.
Purpose of the Study:
- To investigate the impact of specific DNA polymerase mutations (antimutator tsCB87 and mutator tsL56) on mutation frequencies at rII nonsense codons in phage T4.
- To analyze how these polymerase variants affect spontaneous and base-analogue-induced (2-aminopurine, 5-bromouracil) mutation pathways, including interconversions and reversions.
- To determine if base-analogue-induced mutations show a preference for AT or GC base pairs under the influence of these polymerase alleles.
Main Methods:
- Utilized phage T4 strains with wild-type and mutant gene 43 alleles (tsCB87 and tsL56).
- Induced mutations using 2-aminopurine and 5-bromouracil.
- Quantified spontaneous and induced mutation frequencies at six specific rII nonsense codons.
- Analyzed mutation pathways, including nonsense codon reversions and interconversions (e.g., UAA to UAG, UAA to UGA).
Main Results:
- The antimutator tsCB87 allele reduced base-analogue-induced mutation frequencies across all pathways, with less impact on GC than AT base pairs.
- Spontaneous UAA to UAG conversions decreased with tsCB87, but UAA to UGA conversions often increased.
- The mutator tsL56 allele increased mutation frequencies along all pathways without AT/GC preference.
- Mutation frequencies with both polymerase mutants were highly variable.
- A strong correlation was observed between 2-aminopurine-induced mutation frequencies and reversion/UAA to UAG pathways in ts+ tsCB87 phage.
Conclusions:
- DNA polymerase genotype significantly influences spontaneous and induced mutation frequencies and pathways.
- Antimutator polymerases can reduce certain types of base-analogue mutagenesis, but specific pathways may be differentially affected.
- Mutator polymerases broadly increase mutation rates.
- The study highlights the complex interplay between DNA polymerase activity, mutagens, and specific DNA sequence contexts in determining mutation outcomes.