Stimulation of sister chromatid exchanges and mutation by aflatoxin B ₁-DNA adducts in sSaccharomyces cerevisiae requires MEC1 (ATR), RAD53, and DUN1

The hepatocarcinogen aflatoxin B₁ (AFB₁) is a potent recombinagen but weak mutagen in the yeast Saccharomyces cerevisiae. AFB ₁ exposure induces DNA damage-inducible genes, such as RAD51 and those encoding ribonucleotide reductase (RNR), through a MEC1 (ATR homolog)-dependent pathway. Previous studies have indicated that MEC1 is required for both AFB₁-associated recombination and mutation, and suggested that AFB₁-DNA adduces are common substrates for recombination and mutagenesis. However, little is known about the downstream effectors of MEC1 required for genotoxic events associated with AFB₁ exposure. Here we show that AFB₁ exposure increases frequencies of RAD51-dependent unequal sister chromatid exchange (SCE) and activates Rad53 (CHK2). We found that MEC1, RAD53, and DUN1 are required for both AFB ₁-associated mutation and SCE. Deletion of SML1, which encodes an inhibitor of RNR, did not suppress the DUN1-dependent requirement for AFB ₁-associated genetic events, indicating that higher dNTP levels could not suppress the dun1 phenotype. We identified AFB₁-DNA adducts and show that approximately the same number of adducts are obtained in both wild type and rad53 mutants. Since DUN1 is not required for UV-associated mutation and recombination, these studies define a distinct role for DUN1 in AFB ₁-associated mutagenesis and recombination. We speculate that AFB₁-associated DNA adducts stall DNA replication, a consequence of which can either be mutation or recombination.