DNA MISMATCH REPAIR MUTANTS DO NOT INCREASE N-METHYL-N'-NITRO-N-NITROSOGUANIDINE TOLERANCE IN O-6 METHYLGUANINE DNA METHYLTRANSFERASE-DEFICIENT YEAST-CELLS

Treatment of cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) pr oduces, among other lesions, mutagenic and carcinogenic lesions such a s O-6-methylguanine (O(6)MeG) and O-4-methylthymine in DNA, An O(6)MeG DNA methyl-transferase (MTase) specifically and efficiently repairs s uch lesions, MTase-deficient bacterial, yeast and mammalian cells exhi bit increased sensitivity not only to MNNG-induced mutagenesis, but al so to MNNG-induced kilting, suggesting that O(6)MeG-type lesions are a lso lethal to the cells, The lethal effect caused by O(6)MeG is not cl ear, Results from several recent experiments indicate that some MNNG-t olerant cell lines exhibit a loss of DNA mismatch binding/repair activ ity, suggesting that functional mismatch repair is probably responsibl e for the cellular sensitivity to DNA methylating agents, We tested th is abortive O(6)MeG-T mismatch repair hypothesis in a well-defined low er eukaryote, Saccharomyces cerevisiae, We found that while mgt1-delet ed MTase-deficient yeast strains are hypersensitive to MNNG-induced ki lling, combination of this mutation with any of the mlh1, msh2 or pms1 mutations did not render cells more tolerant to killing, msh3 mutatio n also did not rescue MNNG-induced genotoxicity. Furthermore, through the isolation and characterization of MNNG-tolerant cell lines from th e MTase-deficient mutants we demonstrated that a DNA mismatch repair d efect is neither sufficient nor required for this process, Since both DNA repair MTases and mismatch repair proteins are highly conserved be tween yeast and mammalian cells, our results could suggest alternative mechanisms in the cellular tolerance to O(6)MeG-induced killing.