DNA methylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (M
NNG) are potent carcinogens; their carcinogenic effect is mainly due t
o the effect of production of O-6-methylguanine (O-6 MeG) on DNA. O-6
MeG is not only mutagenic hut also toxic to the cell because Mer(-)/Me
x(-) cells unable to remove O-6 MeG are very sensitive to killing by M
NNG. It has been proposed that repeated futile mismatch correction of
O-6 MeG-containing bp is responsible for the genotoxicity of the O-6 M
eG lesion and that loss of mismatch repair activity results in cellula
r tolerance to O-6 MeG, but the hypothesis has not been proved. We use
d yeast as a model to test this hypothesis and found that chromosome d
eletion of any known nuclear mitotic mismatch repair genes, including
MLH1, MSH2, MSH3, MSH6, and PMS1, did not rescue mgt1 Delta O-6 MeG DN
A repair methyltransferase-deficient cells from killing by MNNG. A lar
ge number of mgt1 Delta, MNNG-tolerant revertants were isolated, among
which one cell line, XS-14, has been found to carry a mutated allele
of the MSH5 gene. The mutation also affected spore survival but did no
t increase the spontaneous mutation rate. We further demonstrated that
a mutated form of the MSH5 gene, msh5-14, not the msh5 Delta-null mut
ation, is responsible for the cellular tolerance to MNNG in XS-14 cell
s. This observation offers an alternative model that may reconcile see
mingly contradictory observations of yeast and mammalian cells.