Figure 1.

(A) MNNG survival curves: strains were exposed for 45 min to increasing amounts of MNNG. Error bars represent standard error of the mean. The LD₅₀ values for mgt1Δ, rad52Δ mgt1Δ dcd1Δ pms1Δ, rad52Δ mgt1Δ dcd1Δ, rad52Δ mgt1Δ dcd1-S178F, rad52Δ mgt1Δ pms1Δ, rad52Δ mgt1Δ mlh1Δ and rad52Δ mgt1Δ strains were 9.7 μM, 2.4 μM, 2.2 μM, 2.1 μM, 1.8 μM, 1.6 μM and 0.25 μM MNNG, respectively. (B) A model for the cellular response to MNNG treatment in budding yeast. The scheme is modified from a previous report [2] to include the role of DCD1, which, when absent, results in reduced O^6metG/T mispair formation following replication, reduced MMR-dependent processing and resistance to cell death. With the exception of the ‘heavy’ and ‘dashed’ arrows signifying consequences in the dcd1Δ strain, the model applies to wild-type yeast. Further, following replication, remaining O^6metG/C residues will either be detoxified by the methyltransferase or diluted out by subsequent cell cycles. Although as shown the model infers MMR-dependent ‘futile repair’, ‘direct signaling’ must be considered (for reviews, see [1,12]). Support for the futile cycling model comes from several studies including in vitro results showing reiterative MMR excision cycles on O^6metG/T mismatch-containing plasmid substrates [13]. Support for direct signaling comes from a ‘separation-of-function’ allele of mouse Msh6 that compromises spellchecking without affecting the DNA-damage response [14] and from studies showing that MutSα–MutLα complexes are required for ATR–ATRIP signaling from methylation damage sites [15].