E. coli mismatch repair enhances AT-to-GC mutagenesis caused by alkylating agents

Kota Nakano, Yoko Yamada, Eizo Takahashi, Sakae Arimoto, Keinosuke Okamoto, Kazuo Negishi, Tomoe Negishi

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)


Alkylating agents are known to induce the formation of O6-alkylguanine (O6-alkG) and O4-alkylthymine (O4-alkT) in DNA. These lesions have been widely investigated as major sources of mutations. We previously showed that mismatch repair (MMR) facilitates the suppression of GC-to-AT mutations caused by O6-methylguanine more efficiently than the suppression of GC-to-AT mutations caused by O6-ethylguanine. However, the manner by which O4-alkyT lesions are repaired remains unclear. In the present study, we investigated the repair pathway involved in the repair of O4-alkT. The E. coli CC106 strain, which harbors Δprolac in its genomic DNA and carries the F'CC106 episome, can be used to detect AT-to-GC reverse-mutation of the gene encoding β-galactosidase. Such AT-to-GC mutations should be induced through the formation of O4-alkT at AT base pairs. As expected, an O6-alkylguanine-DNA alkyltransferase (AGT) -deficient CC106 strain, which is defective in both ada and agt genes, exhibited elevated mutant frequencies in the presence of methylating agents and ethylating agents. However, in the UvrA-deficient strain, the methylating agents were less mutagenic than in wild-type, while ethylating agents were more mutagenic than in wild-type, as observed with agents that induce O6-alkylguanine modifications. Unexpectedly, the mutant frequencies decreased in a MutS-deficient strain, and a similar tendency was observed in MutL- or MutH-deficient strains. Thus, MMR appears to promote mutation at AT base pairs. Similar results were obtained in experiments employing double-mutant strains harboring defects in both MMR and AGT, or MMR and NER. E. coli MMR enhances AT-to-GC mutagenesis, such as that caused by O4-alkylthymine. We hypothesize that the MutS protein recognizes the O4-alkT:A base pair more efficiently than O4-alkT:G. Such a distinction would result in misincorporation of G at the O4-alkT site, followed by higher mutation frequencies in wild-type cells, which have MutS protein, compared to MMR-deficient strains.

Original languageEnglish
Pages (from-to)22-27
Number of pages6
JournalMutation Research - Genetic Toxicology and Environmental Mutagenesis
Publication statusPublished - Mar 1 2017


  • E. coli
  • Mismatch repair (MMR)
  • Mutagenesis
  • Mutagenic repair
  • Nucleotide excision repair (NER)
  • O-alkylguanine-DNA alkyltransferase (AGT)
  • O-alkylthymine

ASJC Scopus subject areas

  • Genetics
  • Health, Toxicology and Mutagenesis


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