title: Distinct pathways for repairing mutagenic lesions induced by methylating and ethylating agents.
abstract: DNA alkylation damage can be repaired by nucleotide excision repair (NER), base excision repair (BER) or by direct removal of alkyl groups from modified bases by O(6)-alkylguanine DNA alkyltransferase (AGT; E.C. 2.1.1.63). DNA mismatch repair (MMR) is also likely involved in this repair. We have investigated alkylation-induced mutagenesis in a series of NER- or AGT-deficient Escherichia coli strains, alone or in combination with defects in the MutS, MutL or MutH components of MMR. All strains used contained the F'prolac from strain CC102 (F'CC102) episome capable of detecting specifically lac GC to AT reverse mutations resulting from O(6)-alkylguanine. The results showed the repair of O(6)-methylguanine to be performed by AGT ≫ MMR > NER in order of importance, whereas the repair of O(6)-ethylguanine followed the order NER > AGT > MMR. Studies with double mutants showed that in the absence of AGT or NER repair pathways, the lack of MutS protein generally increased mutant frequencies for both methylating and ethylating agents, suggesting a repair or mutation avoidance role for this protein. However, lack of MutL or MutH protein did not increase alkylation-induced mutagenesis under these conditions and, in fact, reduced mutagenesis by the N-alkyl-N-nitrosoureas MNU and ENU. The combined results suggest that little or no alkylation damage is actually corrected by the mutHLS MMR system; instead, an as yet unspecified interaction of MutS protein with alkylated DNA may promote the involvement of a repair system other than MMR to avoid a mutagenic outcome. Furthermore, both mutagenic and antimutagenic effects of MMR were detected, revealing a dual function of the MMR system in alkylation-exposed cells.
authors: Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Tsushima, Okayama 700-8530, Japan.
authors: Kentaro
authors: K
authors: Taira
authors: Satomi
authors: S
authors: Kaneto
authors: Kota
authors: K
authors: Nakano
authors: Shinji
authors: S
authors: Watanabe
authors: Eizo
authors: E
authors: Takahashi
authors: Sakae
authors: S
authors: Arimoto
authors: Keinosuke
authors: K
authors: Okamoto
authors: Roel M
authors: RM
authors: Schaaper
authors: Kazuo
authors: K
authors: Negishi
authors: Tomoe
authors: T
authors: Negishi
mesh_terms: D000477:Alkylating Agents / Q000633:toxicity*; D004249:DNA Damage / Q000187:drug effects*; D004260:DNA Repair / Q000502:physiology*; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D055786:Gene Knockout Techniques; D023061:Gene Order; D005816:Genetic Complementation Test; D016296:Mutagenesis / Q000187:drug effects*; D009153:Mutagens / Q000633:toxicity*; D009154:Mutation; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000235:genetics / Q000378:metabolism; D010957:Plasmids / Q000235:genetics
chemical_list: D000477:Alkylating Agents; D009153:Mutagens; D019853:O(6)-Methylguanine-DNA Methyltransferase