title: Removal of O6-methylguanine from plant DNA in vivo is accelerated under conditions of clastogenic adaptation.
abstract: Previously it was shown that the clastogenic efficiency of high doses of alkylating agents in plant root meristems can be reduced significantly by conditioning pretreatment with either a low dose of the same agents, a sublethal heat shock, or heavy metal salts. The molecular mechanisms responsible for these protective effects are still unclear. Here we report on the quantification of O6-methylguanine [O6-MeG] by immuno-slot-blot analysis in DNA of root tip meristems of field bean (Vicia faba) seedlings under conditions of clastogenic adaptation. When root tips were pretreated with a low, conditioning dose of N-methyl-N-nitrosourea (MNU, 10(-4) M, 1 hour) 2 hours before exposure to a high dose of the same clastogen (10(-3) M, 1 hour), the frequency of chromatid aberrations was reduced by more than 50% at a recovery time of 1 B hours, as compared to treatment with the high dose alone. The same was observed when conditioning pretreatment was by a sublethal heat shock [10 minutes, 40 degrees C] or a heavy metal salt (Cd(NO)3, 10(-7) M, 1 hour). The frequency of O6-MeG immediately after exposure to a conditioning and a subsequent challenge treatment was reduced by 43% as compared to treatment with only the high dose. At a recovery time of 18 hours the corresponding frequency of adducts was reduced by 68.3% (related to the initial level) after treatment with the high dose alone, and by 81.3% under adaptive conditions. Sublethal heat shock or heavy metal salt used as conditioning pretreatments also resulted in a decrease of adducts immediately after treatment with the challenge dose. From these data and from prevention of the effects by pretreatment with cycloheximide or O6-benzylguanine we conclude that under conditions of clastogenic adaptation O6-MeG is more efficiently removed from the DNA, presumably by induction of an alkyl acceptor protein such as O6-methylguanine-DNA methyltransferase [MGMT]. This could explain the observed protective effects (clastogenic adaptation.
authors.affiliation: Institut für Pflanzengenetik und Kulturpflanzenforschung, Gatersleben, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Baranczewski
authors.forename: P
authors.initials: P
authors.lastname: Nehls
authors.forename: R
authors.initials: R
authors.lastname: Rieger
authors.forename: M F
authors.initials: MF
authors.lastname: Rajewsky
authors.forename: I
authors.initials: I
authors.lastname: Schubert
mesh_terms: D000222:Adaptation, Physiological / Q000187:drug effects / Q000235:genetics*; D000477:Alkylating Agents / Q000633:toxicity; D002869:Chromosome Aberrations; D018736:DNA Adducts / Q000032:analysis / Q000187:drug effects* / Q000378:metabolism; D004249:DNA Damage / Q000187:drug effects; D018744:DNA, Plant / Q000187:drug effects*; D007887:Fabaceae / Q000187:drug effects / Q000235:genetics; D006147:Guanine / Q000031:analogs & derivatives* / Q000032:analysis / Q000378:metabolism; D015151:Immunoblotting; D018519:Meristem / Q000187:drug effects / Q000235:genetics; D008770:Methylnitrosourea / Q000633:toxicity; D009153:Mutagens / Q000633:toxicity*; D010946:Plants, Medicinal
chemical_list: D000477:Alkylating Agents; D018736:DNA Adducts; D018744:DNA, Plant; D009153:Mutagens; D006147:Guanine; D008770:Methylnitrosourea; C008449:O-(6)-methylguanine