The relationship between mutagenicity in His G46 Salmonella and the O(6)-guanine alkylation in bacterial DNA by monofunctional methanesulphonates.
The influence of the O(6)-alkylation in Salmonella typhimurium cells by monofunctional methanesulphonates on their mutagenicity in strains TA1535 and TA100 was examined using a specially modified method that allows DNA-binding studies in bacteria. All the tested monofunctional compounds, methylmethanesulphonate (MMS; very strong Sn2 reactivity), ethylmethanesulphonate (EMS; moderate Sn1 and Sn2 reactivity) and isopropylmethanesulphonate (iPMS; strong Sn1 reactivity) formed significant amounts of O(6)-Palkylguanine. A comparison between the results in a pure in vitro system and the situation in the bacterial cells revealed significant differences. Despite having different alkylating reactivities, MMS and iPMS produced similar amounts of O(6)-adducts in bacterial DNA, probably owing to the much lower hydrolysis rate of MMS when compared with that of iPMS. The O(6):N7 ratio was, however, ten-fold higher in the case of iPMS, which reflects its clearly higher Sn1 reactivity. Owing to the formation of O(6)-adducts, all three test compounds induced back mutations in strain TA1535, which is not capable of performing error-prone repair. Whereas MMS showed a much higher mutagenic activity in strain TA100, because of the large amount of N-alkylation, iPMS induced practically identical mutagenicity in both strains, which indicates that the pkM101 plasmid is not significantly involved in iPMS mutagenicity. Evidently only strong Sn2-type monofunctional alkylating compounds induce back mutation in strain TA100 by the pkM101-mediated error-prone mechanism. This underlines the importance of the O(6)-alkylation as promutagenic lesion in strains containing the His G46 mutation. Surprisingly, EMS showed a higher mutagenicity in strain TA1535 than in TA100.
Eder
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Institute of Toxicology, University of Würzburg, Versbacher Str. 9, D-8700 Würzburg, Federal Republic of Germany.
Wiedenmann
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Deininger
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C
Kütt
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