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DNA-synthesis with methylated poly(dA-dT) templates: possible role of O4-methylthymine as a pro-mutagenic base
Abstract:
The alternating copolymer poly(dA-dT) has been methylated with either dimethyl sulphate (DMS) or N-methyl-N-nitrosourea (MNU) and the levels of the various methylation products determined. In addition to the methylated adenines formed by both methylating agents, MNU resulted also in the formation of 3-methylthymine, O4-methylthymine and phosphotriesters. The methylated polymers have been ution of complementary and non-complementary nucleotides determined. With the DMS methylated template no wrong nucleotide incorporation was detectable, but with the MNU methylated polymer the incorporation of dGMP was observed. The amount of dGMP incorporated correlated with the level of O4-methylthymine in the template over the range of methylation studied. The results indicate that O4-methylthymine is capable of miscoding on a one-to-one basis while the products of DMS methylation (1-, 3- and 7-methyladenines), and also possibly the phosphotriesters, do not lead to any misincorporation.
Insights
Dimethyl sulfate (DMS) and N-methyl-N-nitrosourea (MNU) methylation of poly(dA-dT) revealed O4-methylthymine as a potent mutagen. This specific DNA lesion causes misincorporation of guanine nucleotides, impacting genetic stability.
Area of Science:
- Molecular Biology
- Biochemistry
- Chemical Mutagenesis
Background:
- DNA methylation is a critical process in cellular function and can be induced by various chemical agents.
- Understanding the mutagenic potential of different DNA adducts is essential for assessing chemical toxicity and DNA repair mechanisms.
Purpose of the Study:
- To investigate the mutagenic effects of DNA methylation on the alternating copolymer poly(dA-dT) using dimethyl sulfate (DMS) and N-methyl-N-nitrosourea (MNU).
- To determine the specific DNA lesions responsible for miscoding during DNA replication.
Main Methods:
- Methylation of poly(dA-dT) using DMS and MNU.
- Quantification of various methylation products, including methylated adenines, methylthymines, and phosphotriesters.
- Assessing nucleotide incorporation opposite methylated bases using DNA polymerase assays.
Main Results:
- MNU methylation produced 3-methylthymine, O4-methylthymine, and phosphotriesters, in addition to methylated adenines found with DMS.
- DMS-methylated poly(dA-dT) showed no detectable nucleotide misincorporation.
- MNU-methylated poly(dA-dT) exhibited significant dGMP incorporation, directly correlating with the level of O4-methylthymine.
Conclusions:
- O4-methylthymine is a highly mutagenic lesion, capable of causing one-to-one miscoding by directing the incorporation of guanine.
- Methylated adenines (1-, 3-, 7-methyladenines) and potentially phosphotriesters do not appear to cause significant misincorporation under these conditions.