Chetsanga C J, Lindahl T
Nucleic Acids Res. 1979 Aug 10;6(11):3673-84. doi: 10.1093/nar/6.11.3673.
Double-stranded DNA containing 7-methylguanine residues whose imidazole rings have been opened, i.e. 2,6-diamino-4-hydroxy-5-N-methylformamido-pyrimidine residues, may be prepared by treatment of DNA with dimethyl sulfate followed by prolonged incubation at pH 11.4. These substituted formamidopyrimidine residues are actively removed from DNA by a DNA glycosylase present in E. coli cell extracts. The enz;me shows no apparent cofactor requirement and has a molecular weight of about 30 000. The release of ring-opened 7-methyl-guanine residues is due to a previously unrecognized activity, different from the three known E. coli DNA glycosylases that release uracil, 3-methyladenine, and hypoxanthine from DNA. This enzyme may serve to repair a major secondary alkylation product in DNA. In addition, it may remove nonmethylated purines, whose imidazole rings have been opened, from X-irradiated DNA.
含有咪唑环已打开的7-甲基鸟嘌呤残基（即2,6-二氨基-4-羟基-5-N-甲基甲酰胺基嘧啶残基）的双链DNA，可以通过用硫酸二甲酯处理DNA，然后在pH 11.4下长时间孵育来制备。这些取代的甲酰胺基嘧啶残基可被大肠杆菌细胞提取物中存在的一种DNA糖基化酶从DNA中有效去除。该酶没有明显的辅因子需求，分子量约为30000。开环的7-甲基鸟嘌呤残基的释放是由于一种以前未被认识的活性，不同于三种已知的从DNA中释放尿嘧啶、3-甲基腺嘌呤和次黄嘌呤的大肠杆菌DNA糖基化酶。这种酶可能用于修复DNA中的一种主要二级烷基化产物。此外，它可能从X射线照射的DNA中去除咪唑环已打开的非甲基化嘌呤。
