Olsson M, Lindahl T
J Biol Chem. 1980 Nov 25;255(22):10569-71.
O6-Methylguanine residues disappear from alkylated DNA by an inducible repair process in Escherichia coli. The reaction can be studied in a cell-free system, using DNA treated with a radioactive methylating agent as substrate. The disappearance of labeled O6-methylguanine from DNA is not accompanied by release of radioactive material in an acid-soluble form. Instead, the methyl group of O6-methylguanine appears to be transferred enzymatically to a protein cysteine residue. Radioactively labeled S-methylcysteine has been identified in protein hydrolysates after incubation of the alkylated DNA with a partly purified E. coli methyltransferase activity. The radioactive amino acid residue shows properties identical with those of S-methylcysteine by automatic amino acid analysis and paper chromatography in several solvent systems. Moreover, oxidation of the compound with hydrogen peroxide yields a product which co-chromatographs with S-methylcysteine sulfone.
在大肠杆菌中，O6-甲基鸟嘌呤残基通过一种可诱导的修复过程从烷基化DNA中消失。该反应可以在无细胞系统中进行研究，使用经放射性甲基化剂处理的DNA作为底物。DNA中标记的O6-甲基鸟嘌呤的消失并没有伴随着放射性物质以酸溶性形式释放出来。相反，O6-甲基鸟嘌呤的甲基似乎通过酶促反应转移到蛋白质的半胱氨酸残基上。在用部分纯化的大肠杆菌甲基转移酶活性孵育烷基化DNA后，在蛋白质水解物中鉴定出了放射性标记的S-甲基半胱氨酸。通过自动氨基酸分析以及在几种溶剂系统中的纸色谱分析，放射性氨基酸残基显示出与S-甲基半胱氨酸相同的性质。此外，用过氧化氢氧化该化合物会产生一种与S-甲基半胱氨酸砜共色谱的产物。
