Sassanfar M, Samson L
Charles A. Dana Laboratory of Toxicology, Harvard School of Public Health, Boston, Massachusetts 02115.
J Biol Chem. 1990 Jan 5;265(1):20-5.
Saccharomyces cerevisiae contains a DNA repair methyltransferase (MTase) that repairs O6-methylguanine. Methyl groups are irreversibly transferred from O6-methylguanine in DNA to a 25-kilodalton protein in S. cerevisiae cell extracts, and methyl transfer is accompanied by the formation of S-methylcysteine. The yeast MTase is expressed at approximately 150 molecules/cell in exponentially growing yeast cultures but is not detectable in stationary phase cells. Unlike mammalian and bacterial MTases, the yeast MTase is very temperature-sensitive, having a half-life of about 4 min at 37 degrees C, which may explain why others have failed to detect it. Like other DNA repair MTases, the S. cerevisiae MTase repairs O6-methylguanine more efficiently in double-stranded DNA than in single-stranded DNA. Synthesis of the yeast DNA MTase is apparently not inducible by sublethal exposures to alkylating agent, but rather MTase activity is depleted in cells exposed to low doses of alkylating agent. Judging from its molecular weight and substrate specificity, the yeast DNA MTase is more closely related to mammalian MTases than to Escherichia coli MTases.
酿酒酵母含有一种修复O6-甲基鸟嘌呤的DNA修复甲基转移酶（MTase）。甲基从DNA中的O6-甲基鸟嘌呤不可逆地转移到酿酒酵母细胞提取物中的一种25千道尔顿的蛋白质上，并且甲基转移伴随着S-甲基半胱氨酸的形成。酵母MTase在指数生长的酵母培养物中以大约150个分子/细胞的水平表达，但在稳定期细胞中无法检测到。与哺乳动物和细菌的MTase不同，酵母MTase对温度非常敏感，在37℃下半衰期约为4分钟，这可能解释了为什么其他人未能检测到它。与其他DNA修复MTase一样，酿酒酵母MTase在双链DNA中比在单链DNA中更有效地修复O6-甲基鸟嘌呤。酵母DNA MTase的合成显然不会因亚致死剂量的烷基化剂暴露而被诱导，而是在暴露于低剂量烷基化剂的细胞中MTase活性会降低。从其分子量和底物特异性判断，酵母DNA MTase与哺乳动物MTase的关系比与大肠杆菌MTase的关系更密切。
