Xiao W, Samson L
Department of Molecular and Cellular Toxicology, Harvard School of Public Health, Boston, MA 02115.
Proc Natl Acad Sci U S A. 1993 Mar 15;90(6):2117-21. doi: 10.1073/pnas.90.6.2117.
Three genes that participate in the repair of DNA alkylation damage were recently cloned from Saccharomyces cerevisiae: the MGT1 O6-methylguanine DNA methyltransferase gene, the MAG 3-methyladenine DNA glycosylase gene, and the APN1 apurinic/apyrimidinic (AP) endonuclease gene. Altering the expression levels of these three genes produced significant changes in the S. cerevisiae spontaneous mutation rate. Spontaneous mutation increased in the absence of the MGT1 DNA methyltransferase, presumably because unrepaired, spontaneously produced, O6-alkylguanine lesions mispair during replication. Moreover, changing the ratios of the MAG 3-methyladenine DNA glycosylase and the APN1 AP endonuclease had profound effects on spontaneous mutation rates. In the absence of APN1, the overexpression of MAG increased spontaneous mutation, and the underexpression of MAG decreased spontaneous mutation. We infer that the MAG glycosylase acts upon spontaneously produced 3-alkyladenine and 7-alkylguanine DNA lesions to produce mutagenic abasic sites, and that if the repair of these abasic sites is not initiated by the APN1 AP endonuclease they cause mutations during replication. Our results indicate that eukaryotic cells harbor endogenous metabolites that alkylate nuclear DNA at both oxygens and nitrogens.
最近从酿酒酵母中克隆出了三个参与DNA烷基化损伤修复的基因：MGT1 O6-甲基鸟嘌呤DNA甲基转移酶基因、MAG 3-甲基腺嘌呤DNA糖基化酶基因和APN1无嘌呤/无嘧啶（AP）核酸内切酶基因。改变这三个基因的表达水平会使酿酒酵母的自发突变率发生显著变化。在缺乏MGT1 DNA甲基转移酶的情况下，自发突变增加，这可能是因为未修复的、自发产生的O6-烷基鸟嘌呤损伤在复制过程中发生错配。此外，改变MAG 3-甲基腺嘌呤DNA糖基化酶和APN1 AP核酸内切酶的比例对自发突变率有深远影响。在缺乏APN1的情况下，MAG的过表达会增加自发突变，而MAG的低表达则会降低自发突变。我们推断，MAG糖基化酶作用于自发产生的3-烷基腺嘌呤和7-烷基鸟嘌呤DNA损伤，产生诱变的无碱基位点，并且如果这些无碱基位点的修复不是由APN1 AP核酸内切酶启动，它们会在复制过程中导致突变。我们的结果表明，真核细胞含有在内源代谢产物作用下会使核DNA的氧原子和氮原子都发生烷基化的物质。
