Takano K, Nakamura T, Sekiguchi M
Department of Biochemistry, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
Mutat Res. 1991 Jan;254(1):37-44. doi: 10.1016/0921-8777(91)90038-q.
Escherichia coli possesses 2 types of O6-methylguanine-DNA methyltransferases, one inducible and the other constitutive. These enzymes are coded by the ada and the ogt genes, respectively. Using a synthetic ogt-specific probe, we mapped ogt at 29.4 min, near the 5'-flanking region of the nirR gene, on the E. coli chromosome. To elucidate the roles of the 2 types of methyltransferases in DNA repair, we constructed mutant strains which lack either one or both of the genes. In either the ada+ or the ada- background, the ogt mutation had no effect on cell survival after N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment. On the other hand, ada- ogt- cells were more prone to mutation as compared to the ada- ogt+ cells exposed to MNNG. The frequency of spontaneous mutation of cells defective in either one or both of the genes was the same, however, the introduction of the ogt+ plasmid into the cells produced a 2-3-fold decrease in the frequency of spontaneous mutation. O6-Methylguanine-DNA methyltransferases appear to eliminate premutagenic DNA lesions not only from cells exposed to alkylating agents but also from those grown in the absence of the agents.
大肠杆菌有两种O6-甲基鸟嘌呤-DNA甲基转移酶，一种是诱导型的，另一种是组成型的。这些酶分别由ada和ogt基因编码。我们使用合成的ogt特异性探针，将ogt定位在大肠杆菌染色体上nirR基因5'侧翼区域附近的29.4分钟处。为了阐明这两种甲基转移酶在DNA修复中的作用，我们构建了缺失其中一个或两个基因的突变菌株。在ada+或ada-背景下，ogt突变对经N-甲基-N'-硝基-N-亚硝基胍（MNNG）处理后的细胞存活没有影响。另一方面，与暴露于MNNG的ada-ogt+细胞相比，ada-ogt-细胞更容易发生突变。任一基因或两个基因都有缺陷的细胞自发突变频率相同，然而，将ogt+质粒导入细胞后，自发突变频率降低了2至3倍。O6-甲基鸟嘌呤-DNA甲基转移酶似乎不仅能从暴露于烷化剂的细胞中，也能从未接触这些试剂而生长的细胞中消除致突变前的DNA损伤。
