Carraway M, Youderian P, Marinus M G
Genetics. 1987 Jul;116(3):343-7. doi: 10.1093/genetics/116.3.343.
The mismatch repair system of Escherichia coli K12 removes mispaired bases from DNA. Mismatch repair can occur on either strand of DNA if it lacks N6-methyladenines within 5'-GATC-3' sequences. In hemimethylated heteroduplexes, repair occurs preferentially on the unmethylated strand. If both strands are fully methylated, repair is inhibited. Mutant (dam-) strains of E. coli defective in the adenine methylase that recognizes 5'-GATC-3' sequences (Dam), and therefore defective in mismatch repair, show increased spontaneous mutation rates compared to otherwise isogenic dam+ hosts. We have isolated and characterized 91 independent mutations that arise as a consequence of the Dam- defect in a plasmid-borne phage P22 repressor gene, mnt. The majority of these mutations are A:T----G:C transitions that occur within six base pairs of the two 5'-GATC-3' sequences in the mnt gene. In contrast, the spectrum of mnt- mutations in a dam+ host is comprised of a majority of insertions of IS elements and deletions that do not cluster near Dam recognition sites. These results show that Dam-directed post-replicative mismatch repair plays a significant role in the rectification of potential transition mutations in vivo, and suggest that sequences associated with Dam recognition sites are particularly prone to replication or repair errors.
大肠杆菌K12的错配修复系统可从DNA中去除错配碱基。如果DNA的任何一条链在5'-GATC-3'序列中缺乏N6-甲基腺嘌呤，错配修复就会发生。在半甲基化的异源双链体中，修复优先发生在未甲基化的链上。如果两条链都完全甲基化，修复就会受到抑制。与其他同基因的dam+宿主相比，识别5'-GATC-3'序列（Dam）的腺嘌呤甲基化酶有缺陷，因此错配修复有缺陷的大肠杆菌突变体（dam-）菌株显示出自发突变率增加。我们已经分离并鉴定了91个独立的突变，这些突变是由质粒携带的噬菌体P22阻遏基因mnt中的Dam-缺陷导致的。这些突变中的大多数是A:T----G:C转换，发生在mnt基因中两个5'-GATC-3'序列的六个碱基对内。相比之下，dam+宿主中mnt-突变的谱主要由IS元件的插入和缺失组成，这些插入和缺失不会聚集在Dam识别位点附近。这些结果表明，Dam指导的复制后错配修复在体内纠正潜在的转换突变中起重要作用，并表明与Dam识别位点相关的序列特别容易发生复制或修复错误。
