Mol Clifford D, Arvai Andrew S, Begley Thomas J, Cunningham Richard P, Tainer John A
Department of Molecular Biology MB4, Skaggs Institute for Chemical Biology, 10550 North Torrey Pines Rd, La Jolla, CA 92037, USA.
J Mol Biol. 2002 Jan 18;315(3):373-84. doi: 10.1006/jmbi.2001.5264.
The repair of T:G mismatches in DNA is key for maintaining bacterial restriction/modification systems and gene silencing in higher eukaryotes. T:G mismatch repair can be initiated by a specific mismatch glycosylase (MIG) that is homologous to the helix-hairpin-helix (HhH) DNA repair enzymes. Here, we present a 2.0 A resolution crystal structure and complementary mutagenesis results for this thermophilic HhH MIG enzyme. The results suggest that MIG distorts the target thymine nucleotide by twisting the thymine base approximately 90 degrees away from its normal anti position within DNA. We propose that functionally significant differences exist in DNA repair enzyme extrahelical nucleotide binding and catalysis that are characteristic of whether the target base is damaged or is a normal base within a mispair. These results explain why pure HhH DNA glycosylases and combined glycosylase/AP lyases cannot be interconverted by simply altering their functional group chemistry, and how broad-specificity DNA glycosylase enzymes may weaken the glycosylic linkage to allow a variety of damaged DNA bases to be excised.
DNA中T:G错配的修复对于维持细菌限制/修饰系统以及高等真核生物中的基因沉默至关重要。T:G错配修复可由一种与螺旋-发夹-螺旋（HhH）DNA修复酶同源的特异性错配糖基化酶（MIG）启动。在此，我们展示了这种嗜热HhH MIG酶的2.0埃分辨率晶体结构及互补诱变结果。结果表明，MIG通过将胸腺嘧啶碱基从其在DNA中正常的反式位置扭转约90度，使目标胸腺嘧啶核苷酸发生扭曲。我们提出，DNA修复酶的螺旋外核苷酸结合和催化存在功能上的显著差异，这取决于目标碱基是受损的还是错配中的正常碱基。这些结果解释了为什么单纯的HhH DNA糖基化酶和组合的糖基化酶/AP裂解酶不能通过简单改变其官能团化学结构而相互转换，以及广谱特异性DNA糖基化酶如何削弱糖苷键以切除多种受损的DNA碱基。
