Niu Hengyao, Potenski Catherine J, Epshtein Anastasiya, Sung Patrick, Klein Hannah L
a Molecular Biophysics and Biochemistry, Yale University School of Medicine , New Haven , CT, USA.
b Department of Molecular and Cellular Biochemistry , Indiana University , Bloomington , IN , USA.
Cell Cycle. 2016;15(3):331-6. doi: 10.1080/15384101.2015.1128594. Epub 2015 Dec 30.
The replicative DNA polymerases insert ribonucleotides into DNA at a frequency of approximately 1/6500 nucleotides replicated. The rNMP residues make the DNA backbone more susceptible to hydrolysis and can also distort the helix, impeding the transcription and replication machineries. rNMPs in DNA are efficiently removed by RNaseH2 by a process called ribonucleotides excision repair (RER). In the absence of functional RNaseH2, rNMPs are subject to cleavage by Topoisomerase I, followed by further processing to result in deletion mutations due to slippage in simple DNA repeats. The topoisomerase I-mediated cleavage at rNMPs results in DNA ends that cannot be ligated by DNA ligase I, a 5'OH end and a 2'-3' cyclic phosphate end. In the budding yeast, the mutation level in RNaseH2 deficient cells is kept low via the action of the Srs2 helicase and the Exo1 nuclease, which collaborate to process the Top1-induced nick with subsequent non-mutagenic gap filling. We have surveyed other helicases and nucleases for a possible role in reducing mutagenesis at Top1 nicks at rNMPs and have uncovered a novel role for the RecQ family helicase Sgs1 in this process.
复制性DNA聚合酶以大约每复制6500个核苷酸就插入1个核糖核苷酸的频率将核糖核苷酸插入DNA中。核糖核苷酸单磷酸（rNMP）残基使DNA主链更容易被水解，还会扭曲螺旋结构，阻碍转录和复制机制。DNA中的rNMP可通过一种称为核糖核苷酸切除修复（RER）的过程被核糖核酸酶H2（RNaseH2）有效去除。在缺乏功能性RNaseH2的情况下，rNMP会被拓扑异构酶I切割，随后进一步加工，由于简单DNA重复序列中的滑动而导致缺失突变。拓扑异构酶I在rNMP处介导的切割会产生DNA末端，这些末端无法被DNA连接酶I连接，一个是5'羟基末端，另一个是2'-3'环磷酸末端。在芽殖酵母中，RNaseH2缺陷细胞中的突变水平通过Srs2解旋酶和Exo1核酸酶的作用保持在较低水平，它们协同作用处理Top1诱导的切口，随后进行非诱变的缺口填充。我们已经研究了其他解旋酶和核酸酶在降低rNMP处Top1切口诱变中的可能作用，并在这一过程中发现了RecQ家族解旋酶Sgs1的新作用。
