Laboratory of Molecular Genetics, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709.
Proc Natl Acad Sci U S A. 2013 Nov 12;110(46):18596-601. doi: 10.1073/pnas.1310849110. Epub 2013 Oct 28.
The enzyme ribonucleotide reductase (RNR) plays a critical role in the production of deoxynucleoside-5'-triphosphates (dNTPs), the building blocks for DNA synthesis and replication. The levels of the cellular dNTPs are tightly controlled, in large part through allosteric control of RNR. One important reason for controlling the dNTPs relates to their ability to affect the fidelity of DNA replication and, hence, the cellular mutation rate. We have previously isolated a set of mutants of Escherichia coli RNR that are characterized by altered dNTP pools and increased mutation rates (mutator mutants). Here, we show that one particular set of RNR mutants, carrying alterations at the enzyme's allosteric specificity site, is characterized by relatively modest dNTP pool deviations but exceptionally strong mutator phenotypes, when measured in a mutational forward assay (>1,000-fold increases). We provide evidence indicating that this high mutability is due to a saturation of the DNA mismatch repair system, leading to hypermutability and error catastrophe. The results indicate that, surprisingly, even modest deviations of the cellular dNTP pools, particularly when the pool deviations promote particular types of replication errors, can have dramatic consequences for mutation rates.
酶核苷酸还原酶（RNR）在脱氧核苷-5'-三磷酸（dNTP）的生成中起着关键作用，dNTP 是 DNA 合成和复制的构建块。细胞内 dNTP 的水平受到严格控制，这在很大程度上是通过 RNR 的变构控制实现的。控制 dNTP 的一个重要原因与它们影响 DNA 复制保真度的能力有关，从而影响细胞突变率。我们之前已经分离出一组大肠杆菌 RNR 的突变体，其特点是 dNTP 池发生改变，突变率增加（诱变突变体）。在这里，我们表明，一组特定的 RNR 突变体，其酶的变构特异性位点发生改变，当在突变正向测定中测量时，其具有相对适度的 dNTP 池偏差，但表现出异常强的诱变表型（>1000 倍增加）。我们提供的证据表明，这种高突变率是由于 DNA 错配修复系统的饱和，导致高突变率和错误灾难。结果表明，令人惊讶的是，即使细胞内 dNTP 池的适度偏差，特别是当池偏差促进特定类型的复制错误时，也会对突变率产生巨大影响。
