Bruner S D, Nash H M, Lane W S, Verdine G L
Harvard University, Department of Chemistry and Chemical Biology, 12 Oxford Street, Cambridge, Massachusetts 02138, USA.
Curr Biol. 1998 Mar 26;8(7):393-403. doi: 10.1016/s0960-9822(98)70158-7.
Transversion mutations are caused by 8-oxoguanine (OG), a DNA lesion produced by the spontaneous oxidation of guanine nucleotides, which mis-pairs with adenine during replication. Resistance to this mutagenic threat is mediated by the GO system, the components of which are functionally conserved in bacteria and mammals. To date, only one of three GO system components has been identified in the budding yeast Saccharomyces cerevisiae, namely the OG:C-specific glycosylase/lyase yOgg1. Furthermore, S. cerevisiae has been reported to contain a unique glycosylase/lyase activity, yOgg2, which excises OG residues opposite adenines. Paradoxically, according to the currently accepted model, yOgg2 activity should increase the mutagenicity of OG lesions. Here we report the isolation of yOgg2 and the elucidation of its role in oxidative mutagenesis.
Borohydride-dependent cross-linking using an OG-containing oligonucleotide substrate led to the isolation of yOgg1 and a second protein, Ntg1, which had previously been shown to process oxidized pyrimidines in DNA. We demonstrate that Ntg1 has OG-specific glycosylase/lyase activity indistinguishable from that of yOgg2. Targeted disruption of the NTG1 gene resulted in complete loss of yOgg2 activity and yeast lacking NTG1 had an elevated rate of A:T to C:G transversions.
The Ntg1 and yOgg2 activities are encoded by a single gene. We propose that yOgg2 has evolved to process OG:A mis-pairs that have arisen through mis-incorporation of 8-oxo-dGTP during replication. Thus, the GO system in S. cerevisiae is fundamentally distinct from that in bacteria and mammals.
颠换突变由8-氧鸟嘌呤（OG）引起，OG是鸟嘌呤核苷酸自发氧化产生的一种DNA损伤，在复制过程中它会与腺嘌呤错配。对这种诱变威胁的抗性由GO系统介导，该系统的组成部分在细菌和哺乳动物中功能保守。迄今为止，在芽殖酵母酿酒酵母中仅鉴定出GO系统三个组成部分中的一个，即OG:C特异性糖基化酶/裂解酶yOgg1。此外，据报道酿酒酵母含有一种独特的糖基化酶/裂解酶活性，即yOgg2，它能切除与腺嘌呤相对的OG残基。矛盾的是，根据目前被广泛接受的模型，yOgg2活性应该会增加OG损伤的致突变性。在此我们报告yOgg2的分离及其在氧化诱变中的作用解析。
使用含OG的寡核苷酸底物进行硼氢化钠依赖性交联，导致yOgg1和第二种蛋白质Ntg1的分离，Ntg1此前已被证明可处理DNA中的氧化嘧啶。我们证明Ntg1具有与yOgg2无法区分的OG特异性糖基化酶/裂解酶活性。NTG1基因的靶向破坏导致yOgg2活性完全丧失，缺乏NTG1的酵母中A:T到C:G颠换的发生率升高。
Ntg1和yOgg2活性由单个基因编码。我们提出yOgg2已经进化以处理复制过程中因8-氧代-dGTP错掺入而产生的OG:A错配。因此，酿酒酵母中的GO系统与细菌和哺乳动物中的GO系统根本不同。
