Johnson R E, Haracska L, Prakash S, Prakash L
Sealy Center for Molecular Science, University of Texas Medical Branch, Galveston, Texas 77555-1061, USA.
Mol Cell Biol. 2001 May;21(10):3558-63. doi: 10.1128/MCB.21.10.3558-3563.2001.
UV light-induced DNA lesions block the normal replication machinery. Eukaryotic cells possess DNA polymerase eta (Poleta), which has the ability to replicate past a cis-syn thymine-thymine (TT) dimer efficiently and accurately, and mutations in human Poleta result in the cancer-prone syndrome, the variant form of xeroderma pigmentosum. Here, we test Poleta for its ability to bypass a (6-4) TT lesion which distorts the DNA helix to a much greater extent than a cis-syn TT dimer. Opposite the 3' T of a (6-4) TT photoproduct, both yeast and human Poleta preferentially insert a G residue, but they are unable to extend from the inserted nucleotide. DNA Polzeta, essential for UV induced mutagenesis, efficiently extends from the G residue inserted opposite the 3' T of the (6-4) TT lesion by Poleta, and Polzeta inserts the correct nucleotide A opposite the 5' T of the lesion. Thus, the efficient bypass of the (6-4) TT photoproduct is achieved by the combined action of Poleta and Polzeta, wherein Poleta inserts a nucleotide opposite the 3' T of the lesion and Polzeta extends from it. These biochemical observations are in concert with genetic studies in yeast indicating that mutations occur predominantly at the 3' T of the (6-4) TT photoproduct and that these mutations frequently exhibit a 3' T-->C change that would result from the insertion of a G opposite the 3' T of the (6-4) TT lesion.
紫外线诱导的DNA损伤会阻碍正常的复制机制。真核细胞拥有DNA聚合酶η（Poleta），它能够高效且准确地越过顺式胸腺嘧啶-胸腺嘧啶（TT）二聚体进行复制，而人类Poleta的突变会导致易患癌症的综合征——着色性干皮病的变异形式。在此，我们测试Poleta越过（6-4）TT损伤的能力，该损伤比顺式胸腺嘧啶-胸腺嘧啶二聚体更严重地扭曲了DNA螺旋结构。在（6-4）TT光产物的3'端胸腺嘧啶对面，酵母和人类Poleta都优先插入一个鸟嘌呤残基，但它们无法从插入的核苷酸处延伸。DNA聚合酶ζ（Polzeta）对于紫外线诱导的诱变至关重要，它能有效地从Poleta在（6-4）TT损伤的3'端胸腺嘧啶对面插入的鸟嘌呤残基处延伸，并且Polzeta在损伤的5'端胸腺嘧啶对面插入正确的核苷酸腺嘌呤。因此，（6-4）TT光产物的有效越过是通过Poleta和Polzeta的联合作用实现的，其中Poleta在损伤的3'端胸腺嘧啶对面插入一个核苷酸，而Polzeta从该核苷酸处延伸。这些生化观察结果与酵母中的遗传学研究一致，表明突变主要发生在（6-4）TT光产物的3'端胸腺嘧啶处，并且这些突变经常表现出3'端胸腺嘧啶到胞嘧啶的变化，这是由于在（6-4）TT损伤的3'端胸腺嘧啶对面插入一个鸟嘌呤所导致的。
