Shivji M K, Grey S J, Strausfeld U P, Wood R D, Blow J J
Imperial Cancer Research Fund, Clare Hall Laboratories, South Mimms, Hertfordshire, UK.
Curr Biol. 1994 Dec 1;4(12):1062-8. doi: 10.1016/s0960-9822(00)00244-x.
DNA that is damaged by ultraviolet (UV) light is repaired predominantly by nucleotide excision-repair, a process requiring the DNA polymerase auxiliary factor PCNA. UV-irradiation also induces the production of Cip1 protein via activation of p53. Cip1 is an inhibitor of the cyclin-dependent kinases, which are required for the cell cycle to proceed through the G1/S-phase transition and initiate DNA replication. Inhibition by Cip1 probably causes the block to initiation of DNA replication that is seen in irradiated cells. Cip1 also directly inhibits the function of PCNA during DNA synthesis. As nucleotide excision-repair requires PCNA, the physiological relevance of PCNA inhibition by Cip1 is currently unclear.
We show that nucleotide excision-repair of UV-damaged DNA occurs in extracts of Xenopus eggs, and that this reaction is PCNA-dependent. The repair reaction is not inhibited by Cip1, even when the level of PCNA is reduced 100-fold so that it becomes limiting for DNA repair. By contrast, Cip1 strongly suppresses the function of PCNA in replicative DNA synthesis under these conditions.
Cip1 can potentially inhibit DNA replication in Xenopus egg extracts by inhibiting the cyclin-dependent kinase function required for the initiation of replication forks, and also by inhibiting PCNA function. The inhibition of PCNA is selective for its function in DNA replication, however, as Cip1 does not affect PCNA function in nucleotide excision-repair. The induction of Cip1 in response to DNA damage, therefore, allows repair to continue in the genome under conditions in which replication is severely inhibited.
受紫外线（UV）损伤的DNA主要通过核苷酸切除修复来修复，这一过程需要DNA聚合酶辅助因子PCNA。紫外线照射还通过激活p53诱导Cip1蛋白的产生。Cip1是细胞周期蛋白依赖性激酶的抑制剂，细胞周期需要这些激酶来推进G1/S期转换并启动DNA复制。Cip1的抑制作用可能导致受照射细胞中DNA复制起始受阻。Cip1在DNA合成过程中也直接抑制PCNA的功能。由于核苷酸切除修复需要PCNA，目前尚不清楚Cip1对PCNA的抑制作用在生理上的相关性。
我们发现，非洲爪蟾卵提取物中发生了紫外线损伤DNA的核苷酸切除修复，且该反应依赖于PCNA。即使PCNA水平降低100倍，使其成为DNA修复的限制因素，修复反应也不会受到Cip1的抑制。相比之下，在这些条件下，Cip1强烈抑制PCNA在复制性DNA合成中的功能。
Cip1可能通过抑制复制叉起始所需的细胞周期蛋白依赖性激酶功能以及抑制PCNA功能，来抑制非洲爪蟾卵提取物中的DNA复制。然而，Cip1对PCNA的抑制作用对其在DNA复制中的功能具有选择性，因为Cip1不影响PCNA在核苷酸切除修复中的功能。因此，在DNA损伤时诱导产生的Cip1，使得在复制受到严重抑制的情况下，基因组中的修复仍能继续进行。
