Teo S H, Jackson S P
Wellcome/CRC Institute and Department of Zoology, University of Cambridge, UK.
EMBO J. 1997 Aug 1;16(15):4788-95. doi: 10.1093/emboj/16.15.4788.
DNA ligases catalyse the joining of single and double-strand DNA breaks, which is an essential final step in DNA replication, recombination and repair. Mammalian cells have four DNA ligases, termed ligases I-IV. In contrast, other than a DNA ligase I homologue (encoded by CDC9), no other DNA ligases have hitherto been identified in Saccharomyces cerevisiae. Here, we report the identification and characterization of a novel gene, LIG4, which encodes a protein with strong homology to mammalian DNA ligase IV. Unlike CDC9, LIG4 is not essential for DNA replication, RAD52-dependent homologous recombination nor the repair of UV light-induced DNA damage. Instead, it encodes a crucial component of the non-homologous end-joining (NHEJ) apparatus, which repairs DNA double-strand breaks that are generated by ionizing radiation or restriction enzyme digestion: a function which cannot be complemented by CDC9. Lig4p acts in the same DNA repair pathway as the DNA end-binding protein Ku. However, unlike Ku, it does not function in telomere length homeostasis. These findings indicate diversification of function between different eukaryotic DNA ligases. Furthermore, they provide insights into mechanisms of DNA repair and suggest that the NHEJ pathway is highly conserved throughout the eukaryotic kingdom.
DNA连接酶催化单链和双链DNA断裂的连接，这是DNA复制、重组和修复过程中必不可少的最后一步。哺乳动物细胞有四种DNA连接酶，分别称为连接酶I-IV。相比之下，除了一种DNA连接酶I同源物（由CDC9编码）外，酿酒酵母中迄今尚未鉴定出其他DNA连接酶。在此，我们报告了一个新基因LIG4的鉴定和特征，该基因编码一种与哺乳动物DNA连接酶IV具有高度同源性的蛋白质。与CDC9不同，LIG4对于DNA复制、RAD52依赖性同源重组以及紫外线诱导的DNA损伤修复并非必不可少。相反，它编码非同源末端连接（NHEJ）装置的一个关键组分，该装置修复由电离辐射或限制酶消化产生的DNA双链断裂：这一功能不能由CDC9互补。Lig4p与DNA末端结合蛋白Ku在相同的DNA修复途径中起作用。然而，与Ku不同，它在端粒长度稳态中不起作用。这些发现表明不同真核生物DNA连接酶之间功能的多样化。此外，它们为DNA修复机制提供了见解，并表明NHEJ途径在整个真核生物界高度保守。
