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Nucleic Acids Res. 2007;35(17):5755-62. Epub 2007 Aug 23.

Single-stranded DNA ligation and XLF-stimulated incompatible DNA end ligation by the XRCC4-DNA ligase IV complex: influence of terminal DNA sequence.

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Department of Pathology, Biochemistry & Molecular Biology, Biological Sciences, Norris Comprehensive Cancer Center, University of Southern California Keck School of Medicine, Los Angeles, CA, USA.


The double-strand DNA break repair pathway, non-homologous DNA end joining (NHEJ), is distinctive for the flexibility of its nuclease, polymerase and ligase activities. Here we find that the joining of ends by XRCC4-ligase IV is markedly influenced by the terminal sequence, and a steric hindrance model can account for this. XLF (Cernunnos) stimulates the joining of both incompatible DNA ends and compatible DNA ends at physiologic concentrations of Mg2+, but only of incompatible DNA ends at higher concentrations of Mg2+, suggesting charge neutralization between the two DNA ends within the ligase complex. XRCC4-DNA ligase IV has the distinctive ability to ligate poly-dT single-stranded DNA and long dT overhangs in a Ku- and XLF-independent manner, but not other homopolymeric DNA. The dT preference of the ligase is interesting given the sequence bias of the NHEJ polymerase. These distinctive properties of the XRCC4-DNA ligase IV complex explain important aspects of its in vivo roles.

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