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CUL-4 ubiquitin ligase maintains genome stability by restraining DNA-replication licensing


To maintain genome stability, DNA replication is strictly regulated to occur only once per cell cycle. In eukaryotes, the presence of ‘licensing proteins’ at replication origins during the G1 cell-cycle phase allows the formation of the pre-replicative complex1. The removal of licensing proteins from chromatin during the S phase ensures that origins fire only once per cell cycle1. Here we show that the CUL-4 ubiquitin ligase temporally restricts DNA-replication licensing in Caenorhabditis elegans. Inactivation of CUL-4 causes massive DNA re-replication, producing cells with up to 100C DNA content. The C. elegans orthologue of the replication-licensing factor Cdt1 (refs 2, 3) is required for DNA replication. C. elegans CDT-1 is present in G1-phase nuclei but disappears as cells enter S phase. In cells lacking CUL-4, CDT-1 levels fail to decrease during S phase and instead remain constant in the re-replicating cells. Removal of one genomic copy of cdt-1 suppresses the cul-4 re-replication phenotype. We propose that CUL-4 prevents aberrant re-initiation of DNA replication, at least in part, by facilitating the degradation of CDT-1.

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Figure 1: cul-4 mRNA levels.
Figure 2: cul-4 RNAi phenotype.
Figure 3: DNA re-replication in cul-4 RNAi seam cells.
Figure 4: CDT-1 expression in cul-4 RNAi animals.


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We thank Y. Kohara, A. Fire, G. Seydoux, R. Roy and V. Ambros for reagents; the Caenorhabditis Genetics Center for nematode strains; R. Santurri for technical help; and H. Cai and P. Shen for critical reading of the manuscript. This work was supported by a grant from the National Institutes of Health to E.T.K.

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Correspondence to Edward T. Kipreos.

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Zhong, W., Feng, H., Santiago, F. et al. CUL-4 ubiquitin ligase maintains genome stability by restraining DNA-replication licensing. Nature 423, 885–889 (2003).

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