Schizosaccharomyces pombe Ddb1 Is functionally Linked to the Replication Checkpoint Pathway*

Schizosaccharomyces pombe Ddb1 is homologous to the mammalian DDB1 protein, which has been implicated in damaged-DNA recognition and global genomic repair. However, a recent study suggested that the S. pombe Ddb1 is involved in cell division and chromosomal segregation. Here, we provide evidence that the S. pombe Ddb1 is functionally linked to the replication checkpoint control gene cds1. We show that the S. pombe strain lacking ddb1 has slow growth due to delayed replication progression. Flow cytometric analysis shows an extensive heterogeneity in DNA content. Furthermore, the Δddb1 strain is hypersensitive to UV irradiation in S phase and is unable to tolerate a prolonged replication block imposed by hydroxyurea. Interestingly, the Δddb1 strain exhibits a high level of the Cds1 kinase activity during passage through S phase. Moreover, mutation of the cds1 gene relieves the defects observed in Δddb1 strain. The results suggest that many of the defects observed in Δddb1 cells are linked to an aberrant activation of Cds1, and that Ddb1 is functionally linked to Cds1.