In the fission yeast, Schizosacchromyces pombe, blocks to DNA replication elongation trigger the intra-S phase checkpoint that leads to the activation of the Cds1 kinase. Cds1 is required to both stabilize stalled replication forks and to prevent premature entry into mitosis. Interestingly, although Cds1 is essential to maintain the viability of mutants defective in DNA replication elongation, my study shows that mutants defective in DNA replication initiation require the Chk1 kinase, rather than Cds1. This suggests that failed initiation events can lead to activation of the DNA damage checkpoint independent of the intra-S phase checkpoint. This might result from reduced origin firing that leads to an increase in replication fork stalling or replication fork collapse that activates the G2 DNA damage checkpoint. I refer to the Chk1-dependent, Cds1-independent phenotype as the rid phenotype (for replication initiation defective). The data shows that Chk1 is active in rid mutants when grown under semi-permissive conditions, and rid mutant viability is dependent on the DNA damage checkpoint, and surprisingly Mrc1, an adaptor protein required for activation of Cds1. Mutations in Mrc1 that prevent activation of Cds1 have no effect on its ability to support rid mutant viability, suggesting that Mrc1 has a checkpoint-independent role in maintaining the viability of mutants defective in DNA replication initiation. Like Mrc1, Swi1 and Swi3 have been hypothesized as a part of the replication fork protection complex (RFPC). They are required for maintaining the viability of rid mutants, but are not essential for activation of Chk1 in response to failed initiation events. This suggests that Mrc1 in conjunction with Swi1 and Swi3 function in a similar pathway to alleviate replicative stress resulting from defects in DNA replication initiation. Using flow cytometry, I demonstrate that inhibition of DNA replication initiation has no significant impact on the duration of S phase, suggesting dormant origins might be activated in response to defects in DNA replication initiation. Fission yeast Rad22 is implicated in forming nuclear foci in response to damaged DNA. By tracking YFP-labeled Rad22, I screened for potential DNA damage in rid mutants grown at semi-permissive temperatures, and the results show that DNA damage occurs as the result of defects in DNA replication initiation. I also identified camptothecin, a DNA topoisomerase I inhibitor that can at low dose (2 µM) induce the rid phenotype, suggesting our assay (Chk1-dependent, Cds1-independent) can be used to screen small molecule inhibitors that interfere with the initiation step of DNA replication.

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