Abstract
Cells slow replication in response to DNA damage. This slowing was the first DNA damage checkpoint response discovered and its study led to the discovery of the central checkpoint kinase, Ataxia Telangiectasia Mutated (ATM). Nonetheless, the manner by which the S-phase DNA damage checkpoint slows replication is still unclear. The checkpoint could slow bulk replication by inhibiting replication origin firing or slowing replication fork progression, and both mechanisms appear to be used. However, assays in various systems using different DNA damaging agents have produced conflicting results as to the relative importance of the two mechanisms. Furthermore, although progress has been made in elucidating the mechanism of origin regulation in vertebrates, the mechanism by which forks are slowed remains unknown. We review both past and present efforts towards determining how cells slow replication in response to damage and try to resolve apparent conflicts and discrepancies within the field. We propose that inhibition of origin firing is a global checkpoint mechanism that reduces overall DNA synthesis whenever the checkpoint is activated, whereas slowing of fork progression reflects a local checkpoint mechanism that only affects replisomes as they encounter DNA damage and therefore only affects overall replication rates in cases of high lesion density.
Highlights
DNA damage is a constant problem that cells must deal with to maintain viability
Merrick et al observed in vertebrate cells that low doses of ionizing radiation (IR) (1-5Gy), which cause 100s of lesions, prevented origin firing and did not slow forks but that MMS, which causes 10,000s of lesions, prevented origin firing and robustly slowed forks [96]. These observations suggest that DNA damaging agents reduce replication by different means; IR primarily prevents origin firing whereas MMS much more robustly slows replication forks
Despite some important differences between model systems, we propose checkpoint regulation of both origin firing and fork progression play important roles in checkpoint-dependent replication slowing
Summary
DNA damage is a constant problem that cells must deal with to maintain viability. Proper duplication and segregation of undamaged genetic material to daughter cells is essential for survival. MRN mutants display residual slowing even in response to IR fully capable of activating Chk and preventing origin firing [41] These observations suggest the ATM RDS phenotype is due to defects in origindependent and independent, presumably fork slowing, events. Vertebrates slow replication in response to low doses of IR while fission yeast do not [27,95] In these model systems, exposure to IR prevents origin firing, demonstrating a global checkpoint response. These observations suggest that DNA damaging agents reduce replication by different means; IR primarily prevents origin firing whereas MMS much more robustly slows replication forks These observations support a direct correlation between DNA lesion density and the observed effect on replication fork progression. All doses of IR effectively prevent origin firing in an ATM-dependent manner, but higher doses of IR may induce slightly more slowing because lesion density is moderately increased and more forks directly affected and slowed, in an ATM-dependent manner
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