Although p21 upregulation is required to block cell-cycle progression following many types of genotoxic insult, UV irradiation triggers p21 proteolysis. The significance of the increased p21 turnover is unclear and might be associated with DNA repair. While the role of p21 in nucleotide excision repair (NER) remains controversial, recent reports have explored its effect on translesion DNA synthesis (TLS), a process that avoids replication blockage during S phase. Herein, we analyze the effect of p21 on different PCNA-driven processes including DNA replication, NER and TLS. Whereas only the CDK-binding domain of p21 is required for cell-cycle arrest in unstressed cells, neither the CDK-binding nor the PCNA-binding domain of p21 is able to block early and late steps of NER. Intriguingly, through its PCNA-binding domain, p21 inhibits the interaction of the TLS polymerase, pol η (pol eta), with PCNA and impairs the assembly of pol η foci after UV. Moreover, this obstruction correlates with accumulation of phosphorylated H2AX and increased apoptosis. By showing that p21 is a negative regulator of PCNA-pol η interaction, our data unveil a link between efficient TLS and UV-induced degradation of p21.

The checkpoint kinases Chk1 and ATR are broadly known for their role in the response to the accumulation of damaged DNA. Because Chk1 activation requires its phosphorylation by ATR, it is expected that ATR or Chk1 down-regulation should cause similar alterations in the signals triggered by DNA lesions. Intriguingly, we found that Chk1, but not ATR, promotes the progression of replication forks after UV irradiation. Strikingly, this role of Chk1 is independent of its kinase-domain and of its partnership with Claspin. Instead, we demonstrate that the ability of Chk1 to promote replication fork progression on damaged DNA templates relies on its recently identified proliferating cell nuclear antigen-interacting motif, which is required for its release from chromatin after DNA damage. Also supporting the importance of Chk1 release, a histone H2B-Chk1 chimera, which is permanently immobilized in chromatin, is unable to promote the replication of damaged DNA. Moreover, inefficient chromatin dissociation of Chk1 impairs the efficient recruitment of the specialized DNA polymerase η (pol η) to replication-associated foci after UV. Given the critical role of pol η during translesion DNA synthesis (TLS), these findings unveil an unforeseen facet of the regulation by Chk1 of DNA replication. This kinase-independent role of Chk1 is exclusively associated to the maintenance of active replication forks after UV irradiation in a manner in which Chk1 release prompts TLS to avoid replication stalling. T he checkpoint kinases ATR and Chk1 are central factors in the DNA damage response (1). During the S phase checkpoint, ATR is activated at single-stranded DNA (ssDNA) and this event, in turn, activates the effector kinase Chk1. Although ATR remains associated with the DNA, activated Chk1 rapidly spreads throughout the whole nucleus. Within the nucleoplasm, Chk1 delays the progression through S phase via phosphorylation of key target genes (2, 3).Several lines of evidence suggest that the activities of ATR and Chk1 are also required for proper unperturbed S phase progression. In fact, ATR or Chk1 loss leads to embryonic lethality (4-7), and Chk1 heterozygosity is associated with multiple defects, including a miscoordinated cell cycle and increased apoptosis (8).A contribution of Chk1 to replication fork stability during unperturbed DNA replication was identified and characterized in detail (9-11). Chk1 activity promotes the maintenance of global replication rates by regulating origin firing. In line with these observations, the monoallelic expression of the mutant Chk1 S317A, which is not phosphorylated by ATR, impairs fork elongation (12). Together, these results reveal an unambiguous role of the Chk1 kinase during unperturbed DNA replication. Intriguingly, recent reports described a kinase-independent effect of Chk1 on DNA replication-associated events. Scorah and colleagues discovered a proliferating cell nuclear antigen (PCNA) binding motif of Chk1 (Chk1_TRFF motif) required for the efficient dissociation of Chk1 from chromatin a...