p53 accumulates but is functionally impaired when DNA synthesis is blocked

p53 is required for the induction of a G(1) and/or G(2) irreversible arrest after γ irradiation (IR), whereas blocked DNA replication causes a p53-independent S-phase arrest. We have examined the response to p53 when DNA synthesis is blocked by hydroxyurea (HU) or aphidicolin or when DNA is damaged by γ IR. Similarly to γ IR, blocked DNA synthesis induces high levels of phosphorylated nuclear p53. Surprisingly, several (but not all) p53 transcriptional targets that are rapidly induced by γ IR are weakly or not induced when DNA replication is blocked. Moreover, the p53 response to γ IR is inhibited by pretreatment of cells with HU or aphidicolin, suggesting that blocked DNA replication prevents p53 from being fully active as a transcription factor. HU-induced stabilization of p53 neither requires functional ATM (ataxia telangiectasia mutated), nor interferes with the γ IR-dependent activation of the ATM kinase. Thus, stalled replication forks activate kinases that modify and stabilize p53, yet act downstream of ATM to impair p53 transcriptional activity. The ramifications of this novel regulation of p53 are discussed.