Initiation of DNA replication requires the function of MCM gene products, which participate in ensuring that DNA replication occurs only once in the cell cycle. Expression of all mammalian genes of the MCM family is induced by growth stimulation, unlike yeast, and the mRNA levels peak at G1/S boundary. In this study, we examined the transcriptional activities of isolated human MCM gene promoters. Human MCM5 and MCM6 promoters with mutation in the E2F sites failed in promoter regulation following serum stimulation and exogenous E2F expression. In addition, we identi®ed a novel E2F-like sequence in human MCM6 promoter which cooperates with the authentic E2F sites in E2F-dependent regulation. Forced expression of E2F1 could induce expression of all members of the endogenous MCM genes in rat embryonal ®broblast REF52 cells. Our results demonstrated that the growth-regulated expression of mammalian MCM5 and MCM6 genes, and presumably other MCM members, is primarily regulated by E2F through binding to multiple E2F sites in the promoters.

DNA damage as a result of environmental stress is recognized by sensor proteins that trigger repair mechanisms, or, if repair is unsuccessful, initiate apoptosis. Defects in DNA damage-induced apoptosis promote genomic instability and tumorigenesis. The protein ataxia-telangiectasia mutated (ATM) is activated by DNA double strand breaks and regulates apoptosis via p53. Here we show that FOXO3 interacts with the ATM-Chk2-p53 complex, augments phosphorylation of the complex and induces the formation of nuclear foci in cells upon DNA damage. FOXO3 is essential for DNA damage-induced apoptosis and conversely FOXO3 requires ATM, Chk2, and phosphorylated p53 isoforms to trigger apoptosis as a result of DNA damage. Under these conditions FOXO3 may also play a role in regulating chromatin retention of phosphorylated p53. These results suggest an essential link between FOXO3 and the ATM-Chk2-p53-mediated apoptotic program following DNA damage.

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