The current concept regarding cell cycle regulation of DNA replication is that Cdt1, together with origin recognition complex and CDC6 proteins, constitutes the machinery that loads the minichromosome maintenance complex, a candidate replicative helicase, onto chromatin during the G 1 phase. The actions of origin recognition complex and CDC6 are suppressed through phosphorylation by cyclin-dependent kinases (Cdks) after S phase to prohibit rereplication. It has been suggested in metazoan cells that the function of Cdt1 is blocked through binding to an inhibitor protein, geminin. However, the functional relationship between the Cdt1-geminin system and Cdks remains to be clarified. In this report, we demonstrate that human Cdt1 is phosphorylated by cyclin A-dependent kinases dependent on its cyclin-binding motif. Cdk phosphorylation resulted in the binding of Cdt1 to the F-box protein Skp2 and subsequent degradation. In contrast, in vitro DNA binding activity of Cdt1 was inhibited by the phosphorylation. However, geminin binding to Cdt1 was not affected by the phosphorylation. Finally we provide evidence that inactivation of Cdk1 results in Cdt1 dephosphorylation and rebinding to chromatin in murine FT210 cells synchronized around the G 2 /M phase. Taken together, these findings suggest that Cdt1 function is also negatively regulated by the Cdk phosphorylation independent of geminin binding.

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The Epstein-Barr virus (EBV) can choose between two alternative lifestyles; latent or lytic replication. In the latent state, the EBV genomic DNA, which exists as a closed circular plasmid, appears to behave just like host chromosomal DNA and it has been demonstrated recently that replication of OriP-containing plasmids is indeed dependent on the chromosomal initiation factors, ORC2 and Cdt1. On the other hand, in the viral productive cycle, the EBV genome is amplified 100- to 1000-fold by the viral replication machinery. EBV productive DNA replication occurs at discrete sites in nuclei, called replication compartments and the lytic programme arrests cell cycle progression and changes the cellular environment greatly. It has been revealed recently that the EBV lytic programme promotes an S-phase like cellular condition, which most favours viral lytic replication. This review describes recent advances regarding the molecular basis of EBV DNA replication during latent and lytic infections and then refers to cellular circumstances after induction of the lytic replication of EBV. Based on the molecular mechanism for the EBV lifestyle, purposeful induction of the lytic form of EBV infection is now advocated as one of the strategies for specific destruction of Epstein-Barr virus (EBV)-associated malignancies where the virus is latently infected.

Eukaryotic cells are equipped with machinery to monitor and repair damaged DNA. Herpes simplex virus (HSV) DNA replication occurs at discrete sites in nuclei, the replication compartment, where viral replication proteins cluster and synthesize a large amount of viral DNA. In the present study, HSV infection was found to elicit a cellular DNA damage response, with activation of the ataxia-telangiectasia-mutated (ATM) signal transduction pathway, as observed by autophosphorylation of ATM and phosphorylation of multiple downstream targets including Nbs1, Chk2, and p53, while infection with a UV-inactivated virus or with a replication-defective virus did not. Activated ATM and the DNA damage sensor MRN complex composed of Mre11, Rad50, and Nbs1 were recruited and retained at sites of viral DNA replication, probably recognizing newly synthesized viral DNAs as abnormal DNA structures. These events were not observed in ATM-deficient cells, indicating ATM dependence. In Nbs1-deficient cells, HSV infection induced an ATM DNA damage response that was delayed, suggesting a functional MRN complex requirement for efficient ATM activation. However, ATM silencing had no effect on viral replication in 293T cells. Our data open up an interesting question of how the virus is able to complete its replication, although host cells activate ATM checkpoint signaling in response to the HSV infection.

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In an earlier report, we described a DNA helicase that is specifically induced upon infection of Vero cells with herpes simplex virus 1. We have purified this enzyme to near homogeneity and found it to consist of three polypeptides with molecular weights of 120,000, 97,000, and 70,000. Immunochemical analysis has shown these polypeptides to be the products of three of the genes UL52, UL5, and UL8 that are required for replication of a plasmid containing a herpes simplex 1 origin (oris). In addition to helicase activity, the enzyme contains a tightly associated DNA primase. Thus, the three-subunit enzyme is a helicase-primase complex that may prime lagging-strand synthesis as it unwinds DNA at the viral replication fork.The 153-kilobase genome of herpes simplex virus 1 (HSV-1) contains both cis-and trans-acting elements that function in viral DNA replication (1). The cis-acting elements correspond to the origins of DNA replication (oris and oriL) (2-4), and the trans-acting elements very likely code for most and possibly all of the enzymes required for HSV-1 DNA replication. The nucleotide sequence of the entire HSV-1 genome has been determined (5), allowing assignment ofHSV-1 genes and their products to specific open reading frames. Seven of these open reading frames have been shown to be necessary and sufficient for the replication in trans of plasmids containing either origin of DNA replication, oriL or oris (6).These open reading frames also correspond to seven complementation groups known to be essential for HSV-1 DNA replication (7)(8)(9). Of the seven open reading frames, three have thus far been identified and shown to encode the herpes DNA polymerase (Pol) (10), a single-stranded DNA-binding protein (ICP8) (11), and the oris-binding protein (UL9) (12).A double-stranded DNA-binding protein whose role in DNA replication is unknown is encoded by the fourth open reading frame (UL42) (13).In this report we show that the HSV-1-induced DNA helicase that we have identified (14) consists of three polypeptides encoded by the three remaining open reading frames UL5, UL8, and UL52. We have also found that a DNA primase activity is tightly associated with the three-subunit enzyme, establishing the presence of an HSV-1-encoded helicase-primase complex in HSV-1-infected cells.

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