Physical interactions among Mcm proteins and effects of Mcm dosage on DNA replication in Saccharomyces cerevisiae

Reconstitution of licensed replication origins on Xenopus sperm nuclei using purified proteins

BACKGROUND

In order to ensure precise chromosome duplication, eukaryotes "license" their replication origins during late mitosis and early G1 by assembling complexes of Mcm2-7 onto them. Mcm2-7 are essential for DNA replication, but are displaced from origins as they initiate, thus ensuring that no origin fires more than once in a single cell cycle.

RESULTS

Here we show that a combination of purified nucleoplasmin, the origin recognition complex (ORC), Cdc6, RLF-B/Cdt1 and Mcm2-7 can promote functional origin licensing and the assembly of Mcm2-7 onto Xenopus sperm nuclei. The reconstituted reaction is inhibited by geminin, a specific RLF-B/Cdt1 inhibitor. Interestingly, the purified ORC used in the reconstitution had apparently lost the Orc6 subunit, suggesting that Orc6 is not essential for replication licensing. We use the reconstituted system to make a preliminary analysis of the different events occurring during origin assembly, and examine their nucleotide requirements. We show that the loading of Xenopus ORC onto chromatin is strongly stimulated by both ADP, ATP and ATP-gamma-S whilst the loading of Cdc6 and Cdt1 is stimulated only by ATP or ATP-gamma-S.

CONCLUSIONS

Nucleoplasmin, ORC, Cdc6, RLF-B/Cdt1 and Mcm2-7 are the only proteins required for functional licensing and the loading of Mcm2-7 onto chromatin. The requirement for nucleoplasmin probably only reflects a requirement to decondense sperm chromatin before ORC can bind to it. Use of this reconstituted system should allow a full biochemical analysis of origin licensing and Mcm2-7 loading.

MCM2 is an independent predictor of survival in patients with non-small-cell lung cancer

PURPOSE

Minichromosome maintenance protein 2 (MCM2) is a component of the prereplicative complex. It is essential for eukaryotic DNA replication and is only expressed in proliferating cells. The prognostic utility of MCM2 compared with Ki-67, another marker of proliferating cells, on survival of patients with non-small-cell lung cancer (NSCLC) was studied.

PATIENTS AND METHODS

We examined the immunohistochemical expression of MCM2 and Ki-67 in primary pathologic tumor specimens from 221 NSCLC patients. For each marker, the fraction of tumor cells with positive staining was assessed as a percentage and categorized into four groups: 0% to 24%, 25% to 49%, 50% to 74%, and > or = 75%. MCM2 and Ki-67 immunoreactivities were compared with each other, and associations with pathologic and clinical parameters predictive of survival were analyzed with the chi(2) test. Cox regression models were used to assess associations between MCM2 and Ki-67 and survival while controlling for confounders.

RESULTS

Independent variables significantly associated with survival were tumor stage, performance status, and staining category. Patients with less than 25% MCM2 immunoreactivity had a longer median survival time than patients with > or = 25% MCM2 immunoreactivity (46 v 31 months; P =.039) and a lower relative risk (RR) of death (RR, 0.55, 95% confidence interval, 0.34 to 0.88). There was no significant association between survival and Ki-67 expression.

CONCLUSION

Immunostaining of tumor cells for MCM2 is an independent prognostic parameter of survival for patients with NSCLC. Interpretable results can be obtained on more than 96% of paraffin-embedded specimens, and approximately 35% will be in the favorable subgroup, with less than 25% positively stained tumor cells. Whether MCM2 is predictive of response to therapy needs to be studied.

MCM2-7 proteins are essential components of prereplicative complexes that accumulate cooperatively in the nucleus during G1-phase and are required to establish, but not maintain, the S-phase checkpoint

A prereplicative complex (pre-RC) of proteins is assembled at budding yeast origins of DNA replication during the G1-phase of the cell cycle, as shown by genomic footprinting. The proteins responsible for this prereplicative footprint have yet to be identified but are likely to be involved in the earliest stages of the initiation step of chromosome replication. Here we show that MCM2-7 proteins are essential for both the formation and maintenance of the pre-RC footprint at the origin ARS305. It is likely that pre-RCs contain heteromeric complexes of MCM2-7 proteins, since degradation of Mcm2, 3, 6, or 7 during G1-phase, after pre-RC formation, causes loss of Mcm4 from the nucleus. It has been suggested that pre-RCs on unreplicated chromatin may generate a checkpoint signal that inhibits premature mitosis during S-phase. We show that, although mitosis does indeed occur in the absence of replication if MCM proteins are degraded during G1-phase, anaphase is prevented if MCMs are degraded during S-phase. Our data indicate that pre-RCs do not play a direct role in checkpoint control during chromosome replication.

Identification of Cdc6 protein domains involved in interaction with Mcm2 protein and Cdc4 protein in budding yeast cells

The Cdc6 protein (Cdc6p) has essential roles in regulating initiation of DNA replication. Cdc6p is recruited to origins of replication by the origin recognition complex (ORC) late in mitosis; Cdc6p in turn recruits minichromosome maintenance (Mcm) proteins to form the pre-replicative complex. Cdc6p is thought to interact with one or more Mcm proteins but this point has not yet been demonstrated. In the present study we observed that Cdc6p interacted significantly only with Mcm2p out of six Mcm proteins in yeast two-hybrid cells. Our results indicate that the interaction of Cdc6p with Mcm2p is specific, although we cannot exclude the possibility that the interaction might not be direct. In attempts to identify domains of Cdc6p important for interaction with Mcm2p, we tested interactions of various deleted versions of Cdc6p with Mcm2p and also with Cdc4p, which was previously known to interact with Cdc6p. The portion of Cdc6p from amino acid residues 51 to 394 was able to interact with Mcm2p. During the course of the studies we also discovered a previously undetected Cdc4p interaction domain between residues 51 and 394. Interestingly, when all six putative Cdc28 phosphorylation sites in Cdc6p were changed to alanine, a 6-7-fold increase in binding to Mcm2p was observed. This result suggests that unphosphorylated Cdc6p has higher affinity than phosphorylated Cdc6p for Mcm2p; this might partly explain the previous observation that Cdc6p failed to load Mcm proteins on replication origins during S phase when the cyclin-dependent protein kinase was active, thus helping to prevent the reinitiation of activated replicons.

Identification of two residues in MCM5 critical for the assembly of MCM complexes and Stat1-mediated transcription activation in response to IFN-gamma

In response to IFN-gamma, the latent cytoplasmic Stat1 (signal transducer and activator of transcription) proteins translocate into the nucleus and activate transcription. We showed previously that Stat1 recruits a group of nuclear proteins, among them MCM5 (minichromosome maintenance) and MCM3, for transcription activation. MCM5 directly interacts with the transcription activation domain (TAD) of Stat1 and enhances Stat1-mediated transcription activation. In this report, we identified two specific residues (R732, K734) in MCM5 that are required for the direct interaction between Stat1 and MCM5 both in vitro and in vivo. MCM5 containing mutations of R732/K734 did not enhance Stat1-mediated transcription activation in response to IFN-gamma. In addition, it also failed to form complexes with other MCM proteins in vivo, suggesting that these two residues may be important for an interaction domain in MCM5. Furthermore, MCM5 bearing mutations in its ATPase and helicase domains did not enhance Stat1 activity. In vitro binding assays indicate that MCM3 does not interact directly with Stat1, suggesting that the presence of MCM3 in the group of Stat1TAD-interacting proteins is due to the association of MCM3 with MCM5. Finally, gel filtration analyses of nuclear extracts from INF-gamma-treated cells demonstrate that there is a MCM5/3 subcomplex coeluting with Stat1. Together, these results strongly suggest that Stat1 recruits a MCM5/3 subcomplex through direct interaction with MCM5 in the process of IFN-gamma-induced gene activation.

Interactions between Mcm10p and other replication factors are required for proper initiation and elongation of chromosomal DNA replication in Saccharomyces cerevisiae

BACKGROUND

MCM10 is essential for the initiation of chromosomal DNA replication in Saccharomyces cerevisiae. Previous work showed that Mcm10p interacts with the Mcm2-7 protein complex that may be functioning as the replication-licensing factor. In addition, Mcm10p is required during origin activation and disassembly of the prereplicative complex, which allows smooth passage of replication forks.

RESULTS

We show that an mcm10 mutation causes a slow progression of DNA synthesis and a loss of chromosome integrity during the S phase and prevents entry into mitosis, despite apparent completion of chromosomal DNA replication at nonpermissive temperatures. Furthermore, Mcm10p interacts genetically with the origin recognition complex (ORC) and various replication elongation factors, including a subunit of DNA polymerases epsilon and delta. Mcm10p is an abundant protein (approximately 4 x 10(4) copies per haploid cell) that is almost exclusively localized in the chromatin and/or nuclear matrix fractions during all phases of the cell cycle. When it is visualized by the chromosome-spreading method followed by immunostaining, Mcm10p forms punctate foci on chromatin throughout the cell cycle and these foci mostly overlap with those of Orc1p, a component of ORC.

CONCLUSIONS

These results suggest that Mcm10p, like the Mcm2-7 proteins, is a critical component of the prereplication chromatin and acts together with ORC during the initiation of chromosomal DNA replication; in addition, Mcm10p plays an important role during the elongation of DNA replication.

Characterization of the yeast Cdc7p/Dbf4p complex purified from insect cells. Its protein kinase activity is regulated by Rad53p

The yeast Saccharomyces cerevisiae Cdc7p/Dbf4p protein kinase complex was purified to near homogeneity from insect cells. The complex efficiently phosphorylated yeast Mcm2p and less efficiently the remaining Mcm proteins or other replication proteins. Significantly, when pretreated with alkaline phosphatase, Mcm2p became completely inactive as a substrate, suggesting that it must be phosphorylated by other protein kinase(s) to be a substrate for the Cdc7p/Dbf4p complex. Mutant Cdc7p/Dbf4p complexes containing either Cdc7-1p or Dbf4-1 approximately 5p were also partially purified from insect cells and characterized in vitro. Furthermore, the autonomously replicating sequence binding activity of various dbf4 mutants was also analyzed. These studies suggest that the autonomously replicating sequence-binding and Cdc7p protein kinase activation domains of Dbf4p collaborate to form an active Cdc7p/Dbf4p complex and function during S phase in S. cerevisiae. It is shown that Rad53p phosphorylates the Cdc7p/Dbf4p complex in vitro and that this phosphorylation greatly inhibits the kinase activity of Cdc7p/Dbf4p. This result suggests that Rad53p controls the initiation of chromosomal DNA replication by regulating the protein kinase activity associated with the Cdc7p/Dbf4p complex.

MCM proteins in DNA replication

The MCM proteins are essential replication initiation factors originally identified as proteins required for minichromosome maintenance in Saccharomyces cerevisiae. The best known among them are a family of six structurally related proteins, MCM2-7, which are evolutionally conserved in all eukaryotes. The MCM2-7 proteins form a hexameric complex. This complex is a key component of the prereplication complex that assembles at replication origins during early G1 phase. New evidence suggests that the MCM2-7 proteins may be involved not only in the initiation but also in the elongation of DNA replication. Orchestration of the functional interactions between the MCM2-7 proteins and other components of the prereplication complex by cell cycle-dependent protein kinases results in initiation of DNA synthesis once every cell cycle.

Loss control of Mcm5 interaction with chromatin in cdc6-1 mutated in CDC-NTP motif

Saccharomyces cerevisiae Cdc6 plays an essential role in establishing and maintaining the prereplicative complex (pre-RC) by interacting with the origin recognition complex (ORC) and associating with chromatin origins. These interactions are required to load minichromosome maintenance proteins (MCMs) and other initiator proteins onto replication origins. Although the temperature-sensitive cdc6 mutant, cdc6-1, has been widely used for these studies, the molecular mechanism of the cdc6-1 mutation has been unclear. In this study, we have identified a base substitution at Gly260-->Asp, near the CDC-NTP motif. Using a chromatin immunoprecipitation assay (CHIP), we found that cdc6-1 fails to load Mcm5 onto the replication origins. Chromatin fractions were used to study Mcm5 binding in both the wildtype and mutant background. These studies indicated that Cdc6 is also involved in unloading Mcm5 from chromatin. Specifically, the cdc6-1 mutation protein, cdc6(G260D), which failed to load Mcm5 onto replication origins, also failed to unload the Mcm5 protein. Furthermore, the overexpression of wildtype CDC6 accelerated the unloading of Mcm5 from chromatin fractions. In the absence of functional Cdc6, the Mcm5 protein showed nonorigin binding to chromatin with the cell cycle arrested at the G1S phase transition. Our results suggested that the cdc6(G260D) mutant protein fails to assemble an operational replicative complex and that wildtype Cdc6 plays a role in preventing re-replication by controlling the unloading the MCMs from chromatin origins.

Isolation and characterization of various complexes of the minichromosome maintenance proteins of Schizosaccharomyces pombe

Minichromosome maintenance (Mcm) proteins 2-7 are highly conserved in eukaryotes and play an essential role in DNA replication. Here, we describe the reconstitution of the various complexes of the Mcm proteins of Schizosaccharomyces pombe using the baculovirus expression system. The simultaneous expression of all six of the Mcm proteins, as well as different combinations of these proteins, yielded several stable complexes that included the heterohexamer of Mcm2/3/4/5/6/7, the Mcm2/4/6/7 heterotetramer, the dimer of the Mcm4/6/7 heterotrimer, and the Mcm3/5 heterodimer. The purification and characterization of the biochemical properties of these complexes showed that only the dimeric complex of the Mcm4/6/7 heterotrimer possessed single stranded DNA-dependent ATPase, ATP-dependent single stranded DNA binding, and 3' to 5' DNA helicase activities. Consistent with these results, the interaction of either Mcm2 or Mcm3/5 with the Mcm4/6/7 complex resulted in the disassembly of the dimeric complex of Mcm4/6/7 and the loss of DNA helicase activity. These results suggest that the Mcm4/6/7 complex is a catalytic core of the Mcm complex and that Mcm2 and Mcm3/5 may be involved in the regulation of the activity of this complex.

DNA synthesis at individual replication forks requires the essential initiation factor Cdc45p

Cdc45p assembles at replication origins before initia tion and is required for origin firing in Saccharomyces cerevisiae. A heat-inducible cdc45 degron mutant was constructed that promotes rapid degradation of Cdc45p at the restrictive temperature. Consistent with a role in initiation, loss of Cdc45p in G(1) prevents all detectable DNA replication without preventing subsequent entry into mitosis. Loss of Cdc45p activity during S-phase blocks S-phase completion but not activation of replication checkpoints. Using density substitution, we show that after allowing replication fork establishment, Cdc45p inactivation prevents the subsequent progression of individual replication forks. This provides the first direct functional evidence that Cdc45p plays an essential role during elongation. Thus, like the large T antigen in SV40 replication, Cdc45p plays a central role in both initiation and elongation phases of chromosomal DNA replication.

Dpb11 controls the association between DNA polymerases alpha and epsilon and the autonomously replicating sequence region of budding yeast

Dpb11 is required for chromosomal DNA replication and the S-phase checkpoint in Saccharomyces cerevisiae. Here, we report detection of a physical complex containing Dpb11 and DNA polymerase epsilon (Dpb11-Polepsilon complex). During the S phase of the cell cycle, Dpb11 associated preferentially with DNA fragments containing autonomously replicating sequences (ARSs), at the same time as Polepsilon associated with these fragments. Association of Dpb11 and Polepsilon with these fragments was mutually dependent, suggesting that the Dpb11-Polepsilon complex associates with the ARS. Moreover, Dpb11 was required for the association of Polalpha-primase with the fragments. Thus, it seems likely that association of the Dpb11-Polepsilon complex with the ARS fragments is required for the association of the Polalpha-primase complex. Hydroxyurea inhibits late-origin firing in S. cerevisiae, and the checkpoint genes, RAD53 and MEC1, are involved in this inhibition. In the presence of hydroxyurea at temperatures permissive for cell growth, Polepsilon in dpb11-1 cells associated with early- and late-origin fragments. In wild-type cells, however, it associated only with early-origin fragments. This indicates that Dpb11 may also be involved in the regulation of late-origin firing. Overall, these results suggest that Dpb11 controls the association between DNA polymerases alpha and epsilon and the ARS.

Stepwise regulated chromatin assembly of MCM2-7 proteins

Acquisition of the competence to replicate requires the assembly of the MCM2-7 (minichromosome maintenance) protein complex onto pre-replicative chromatin, a step of the licensing reaction. This step is thought to occur through binding of a heterohexameric MCM complex containing the six related MCM subunits. Here we show that assembly of the MCM complex onto pre-replicative chromatin occurs through sequential stabilization of specific MCM subunits. Inhibition of licensing with 6-dimethylaminopurine results in chromatin containing specifically bound MCM4 and MCM6. A similar result was obtained by interference of the assembly reaction with an MCM3 antibody. The presence of chromatin-bound MCM intermediates was confirmed by reconstitution experiments in vitro with purified proteins and by the observation of an ordered association of MCM subunits with chromatin. These results indicate that the assembly of the MCM complex onto pre-replicative chromatin is regulated at the level of distinct subunits, suggesting an additional regulatory step in the formation of pre-replication complexes.

Clb/Cdc28 kinases promote nuclear export of the replication initiator proteins Mcm2-7

BACKGROUND

In the budding yeast Saccharomyces cerevisiae, the cyclin-dependent kinases of the Clb/Cdc28 family restrict the initiation of DNA replication to once per cell cycle by preventing the re-assembly of pre-replicative complexes (pre-RCs) at replication origins that have already initiated replication. This assembly involves the Cdc6-dependent loading of six minichromosome maintenance (Mcm) proteins, Mcm2-7, onto origins. How Clb/Cdc28 kinases prevent pre-RC assembly is not understood.

RESULTS

In living cells, the Mcm proteins were found to colocalize in a cell-cycle-regulated manner. Mcm2-4, 6 and 7 were concentrated in the nucleus in G1 phase, gradually exported to the cytoplasm during S phase, and excluded from the nucleus by G2 and M phase. Tagging any single Mcm protein with the SV40 nuclear localization signal made all Mcm proteins constitutively nuclear. In the absence of functional Cdc6, Clb/Cdc28 kinases were necessary and sufficient for efficient net nuclear export of a fusion protein between Mcm7 and the green fluorescent protein (Mcm7-GFP), whereas inactivation of these kinases at the end of mitosis coincided with the net nuclear import of Mcm7-GFP. In contrast, in the presence of functional Cdc6, which loads Mcm proteins onto chromatin, S-phase progression as well as Clb/Cdc28 kinases was required for Mcm-GFP export.

CONCLUSIONS

We propose that Clb/Cdc28 kinases prevent pre-RC reassembly in part by promoting the net nuclear export of Mcm proteins. We further propose that Mcm proteins become refractory to this regulation when they load onto chromatin and must be dislodged by DNA replication before they can be exported. Such an arrangement could ensure that Mcm proteins complete their replication function before they are removed from the nucleus.

Sequential MCM/P1 subcomplex assembly is required to form a heterohexamer with replication licensing activity

Replication licensing factor (RLF) is a multiprotein complex involved in ensuring that chromosomal DNA replicates only once in a single cell cycle. It comprises two components, termed RLF-M and RLF-B. Purified RLF-M consists of a mixture of complexes containing all six members of the MCM/P1 family of minichromosome maintenance proteins. The precise composition of these different complexes and their contribution to RLF-M activity has been unclear. Here we show that in Xenopus extracts, MCM/P1 proteins mainly form heterohexamers containing each of the six proteins. This heterohexamer is readily split into subcomplexes, whose interactions and subunit composition we characterize in detail. We show for the first time an ordered multistep assembly pathway by which the heterohexamer can be reformed from the subcomplexes. Importantly, this novel pathway is essential for DNA replication, since only the full heterohexamer can bind productively to chromatin and provide RLF-M activity.

Cell cycle regulation of human CDC6 protein. Intracellular localization, interaction with the human mcm complex, and CDC2 kinase-mediated hyperphosphorylation

The binding of mammalian MCM complexes to chromatin is cell cycle-regulated and under CDC2 kinase negative control. Here, we investigated the properties of mammalian CDC6 protein, a candidate regulator of MCM. The levels of CDC6 were relatively constant during the HeLa cell cycle. In asynchronous cells, CDC6 was mainly detected in the nuclei with immunostaining, but some CDC6 was not extractable with nonionic detergent. In contrast to the chromatin-bound MCM, this fraction of CDC6 was resistant to DNase I treatment, suggesting that it binds to the detergent- and nuclease-resistant nuclear structure. In S phase cells, CDC6 became detectable in the cytoplasm with immunostaining; however, the level of the bound CDC6 was unchanged. In G(2)/M phase cells, the level of the bound CDC6 was still maintained, which was hyperphosphorylated by CDC2 kinase. These data suggest that some CDC6 protein is associated with the specific nuclear structure throughout the cell cycle and that major binding sites on chromatin differ between MCM and CDC6. However, co-immunoprecipitation assays with chemical cross-linking indicated that a small part of the chromatin-bound MCM is present close to the bound CDC6.

Regulation of eukaryotic DNA replication and nuclear structure

In eukaryote, nuclear structure is a key component for the functions of eukaryotic cells. More and more evidences show that the nuclear structure plays important role in regulating DNA replication. The nuclear structure provides a physical barrier for the replication licensing, participates in the decision where DNA replication initiates, and organizes replication proteins as replication factory for DNA replication. Through these works, new concepts on the regulation of DNA replication have emerged, which will be discussed in this minireview.

Cell cycle control of Cdc7p kinase activity through regulation of Dbf4p stability

In Saccharomyces cerevisiae, the heteromeric kinase complex Cdc7p-Dbf4p plays a pivotal role at replication origins in triggering the initiation of DNA replication during the S phase. We have assayed the kinase activity of endogenous levels of Cdc7p kinase by using a likely physiological target, Mcm2p, as a substrate. Using this assay, we have confirmed that Cdc7p kinase activity fluctuates during the cell cycle; it is low in the G1 phase, rises as cells enter the S phase, and remains high until cells complete mitosis. These changes in kinase activity cannot be accounted for by changes in the levels of the catalytic subunit Cdc7p, as these levels are constant during the cell cycle. However, the fluctuations in kinase activity do correlate with levels of the regulatory subunit Dbf4p. The regulation of Dbf4p levels can be attributed in part to increased degradation of the protein in G1 cells. This G1-phase instability is cdc16 dependent, suggesting a role of the anaphase-promoting complex in the turnover of Dbf4p. Overexpression of Dbf4p in the G1 phase can partially overcome this elevated turnover and lead to an increase in Cdc7p kinase activity. Thus, the regulation of Dbf4p levels through the control of Dbf4p degradation has an important role in the regulation of Cdc7p kinase activity during the cell cycle.

Minichromosome maintenance as a genetic assay for defects in DNA replication

Minichromosome maintenance (mcm) is an effective genetic assay for mutants defective in DNA replication. Two classes of mcm mutants have been identified using this screen: those that differentially affect the activities of certain autonomously replicating sequences (ARSs) and those that uniformly affect the activities of all ARSs. The ARS-specific MCM genes are essential for the initiation of DNA replication. Among these are members of the MCM2-7 family that encode subunits of the preinitiation complex and MCM10, whose gene product interacts with members of the Mcm2-7 proteins. Among the ARS-nonspecific MCM gene products are chromosome transmission factors. Refinement of this genetic assay as a screening tool and further analysis of existing mcm mutants may reveal new replication initiation proteins.

Changes in association of the Xenopus origin recognition complex with chromatin on licensing of replication origins

During late mitosis and early G1, a series of proteins are assembled onto replication origins that results in them becoming ‘licensed’ for replication in the subsequent S phase. In Xenopus this first involves the assembly onto chromatin of the Xenopus origin recognition complex XORC, and then XCdc6, and finally the RLF-M component of the replication licensing system. In this paper we examine changes in the way that XORC associates with chromatin in the Xenopus cell-free system as origins become licensed. Restricting the quantity of XORC on chromatin reduced the extent of replication as expected if a single molecule of XORC is sufficient to specify a single replication origin. During metaphase, XOrc1 associated only weakly with chromatin. In early interphase, XOrc1 formed a strong complex with chromatin, as evidenced by its resistance to elution by 200 mM salt, and this state persisted when XCdc6 was assembled onto the chromatin. As a consequence of origins becoming licensed the association of XOrc1 and XCdc6 with chromatin was destabilised, and XOrc1 became susceptible to removal from chromatin by exposure to either high salt or high Cdk levels. At this stage the essential function for XORC and XCdc6 in DNA replication had already been fulfilled. Since high Cdk levels are required for the initiation of DNA replication, this ‘licensing-dependent origin inactivation’ may contribute to mechanisms that prevent re-licensing of replication origins once S phase has started.

Reduced dosage of a single fission yeast MCM protein causes genetic instability and S phase delay

MCM proteins are a conserved family of eukaryotic replication factors implicated in the initiation of DNA replication and in the discrimination between replicated and unreplicated chromatin. However, most mcm mutants in yeast arrest the cell cycle after bulk DNA synthesis has occurred. We investigated the basis for this late S phase arrest by analyzing the effects of a temperature-sensitive mutation in fission yeast cdc19(+)(mcm2(+)). cdc19-P1 cells show a dramatic loss of viability at the restrictive temperature, which is not typical of all S phase mutants. The cdc19-P1 cell cycle arrest requires an intact damage-response checkpoint and is accompanied by increased rates of chromosome loss and mitotic recombination. Chromosomes from cdc19-P1 cells migrate aberrantly in pulsed-field gels, typical of strains arrested with unresolved replication intermediates. The cdc19-P1 mutation reduces the level of the Cdc19 protein at all temperatures. We compared the effects of disruptions of cdc19(+)(mcm2(+)), cdc21(+)(mcm4(+)), nda4(+)(mcm5(+)) and mis5(+)(mcm6(+)); in all cases, the null mutants underwent delayed S phase but were unable to proceed through the cell cycle. Examination of protein levels suggests that this delayed S phase reflects limiting, but not absent, MCM proteins. Thus, reduced dosage of MCM proteins allows replication initiation, but is insufficient for completion of S phase and cell cycle progression.

The regulation of replication origin activation

At the start of the cell-division programme, proteins must be assembled onto replication origins to establish competence for initiation of DNA synthesis. At the correct moment, other effectors must then coordinate appropriate firing of the various origins to control entry into and progress through S phase. These processes are key targets of cell-cycle control, and understanding their regulation will provide a deeper knowledge of the mechanisms controlling cell proliferation.

Evidence for different MCM subcomplexes with differential binding to chromatin in Xenopus

MCM proteins are molecular components of the DNA replication licensing system in Xenopus. These proteins comprise a conserved family made up of six distinct members which have been found to associate in large protein complexes. We have used a combination of biochemical and cytological methods to study the association of soluble and chromatin-bound Xenopus MCM proteins during the cell cycle. In interphase, soluble MCM proteins are found organized in a core salt-resistant subcomplex that includes MCM subunits which are known to have high affinity for histones. The interphasic complex is modified at mitosis and the subunit composition of the resulting mitotic subcomplexes is distinct, indicating that the stability of the MCM complex is under cell cycle control. Moreover, we provide evidence that the binding of MCM proteins to chromatin may occur in sequential steps involving the loading of distinct MCM subunits. Comparative analysis of the chromatin distribution of MCM2, 3, and 4 shows that the binding of MCM4 is distinct from that of MCM2 and 3. Altogether, these data suggest that licensing of chromatin by MCMs occurs in an ordered fashion involving discrete subcomplexes.

Emerging mechanisms of eukaryotic DNA replication initiation

Recent research has focused on proteins important for early steps in replication in eukaryotes, and particularly on Cdc6/Cdc18, the MCMs, and Cdc45. Although it is still unclear exactly what role these proteins play, it is possible that they are analogous to initiation proteins in prokaryotes. One specific model is that MCMs form a hexameric helicase at replication forks, and Cdc6/Cdc18 acts as a 'clamp-loader' required to lock the MCMs around DNA. The MCMs appear to be the target of Cdc7-Dbf4 kinase acting at individual replication origins. Finally, Cdc45 interacts with MCMs and may shed light on how cyclin-dependent kinases activate DNA replication.

Ser727-dependent recruitment of MCM5 by Stat1alpha in IFN-gamma-induced transcriptional activation

Stat1alpha is a latent cytoplasmic transcription factor activated in response to interferon-gamma (IFN-gamma). The C-terminal 38 amino acids of Stat1alpha are required to trigger transcription and therefore may possibly serve as a transcription activation domain (TAD). Here we show that the C-terminus of Stat1alpha is an independent TAD which can interact with a specific group of nuclear proteins. Mutation of the Stat1 Ser727 and Leu724 decreases its transcriptional activity and affinity for the nuclear proteins. One of the interacting proteins was identified as MCM5, a member of the mini-chromosome maintenance (MCM) family involved in DNA replication. Both in vitro and in vivo interaction of Stat1alpha and MCM5 were demonstrated. Furthermore, the in vitro interaction required Ser727 and was enhanced by its phosphorylation. Transient over-expression of MCM5 enhanced transcriptional activation by Stat1alpha in a Ser727-dependent manner. Finally, changes in the level of nuclear localized MCM5 during the cell cycle correlated with the changes in transcriptional response to IFN-gamma acting through Stat1alpha. These results strongly suggest that MCM5 is recruited through interaction with Stat1alpha in a Ser727- and Leu724-dependent manner to play a role in optimal transcriptional activation.

Sld2, which interacts with Dpb11 in Saccharomyces cerevisiae, is required for chromosomal DNA replication

The DPB11 gene, which genetically interacts with DNA polymerase II (epsilon), one of three replicative DNA polymerases, is required for DNA replication and the S phase checkpoint in Saccharomyces cerevisiae. To identify factors interacting with Dbp11, we have isolated sld (synthetically lethal with dpb11-1) mutations which fall into six complementation groups (sld1 to -6). In this study, we characterized SLD2, encoding an essential 52-kDa protein. High-copy SLD2 suppressed the thermosensitive growth defect caused by dpb11-1. Conversely, high-copy DPB11 suppressed the temperature-sensitive growth defect caused by sld2-6. The interaction between Sld2 and Dpb11 was detected in a two-hybrid assay. This interaction was evident at 25 degreesC but not at 34 degreesC when Sld2-6 or Dpb11-1 replaced its wild-type protein. No interaction between Sld2-6 and Dpb11-1 could be detected even at 25 degreesC. Immunoprecipitation experiments confirmed that Dpb11 physically interacts with Sld2. sld2-6 cells were defective in DNA replication at the restrictive temperature, as were dpb11-1 cells. Further, in dpb11-1 and sld2-6 cells, the bubble-shaped replication intermediates formed in the region of the autonomously replicating sequence reduced quickly after a temperature shift. These results strongly suggest the involvement of the Dpb11-Sld2 complex in a step close to the initiation of DNA replication.

Schizosaccharomyces pombe Mcm3p, an essential nuclear protein, associates tightly with Nda4p (Mcm5p)

MCM proteins are required for the proper regulation of DNA replication. There are six MCM proteins in all eukaryotes which interact to form a large complex. We report the cloning of fission yeast mcm3 +. mcm3 + is essential and spores carrying a Delta mcm3 disruption arrest with an apparently replicated DNA content. The protein is found constitutively in the nucleus and levels remain constant throughout the cell cycle. Mcm3p binds particularly tightly to Nda4p (Mcm5p), but is loosely associated with the other Schizosaccharomyces pombe MCM proteins. Thus, Mcm3p is a peripheral MCM subunit.

Cell cycle- and chromatin binding state-dependent phosphorylation of human MCM heterohexameric complexes. A role for cdc2 kinase

The mammalian MCM protein family, presently with six members, exists in the nuclei in two forms, chromatin-bound and unbound. The former dissociates from chromatin with progression through the S phase. Recently, we have established a procedure to isolate chromatin-bound and unbound complexes containing all six human MCM (hMCM) proteins by immunoprecipitation. In the present study, we applied this procedure to HeLa cells synchronized in each of the G1, S, and G2/M phases and could detect hMCM heterohexameric complexes in all three. In addition, depending on the cell cycle and the state of chromatin association, hMCM2 and 4 in the complexes were found to variously change their phosphorylation states. Concentrating attention on G2/M phase hyperphosphorylation, we found hMCM2 and 4 in the complexes to be good substrates for cdc2/cyclin B in vitro. Furthermore, when cdc2 kinase was inactivated in temperature-sensitive mutant murine FT210 cells, the G2/M hyperphosphorylation of the murine MCM2 and MCM4 and release of the MCMs from chromatin in the G2 phase were severely impaired. Taken together, the data suggest that the six mammalian MCM proteins function and undergo cell cycle-dependent regulation as heterohexameric complexes and that phosphorylation of the complexes by cdc2 kinase may be one of mechanisms negatively regulating the MCM complex-chromatin association.

Multiple domains of fission yeast Cdc19p (MCM2) are required for its association with the core MCM complex

The members of the MCM protein family are essential eukaryotic DNA replication factors that form a six-member protein complex. In this study, we use antibodies to four MCM proteins to investigate the structure of and requirements for the formation of fission yeast MCM complexes in vivo, with particular regard to Cdc19p (MCM2). Gel filtration analysis shows that the MCM protein complexes are unstable and can be broken down to subcomplexes. Using coimmunoprecipitation, we find that Mis5p (MCM6) and Cdc21p (MCM4) are tightly associated with one another in a core complex with which Cdc19p loosely associates. Assembly of Cdc19p with the core depends upon Cdc21p. Interestingly, there is no obvious change in Cdc19p-containing MCM complexes through the cell cycle. Using a panel of Cdc19p mutants, we find that multiple domains of Cdc19p are required for MCM binding. These studies indicate that MCM complexes in fission yeast have distinct substructures, which may be relevant for function.

Cell cycle modulation of protein-DNA interactions at a human replication origin

We followed the variations of protein-DNA interactions occurring in vivo over the early firing replication origin located near the human lamin B2 gene, in IMR-90 cells synchronized in different moments of the cell cycle. In G0 phase cells no protection is present; as the cells progress in G1 phase an extended footprint covering over 100 bp appears, particularly marked at the G1/S border. As the cells enter S phase the protection shrinks to 70 bp and remains unchanged throughout this phase. In mitosis the protection totally disappears, only to reappear in its extended form as the cells move into the next G1. These variations are reminiscent of those corresponding to the formation of the pre- and post-replicative complexes described in yeast and Xenopus cells.

Cell cycle regulation of S phase entry in Saccharomyces cerevisiae

Eukaryotic DNA replication is restricted to a narrow window of the cell cycle called S phase, and occurs once and only once during each cell cycle. The combination of genetic and biochemical approaches in the budding yeast Saccharomyces cerevisiae has proven extremely helpful for studying the cell cycle regulation of S phase entry. This review will try to summarise the most recent discoveries which led to a new model to explain how entry into S phase is regulated in eukaryotic cells.

Initiation of DNA replication in eukaryotic cells

The recent identification of proteins that recognize origins of DNA replication and control the initiation of eukaryotic DNA replication has provided critical molecular tools to dissect this process. Dynamic changes in the assembly and disassembly of protein complexes at origins are important for the initiation of DNA replication and occur throughout the cell cycle. Herein, we review the key proteins required for the initiation of DNA replication, their involvement in the protein complex assembly at replication origins, and how the cell cycle machinery regulates this process.

The replication licensing system

The replication licensing system acts to ensure that no section of the genome is replicated more than once in a single cell cycle. Experiments using Xenopus egg extracts have revealed that the licensing system consists of two components, named RLF-M and RLF-B. Whereas the function of RLF-B is still unclear, RLF-M has been shown to consist of all six members of the MCM/P1 family proteins, which appear to be the structural component of the licensing system. The origin recognition complex (ORC) and Cdc6/Cdc18 are needed on chromatin before the licensing reaction can take place, although they are not themselves components of the licensing system. Cell cycle events and cyclin-dependent protein kinases (Cdks) also seem to be involved in controlling the licensing system to ensure once per cell cycle DNA replication. The subject of this review is to detail our current understanding of the licensing system and the way that it interacts with other components of the cell cycle machinery.

Persistent initiation of DNA replication and chromatin-bound MCM proteins during the cell cycle in cdc6 mutants

Faithful inheritance of genetic information requires that DNA be copied only once each cell cycle. Initiation of DNA replication involves the establishment of a prereplication complex (pre-RC) and subsequent activation by CDK/cyclins, converting the pre-RC to a post-RC. The origin recognition complex (ORC), Cdc6p, and the MCM proteins are required for establishing the pre-RC. We show that all six ORC subunits remain bound to chromatin throughout the cell cycle, whereas the MCM proteins cycle on and off, corresponding precisely to transitions of the RC. A newly isolated cdc6 mutant displays promiscuous initiation of DNA replication, increased nuclear DNA content, and constant MCM protein association with chromatin throughout the cell cycle. This gain-of-function cdc6 mutant ignores the negative controls imposed normally on initiation by the CDK/cyclins, suggesting that Cdc6p is a key mediator of once-per-cell-cycle control of DNA replication.

Mcm2 is a target of regulation by Cdc7-Dbf4 during the initiation of DNA synthesis

The initiation of DNA synthesis is an important cell cycle event that defines the beginning of S phase. This critical event involves the participation of proteins whose functions are regulated by cyclin dependent protein kinases (Cdks). The Mcm2-7 proteins are a family of six conserved proteins that are essential for the initiation of DNA synthesis in all eukaryotes. In Saccharomyces cerevisiae, members of the Mcm2-7 family undergo cell cycle-specific phosphorylation. Phosphorylation of Mcm proteins at the beginning of S phase coincides with the removal of these proteins from chromatin and the onset of DNA synthesis. In this study, we identified DBF4, which encodes the regulatory subunit of a Cdk-like protein kinase Cdc7-Dbf4, in a screen for second site suppressors of mcm2-1. The dbf4 suppressor mutation restores competence to initiate DNA synthesis to the mcm2-1 mutant. Cdc7-Dbf4 interacts physically with Mcm2 and phosphorylates Mcm2 and three other members of the Mcm2-7 family in vitro. Blocking the kinase activity of Cdc7-Dbf4 at the G1-to-S phase transition also blocks the phosphorylation of Mcm2 at this defined point of the cell cycle. Taken together, our data suggest that phosphorylation of Mcm2 and probably other members of the Mcm2-7 proteins by Cdc7-Dbf4 at the G1-to-S phase transition is a critical step in the initiation of DNA synthesis at replication origins.

Mutational analysis of Cdc19p, a Schizosaccharomyces pombe MCM protein

The cdc19+ gene encodes an essential member of the MCM family of replication proteins in Schizosaccharomyces pombe. We have examined the structure and function of the Cdc19p protein using molecular and genetic approaches. We find that overproduction of wild-type Cdc19p in wild-type cells has no effect, but cdc19-P1 mutant cells do not tolerate elevated levels of other MCM proteins or overexpression of mutant forms of Cdc19p. We have found genetic interactions between cdc19+ and genes encoding subunits of DNA polymerase delta and the replication initiator cdc18+. We have constructed a series of point mutations and sequence deletions throughout Cdc19p, which allow us to distinguish essential from nonessential regions of the protein. Not surprisingly, conserved residues in the MCM homology domain are required for protein function, but some residues outside the core homology domain are dispensable.

Characterization of Cdc47p-minichromosome maintenance complexes in Saccharomyces cerevisiae: identification of Cdc45p as a subunit

Cdc47p is a member of the minichromosome maintenance (MCM) family of polypeptides, which have a role in the early stages of chromosomal DNA replication. Here, we show that Cdc47p assembles into stable complexes with two other members of the MCM family, Cdc46p and Mcm3p. The assembly of Cdc47p into complexes with Cdc46p does not appear to be cell cycle regulated, making it unlikely that these interactions per se are a rate-limiting step in the control of S phase. Cdc45p is also shown to interact with Cdc47p in vivo and to be a component of high-molecular-weight MCM complexes in cell lysates. Like MCM polypeptides, Cdc45p is essential for the initiation of chromosomal DNA replication in Saccharomyces cerevisiae; however, Cdc45p remains in the nucleus throughout the cell cycle, whereas MCMs are nuclear only during G1. We characterize two mutations in CDC47 and CDC46 which arrest cells with unduplicated DNA as a result of single base substitutions. The corresponding amino acid substitutions in Cdc46p and Cdc47p severely reduce the ability of these polypeptides to assemble in a complex with each other in vivo and in vitro. This argues that assembly of Cdc47p into complexes with other MCM polypeptides is important for its role in the initiation of chromosomal DNA replication.

Loading of an Mcm protein onto DNA replication origins is regulated by Cdc6p and CDKs

In eukaryotic cells, firing of DNA replication origins normally does not recur until after M phase. This characteristic is thought to be due to the properties of "initiation" proteins like Orc, Cdc6, and Mcms. Using formaldehyde cross-linking, we show that Cdc6p and Mcm7p associate specifically with replication origins during G1 but not during G2 in S. cerevisiae. Mcm7p's association with origins depends on Cdc6p. Ectopic expression of Cdc6p enables it to associate with origins during G2, but this fails to recruit Mcm7p. Our data suggest that the loading of Mcm proteins onto origins is regulated by two mechanisms: first, by Cdc6p occupancy, and second, by S- and M-CDKs, whose activity during S, G2, and M phases prevents Mcm loading.

Mcm2 and Mcm3 are constitutive nuclear proteins that exhibit distinct isoforms and bind chromatin during specific cell cycle stages of Saccharomyces cerevisiae

The Mcm2-7 proteins are a family of conserved proteins whose functions are essential for the initiation of DNA synthesis in all eukaryotes. These patients are constitutively present in high abundance in actively proliferating cells. In Saccharomyces cerevisiae, the intracellular concentrations of Mcms are between 100 and 500 times the number of replication origins. However, these proteins are limiting for the initiation of DNA synthesis at replication origins. Our studies indicate that only a small fraction of Mcm2 and Mcm3 tightly associates with chromatin, from late M phase to the beginning of the S phase. The rest of the Mcm2 and Mcm3 proteins are disturbed to both the cytoplasm and nucleoplasm in relatively constant levels throughout the cell cycle. We also show that S. cerevisiae Mcm3 is a phosphoprotein that exists in multiple isoforms and that distinct isoforms of Mcm2 and Mcm3 can be detected at specific stages of the cell cycle. These results suggest that the localization and function of the Mcm proteins are regulated by posttranslational phosphorylation in a manner that is consistent with a role for the Mcm proteins in restricting DNA replication to once per cell cycle.

High-molecular-mass complexes of human minichromosome-maintenance proteins in mitotic cells

Minichromosome-maintenance (Mcm) proteins perform essential functions regulating the replication of eukaryotic genomes. In interphase cells they are either bound to a nuclear structure, most probably chromatin, or occur as free multiprotein complexes in the nucleoplasm. Mcm proteins are displaced from their chromatin sites during S phase, and several become highly phosphorylated during mitosis. We investigated whether phosphorylation affects the ability of mitotic Mcm proteins to form multiprotein complexes. Our results clearly show that phosphorylated mitotic Mcm proteins form a 14-15-S complex, probably consisting of one molecule each of the six known human Mcm proteins.

Licensing of DNA replication by a multi-protein complex of MCM/P1 proteins in Xenopus eggs

In eukaryotes, chromosomal DNA is licensed for a single round of replication in each cell cycle. Xenopus MCM3 protein has been implicated in the licensing of replication in egg extract. We have cloned cDNAs encoding five immunologically distinct proteins associated with Xenopus MCM3 as members of the MCM/P1 family. Six Xenopus MCM proteins formed a physical complex in the egg extract, bound to unreplicated chromatin before the formation of nuclei, and apparently displaced from replicated chromatin. The requirement of six XMCM proteins for the replication activity of the egg extract before nuclear formation suggests that their re-association with replicated chromatin at the end of the mitotic cell cycle is a key step for the licensing of replication.

A lesion in the DNA replication initiation factor Mcm10 induces pausing of elongation forks through chromosomal replication origins in Saccharomyces cerevisiae

We describe a new minichromosome maintenance factor, Mcm10, and show that this essential protein is involved in the initiation of DNA replication in Saccharomyces cerevisiae. The mcm10 mutant has an autonomously replicating sequence-specific minichromosome maintenance defect and arrests at the nonpermissive temperature with dumbbell morphology and 2C DNA content. Mcm10 is a nuclear protein that physically interacts with several members of the MCM2-7 family of DNA replication initiation factors. Cloning and sequencing of the MCM10 gene show that it is identical to DNA43, a gene identified independently for its putative role in replicating DNA. Two-dimensional DNA gel analysis reveals that the mcm10-1 lesion causes a dramatic reduction in DNA replication initiation at chromosomal origins, including ORI1 and ORI121. Interestingly, the mcm10-1 lesion also causes replication forks to pause during elongation through these same loci. This novel phenotype suggests a unique role for the Mcm10 protein in the initiation of DNA synthesis at replication origins.

Cdc6p-dependent loading of Mcm proteins onto pre-replicative chromatin in budding yeast

The Cdc6 protein is essential for the assembly of pre-replicative complexes (pre-RCs) at origins of DNA replication in the budding yeast Saccharomyces cerevisiae. This reaction is blocked in vivo by the cyclin-dependent kinase Cdc28p, together with its regulatory subunits, the B type cyclins that are present throughout S, G2, and M phases. Because the destruction of B type cyclins and the consequent inactivation of the kinase are essential for exit from mitosis, pre-RC formation can only occur after passage through mitosis. Therefore, pre-RC formation has been proposed to be essential for coupling S phase and mitosis and for limiting DNA replication to once per cell cycle. The Mcm2-7 family of proteins has been implicated in limiting replication to once per cell cycle from experiments with Xenopus egg extracts. Here we show that the Mcm proteins of budding yeast are abundant and are quantitatively found in a chromatin-enriched fraction specifically during the G1 phase of the cell cycle. This chromatin binding depends on the de novo synthesis of Cdc6p, providing evidence that a conserved biochemical pathway plays a critical role in coordinating DNA replication with mitosis in both yeast and higher eukaryotes. Cdc6p and the origin recognition complex can be selectively removed from this chromatin-enriched fraction without removing the Mcm proteins. From these results, we propose that Cdc6p (and the origin recognition complex) nucleates the binding of Mcm proteins to chromatin, but once bound, the Mcm proteins appear to interact tightly with some other component of chromatin.

In vivo interaction of human MCM heterohexameric complexes with chromatin. Possible involvement of ATP

The MCM protein family, which consists of at least six members, has been implicated in the regulatory machinery causing DNA to replicate once in the S phase. Mammalian MCM proteins are present in the nucleus in two different forms, one extractable by nonionic detergents and the other resistant to such extraction. The latter is assumed to be tightly associated with nuclear structures and released at the time of initiation of replication. However, details of the mode of binding remain unclear. In the present study, we found that, in nonionic detergent-permeabilized nuclei, the association of human MCM (hMCM) proteins with them could be stabilized by the addition of ATP. The hMCMs bound to the nuclei in the presence of ATP were released by digestion with nucleases, suggesting that they are chromatin-associated. The nuclease-directed solubilization of the chromatin-bound hMCMs thus provided a means to analyze them as well as soluble hMCMs by co-immunoprecipitation. The results indicate that the six hMCM members exist as heterocomplexes, whether bound or unbound. We therefore propose that hMCM proteins may function in DNA replication as heterohexamers associated with chromatin and that ATP is possibly involved in the association. Nuclease digestion-immunoprecipitation techniques of the type described here should facilitate further elucidation of the mode of interaction between hMCMs and chromatin.

Chromatin proteins involved in the initiation of DNA replication

Eukaryotic DNA replication is regulated at least in part by the assembly of initiation proteins onto origins of replication. The origin recognition complex (ORC) is bound to origins throughout most of the cell cycle. Other initiation proteins, such as Cdc6 and the MCM/P1 proteins, are assembled onto ORC-containing chromatin during G1 to define a prereplicative complex. During S phase, these proteins are displaced from chromatin and their reassembly is inhibited by protein-dependent kinases.

Cell cycle regulation of the replication licensing system: involvement of a Cdk-dependent inhibitor

The replication licensing factor (RLF) is an essential initiation factor that is involved in preventing re-replication of chromosomal DNA in a single cell cycle. In Xenopus egg extracts, it can be separated into two components: RLF-M, a complex of MCM/P1 polypeptides, and RLF-B, which is currently unpurified. In this paper we investigate variations in RLF activity throughout the cell cycle. Total RLF activity is low in metaphase, due to a lack of RLF-B activity and the presence of an RLF inhibitor. RLF-B is rapidly activated on exit from metaphase, and then declines during interphase. The RLF inhibitor present in metaphase extracts is dependent on the activity of cyclin-dependent kinases (Cdks). Affinity depletion of Cdks from metaphase extracts removed the RLF inhibitor, while Cdc2/cyclin B directly inhibited RLF activity. In metaphase extracts treated with the protein kinase inhibitor 6-dimethylaminopurine (6-DMAP), both cyclin B and the RLF inhibitor were stabilized although the extracts morphologically entered interphase. These results are consistent with studies in other organisms that invoke a key role for Cdks in preventing re-replication of DNA in a single cell cycle.

Cell cycle control of DNA replication

The initiation of DNA replication in eukaryotic cells is a highly regulated process that leads to the duplication of the genetic information for the next cell generation. This requires the ordered assembly of many proteins at the origins of DNA replication to form a competent, pre-replicative chromosomal state. In addition to this competent complex, at least two cell cycle regulated protein kinase pathways are required to affect a transition to a post-replicative chromosomal state. Protein kinases required to establish mitosis prevent re-replication of the DNA. As cells exit mitosis, the cell cycle is reset, allowing the establishment of a new, competent replication state.