Selective instability of Orc1 protein accounts for the absence of functional origin recognition complexes during the M-G(1) transition in mammals

Distinct roles of Arabidopsis ORC1 proteins in DNA replication and heterochromatic H3K27me1 deposition

Most cellular proteins involved in genome replication are conserved in all eukaryotic lineages including yeast, plants and animals. However, the mechanisms controlling their availability during the cell cycle are less well defined. Here we show that the Arabidopsis genome encodes for two ORC1 proteins highly similar in amino acid sequence and that have partially overlapping expression domains but with distinct functions. The ancestral ORC1b gene, present before the partial duplication of the Arabidopsis genome, has retained the canonical function in DNA replication. ORC1b is expressed in both proliferating and endoreplicating cells, accumulates during G1 and is rapidly degraded upon S-phase entry through the ubiquitin-proteasome pathway. In contrast, the duplicated ORC1a gene has acquired a specialized function in heterochromatin biology. ORC1a is required for efficient deposition of the heterochromatic H3K27me1 mark by the ATXR5/6 histone methyltransferases. The distinct roles of the two ORC1 proteins may be a feature common to other organisms with duplicated ORC1 genes and a major difference with animal cells.

Stability of proteins involved in initiation of DNA replication in UV damaged human cells

The protein stability of the initiation factors Orc2, Orc3, Orc4, and Cdc6 was analyzed after UV light exposure in two human cell lines. In the cell line with higher repair capacity, HEK 293, no changes in the cell cycle distribution or in the protein levels of the investigated factors were detected. In HeLa cells that are characterized by lower repair capacity, UV irradiation caused a reduction of the levels of Cdc6, Orc2 and Orc3, but not of Orc4 or triggered apoptosis. The appearance of the truncated 49 kDa form of Cdc6 suggested the involvement of the caspase pathway in the degradation of the proteins. Reduced protein levels of Cdc6 were detected in UV damaged HeLa cells in which the apoptotic process was blocked with the caspase inhibitor Z-VAD-fmk, indicating that the degradation of Cdc6 is mediated by the proteasome pathway instead. In the presence of caffeine, an inhibitor of the cell cycle checkpoint kinases, Cdc6 was stabilized, demonstrating that its degradation is controlled by the DNA damage cell cycle checkpoint. We conclude that in response to DNA damage, the activation of origins of replication can be prevented by the degradation of Cdc6, most likely through the proteasome pathway.

A new class of disordered elements controls DNA replication through initiator self-assembly

The initiation of DNA replication in metazoans occurs at thousands of chromosomal sites known as origins. At each origin, the Origin Recognition Complex (ORC), Cdc6, and Cdt1 co-assemble to load the Mcm2-7 replicative helicase onto chromatin. Current replication models envisage a linear arrangement of isolated origins functioning autonomously; the extent of inter-origin organization and communication is unknown. Here, we report that the replication initiation machinery of D. melanogaster unexpectedly undergoes liquid-liquid phase separation (LLPS) upon binding DNA in vitro. We find that ORC, Cdc6, and Cdt1 contain intrinsically disordered regions (IDRs) that drive LLPS and constitute a new class of phase separating elements. Initiator IDRs are shown to regulate multiple functions, including chromosome recruitment, initiator-specific co-assembly, and Mcm2-7 loading. These data help explain how CDK activity controls replication initiation and suggest that replication programs are subject to higher-order levels of inter-origin organization.

Identification and characterization of ARS-like sequences as putative origin(s) of replication in human malaria parasite Plasmodium falciparum

DNA replication is a fundamental process in genome maintenance, and initiates from several genomic sites (origins) in eukaryotes. In Saccharomyces cerevisiae, conserved sequences known as autonomously replicating sequences (ARSs) provide a landing pad for the origin recognition complex (ORC), leading to replication initiation. Although origins from higher eukaryotes share some common sequence features, the definitive genomic organization of these sites remains elusive. The human malaria parasite Plasmodium falciparum undergoes multiple rounds of DNA replication; therefore, control of initiation events is crucial to ensure proper replication. However, the sites of DNA replication initiation and the mechanism by which replication is initiated are poorly understood. Here, we have identified and characterized putative origins in P. falciparum by bioinformatics analyses and experimental approaches. An autocorrelation measure method was initially used to search for regions with marked fluctuation (dips) in the chromosome, which we hypothesized might contain potential origins. Indeed, S. cerevisiae ARS consensus sequences were found in dip regions. Several of these P. falciparum sequences were validated with chromatin immunoprecipitation-quantitative PCR, nascent strand abundance and a plasmid stability assay. Subsequently, the same sequences were used in yeast to confirm their potential as origins in vivo. Our results identify the presence of functional ARSs in P. falciparum and provide meaningful insights into replication origins in these deadly parasites. These data could be useful in designing transgenic vectors with improved stability for transfection in P. falciparum.

Mechanisms and regulation of DNA replication initiation in eukaryotes

Cellular DNA replication is initiated through the action of multiprotein complexes that recognize replication start sites in the chromosome (termed origins) and facilitate duplex DNA melting within these regions. In a typical cell cycle, initiation occurs only once per origin and each round of replication is tightly coupled to cell division. To avoid aberrant origin firing and re-replication, eukaryotes tightly regulate two events in the initiation process: loading of the replicative helicase, MCM2-7, onto chromatin by the origin recognition complex (ORC), and subsequent activation of the helicase by its incorporation into a complex known as the CMG. Recent work has begun to reveal the details of an orchestrated and sequential exchange of initiation factors on DNA that give rise to a replication-competent complex, the replisome. Here, we review the molecular mechanisms that underpin eukaryotic DNA replication initiation - from selecting replication start sites to replicative helicase loading and activation - and describe how these events are often distinctly regulated across different eukaryotic model organisms.

Proteomics Analysis with a Nano Random Forest Approach Reveals Novel Functional Interactions Regulated by SMC Complexes on Mitotic Chromosomes

Packaging of DNA into condensed chromosomes during mitosis is essential for the faithful segregation of the genome into daughter nuclei. Although the structure and composition of mitotic chromosomes have been studied for over 30 years, these aspects are yet to be fully elucidated. Here, we used stable isotope labeling with amino acids in cell culture to compare the proteomes of mitotic chromosomes isolated from cell lines harboring conditional knockouts of members of the condensin (SMC2, CAP-H, CAP-D3), cohesin (Scc1/Rad21), and SMC5/6 (SMC5) complexes. Our analysis revealed that these complexes associate with chromosomes independently of each other, with the SMC5/6 complex showing no significant dependence on any other chromosomal proteins during mitosis. To identify subtle relationships between chromosomal proteins, we employed a nano Random Forest (nanoRF) approach to detect protein complexes and the relationships between them. Our nanoRF results suggested that as few as 113 of 5058 detected chromosomal proteins are functionally linked to chromosome structure and segregation. Furthermore, nanoRF data revealed 23 proteins that were not previously suspected to have functional interactions with complexes playing important roles in mitosis. Subsequent small-interfering-RNA-based validation and localization tracking by green fluorescent protein-tagging highlighted novel candidates that might play significant roles in mitotic progression.

Orc1 Binding to Mitotic Chromosomes Precedes Spatial Patterning during G1 Phase and Assembly of the Origin Recognition Complex in Human Cells

Replication of eukaryotic chromosomes occurs once every cell division cycle in normal cells and is a tightly controlled process that ensures complete genome duplication. The origin recognition complex (ORC) plays a key role during the initiation of DNA replication. In human cells, the level of Orc1, the largest subunit of ORC, is regulated during the cell division cycle, and thus ORC is a dynamic complex. Upon S phase entry, Orc1 is ubiquitinated and targeted for destruction, with subsequent dissociation of ORC from chromosomes. Time lapse and live cell images of human cells expressing fluorescently tagged Orc1 show that Orc1 re-localizes to condensing chromatin during early mitosis and then displays different nuclear localization patterns at different times during G1 phase, remaining associated with late replicating regions of the genome in late G1 phase. The initial binding of Orc1 to mitotic chromosomes requires C-terminal amino acid sequences that are similar to mitotic chromosome-binding sequences in the transcriptional pioneer protein FOXA1. Depletion of Orc1 causes concomitant loss of the mini-chromosome maintenance (Mcm2-7) helicase proteins on chromatin. The data suggest that Orc1 acts as a nucleating center for ORC assembly and then pre-replication complex assembly by binding to mitotic chromosomes, followed by gradual removal from chromatin during the G1 phase.

Regulating DNA replication in eukarya

DNA replication is tightly controlled in eukaryotic cells to ensure that an exact copy of the genetic material is inherited by both daughter cells. Oscillating waves of cyclin-dependent kinase (CDK) and anaphase-promoting complex/cyclosome (APC/C) activities provide a binary switch that permits the replication of each chromosome exactly once per cell cycle. Work from several organisms has revealed a conserved strategy whereby inactive replication complexes are assembled onto DNA during periods of low CDK and high APC activity but are competent to execute genome duplication only when these activities are reversed. Periods of high CDK and low APC/C serve an essential function by blocking reassembly of replication complexes, thereby preventing rereplication. Higher eukaryotes have evolved additional CDK-independent mechanisms for preventing rereplication.

Epstein-Barr nuclear antigen 1 (EBNA1)-dependent recruitment of origin recognition complex (Orc) on oriP of Epstein-Barr virus with purified proteins: stimulation by Cdc6 through its direct interaction with EBNA1

Origin recognition complex (Orc) plays an essential role in directing assembly of prereplicative complex at selective sites on chromosomes. However, Orc from vertebrates is reported to bind to DNA in a sequence-nonspecific manner, and it is still unclear how it selects specific genomic loci and how Cdc6, another conserved AAA(+) factor known to interact with Orc, participates in this process. Replication from oriP, the latent origin of Epstein-Barr virus, provides an excellent model system for the study of initiation on the host chromosomes because it is known to depend on prereplicative complex factors, including Orc and Mcm. Here, we show that Orc is recruited selectively at the essential dyad symmetry element in nuclear extracts in a manner dependent on EBNA1, which specifically binds to dyad symmetry. With purified proteins, EBNA1 can recruit both Cdc6 and Orc independently on a DNA containing EBNA1 binding sites, and Cdc6 facilitates the Orc recruitment by EBNA1. Purified Cdc6 directly binds to EBNA1, whereas association of Orc with EBNA1 requires the presence of the oriP DNA. Nuclease protection assays suggest that Orc associates with DNA segments on both sides adjacent to the EBNA1 binding sites and that this process is stimulated by the presence of Cdc6. Thus, EBNA1 can direct localized assembly of Orc in a process that is facilitated by Cdc6. The possibility of similar modes of recruitment of Orc/Cdc6 at the human chromosomal origins will be discussed.

Mediators of reprogramming: transcription factors and transitions through mitosis

It is thought that most cell types of the human body share the same genetic information as that contained in the zygote from which they originate. Consistent with this view, animal cloning studies demonstrated that the intact genome of a differentiated cell can be reprogrammed to support the development of an entire organism and allow the production of pluripotent stem cells. Recent progress in reprogramming research now points to an important role for transcription factors in the establishment and the maintenance of cellular phenotypes, and to cell division as a mediator of transitions between different states of gene expression.

Global regulation of genome duplication in eukaryotes: an overview from the epifluorescence microscope

In eukaryotes, DNA replication is initiated along each chromosome at multiple sites called replication origins. Locally, each replication origin is "licensed" or specified at the end of the M and the beginning of the G1 phases of the cell cycle. During the S phase when DNA synthesis takes place, origins are activated in stages corresponding to early and late-replicating domains. The staged and progressive activation of replication origins reflects the need to maintain a strict balance between the number of active replication forks and the rate at which DNA synthesis proceeds. This suggests that origin densities (frequency of initiation) and replication fork movement (rates of elongation) must be coregulated to guarantee the efficient and complete duplication of each subchromosomal domain. Emerging evidence supports this proposal and suggests that the ATM/ATR intra-S phase checkpoint plays an important role in the coregulation of initiation frequencies and rates of elongation. In this paper, we review recent results concerning the mechanisms governing the global regulation of DNA replication and discuss the roles these mechanisms play in maintaining genome stability during both a normal and perturbed S phase.

Identification of genes mediating thyroid hormone action in the developing mouse cerebellum

Despite the indispensable role thyroid hormone (TH) plays in brain development, only a small number of genes have been identified to be directly regulated by TH and its precise mechanism of action remains largely unknown, partly because most of the previous studies have been carried out at postnatal day 15 or later. In the present study, we screened for TH-responsive genes in the developing mouse cerebellum at postnatal day 4 when morphological alterations because of TH status are not apparent. Among the new candidate genes selected by comparing gene expression profiles of experimentally hypothyroid, hypothyroid with postnatal thyroxine replacement, and control animals using oligoDNA microarrays, six genes were confirmed by real-time PCR to be positively (orc1l, galr3, sort1, nlgn3, cdk5r2, and zfp367) regulated by TH. Among these, sort1, cdk5r2, and zfp367 were up-regulated already at 1 h after a single injection of thyroxine to the hypothyroid or control animal, suggesting them to be possible primary targets of the hormone. Cell proliferation and apoptosis examined by BrdU incorporation and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling assay revealed that hypothyroidism by itself did not enhance apoptosis at this stage, but rather increased cell survival, possibly through regulation of these newly identified genes.

Site-specific interaction of the murine pre-replicative complex with origin DNA: assembly and disassembly during cell cycle transit and differentiation

Eukaryotic DNA replication initiates at origins of replication by the assembly of the highly conserved pre-replicative complex (pre-RC). However, exact sequences for pre-RC binding still remain unknown. By chromatin immunoprecipitation we identified in vivo a pre-RC-binding site within the origin of bidirectional replication in the murine rDNA locus. At this sequence, ORC1, -2, -4 and -5 are bound in G1 phase and at the G1/S transition. During S phase, ORC1 is released. An ATP-dependent and site-specific assembly of the pre-RC at origin DNA was demonstrated in vitro using partially purified murine pre-RC proteins in electrophoretic mobility shift assays. By deletion experiments the sequence required for pre-RC binding was confined to 119 bp. Nucleotide substitutions revealed that two 9 bp sequence elements, CTCGGGAGA, are essential for the binding of pre-RC proteins to origin DNA within the murine rDNA locus. During myogenic differentiation of C2C12 cells, we demonstrated a reduction of ORC1 and ORC2 by immunoblot analyses. ChIP analyses revealed that ORC1 completely disappears from chromatin of terminally differentiated myotubes, whereas ORC2, -4 and -5 still remain associated.

ATP-dependent assembly of the human origin recognition complex

The origin recognition complex (ORC) was initially discovered in budding yeast extracts as a protein complex that binds with high affinity to autonomously replicating sequences in an ATP-dependent manner. We have cloned and expressed the human homologs of the ORC subunits as recombinant proteins. In contrast to other eukaryotic initiators examined thus far, assembly of human ORC in vitro is dependent on ATP binding. Mutations in the ATP-binding sites of Orc4 or Orc5 impair complex assembly, whereas Orc1 ATP binding is not required. Immunofluorescence staining of human cells with anti-Orc3 antibodies demonstrate cell cycle-dependent association with a nuclear structure. Immunoprecipitation experiments show that ORC disassembles as cells progress through S phase. The Orc6 protein binds directly to the Orc3 subunit and interacts as part of ORC in vivo. These data suggest that the assembly and disassembly of ORC in human cells is uniquely regulated and may contribute to restricting DNA replication to once in every cell division cycle.

The BAH domain facilitates the ability of human Orc1 protein to activate replication origins in vivo

Selection of initiation sites for DNA replication in eukaryotes is determined by the interaction between the origin recognition complex (ORC) and genomic DNA. In mammalian cells, this interaction appears to be regulated by Orc1, the only ORC subunit that contains a bromo-adjacent homology (BAH) domain. Since BAH domains mediate protein-protein interactions, the human Orc1 BAH domain was mutated, and the mutant proteins expressed in human cells to determine their affects on ORC function. The BAH domain was not required for nuclear localization of Orc1, association of Orc1 with other ORC subunits, or selective degradation of Orc1 during S-phase. It did, however, facilitate reassociation of Orc1 with chromosomes during the M to G1-phase transition, and it was required for binding Orc1 to the Epstein-Barr virus oriP and stimulating oriP-dependent plasmid DNA replication. Moreover, the BAH domain affected Orc1's ability to promote binding of Orc2 to chromatin as cells exit mitosis. Thus, the BAH domain in human Orc1 facilitates its ability to activate replication origins in vivo by promoting association of ORC with chromatin.

Binding of AlF-C, an Orc1-binding transcriptional regulator, enhances replicator activity of the rat aldolase B origin

A region encompassing the rat aldolase B gene (aldB) promoter acts as a chromosomal origin of DNA replication (origin) in rat aldolase B-nonexpressing hepatoma cells. To examine replicator function of the aldB origin, we constructed recombinant mouse cell lines in which the rat aldB origin and the mutant derivatives were inserted into the same position at the mouse chromosome 8 by cre-mediated recombination. Nascent strand abundance assays revealed that the rat origin acts as a replicator at the ectopic mouse locus. Mutation of site C in the rat origin, which binds an Orc1-binding protein AlF-C in vitro, resulted in a significant reduction of the replicator activity in the mouse cells. Chromatin immunoprecipitation (ChIP) assays indicated that the reduction of replicator activity was paralleled with the reduced binding of AlF-C and Orc1, suggesting that sequence-specific binding of AlF-C to the ectopic rat origin leads to enhanced replicator activity in cooperation with Orc1. Involvement of AlF-C in replication in vivo was further examined for the aldB origin at its original rat locus and for a different rat origin identified in the present study, which contained an AlF-C-binding site. ChIP assays revealed that both replication origins bind AlF-C and Orc1. We think that the results presented here may represent one mode of origin recognition in mammalian cells.

Identification of new human origins of DNA replication by an origin-trapping assay

Metazoan genomes contain thousands of replication origins, but only a limited number have been characterized so far. We developed a two-step origin-trapping assay in which human chromatin fragments associated with origin recognition complex (ORC) in vivo were first enriched by chromatin immunoprecipitation. In a second step, these fragments were screened for transient replication competence in a plasmid-based assay utilizing the Epstein-Barr virus latent origin oriP. oriP contains two elements, an origin (dyad symmetry element [DS]) and the family of repeats, that when associated with the viral protein EBNA1 facilitate extrachromosomal stability. Insertion of the ORC-binding human DNA fragments in oriP plasmids in place of DS enabled us to screen functionally for their abilities to restore replication. Using the origin-trapping assay, we isolated and characterized five previously unknown human origins. The assay was validated with nascent strand abundance assays that confirm these origins as active initiation sites in their native chromosomal contexts. Furthermore, ORC and MCM2-7 components localized at these origins during G(1) phase of the cell cycle but were not detected during mitosis. This finding extends the current understanding of origin-ORC dynamics by suggesting that replication origins must be reestablished during the early stages of each cell division cycle and that ORC itself participates in this process.

Regulating the licensing of DNA replication origins in metazoa

Eukaryotic DNA replication is a highly conserved process; the proteins and sequence of events that replicate animal genomes are remarkably similar to those that replicate yeast genomes. Moreover, the assembly of prereplication complexes at DNA replication origins ('DNA licensing') is regulated in all eukaryotes so that no origin fires more than once in a single cell cycle. And yet there are significant differences between species both in the selection of replication origins and in the way in which these origins are licensed to operate. Moreover, these differences impart advantages to multicellular animals and plants that facilitate their development, such as better control over endoreduplication, flexibility in origin selection, and discrimination between quiescent and proliferative states.

Ubiquitylation, phosphorylation and Orc2 modulate the subcellular location of Orc1 and prevent it from inducing apoptosis

Previous studies have suggested that the activity of the mammalian origin recognition complex (ORC) is regulated by cell-cycle-dependent changes in its Orc1 subunit. Here, we show that Orc1 modifications such as mono-ubiquitylation and hyperphosphorylation that occur normally during S and G2-M phases, respectively, can cause Orc1 to accumulate in the cytoplasm. This would suppress reassembly of pre-replication complexes until mitosis is complete. In the absence of these modifications, transient expression of Orc1 rapidly induced p53-independent apoptosis, and Orc1 accumulated perinuclearly rather than uniformly throughout the nucleus. This behavior mimicked the increased concentration and perinuclear accumulation of endogenous Orc1 in apoptotic cells that arise spontaneously in proliferating cell cultures. Remarkably, expression of Orc1 in the presence of an equivalent amount of Orc2, the only ORC subunit that did not induce apoptosis, prevented induction of apoptosis and restored uniform nuclear localization of Orc1. This would promote assembly of ORC-chromatin sites, such as occurs during the transition from M to G1 phase. These results provide direct evidence in support of the regulatory role proposed for Orc1, and suggest that aberrant DNA replication during mammalian development could result in apoptosis through the appearance of 'unmodified' Orc1.

DNA replication in protein extracts from human cells requires ORC and Mcm proteins

We used protein extracts from proliferating human HeLa cells to support plasmid DNA replication in vitro. An extract with soluble nuclear proteins contains the major replicative chain elongation functions, whereas a high salt extract from isolated nuclei contains the proteins for initiation. Among the initiator proteins active in vitro are the origin recognition complex (ORC) and Mcm proteins. Recombinant Orc1 protein stimulates in vitro replication presumably in place of endogenous Orc1 that is known to be present in suboptimal amounts in HeLa cell nuclei. Partially purified endogenous ORC, but not recombinant ORC, is able to rescue immunodepleted nuclear extracts. Plasmid replication in the in vitro replication system is slow and of limited efficiency but robust enough to serve as a basis to investigate the formation of functional pre-replication complexes under biochemically defined conditions.

Chinese hamster ORC subunits dynamically associate with chromatin throughout the cell-cycle

In yeast, the Origin Recognition Complex (ORC) is bound to replication origins throughout the cell-cycle, but in animal cells, there are conflicting data as to whether and when ORC is removed from chromatin. We find ORC1, 2 and ORC4 to be metabolically stable proteins that co-fractionate with chromatin throughout the cell-cycle in Chinese hamster fibroblasts. Since cellular extraction methods cannot directly examine the chromatin binding properties of proteins in vivo, we examined ORC:chromatin interactions in living cells. Fluorescence loss in photobleaching (FLIP) studies revealed ORC1 and ORC4 to be highly dynamic proteins during the cell-cycle with exchange kinetics similar to other regulatory chromatin proteins. In vivo interaction with chromatin was not significantly altered throughout the cell-cycle, including S-phase. These data support a model in which ORC subunits dynamically interact with chromatin throughout the cell-cycle.

Origin recognition and the chromosome cycle

Prior to the initiation of DNA replication, chromosomes must establish a biochemical mark that permits the recruitment in S phase of the DNA replication machinery that copies DNA. The process of chromosome replication in eukaryotes also must be coordinated with segregation of the duplicated chromosomes to daughter cells during mitosis. Protein complexes that utilize ATP coordinate events at origins of DNA replication and later they participate in the initiation of DNA replication. In eukaryotes, some of these proteins also play a part in later processes that ensure accurate inheritance of chromosomes in mitosis, including spindle attachment of chromosomes, accurate duplication of centrosomes and cytokinesis. A perspective of how ATP-dependent proteins accomplish this task in eukaryotes is discussed.

Role for Cdk1 (Cdc2)/cyclin A in preventing the mammalian origin recognition complex's largest subunit (Orc1) from binding to chromatin during mitosis

The eukaryotic origin recognition complex (ORC) selects the genomic sites where prereplication complexes are assembled and DNA replication begins. In proliferating mammalian cells, ORC activity appears to be regulated by reducing the affinity of the Orc1 subunit for chromatin during S phase and then preventing reformation of a stable ORC-chromatin complex until mitosis is completed and a nuclear membrane is assembled. Here we show that part of the mechanism by which this is accomplished is the selective association of Orc1 with Cdk1 (Cdc2)/cyclin A during the G(2)/M phase of cell division. This association accounted for the appearance in M-phase cells of hyperphosphorylated Orc1 that was subsequently dephosphorylated during the M-to-G(1) transition. Moreover, inhibition of Cdk activity in metaphase cells resulted in rapid binding of Orc1 to chromatin. However, chromatin binding was not mediated through increased affinity of Orc1 for Orc2, suggesting that additional events are involved in the assembly of functional ORC-chromatin sites. These results reveal that the same cyclin-dependent protein kinase that initiates mitosis in mammalian cells also concomitantly inhibits assembly of functional ORC-chromatin sites.

Human Orc2 localizes to centrosomes, centromeres and heterochromatin during chromosome inheritance

The initiation of DNA replication in S phase requires the prior assembly of an origin recognition complex (ORC)-dependent pre-replicative complex on chromatin during G1 phase of the cell division cycle. In human cells, the Orc2 subunit localized to the nucleus as expected, but it also localized to centrosomes throughout the entire cell cycle. Furthermore, Orc2 was tightly bound to heterochromatin and heterochromatin protein 1alpha (HP1alpha) and HP1beta in G1 and early S phase, but during late S, G2 and M phases tight chromatin association was restricted to centromeres. Depletion of Orc2 by siRNA caused multiple phenotypes. A population of cells showed an S-phase defect with little proliferating cell nuclear antigen (PCNA) on chromatin, although MCM proteins remained. Orc2 depletion also disrupted HP1 localization, but not histone-H3-lysine-9 methylation at prominent heterochromatic foci. Another subset of Orc2-depleted cells containing replicated DNA arrested with abnormally condensed chromosomes, failed chromosome congression and multiple centrosomes. These results implicate Orc2 protein in chromosome duplication, chromosome structure and centrosome copy number control, suggesting that it coordinates all stages of the chromosome inheritance cycle.

Dynamics of pre-replication complex proteins during the cell division cycle

Replication of the human genome every time a cell divides is a highly coordinated process that ensures accurate and efficient inheritance of the genetic information. The molecular mechanism that guarantees that many origins of replication fire only once per cell-cycle has been the area of intense research. The origin recognition complex (ORC) marks the position of replication origins in the genome and serves as the landing pad for the assembly of a multiprotein, pre-replicative complex (pre-RC) at the origins, consisting of ORC, cell division cycle 6 (Cdc6), Cdc10-dependent transcript (Cdt1) and mini-chromosome maintenance (MCM) proteins. The MCM proteins serve as key participants in the mechanism that limits eukaryotic DNA replication to once-per-cell-cycle and its binding to the chromatin marks the final step of pre-RC formation, a process referred to as 'replication licensing'. We present data demonstrating how the MCM proteins associate with the chromatin during the G1 phase, probably defining pre-RCs and then anticipate replication fork movement in a precisely coordinated manner during the S phase of the cell cycle. The process of DNA replication must also be carefully coordinated with other cell-cycle processes including mitosis and cytokinesis. Some of the proteins that control initiation of DNA replication are likely to interact with the pathways that control these important cell-cycle transitions. Herein, we discuss the participation of human ORC proteins in other vital functions, in addition to their bona fide roles in replication.

Degradation of origin recognition complex large subunit by the anaphase-promoting complex in Drosophila

The initiation of DNA synthesis is thought to occur at sites bound by a heteromeric origin recognition complex (ORC). Previously, we have shown that in Drosophila, the level of the large subunit, ORC1, is modulated during cell cycle progression and that changes in ORC1 concentration alter origin utilization during development. Here, we investigate the mechanisms underlying cell cycle-dependent degradation of ORC1. We show that signals in the non-conserved N-terminal domain of ORC1 mediate its degradation upon exit from mitosis and in G1 phase by the anaphase-promoting complex (APC) in vivo. Degradation appears to be the result of direct action of the APC, as the N-terminal domain is ubiquitylated by purified APC in vitro. This regulated proteolysis is potent, sufficient to generate a normal temporal distribution of protein even when transcription of ORC1 is driven by strong constitutive promoters. These observations suggest that in Drosophila, ORC1 regulates origin utilization much as does Cdc6 in budding yeast.

Enigmatic variations: divergent modes of regulating eukaryotic DNA replication

Proteins involved in DNA replication are conserved from yeast to mammals, suggesting that the mechanism was established at an early stage of eukaryotic evolution. In spite of this common origin, recent findings have revealed surprising variations in how replication initiation is controlled, implying that a conserved mechanism has not necessarily resulted in regulatory conservation.

Complex protein-DNA dynamics at the latent origin of DNA replication of Epstein-Barr virus

The sequential binding of the origin recognition complex (ORC), Cdc6p and the minichromosome maintenance proteins (MCM2-7) mediates replication competence at eukaryotic origins of DNA replication. The latent origin of Epstein-Barr virus, oriP, is a viral origin known to recruit ORC. OriP also binds EBNA1, a virally encoded protein that lacks any activity predicted to be required for replication initiation. Here, we used chromatin immunoprecipitation and chromatin binding to compare the cell-cycle-dependent binding of pre-RC components and EBNA1 to oriP and to global cellular chromatin. Prereplicative-complex components such as the Mcm2p-Mcm7p proteins and HsOrc1p are regulated in a cell-cycle-dependent fashion, whereas other HsOrc subunits and EBNA1 remain constantly bound. In addition, HsOrc1p becomes sensitive to the 26S proteasome after release from DNA during S phase. These results show that the complex protein-DNA dynamics at the viral oriP are synchronized with the cell division cycle. Chromatin-binding and chromatin-immunoprecipitation experiments on G0 arrested cells indicated that the ORC core complex (ORC2-5) and EBNA1 remain bound to chromatin and oriP. HsOrc6p and the MCM2-7 complex are released in resting cells. HsOrc1p is partly liberated from chromatin. Our data suggest that origins remain marked in resting cells by the ORC core complex to ensure a rapid and regulated reentry into the cell cycle. These findings indicate that HsOrc is a dynamic complex and that its DNA binding activity is regulated differently in the various stages of the cell cycle.

Localization of proteins bound to a replication origin of human DNA along the cell cycle

The proteins bound in vivo at the human lamin B2 DNA replication origin and their precise sites of binding were investigated along the cell cycle utilizing two novel procedures based on immunoprecipitation following UV irradiation with a pulsed laser light source. In G(1), the pre-replicative complex contains CDC6, MCM3, ORC1 and ORC2 proteins; of these, the post-replicative complex in S phase contains only ORC2; in M phase none of them are bound. The precise nucleotide of binding was identified for the two ORC and the CDC6 proteins near the start sites for leading-strand synthesis; the transition from the pre- to the post-replicative complex is accompanied by a 17 bp displacement of the ORC2 protein towards the start site.

Sequence-independent DNA binding and replication initiation by the human origin recognition complex

We report that a highly purified human origin recognition complex (HsORC) has intrinsic DNA-binding activity, and that this activity is modestly stimulated by ATP. HsORC binds preferentially to synthetic AT-rich polydeoxynucleotides, but does not effectively discriminate between natural DNA fragments that contain known human origins and control fragments. The complex fully restores DNA replication to ORC-depleted Xenopus egg extracts, providing strong evidence for its initiator function. Strikingly, HsORC stimulates initiation from any DNA sequence, and it does not preferentially replicate DNA containing human origin sequences. These data provide a biochemical explanation for the observation that in metazoans, initiation of DNA replication often occurs in a seemingly random pattern, and they have important implications for the nature of human origins of DNA replication.

The 'ORC cycle': a novel pathway for regulating eukaryotic DNA replication

The function of the 'origin recognition complex' (ORC) in eukaryotic cells is to select genomic sites where pre-replication complexes (pre-RCs) can be assembled. Subsequent activation of these pre-RCs results in bi-directional DNA replication that originates at or close to the ORC DNA binding sites. Recent results have revealed that one or more of the six ORC subunits is modified during the G1 to S-phase transition in such a way that ORC activity is inhibited until mitosis is complete and a nuclear membrane is assembled. In yeast, Cdk1/Clb phosphorylates ORC. In frog eggs, pre-RC assembly destabilizes ORC/chromatin sites, and ORC is eventually hyperphosphorylated and released. In mammals, the affinity of Orc1 for chromatin is selectively reduced during S-phase and restored during early G1-phase. Unbound Orc1 is ubiquitinated during S-phase and in some cases degraded. Thus, most, perhaps all, eukaryotes exhibit some manifestation of an 'ORC cycle' that restricts the ability of ORC to initiate pre-RC assembly to the early G1-phase of the cell cycle, making the 'ORC cycle' the premier step in determining when replication begins.

Interaction and assembly of murine pre-replicative complex proteins in yeast and mouse cells

Eukaryotic cells coordinate chromosome duplication by the assembly of protein complexes at origins of DNA replication by sequential binding of member proteins of the origin recognition complex (ORC), CDC6, and minichromosome maintenance (MCM) proteins. These pre-replicative complexes (pre-RCs) are activated by cyclin-dependent kinases and DBF4/CDC7 kinase. Here, we carried out a comprehensive yeast two-hybrid screen to establish sequential interactions between two individual proteins of the mouse pre-RC that are probably required for the initiation of DNA replication. The studies revealed multiple interactions among ORC subunits and MCM proteins as well as interactions between individual ORC and MCM proteins. In particular CDC6 was found to bind strongly to ORC1 and ORC2, and to MCM7 proteins. DBF4 interacts with the subunits of ORC as well as with MCM proteins. It was also demonstrated that CDC7 binds to different ORC and MCM proteins. CDC45 interacts with ORC1 and ORC6, and weakly with MCM3, -6, and -7. The three subunits of the single-stranded DNA binding protein RPA show interactions with various ORC subunits as well as with several MCM proteins. The data obtained by yeast two-hybrid analysis were paradigmatically confirmed in synchronized murine FM3A cells by immunoprecipitation of the interacting partners. Some of the interactions were found to be cell-cycle-dependent; however, most of them were cell-cycle-independent. Altogether, 90 protein-protein interactions were detected in this study, 52 of them were found for the first time in any eukaryotic pre-RC. These data may help to understand the complex interplay of the components of the mouse pre-RC and should allow us to refine its structural architecture as well as its assembly in real time.

The role of the replication licensing system in cell proliferation and cancer

The precise duplication of chromosomal DNA during each cell cycle is essential for the maintenance of genetic stability. Failure to correctly regulate chromosomal DNA replication could lead to losses or duplication of chromosome segments. The precise duplication of chromosomes is normally achieved by correct regulation of the replication licensing system. Here we review our current knowledge of the licensing system and how this might be defective in cancer cells. We also review how detection of licensing components can be used for the diagnosis and prognosis of cancer. Finally we discuss the potential of the replication licensing system as a novel anti-cancer target.

Functional domains involved in the interaction between Orc1 and transcriptional repressor AlF-C that bind to an origin/promoter of the rat aldolase B gene

The promoter of the rat aldolase B (AldB) gene functions in vivo as an origin of DNA replication in the cells in which transcription of the gene is repressed. Previously, we identified two closely related DNA-binding proteins, AlF-C1 and AlF-C2, which repressed the AldB gene promoter. We also reported that the binding site of these proteins, site C, is one of the required DNA elements of the AldB gene origin/promoter for autonomously replicating activity in transfected cells. In the present study, we show that AlF-C1 and AlF-C2 bind directly to Orc1, a subunit of the origin recognition complex (ORC). Deletion analyses revealed a functional domain in AlF-C2 for binding to Orc1, which is located separately from the DNA-binding domain. In addition, we found a novel protein-interacting domain in Orc1 required for the binding of AlF-C2, which was conserved in human, mouse and Chinese hamster, but not in Drosophila, frog and yeast. Thus, it is assumed that in mammalian cells, sequence- specific DNA-binding proteins are involved in recruiting ORC to regulate replication initiation and/or transcription repression.

DNA replication in eukaryotic cells

The maintenance of the eukaryotic genome requires precisely coordinated replication of the entire genome each time a cell divides. To achieve this coordination, eukaryotic cells use an ordered series of steps to form several key protein assemblies at origins of replication. Recent studies have identified many of the protein components of these complexes and the time during the cell cycle they assemble at the origin. Interestingly, despite distinct differences in origin structure, the identity and order of assembly of eukaryotic replication factors is highly conserved across all species. This review describes our current understanding of these events and how they are coordinated with cell cycle progression. We focus on bringing together the results from different organisms to provide a coherent model of the events of initiation. We emphasize recent progress in determining the function of the different replication factors once they have been assembled at the origin.

Cdc6 synthesis regulates replication competence in Xenopus oocytes

The early division cycles of an embryo rely on the oocyte's ability to replicate DNA. During meiosis, oocytes temporarily lose this ability. After a single round of pre-meiotic S-phase, oocytes enter meiosis and rapidly arrest at prophase of meiosis I (G2). Upon hormonal stimulation, arrested oocytes resume meiosis, re-establish DNA replication competence in meiosis I shortly after germinal vesicle breakdown (GVBD), but repress replication until fertilization. How oocytes lose and regain replication competence during meiosis are important questions underlying the production of functional gametes. Here we show that the inability of immature Xenopus oocytes to replicate is linked to the absence of the Cdc6 protein and the cytoplasmic localization of other initiation proteins. Injection of Cdc6 protein into immature oocytes does not induce DNA replication. However, injection of Cdc6 into oocytes undergoing GVBD is sufficient to induce DNA replication in the absence of protein synthesis. Our results show that GVBD and Cdc6 synthesis are the only events that limit the establishment of the oocyte's replication competence during meiosis.

Site-specific ORC binding, pre-replication complex assembly and DNA synthesis at Schizosaccharomyces pombe replication origins

Previous studies have shown that the Schizo saccharomyces pombe Orc4 subunit is solely responsible for in vitro binding of origin recognition complex (ORC) to specific AT-rich sites within S.pombe replication origins. Using ARS3001, a S.pombe replication origin consisting of four genetically required sites, we show that, in situ as well as in vitro, Orc4 binds strongly to the Delta3 site, weakly to the Delta6 site and not at all to the remaining sequences. In situ, the footprint over Delta3 is extended during G(1) phase, but only when Cdc18 is present and Mcm proteins are bound to chromatin. Moreover, this footprint extends into the adjacent Delta2 site, where leading strand DNA synthesis begins. Therefore, we conclude that ARS3001 consists of a single primary ORC binding site that assembles a pre-replication complex and initiates DNA synthesis, plus an additional novel origin element (Delta9) that neither binds ORC nor functions as a centromere, but does bind an as yet unidentified protein throughout the cell cycle. Schizosaccharomyces pombe may be an appropriate paradigm for the complex origins found in the metazoa.

Initiation of eukaryotic DNA replication: regulation and mechanisms

The accurate and timely duplication of the genome is a major task for eukaryotic cells. This process requires the cooperation of multiple factors to ensure the stability of the genetic information of each cell. Mutations, rearrangements, or loss of chromosomes can be detrimental to a single cell as well as to the whole organism, causing failures, disease, or death. Because of the size of eukaryotic genomes, chromosomal duplication is accomplished in a multiparallel process. In human somatic cells between 10,000 and 100,000 parallel synthesis sites are present. This raises fundamental problems for eukaryotic cells to coordinate the start of DNA replication at each origin and to prevent replication of already duplicated DNA regions. Since these general phenomena were recognized in the middle of the 20th century the regulation and mechanisms of the initiation of eukaryotic DNA replication have been intensively investigated. These studies were carried out to find the essential factors involved in the process and to determine their functions during DNA replication. These studies gave rise to a model of the organization and the coordination of DNA replication within the eukaryotic cell. The elegant experiments carried out by Rao and Johnson (1970) (1), who fused cells in different phases of the cell cycle, showed that G1 cells are competent for replication of their chromosomes, but lack a specific diffusible factor required to activate their replicaton machinery and showed that G2 cells are incompetent for DNA replication. These findings suggested that eukaryotic cells exist in two states. In G1 phase, cells are competent to initiate DNA replication, which is subsequently triggered in S phase. After completion of S phase, cells in G2 are no longer able to initiate DNA replication and they require a transition through mitosis to reenable initiation of DNA replication to take place in the next S phase. The Xenopus cell-free replication system has proved a good model system in which to study DNA replication in vitro as well as the mechanism preventing rereplication within a single cell cycle (2). Studies using this system resulted in the development of a model postulating the existence of a replication licensing factor, which binds to chromatin before the G1-S transition and which is displaced during replication (2, 3). These results were supported by genetic and biochemical experiments in Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) (4, 5). The investigation of cell division cycle mutants and the budding yeast origin of replication resulted in the concept of a prereplicative and a postreplicative complex of initiation proteins (6-9). These three individual concepts have recently started to merge and it has become obvious that initiation in eukaryotes is generally governed by the same ubiquitous mechanisms.

Genes involved in the initiation of DNA replication in yeast

Replication and segregation of the information contained in genomic DNA are strictly regulated processes that eukaryotic cells alternate to divide successfully. Experimental work on yeast has suggested that this alternation is achieved through oscillations in the activity of a serine/threonine kinase complex, CDK, which ensures the timely activation of DNA synthesis. At the same time, this CDK-mediated activation sets up the basis of the mechanism that ensures ploidy maintenance in eukaryotes. DNA synthesis is initiated at discrete sites of the genome called origins of replication on which a prereplicative complex (pre-RC) of different protein subunits is formed during the G1 phase of the cell division cycle. Only after pre-RCs are formed is the genome competent to be replicated. Several lines of evidence suggest that CDK activity prevents the assembly of pre-RCs ensuring single rounds of genome replication during each cell division cycle. This review offers a descriptive discussion of the main molecular events that a unicellular eukaryote such as the budding yeast Saccharomyces cerevisiae undergoes to initiate DNA replication.

Xenopus Cdc6 performs separate functions in initiating DNA replication

Cdc6 performs an essential role in the initiation of eukaryotic DNA replication by recruiting the minichromosome maintenance (MCM) complex onto DNA. Using immunodepletion/add-back experiments in Xenopus egg extracts, we have determined that both Walker A (ATP binding) and Walker B (ATP hydrolysis) motifs of Xenopus Cdc6 (Xcdc6) are essential, but have distinct functional roles. Although Walker B mutant protein binds chromatin well, Walker A mutant protein binds chromatin poorly. Neither Walker A nor Walker B mutant protein, however, load appreciable MCM onto DNA. Herein, we provide evidence that Cdc6 functions as a multimer: 1) mutant and wild-type Xcdc6 form multimers; 2) either mutant protein is dominant negative when added before wild-type Xcdc6, but stimulates DNA replication when added simultaneously with wild-type Xcdc6; and 3) the two mutants restore DNA replication when added together, in the absence of wild-type Xcdc6. Our findings suggest that ATP may play a key regulatory role within this multimer: its binding to Cdc6 promotes chromatin association and its hydrolysis facilitates MCM loading. Moreover, ATP binding and hydrolysis may occur in trans between Cdc6 subunits within the complex.

DNA replication and chromatin

The study of DNA replication in eukaryotic chromosomes has revealed a multitude of different regulatory levels. Nuclear and chromosomal location as well as chromatin structure may affect the activity of replication origins and their modulation during development.

Cell cycle-dependent regulation of the association between origin recognition proteins and somatic cell chromatin

Previous studies have suggested that cell cycle-dependent changes in the affinity of the origin recognition complex (ORC) for chromatin are involved in regulating initiation of DNA replication. To test this hypothesis, chromatin lacking functional ORCs was isolated from metaphase hamster cells and incubated in Xenopus egg extracts to initiate DNA replication. Intriguingly, Xenopus ORC rapidly bound to hamster somatic chromatin in a Cdc6-dependent manner and was then released, concomitant with initiation of DNA replication. Once pre-replication complexes (pre-RCs) were assembled either in vitro or in vivo, further binding of XlORC was inhibited. Neither binding nor release of XlORC was affected by inhibitors of either cyclin-dependent protein kinase activity or DNA synthesis. In contrast, inhibition of pre-RC assembly, either by addition of Xenopus geminin or by depletion of XlMcm proteins, augmented ORC binding by inhibiting ORC release. These results demonstrate a programmed release of XlORC from somatic cell chromatin as it enters S phase, consistent with the proposed role for ORC in preventing re-initiation of DNA replication during S phase.

Human origin recognition complex large subunit is degraded by ubiquitin-mediated proteolysis after initiation of DNA replication

Eukaryotic cells possess overlapping mechanisms to ensure that DNA replication is restricted to the S phase of the cell cycle. The levels of hOrc1p, the largest subunit of the human origin recognition complex, vary during the cell division cycle. In rapidly proliferating cells, hOrc1p is expressed and targeted to chromatin as cells exit mitosis and prereplicative complexes are formed. Later, as cyclin A accumulates and cells enter S phase, hOrc1p is ubiquitinated on chromatin and then degraded. hOrc1p destruction occurs through the proteasome and is signaled in part by the SCF(Skp2) ubiquitin-ligase complex. Other hORC subunits are stable throughout the cell cycle. The regulation of hOrc1p may be an important mechanism in maintaining the ploidy in human cells.

Identification of a binding region for human origin recognition complex proteins 1 and 2 that coincides with an origin of DNA replication

We investigated the binding regions of components of the origin recognition complex (ORC) in the human genome. For this purpose, we performed chromatin immunoprecipitation assays with antibodies against human Orc1 and Orc2 proteins. We identified a binding region for human Orc proteins 1 and 2 in a <1-kbp segment between two divergently transcribed human genes. The region is characterized by CpG tracts and a central sequence rich in AT base pairs. Both, Orc1 and Orc2 proteins are found at the intergenic region in the G(1) phase, but S-phase chromatin contains only Orc2 protein, supporting the notion that Orc1p dissociates from its binding site in the S phase. Sequences corresponding to the intergenic region are highly abundant in a fraction of nascent DNA strands, strongly suggesting that this region not only harbors the binding sites for Orc1 protein and Orc2 protein but also serves as an origin of bidirectional DNA replication.

Replication licensing--defining the proliferative state?

The proliferation of eukaryotic cells is a highly regulated process that depends on the precise duplication of chromosomal DNA in each cell cycle. Regulation of the replication licensing system, which promotes the assembly of complexes of proteins termed Mcm2-7 onto replication origins, is responsible for preventing re-replication of DNA in a single cell cycle. Recent work has shown how the licensing system is directly controlled by cyclin-dependent kinases (CDKs). Repression of origin licensing is emerging as a ubiquitous route by which the proliferative capacity of cells is lowered, and Mcm2-Mcm7 proteins show promise as diagnostic markers of early cancer stages. These results have prompted us to propose a functional distinction between the proliferative state and the non-proliferative state (including G0) depending on whether origins are licensed.

Drosophila minichromosome maintenance 6 is required for chorion gene amplification and genomic replication

Duplication of the eukaryotic genome initiates from multiple origins of DNA replication whose activity is coordinated with the cell cycle. We have been studying the origins of DNA replication that control amplification of eggshell (chorion) genes during Drosophila oogenesis. Mutation of genes required for amplification results in a thin eggshell phenotype, allowing a genetic dissection of origin regulation. Herein, we show that one mutation corresponds to a subunit of the minichromosome maintenance (MCM) complex of proteins, MCM6. The binding of the MCM complex to origins in G1 as part of a prereplicative complex is critical for the cell cycle regulation of origin licensing. We find that MCM6 associates with other MCM subunits during amplification. These results suggest that chorion origins are bound by an amplification complex that contains MCM proteins and therefore resembles the prereplicative complex. Lethal alleles of MCM6 reveal it is essential for mitotic cycles and endocycles, and suggest that its function is mediated by ATP. We discuss the implications of these findings for the role of MCMs in the coordination of DNA replication during the cell cycle.

A potential role for mini-chromosome maintenance (MCM) proteins in initiation at the dihydrofolate reductase replication origin

Mini-chromosome maintenance (MCM) proteins were originally identified in yeast, and homologues have been identified in several other eukaryotic organisms, including mammals. These findings suggest that the mechanisms by which eukaryotic cells initiate and regulate DNA replication have been conserved throughout evolution. However, it is clear that many mammalian origins are much more complex than those of yeast. An example is the Chinese hamster dihydrofolate reductase (DHFR) origin, which resides in the spacer between the DHFR and 2BE2121 genes. This origin consists of a broad zone of potential sites scattered throughout the 55-kb spacer, with several subregions (e.g. ori-beta, ori-beta', and ori-gamma) being preferred. We show here that antibodies to human MCMs 2-7 recognize counterparts in extracts prepared from hamster cells; furthermore, co-immunoprecipitation data demonstrate the presence of an MCM2-3-5 subcomplex as observed in other species. To determine whether MCM proteins play a role in initiation and/or elongation in Chinese hamster cells, we have examined in vivo protein-DNA interactions between the MCMs and chromatin in the DHFR locus using a chromatin immunoprecipitation (ChIP) approach. In synchronized cultures, MCM complexes associate preferentially with DNA in the intergenic initiation zone early in S-phase during the time that replication initiates. However, significant amounts of MCMs were also detected over the two genes, in agreement with recent observations that the MCM complex co-purifies with RNA polymerase II. As cells progress through S-phase, the MCMs redistribute throughout the DHFR domain, suggesting a dynamic interaction with DNA. In asynchronous cultures, in which replication forks should be found at any position in the genome, MCM proteins were distributed relatively evenly throughout the DHFR locus. Altogether, these data are consistent with studies in yeast showing that MCM subunits localize to origins during initiation and then migrate outward with the replication forks. This constitutes the first evidence that mammalian MCM complexes perform a critical role during the initiation and elongation phases of replication at the DHFR origin in hamster cells.

Mammalian nuclei become licensed for DNA replication during late telophase

Mcm 2-7 are essential replication proteins that bind to chromatin in mammalian nuclei during late telophase. Here, we have investigated the relationship between Mcm binding, licensing of chromatin for replication, and specification of the dihydrofolate reductase (DHFR) replication origin. Approximately 20% of total Mcm3 protein was bound to chromatin in Chinese hamster ovary (CHO) cells during telophase, while an additional 25% bound gradually and cumulatively throughout G1-phase. To investigate the functional significance of this binding, nuclei prepared from CHO cells synchronized at various times after metaphase were introduced into Xenopus egg extracts, which were either immunodepleted of Mcm proteins or supplemented with geminin, an inhibitor of the Mcm-loading protein Cdt1. Within 1 hour after metaphase, coincident with completion of nuclear envelope formation, CHO nuclei were fully competent to replicate in both of these licensing-defective extracts. However, sites of initiation of replication in each of these extracts were found to be dispersed throughout the DHFR locus within nuclei isolated between 1 to 5 hours after metaphase, but became focused to the DHFR origin within nuclei isolated after 5 hours post-metaphase. Importantly, introduction of permeabilized post-ODP, but not pre-ODP, CHO nuclei into licensing-deficient Xenopus egg extracts resulted in the preservation of a significant degree of DHFR origin specificity, implying that the previously documented lack of specific origin selection in permeabilized nuclei is at least partially due to the licensing of new initiation sites by proteins in the Xenopus egg extracts. We conclude that the functional association of Mcm proteins with chromatin (i.e. replication licensing) in CHO cells takes place during telophase, several hours prior to the specification of replication origins at the DHFR locus.

Mammalian Orc1 protein is selectively released from chromatin and ubiquitinated during the S-to-M transition in the cell division cycle

Previous studies have shown that changes in the affinity of the hamster Orc1 protein for chromatin during the M-to-G(1) transition correlate with the activity of hamster origin recognition complexes (ORCs) and the appearance of prereplication complexes at specific sites. Here we show that Orc1 is selectively released from chromatin as cells enter S phase, converted into a mono- or diubiquitinated form, and then deubiquitinated and re-bound to chromatin during the M-to-G(1) transition. Orc1 is degraded by the 26S proteasome only when released into the cytosol, and peptide additions to Orc1 make it hypersensitive to polyubiquitination. In contrast, Orc2 remains tightly bound to chromatin throughout the cell cycle and is not a substrate for ubiquitination. Since the concentration of Orc1 remains constant throughout the cell cycle, and its half-life in vivo is the same as that of Orc2, ubiquitination of non-chromatin-bound Orc1 presumably facilitates the inactivation of ORCs by sequestering Orc1 during S phase. Thus, in contrast to yeast (Saccharomyces cerevisiae and Schizosaccharomyces pombe), mammalian ORC activity appears to be regulated during each cell cycle through selective dissociation and reassociation of Orc1 from chromatin-bound ORCs.