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

Hepatitis B virus X protein affects S phase progression leading to chromosome segregation defects by binding to damaged DNA binding protein 1

Chronic hepatitis B virus (HBV) infection is a leading cause of hepatocellular carcinoma (HCC), but its role in the transformation process remains unclear. HBV encodes a small protein, known as HBx, which is required for infection and has been implicated in hepatocarcinogenesis. Here we show that HBx induces lagging chromosomes during mitosis, which in turn leads to formation of aberrant mitotic spindles and multinucleated cells. These effects require the binding of HBx to UV-damaged DNA binding protein 1 (DDB1), a protein involved in DNA repair and cell cycle regulation, and are unexpectedly attributable to HBx interfering with S-phase progression and not directly with mitotic events. HBx also affects S-phase and induces lagging chromosomes when expressed from its natural viral context and, consequently, exhibits deleterious activities in dividing, but not quiescent, hepatoma cells.

CONCLUSION

In addition to its reported role in promoting HBV replication, the binding of HBx to DDB1 may induce genetic instability in regenerating hepatocytes and thereby contribute to HCC development, thus making this HBV-host protein interaction an attractive target for new therapeutic intervention.

Binding of Drosophila ORC proteins to anaphase chromosomes requires cessation of mitotic cyclin-dependent kinase activity

The initial step in the acquisition of replication competence by eukaryotic chromosomes is the binding of the multisubunit origin recognition complex, ORC. We describe a transgenic Drosophila model which enables dynamic imaging of a green fluorescent protein (GFP)-tagged Drosophila melanogaster ORC subunit, DmOrc2-GFP. It is functional in genetic complementation, expressed at physiological levels, and participates quantitatively in complex formation. This fusion protein is therefore able to depict both the holocomplex DmOrc1-6 and the core complex DmOrc2-6 formed by the Drosophila initiator proteins. Its localization can be monitored in vivo along the cell cycle and development. DmOrc2-GFP is not detected on metaphase chromosomes but binds rapidly to anaphase chromatin in Drosophila embryos. Expression of either stable cyclin A, B, or B3 prevents this reassociation, suggesting that cessation of mitotic cyclin-dependent kinase activity is essential for binding of the DmOrc proteins to chromosomes.

Involvement of human ORC and TRF2 in pre-replication complex assembly at telomeres

The origin recognition complex (ORC) binds to replication origins to regulate the cell cycle-dependent assembly of pre-replication complexes (pre-RCs). We have found a novel link between pre-RC assembly regulation and telomere homeostasis in human cells. Biochemical analyses showed that human ORC binds to TRF2, a telomere sequence-binding protein that protects telomeres and functions in telomere length homeostasis, via the ORC1 subunit. Immunostaining further revealed that ORC and TRF2 partially co-localize in nuclei, whereas chromatin immunoprecipitation analyses confirmed that pre-RCs are assembled at telomeres in a cell cycle-dependent manner. Over-expression of TRF2 stimulated ORC and MCM binding to chromatin and RNAi-directed TRF2 silencing resulted in reduced ORC binding and pre-RC assembly at telomeres. As expected from previous reports, TRF2 silencing induced telomere elongation. Interestingly, ORC1 silencing by RNAi weakened the TRF2 binding as well as the pre-RC assembly at telomeres, suggesting that ORC and TRF2 interact with each other to achieve stable binding. Furthermore, ORC1 silencing also resulted in modest telomere elongation. These data suggest that ORC might be involved in telomere homeostasis in human cells.

Plasmodium falciparum origin recognition complex subunit 5: functional characterization and role in DNA replication foci formation

The mechanism of DNA replication initiation and progression is poorly understood in the parasites, including human malaria parasite Plasmodium falciparum. Using bioinformatics tools and yeast complementation assay, we identified a putative homologue of Saccharomyces cerevisiaeorigin recognition complex subunit 5 in P. falciparum (PfORC5). PfORC5 forms distinct nuclear foci colocalized with the replication foci marker proliferating cell nuclear antigen (PfPCNA) and co-immunoprecipitates with PCNA during early-to-mid trophozoite stage replicating parasites. Interestingly, these proteins separate from each other at the non-replicating late schizont stage, citing the evidence of the presence of both PCNA and ORC components in replication foci during eukaryotic DNA replication. PfORC1, another ORC subunit, colocalizes with PfPCNA and PfORC5 at the beginning of DNA replication, but gets degraded at the late schizont stage, ensuring the regulation of DNA replication in the parasites. Further, we have identified putative PCNA-interacting protein box in PfORC1 that may explain in part the colocalization of PfORC and PfPCNA. Additionally, use of specific DNA replication inhibitor hydroxyurea affects ORC5/PCNA foci formation and parasitic growth. These results strongly favour replication factory model in the parasites and confer great potential to understand the co-ordination between ORC and PCNA during eukaryotic DNA replication in general.

Chromosome cohesion - rings, knots, orcs and fellowship

Sister-chromatid cohesion is essential for accurate chromosome segregation. A key discovery towards our understanding of sister-chromatid cohesion was made 10 years ago with the identification of cohesins. Since then, cohesins have been shown to be involved in cohesion in numerous organisms, from yeast to mammals. Studies of the composition, regulation and structure of the cohesin complex led to a model in which cohesin loading during S-phase establishes cohesion, and cohesin cleavage at the onset of anaphase allows sister-chromatid separation. However, recent studies have revealed activities that provide cohesion in the absence of cohesin. Here we review these advances and propose an integrative model in which chromatid cohesion is a result of the combined activities of multiple cohesion mechanisms.

Phosphorylation of MCM3 on Ser-112 regulates its incorporation into the MCM2-7 complex

During late M and early G(1), MCM2-7 assembles and is loaded onto chromatin in the final step of prereplicative complex (pre-RC) formation. However, the regulation of MCM assembly remains poorly understood. Cyclin-dependent kinase (CDK)-dependent phosphorylation contributes to DNA replication by initially activating pre-RCs and subsequently inhibiting refiring of origins during S and M phases, thus limiting DNA replication to a single round. Although the precise roles of specific MCM phosphorylation events are poorly characterized, we now demonstrate that CDK1 phosphorylates MCM3 at Ser-112, Ser-611, and Thr-719. In vivo, CDK1-dependent phosphorylation of Ser-112 triggers the assembly of MCM3 with the remaining MCM subunits and subsequent chromatin loading of MCMs. Strikingly, loss of MCM3 triggers the destabilization of other MCM proteins, suggesting that phosphorylation-dependent assembly is essential for stable accumulation of MCM proteins. These data reveal that CDK-dependent MCM3 phosphorylation contributes to the regulated formation of the MCM2-7 complex.

A topoisomerase II-dependent mechanism for resetting replicons at the S-M-phase transition

Topoisomerase II (topo II) is required for chromosome segregation and for reprogramming replicons. Here, we show that topo II couples DNA replication termination with the clearing of replication complexes for resetting replicons at mitosis. Topo II inhibition impairs completion of DNA replication, accounting for replication protein A (RPA) stabilization onto ssDNA. Topo II inhibition does not affect the caffeine-sensitive ORC1 degradation found upon origin firing, but it impairs the cdk-dependent degradation/chromatin dissociation of an ORC1/2 reservoir at mitosis. Our results show that ORC1 degradation is rescued by Pin1 depletion and that this topo II-dependent clearing of ORC1/2 from chromatin involves the APC.

Deletion of histone deacetylase 3 reveals critical roles in S phase progression and DNA damage control

Histone deacetylases (HDACs) are enzymes that modify key residues in histones to regulate chromatin architecture, and they play a vital role in cell survival, cell-cycle progression, and tumorigenesis. To understand the function of Hdac3, a critical component of the N-CoR/SMRT repression complex, a conditional allele of Hdac3 was engineered. Cre-recombinase-mediated inactivation of Hdac3 led to a delay in cell-cycle progression, cell-cycle-dependent DNA damage, and apoptosis in mouse embryonic fibroblasts (MEFs). While no overt defects in mitosis were observed in Hdac3-/- MEFs, including normal H3Ser10 phosphorylation, DNA damage was observed in Hdac3-/- interphase cells, which appears to be associated with defective DNA double-strand break repair. Moreover, we noted that Hdac3-/- MEFs were protected from DNA damage when quiescent, which may provide a mechanistic basis for the action of HDAC inhibitors on cycling tumor cells.

Downregulation of protein 4.1R, a mature centriole protein, disrupts centrosomes, alters cell cycle progression, and perturbs mitotic spindles and anaphase

Centrosomes nucleate and organize interphase microtubules and are instrumental in mitotic bipolar spindle assembly, ensuring orderly cell cycle progression with accurate chromosome segregation. We report that the multifunctional structural protein 4.1R localizes at centrosomes to distal/subdistal regions of mature centrioles in a cell cycle-dependent pattern. Significantly, 4.1R-specific depletion mediated by RNA interference perturbs subdistal appendage proteins ninein and outer dense fiber 2/cenexin at mature centrosomes and concomitantly reduces interphase microtubule anchoring and organization. 4.1R depletion causes G(1) accumulation in p53-proficient cells, similar to depletion of many other proteins that compromise centrosome integrity. In p53-deficient cells, 4.1R depletion delays S phase, but aberrant ninein distribution is not dependent on the S-phase delay. In 4.1R-depleted mitotic cells, efficient centrosome separation is reduced, resulting in monopolar spindle formation. Multipolar spindles and bipolar spindles with misaligned chromatin are also induced by 4.1R depletion. Notably, all types of defective spindles have mislocalized NuMA (nuclear mitotic apparatus protein), a 4.1R binding partner essential for spindle pole focusing. These disruptions contribute to lagging chromosomes and aberrant microtubule bridges during anaphase/telophase. Our data provide functional evidence that 4.1R makes crucial contributions to the structural integrity of centrosomes and mitotic spindles which normally enable mitosis and anaphase to proceed with the coordinated precision required to avoid pathological events.

Mitotic functions for SNAP45, a subunit of the small nuclear RNA-activating protein complex SNAPc

The small nuclear RNA-activating protein complex SNAP_c is required for transcription of small nuclear RNA genes and binds to a proximal sequence element in their promoters. SNAP_c contains five types of subunits stably associated with each other. Here we show that one of these polypeptides, SNAP45, also known as PTF δ, localizes to centrosomes during parts of mitosis, as well as to the spindle midzone during anaphase and the mid-body during telophase. Consistent with localization to these mitotic structures, both down- and up-regulation of SNAP45 lead to a G₂/M arrest with cells displaying abnormal mitotic structures. In contrast, down-regulation of SNAP190, another SNAP_c subunit, leads to an accumulation of cells with a G₀/G₁ DNA content. These results are consistent with the proposal that SNAP45 plays two roles in the cell, one as a subunit of the transcription factor SNAP_c and another as a factor required for proper mitotic progression.

The origin recognition complex localizes to telomere repeats and prevents telomere-circle formation

Chromosome ends are maintained by telomere-repeat-binding factors (TRFs) that coordinate DNA end protection with telomere replication. The origin recognition complex (ORC) coordinates bidirectional DNA replication at most chromosomal sites, but it is also known to function in transcriptional silencing, heterochromatin formation, and sister-chromatid cohesion. We now show that ORC localizes to telomere repeats and contributes to telomere maintenance. We found that ORC subunits can be affinity purified with telomere-repeat DNA along with other components of the known "shelterin" complex. ORC subunits colocalized with telomere-repeat foci and coimmunoprecipitated with TRF2 but not TRF2 lacking its amino-terminal basic domain (TRF2DeltaB). ORC2 depletion or hypomorphic cell lines caused a loss of telomere-repeat signal intensity and the appearance of dysfunctional telomeres, including telomere-signal-free ends and telomere-repeat-containing double minutes. Two-dimensional agarose gel electrophoresis revealed that ORC2 depletion increased telomere circle formation, comparable to the overexpression of TRF2DeltaB. A similar increase in telomere circle formation was induced by hydroxyurea treatment, providing evidence that replication stress produces telomere circles. These findings suggest that ORC recruitment by TRF2 contributes to telomere integrity by facilitating efficient telomere DNA replication and preventing the generation of telomere-repeat-containing circles.

Dictyostelium Sun-1 connects the centrosome to chromatin and ensures genome stability

The centrosome-nucleus attachment is a prerequisite for faithful chromosome segregation during mitosis. We addressed the function of the nuclear envelope (NE) protein Sun-1 in centrosome-nucleus connection and the maintenance of genome stability in Dictyostelium discoideum. We provide evidence that Sun-1 requires direct chromatin binding for its inner nuclear membrane targeting. Truncation of the cryptic N-terminal chromatin-binding domain of Sun-1 induces dramatic separation of the inner from the outer nuclear membrane and deformations in nuclear morphology, which are also observed using a Sun-1 RNAi construct. Thus, chromatin binding of Sun-1 defines the integrity of the nuclear architecture. In addition to its role as a NE scaffold, we find that abrogation of the chromatin binding of Sun-1 dissociates the centrosome-nucleus connection, demonstrating that Sun-1 provides an essential link between the chromatin and the centrosome. Moreover, loss of the centrosome-nucleus connection causes severe centrosome hyperamplification and defective spindle formation, which enhances aneuploidy and cell death significantly. We highlight an important new aspect for Sun-1 in coupling the centrosome and nuclear division during mitosis to ensure faithful chromosome segregation.

Interaction between HMGA1a and the origin recognition complex creates site-specific replication origins

In all eukaryotic cells, origins of DNA replication are characterized by the binding of the origin recognition complex (ORC). How ORC is positioned to sites where replication initiates is unknown, because metazoan ORC binds DNA without apparent sequence specificity. Thus, additional factors might be involved in ORC positioning. Our experiments indicate that a family member of the high-mobility group proteins, HMGA1a, can specifically target ORC to DNA. Coimmunoprecipitations and imaging studies demonstrate that HMGA1a interacts with different ORC subunits in vitro and in vivo. This interaction occurs mainly in AT-rich heterochromatic regions to which HMGA1a localizes. Fusion proteins of HMGA1a and the DNA-binding domain of the viral factor EBNA1 or the prokaryotic tetracycline repressor, TetR, can recruit ORC to cognate operator sites forming functional origins of DNA replication. When HMGA1a is targeted to plasmid DNA, the prereplicative complex is assembled during G(1) and the amount of ORC correlates with the local concentration of HMGA1a. Nascent-strand abundance assays demonstrate that DNA replication initiates at or near HMGA1a-rich sites. Our experiments indicate that chromatin proteins can target ORC to DNA, suggesting they might specify origins of DNA replication in metazoan cells.

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.

Dendrite development: a surprising origin

Neurons extend elaborate dendrites studded with spines. Unexpectedly, this cellular sculpting is regulated by the origin recognition complex—the core machinery for initiating DNA replication.

An autonomous replicating element within the KSHV genome

Members of the herpesviridae family including Kaposi's sarcoma-associated herpesvirus (KSHV) persist latently in their hosts and harbor their genomes as closed circular episomes. Propagation of the KSHV genome into new daughter cells requires replication of the episome once every cell division and is considered critically dependent on expression of the virus encoded latency-associated nuclear antigen (LANA). This study demonstrates a LANA-independent mechanism of KSHV latent DNA replication. A cis-acting DNA element within a discreet KSHV genomic region termed the long unique region (LUR) can initiate and support replication of plasmids lacking LANA-binding sequences or a eukaryotic replication origin. The human cellular replication machinery proteins ORC2 and MCM3 associated with the LUR element and depletion of cellular ORC2 abolished replication of the plasmids indicating that recruitment of the host cellular replication machinery is important for LUR-dependent replication. Thus, KSHV can initiate replication of its genome independent of any trans-acting viral factors.

Human Mcm10 regulates the catalytic subunit of DNA polymerase-alpha and prevents DNA damage during replication

In Saccharomyces cerevisiae, minichromosome maintenance protein (Mcm) 10 interacts with DNA polymerase (pol)-alpha and functions as a nuclear chaperone for the catalytic subunit, which is rapidly degraded in the absence of Mcm10. We report here that the interaction between Mcm10 and pol-alpha is conserved in human cells. We used a small interfering RNA-based approach to deplete Mcm10 in HeLa cells, and we observed that the catalytic subunit of pol-alpha, p180, was degraded with similar kinetics as Mcm10, whereas the regulatory pol-alpha subunit, p68, remained unaffected. Simultaneous loss of Mcm10 and p180 inhibited S phase entry and led to an accumulation of already replicating cells in late S/G2 as a result of DNA damage, which triggered apoptosis in a subpopulation of cells. These phenotypes differed considerably from analogous studies in Drosophila embryo cells that did not exhibit a similar arrest. To further dissect the roles of Mcm10 and p180 in human cells, we depleted p180 alone and observed a significant delay in S phase entry and fork progression but little effect on cell viability. These results argue that cells can tolerate low levels of p180 as long as Mcm10 is present to "recycle" it. Thus, human Mcm10 regulates both replication initiation and elongation and maintains genome integrity.

The role of pre‐replicative complex (pre‐RC) components in oncogenesis

Normal DNA replication is stringently regulated to ensure a timely occurrence no more than once per cell cycle. Abrogation of the exquisite control mechanisms that maintain this process results in detrimental gains and losses of genomic DNA commonly seen in cancer and developmental defects. Replication initiation proteins, known as prereplicative complex (pre-RC) proteins, serve as a primary level of regulation, controlling when DNA replication can begin. Unsurprisingly, several pre-RC proteins are overexpressed in cancer and serve as good tumor markers. However, their direct correlation with increasing tumor grade and poor prognosis has posed a long-standing question: Are pre-RC proteins oncogenic? Recently, a growing body of data indicates that deregulation of individual pre-RC proteins, either by overexpression or functional deficiency in several organismal models, results in significant and consistently perturbed cell cycle regulation, genomic instability, and, potentially, tumorigenesis. In this review, we examine this broad range of evidence suggesting that pre-RC proteins play roles during oncogenesis that are more than simply indicative of proliferation, supporting the notion that pre-RC proteins may potentially have significant diagnostic and therapeutic value.

Interactions and subcellular distribution of DNA replication initiation proteins in eukaryotic cells

For initiation of eukaryotic DNA replication the origin recognition complex (ORC) associates with chromatin sites and constitutes a landing pad allowing Cdc6, Cdt1 and MCM proteins to accomplish the pre-replication complex (pre-RC). In S phase, the putative MCM helicase is assumed to move away from the ORC to trigger DNA unwinding. By using the fluorescence-based assays bioluminescence resonance energy transfer (BRET) and bimolecular fluorescence complementation (BiFC) we show in live mammalian cells that one key interaction in pre-RC assembly, the interaction between Orc2 and Orc3, is not restricted to the nucleus but also occurs in the cytoplasm. BRET assays also revealed a direct interaction between Orc2 and nuclear localization signal (NLS)-depleted Orc3. Further, we assessed the subcellular distribution of Orc2 and Orc3 in relation to MCM proteins Mcm3 and Mcm6 as well as to a key protein involved in elongation of DNA replication, proliferating nuclear cell antigen (PCNA). Our findings illustrate the spatial complexity of the elaborated process of DNA replication as well as that the BRET and BiFC techniques are novel tools that could contribute to our understanding of the processes at the very beginning of the duplication of the genome.

Visualizing the dynamics of chromosome structure formation coupled with DNA replication

A basic question of cell biology is how DNA folds to chromosome. Numbers of examples have suggested the involvement of DNA replication in chromosome structure formation. To visualize and identify the dynamics of chromosome structure formation and to elucidate the involvement of DNA replication in chromosome construction, Cy3-2'-deoxyuridine-5'-triphosphate direct-labeled active replicating DNA was observed in prematurely condensed chromosomes (PCCs) under a confocal scanning microscope utilized with drug-induced premature chromosome condensation (PCC) technique that facilitates the visualization of interphase chromatin as condensed chromosome form. S-phase PCCs revealed clearly the drastic dynamics of chromosome formation that transits during S-phase from a 'cloudy nebula' to numerous numbers of 'beads on a string' and finally to 'striped arrays of banding structured chromosome' along with the progress of DNA replication. The number, distribution, and shape of replication foci were also measured in individual subphases of S-phase more precisely than reported previously; maximally, approximately 1,400 foci of 0.35 microm average radius size were scored at the beginning of the S-phase, and the number reduced to approximately 100 at the end of the S-phase. Drug-induced PCC clearly provided the new insight that eukaryote DNA replication is tightly coupled with the chromosome condensation/compaction for the construction of the higher-ordered structure of the eukaryote chromosome.

Localization and organization of protein factors involved in chromosome inheritance in Dictyostelium discoideum

Heterochromatin protein 1 (HP1) proteins are highly conserved heterochromatin components required for genomic integrity. We have previously shown that the two HP1 isoforms expressed in Dictyostelium, HcpA and HcpB, are mainly localized to (peri-)centromeric heterochromatin and have largely overlapping functions. However, they cause distinct phenotypes when overexpressed. We show here that these isoforms display quantitative differences in dimerization behavior. Dimerization preference, as well as the mutant phenotype in overexpression strains, depends on the C-terminus containing the hinge and chromo shadow domains. Both Hcp proteins are targeted to distinct subnuclear regions by different chromo shadow domain-dependent and -independent mechanisms. In addition, both proteins bind to DNA and RNA in vitro and binding is independent of the chromo shadow domain. Thus, this DNA and/or RNA binding activity may contribute to protein targeting. To further characterize heterochromatin, we cloned the Dictyostelium homolog of the origin recognition complex subunit 2 (OrcB). OrcB localizes to distinct subnuclear foci that were also targeted by HcpA. In addition, it is associated with the centrosome throughout the cell cycle. The results indicate that, similar to Orc2 homologs from other organisms, it is required for different processes in chromosome inheritance.

The many faces of the origin recognition complex

The hetero-hexameric origin recognition complex (ORC) is well known for its separable roles in DNA replication and heterochromatin assembly. However, ORC and its individual subunits have been implicated in diverse cellular activities in both the nucleus and the cytoplasm. Some of ORC's implied functions, such as cell cycle checkpoint control and mitotic chromosome assembly, may be indirectly related to its roles in replication control and/or heterochromatin assembly. Other suggested roles in ribosomal biogenesis and in centrosome and kinetochore function are based on localization/interaction data and are as yet inconclusive. However, recent findings directly link ORC to sister chromatin cohesion, cytokinesis and neural dendritic branching.

Identification of new centrosome proteins by autoimmune patient sera

Compared to other subcellular organelles, centrosome proteome can hardly be studied, due to the difficulties in separation and purification of centrosome. Auto-antisera from 6 autoimmune patients, which recognized centrosome specifically in immunofluorescence, were used to identify the corresponding centrosomal proteins. The sera were first tested by Western blot on whole cell lysate, and all bound antibodies were then eluted from each single band in Western blot membrane to assure which antibody was responsible for the centrosome specific immunofluorescence staining. The corresponding proteins were obtained by immunoprecipitation and identified by mass spectrometry. Six centrosomal proteins, including 2 known centrosomal proteins and 4 proteins with unknown localization or reportedly non-centrosomal localization, were identified. These proteins apparently involve in cell cycle regulation, signal transduction pathways, molecular chaperons, and metabolism enzymes, which may reflect the expected functional diversity of centrosome.

Genome-wide localization of pre-RC sites and identification of replication origins in fission yeast

DNA replication of eukaryotic chromosomes initiates at a number of discrete loci, called replication origins. Distribution and regulation of origins are important for complete duplication of the genome. Here, we determined locations of Orc1 and Mcm6, components of pre-replicative complex (pre-RC), on the whole genome of Schizosaccharomyces pombe using a high-resolution tiling array. Pre-RC sites were identified in 460 intergenic regions, where Orc1 and Mcm6 colocalized. By mapping of 5-bromo-2'-deoxyuridine (BrdU)-incorporated DNA in the presence of hydroxyurea (HU), 307 pre-RC sites were identified as early-firing origins. In contrast, 153 pre-RC sites without BrdU incorporation were considered to be late and/or inefficient origins. Inactivation of replication checkpoint by Cds1 deletion resulted in BrdU incorporation with HU specifically at the late origins. Early and late origins tend to distribute separately in large chromosome regions. Interestingly, pericentromeric heterochromatin and the silent mating-type locus replicated in the presence of HU, whereas the inner centromere or subtelomeric heterochromatin did not. Notably, MCM did not bind to inner centromeres where origin recognition complex was located. Thus, replication is differentially regulated in chromosome domains.

The origin recognition complex functions in sister-chromatid cohesion in Saccharomyces cerevisiae

High-fidelity chromosomal segregation requires the properly timed establishment of sister-chromatid cohesion mediated by the Cohesin complex, and its resolution at the metaphase-to-anaphase transition. We have examined cell-cycle progression in a yeast strain from which the origin recognition complex protein Orc2 was depleted after the assembly of prereplication complexes. We find that Orc2 depletion causes a delay in progression through mitosis, reflecting activation of both the DNA-damage and Mad2-spindle checkpoints. Surprisingly, sister-chromatid cohesion is impaired in Orc2-depleted cells, although Cohesin subunits are properly associated with chromatin. Reexpression of Orc2 in late G2/M phase restores chromatid cohesion. Finally, the targeting of Orc2 to a specific chromosomal locus suppresses premature sister-chromatid separation locally in a temperature-sensitive cohesin mutant. We conclude that ORC mediates sister-chromatid interaction on a pathway that is additive with Cohesin-mediated pairing.

Multiple functions of the origin recognition complex

The origin recognition complex (ORC), a heteromeric six-subunit protein, is a central component for eukaryotic DNA replication. The ORC binds to DNA at replication origin sites in an ATP-dependent manner and serves as a scaffold for the assembly of other key initiation factors. Sequence rules for ORC-DNA binding appear to vary widely. In budding yeast the ORC recognizes specific ori elements, however, in higher eukaryotes origin site selection does not appear to depend on the specific DNA sequence. In metazoans, during cell cycle progression, one or more of the ORC subunits can be modified in such a way that ORC activity is inhibited until mitosis is complete and a nuclear membrane is assembled. In addition to its well-documented role in the initiation of DNA replication, the ORC is also involved in other cell functions. Some of these activities directly link cell cycle progression with DNA replication, while other functions seem distinct from replication. The function of ORCs in the establishment of transcriptionally repressed regions is described for many species and may be a conserved feature common for both unicellular eukaryotes and metazoans. ORC subunits were found at centrosomes, at the cell membranes, at the cytokinesis furrows of dividing cells, as well as at the kinetochore. The exact mechanism of these localizations remains to be determined, however, latest results support the idea that ORC proteins participate in multiple aspects of the chromosome inheritance cycle. In this review, we discuss the participation of ORC proteins in various cell functions, in addition to the canonical role of ORC in initiating DNA replication.

Mouse pre-replicative complex proteins colocalise and interact with the centrosome

Initiation of eukaryotic DNA replication is achieved by the sequential binding of different proteins to origins of DNA replication. Using EGFP-tagged initiator proteins and immunofluorescence techniques we found that most of the ORC and the MCM subunits are localised at centrosomes and are colocalised with the polo-like protein kinase, Plk1. Yeast two-hybrid studies revealed interactions of Plk1 with the Mcm2 as well as the Orc2 protein. Co-immunoprecipitations showed an interaction of Plk1 with Mcm2 as well as interactions of gamma-tubulin with Mcm3 and Orc2, respectively. An in vitro phosphorylation assay showed that the Orc2 protein is a substrate of Plk1. Depletion of Orc2 and Mcm3 by siRNA leads to an inhibition of cell proliferation, an altered cell cycle distribution as well as to multinucleated cells with insufficiently organised microtubules. These results indicate an important role of the MCM and ORC proteins in mitosis besides their described role in the establishment of the pre-replicative complex.

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.

Cell cycle execution point analysis of ORC function and characterization of the checkpoint response to ORC inactivation in Saccharomyces cerevisiae

Chromosomal replication initiates through the assembly of a prereplicative complex (pre-RC) at individual replication origins in the G1-phase, followed by activation of these complexes in the S-phase. In Saccharomyces cerevisiae, the origin recognition complex (ORC) binds replication origins throughout the cell cycle and participates in pre-RC assembly. Whether the ORC plays an additional role subsequent to pre-RC assembly in replication initiation or any other essential cell cycle process is not clear. To study the function of the ORC during defined cell cycle periods, we performed cell cycle execution point analyses with strains containing a conditional mutation in the ORC1, ORC2 or ORC5 subunit of ORC. We found that the ORC is essential for replication initiation, but is dispensable for replication elongation or later cell cycle events. Defective initiation in ORC mutant cells results in incomplete replication and mitotic arrest enforced by the DNA damage and spindle assembly checkpoint pathways. The involvement of the spindle assembly checkpoint implies a defect in kinetochore-spindle attachment or sister chromatid cohesion due to incomplete replication and/or DNA damage. Remarkably, under semipermissive conditions for ORC1 function, the spindle checkpoint alone suffices to block proliferation, suggesting this checkpoint is highly sensitive to replication initiation defects. We discuss the potential significance of these overlapping checkpoints and the impact of our findings on previously postulated role(s) of ORCs in other cell cycle functions.

Centrosome control of the cell cycle

Early observations of centrosomes, made a century ago, revealed a tiny dark structure surrounded by a radial array of cytoplasmic fibers. We now know that the fibers are microtubules and that the dark organelles are centrosomes that mediate functions far beyond the more conventional role of microtubule organization. More recent evidence demonstrates that the centrosome serves as a scaffold for anchoring an extensive number of regulatory proteins. Among these are cell-cycle regulators whose association with the centrosome is an essential step in cell-cycle control. Such studies show that the centrosome is required for several cell-cycle transitions, including G(1) to S-phase, G(2) to mitosis and metaphase to anaphase. In this review (which is part of the Chromosome Segregation and Aneuploidy series), we discuss recent data that provide the most direct links between centrosomes and cell-cycle progression.

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.

Interaction between HP1alpha and replication proteins in mammalian cells

HP1 is an essential heterochromatin-associated protein known to play an important role in the organization of heterochromatin as well as in the transcriptional regulation of heterochromatic and euchromatic genes both in repression and activation. Using the yeast two-hybrid system and immunoprecipitation, we report here that murine HP1alpha interacts with the preRC proteins ORC1, ORC2 and CDC6. Immunofluorescence staining and EGFP/DsRed fusion proteins revealed a colocalization of HP1alpha with ORC1, ORC2 and CDC6 in heterochromatin, supporting the notion that ORC and probably CDC6 play an important role in murine HP1alpha function. Besides that, we also observed a colocalization of HP1alpha with gamma-tubulin suggesting a centrosomal localization of HP1alpha in murine cells. To gain insight into HP1alpha function, we applied the RNAi technique. Depletion of HP1alpha leads to a slow down of cell proliferation, an aberrant cell cycle progression as well as to multinucleated cells with insufficiently organized microtubule. These results together indicate that HP1alpha exerts functions in mitosis and cytokinesis.

Genetic analysis of human Orc2 reveals specific domains that are required in vivo for assembly and nuclear localization of the origin recognition complex

Eukaryotic DNA replication begins with the binding of a six subunit origin recognition complex (ORC) to DNA. To study the assembly and function of mammalian ORC proteins in their native environment, HeLa cells were constructed that constitutively expressed an epitope-tagged, recombinant human Orc2 subunit that had been genetically altered. Analysis of these cell lines revealed that Orc2 contains a single ORC assembly domain that is required in vivo for interaction with all other ORC subunits, as well as two nuclear localization signals (NLSs) that are required for ORC accumulation in the nucleus. The recombinant Orc2 existed in the nucleus either as an ORC-(2-5) or ORC-(1-5) complex; no other combinations of ORC subunits were detected. Moreover, only ORC-(1-5) was bound to the chromatin fraction, suggesting that Orc1 is required in vivo to load ORC-(2-5) onto chromatin. Surprisingly, recombinant Orc2 suppressed expression of endogenous Orc2, revealing that mammalian cells limit the intracellular level of Orc2, and thereby limit the amount of ORC-(2-5) in the nucleus. Because this suppression required only the ORC assembly and NLS domains, these domains appear to constitute the functional domain of Orc2.

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.

Methylation-independent Binding to Histone H3 and Cell Cycle-dependent Incorporation of HP1β into Heterochromatin

We have examined HP1beta-chromatin interactions in different molecular contexts in vitro and in vivo. Employing purified components we show that HP1beta exhibits selective, stoichiometric, and salt-resistant binding to recombinant histone H3, associating primarily with the helical "histone fold" domain. Furthermore, using "bulk" nucleosomes released by MNase digestion, S-phase extracts, and fragments of peripheral heterochromatin, we demonstrate that HP1beta associates more tightly with destabilized or disrupted nucleosomes (H3/H4 subcomplexes) than with intact particles. Western blotting and mass spectrometry data indicate that HP1beta-selected H3/H4 particles and subparticles possess a complex pattern of posttranslational modifications but are not particularly enriched in me3K9-H3. Consistent with these results, mapping of HP1beta and me3K9-H3 sites in vivo reveals overlapping, yet spatially distinct patterns, while transient transfection assays with synchronized cells show that stable incorporation of HP1beta-gfp into heterochromatin requires passage through the S-phase. The data amassed challenge the dogma that me3K9H3 is necessary and sufficient for HP1 binding and unveil a new mode of HP1-chromatin interactions.

ORC is necessary at the interphase-to-mitosis transition to recruit cdc2 kinase and disassemble RPA foci

The origin-recognition complex (ORC) has an essential role in defining DNA replication origins and in chromosome segregation. Recent studies in Drosophila orc2 mutants, and in human cells depleted of ORC2, have suggested that this factor is also implicated in mitotic chromosome assembly. We asked whether ORC was required for M phase chromosome assembly independently of its function in DNA replication. We performed depletion assays and reconstitution experiments in Xenopus egg extracts, in conditions of M phase chromosome assembly coupled or uncoupled from DNA replication. We show that, although ORC is dispensable for mitotic chromosome condensation, it is necessary at the interphase-mitosis transition for proper mitotic chromosome assembly to occur in a reaction not strictly dependent on DNA replication. This function involves the recruitment to chromatin of cdc2 kinase and the chromatin disassembly of interphasic replication protein A (RPA) foci. Furthermore, we show that mutations of RPA at the cdc2 kinase site prevents RPA dissociation from chromatin and impairs mitotic chromosome assembly without affecting DNA replication. Our results support the conclusion that in addition to its role in the assembly of prereplication complexes (pre-RCs), at the G1-S transition, ORC is also required for their disassembly at mitotic entry.

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.

Cell cycle perturbations induced by infection with the coronavirus infectious bronchitis virus and their effect on virus replication

In eukaryotic cells, cell growth and division occur in a stepwise, orderly fashion described by a process known as the cell cycle. The relationship between positive-strand RNA viruses and the cell cycle and the concomitant effects on virus replication are not clearly understood. We have shown that infection of asynchronously replicating and synchronized replicating cells with the avian coronavirus infectious bronchitis virus (IBV), a positive-strand RNA virus, resulted in the accumulation of infected cells in the G2/M phase of the cell cycle. Analysis of various cell cycle-regulatory proteins and cellular morphology indicated that there was a down-regulation of cyclins D1 and D2 (G1 regulatory cyclins) and that a proportion of virus-infected cells underwent aberrant cytokinesis, in which the cells underwent nuclear, but not cytoplasmic, division. We assessed the impact of the perturbations on the cell cycle for virus-infected cells and found that IBV-infected G2/M-phase-synchronized cells exhibited increased viral protein production when released from the block when compared to cells synchronized in the G0 phase or asynchronously replicating cells. Our data suggested that IBV induces a G2/M phase arrest in infected cells to promote favorable conditions for viral replication.

A structural role for ATP in the formation and stability of the human origin recognition complex

The locally restricted recruitment of the multisubunit origin recognition complex (ORC) to eukaryotic chromosomes defines the position of origins of DNA replication. In budding yeast and metazoans the DNA binding activity of ORC is stimulated by ATP and requires an AAA+-type nucleotide binding domain in the largest subunit. Little else is known about the mechanisms behind the ATP requirement for ORC in its initiator function and, specifically, the relevance of nucleotide binding domains present on other subunits. Here we show that ATP is required for specific subunit interactions in the human ORC, with the Orc4 subunit playing a critical role in this dynamic process. ATP is essential for the maintenance of ORC integrity and facilitates complex formation. Thus, besides its previously identified role in DNA binding, ATP serves also as a structural cofactor for human ORC.

Overexpression of ORC subunits and increased ORC-chromatin association in transformed mammalian cells

The origin recognition complex (ORC) is a conserved heterohexamer required for the formation of pre-replication (pre-RC) complexes at origins of DNA replication. Many studies of ORC subunits have been carried out in transformed human cell lines but the properties of ORC in primary cells have not been addressed. Here, we compare the expression levels and chromatin-association of ORC subunits in HeLa cells to the primary human cell line, WI38, and a virally transformed derivative of WI38, VA13. ORC subunits 2 and 4 were highly overexpressed in both HeLa and VA13, whereas ORC1 levels were elevated in VA13 but considerably higher in HeLa cells. Cellular extraction revealed that the proportion of ORC2 and ORC4 subunits bound to chromatin was similar in all three cell lines throughout the cell-cycle. In contrast, very little ORC1 was associated with chromatin after extraction of primary WI38 cells, whereas the majority of overexpressed ORC1 in both HeLa and VA13 co-fractionated with chromatin throughout the cell-cycle. Although none of the cell lines displayed significant changes in the levels or chromatin-association of ORC during the cell-cycle, the chromatin-associated fraction of ORC1 displayed an increase in apparent molecular weight during S-phase. Similar experiments comparing immortalized CHO cells to an isogenic virally transformed derivative revealed no changes in levels of ORC subunits but an increase in the proportion of all three ORC subunits associated with chromatin. These results demonstrate a complex influence of cellular immortalization and transformation properties on the expression and regulation of ORC subunits. These results extend the potential link between cancer and deregulation of pre-RC proteins, and underscore the importance of considering the transformation status of cell lines when working with these proteins.

Proliferating human cells hypomorphic for origin recognition complex 2 and pre-replicative complex formation have a defect in p53 activation and Cdk2 kinase activation

The Origin Recognition Complex (ORC) is a critical component of replication initiation. We have previously reported generation of an Orc2 hypomorph cell line (Delta/-) that expresses very low levels of Orc2 but is viable. We have shown here that Chk2 is phosphorylated, suggesting that DNA damage checkpoint pathways are activated. p53 was inactivated during the derivation of the Orc2 hypomorphic cell lines, accounting for their survival despite active Chk2. These cells also show a defect in the G1 to S-phase transition. Cdk2 kinase activation in G1 is decreased due to decreased Cyclin E levels, preventing progression into S-phase. Molecular combing of bromodeoxyuridine-labeled DNA revealed that once the Orc2 hypomorphic cells enter S-phase, fork density and fork progression are approximately comparable with wild type cells. Therefore, the low level of Orc2 hinders normal cell cycle progression by delaying the activation of G1 cyclin-dependent kinases. The results suggest that hypomorphic mutations in initiation factor genes may be particularly deleterious in cancers with mutant p53 or increased activity of Cyclin E/Cdk2.

A question of timing: emerging links between transcription and replication

The coordination of transcription and timing of DNA replication during the S phase of the cell cycle has recently been studied chromosome-wide in high resolution. This revealed that in the complex genome of higher eukaryotes actively transcribed genes are more likely to replicate early in S phase. Dynamic changes in chromatin structure and nuclear organization appear to provide the underlying mechanism to link transcription and replication. A possible evolutionary benefit for this connection might result from differential replication fidelity during S phase, and comparisons of the human and chimpanzee genomes are compatible with this hypothesis.

The functional role of Cdc6 in S-G2/M in mammalian cells

The Cdc6 protein is required for licensing of replication origins before the onset of DNA replication in eukaryotic cells. Here, we examined whether Cdc6 has other roles in mammalian cell-cycle progression from S to G2/M phase. Using RNA interference, we showed that depletion of Cdc6 in synchronous G1 cells blocks G1 to S transition, confirming the essential role of Cdc6 in the initiation of DNA replication. In contrast, depletion of Cdc6 in synchronous S-phase cells slowed DNA replication and led to mitotic lethality. The Cdc6-depleted S-phase cells showed fewer newly fired origins; however, established replication forks remained active, even during chromatin condensation. Despite such DNA replication abnormalities, loss of Cdc6 failed to activate Chk1 kinase. These results show that Cdc6 is not only required for G1 origin licensing, but is also crucial for proper S-phase DNA replication that is essential for DNA segregation during mitosis.

Eukaryotic DNA Replication in a Chromatin Context

There has been remarkable progress in the last 20 years in defining the molecular mechanisms that regulate initiation of DNA synthesis in eukaryotic cells. Replication origins in the DNA nucleate the ordered assembly of protein factors to form a prereplication complex (preRC) that is poised for DNA synthesis. Transition of the preRC to an active initiation complex is regulated by cyclin-dependent kinases and other signaling molecules, which promote further protein assembly and activate the mini chromosome maintenance helicase. We will review these mechanisms and describe the state of knowledge about the proteins involved. However, we will also consider an additional layer of complexity. The DNA in the cell is packaged with histone proteins into chromatin. Chromatin structure provides an additional layer of heritable information with associated epigenetic modifications. Thus, we will begin by describing chromatin structure, and how the cell generally controls access to the DNA. Access to the DNA requires active chromatin remodeling, specific histone modifications, and regulated histone deposition. Studies in transcription have revealed a variety of mechanisms that regulate DNA access, and some of these are likely to be shared with DNA replication. We will briefly describe heterochromatin as a model for an epigenetically inherited chromatin state. Next, we will describe the mechanisms of replication initiation and how these are affected by constraints of chromatin. Finally, chromatin must be reassembled with appropriate modifications following passage of the replication fork, and our third major topic will be the reassembly of chromatin and its associated epigenetic marks. Thus, in this chapter, we seek to bring together the studies of replication initiation and the studies of chromatin into a single holistic narrative.

Identification and characterization of mouse PSF1-binding protein, SLD5

Although most somatic cells cannot proliferate, immature cells proliferate continuously to produce mature cells. Recently, we cloned mouse PSF1 from a hematopoietic stem cell specific cDNA library and reported that PSF1 is indispensable for the proliferation of immature cells. To identify the PSF1-binding protein, we used the yeast two-hybrid system with PSF1 as bait, and identified and cloned SLD5. SLD5 interacted with a central region of PSF1. Tissue distribution of SLD5 was quite similar to that of PSF1. When overexpressed, SLD5 protein was co-localized with PSF1. These data suggest that PSF1 and SLD5 may cooperate in the proliferation of immature cell populations.

PSF1 is essential for early embryogenesis in mice

Psf1 (partner of sld five 1) forms a novel heterotetramer complex, GINS (Go, Ichi, Nii, and San; five, one, two, and three, respectively, in Japanese), with Sld5, Psf2, and Psf3. The formation of this complex is essential for the initiation of DNA replication in yeast and Xenopus laevis egg extracts. Although all of the components are well conserved in higher eukaryotes, the biological function in vivo is largely unknown. We originally cloned the mouse ortholog of PSF1 from a hematopoietic stem cell cDNA library and found that PSF1 is expressed in blastocysts, adult bone marrow, and testis, in which the stem cell system is active. Here we used the gene-targeting technique to determine the physiological function of PSF1 in vivo. Mice homozygous for a nonfunctional mutant of PSF1 died in utero around the time of implantation. PSF1-/- blastocysts failed to show outgrowth in culture and exhibited a cell proliferation defect. Our data clearly indicate that PSF1 is required for early embryogenesis.

Recruitment of ORC or CDC6 to DNA is sufficient to create an artificial origin of replication in mammalian cells

Origins of replication are expected to recruit initiation proteins like origin recognition complex (ORC) and Cdc6 in eukaryotes and provide a platform for unwinding DNA. Here we test whether localization of initiation proteins onto DNA is sufficient for origin function. Different components of the ORC complex and Cdc6 stimulated prereplicative complex (pre-RC) formation and replication initiation when fused to the GAL4 DNA-binding domain and recruited to plasmid DNA containing a tandem array of GAL4-binding sites. Replication occurred once per cell cycle and was inhibited by Geminin, indicating that the plasmid was properly licensed during the cell cycle. The GAL4 fusion protein recruits other polypeptides of the ORC-Cdc6 complex, and nascent strand abundance was highest near the GAL4-binding sites. Therefore, the artificial origin recapitulates many of the regulatory features of physiological origins and is valuable for studies on replication initiation in mammalian cells. We demonstrated the utility of this system by showing the functional importance of the ATPase domains of human Cdc6 and Orc1 and the dispensability of the N-terminal segments of Orc1 and Orc2 in this assay. Artificial recruitment of a eukaryotic cellular replication initiation factor to a DNA sequence can create a functional origin of replication, providing a robust genetic assay for these factors and a novel approach to generating episomal vectors for gene therapy.

Replication of heterochromatin: insights into mechanisms of epigenetic inheritance

Heterochromatin is composed of tightly condensed chromatin in which the histones are deacetylated and methylated, and specific nonhistone proteins are bound. Additionally, in vertebrates and plants, the DNA within heterochromatin is methylated. As the heterochromatic state is stably inherited, replication of heterochromatin requires not only duplication of the DNA but also a reinstallment of the appropriate protein and DNA modifications. Thus replication of heterochromatin provides a framework for understanding mechanisms of epigenetic inheritance. In recent studies, roles have been identified for replication factors in reinstating heterochromatin, particularly functions for origin recognition complex, proliferating cell nuclear antigen, and chromatin-assembly factor 1 in recruiting the heterochromatin binding protein HP1, a histone methyltransferase, a DNA methyltransferase, and a chromatin remodeling complex. Potential mechanistic links between these factors are discussed. In some cells, replication of the heterochromatin is blocked, and in Drosophila this inhibition is mediated by a chromatin binding protein SuUR.