Emerging mechanisms of eukaryotic DNA replication initiation

Effective sequential combined therapy with carboplatin and a CDC7 inhibitor in ovarian cancer

BACKGROUND

The enhancement of DNA damage repair is one of the important mechanisms of platinum resistance. Protein cell division cycle 7 (CDC7) is a conserved serine/threonine kinase that plays important roles in the initiation of DNA replication and is associated with chemotherapy resistance in ovarian cancer. However, whether the CDC7 inhibitor XL413 has antitumor activity against ovarian cancer and its relationship with chemosensitivity remain poorly elucidated.

METHODS

We evaluated the antitumor effects of carboplatin combined with XL413 for ovarian cancer in vitro and in vivo. Cell viability inhibition, colony formation and apoptosis were assessed. The molecules related to DNA repair and damage were investigated. The antitumor effects of carboplatin combined with XL413 were also evaluated in SKOV-3 and OVCAR-3 xenografts in subcutaneous and intraperitoneal tumor models.

RESULTS

Sequential administration of XL413 after carboplatin (CBP) prevented cellular proliferation and promoted apoptosis in ovarian cancer (OC) cells. Compared with the CBP group, the expression level of RAD51 was significantly decreased and the expression level of γH2AX was significantly increased in the sequential combination treatment group. The equential combination treatment could significantly inhibit tumor growth in the subcutaneous and intraperitoneal tumor models, with the expression of RAD51 and Ki67 significantly decreased and the expression of γH2AX increased.

CONCLUSIONS

Sequential administration of CDC7 inhibitor XL413 after carboplatin can enhance the chemotherapeutic effect of carboplatin on ovarian cancer cells. The mechanism may be that CDC7 inhibitor XL413 increases the accumulation of chemotherapy-induced DNA damage by inhibiting homologous recombination repair activity.

Comparative analysis of genome-wide transcriptional responses to continuous heat stress in Pleurotus tuoliensis

Temperature plays an impactful role in mushroom cultivation. To obtain insights of transcriptomic response in macrofungi against heat stress, we performed RNA-seq analysis using Pleurotus tuoliensis mycelium cells that were treated under 32 °C and 36 °C for consecutive 96 h. By comparing the growth rate data, we found mycelium cells could maintain normal growth rate almost the same as control under 32 °C, yet halted the growths under 36 °C. In total, 2724 differential expressed genes were identified from the three pair-wise comparisons, which were classified to four clusters based on their expression patterns. We also performed gene set enrichment analysis using both GO and KEGG databases, and revealed 48, 113 and 105 enriched GO terms, and 1, 5, and 6 enriched KEGG pathways for three pair-wise comparisons accordingly. In addition, we identified 9 overlapping GO terms and 1 overlapping KEGG pathway shared by the three comparisons. Differentially expressed genes (DEGs) involved in cell communication, amino acid metabolic process, intracellular signal transduction and small molecule biosynthesis were identified in two heat stress treatments despite of the stress intensity. However, the expression of two heat shock protein genes (HSP10 and HSP60) were induced by increasing temperature. Our findings also suggested the DEGs associated with cell cycle regulation had various expression patterns under two heat stress conditions possibly due to different functions. Furthermore, 11 DEGs related to ergosterol biosynthesis were identified with similar expression trends, indicating the ergosterol levels and cell membrane composition may have a tight connection to the acquisition of thermotolerance, which warrant further investigations for deeper understanding of molecular mechanisms in fungal stress responses.

GATA2-Mediated Transcriptional Activation of Notch3 Promotes Pancreatic Cancer Liver Metastasis

The liver is the predominant metastatic site for pancreatic cancer. However, the factors that determine the liver metastasis and the specific molecular mechanisms are still unclear. In this study, we used human pancreatic cancer cell line Hs766T to establish Hs766T-L3, a subline of Hs766T with stable liver metastatic ability. We performed RNA sequencing of Hs766T-L3 and its parental cell line Hs766T, and revealed huge differences in gene expression patterns and pathway activation between these two cell lines. We correlated the difference in pathway activation with the expression of the four core transcriptional factors including STAT1, NR2F2, GATA2, and SMAD4. Using the TCGA database, we examined the relative expression of these transcription factors (TFs) in pan-cancer and their relationship with the prognosis of the pancreatic cancer. Among these TFs, we considered GATA2 is closely involved in tumor metastasis and may serve as a potential metastatic driver. Further in vitro and in vivo experiments confirmed that GATA2-mediated transcriptional activation of Notch3 promotes the liver metastasis of Hs766T-L3, and knockdown of either GATA2 or Notch3 reduces the metastatic ability of Hs766T-L3. Therefore, we claim that GATA2 may serve as a metastatic driver of pancreatic cancer and a potential therapeutic target to treat liver metastasis of pancreatic cancer.

Fungi.guru: Comparative genomic and transcriptomic resource for the fungi kingdom

The fungi kingdom is composed of eukaryotic heterotrophs, which are responsible for balancing the ecosystem and play a major role as decomposers. They also produce a vast diversity of secondary metabolites, which have antibiotic or pharmacological properties. However, our lack of knowledge of gene function in fungi precludes us from tailoring them to our needs and tapping into their metabolic diversity. To help remedy this, we gathered genomic and gene expression data of 19 most widely-researched fungi to build an online tool, fungi.guru, which contains tools for cross-species identification of conserved pathways, functional gene modules, and gene families. We exemplify how our tool can elucidate the molecular function, biological process and cellular component of genes involved in various biological processes, by identifying a secondary metabolite pathway producing gliotoxin in Aspergillus fumigatus, the catabolic pathway of cellulose in Coprinopsis cinerea and the conserved DNA replication pathway in Fusarium graminearum and Pyricularia oryzae. The tool is available at www.fungi.guru.

A stochastic model of size control in the budding yeast cell cycle

BACKGROUND

Cell size is a key characteristic that significantly affects many aspects of cellular physiology. There are specific control mechanisms during cell cycle that maintain the cell size within a range from generation to generation. Such control mechanisms introduce substantial variabilities to important properties of the cell cycle such as growth and division. To quantitatively study the effect of such variability in progression through cell cycle, detailed stochastic models are required.

RESULTS

In this paper, a new hybrid stochastic model is proposed to study the effect of molecular noise and size control mechanism on the variabilities in cell cycle of the budding yeast Saccharomyces cerevisiae. The proposed model provides an accurate, yet computationally efficient approach for simulation of an intricate system by integrating the deterministic and stochastic simulation schemes. The developed hybrid stochastic model can successfully capture several key features of the cell cycle observed in experimental data. In particular, the proposed model: 1) confirms that the majority of noise in size control stems from low copy numbers of transcripts in the G1 phase, 2) identifies the size and time regulation modules in the size control mechanism, and 3) conforms with phenotypes of early G1 mutants in exquisite detail.

CONCLUSIONS

Hybrid stochastic modeling approach can be used to provide quantitative descriptions for stochastic properties of the cell cycle within a computationally efficient framework.

Identification and characterization of LysM effectors in Penicillium expansum

P. expansum is regarded as one of the most important postharvest rots of apple fruit and is also of great concern to fruit processing industries. Elucidating the pathogenicity mechanism of this pathogen is of utmost importance for the development of effective and safe management strategies. Although, many studies on modification of the host environment by the pathogen were done, its interactions with fruit during the early stages of infection and the virulence factors that mediate pathogenicity have not been fully defined. Effectors carrying LysM domain have been identified in numerous pathogenic fungi and their role in the first stages of infection has been established. In this study, we identified 18 LysM genes in the P. expansum genome. Amino acid sequence analysis indicated that P. expansum LysM proteins belong to a clade of fungal-specific LysM. Eleven of the discovered LysM genes were found to have secretory pathway signal peptide, among them, 4 (PeLysM1 PeLysM2, PeLysM3 and PeLysM4) were found to be highly expressed during the infection and development of decay of apple fruit. Effect of targeted deletion of the four putative PeLysM effectors on the growth and pathogenicity was studied. Possible interactions of PeLysM with host proteins was investigated using the yeast-two-hybrid system.

Papillomaviruses: a systematic review

In the last decades, a group of viruses has received great attention due to its relationship with cancer development and its wide distribution throughout the vertebrates: the papillomaviruses. In this article, we aim to review some of the most relevant reports concerning the use of bovines as an experimental model for studies related to papillomaviruses. Moreover, the obtained data contributes to the development of strategies against the clinical consequences of bovine papillomaviruses (BPV) that have led to drastic hazards to the herds. To overcome the problem, the vaccines that we have been developing involve recombinant DNA technology, aiming at prophylactic and therapeutic procedures. It is important to point out that these strategies can be used as models for innovative procedures against HPV, as this virus is the main causal agent of cervical cancer, the second most fatal cancer in women.

Structural Basis for the Unique Multivalent Readout of Unmodified H3 Tail by Arabidopsis ORC1b BAH-PHD Cassette

DNA replication initiation relies on the formation of the origin recognition complex (ORC). The plant ORC subunit 1 (ORC1) protein possesses a conserved N-terminal BAH domain with an embedded plant-specific PHD finger, whose function may be potentially regulated by an epigenetic mechanism. Here, we report structural and biochemical studies on the Arabidopsis thaliana ORC1b BAH-PHD cassette which specifically recognizes the unmodified H3 tail. The crystal structure of ORC1b BAH-PHD cassette in complex with an H3(1-15) peptide reveals a strict requirement for the unmodified state of R2, T3, and K4 on the H3 tail and a novel multivalent BAH and PHD readout mode for H3 peptide recognition. Such recognition may contribute to epigenetic regulation of the initiation of DNA replication.

Acetylation by GCN5 regulates CDC6 phosphorylation in the S phase of the cell cycle

In eukaryotic cells, the cell-division cycle (CDC)-6 protein is essential to promote the assembly of pre-replicative complexes in the early G1 phase of the cell cycle, a process requiring tight regulation to ensure that proper origin licensing occurs once per cell cycle. Here we show that, in late G1 and early S phase, CDC6 is found in a complex also containing Cyclin A, cyclin-dependent kinase (CDK)-2 and the acetyltransferase general control nonderepressible 5 (GCN5). GCN5 specifically acetylates CDC6 at three lysine residues flanking its cyclin-docking motif, and this modification is crucial for the subsequent phosphorylation of the protein by Cyclin A-CDKs at a specific residue close to the acetylation site. GCN5-mediated acetylation and site-specific phosphorylation of CDC6 are both necessary for the relocalization of the protein to the cell cytoplasm in the S phase, as well as to regulate its stability. This two-step, intramolecular regulatory program by sequential modification of CDC6 seems to be essential for proper S-phase progression.

The Xenopus cell cycle: an overview

Oocytes, eggs and embryos from the frog Xenopus laevis have been an important model system for studying cell-cycle regulation for several decades. First, progression through meiosis in the oocyte has been extensively investigated. Oocyte maturation has been shown to involve complex networks of signal transduction pathways, culminating in the cyclic activation and inactivation of Maturation Promoting Factor (MPF), composed of cyclin B and cdc2. After fertilisation, the early embryo undergoes rapid simplified cell cycles which have been recapitulated in cell-free extracts of Xenopus eggs. Experimental manipulation of these extracts has given a wealth of biochemical information about the cell cycle, particularly concerning DNA replication and mitosis. Finally, cells of older embryos adopt a more somatic-type cell cycle and have been used to study the balance between cell cycle and differentiation during development.

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.

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.

Arabidopsis CDKA;1, a cdc2 homologue, controls proliferation of generative cells in male gametogenesis

The protein kinase cdc2 is conserved throughout eukaryotes and acts as a key regulator of the cell cycle. In plants, A-type cyclin-dependent kinase (CDKA), a homologue of cdc2, has a role throughout the cell cycle. Here we show that a loss-of-function mutation in CDKA;1, encoding the only Arabidopsis CDKA, results in lethality of the male gametophyte. Heterozygous plants produced mature siliques containing about 50% aborted seeds, and segregation distortion was observed in paternal inheritance. Microspores normally undergo an asymmetric cell division, pollen mitosis I (PMI), to produce bicellular pollen grains. The larger vegetative cell does not divide, but the smaller generative cell undergoes mitosis, PMII, to form the two sperm cells, thereby generating tricellular pollen grains. The cdka-1 mutant, however, produces mature bicellular pollen grains, consisting of a single sperm-like cell and a vegetative cell, due to failure of PMII. The mutant sperm-like cell is fertile, and preferentially fuses with the egg cell to initiate embryogenesis. As the central cell nucleus remains unfertilized, however, double fertilization does not occur. In heterozygous plants, the embryo is arrested at the globular stage, most likely because of loss of endosperm development, whereas it is arrested at the one- or two-cell stage in presumptive homozygous plants. Thus, CDKA;1 is essential for cell division of the generative cell in male gametogenesis.

A plasma membrane-associated AAA-ATPase from Glycine max

An AAA-ATPase (ATPases Associated with a Variety of Cellular Activities) localized to the plasma membrane of soybean (Glycine max) was isolated, partially sequenced and cloned (SBPM AAA-ATPase). The protein with an apparent monomer molecular mass of about 97 kDa was isolated using a combination of anion exchange, preparative SDS-PAGE, reverse phase HPLC, and ATP affinity chromatography. The cDNA for the full-length SBPM AAA-ATPase was cloned by screening an expression library using an antibody against the highly conserved Walker B AAA-ATP-binding motif. Northern blot analysis detected one transcript of approximately 2700 bp. The full-length cDNA sequence was that previously obtained (GenBank Database; U20213) encoding a protein with two copies of the conserved AAA-ATP-binding motif and regions of sequence homology with other AAA-ATPases. Electron microscopic preparations of the recombinant SBPM AAA-ATPase revealed hexamers typically formed by these proteins. The cloned and expressed protein was identical to the protein isolated from the soybean plasma membrane as confirmed using antisera raised to a non-conserved region of the derived protein sequence and by N-terminal sequencing of peptides derived from the isolated protein.

Proliferating cell nuclear antigen recruits cyclin-dependent kinase inhibitor Xic1 to DNA and couples its proteolysis to DNA polymerase switching

The Xenopus cyclin-dependent kinase (CDK) inhibitor, p27(Xic1) (Xic1), binds to CDK2-cyclins and proliferating cell nuclear antigen (PCNA), inhibits DNA synthesis in Xenopus extracts, and is targeted for ubiquitin-mediated proteolysis. Previous studies suggest that Xic1 ubiquitination and degradation are coupled to the initiation of DNA replication, but the precise timing and molecular mechanism of Xic1 proteolysis has not been determined. Here we demonstrate that Xic1 proteolysis is temporally restricted to late replication initiation following the requirements for DNA polymerase alpha-primase, replication factor C, and PCNA. Our studies also indicate that Xic1 degradation is absolutely dependent upon the binding of Xic1 to PCNA in both Xenopus egg and gastrulation stage extracts. Additionally, extracts depleted of PCNA do not support Xic1 proteolysis. Importantly, while the addition of recombinant wild-type PCNA alone restores Xic1 degradation, the addition of a PCNA mutant defective for trimer formation does not restore Xic1 proteolysis in PCNA-depleted extracts, suggesting Xic1 proteolysis requires both PCNA binding to Xic1 and the ability of PCNA to be loaded onto primed DNA by replication factor C. Taken together, our studies suggest that Xic1 is targeted for ubiquitination and degradation during DNA polymerase switching through its interaction with PCNA at a site of initiation.

Control of DNA replication initiation by recruitment of an essential initiation protein to the membrane of Bacillus subtilis

The Bacillus subtilis proteins DnaD and DnaB are essential for replication initiation and are conserved in low G+C content Gram-positive bacteria. Previous work indicated that DnaD and DnaB are involved in helicase loading during the process of restarting stalled replication forks. We have investigated the roles of DnaD and DnaB in replication initiation at oriC in vivo. Using chromatin immunoprecipitation (ChIP), we found that DnaD and DnaB functions are needed to load the replicative helicase at oriC. To investigate further the functions of DnaD and DnaB in replication initiation, we isolated and characterized suppressors of the temperature sensitivity of dnaD and dnaB mutant cells. In both cases, we isolated the identical missense mutation in dnaB, dnaBS371P. Using yeast two-hybrid analysis, we found that dnaBS371P uncovers a previously undetected physical interaction between DnaD and DnaB. We also found that DnaBS371P constitutively recruits DnaD to the membrane fraction of cells, where DnaB and oriC are enriched. Phenotypes of cells expressing DnaBS371P are consistent with aberrant replication control. We hypothesize that B. subtilis regulates replication initiation by regulating a physical interaction between two proteins essential for helicase loading at chromosomal origins.

Putative subunits of the maize origin of replication recognition complex ZmORC1-ZmORC5

The finding in animal species of complexes homologous to the products of six Saccharomyces cerevisiae genes, origin of replication recognition complex (ORC), has suggested that ORC-related mechanisms have been conserved in all eukaryotes. In plants, however, the only cloned putative homologs of ORC subunits are the Arabidopsis ORC2 and the rice ORC1. Homologs of other subunits of plant origin have not been cloned and characterized. A striking observation was the absence from the Arabidopsis genome of an obvious candidate gene-homolog of ORC4. This fact raised compelling questions of whether plants, in general, and Arabidopsis, in particular, may have lost the ORC4 gene, whether ORC-homologous subunits function within a complex in plants, whether an ORC complex may form and function without an ORC4 subunit, whether a functional (but not sequence) protein homolog may have taken up the role of ORC4 in Arabidopsis, and whether lack of ORC4 is a plant feature, in general. Here, we report the first cloned and molecularly characterized five genes coding for the maize putative homologs of ORC subunits ZmORC1, ZmORC2, ZmORC3, ZmORC4 and ZmORC5. Their expression profiles in tissues with different cell-dividing activities are compatible with a role in DNA replication. Based on the potential of ORC-homologous maize proteins to bind each other in yeast, we propose a model for their possible assembly within a maize ORC. The isolation and molecular characterization of an ORC4-homologous gene from maize argues that, in its evolution, Arabidopsis may have lost the homologous ORC4 gene.

Regulation of CDC6, geminin, and CDT1 in human cells that undergo polyploidization

Endomitosis is the process by which mammalian megakaryocytes become polyploid during terminal differentiation. As in other endoreplicating cells, cyclin-cdk complexes are distinctly regulated, probably to overcome the strict mechanisms that prevent rereplication in most somatic cells. We have asked whether key factors involved in the assembly and licensing of replication origins are equally regulated during endomitosis. Cdc6, cdt1, and geminin expression was analyzed during differentiation of two human megakaryoblastic cell lines, HEL and K562, which respectively do and do not establish endoreplication cycles. Geminin was downregulated, whereas cdt1 levels were maintained upon differentiation of both cell lines, independently of whether cells entered extra S-phases. In contrast, cdc6 was present and remained nuclear only in differentiated endoreplicating cells. Interestingly, cdc6 protein expression was reestablished in K562 cells that underwent endomitosis after transient or stable cyclin E overexpression. The high levels of cyclin E reached in these cells appeared to influence the stabilization of cdc6 protein rather than its RNA transcription rate. Finally, cdc6 overexpression drove HEL cells into endoreplication cycles in the absence of differentiation stimuli. Our results show that both cdt1 and cdc6 are differentially regulated during megakaryocytic differentiation and suggest an active role of cdc6 in endomitosis.

Regulation of bovine papillomavirus replicative helicase e1 by the ubiquitin-proteasome pathway

Papillomaviruses maintain their genomes in a relatively constant copy number as stable extrachromosomal plasmids in the nuclei of dividing host cells. The viral initiator of replication, E1, is not detected in papillomavirus-infected cells. Here, we present evidence that E1 encoded by bovine papillomavirus type 1 is an unstable protein that is degraded through the ubiquitin-proteasome pathway. In a cell-free system derived from Xenopus egg extracts, E1 degradation is regulated by both cyclin E/Cdk2 binding and E1 replication activity. Free E1 is readily ubiquitinated and degraded by the proteasome, while it becomes resistant to this degradation pathway when bound to cyclin E/Cdk2 complexes before the start of DNA synthesis. This stabilization is reversed in a process involving E1-dependent replication activity. In transiently transfected cells, E1 is also polyubiquitinated and accumulates when proteasome activity is inhibited. Thus, the establishment and maintenance of a stable number of papillomavirus genomes in latently infected cells are in part a function of regulated ubiquitin-mediated degradation of E1.

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.

A novel chk1-dependent G1/M checkpoint in fission yeast

Fission yeast cells with a temperature-sensitive Orp1 protein, a component of the origin recognition complex, cannot perform DNA replication at the restrictive temperature. Seventy percent of orp1-4 cells arrest with a 1C DNA content, whereas 30% proceed to mitosis ('cut'). The arrest depends upon the checkpoint Rad proteins and, surprisingly, the Chk1 protein, which is thought to act only from late S phase. The arrested cells maintain a 1C DNA content, as judged by flow cytometry, and the early origin ars3001 has not been initiated, as judged by 2D gel analysis. We show that in G1-arrested orp1-4 cells, Wee1 phosphorylates and inactivates Cdc2. Activation of Chk1 occurs earlier than Cdc2 phosphorylation, indicating a novel role for Chk1, namely to induce and/or maintain Cdc2 phosphorylation upon checkpoint activation in G1. We also show that commitment to cutting occurs already in early G1 phase.

A conserved domain of Schizosaccharomyces pombe dfp1(+) is uniquely required for chromosome stability following alkylation damage during S phase

The fission yeast Dbf4 homologue Dfp1 has a well-characterized role in regulating the initiation of DNA replication. Sequence analysis of Dfp1 homologues reveals three highly conserved regions, referred to as motifs N, M, and C. To determine the roles of these conserved regions in Dfp1 function, we have generated dfp1 alleles with mutations in these regions. Mutations in motif N render cells sensitive to a broad range of DNA-damaging agents and replication inhibitors, yet these mutant proteins are efficient activators of Hsk1 kinase in vitro. In contrast, mutations in motif C confer sensitivity to the alkylating agent methyl methanesulfonate (MMS) but, surprisingly, not to UV, ionizing radiation, or hydroxyurea. Motif C mutants are poor activators of Hsk1 in vitro but can fulfill the essential function(s) of Dfp1 in vivo. Strains carrying dfp1 motif C mutants have an intact mitotic and intra-S-phase checkpoint, and epistasis analysis indicates that dfp1 motif C mutants function outside of the known MMS damage repair pathways, suggesting that the observed MMS sensitivity is due to defects in recovery from DNA damage. The motif C mutants are most sensitive to MMS during S phase and are partially suppressed by deletion of the S-phase checkpoint kinase cds1. Following treatment with MMS, dfp1 motif C mutants exhibit nuclear fragmentation, chromosome instability, precocious recombination, and persistent checkpoint activation. We propose that Dfp1 plays at least two genetically separable roles in the DNA damage response in addition to its well-characterized role in the initiation of DNA replication and that motif C plays a critical role in the response to alkylation damage, perhaps by restarting or stabilizing stalled replication forks.

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.

Xic1 degradation in Xenopus egg extracts is coupled to initiation of DNA replication

CDK2 activity is regulated by phosphorylation/dephosphorylation, subcellular localization, cyclin levels, and cyclin dependent kinase inhibitors (CKIs). Using Xenopus egg extracts, we find that degradation of Xic1, a Xenopus p21(cip1)/p27(kip1) family member, is coupled to initiation of DNA replication. Xic1 turnover requires the formation of a prereplication complex (pre-RC). Additionally, downstream initiation factors including CDK2, Cdc7, and Cdc45, but not RPA or DNA polymerase alpha, are necessary for activating the degradation system. Xic1 degradation is attenuated following completion of DNA replication. Unlike degradation of p27(kip1) in mammalian cells, CDK2 activity is not directly involved in Xic1 degradation and interactions between Xic1 and CDK2/cyclin E are dispensable for Xic1 turnover. Interestingly, a C-terminal region (162-192) of Xic1 is essential and apparently sufficient for triggering Xic1 ubiquitination prior to degradation. These observations demonstrate that a direct link exists between DNA replication and CKI degradation.

Functions of sensor 1 and sensor 2 regions of Saccharomyces cerevisiae Cdc6p in vivo and in vitro

Cdc6p is a key regulator of the cell cycle in eukaryotes and is a member of the AAA(+) (ATPases associated with a variety of cellular activities) family of proteins. In this family of proteins, the sensor 1 and sensor 2 regions are important for their function and ATPase activity. Here, site-directed mutagenesis has been used to examine the role of these regions of Saccharomyces cerevisiae Cdc6p in controlling the cell cycle progression and initiation of DNA replication. Two important amino acid residues (Asn(263) in sensor 1 and Arg(332) in sensor 2) were identified as key residues for Cdc6p function in vivo. Cells expressing mutant Cdc6p (N263A or R332E) grew slowly and accumulated in the S phase. In cells expressing mutant Cdc6p, loading of the minichromosome maintenance (MCM) complex of proteins was decreased, suggesting that the slow progression of S phase in these cells was due to inefficient MCM loading on chromatin. Purified wild type Cdc6p but not mutant Cdc6p (N263A and R332E) caused the structural modification of origin recognition complex proteins. These results are consistent with the idea that Cdc6p uses its ATPase activity to change the conformation of origin recognition complex, and then together they recruit the MCM complex.

SpSld3 is required for loading and maintenance of SpCdc45 on chromatin in DNA replication in fission yeast

Initiation of DNA replication in eukaryotic cells is regulated through the ordered assembly of replication complexes at origins of replication. Association of Cdc45 with the origins is a crucial step in assembly of the replication machinery, hence can be considered a target for the regulation of origin activation. To examine the process required for SpCdc45 loading, we isolated fission yeast SpSld3, a counterpart of budding yeast Sld3 that interacts with Cdc45. SpSld3 associates with the replication origin during G1-S phases and this association depends on Dbf4-dependent (DDK) kinase activity. In the corresponding period, SpSld3 interacts with minichromosome maintenance (MCM) proteins and then with SpCdc45. A temperature-sensitive sld3-10 mutation suppressed by the multicopy of the sna41+ encoding SpCdc45 impairs loading of SpCdc45 onto chromatin. In addition, this mutation leads to dissociation of preloaded Cdc45 from chromatin in the hydroxyurea-arrested S phase, and DNA replication upon removal of hydroxyurea is retarded. Thus, we conclude that SpSld3 is required for stable association of Cdc45 with chromatin both in initiation and elongation of DNA replication. The DDK-dependent origin association suggests that SpSld3 is involved in temporal regulation of origin firing.

A tale of two HSV-1 helicases: roles of phage and animal virus helicases in DNA replication and recombination

Helicases play essential roles in many important biological processes such as DNA replication, repair, recombination, transcription, splicing, and translation. Many bacteriophages and plant and animal viruses encode one or more helicases, and these enzymes have been shown to play many roles in their respective viral life cycles. In this review we concentrate primarily on the roles of helicases in DNA replication and recombination with special emphasis on the bacteriophages T4, T7, and A as model systems. We explore comparisons between these model systems and the herpesviruses--primarily herpes simplex virus. Bacteriophage utilize various pathways of recombination-dependent DNA replication during the replication of their genomes. In fact the study of recombination in the phage systems has greatly enhanced our understanding of the importance of recombination in the replication strategies of bacteria, yeast, and higher eukaryotes. The ability to "restart" the replication process after a replication fork has stalled or has become disrupted for other reasons is a critical feature in the replication of all organisms studied. Phage helicases and other recombination proteins play critical roles in the "restart" process. Parallels between DNA replication and recombination in phage and in the herpesviruses is explored. We and others have proposed that recombination plays an important role in the life cycle of the herpesviruses, and in this review, we discuss models for herpes simplex virus type 1 (HSV-1) DNA replication. HSV-1 encodes two helicases. UL9 binds specifically to the origins of replication and is believed to initiate HSV DNA replication by unwinding at the origin; the heterotrimeric helicase-primase complex, encoded by UL5, UL8, and UL52 genes, is believed to unwind duplex viral DNA at replication forks. Structure-function analyses of UL9 and the helicase-primase are discussed with attention to the roles these proteins might play during HSV replication.

Novel localization and possible functions of cyclin E in early sea urchin development

In somatic cells, cyclin E-cdk2 activity oscillates during the cell cycle and is required for the regulation of the G1/S transition. Cyclin E and its associated kinase activity remain constant throughout early sea urchin embryogenesis, consistent with reports from studies using several other embryonic systems. Here we have expanded these studies and show that cyclin E rapidly and selectively enters the sperm head after fertilization and remains concentrated in the male pronucleus until pronuclear fusion, at which time it disperses throughout the zygotic nucleus. We also show that cyclin E is not concentrated at the centrosomes but is associated with condensed chromosomes throughout mitosis for at least the first four cell cycles. Isolated mitotic spindles are enriched for cyclin E and cdk2, which are localized to the chromosomes. The chromosomal cyclin E is associated with active kinase during mitosis. We propose that cyclin E may play a role in the remodeling of the sperm head and re-licensing of the paternal genome after fertilization. Furthermore, cyclin E does not need to be degraded or dissociated from the chromosomes during mitosis; instead, it may be required on chromosomes during mitosis to immediately initiate the next round of DNA replication.

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.

Immortalization of human WI38 cells is associated with differential activation of the c-myc origins

To study the possible relationships between origin activities and cellular processes leading to malignancy, we used an isogenic system of human embryo lung fibroblast cells WI38 and a SV40-transformed variant, WI38 VA13 2RA (WI38(SV40)). We found that the activities of all initiation sites at the c-myc locus were approximately two-fold as high in WI38(SV40) cells as in WI38 cells. Thus, higher initiation frequency of origins at certain loci is induced with cell immortalization, one of the steps in the multi-step process leading to malignancy. We measured the activities of the four c-myc promoters P0, P1, P2, and P3 with nuclear runon assay in the two cell lines in order to detect potential individual promoter changes that may be also associated with immortalization by SV40 virus. The results show that the activities of the promoters P0, P1, and P3 did not significantly change, but the activity of the major promoter P2 in WI38(SV40) cells was about 7.5- to 8.0-fold as high as that in WI38 cells. The increased activity of promoter P2, although approximately 600 bp downstream of one of the major DNA replication initiation sites, had no preferential influence on the major sites of origin activity. Since the distribution of nascent strand abundance was not significantly altered, binding of transcription factors does not seem to facilitate the assembly of pre-replication complex (pre-RC) or otherwise preferentially alter the activities of the DNA replication proteins at this major initiation site.

Expression of Cdc18/Cdc6 and Cdt1 during G2 phase induces initiation of DNA replication

Cdc18/Cdc6 and Cdt1 are essential initiation factors for DNA replication. In this paper we show that expression of Cdc18 in fission yeast G2 cells is sufficient to override the controls that ensure one S phase per cell cycle. Cdc18 expression in G2 induces DNA synthesis by re-firing replication origins and recruiting the MCM Cdc21 to chromatin in the presence of low levels of Cdt1. However, when Cdt1 is expressed together with Cdc18 in G2, cells undergo very rapid, uncontrolled DNA synthesis, accumulating DNA contents of 64C or more. Our data suggest that Cdt1 may potentiate re-replication by inducing origins to fire more persistently, possibly by stabilizing Cdc18 on chromatin. In addition, low level expression of a mutant form of Cdc18 that cannot be phosphorylated by cyclin-dependent kinases is not sufficient to induce replication in G2, but does so only when co-expressed with Cdt1. Thus, regulation of both Cdc18 and Cdt1 in G2 plays a crucial role in preventing the re-initiation of DNA synthesis until the next cell cycle.

Identification of an MCM4 homologue expressed specifically in the sexual stage of Plasmodium falciparum

Mini-chromosome maintenance (MCM) proteins play an essential role in DNA replication initiation. We have isolated a novel gene encoding an MCM-like protein from the human malaria parasite Plasmodium falciparum using the vectorette technique. The gene has no introns and comprises an open reading frame encoding 1005 amino acid residues with a predicted Mr of 115 kDa. The encoded protein, termed PfMCM4, contains all conserved sequences in the MCM family and displays the highest homology to the Cdc54 (MCM4) of Saccharomyces cerevisiae. However, PfMCM4 possesses five unique amino acid inserts with sizes ranging from seven to 75 residues. Southern blotting of genomic DNA digests and chromosomal separations showed that the Pfmcm4 gene is present as a single copy per haploid genome and is located on chromosome 13. A 4000-nucleotide transcript of this gene is expressed specifically in the sexual erythrocytic stage, indicating that PfMCM4 may be involved in gametogenesis in which DNA is quickly replicated.

Control of DNA rereplication via Cdc2 phosphorylation sites in the origin recognition complex

Cdc2 kinase is a master regulator of cell cycle progression in the fission yeast Schizosaccharomyces pombe. Our data indicate that Cdc2 phosphorylates replication factor Orp2, a subunit of the origin recognition complex (ORC). Cdc2 phosphorylation of Orp2 appears to be one of multiple mechanisms by which Cdc2 prevents DNA rereplication in a single cell cycle. Cdc2 phosphorylation of Orp2 is not required for Cdc2 to activate DNA replication initiation. Phosphorylation of Orp2 appears first in S phase and becomes maximal in G(2) and M when Cdc2 kinase activity is required to prevent reinitiation of DNA replication. A mutant lacking Cdc2 phosphorylation sites in Orp2 (orp2-T4A) allowed greater rereplication of DNA than congenic orp2 wild-type strains when the limiting replication initiation factor Cdc18 was deregulated. Thus, Cdc2 phosphorylation of Orp2 may be redundant with regulation of Cdc18 for preventing reinitiation of DNA synthesis. Since Cdc2 phosphorylation sites are present in Orp2 (also known as Orc2) from yeasts to metazoans, we propose that cell cycle-regulated phosphorylation of the ORC provides a safety net to prevent DNA rereplication and resulting genetic instability.

Stability, chromatin association and functional activity of mammalian pre-replication complex proteins during the cell cycle

We have examined the behavior of pre-replication complex (pre-RC) proteins in relation to key cell cycle transitions in Chinese Hamster Ovary (CHO) cells. ORC1, ORC4 and Cdc6 were stable (T1/2 >2 h) and associated with a chromatin-containing fraction throughout the cell cycle. Green fluorescent protein-tagged ORC1 associated with chromatin throughout mitosis in living cells and co-localized with ORC4 in metaphase spreads. Association of Mcm proteins with chromatin took place during telophase, approximately 30 min after the destruction of geminin and cyclins A and B, and was coincident with the licensing of chromatin to replicate in geminin-supplemented Xenopus egg extracts. Neither Mcm recruitment nor licensing required protein synthesis throughout mitosis. Moreover, licensing could be uncoupled from origin specification in geminin-supplemented extracts; site-specific initiation within the dihydrofolate reductase locus required nuclei from cells that had passed through the origin decision point (ODP). These results demonstrate that mammalian pre-RC assembly takes place during telophase, mediated by post-translational modifications of pre-existing proteins, and is not sufficient to select specific origin sites. A subsequent, as yet undefined, step selects which pre-RCs will function as replication origins.

Oncogenic cell cycle start control

The ability to proliferate in the absence of anchorage is a fundamental attribute of cancer cells, yet how it is acquired is one central problem in cancer biology. By utilizing growth factor-transformable NRK cells and its insensitive mutants, we recently found that oncogenic stimulation invokes Cdk6 to participate in a critical step of the cell cycle start, but not via the regulation of its catalytic activity and that Cdk6 participation closely correlates with the anchorage-independent growth ability. Since many hematopoietic cells employ predominantly Cdk6 for the cell cycle start and perform anchorage-independent growth by nature, this finding raises the possibility that the mechanism by which oncogenic stimulation invokes anchorage-independent growth of NRK cells is similar to the one used for hematopoietic cell proliferation. We discuss this novel mechanism and its implication.

Kick-starting the cell cycle: From growth-factor stimulation to initiation of DNA replication

The essential genes, proteins and associated regulatory networks involved in the entry into the mammalian cell cycle are identified, from activation of growth-factor receptors to intracellular signal transduction pathways that impinge on the cell cycle machinery and ultimately on the initiation of DNA replication. Signaling pathways mediated by the oncoproteins Ras and Myc induce the activation of cyclin-dependent kinases CDK4 and CDK2, and the assembly and firing of pre-replication complexes require a collaboration among E2F, CDK2, and Cdc7 kinase. A proposed core mechanism of the restriction point, the major checkpoint prior to commitment to DNA synthesis, involves cyclin E/CDK2, the phosphatase Cdc25A, and the CDK inhibitor p27Kip1. (c) 2001 American Institute of Physics.

Regulation of chromosome replication

The initiation of DNA replication in eukaryotic cells is tightly controlled to ensure that the genome is faithfully duplicated once each cell cycle. Genetic and biochemical studies in several model systems indicate that initiation is mediated by a common set of proteins, present in all eukaryotic species, and that the activities of these proteins are regulated during the cell cycle by specific protein kinases. Here we review the properties of the initiation proteins, their interactions with each other, and with origins of DNA replication. We also describe recent advances in understanding how the regulatory protein kinases control the progress of the initiation reaction. Finally, we describe the checkpoint mechanisms that function to preserve the integrity of the genome when the normal course of genome duplication is perturbed by factors that damage the DNA or inhibit DNA synthesis.

Where it all starts: eukaryotic origins of DNA replication

Chromosomal origins of DNA replication in eukaryotic cells not only are crucial for understanding the basic process of DNA duplication but also provide a tool to analyze how cell cycle regulators are linked to the replication machinery. During the past decade much progress has been made in identifying replication origins in eukaryotic genomes. More recently, replication initiation point (RIP) mapping has allowed us to detect start sites for DNA synthesis at the nucleotide level and thus to monitor replication initiation events at the origin very precisely. Beyond giving us the precise positions of start sites, the application of RIP mapping in yeast and human cells has revealed a single, defined start point at which replication initiates, a scenario very reminiscent of transcription initiation. More importantly, studies in yeast have shown that the binding site for the initiator, the origin recognition complex (ORC), lies immediately adjacent to the replication start point, which suggests that ORC directs the initiation machinery to a distinct site. Therefore, in our pursuit of identifying ORC-binding sites in higher eukaryotes, RIP mapping may lead the way.

Investigating the hows and whys of DNA endoreduplication

Endoreduplication is a form of nuclear polyploidization that results in multiple, uniform copies of chromosomes. This process is common in plants and animals, especially in tissues with high metabolic activity, and it generally occurs in cells that are terminally differentiated. In plants, endoreduplication is well documented in the endosperm and cotyledons of developing seeds, but it also occurs in many tissues throughout the plant. It is thought that endoreduplication provides a mechanism to increase the level of gene expression, but the function of this process has not been thoroughly investigated. Numerous observations have been made of endoreduplication, or at least extra cycles of S-phase, as a consequence of mutations in genes controlling several aspects of cell cycle regulation. However, until recently there were few studies directed at the molecular mechanisms responsible for this specialized cell cycle. It is suggested that endoreduplication requires nothing more elaborate than a loss of M-phase cyclin-dependent kinase activity and oscillations in the activity of S-phase cyclin-dependent kinase.

Processive DNA helicase activity of the minichromosome maintenance proteins 4, 6, and 7 complex requires forked DNA structures

The minichromosome maintenance (Mcm) proteins 2-7 are required for both the initiation and elongation steps of chromosomal DNA replication. Previous studies have shown that the Mcm complex consisting of the Mcm 4, 6, and 7 proteins contains 3' to 5' DNA helicase activity with limited processivity (displacing duplex DNA regions up to 30 nt). In this report, we show that the presence of both 5' and 3' single-stranded tails in DNA helicase substrates is essential for the processive helicase activity of the Mcm complex. The presence of both 5' and 3' tails facilitated the formation of double heterohexameric complexes of Mcm4/6/7 on substrate DNA, which appeared to be essential for the processive helicase activity. The double heterohexameric complex of Mcm4/6/7, in the presence of a single-strand DNA binding protein, is capable of unwinding duplex DNA region of about 600 bp in length. These results support the hypothesis that the Mcm4/6/7 complex can function as a replication helicase.

The Ran-GTPase and cell-cycle control

RCC1, the chromatin-bound guanine-nucleotide exchange factor (GEF) for the small nuclear GTPase, Ran, is required for coordinating the onset of mitosis with S-phase completion in mammalian cells. Other defects in the Ran-GTPase network also result in disruption of cell-cycle processes such as DNA replication, exit from mitosis and, at least in budding yeast, accurate chromosome segregation. However, the Ran system is now best known for its pivotal role in nucleocytoplasmic transport, where RanGTP is used as a positional flag for the nucleus during interphase. Ran's effectors are the shuttling transport factors, importins and exportins, which facilitate the transit of cargoes between the nucleus and cytoplasm: RanGTP regulates their cargo-binding properties so that they can move their cargo in the correct direction. RanGTP also plays a separate role during mitosis, influencing microtubule polymerisation, possibly specifically in the vicinity of chromosomes. Most recently, Ran has been shown to be crucial for the regeneration of a nuclear envelope after exit from mitosis. So, can the problems with cell-cycle progression and control induced by perturbing the Ran-system be attributed to defects in these three processes? This article examines this issue, concentrating on vertebrate systems. BioEssays 23:77-85, 2001.

Eukaryotic DNA replication: from pre-replication complex to initiation complex

A common mechanism has emerged for the control of the initiation of eukaryotic DNA replication. The minichromosome maintenance protein complex (MCM) and Cdc45 have now been recognized as central components of the initiation machinery. In addition, two types of S phase promoting kinases conserved between yeast and humans play critical roles in the initiation reaction. At the onset of S phase, S phase kinases promote the association of Cdc45 with MCM at origins. Upon the formation of the MCM-Cdc45 complex at origins, the duplex DNA is unwound and various replication proteins, including DNA polymerases, are recruited onto unwound DNA. The increasing number of newly identified factors involved in the initiation reaction indicates that the control of initiation requires highly evolved machinery in eukaryotic cells.

Schizosaccharomyces pombe Hsk1p is a potential cds1p target required for genome integrity

The fission yeast Hsk1p kinase is an essential activator of DNA replication. Here we report the isolation and characterization of a novel mutant allele of the gene. Consistent with its role in the initiation of DNA synthesis, hsk1(ts) genetically interacts with several S-phase mutants. At the restrictive temperature, hsk1(ts) cells suffer abnormal S phase and loss of nuclear integrity and are sensitive to both DNA-damaging agents and replication arrest. Interestingly, hsk1(ts) mutants released to the restrictive temperature after early S-phase arrest in hydroxyurea (HU) are able to complete bulk DNA synthesis but they nevertheless undergo an abnormal mitosis. These findings indicate a second role for hsk1 subsequent to HU arrest. Consistent with a later S-phase role, hsk1(ts) is synthetically lethal with Deltarqh1 (RecQ helicase) or rad21ts (cohesin) mutants and suppressed by Deltacds1 (RAD53 kinase) mutants. We demonstrate that Hsk1p undergoes Cds1p-dependent phosphorylation in response to HU and that it is a direct substrate of purified Cds1p kinase in vitro. These results indicate that the Hsk1p kinase is a potential target of Cds1p regulation and that its activity is required after replication initiation for normal mitosis.

Cell cycle- and cell growth-regulated proteolysis of mammalian CDC6 is dependent on APC-CDH1

CDC6 is conserved during evolution and is essential and limiting for the initiation of eukaryotic DNA replication. Human CDC6 activity is regulated by periodic transcription and CDK-regulated subcellular localization. Here, we show that, in addition to being absent from nonproliferating cells, CDC6 is targeted for ubiquitin-mediated proteolysis by the anaphase promoting complex (APC)/cyclosome in G(1). A combination of point mutations in the destruction box and KEN-box motifs in CDC6 stabilizes the protein in G(1) and in quiescent cells. Furthermore, APC, in association with CDH1, ubiquitinates CDC6 in vitro, and both APC and CDH1 are required and limiting for CDC6 proteolysis in vivo. Although a stable mutant of CDC6 is biologically active, overexpression of this mutant or wild-type CDC6 is not sufficient to induce multiple rounds of DNA replication in the same cell cycle. The APC-CDH1-dependent proteolysis of CDC6 in early G(1) and in quiescent cells suggests that this process is part of a mechanism that ensures the timely licensing of replication origins during G(1).

Molecular cloning and characterization of a plant homologue of the origin recognition complex 1 (ORC1)

By using the rice EST database, we have isolated a 2.8 kb cDNA, termed Oryza sativa ORC1 (OsORC1), from rice (O. sativa) encoding a protein that shows homology with the eukaryotic ORC1 proteins. Alignment of the OsORC1 protein sequence with the sequence of ORC1 from human and yeasts S. cerevisiae and S. pombe showed a high degree of sequence homology (38.7, 32.9 and 35.0% identity, respectively), particularly around the C-terminal region containing the CDC-NTP domain. Interestingly, the OsORC1 protein had an A+T hook-like motif, which was not present in the human or yeast genes. Genomic analysis indicated that OsORC1 existed as a single copy per genome. OsORC1 transcripts were expressed strongly in root tips and weakly in young leaves containing root apical meristem and marginal meristem, respectively. No expression was detected in the mature leaves. The level of OsORC1 expression was significantly reduced when cell proliferation was temporarily halted by the removal of sucrose from the growth medium. When the growth-halted cells began to re-grow following addition of sucrose to the medium, OsORC1 was again expressed at high levels. These results suggested that OsORC1 is required for cell proliferation. The role of OsORC1 in plant DNA replication will be discussed.

Cdc6p modulates the structure and DNA binding activity of the origin recognition complex in vitro

An interaction between the origin recognition complex (ORC) and Cdc6p is the first and a key step in the initiation of chromosomal DNA replication. We describe the assembly of an origin-dependent complex containing ORC and Cdc6p from Saccharomyces cerevisiae. Cdc6p increases the DNA binding specificity of ORC by inhibiting non-specific DNA binding of ORC. Cdc6p induces a concomitant change in the conformation of ORC and mutations in the Cdc6p Walker A and Walker B motifs, or ATP-γ-S inhibited these activities of Cdc6p. These data suggest that Cdc6p modifies ORC function at DNA replication origins. On the basis of these results in yeast, we propose that Cdc6p may be an essential determinant of origin specificity in metazoan species.

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

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