The MCM2-3-5 proteins: are they replication licensing factors?

Starting DNA Synthesis: Initiation Processes during the Replication of Chromosomal DNA in Humans

The initiation reactions of DNA synthesis are central processes during human chromosomal DNA replication. They are separated into two main processes: the initiation events at replication origins, the start of the leading strand synthesis for each replicon, and the numerous initiation events taking place during lagging strand DNA synthesis. In addition, a third mechanism is the re-initiation of DNA synthesis after replication fork stalling, which takes place when DNA lesions hinder the progression of DNA synthesis. The initiation of leading strand synthesis at replication origins is regulated at multiple levels, from the origin recognition to the assembly and activation of replicative helicase, the Cdc45-MCM2-7-GINS (CMG) complex. In addition, the multiple interactions of the CMG complex with the eukaryotic replicative DNA polymerases, DNA polymerase α-primase, DNA polymerase δ and ε, at replication forks play pivotal roles in the mechanism of the initiation reactions of leading and lagging strand DNA synthesis. These interactions are also important for the initiation of signalling at unperturbed and stalled replication forks, "replication stress" events, via ATR (ATM-Rad 3-related protein kinase). These processes are essential for the accurate transfer of the cells' genetic information to their daughters. Thus, failures and dysfunctions in these processes give rise to genome instability causing genetic diseases, including cancer. In their influential review "Hallmarks of Cancer: New Dimensions", Hanahan and Weinberg (2022) therefore call genome instability a fundamental function in the development process of cancer cells. In recent years, the understanding of the initiation processes and mechanisms of human DNA replication has made substantial progress at all levels, which will be discussed in the review.

Regulation of cancer stem cells by CXCL1, a chemokine whose secretion is controlled by MCM2

BACKGROUND

A high expression pattern of minichromosome maintenance 2 (MCM2) has been observed in various cancers. MCM2 is a protein involved in the cell cycle and plays a role in cancer growth and differentiation by binding to six members of the MCM subfamily. The MCM protein family includes MCM2 through MCM7.

METHODS

MCM2 has shown high expression in both lung cancer stem cells (LCSCs) and glioma stem cells (GSCs). We investigated the characteristics of CSCs and the regulation of the epithelial-to-mesenchymal transition (EMT) phenomenon in LCSCs and GSCs by MCM2. Additionally, we explored secreted factors regulated by MCM2.

RESULTS

There was a significant difference in survival rates between lung cancer patients and brain cancer patients based on MCM2 expression. MCM2 was found to regulate both markers and regulatory proteins in LCSCs. Moreover, MCM2 is thought to be involved in cancer metastasis by regulating cell migration and invasion, not limited to lung cancer but also identified in glioma. Among chemokines, chemokine (C-X-C motif) ligand 1 (CXCL1) was found to be regulated by MCM2.

CONCLUSIONS

MCM2 not only participates in the cell cycle but also affects cancer cell growth by regulating the external microenvironment to create a favorable environment for cells. MCM2 is highly expressed in malignant carcinomas, including CSCs, and contributes to the malignancy of various cancers. Therefore, MCM2 may represent a crucial target for cancer therapeutics.

MCM10 expression is linked to cervical cancer aggressiveness

Cervical cancer screening is a challenge mainly in developing countries. In developed countries, both incidence and mortality rates have been decreasing due to well organized screening programs. One of the potential biomarkers being exploited are the minichromosome maintenance proteins (MCMs), which show both specificity and sensitivity. MCM2-7 are involved in DNA replication initiation and elongation, and the MCM subunits are highly expressed in malignant tissues. Unlike other MCMs, MCM10, which is not part of the core helicase complex, is a critical determinant of origin activation and its levels are limiting in cancer cells. In this study, we performed bioinformatic analysis on the expression profile of all DNA replication associated MCM proteins in cervical cancer. MCM10 showed a relatively higher expression profile compared to the other MCMs. The mRNA expression levels of the MCMs were significantly increased in tumour tissues compared to normal, and MCM10 showed a fold change of 3.4. In order to understand if MCM10 is associated with the aggressiveness of cervical cancer, we looked into the mRNA expression pattern of MCM10 in three cervical cancer cell lines and one normal cervical cell line. MCM10 expression was significantly higher in the case of the more aggressive cancer cell line HeLa compared to controls. MCM10, therefore, can serve as a prominent biomarker for cancer progression and thus aid in early detection to control the spread of cancer cells. Our results show that MCM10 expression levels in cervical cancer cell lines are associated with cancer aggressiveness, demonstrating its clinical significance.

Host cell interactome of PB1 N40 protein of H5N1 influenza A virus in chicken cells

H5N1 influenza A virus (IAV) causes seasonal epidemics that represent a worldwide threat to public health. IAV relies on host factors for viral replication. PB1 N40 is translated from the fifth starting code (AUG) of PB1 mRNA, which is the product of the ribosomal scan omission. Here, we report the interactome landscape of H5N1 IAV PB1 N40 protein in chicken cells. The interacting complexes were captured by co-immunoprecipitation and analyzed by mass spectrometry. We identified 135 proteins as PB1 N40-interacting proteins. GO and Pathway analysis showed that proteins with biological functions such as protein localization and viral transcription and proteins related to signaling pathways of DNA replication and cell cycle were significantly enriched in virus-host interactions, suggesting the potential roles of them in infection with H5N1 IAV. Comparative analysis among H1N1 and H5N1 revealed conservation of the virus-host protein interaction between different subtypes or strains of influenza virus. ARCN1 was identified as a host interacting factor of H5N1 IAV PB1 N40 protein, which is the component of the coatomer. Knockdown of ARCN1 significantly decreased the titer of H5N1 IAV in chicken cells. BIOLOGICAL SIGNIFICANCE: Influenza A virus (IAV) is a great threat to public health and avian production. However, the manner in which avian IAV recruits the host cellular machinery for replication and how the host antagonizes the IAV infection was previously poorly understood. Here we present the viral-host interactome of the H5N1 IAV PB1 N40 protein and reveal its involvement with dozens of important host genes during the course of IAV infection.

Low Temperature Affects Stem Cell Maintenance in Brassica oleracea Seedlings

Most of the above ground tissues in higher plants originate from stem cells located in the shoot apical meristem (SAM). Several plant species can suffer from spontaneous stem cell arrest resulting in lack of further shoot development. In Brassica oleracea this SAM arrest is known as blindness and occurs in an unpredictable manner leading to considerable economic losses for plant raisers and farmers. Detailed analyses of seedlings showed that stem cell arrest is triggered by low temperatures during germination. To induce this arrest reproducibly and to study the effect of the environment, an assay was developed. The role of genetic variation on the susceptibility to develop blind seedlings was analyzed by a quantitative genetic mapping approach, using seeds from a double haploid population from a cross between broccoli and Chinese kale, produced at three locations. The analysis revealed, besides an effect of the seed production location, a region on linkage group C3 associated with blindness sensitivity. A subsequent dynamic genome-wide transcriptome analysis resulted in the identification of around 3000 differentially expressed genes early after blindness induction. A large number of cell cycle genes were en masse induced early during the development of blindness, whereas shortly after, all were down-regulated. This miss-regulation of core cell cycle genes is accompanied with a strong reduction of cells reaching the DNA replication phase. From the differentially expressed genes, 90 were located in the QTL region C3. Among them are two genes belonging to the MINICHROMOSOMAL MAINTENANCE gene family, known to be involved in DNA replication, a RETINOBLASTOMA-RELATED gene, a key regulator for cell cycle initiation, and several MutS homologs genes, involved in DNA repair. These genes are potential candidates for being involved in the development of blindness in Brassica oleracea sensitive genotypes.

GhH2A12, a replication-dependent histone H2A gene from Gossypium hirsutum, is negatively involved in the development of cotton fiber cells

GhH2A12 was preferentially expressed at the initiation and early elongation stage of cotton fiber development, and overexpression of GhH2A12 caused retardation of fiber initiation and produced shorter fibers. Histone H2A is a component of eukaryotic chromatin whose function has not been studied in cotton. We have isolated an H2A gene encoding 156 amino acids, named GhH2A12. Like other plant histone H2As, GhH2A12 contains a typical SPKK motif in the carboxy-terminal and a plant-unique peptide-binding A/T-rich DNA region, and it was localized to the nucleus. GhH2A12 was preferentially expressed at the initiation and early elongation stage of cotton fiber, from 0 to 5 days post anthesis and the transcript level declined rapidly when the fiber entered the fast elongation stage, suggesting that GhH2A12 was involved in fiber differentiation. Therefore, GhH2A12 overexpression and RNAi transgenic cotton lines were developed via Agrobacterium tumefaciens-mediated transformation. Overexpression of GhH2A12 caused retardation of fiber initiation and produced shorter fibers and lower lint percentages. Moreover, the overexpressors showed negative effects on seedling growth, and the leaf emergence was delayed compared to wild type. However, no significant change in the GhH2A12 suppression line was observed. Coupled with retardation of fiber initiation, upregulation of GhH2A12 downregulated the expression of genes involved in cell-cycle performance. These results suggest that GhH2A12 might regulate fiber differentiation via regulating the cell cycle-related genes.

The replication fork: understanding the eukaryotic replication machinery and the challenges to genome duplication

Eukaryotic cells must accurately and efficiently duplicate their genomes during each round of the cell cycle. Multiple linear chromosomes, an abundance of regulatory elements, and chromosome packaging are all challenges that the eukaryotic DNA replication machinery must successfully overcome. The replication machinery, the “replisome” complex, is composed of many specialized proteins with functions in supporting replication by DNA polymerases. Efficient replisome progression relies on tight coordination between the various factors of the replisome. Further, replisome progression must occur on less than ideal templates at various genomic loci. Here, we describe the functions of the major replisome components, as well as some of the obstacles to efficient DNA replication that the replisome confronts. Together, this review summarizes current understanding of the vastly complicated task of replicating eukaryotic DNA.

Expression, purification and biochemical characterization of Schizosaccharomyces pombe Mcm4, 6 and 7

Background

The hetero-hexamer of the eukaryotic minichromosome maintenance (MCM) proteins plays an essential role in replication of genomic DNA. The ring-shaped Mcm2-7 hexamers comprising one of each subunit show helicase activity in vitro, and form double-hexamers on DNA. The Mcm4/6/7 also forms a hexameric complex with helicase activity in vitro.

Results

We used an Escherichiai coli expression system to express various domains of Schizosaccharomyces pombe Mcm4, 6 and 7 in order to characterize their domain structure, oligomeric states, and possible inter-/intra-subunit interactions. We also successfully employed a co-expression system to express Mcm4/6/7 at the same time in Escherichiai coli, and have purified functional Mcm4/6/7 complex in a hexameric state in high yield and purity, providing a means for generating large quantity of proteins for future structural and biochemical studies.

Conclusions

Based on our results and those of others, models were proposed for the subunit arrangement and architecture of both the Mcm4/6/7 hexamer and the Mcm2-7 double-hexamer.

Developmental gene amplification and origin regulation

Developmentally regulated gene amplification serves to increase the number of templates for transcription, yielding greatly increased protein and/or RNA product for gene(s) at the amplified loci. It is observed with genes that are very actively transcribed and during narrow windows of developmental time where copious amounts of those particular gene products are required. Amplification results from repeated firing of origins at a few genomic loci, while the rest of the genome either does not replicate, or replicates to a lesser extent. As such, amplification is a striking exception to the once-and-only-once rule of DNA replication and may be informative as to that mechanism. Drosophila amplifies eggshell (chorion) genes in the follicle cells of the ovary to allow for rapid eggshell synthesis. Sciara amplifies multiple genes in larval salivary gland cells that encode proteins secreted in the saliva for the pupal case. Finally, Tetrahymena amplifies its rRNA genes several thousand-fold in the creation of the transcriptionally active macronucleus. Due to the ease of molecular and genetic analysis with these systems, the study of origin regulation has advanced rapidly. Comparisons reveal an evolutionarily conserved trans-regulatory apparatus and a similar organization of sequence-specific cis-regulatory replicator and origin elements. The studies indicate a regulatory role for chromatin structure and transcriptionally active genes near the origins.

Minichromosome maintenance 2 expression is correlated with mode of invasion and prognosis in oral squamous cell carcinomas

BACKGROUND

This study examined the immunohistochemical expression of cell-cycle related molecules as well as cell proliferation and pathologic findings in oral squamous cell carcinoma (SCC) in order to clarify their pathobiologic and prognostic significance.

METHODS

A total of 46 oral SCC specimens were analyzed using Ki-67, minichromosome maintenance 2 (MCM2), p53, p27, p21, and TUNEL. Aspects including tumor differentiation, mode of carcinoma invasion, tumor metastasis, and patient prognosis were compared among the specimens.

RESULTS

A significantly higher MCM2 labeling index (LI) was observed in the moderately differentiated SCCs when compared to the well-differentiated SCCs (P<0.05). The higher MCM2 LI was correlated with mode of invasion Grade 4 (infiltrative growth) and patient prognosis. In contrast, the LIs of Ki-67, TUNEL-signal, p53, p27, and p21 were not correlated with patient prognosis.

CONCLUSION

Higher MCM2 LI provides useful information for patient prognosis in oral SCCs.

Expression of minichromosome maintenance 2 (MCM2), Ki-67, and cell-cycle-related molecules, and apoptosis in the normal-dysplasia-carcinoma sequence of the oral mucosa

OBJECTIVE

We examined cell cycle and cell death biomarker trends with the normal-dysplasia-carcinoma sequence of the oral epithelia analyzing the pathological significance of a new biomarker, minichromosome maintenance 2 (MCM2).

METHODS

This study analyzed 12 patients with normal oral epithelia, 69 with dysplasia, and 35 with squamous cell carcinoma (SCC); in 13 patients, SCCs were preceded by dysplasia. The sections were immunostained for MCM2, Ki-67, P53, P27(Kip1) and P21(CIP1/WAF1), and conducted by TUNEL methods. Western blot analysis of MCM2 was performed in the 4 human cultured oral SCCs, all of which showed the expression.

RESULTS

Significantly higher labeling indices (LI; %) of MCM2, Ki-67, and P53, as well as lower LI of TUNEL indices (TI; %), P27, and P21 were noted in the SCCs than in the dysplasias. The 13 dysplasias developed SCC with significantly higher LI of MCM2 and P53, and lower LI of P21 than the other dysplasias (each p < 0.05). The LI of MCM2, P21 and the TI were not correlated with P53 expression.

CONCLUSIONS

Oral dysplasia was characterized by lower cell proliferation and a higher frequency of cell death compared to SCCs. The higher LI of MCM2 and P53 and the lower LI of P21 might predict malignant transformation of oral dysplasia. MCM2 is regulated via a P53-independent pathway, and a useful biomarker of proliferating cells.

Germinal center-associated nuclear protein (GANP) has a phosphorylation-dependent DNA-primase activity that is up-regulated in germinal center regions

Antigen stimulation induces a rapid proliferation of B cells for expansion of specific B cell clones and their further differentiation into antibody-producing cells in germinal centers of T-dependent antigen-immunized mice. Previously, we identified a 210-kDa germinal center-associated nuclear protein (GANP) that is up-regulated selectively in germinal centers and carries an MCM-binding domain in the carboxyl-terminal side. In addition, here, we found a region (from 414 to 550 aa) in GANP molecule that is slightly similar to the known DNA-primase component p49. The recombinant GANP fragment covering this region synthesizes RNA primers for extension by DNA polymerase I with single-stranded DNA templates in vitro. GANP DNA-primase activity is controlled by phosphorylation at Ser(502) that is induced by CD40-mediated signaling in vitro and in the germinal center B cells stimulated with antigen in vivo. Overexpression of ganp cDNA in Daudi B cells caused the increased DNA synthesis more than the levels of the mock-transfectants. These evidences suggested that the novel DNA-primase GANP is involved in regulation of cell proliferation of antigen-driven B cells in germinal centers.

Regulation of initiation of S phase, replication checkpoint signaling, and maintenance of mitotic chromosome structures during S phase by Hsk1 kinase in the fission yeast

Hsk1, Saccharomyces cerevisiae Cdc7-related kinase in Shizosaccharomyces pombe, is required for G1/S transition and its kinase activity is controlled by the regulatory subunit Dfp1/Him1. Analyses of a newly isolated temperature-sensitive mutant, hsk1-89, reveal that Hsk1 plays crucial roles in DNA replication checkpoint signaling and maintenance of proper chromatin structures during mitotic S phase through regulating the functions of Rad3 (ATM)-Cds1 and Rad21 (cohesin), respectively, in addition to expected essential roles for initiation of mitotic DNA replication through phosphorylating Cdc19 (Mcm2). Checkpoint defect in hsk1-89 is indicated by accumulation of cut cells at 30 degrees C. hsk1-89 displays synthetic lethality in combination with rad3 deletion, indicating that survival of hsk1-89 depends on Rad3-dependent checkpoint pathway. Cds1 kinase activation, which normally occurs in response to early S phase arrest by nucleotide deprivation, is largely impaired in hsk1-89. Furthermore, Cds1-dependent hyperphosphorylation of Dfp1 in response to hydroxyurea arrest is eliminated in hsk1-89, suggesting that sufficient activation of Hsk1-Dfp1 kinase is required for S phase entry and replication checkpoint signaling. hsk1-89 displays apparent defect in mitosis at 37 degrees C leading to accumulation of cells with near 2C DNA content and with aberrant nuclear structures. These phenotypes are similar to those of rad21-K1 and are significantly enhanced in a hsk1-89 rad21-K1 double mutant. Consistent with essential roles of Rad21 as a component for the cohesin complex, sister chromatid cohesion is partially impaired in hsk1-89, suggesting a possibility that infrequent origin firing of the mutant may affect the cohesin functions during S phase.

Cyclin E uses Cdc6 as a chromatin-associated receptor required for DNA replication

Using an in vitro chromatin assembly assay in Xenopus egg extract, we show that cyclin E binds specifically and saturably to chromatin in three phases. In the first phase, the origin recognition complex and Cdc6 prereplication proteins, but not the minichromosome maintenance complex, are necessary and biochemically sufficient for ATP-dependent binding of cyclin E--Cdk2 to DNA. We find that cyclin E binds the NH(2)-terminal region of Cdc6 containing Cy--Arg-X-Leu (RXL) motifs. Cyclin E proteins with mutated substrate selection (Met-Arg-Ala-Ile-Leu; MRAIL) motifs fail to bind Cdc6, fail to compete with endogenous cyclin E--Cdk2 for chromatin binding, and fail to rescue replication in cyclin E--depleted extracts. Cdc6 proteins with mutations in the three consensus RXL motifs are quantitatively deficient for cyclin E binding and for rescuing replication in Cdc6-depleted extracts. Thus, the cyclin E--Cdc6 interaction that localizes the Cdk2 complex to chromatin is important for DNA replication. During the second phase, cyclin E--Cdk2 accumulates on chromatin, dependent on polymerase activity. In the third phase, cyclin E is phosphorylated, and the cyclin E--Cdk2 complex is displaced from chromatin in mitosis. In vitro, mitogen-activated protein kinase and especially cyclin B--Cdc2, but not the polo-like kinase 1, remove cyclin E--Cdk2 from chromatin. Rebinding of hyperphosphorylated cyclin E--Cdk2 to interphase chromatin requires dephosphorylation, and the Cdk kinase-directed Cdc14 phosphatase is sufficient for this dephosphorylation in vitro. These three phases of cyclin E association with chromatin may facilitate the diverse activities of cyclin E--Cdk2 in initiating replication, blocking rereplication, and allowing resetting of origins after mitosis.

A coordinated interplay: proteins with multiple functions in DNA replication, DNA repair, cell cycle/checkpoint control, and transcription

In eukaryotic cells, DNA transactions such as replication, repair, and transcription require a large set of proteins. In all of these events, complexes of more than 30 polypetides appear to function in highly organized and structurally well-defined machines. We have learned in the past few years that the three essential macromolecular events, replication, repair, and transcription, have common functional entities and are coordinated by complex regulatory mechanisms. This can be documented for replication and repair, for replication and checkpoint control, and for replication and cell cycle control, as well as for replication and transcription. In this review we cover the three different protein classes: DNA polymerases, DNA polymerase accessory proteins, and selected transcription factors. The "common enzyme-different pathway strategy" is fascinating from several points of view: first, it might guarantee that these events are coordinated; second, it can be viewed from an evolutionary angle; and third, this strategy might provide cells with backup mechanisms for essential physiological tasks.

Structure, expression, and chromosomal localization of the human gene encoding a germinal center-associated nuclear protein (GANP) that associates with MCM3 involved in the initiation of DNA replication

A 210kDa protein named GANP is upregulated in germinal center (GC)-B cells in the spleen of antigen-immunized mouse. We studied a human ganp gene (hganp) encoding a putative polypeptide of 1980 amino acids. The carboxyl-terminal 721-amino-acid sequence of hGANP is identical to Map80, that is presumably generated by alternative splicing of hganp/Map80 gene. The genomic segment carrying hganp and Map80 genes was isolated, and the chromosomal location was determined on 21q22.3. Northern blot analysis with RNAs from various organs demonstrated a single band of 7kb hganp mRNA, which suggests a preferential transcription of hganp gene from the hganp/Map80 locus. The hGANP expression was upregulated in GCs of the tonsil, as demonstrated by in-situ RNA hybridization and immunohistochemical analyses. The hGANP, with the domain (Map-box) capable of binding to MCM3 in B cells, might be involved in regulation of cell-cycle progression and DNA replication of GC-B cells.

MCM proteins in DNA replication

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

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

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

A novel nuclear phosphoprotein, GANP, is up-regulated in centrocytes of the germinal center and associated with MCM3, a protein essential for DNA replication

Antigen (Ag) immunization induces formation of the germinal center (GC), with large, rapidly proliferating centroblasts in the dark zone, and small, nondividing centrocytes in the light zone. We identified a novel nuclear protein, GANP, that is up-regulated in centrocytes. We found that GANP was up-regulated in GC B cells of Peyer's patches in normal mice and in spleens from Ag-immunized mice. GANP-positive cells appeared in the light zone of the GC, with coexpression of the peanut agglutinin (PNA) (PNA)-positive B220-positive phenotype. The expression of GANP was strikingly correlated with GC formation because Bcl6-deficient mice did not show the up-regulation of GANP. GANP-positive cells were mostly surrounded by follicular dendritic cells. Stimulation with anti-μ and anti-CD40 induced up-regulation of ganp messenger RNA as well as GANP protein in B220-positive B cells in vitro. GANP is a 210-kd protein localized in both the cytoplasm and nuclei, with a homologous region to Map80 that is associated with MCM3, a protein essential for DNA replication. Remarkably, GANP is associated with MCM3 in B cells and MCM3 is also up-regulated in the GC area. These results suggest that the up-regulation of GANP might participate in the development of Ag-driven B cells in GCs through its interaction with MCM3.

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

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

A novel nuclear phosphoprotein, GANP, is up-regulated in centrocytes of the germinal center and associated with MCM3, a protein essential for DNA replication

Antigen (Ag) immunization induces formation of the germinal center (GC), with large, rapidly proliferating centroblasts in the dark zone, and small, nondividing centrocytes in the light zone. We identified a novel nuclear protein, GANP, that is up-regulated in centrocytes. We found that GANP was up-regulated in GC B cells of Peyer's patches in normal mice and in spleens from Ag-immunized mice. GANP-positive cells appeared in the light zone of the GC, with coexpression of the peanut agglutinin (PNA) (PNA)-positive B220-positive phenotype. The expression of GANP was strikingly correlated with GC formation because Bcl6-deficient mice did not show the up-regulation of GANP. GANP-positive cells were mostly surrounded by follicular dendritic cells. Stimulation with anti-μ and anti-CD40 induced up-regulation of ganp messenger RNA as well as GANP protein in B220-positive B cells in vitro. GANP is a 210-kd protein localized in both the cytoplasm and nuclei, with a homologous region to Map80 that is associated with MCM3, a protein essential for DNA replication. Remarkably, GANP is associated with MCM3 in B cells and MCM3 is also up-regulated in the GC area. These results suggest that the up-regulation of GANP might participate in the development of Ag-driven B cells in GCs through its interaction with MCM3.

Biochemical analysis of the intrinsic Mcm4-Mcm6-mcm7 DNA helicase activity

Mcm proteins play an essential role in eukaryotic DNA replication, but their biochemical functions are poorly understood. Recently, we reported that a DNA helicase activity is associated with an Mcm4-Mcm6-Mcm7 (Mcm4,6,7) complex, suggesting that this complex is involved in the initiation of DNA replication as a DNA-unwinding enzyme. In this study, we have expressed and isolated the mouse Mcm2, 4,6,7 proteins from insect cells and characterized various mutant Mcm4,6,7 complexes in which the conserved ATPase motifs of the Mcm4 and Mcm6 proteins were mutated. The activities associated with such preparations demonstrated that the DNA helicase activity is intrinsically associated with the Mcm4,6,7 complex. Biochemical analyses of these mutant Mcm4,6,7 complexes indicated that the ATP binding activity of the Mcm6 protein in the complex is critical for DNA helicase activity and that the Mcm4 protein may play a role in the single-stranded DNA binding activity of the complex. The results also indicated that the two activities of DNA helicase and single-stranded DNA binding can be separated.

Regulation of eukaryotic DNA replication and nuclear structure

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

Identification and complete cDNA sequence of the missing Drosophila MCMs: DmMCM3, DmMCM6 and DmMCM7

The minichromosome maintenance (MCM) gene family consists of six members (MCM2, 3, 4, 5, 6 and 7) in Saccharomyces cerevisiae as well as in humans. Each family member plays an essential role in the replication of DNA. In Drosophila melanogaster only three members, DmMCM2, DmMCM4/dpa and DmMCM5/DmCDC46, have been studied. In addition, two other partial sequences were recently reported. Using degenerate primers and low stringency PCR conditions six different DNA sequences were identified with highest sequence similarity to MCM2, 3, 4, 5, 6 and 7. Sequence analysis of full length cDNA clones corresponding to the MCM3, 6 and 7 fragment proves the existence of six MCM genes in Drosophila melanogaster. Strong homology to the human counterparts, mRNA expression analysis and physico-chemical properties suggest a conserved function in DNA replication for DmMCM3, 6 and 7.

Fission yeast cdc24 is a replication factor C- and proliferating cell nuclear antigen-interacting factor essential for S-phase completion

At the nonpermissive temperature the fission yeast cdc24-M38 mutant arrests in the cell cycle with incomplete DNA replication as indicated by pulsed-field gel electrophoresis. The cdc24(+) gene encodes a 501-amino-acid protein with no significant homology to any known proteins. The temperature-sensitive cdc24 mutant is effectively rescued by pcn1(+), rfc1(+) (a fission yeast homologue of RFC1), and hhp1(+), which encode the proliferating cell nuclear antigen (PCNA), the large subunit of replication factor C (RFC), and a casein kinase I involved in DNA damage repair, respectively. The Cdc24 protein binds PCNA and RFC1 in vivo, and the domains essential for Cdc24 function and for RFC1 and PCNA binding colocalize in the N-terminal two-thirds of the molecule. In addition, cdc24(+) genetically interacts with the gene encoding the catalytic subunit of DNA polymerase epsilon, which is stimulated by PCNA and RFC, and with those encoding the fission yeast counterparts of Mcm2, Mcm4, and Mcm10. These results indicate that Cdc24 is an RFC- and PCNA-interacting factor required for DNA replication and might serve as a target for regulation.

MCM21 and MCM22, two novel genes of the yeast Saccharomyces cerevisiae are required for chromosome transmission

The minichromosome maintenance genes, MCM21 and MCM22, have been cloned and are shown to code for the ORFs YDR318W and YJR135C respectively. Mutations in these genes caused a decrease in the stability of the minichromosome. This decrease in stability was associated with an increase in the copy number of the minichromosome in cells carrying it. Small circular dicentric plasmids were maintained relatively stably and structurally intact in the mutants compared with the wild-type strain. In the latter, such plasmids were mitotically unstable and, upon recovery, showed frequent rearrangements of their DNA. A centromere offered less obstruction to transcription in mutant cells than in the wild type, showing that both these mutants had a more relaxed kinetochore assembly. The mutant strains showed elevated rates of chromosome loss but not those of recombination. Both the mutations caused the cells to display a higher sensitivity towards the anti-mitotic drug benomyl. All these observations suggest that MCM21 and MCM22 are important for chromosome segregation with a potential role in kinetochore function. These genes are non-essential, as their deletions from chromosomes did not cause loss of cell viability. However, exponentially growing mutant cells carrying the deletion of the MCM21 gene had a significant population of large-budded cells with a single nucleus at the neck. Furthermore, the DNA content of these cells showed a shift towards 2N, suggesting a temporary pause of cells in G2 or in an early phase of mitosis. The mcm21 and mcm22 mutations do not show synthetic lethality or any further enhancement of growth defects, implying that they could be carrying out non-overlapping functions in chromosome segregation.

[Is the replicon model applicable to higher eukaryotes?]

Thirty-five years ago, the Replicon model was proposed by Jacob, Brenner and Cuzin to explain the regulation of the Escherichia coli DNA replication. In this model, a genetic element, the replicator, would function as a target for a positive-acting initiator protein to drive the initiation of replication. This simple idea has been extremely useful in providing a framework to explain how the initiation of DNA replication occurs in all organisms. The identification of autonomously replicating sequences (ARSs) in budding yeast was the first extension of the Replicon model to eukaryotic chromosomes. In the higher eukaryotes, many biochemically defined replication start sites have been identified; nevertheless there is little genetic data indicating that these sites contain DNA sequences that are essential for replication. Moreover, in early Xenopus or Drosophila embryos, specific DNA sequences are not required either for initiating DNA replication or for preventing rereplication within a single cell cycle. This apparently fundamental difference between replicators in yeast and metazoan embryos may be more superficial than initially thought. In fact, during the past several years, an eukaryotic initiator conserved from yeast to man and also present in embryonic cells, the origin recognition complex (ORC), has been characterized, suggesting that the initiation mechanism should be essentially the same in prokaryotes and eukaryotes. In addition, the efficient once-per-cell-cycle replication of DNA is ensured in eukaryotes by a simple two-step mechanism in which the assembly of stable prereplicative complexes (PreRCs) at origins precedes and is temporally separated from the firing of these origins. Regulation of this process by cyclin-dependent kinases ensures that when origins fire, the cell is no longer competent to form new PreRCs. Now, it is important to understand how these complexes are remodeled or disassembled during replication initiation to trigger the transition from a stable origin-bound complex to a mobile replication machine.

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

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

Human minichromosome maintenance proteins and human origin recognition complex 2 protein on chromatin

Minichromosome maintenance (Mcm) proteins and the constituents of the origin recognition complex (Orc) are essential components of the eukaryotic replication initiation apparatus. Published evidence strongly suggests that the binding of Mcm proteins to chromatin is contingent upon the prior binding of Orc proteins. Here we use two different approaches to investigate the presence of the human ORC2 protein and of Mcm proteins on chromatin of HeLa cells in various cell cycle phases. First, we mobilized chromatin-bound proteins by micrococcal nuclease and analyzed the resulting digestion products by sucrose gradient centrifugations. Under digestion conditions when Mcm proteins were almost entirely released from chromatin, ORC2 protein was found to be associated with chromatin fragments containing several hundred base pairs of DNA. Second, we used an in vivo cross-linking procedure to covalently link Mcm proteins and ORC2 to DNA by short exposure of intact HeLa cells to formaldehyde. Specific immunoprecipitations revealed that cross-linked nucleoprotein fragments carried either Mcm proteins or ORC2 protein, but not both. Based on the lengths of the DNA fragments in immunoprecipitates, we estimate that the distance between chromatin-bound ORC2 protein and chromatin-bound Mcm proteins must be at least 500-1000 base pairs in HeLa cells.

cDNA cloning and expression during development of Drosophila melanogaster MCM3, MCM6 and MCM7

cDNAs encoding three Drosophila melanogaster MCM proteins, DmMCM3, DmMCM6 and DmMCM7, candidates of DNA replication-licensing factors, were cloned and sequenced. The deduced amino-acid sequences displayed 60, 59 and 68% identities with the respective Xenopus laevis homologues, XMCM3, XMCM6 and XMCM7. Six members of the D. melanogaster MCM family were found to share 31-36% identities in their amino-acid sequences, and to possess the five common domains carrying conserved amino-acid sequences as reported with X. laevis MCM proteins. DmMCM3, DmMCM6 and DmMCM7 genes were mapped to the 4F region on the X chromosome, the 6B region on the X chromosome and the 66E region on the third chromosome, respectively, by in situ hybridization. Contents of their mRNAs were proved to be high in unfertilized eggs and early embryos (0-4h after fertilization), then decrease gradually by the 12h time point, with only low levels detected at later stages of development except in adult females. This fluctuation pattern is similar to those of genes for proteins involved in DNA replication, such as DNA polymerase alpha and proliferating cell nuclear antigen, suggesting that expression of DmMCM genes is under the regulatory mechanism which regulates expression of other genes involved in DNA replication.

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

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

Mobilization of chromatin-bound Mcm proteins by micrococcal nuclease

Mcm (minichromosome maintenance) proteins are important components of the eukaryotic replication initiation apparatus. We investigate the binding of human Mcm proteins to HeLa cell chromatin using micrococcal nuclease as a tool. In previous work we prepared chromatin under low ionic strength conditions. The use of a low salt buffer was necessary to prevent the dissociation of Mcm proteins. Here we use chromatin prepared at more physiological salt concentrations (100 mM NaCl) following the procedure of Fujita et al. (J. Biol. Chem. 272, 10928-10935; 1997) who had shown that ATP stabilizes the interaction of Mcm proteins with chromatin. We show here that micrococcal nuclease released Mcm proteins early during the digestion process suggesting that Mcm proteins reside on chromatin sites which are more open to nuclease attack than bulk chromatin. Released Mcm proteins sedimented through glycerol gradients as a multiprotein complex comprising several of the six known human Mcm proteins.

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

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

Stability of the replicative Mcm3 protein in proliferating and differentiating human cells

Mcm proteins are abundant nuclear proteins involved in the regulation of genome replication. Previous experiments had shown that levels of Mcm-specific mRNAs increase at the G1/S phase transition of the cell cycle, but that the amounts of Mcm proteins do not change much during the cell cycle. To learn more about the stability of an Mcm protein we performed experiments which showed that: (i) more than 60% of [35S]methionine pulse-labeled Mcm3 protein appears to be degraded during a 24-h chase in HeLa cells; (ii) the amount of Mcm3 protein significantly decreases during the differentiation of HL60 cells in vitro (whereas another replication-initiation protein, hOrc2, remains fairly constant); and (iii) according to immunohistochemical staining, Mcm3 protein is present in nuclei of cells in the proliferating zone of human epidermal tissue, but in decreasing amounts in nuclei of differentiating cells of the upper cell layers. Our interpretation is that Mcm3 protein is no longer synthesized after initiation of differentiation and slowly disappears at a half-life of approximately 24 h.

Negative regulation of DNA replication by the retinoblastoma protein is mediated by its association with MCM7

A yeast two-hybrid screen was employed to identify human proteins that specifically bind the amino-terminal 400 amino acids of the retinoblastoma (Rb) protein. Two independent cDNAs resulting from this screen were found to encode the carboxy-terminal 137 amino acids of MCM7, a member of a family of proteins that comprise replication licensing factor. Full-length Rb and MCM7 form protein complexes in vitro, and the amino termini of two Rb-related proteins, p107 and p130, also bind MCM7. Protein complexes between Rb and MCM7 were also detected in anti-Rb immunoprecipitates prepared from human cells. The amino-termini of Rb and p130 strongly inhibited DNA replication in an MCM7-dependent fashion in a Xenopus in vitro DNA replication assay system. These data provide the first evidence that Rb and Rb-related proteins can directly regulate DNA replication and that components of licensing factor are targets of the products of tumor suppressor genes.

Biochemical function of mouse minichromosome maintenance 2 protein

Minichromosome maintenance (MCM) proteins play an essential role in eukaryotic DNA replication and bind to chromatin before the initiation of DNA replication. We reported that MCM protein complexes consisting of MCM2, -4, -6, and -7 bind strongly to a histone-Sepharose column (Ishimi, Y., Ichinose, S., Omori, A., Sato, K., and Kimura, H. (1996) J. Biol. Chem. 271, 24115-24122). Here, we have analyzed this interaction at the molecular level. We found that among six mouse MCM proteins, only MCM2 binds to histone; amino acid residues 63-153 are responsible for this binding. The region required for nuclear localization of MCM2 was mapped near this histone-binding domain. Far-Western blotting analysis of truncated forms of H3 histone indicated that amino acid residues 26-67 of H3 histone are required for binding to MCM2. We have also shown that mouse MCM2 can inhibit the DNA helicase activity of the human MCM4, -6, and -7 protein complex. These results suggest that MCM2 plays a different role in the initiation of DNA replication than the other MCM proteins.

Human protein MCM6 on HeLa cell chromatin

Minichromosome maintenance (Mcm) proteins perform essential functions regulating the replication of chromatin. Human cells, like other eukaryotic cells, express at least six Mcm proteins conserved in the central region. We have earlier described the primary structures of five human Mcm proteins, but the primary structure of the sixth human Mcm protein, MCM6, was identified only recently. We now use antibodies, specific for the MCM6 protein, to assess its intranuclear distribution. We find that a fraction of MCM6 protein occurs in the nucleosol, forming multiprotein complexes with other Mcm proteins. More importantly, we use for the first time micrococcal nuclease as a tool to investigate the association of MCM6 protein with chromatin. After short digestion times, a considerable fraction of the MCM6 protein is released from chromatin as a multiprotein complex that includes other Mcm proteins as well. In addition, fractions of MCM3 and MCM6 proteins are released by nuclease digestion as monomeric proteins indicating that at least these two Mcm proteins may also occur as single molecules on chromatin. The data also suggest that the chromatin regions with bound Mcm proteins are more vulnerable to nuclease attack than bulk chromatin and may therefore differ in the arrangement of nucleosomes.

Identification of sna41 gene, which is the suppressor of nda4 mutation and is involved in DNA replication in Schizosaccharomyces pombe

BACKGROUND

The replication licensing factor limits DNA replication to once in a cell cycle and is thought to contain MCM proteins as its component parts. Six MCM subtypes have been identified in various species. These MCM proteins are thought to bind each other to make a heteromeric complex. The Nda4 protein of Schizosaccharomyces pombe is one of the MCM proteins and is involved in DNA replication.

RESULTS

The suppressor mutant of nda4 was isolated and the mutant gene was named sna41. The sna41-912 mutant demonstrated the ts phenotype, with an elongated cell shape at the restrictive temperature. Cells with 1C DNA content accumulated 2 h after shifting up to the restrictive temperature. This result suggests that sna41 is also involved in DNA replication. The sna41 genomic clone was isolated by a complementation of the ts phenotype of the mutant strain and was sequenced. The sna41 gene encodes a protein of 638 amino acids, which has low homology with CDC45 in S. cerevisiae. The gene disruption analysis showed that sna41 gene is essential for viability.

CONCLUSIONS

The S. pombe sna41 mutation suppresses the nda4-108 mutation. Sna41 is involved in DNA replication and may play some roles in the regulation of DNA replication by the MCM proteins.

Chromosome association of minichromosome maintenance proteins in Drosophila endoreplication cycles

Minichromosome maintenance (MCM) proteins are essential eukaryotic DNA replication factors. The binding of MCMs to chromatin oscillates in conjunction with progress through the mitotic cell cycle. This oscillation is thought to play an important role in coupling DNA replication to mitosis and limiting chromosome duplication to once per cell cycle. The coupling of DNA replication to mitosis is absent in Drosophila endoreplication cycles (endocycles), during which discrete rounds of chromosome duplication occur without intervening mitoses. We examined the behavior of MCM proteins in endoreplicating larval salivary glands, to determine whether oscillation of MCM-chromosome localization occurs in conjunction with passage through an endocycle S phase. We found that MCMs in polytene nuclei exist in two states: associated with or dissociated from chromosomes. We demonstrate that cyclin E can drive chromosome association of DmMCM2 and that DNA synthesis erases this association. We conclude that mitosis is not required for oscillations in chromosome binding of MCMs and propose that cycles of MCM-chromosome association normally occur in endocycles. These results are discussed in a model in which the cycle of MCM-chromosome associations is uncoupled from mitosis because of the distinctive program of cyclin expression in endocycles.

Selective proteolysis of the nuclear replication factor MCM3 in apoptosis

Cleavage of specific protein subsets is a key event in the execution of apoptosis. Protein degradation may serve for the structural alterations that result in cell self-destruction, but it may also function as a switch in the decisions between apoptosis and necrosis or apoptosis and cell proliferation. Here, we show that MCM3, but not other members of the Mcm family of replicative proteins, is cleaved early in several models of apoptosis. Cleavage of MCM3 can be prevented by caspase inhibitors, and it does not occur when cells are forced to undergo necrosis by energy deprivation. We propose that active destruction of MCM3 inactivates the Mcm complex and serves to prevent untimely DNA replication events during the execution of the cell death program.

Mutations of the Drosophila dDP, dE2F, and cyclin E genes reveal distinct roles for the E2F-DP transcription factor and cyclin E during the G1-S transition

Activation of heterodimeric E2F-DP transcription factors can drive the G1-S transition. Mutation of the Drosophila melanogaster dE2F gene eliminates transcriptional activation of several replication factors at the G1-S transition and compromises DNA replication. Here we describe a mutation in the Drosophila dDP gene. As expected for a defect in the dE2F partner, this mutation blocks G1-S transcription of DmRNR2 and cyclin E as previously described for mutations of dE2F. Mutation of dDP also causes an incomplete block of DNA replication. When S phase is compromised by reducing the activity of dE2F-dDP by either a dE2F or dDP mutation, the first phenotype detected is a reduction in the intensity of BrdU incorporation and a prolongation of the labeling. Notably, in many cells, there was no detected delay in entry into this compromised S phase. In contrast, when cyclin E function was reduced by a hypomorphic allele combination, BrdU incorporation was robust but the timing of S-phase entry was delayed. We suggest that dE2F-dDP contributes to the expression of two classes of gene products: replication factors, whose abundance has a graded effect on replication, and cyclin E, which triggers an all-or-nothing transition from G1 to S phase.

Functional analysis of a replication origin from Saccharomyces cerevisiae: identification of a new replication enhancer

Yeast replication origins have a modular arrangement of essential DNA sequences containing the ARS consensus sequence (ACS) flanked by auxiliary DNA elements which stimulate origin function. One of the auxiliary elements identified at several origins is a DNA replication enhancer that binds the Abf1p protein. We have isolated an ARS sequence from Saccharomyces cerevisiae based on its ability to bind Abf1p. Here we present a detailed molecular dissection of this ARS, designated ARS 1501, and we demonstrate that it functions as a genomic replication origin on chromosome XV . Mutagenesis of the Abf1p DNA-binding sites revealed that these sequences did not contribute significantly to ARS function. Instead, a new DNA element important for replication, designated REN1501, has been located 5' to the T-rich strand of the ACS. We show that REN1501 functions in either orientation and at variable distances from the ACS, defining this element as a DNA replication enhancer. Most significantly, point mutations within this element decreased the stability of plasmids bearing ARS 1501, suggesting that REN1501 binds a protein important for replication initiation. Only three elements found at origins are known to specifically bind proteins. These include the ARS essential sequences and the Abf1p and Rap1p DNA-binding sites. We show that the function of REN1501 at the origin cannot be replaced by a Rap1p DNA-binding site or a site that binds the transcriptional factor Gal4p and can only be partially substituted for by an Abf1p recognition sequence. This implies that the role of the REN1501 element at the ARS 1501 origin is specific, and suggest that the frequency of origin firing in eukaryotic cells may be regulated by origin-specific enhancers.

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

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

CDC45 is required in conjunction with CDC7/DBF4 to trigger the initiation of DNA replication

The initiation of DNA replication in Saccharomyces cerevisiae requires the protein product of the CDC45 gene. We report that although Cdc45p is present at essentially constant levels throughout the cell cycle, it completes its initiation function in late G1, after START and prior to DNA synthesis. Shortly after mitosis, cells prepare for initiation by assembling prereplicative complexes at their replication origins. These complexes are then triggered at the onset of S phase to commence DNA replication. Cells defective for CDC45 are incapable of activating the complexes to initiate DNA replication. In addition, Cdc45p and Cdc7p/Dbf4p, a kinase implicated in the G1/S phase transition, are dependent on one another for function. These data indicate that CDC45 functions in late G1 phase in concert with CDC7/DBF4 to trigger initiation at replication origins after the assembly of the prereplicative complexes.

Chromosome association of minichromosome maintenance proteins in Drosophila mitotic cycles

Minichromosome maintenance (MCM) proteins are essential DNA replication factors conserved among eukaryotes. MCMs cycle between chromatin bound and dissociated states during each cell cycle. Their absence on chromatin is thought to contribute to the inability of a G2 nucleus to replicate DNA. Passage through mitosis restores the ability of MCMs to bind chromatin and the ability to replicate DNA. In Drosophila early embryonic cell cycles, which lack a G1 phase, MCMs reassociate with condensed chromosomes toward the end of mitosis. To explore the coupling between mitosis and MCM-chromatin interaction, we tested whether this reassociation requires mitotic degradation of cyclins. Arrest of mitosis by induced expression of nondegradable forms of cyclins A and/or B showed that reassociation of MCMs to chromatin requires cyclin A destruction but not cyclin B destruction. In contrast to the earlier mitoses, mitosis 16 (M16) is followed by G1, and MCMs do not reassociate with chromatin at the end of M16. dacapo mutant embryos lack an inhibitor of cyclin E, do not enter G1 quiescence after M16, and show mitotic reassociation of MCM proteins. We propose that cyclin E, inhibited by Dacapo in M16, promotes chromosome binding of MCMs. We suggest that cyclins have both positive and negative roles in controlling MCM-chromatin association.

Unusual genome organisation in Entamoeba histolytica leads to two overlapping transcripts

We have isolated homologs of the mini chromosome maintenance (MCM) gene family from the parasitic protozoan Entamoeba histolytica. The full length genomic and cDNA clones for the Eh MCM3 gene have been characterised. The Eh MCM3 gene is much smaller than the Saccharomyces cerevisiae MCM3 gene and other eukaryotic homologs of the MCM3/P1 family. The predicted Eh Mcm3 protein was 597 amino acids long and showed 37 and 46% positional identity with the Sc Mcm3 and the mouse P1 homologs respectively. While proceeding along the chromosome from the Eh MCM3 gene, we have identified a homolog (Eh PAK) of the murine p21 activated kinase (Rn KPAK), or S. cerevisiae STE20. Eh PAK lies 126 bp upstream of the Eh MCM3 gene. The predicted Eh p21 activated kinase protein was 459 amino-acids long and showed 33% positional identity with the murine p21 activated kinase and its yeast homolog Ste20. Analysis of cDNA and genomic sequences shows that the 3' untranslated region (UTR) of the Eh PAK mRNA and the 5' UTR of the Eh MCM3 mRNA are transcribed from a common 40 bp genomic segment. This is the first report of an amoeba gene being physically linked to a second gene such that their transcripts are overlapping and there is no non-transcribed intergenic region between the two genes. Primer extension studies have confirmed that unlike most E. histolytica genes, which have short 5' UTRs, the Eh MCM3 mRNA has a 126 bp long 5' UTR and the Eh PAK mRNA has a 265 bp long 5' UTR.

A DNA helicase activity is associated with an MCM4, -6, and -7 protein complex

All six minichromosome maintenance (MCM) proteins have DNA-dependent ATPase motifs in the central domain which is conserved from yeast to mammals. Our group purified MCM protein complexes consisting of MCM2, -4 (Cdc21), -6 (Mis5), and -7 (CDC47) proteins from HeLa cells by using histone-Sepharose column chromatography (Ishimi, Y., Ichinose, S., Omori, A., Sato K., and Kimura, H. (1996) J. Biol. Chem. 271, 24115-24122). The present study revealed that both ATPase activity and DNA helicase activity that displaces oligonucleotides annealed to single-stranded circular DNA are associated with an MCM protein complex. Both ATPase and DNA helicase activities were co-purified with a 600-kDa protein complex that is consisted of equal amounts of MCM4, -6, and -7 proteins. An immunodepletion of the MCM protein complex from the purified fraction using anti-MCM4 antibody resulted in the severe reduction of the DNA helicase activity. Displacement of DNA fragments by the DNA helicase suggested that it migrated along single-stranded DNA in the 3' to 5' direction, and the DNA helicase activity was detected only in the presence of hydrolyzable ATP or dATP. These results suggest that this helicase may be involved in the initiation of DNA replication as a DNA unwinding enzyme.