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Fong KK, Zelter A, Graczyk B, Hoyt JM, Riffle M, Johnson R, MacCoss MJ, Davis TN. Novel phosphorylation states of the yeast spindle pole body. Biol Open 2018; 7:bio.033647. [PMID: 29903865 PMCID: PMC6215409 DOI: 10.1242/bio.033647] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Phosphorylation regulates yeast spindle pole body (SPB) duplication and separation and likely regulates microtubule nucleation. We report a phosphoproteomic analysis using tandem mass spectrometry of enriched Saccharomyces cerevisiae SPBs for two cell cycle arrests, G1/S and the mitotic checkpoint, expanding on previously reported phosphoproteomic data sets. We present a novel phosphoproteomic state of SPBs arrested in G1/S by a cdc4-1 temperature-sensitive mutation, with particular focus on phosphorylation events on the γ-tubulin small complex (γ-TuSC). The cdc4-1 arrest is the earliest arrest at which microtubule nucleation has occurred at the newly duplicated SPB. Several novel phosphorylation sites were identified in G1/S and during mitosis on the microtubule nucleating γ-TuSC. These sites were analyzed in vivo by fluorescence microscopy and were shown to be required for proper regulation of spindle length. Additionally, in vivo analysis of two mitotic sites in Spc97 found that phosphorylation of at least one of these sites is required for progression through the cell cycle. This phosphoproteomic data set not only broadens the scope of the phosphoproteome of SPBs, it also identifies several γ-TuSC phosphorylation sites that influence microtubule formation. Summary: A phosphoproteome of yeast spindle pole bodies in G1/S or M phase identifies phosphorylation sites involved in spindle length control and provides direction for future phosphorylation analyses of spindle pole components.
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Affiliation(s)
- Kimberly K Fong
- Department of Biochemistry, University of Washington, Seattle, WA 98195, USA
| | - Alex Zelter
- Department of Biochemistry, University of Washington, Seattle, WA 98195, USA
| | - Beth Graczyk
- Department of Biochemistry, University of Washington, Seattle, WA 98195, USA
| | - Jill M Hoyt
- Department of Biochemistry, University of Washington, Seattle, WA 98195, USA
| | - Michael Riffle
- Department of Biochemistry, University of Washington, Seattle, WA 98195, USA
| | - Richard Johnson
- Department of Genome Sciences, University of Washington, Seattle, WA 98195, USA
| | - Michael J MacCoss
- Department of Genome Sciences, University of Washington, Seattle, WA 98195, USA
| | - Trisha N Davis
- Department of Biochemistry, University of Washington, Seattle, WA 98195, USA
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2
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Supady A, Klipp E, Barberis M. A variable fork rate affects timing of origin firing and S phase dynamics in Saccharomyces cerevisiae. J Biotechnol 2013; 168:174-84. [PMID: 23850861 DOI: 10.1016/j.jbiotec.2013.06.022] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2012] [Revised: 05/23/2013] [Accepted: 06/27/2013] [Indexed: 10/26/2022]
Abstract
Activation (in the following referred to as firing) of replication origins is a continuous and irreversible process regulated by availability of DNA replication molecules and cyclin-dependent kinase activities, which are often altered in human cancers. The temporal, progressive origin firing throughout S phase appears as a characteristic replication profile, and computational models have been developed to describe this process. Although evidence from yeast to human indicates that a range of replication fork rates is observed experimentally in order to complete a timely S phase, those models incorporate velocities that are uniform across the genome. Taking advantage of the availability of replication profiles, chromosomal position and replication timing, here we investigated how fork rate may affect origin firing in budding yeast. Our analysis suggested that patterns of origin firing can be observed from a modulation of the fork rate that strongly correlates with origin density. Replication profiles of chromosomes with a low origin density were fitted with a variable fork rate, whereas for the ones with a high origin density a constant fork rate was appropriate. This indeed supports the previously reported correlation between inter-origin distance and fork rate changes. Intriguingly, the calculated correlation between fork rate and timing of origin firing allowed the estimation of firing efficiencies for the replication origins. This approach correctly retrieved origin efficiencies previously determined for chromosome VI and provided testable prediction for other chromosomal origins. Our results gain deeper insights into the temporal coordination of genome duplication, indicating that control of the replication fork rate is required for the timely origin firing during S phase.
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Affiliation(s)
- Adriana Supady
- Institute for Biology, Theoretical Biophysics, Humboldt University Berlin, Invalidenstraβe 42, 10115 Berlin, Germany
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3
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Abstract
The origin recognition complex (ORC) was first discovered in the baker's yeast in 1992. Identification of ORC opened up a path for subsequent molecular level investigations on how eukaryotic cells initiate and control genome duplication each cell cycle. Twenty years after the first biochemical isolation, ORC is now taking on a three-dimensional shape, although a very blurry shape at the moment, thanks to the recent electron microscopy and image reconstruction efforts. In this chapter, we outline the current biochemical knowledge about ORC from several eukaryotic systems, with emphasis on the most recent structural and biochemical studies. Despite many species-specific properties, an emerging consensus is that ORC is an ATP-dependent machine that recruits other key proteins to form pre-replicative complexes (pre-RCs) at many origins of DNA replication, enabling the subsequent initiation of DNA replication in S phase.
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Affiliation(s)
- Huilin Li
- Department of Biochemistry and Cell Biology, Stony Brook University, Stony Brook, NY 11794, USA, And, Biology Department, Brookhaven National Laboratory, Upton, NY 11973, USA, , Tel: 631-344-2931, Fax: 631-344-3407
| | - Bruce Stillman
- Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA, , Tel: 516-367-8383
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4
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Barlow JH, Rothstein R. Rad52 recruitment is DNA replication independent and regulated by Cdc28 and the Mec1 kinase. EMBO J 2009; 28:1121-30. [PMID: 19262568 DOI: 10.1038/emboj.2009.43] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2008] [Accepted: 01/19/2009] [Indexed: 11/09/2022] Open
Abstract
Recruitment of the homologous recombination machinery to sites of double-strand breaks is a cell cycle-regulated event requiring entry into S phase and CDK1 activity. Here, we demonstrate that the central recombination protein, Rad52, forms foci independent of DNA replication, and its recruitment requires B-type cyclin/CDK1 activity. Induction of the intra-S-phase checkpoint by hydroxyurea (HU) inhibits Rad52 focus formation in response to ionizing radiation. This inhibition is dependent upon Mec1/Tel1 kinase activity, as HU-treated cells form Rad52 foci in the presence of the PI3 kinase inhibitor caffeine. These Rad52 foci colocalize with foci formed by the replication clamp PCNA. These results indicate that Mec1 activity inhibits the recruitment of Rad52 to both sites of DNA damage and stalled replication forks during the intra-S-phase checkpoint. We propose that B-type cyclins promote the recruitment of Rad52 to sites of DNA damage, whereas Mec1 inhibits spurious recombination at stalled replication forks.
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Affiliation(s)
- Jacqueline H Barlow
- Department of Genetics & Development, Columbia University Medical Center, New York, NY 10032-2704, USA
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5
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Abstract
High-fidelity chromosome segregation requires that the sister chromatids produced during S phase also become paired during S phase. Ctf7p (Eco1p) is required to establish sister chromatid pairing specifically during DNA replication. However, Ctf7p also becomes active during G2/M in response to DNA damage. Ctf7p is a phosphoprotein and an in vitro target of Cdc28p cyclin-dependent kinase (CDK), suggesting one possible mechanism for regulating the essential function of Ctf7p. Here, we report a novel synthetic lethal interaction between ctf7 and cdc28. However, neither elevated CDC28 levels nor CDC28 Cak1p-bypass alleles rescue ctf7 cell phenotypes. Moreover, cells expressing Ctf7p mutated at all full- and partial-consensus CDK-phosphorylation sites exhibit robust cell growth. These and other results reveal that Ctf7p regulation is more complicated than previously envisioned and suggest that CDK acts in sister chromatid cohesion parallel to Ctf7p reactions.
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6
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Skibbens RV. Mechanisms of sister chromatid pairing. INTERNATIONAL REVIEW OF CELL AND MOLECULAR BIOLOGY 2008; 269:283-339. [PMID: 18779060 DOI: 10.1016/s1937-6448(08)01005-8] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 03/04/2023]
Abstract
The continuance of life through cell division requires high fidelity DNA replication and chromosome segregation. During DNA replication, each parental chromosome is duplicated exactly and one time only. At the same time, the resulting chromosomes (called sister chromatids) become tightly paired along their length. This S-phase pairing, or cohesion, identifies chromatids as sisters over time. During mitosis in most eukaryotes, sister chromatids bi-orient to the mitotic spindle. After each chromosome pair is properly oriented, the cohesion established during S phase is inactivated in a tightly regulated fashion, allowing sister chromatids to segregate away from each other. Recent findings of cohesin structure and enzymology provide new insights into cohesion, while many critical facets of cohesion (how cohesins tether together sister chromatids and how those tethers are established) remain actively debated.
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Affiliation(s)
- Robert V Skibbens
- Department of Biological Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA
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7
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Fierro-Fernández M, Hernández P, Krimer DB, Schvartzman JB. Replication fork reversal occurs spontaneously after digestion but is constrained in supercoiled domains. J Biol Chem 2007; 282:18190-18196. [PMID: 17456472 DOI: 10.1074/jbc.m701559200] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Replication fork reversal was investigated in undigested and linearized replication intermediates of bacterial DNA plasmids containing a stalled fork. Two-dimensional agarose gel electrophoresis, a branch migration and extrusion assay, electron microscopy, and DNA-psoralen cross-linking were used to show that extensive replication fork reversal and extrusion of the nascent-nascent duplex occurs spontaneously after DNA nicking and restriction enzyme digestion but that fork retreat is severely limited in covalently closed supercoiled domains.
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Affiliation(s)
- Marta Fierro-Fernández
- Departamento de Biología Celular y del Desarrollo, Centro de Investigaciones Biológicas (CSIC), Ramiro de Maeztu 9, 28040 Madrid, Spain
| | - Pablo Hernández
- Departamento de Biología Celular y del Desarrollo, Centro de Investigaciones Biológicas (CSIC), Ramiro de Maeztu 9, 28040 Madrid, Spain
| | - Dora B Krimer
- Departamento de Biología Celular y del Desarrollo, Centro de Investigaciones Biológicas (CSIC), Ramiro de Maeztu 9, 28040 Madrid, Spain
| | - Jorge B Schvartzman
- Departamento de Biología Celular y del Desarrollo, Centro de Investigaciones Biológicas (CSIC), Ramiro de Maeztu 9, 28040 Madrid, Spain.
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8
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Arias EE, Walter JC. Strength in numbers: preventing rereplication via multiple mechanisms in eukaryotic cells. Genes Dev 2007; 21:497-518. [PMID: 17344412 DOI: 10.1101/gad.1508907] [Citation(s) in RCA: 319] [Impact Index Per Article: 17.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
In eukaryotic cells, prereplication complexes (pre-RCs) are assembled on chromatin in the G1 phase, rendering origins of DNA replication competent to initiate DNA synthesis. When DNA replication commences in S phase, pre-RCs are disassembled, and multiple initiations from the same origin do not occur because new rounds of pre-RC assembly are inhibited. In most experimental organisms, multiple mechanisms that prevent pre-RC assembly have now been identified, and rereplication within the same cell cycle can be induced through defined perturbations of these mechanisms. This review summarizes the diverse array of inhibitory pathways used by different organisms to prevent pre-RC assembly, and focuses on the challenge of understanding how in any one cell type, various mechanisms cooperate to strictly enforce once per cell cycle regulation of DNA replication.
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Affiliation(s)
- Emily E Arias
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, 240 Longwood Avenue, Boston, Massachusetts 02115, USA
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9
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Abstract
There has been remarkable progress in the last 20 years in defining the molecular mechanisms that regulate initiation of DNA synthesis in eukaryotic cells. Replication origins in the DNA nucleate the ordered assembly of protein factors to form a prereplication complex (preRC) that is poised for DNA synthesis. Transition of the preRC to an active initiation complex is regulated by cyclin-dependent kinases and other signaling molecules, which promote further protein assembly and activate the mini chromosome maintenance helicase. We will review these mechanisms and describe the state of knowledge about the proteins involved. However, we will also consider an additional layer of complexity. The DNA in the cell is packaged with histone proteins into chromatin. Chromatin structure provides an additional layer of heritable information with associated epigenetic modifications. Thus, we will begin by describing chromatin structure, and how the cell generally controls access to the DNA. Access to the DNA requires active chromatin remodeling, specific histone modifications, and regulated histone deposition. Studies in transcription have revealed a variety of mechanisms that regulate DNA access, and some of these are likely to be shared with DNA replication. We will briefly describe heterochromatin as a model for an epigenetically inherited chromatin state. Next, we will describe the mechanisms of replication initiation and how these are affected by constraints of chromatin. Finally, chromatin must be reassembled with appropriate modifications following passage of the replication fork, and our third major topic will be the reassembly of chromatin and its associated epigenetic marks. Thus, in this chapter, we seek to bring together the studies of replication initiation and the studies of chromatin into a single holistic narrative.
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Affiliation(s)
- Angel P Tabancay
- Molecular and Computational Biology Section University of Southern California Los Angeles, California 90089, USA
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10
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Rice LM, Plakas C, Nickels JT. Loss of meiotic rereplication block in Saccharomyces cerevisiae cells defective in Cdc28p regulation. EUKARYOTIC CELL 2005; 4:55-62. [PMID: 15643060 PMCID: PMC544154 DOI: 10.1128/ec.4.1.55-62.2005] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/06/2004] [Accepted: 11/04/2004] [Indexed: 01/01/2023]
Abstract
Cdc28p is the major cyclin-dependent kinase in Saccharomyces cerevisiae. Its activity is required for blocking the reinitiation of DNA replication during mitosis. Here, we show that under conditions where Cdc28p activity is improperly regulated--either through the loss of function of the Schizosaccharomyces pombe wee1 ortholog Swe1p or through the expression of a dominant CDC28 allele, CDC28AF--diploid yeast cells are able to complete several rounds of premeiotic DNA replication within a single meiotic cell cycle. Moreover, a percentage of mutant cells exhibit a "multispore" phenotype, possessing the ability to package more than four spores within a single ascus. These multispored asci contain both even and odd numbers of viable spores. In order for meiotic rereplication and multispore formation to occur, cells must initiate homologous recombination and maintain proper chromosome cohesion during meiosis I. Rad9p- or Rad17p-dependent checkpoint mechanisms are not required for multispore formation and neither are the B-type cyclin Clb6p and the cyclin-dependent kinase inhibitor Sic1p. Finally, we present evidence of a possible role for a Cdc55p-dependent protein phosphatase 2A in initiating meiotic replication.
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Affiliation(s)
- Lyndi M Rice
- Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, 245 North 15th St., NCB#11115, MS#497, Philadelphia, PA 19102, USA
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11
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Abstract
The minichromosome maintenance (or MCM) protein family is composed of six related proteins that are conserved in all eukaryotes. They were first identified by genetic screens in yeast and subsequently analyzed in other experimental systems using molecular and biochemical methods. Early data led to the identification of MCMs as central players in the initiation of DNA replication. More recent studies have shown that MCM proteins also function in replication elongation, probably as a DNA helicase. This is consistent with structural analysis showing that the proteins interact together in a heterohexameric ring. However, MCMs are strikingly abundant and far exceed the stoichiometry of replication origins; they are widely distributed on unreplicated chromatin. Analysis of mcm mutant phenotypes and interactions with other factors have now implicated the MCM proteins in other chromosome transactions including damage response, transcription, and chromatin structure. These experiments indicate that the MCMs are central players in many aspects of genome stability.
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Affiliation(s)
- Susan L Forsburg
- Molecular & Cell Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, California 92037, USA.
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12
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Affiliation(s)
- Philippe Pasero
- Institute of Molecular Genetics, National Center for Scientific Research, Mixed Research Unit 5535, F-34293 Montpellier, France
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13
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Weinreich M, Liang C, Chen HH, Stillman B. Binding of cyclin-dependent kinases to ORC and Cdc6p regulates the chromosome replication cycle. Proc Natl Acad Sci U S A 2001; 98:11211-7. [PMID: 11572976 PMCID: PMC58709 DOI: 10.1073/pnas.201387198] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Cdc6p and the origin recognition complex (ORC) are essential for assembly of a pre-replicative complex (preRC) at origins of replication, before the initiation of DNA synthesis. In the absence of Cdc6p, cells fail to initiate DNA replication and undergo a "reductional" mitosis, in which the unreplicated chromosomes are randomly segregated to the spindle poles. We show here that the cells harboring a mutation in the essential Cdc6p Walker A-box arrest in late mitosis, probably at anaphase. This cell cycle block requires either the three Cdc28p phosphorylation sites within the N terminus of Cdc6p or a short region (aa 8-17) that contains a Cy (Cyclin) interaction sequence. These same two Cdc6p mutants that allow a reductional mitosis are defective in binding Cdc28p kinase. In addition to Cdc6p, ORC also binds to cyclin-dependent kinases (CDKs). Interestingly, Sic1p, a CDK inhibitor protein, blocked the S phase-specific Cdc28p-Clb5p kinase from interacting with ORC, but did not prevent the G(1)-specific Cdc28p-Cln2p kinase-ORC interaction. We suggest that ORC, Cdc6p, and Sic1p bind to different CDKs in a cell cycle-dependent manner to temporally regulate events that (i) allow preRC formation after mitosis, (ii) prevent mitosis before DNA replication can occur, and (iii) promote initiation of DNA replication.
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Affiliation(s)
- M Weinreich
- Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA
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14
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Jacobson MD, Muñoz CX, Knox KS, Williams BE, Lu LL, Cross FR, Vallen EA. Mutations in SID2, a novel gene in Saccharomyces cerevisiae, cause synthetic lethality with sic1 deletion and may cause a defect during S phase. Genetics 2001; 159:17-33. [PMID: 11560884 PMCID: PMC1461789 DOI: 10.1093/genetics/159.1.17] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
SIC1 encodes a nonessential B-type cyclin/CDK inhibitor that functions at the G1/S transition and the exit from mitosis. To understand more completely the regulation of these transitions, mutations causing synthetic lethality with sic1 Delta were isolated. In this screen, we identified a novel gene, SID2, which encodes an essential protein that appears to be required for DNA replication or repair. sid2-1 sic1 Delta strains and sid2-21 temperature-sensitive strains arrest preanaphase as large-budded cells with a single nucleus, a short spindle, and an approximately 2C DNA content. RAD9, which is necessary for the DNA damage checkpoint, is required for the preanaphase arrest of sid2-1 sic1 Delta cells. Analysis of chromosomes in mutant sid2-21 cells by field inversion gel electrophoresis suggests the presence of replication forks and bubbles at the arrest. Deleting the two S phase cyclins, CLB5 and CLB6, substantially suppresses the sid2-1 sic1 Delta inviability, while stabilizing Clb5 protein exacerbates the defects of sid2-1 sic1 Delta cells. In synchronized sid2-1 mutant strains, the onset of replication appears normal, but completion of DNA synthesis is delayed. sid2-1 mutants are sensitive to hydroxyurea indicating that sid2-1 cells may suffer DNA damage that, when combined with additional insult, leads to a decrease in viability. Consistent with this hypothesis, sid2-1 rad9 cells are dead or very slow growing even when SIC1 is expressed.
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Affiliation(s)
- M D Jacobson
- The Rockefeller University, New York, New York 10021, USA
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15
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S and G2 phase roles for Cdk2 revealed by inducible expression of a dominant-negative mutant in human cells. Mol Cell Biol 2001. [PMID: 11283255 DOI: 10.1128/mcb.21.8.27-55-2766.2001] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Cyclin-dependent kinase 2 (Cdk2) is essential for initiation of DNA synthesis in higher eukaryotes. Biochemical studies in Xenopus egg extracts and microinjection studies in human cells have suggested an additional function for Cdk2 in activation of Cdk1 and entry into mitosis. To further examine the role of Cdk2 in human cells, we generated stable clones with inducible expression of wild-type and dominant-negative forms of the enzyme (Cdk2-wt and Cdk2-dn, respectively). Both exogenous proteins associated efficiently with endogenous cyclins. Cdk2-wt had no apparent effect on the cell division cycle, whereas Cdk2-dn inhibited progression through several distinct stages. Cdk2-dn induction could arrest cells at the G1/S transition, as previously observed in transient expression studies. However, under normal culture conditions, Cdk2-dn induction primarily arrested cells with S and G2/M DNA contents. Several observations suggested that the latter cells were in G2 phase, prior to the onset of mitosis: these cells contained uncondensed chromosomes, low levels of cyclin B-associated kinase activity, and high levels of tyrosine-phosphorylated Cdk1. Furthermore, Cdk2-dn did not delay progression through mitosis upon release of cells from a nocodazole block. Although the G2 arrest imposed by Cdk2-dn was similar to that imposed by the DNA damage checkpoint, the former was distinguished by its resistance to caffeine. These findings provide evidence for essential functions of Cdk2 during S and G2 phases of the mammalian cell cycle.
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16
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Hu B, Mitra J, van den Heuvel S, Enders GH. S and G2 phase roles for Cdk2 revealed by inducible expression of a dominant-negative mutant in human cells. Mol Cell Biol 2001; 21:2755-66. [PMID: 11283255 PMCID: PMC86906 DOI: 10.1128/mcb.21.8.2755-2766.2001] [Citation(s) in RCA: 111] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Cyclin-dependent kinase 2 (Cdk2) is essential for initiation of DNA synthesis in higher eukaryotes. Biochemical studies in Xenopus egg extracts and microinjection studies in human cells have suggested an additional function for Cdk2 in activation of Cdk1 and entry into mitosis. To further examine the role of Cdk2 in human cells, we generated stable clones with inducible expression of wild-type and dominant-negative forms of the enzyme (Cdk2-wt and Cdk2-dn, respectively). Both exogenous proteins associated efficiently with endogenous cyclins. Cdk2-wt had no apparent effect on the cell division cycle, whereas Cdk2-dn inhibited progression through several distinct stages. Cdk2-dn induction could arrest cells at the G1/S transition, as previously observed in transient expression studies. However, under normal culture conditions, Cdk2-dn induction primarily arrested cells with S and G2/M DNA contents. Several observations suggested that the latter cells were in G2 phase, prior to the onset of mitosis: these cells contained uncondensed chromosomes, low levels of cyclin B-associated kinase activity, and high levels of tyrosine-phosphorylated Cdk1. Furthermore, Cdk2-dn did not delay progression through mitosis upon release of cells from a nocodazole block. Although the G2 arrest imposed by Cdk2-dn was similar to that imposed by the DNA damage checkpoint, the former was distinguished by its resistance to caffeine. These findings provide evidence for essential functions of Cdk2 during S and G2 phases of the mammalian cell cycle.
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Affiliation(s)
- B Hu
- Department of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA
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17
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Santamaría D, Viguera E, Martínez-Robles ML, Hyrien O, Hernández P, Krimer DB, Schvartzman JB. Bi-directional replication and random termination. Nucleic Acids Res 2000; 28:2099-107. [PMID: 10773078 PMCID: PMC105368 DOI: 10.1093/nar/28.10.2099] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Two-dimensional (2D) agarose gel electrophoresis was used to study termination of DNA replication in a shuttle vector, YRp7', when it replicated in Escherichia coli, Saccharomyces cerevisiae and Xenopus egg extracts. In E. coli, the 2D gel patterns obtained were consistent with uni-directional replication initiated at a specific site, the ColE1 origin. In consequence, termination also occurred precisely at the ColE1 origin. In Xenopus egg extracts, the particular shape of the bubble arc as well as the triangular smear detected to the left of the simple-Y pattern indicated random initiation and termination. In S.cerevisiae, initiation occurred at the ARS1 origin and replication proceeded in a bi-directional manner. However, termination did not always occur at a specific site 180 degrees across from the origin, but almost all along the south hemisphere of the plasmid. Inversion, deletion or replacement of DNA sequences located throughout this hemisphere did not eliminate random termination. Analysis of the replication intermediates of another yeast plasmid bearing a different origin, ARS305, also exhibited random termination. We propose that the random termination events observed in S.cerevisiae could be due to an asynchronous departure of both forks from the bi-directional origin in addition to differences in the rate of fork progression. These observations could be extended to all bi-directional origins.
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Affiliation(s)
- D Santamaría
- Departamento de Biología Celular y del Desarrollo, CIB (CSIC), Velázquez 144, 28006 Madrid, Spain
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18
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Pasero P, Schwob E. Think global, act local--how to regulate S phase from individual replication origins. Curr Opin Genet Dev 2000; 10:178-86. [PMID: 10753785 DOI: 10.1016/s0959-437x(00)00067-8] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
All eukaryotes use similar proteins to licence replication origins but, paradoxically, origin DNA is much less conserved. Specific binding sites for these proteins have now been identified on fission yeast and Drosophila chromosomes, suggesting that the DNA-binding activity of the origin recognition complex has diverged to recruit conserved initiation factors on polymorphic replication origins. Once formed, competent origins are activated by cyclin- and Dbf4-dependent kinases. The latter have been shown to control S phase in several organisms but, in contrast to cyclin-dependent kinases, seem regulated at the level of individual origins. Global and local regulations generate specific patterns of DNA replication that help establish epigenetic chromosome states.
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Affiliation(s)
- P Pasero
- Institut de Génétique Moléculaire, Centre National de la Recherche Scientifique (UMR 5535) & Université Montpellier II, Montpellier, F-34293, France.
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19
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Ritzi M, Knippers R. Initiation of genome replication: assembly and disassembly of replication-competent chromatin. Gene 2000; 245:13-20. [PMID: 10713440 DOI: 10.1016/s0378-1119(00)00020-2] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Considerable progress has been made in research on the initiation of eukaryotic genome replication. This has generated a number of recent review articles. Here, we briefly summarize the major conclusions described in these articles and also include the results of more recent primary articles. The consensus view that has emerged is that a pre-replication complex assembles during the G1 phase of the cell cycle, making chromatin competent for replication. The complex consists of Orc proteins, Cdc6p, and the family of Mcm proteins. Chromatin, thus 'licenced' for replication, is guided into the S phase by the activation of cell-cycle-regulated protein kinases. Upon entry into S phase, the pre-replication complex is partially dissolved, first by the dissociation of Cdc6p and then by the gradual release of Mcm proteins. This appears to be accompanied by a recruitment of chain elongation factors and the establishment of replication forks.
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Affiliation(s)
- M Ritzi
- Department of Biology Universität Konstanz D - 78457, Konstanz, Germany
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Pasero P, Duncker BP, Schwob E, Gasser SM. A role for the Cdc7 kinase regulatory subunit Dbf4p in the formation of initiation-competent origins of replication. Genes Dev 1999; 13:2159-76. [PMID: 10465792 PMCID: PMC316966 DOI: 10.1101/gad.13.16.2159] [Citation(s) in RCA: 92] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Using a reconstituted DNA replication assay from yeast, we demonstrate that two kinase complexes are essential for the promotion of replication in vitro. An active Clb/Cdc28 kinase complex, or its vertebrate equivalent, is required in trans to stimulate initiation in G(1)-phase nuclei, whereas the Dbf4/Cdc7 kinase complex must be provided by the template nuclei themselves. The regulatory subunit of Cdc7p, Dbf4p, accumulates during late G(1) phase, becomes chromatin associated prior to Clb/Cdc28 activation, and assumes a punctate pattern of localization that is similar to, and dependent on, the origin recognition complex (ORC). The association of Dbf4p with a detergent-insoluble chromatin fraction in G(1)-phase nuclei requires ORC but not Cdc6p or Clb/Cdc28 kinase activity, and correlates with competence for initiation. We propose a model in which Dbf4p targets Cdc7p to the prereplication complex prior to the G(1)/S transition, by a pathway parallel to, but independent of, the Cdc6p-dependent recruitment of MCMs.
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Affiliation(s)
- P Pasero
- Swiss Institute for Experimental Cancer Research (ISREC), CH-1066 Epalinges/Lausanne, Switzerland
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Abstract
Two assays have been developed for studying DNA replication in vitro based on nuclear extracts isolated from budding yeast cells synchronized in S phase. In the first, the template DNA for replication is provided in the form of intact yeast nuclei, usually from cells arrested in G(1). In the second assay, bacterially produced supercoiled plasmid is replicated in an S-phase nuclear extract supplemented with nucleotides and an energy-regenerating system. Semiconservative DNA replication is monitored by substitution of newly synthesized DNA with bromodeoxyuridine 5'-triphosphate (BrdUTP) and density gradient analysis. In addition, neutral-neutral two-dimensional gel analyses and, in the case of nuclei, detection of newly synthesized DNA in replication foci by DIG-dUTP incorporation can be used to monitor replication.
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Affiliation(s)
- P Pasero
- Swiss Institute for Experimental Cancer Research (ISREC), Ch. des Boveresses 155, Epalinges/Lausanne, CH-1066, Switzerland
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