1
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Cluett TJ, Akman G, Reyes A, Kazak L, Mitchell A, Wood SR, Spinazzola A, Spelbrink JN, Holt IJ. Transcript availability dictates the balance between strand-asynchronous and strand-coupled mitochondrial DNA replication. Nucleic Acids Res 2019; 46:10771-10781. [PMID: 30239839 PMCID: PMC6237803 DOI: 10.1093/nar/gky852] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2018] [Accepted: 09/12/2018] [Indexed: 11/15/2022] Open
Abstract
Mammalian mitochondria operate multiple mechanisms of DNA replication. In many cells and tissues a strand-asynchronous mechanism predominates over coupled leading and lagging-strand DNA synthesis. However, little is known of the factors that control or influence the different mechanisms of replication, and the idea that strand-asynchronous replication entails transient incorporation of transcripts (aka bootlaces) is controversial. A firm prediction of the bootlace model is that it depends on mitochondrial transcripts. Here, we show that elevated expression of Twinkle DNA helicase in human mitochondria induces bidirectional, coupled leading and lagging-strand DNA synthesis, at the expense of strand-asynchronous replication; and this switch is accompanied by decreases in the steady-state level of some mitochondrial transcripts. However, in the so-called minor arc of mitochondrial DNA where transcript levels remain high, the strand-asynchronous replication mechanism is instated. Hence, replication switches to a strand-coupled mechanism only where transcripts are scarce, thereby establishing a direct correlation between transcript availability and the mechanism of replication. Thus, these findings support a critical role of mitochondrial transcripts in the strand-asynchronous mechanism of mitochondrial DNA replication; and, as a corollary, mitochondrial RNA availability and RNA/DNA hybrid formation offer means of regulating the mechanisms of DNA replication in the organelle.
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Affiliation(s)
- Tricia J Cluett
- MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge CB1 9SY, UK
| | | | - Aurelio Reyes
- MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge CB1 9SY, UK
| | - Lawrence Kazak
- MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge CB1 9SY, UK
| | - Alice Mitchell
- Department of Clinical Movement Neurosciences, Institute of Neurology, Royal Free Campus, University College London, London NW3 2PF, UK
| | - Stuart R Wood
- MRC Mitochondrial Biology Unit, University of Cambridge, Cambridge CB1 9SY, UK
| | - Antonella Spinazzola
- Department of Clinical Movement Neurosciences, Institute of Neurology, Royal Free Campus, University College London, London NW3 2PF, UK.,MRC Centre for Neuromuscular Diseases, UCL Institute of Neurology and National Hospital for Neurology and Neurosurgery, London, UK
| | - Johannes N Spelbrink
- Department of Pediatrics, Radboud Centre for Mitochondrial Medicine, Radboud University Medical Centre, Geert Grooteplein 10, 6500 HB, Nijmegen, The Netherlands
| | - Ian J Holt
- Department of Clinical Movement Neurosciences, Institute of Neurology, Royal Free Campus, University College London, London NW3 2PF, UK.,Biodonostia Health Research Institute, 20014 San Sebastián, Spain and IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain
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2
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Petryk N, Kahli M, d'Aubenton-Carafa Y, Jaszczyszyn Y, Shen Y, Silvain M, Thermes C, Chen CL, Hyrien O. Replication landscape of the human genome. Nat Commun 2016; 7:10208. [PMID: 26751768 PMCID: PMC4729899 DOI: 10.1038/ncomms10208] [Citation(s) in RCA: 191] [Impact Index Per Article: 23.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2015] [Accepted: 11/13/2015] [Indexed: 12/21/2022] Open
Abstract
Despite intense investigation, human replication origins and termini remain elusive. Existing data have shown strong discrepancies. Here we sequenced highly purified Okazaki fragments from two cell types and, for the first time, quantitated replication fork directionality and delineated initiation and termination zones genome-wide. Replication initiates stochastically, primarily within non-transcribed, broad (up to 150 kb) zones that often abut transcribed genes, and terminates dispersively between them. Replication fork progression is significantly co-oriented with the transcription. Initiation and termination zones are frequently contiguous, sometimes separated by regions of unidirectional replication. Initiation zones are enriched in open chromatin and enhancer marks, even when not flanked by genes, and often border ‘topologically associating domains' (TADs). Initiation zones are enriched in origin recognition complex (ORC)-binding sites and better align to origins previously mapped using bubble-trap than λ-exonuclease. This novel panorama of replication reveals how chromatin and transcription modulate the initiation process to create cell-type-specific replication programs. The physical origin and termination sites of DNA replication in human cells have remained elusive. Here the authors use Okazaki fragment sequencing to reveal global replication patterns and show how chromatin and transcription modulate the process.
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Affiliation(s)
- Nataliya Petryk
- Ecole Normale Supérieure, Institut de Biologie de l'ENS (IBENS), and Inserm U1024, and CNRS UMR 8197, 46 rue d'Ulm, Paris F-75005, France.,Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, UMR 9198, FRC 3115, Avenue de la Terrasse, Bâtiment 24, Gif-sur-Yvette, Paris F-91198, France
| | - Malik Kahli
- Ecole Normale Supérieure, Institut de Biologie de l'ENS (IBENS), and Inserm U1024, and CNRS UMR 8197, 46 rue d'Ulm, Paris F-75005, France
| | - Yves d'Aubenton-Carafa
- Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, UMR 9198, FRC 3115, Avenue de la Terrasse, Bâtiment 24, Gif-sur-Yvette, Paris F-91198, France
| | - Yan Jaszczyszyn
- Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, UMR 9198, FRC 3115, Avenue de la Terrasse, Bâtiment 24, Gif-sur-Yvette, Paris F-91198, France
| | - Yimin Shen
- Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, UMR 9198, FRC 3115, Avenue de la Terrasse, Bâtiment 24, Gif-sur-Yvette, Paris F-91198, France
| | - Maud Silvain
- Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, UMR 9198, FRC 3115, Avenue de la Terrasse, Bâtiment 24, Gif-sur-Yvette, Paris F-91198, France
| | - Claude Thermes
- Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, UMR 9198, FRC 3115, Avenue de la Terrasse, Bâtiment 24, Gif-sur-Yvette, Paris F-91198, France
| | - Chun-Long Chen
- Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS, Université Paris-Sud, UMR 9198, FRC 3115, Avenue de la Terrasse, Bâtiment 24, Gif-sur-Yvette, Paris F-91198, France
| | - Olivier Hyrien
- Ecole Normale Supérieure, Institut de Biologie de l'ENS (IBENS), and Inserm U1024, and CNRS UMR 8197, 46 rue d'Ulm, Paris F-75005, France
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3
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Mesner LD, Valsakumar V, Cieślik M, Pickin R, Hamlin JL, Bekiranov S. Bubble-seq analysis of the human genome reveals distinct chromatin-mediated mechanisms for regulating early- and late-firing origins. Genome Res 2013; 23:1774-88. [PMID: 23861383 PMCID: PMC3814878 DOI: 10.1101/gr.155218.113] [Citation(s) in RCA: 64] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2013] [Accepted: 07/08/2013] [Indexed: 11/24/2022]
Abstract
We have devised a method for isolating virtually pure and comprehensive libraries of restriction fragments that contained replication initiation sites (bubbles) in vivo. We have now sequenced and mapped the bubble-containing fragments from GM06990, a near-normal EBV-transformed lymphoblastoid cell line, and have compared origin distributions with a comprehensive replication timing study recently published for this cell line. We find that early-firing origins, which represent ∼32% of all origins, overwhelmingly represent zones, associate only marginally with active transcription units, are localized within large domains of open chromatin, and are significantly associated with DNase I hypersensitivity. Origin "density" falls from early- to mid-S-phase, but rises again in late S-phase to levels only 17% lower than in early S-phase. Unexpectedly, late origin density calculated on the 1-Mb scale increases as a function of increasing chromatin compaction. Furthermore, the median efficiency of origins in late-replicating, heterochromatic domains is only 25% lower than in early-replicating euchromatic loci. Thus, the activation of early- and late-firing origins must be regulated by quintessentially different mechanisms. The aggregate data can be unified into a model in which initiation site selection is driven almost entirely by epigenetic factors that fashion both the long-range and local chromatin environments, with underlying DNA sequence and local transcriptional activity playing only minor roles. Importantly, the comprehensive origin map we have prepared for GM06990 overlaps moderately well with origin maps recently reported for the genomes of four different human cell lines based on the distributions of small nascent strands.
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Affiliation(s)
| | | | - Marcin Cieślik
- Department of Biochemistry & Molecular Genetics, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA
| | - Rebecca Pickin
- Department of Biochemistry & Molecular Genetics, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA
| | - Joyce L. Hamlin
- Department of Biochemistry & Molecular Genetics, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA
| | - Stefan Bekiranov
- Department of Biochemistry & Molecular Genetics, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA
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4
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Reyes A, Kazak L, Wood SR, Yasukawa T, Jacobs HT, Holt IJ. Mitochondrial DNA replication proceeds via a 'bootlace' mechanism involving the incorporation of processed transcripts. Nucleic Acids Res 2013; 41:5837-50. [PMID: 23595151 PMCID: PMC3675460 DOI: 10.1093/nar/gkt196] [Citation(s) in RCA: 82] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The observation that long tracts of RNA are associated with replicating molecules of mitochondrial DNA (mtDNA) suggests that the mitochondrial genome of mammals is copied by an unorthodox mechanism. Here we show that these RNA-containing species are present in living cells and tissue, based on interstrand cross-linking. Using DNA synthesis in organello, we demonstrate that isolated mitochondria incorporate radiolabeled RNA precursors, as well as DNA precursors, into replicating DNA molecules. RNA-containing replication intermediates are chased into mature mtDNA, to which they are thus in precursor-product relationship. While a DNA chain terminator rapidly blocks the labeling of mitochondrial replication intermediates, an RNA chain terminator does not. Furthermore, processed L-strand transcripts can be recovered from gel-extracted mtDNA replication intermediates. Therefore, instead of concurrent DNA and RNA synthesis, respectively, on the leading and lagging strands, preformed processed RNA is incorporated as a provisional lagging strand during mtDNA replication. These findings indicate that RITOLS is a physiological mechanism of mtDNA replication, and that it involves a 'bootlace' mechanism, in which processed transcripts are successively hybridized to the lagging-strand template, as the replication fork advances.
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Affiliation(s)
- Aurelio Reyes
- MRC-Mitochondrial Biology Unit, Wellcome Trust/MRC Building, Hills Road, Cambridge CB2 0XY, UK
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5
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Mesner LD, Valsakumar V, Karnani N, Dutta A, Hamlin JL, Bekiranov S. Bubble-chip analysis of human origin distributions demonstrates on a genomic scale significant clustering into zones and significant association with transcription. Genome Res 2010; 21:377-89. [PMID: 21173031 DOI: 10.1101/gr.111328.110] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Abstract
We have used a novel bubble-trapping procedure to construct nearly pure and comprehensive human origin libraries from early S- and log-phase HeLa cells, and from log-phase GM06990, a karyotypically normal lymphoblastoid cell line. When hybridized to ENCODE tiling arrays, these libraries illuminated 15.3%, 16.4%, and 21.8% of the genome in the ENCODE regions, respectively. Approximately half of the origin fragments cluster into zones, and their signals are generally higher than those of isolated fragments. Interestingly, initiation events are distributed about equally between genic and intergenic template sequences. While only 13.2% and 14.0% of genes within the ENCODE regions are actually transcribed in HeLa and GM06990 cells, 54.5% and 25.6% of zonal origin fragments overlap transcribed genes, most with activating chromatin marks in their promoters. Our data suggest that cell synchronization activates a significant number of inchoate origins. In addition, HeLa and GM06990 cells activate remarkably different origin populations. Finally, there is only moderate concordance between the log-phase HeLa bubble map and published maps of small nascent strands for this cell line.
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Affiliation(s)
- Larry D Mesner
- Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA
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6
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Abstract
Replication forks can be stalled by tightly bound proteins, DNA damage, nucleotide deprivation, or defects in the replication machinery. It is now appreciated that processing of stalled replication forks is critical for completion of DNA replication and maintenance of genome stability. In this chapter, we detail the use of two-dimensional (2D) agarose gels with Southern hybridization for the detection and analysis of blocked replication forks in vivo. This kind of 2D gel electrophoresis has been used extensively for analysis of replication initiation mechanisms for many years, and more recently has become a valuable tool for analysis of fork stalling. Although the method can provide valuable information when forks are stalled in random locations (e.g., after UV damage or nucleotide deprivation), it is even more informative with site-specific fork blockage, for example, blocks caused by tightly bound replication terminator proteins or by drug-stabilized topoisomerase cleavage complexes.
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7
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Yasukawa T, Yang MY, Jacobs HT, Holt IJ. A bidirectional origin of replication maps to the major noncoding region of human mitochondrial DNA. Mol Cell 2005; 18:651-62. [PMID: 15949440 DOI: 10.1016/j.molcel.2005.05.002] [Citation(s) in RCA: 111] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2004] [Revised: 03/01/2005] [Accepted: 05/04/2005] [Indexed: 10/25/2022]
Abstract
In solid tissues of vertebrates, initiation of mitochondrial DNA replication encompasses a broad zone downstream of the major noncoding region (NCR). In contrast, analysis with two-dimensional agarose gel electrophoresis of mitochondrial DNA replication intermediates in cultured human cells revealed initiation concentrated in the NCR. Mapping of prominent free 5' ends on the heavy strand of mitochondrial DNA identified two clusters of potential start sites. One mapped to the previously assigned origin of strand-asynchronous replication (O(H)); the other lay several hundred nucleotides away from O(H), toward the other end of the NCR. The latter cluster is proposed to be the major site of bidirectional replication initiation on the basis of the following: its prominence is enhanced in cells amplifying mitochondrial DNA after experimentally induced mitochondrial DNA depletion; free 5' ends are found in corresponding positions on the opposite strand; it is transient in nature; and it is associated with bubble arcs.
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Affiliation(s)
- Takehiro Yasukawa
- MRC-Dunn Human Nutrition Unit, Wellcome Trust-MRC Building, Hills Road, Cambridge CB2 2XY, United Kingdom
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8
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Bowmaker M, Yang MY, Yasukawa T, Reyes A, Jacobs HT, Huberman JA, Holt IJ. Mammalian mitochondrial DNA replicates bidirectionally from an initiation zone. J Biol Chem 2003; 278:50961-9. [PMID: 14506235 DOI: 10.1074/jbc.m308028200] [Citation(s) in RCA: 157] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Previous data from our laboratory suggested that replication of mammalian mitochondrial DNA initiates exclusively at or near to the formerly designated origin of heavy strand replication, OH, and proceeds unidirectionally from that locus. New results obtained using two-dimensional agarose gel electrophoresis of replication intermediates demonstrate that replication of mitochondrial DNA initiates from multiple origins across a broad zone. After fork arrest near OH, replication is restricted to one direction only. The initiation zone of bidirectional replication includes the genes for cytochrome b and NADH dehydrogenase subunits 5 and 6.
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Affiliation(s)
- Mark Bowmaker
- Dunn Human Nutrition Unit, Wellcome Trust-Medical Research Council Building, Hills Road, Cambridge CB2 2XY, United Kingdom
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9
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Williamson DH, Preiser PR, Moore PW, McCready S, Strath M, Wilson RJM. The plastid DNA of the malaria parasite Plasmodium falciparum is replicated by two mechanisms. Mol Microbiol 2002; 45:533-42. [PMID: 12123462 DOI: 10.1046/j.1365-2958.2002.03033.x] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In common with other apicomplexan parasites, Plasmodium falciparum, a causative organism of human malaria, harbours a residual plastid derived from an ancient secondary endosymbiotic acquisition of an alga. The function of the 35 kb plastid genome is unknown, but its evolutionary origin and genetic content make it a likely target for chemotherapy. Pulsed field gel electrophoresis and ionizing radiation have shown that essentially all the plastid DNA comprises covalently closed circular monomers, together with a tiny minority of linear 35 kb molecules. Using two-dimensional gels and electron microscopy, two replication mechanisms have been revealed. One, sensitive to the topoisomerase inhibitor ciprofloxacin, appears to initiate at twin D-loops located in a large inverted repeat carrying duplicated rRNA and tRNA genes, whereas the second, less drug sensitive, probably involves rolling circles that initiate outside the inverted repeat.
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MESH Headings
- Animals
- Anti-Infective Agents/pharmacology
- Ciprofloxacin/pharmacology
- DNA Replication/drug effects
- DNA, Circular/biosynthesis
- DNA, Circular/genetics
- DNA, Circular/ultrastructure
- DNA, Protozoan/biosynthesis
- DNA, Protozoan/genetics
- DNA, Protozoan/ultrastructure
- Electrophoresis, Gel, Pulsed-Field
- Electrophoresis, Gel, Two-Dimensional
- Microscopy, Electron
- Models, Genetic
- Plasmodium falciparum/drug effects
- Plasmodium falciparum/genetics
- Plasmodium falciparum/ultrastructure
- Plastids/drug effects
- Plastids/genetics
- Plastids/ultrastructure
- Protozoan Proteins/antagonists & inhibitors
- Topoisomerase II Inhibitors
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Affiliation(s)
- D H Williamson
- National Institute for Medical Research, Mill Hill, London NW7 1AA, UK.
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10
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Abstract
The duplication of the mammalian genome is an organized event, but there is limited information about the precision of the duplication program at specific genetic loci. We developed an approach that allows DNA replication events to be visualized in individual DNA molecules. Studying the latent replication of Epstein-Barr virus episomes, we show that different initiation sites are used to commence DNA replication from a specific portion of the viral genome (zone), whereas termination does not seem to be genomically defined. We conclude that initiation zones and pausing sites are major organizers of the duplication program, but initiation, fork progression, and termination of replication can vary in each molecule.
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MESH Headings
- DNA Replication
- DNA, Viral/biosynthesis
- DNA, Viral/genetics
- Electrophoresis, Gel, Pulsed-Field
- Electrophoresis, Gel, Two-Dimensional
- Fluorescent Antibody Technique
- Genes, Viral
- Genome, Viral
- Herpesvirus 4, Human/genetics
- Herpesvirus 4, Human/physiology
- Humans
- Microscopy, Fluorescence
- Nucleic Acid Hybridization
- Plasmids
- Replication Origin
- Tumor Cells, Cultured
- Virus Replication
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Affiliation(s)
- P Norio
- Department of Cell Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
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11
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Abstract
To meet the demand for the rapid synthesis of chorion (eggshell) proteins, Drosophila ovarian follicle cells amplify the chromosomal loci containing the chorion gene clusters up to 60-fold. Amplification occurs by repeated firing of one or more origins located within each gene cluster. Deletion analyses of transgenic constructs derived from the third chromosome cluster have identified a 320-bp amplification control element (ACE3) required for amplification, as well as several stimulatory amplification enhancing regions (AERs). Two-dimensional (2D) gel analyses have identified multiple DNA replication initiation sites (origins) that partially overlap in location with ACE3 and the AERs. To further study sequence requirements for amplification, a vector was used in which transgenic constructs are protected from chromosomal position effects by flanking insulator elements, the suppressor Hairy-wing protein binding site (SHWBS). Using the buffered vector, the 320-bp ACE3 and an 884-bp element designated ori-beta were found to be necessary and sufficient for amplification. Two-dimensional gels revealed that ori-beta was acting as the origin. In contrast, origin activity could not be detected for ACE3. An insulator placed between ACE3 and ori-beta inhibited amplification, indicating that ACE3 activates ori-beta in cis. The results suggest that ACE3 acts as a replicator and support and extend the replicator model for the organization of metazoan chromosomal replicons.
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Affiliation(s)
- L Lu
- Department of Biological Sciences, University of Southern California, Los Angeles, California 90089-1340, USA
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12
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Abstract
Plasmid DNA incubated in interphase Xenopus egg extracts is normally assembled into chromatin and then into synthetic nuclei which undergo one round of regulated replication. During a study of restriction endonuclease cut plasmid replication intermediates (RIs) by the Brewer-Fangman 2D gel electrophoresis technique, we have observed the formation of a strong spike of X-shaped DNA molecules in extracts that otherwise yield very little or no RIs. Formation of these joint molecules is also efficiently induced in replication-competent extracts upon inhibition of replication fork progression by aphidicolin. Although their electrophoretic properties are quite similar to those of Holliday junctions, 2D gels of doubly cut plasmids show that these junctions can link two plasmid molecules at any pair of DNA sequences, with no regard for sequence homology at the branch points. Neutral-neutral-alkaline 3D gels show that the junctions only contain single strands of parental size and no recombinant strands. A hemicatenane, in which one strand of a duplex is wound around one strand of another duplex, is the most likely structure to account for these observations. The mechanism of formation of these novel joint DNA molecules and their biological implications are discussed.
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Affiliation(s)
- I Lucas
- Laboratoire de Génétique Moléculaire, Ecole Normale Supérieure, 46 rue d'Ulm, 75230 Paris Cedex 05, France
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13
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Li CJ, Bogan JA, Natale DA, DePamphilis ML. Selective activation of pre-replication complexes in vitro at specific sites in mammalian nuclei. J Cell Sci 2000; 113 ( Pt 5):887-98. [PMID: 10671378 DOI: 10.1242/jcs.113.5.887] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
As the first step in determining whether or not pre-replication complexes are assembled at specific sites along mammalian chromosomes, nuclei from G(1)-phase hamster cells were incubated briefly in Xenopus egg extract in order to initiate DNA replication. Most of the nascent DNA consisted of RNA-primed DNA chains 0.5 to 2 kb in length, and its origins in the DHFR gene region were mapped using both the early labeled fragment assay and the nascent strand abundance assay. The results revealed three important features of mammalian replication origins. First, Xenopus egg extract can selectively activate the same origins of bi-directional replication (e.g. ori-beta) and (beta') that are used by hamster cells in vivo. Previous reports of a broad peak of nascent DNA centered at ori-(beta/(beta)' appeared to result from the use of aphidicolin to synchronize nuclei and from prolonged exposure of nuclei to egg extracts. Second, these sites were not present until late G(1)-phase of the cell division cycle, and their appearance did not depend on the presence of Xenopus Orc proteins. Therefore, hamster pre-replication complexes appear to be assembled at specific chromosomal sites during G(1)-phase. Third, selective activation of ori-(beta) in late G(1)-nuclei depended on the ratio of Xenopus egg extract to nuclei, revealing that epigenetic parameters such as the ratio of initiation factors to DNA substrate could determine the number of origins activated.
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Affiliation(s)
- C J Li
- National Institute of Child Health and Human Development, Building 6, Room 416, National Institutes of Health, Bethesda, MD 20892-2753, USA
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14
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Abstract
The neutral/neutral and neutral/alkaline two-dimensional gel electrophoretic techniques are sensitive physical mapping methods that have been used successfully to identify replication initiation sites in genomes of widely varying complexity. We present detailed methodology for the preparation of replication intermediates from mammalian cells and their analysis by both neutral/neutral and neutral/alkaline two-dimensional gel approaches. The methods described allow characterization of the replication pattern of single-copy loci, even in mammalian cells. When applied to metazoans, initiation is found to occur at multiple sites scattered throughout zones that can be as long as 50 kb, with some subregions being preferred. Although these observations do not rule out the possibility of genetically defined replicators, they offer the alternative or additional possibility that chromosomal context may play an important role in defining replication initiation sites in complex genomes. We discuss novel recombination strategies that can be used to test for the presence of sequence elements critical for origin function if the origin lies in the vicinity of a selectable gene. Application of this strategy to the DHFR locus shows that loss of sequences more than 25 kb from the local initiation zone can markedly affect origin activity in the zone.
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Affiliation(s)
- P A Dijkwel
- Department of Biochemistry and Molecular Genetics, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA.
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15
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Kalejta RF, Li X, Mesner LD, Dijkwel PA, Lin HB, Hamlin JL. Distal sequences, but not ori-beta/OBR-1, are essential for initiation of DNA replication in the Chinese hamster DHFR origin. Mol Cell 1998; 2:797-806. [PMID: 9885567 DOI: 10.1016/s1097-2765(00)80294-4] [Citation(s) in RCA: 73] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
In the Chinese hamster dihydrofolate reductase replication initiation zone, the ori-beta locus is preferred over other start sites. To test the hypothesis that ori-beta contains a genetic replicator, we restored a deletion in the 3' end of the DHFR gene with a cosmid that provides the missing sequence and simultaneously knocks out the downstream ori-beta locus. Replication initiates normally in ori-beta knockout cell lines, and the DHFR domain is still synthesized in early S phase. However, initiation is completely suppressed in the starting deletion variant lacking the 3' end of the gene. We conclude that ori-beta does not contain an essential replicator, but that distant sequence elements have profound effects on origin activity in this locus.
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Affiliation(s)
- R F Kalejta
- Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville 22908, USA
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16
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Toledo F, Baron B, Fernandez MA, Lachagès AM, Mayau V, Buttin G, Debatisse M. oriGNAI3: a narrow zone of preferential replication initiation in mammalian cells identified by 2D gel and competitive PCR replicon mapping techniques. Nucleic Acids Res 1998; 26:2313-21. [PMID: 9580680 PMCID: PMC147574 DOI: 10.1093/nar/26.10.2313] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
The nature of mammalian origins of DNA replication remains controversial and this is primarily because two-dimensional gel replicon mapping techniques have identified broad zones of replication initiation whereas several other techniques, such as quantitative PCR, have disclosed more discrete sites of initiation at the same chromosomal loci. In this report we analyze the replication of an amplified genomic region encompassing the 3'-end of the GNAI3 gene, the entire GNAT2 gene and the intergenic region between them in exponentially growing Chinese hamster fibroblasts. These cells express GNAI3 but not GNAT2 . The replication pattern was first analyzed by two-dimensional neutral-alkaline gel electrophoresis. Surprisingly, the results revealed a small preferential zone of replication initiation, of at most 1.7 kb, located in a limited part of the GNAI3 - GNAT2 intergenic region. Mapping of this initiation zone was then confirmed by quantitative PCR. The agreement between the two techniques exploited here strengthens the hypothesis that preferred sites of replication initiation do exist in mammalian genomes.
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Affiliation(s)
- F Toledo
- Unité de Génétique Somatique (URA CNRS 1960), Institut Pasteur, 25 rue du Dr Roux, 75724 Paris Cedex 15, France.
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17
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Abstract
The recent flood of information concerning Saccharomyces cerevisiae replication origins and the proteins that interact with them contrasts alarmingly to the trickle of progress in our understanding of metazoan origins. In mammalian cells, origins are complex and heterogeneous, and appear to be selected by features of nuclear architecture that are re-established after each mitosis. Studies in Xenopus egg extracts have shown that once per cell cycle replication does not require specific origin sequences, despite the identification of functional homologues to yeast origin-binding proteins. These observations suggest that initiation of DNA replication in higher eukaryotes is focused to specific genomic regions by features of chromosome structure.
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Affiliation(s)
- D M Gilbert
- Department of Biochemistry and Molecular Biology, SUNY Health Science Center, Syracuse 13210, USA.
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18
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Wang S, Dijkwel PA, Hamlin JL. Lagging-strand, early-labelling, and two-dimensional gel assays suggest multiple potential initiation sites in the Chinese hamster dihydrofolate reductase origin. Mol Cell Biol 1998; 18:39-50. [PMID: 9418851 PMCID: PMC121447 DOI: 10.1128/mcb.18.1.39] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/1997] [Accepted: 10/02/1997] [Indexed: 02/05/2023] Open
Abstract
There is general agreement that DNA synthesis in the single-copy and amplified dihydrofolate reductase (DHFR) loci of CHO cells initiates somewhere within the 55-kb spacer region between the DHFR and 2BE2121 genes. However, results of lagging-strand, early-labelling fragment hybridization (ELFH), and PCR-based nascent-strand abundance assays have been interpreted to suggest a very narrow zone of initiation centered at a single locus known as ori-beta, while two-dimensional (2-D) gel analyses suggest that initiation can occur at any of a large number of potential sites scattered throughout the intergenic region. The results of a leading-strand assay and two intrinsic labelling techniques are compatible with a broad initiation zone in which ori-beta and a second locus (ori-gamma) are somewhat preferred. To determine how these differing views are shaped by differences in experimental manipulations unrelated to the biology itself, we have applied the lagging-strand, ELFH, neutral-neutral, and/or neutral-alkaline 2-D gel assays to CHOC 400 cell populations synchronized and manipulated in the same way. In our experiments, the lagging-strand assay failed to identify a template strand switch at ori-beta; rather, we observed a gradual, undulating change in hybridization bias throughout the intergenic spacer, with hybridization to the two templates being approximately equal near a centered matrix attachment region. In the ELFH assay, all of the fragments in the 55-kb intergenic region were labelled in the first few minutes of the S phase, with the regions encompassing ori-beta and ori-gamma being somewhat preferred. Under the same conditions, neutral-neutral and neutral-alkaline 2-D gel analyses detected initiation sites at multiple locations in the intergenic spacer. Thus, the results of all existing replicon-mapping methods that have been applied to the amplified DHFR locus in CHOC 400 cells are consistent with a model in which two somewhat preferred subzones reside in a larger zone of multiple potential initiation sites in the intergenic region.
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Affiliation(s)
- S Wang
- Department of Biochemistry, University of Virginia School of Medicine, Charlottesville 22908, USA
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19
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Abstract
Neutral/neutral two-dimensional gel electrophoresis is a sensitive physical mapping technique that has been successfully used to unambiguously identify replication initiation sites in genomes of widely varying complexity in vivo. The technique exploits the fact that restriction fragments containing different classes of replicative intermediates (single forks, initiation bubbles, or termination structures) migrate to different and characteristic positions in agarose gels. The replication pattern of any region of interest can then be determined by sequential hybridization with appropriate radioactive probes from that region.
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Affiliation(s)
- P A Dijkwel
- Department of Biochemistry, University of Virginia School of Medicine, Charlottesville 22908, USA.
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20
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Abstract
Neutral/alkaline two-dimensional gel electrophoresis is a robust, easily interpretable, sensitive technique that has yielded insights about the in vivo replication of many types of DNA, from multicopy yeast plasmids to single-copy chromosome regions in unsynchronized mammalian cells. It can provide information about directions of replication fork movement and locations of replication origins, termini, and pause sites. Especially when combined with its partner technique, neutral/neutral two-dimensional gel electrophoresis, it is a method of choice for investigation of unknown situations.
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Affiliation(s)
- J A Huberman
- Department of Molecular and Cellular Biology, Roswell Park Cancer Institute, Buffalo, New York 14263-0001, USA.
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21
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Abstract
The neutral/neutral two-dimensional (2-D) gel replicon mapping technique has been used to great advantage to localize and characterize origins of replication. Interestingly, many yeast origins display a composite pattern consisting of both a bubble arc and a single-fork arc. Moreover, in every instance in which neutral/neutral 2-D gels have been used to analyze origins in higher eukaryotic cells, two or more adjacent fragments display these composite patterns. We believe that composite patterns signal inefficient origin usage in yeast cells because the replicators in question are not active in every cell cycle and in higher eukaryotic replicons because initiation sites are chosen from among many potential sites lying within a zone. However, others have suggested that the single-fork arcs in these composite gel patterns arise from nicking activity that converts replication bubbles to branched structures that comigrate with bona fide single forks. Here, we have used three different replicon mapping strategies to show that broken simian virus 40 replication bubbles trace unique arcs that are clearly distinguishable from classic, intact single forks. Thus, it is likely that composite 2-D gel patterns represent origins that are inefficiently utilized.
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Affiliation(s)
- R F Kalejta
- Department of Biochemistry, University of Virginia School of Medicine, Charlottesvile 22098, USA
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