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The chromatin backdrop of DNA replication: lessons from genetics and genome-scale analyses. BIOCHIMICA ET BIOPHYSICA ACTA-GENE REGULATORY MECHANISMS 2012; 1819:794-801. [PMID: 22342530 DOI: 10.1016/j.bbagrm.2012.01.017] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/06/2011] [Revised: 01/27/2012] [Accepted: 01/31/2012] [Indexed: 01/04/2023]
Abstract
The entire cellular genome must replicate during each cell cycle, but it is yet unclear how replication proceeds along with chromatin condensation and remodeling while ensuring the fidelity of the replicated genome. Mapping replication initiation sites can provide clues for the coordination of DNA replication and transcription on a whole-genome scale. Here we discuss recent insights obtained from genome-scale analyses of replication initiation sites and transcription in mammalian cells and ask how transcription and chromatin modifications affect the frequency of replication initiation events. We also discuss DNA sequences, such as insulators and replicators, which modulate replication and transcription of target genes, and use genome-wide maps of replication initiation sites to evaluate possible commonalities between replicators and chromatin insulators. This article is part of a Special Issue entitled: Chromatin in time and space.
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Hiratani I, Ryba T, Itoh M, Yokochi T, Schwaiger M, Chang CW, Lyou Y, Townes TM, Schübeler D, Gilbert DM. Global reorganization of replication domains during embryonic stem cell differentiation. PLoS Biol 2008; 6:e245. [PMID: 18842067 PMCID: PMC2561079 DOI: 10.1371/journal.pbio.0060245] [Citation(s) in RCA: 409] [Impact Index Per Article: 25.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2008] [Accepted: 08/27/2008] [Indexed: 01/20/2023] Open
Abstract
DNA replication in mammals is regulated via the coordinate firing of clusters of replicons that duplicate megabase-sized chromosome segments at specific times during S-phase. Cytogenetic studies show that these “replicon clusters” coalesce as subchromosomal units that persist through multiple cell generations, but the molecular boundaries of such units have remained elusive. Moreover, the extent to which changes in replication timing occur during differentiation and their relationship to transcription changes has not been rigorously investigated. We have constructed high-resolution replication-timing profiles in mouse embryonic stem cells (mESCs) before and after differentiation to neural precursor cells. We demonstrate that chromosomes can be segmented into multimegabase domains of coordinate replication, which we call “replication domains,” separated by transition regions whose replication kinetics are consistent with large originless segments. The molecular boundaries of replication domains are remarkably well conserved between distantly related ESC lines and induced pluripotent stem cells. Unexpectedly, ESC differentiation was accompanied by the consolidation of smaller differentially replicating domains into larger coordinately replicated units whose replication time was more aligned to isochore GC content and the density of LINE-1 transposable elements, but not gene density. Replication-timing changes were coordinated with transcription changes for weak promoters more than strong promoters, and were accompanied by rearrangements in subnuclear position. We conclude that replication profiles are cell-type specific, and changes in these profiles reveal chromosome segments that undergo large changes in organization during differentiation. Moreover, smaller replication domains and a higher density of timing transition regions that interrupt isochore replication timing define a novel characteristic of the pluripotent state. Microscopy studies have suggested that chromosomal DNA is composed of multiple, megabase-sized segments, each replicated at different times during S-phase of the cell cycle. However, a molecular definition of these coordinately replicated sequences and the stability of the boundaries between them has not been established. We constructed genome-wide replication-timing maps in mouse embryonic stem cells, identifying multimegabase coordinately replicated chromosome segments—“replication domains”—separated by remarkably distinct temporal boundaries. These domain boundaries were shared between several unrelated embryonic stem cell lines, including somatic cells reprogrammed to pluripotency (so-called induced pluripotent stem cells). However, upon differentiation to neural precursor cells, domains encompassing approximately 20% of the genome changed their replication timing, temporally consolidating into fewer, larger replication domains that were conserved between different neural precursor cell lines. Domains that changed replication timing showed a unique sequence composition, a strongly biased directionality for changes in resident gene expression, and altered radial positioning within the three-dimensional space in the cell nucleus, suggesting that changes in replication timing are related to the reorganization of higher-order chromosome structure and function during differentiation. Moreover, the property of smaller discordantly replicating domains may define a novel characteristic of pluripotency. Analyzing the temporal order of DNA replication across the genome during embryonic stem cell differentiation reveals stable boundaries between coordinately replicated regions that consolidate into fewer, larger domains during differentiation.
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Affiliation(s)
- Ichiro Hiratani
- Department of Biological Science, Florida State University, Tallahassee, Florida, United States of America
| | - Tyrone Ryba
- Department of Biological Science, Florida State University, Tallahassee, Florida, United States of America
| | - Mari Itoh
- Department of Biological Science, Florida State University, Tallahassee, Florida, United States of America
| | - Tomoki Yokochi
- Department of Biological Science, Florida State University, Tallahassee, Florida, United States of America
| | - Michaela Schwaiger
- Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
| | - Chia-Wei Chang
- Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Schools of Medicine and Dentistry, Birmingham, Alabama, United States of America
| | - Yung Lyou
- Department of Biochemistry and Molecular Biology, State University of New York, Upstate Medical University, Syracuse, New York, United States of America
| | - Tim M Townes
- Department of Biochemistry and Molecular Genetics, University of Alabama at Birmingham, Schools of Medicine and Dentistry, Birmingham, Alabama, United States of America
| | - Dirk Schübeler
- Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
| | - David M Gilbert
- Department of Biological Science, Florida State University, Tallahassee, Florida, United States of America
- * To whom correspondence should be addressed. E-mail:
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Karnani N, Taylor C, Malhotra A, Dutta A. Pan-S replication patterns and chromosomal domains defined by genome-tiling arrays of ENCODE genomic areas. Genome Res 2007; 17:865-76. [PMID: 17568004 PMCID: PMC1891345 DOI: 10.1101/gr.5427007] [Citation(s) in RCA: 91] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
In eukaryotes, accurate control of replication time is required for the efficient completion of S phase and maintenance of genome stability. We present a high-resolution genome-tiling array-based profile of replication timing for approximately 1% of the human genome studied by The ENCODE Project Consortium. Twenty percent of the investigated segments replicate asynchronously (pan-S). These areas are rich in genes and CpG islands, features they share with early-replicating loci. Interphase FISH showed that pan-S replication is a consequence of interallelic variation in replication time and is not an artifact derived from a specific cell cycle synchronization method or from aneuploidy. The interallelic variation in replication time is likely due to interallelic variation in chromatin environment, because while the early- or late-replicating areas were exclusively enriched in activating or repressing histone modifications, respectively, the pan-S areas had both types of histone modification. The replication profile of the chromosomes identified contiguous chromosomal segments of hundreds of kilobases separated by smaller segments where the replication time underwent an acute transition. Close examination of one such segment demonstrated that the delay of replication time was accompanied by a decrease in level of gene expression and appearance of repressive chromatin marks, suggesting that the transition segments are boundary elements separating chromosomal domains with different chromatin environments.
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Affiliation(s)
- Neerja Karnani
- Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, Virginia 22908, USA
| | - Christopher Taylor
- Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, Virginia 22908, USA
- Department of Computer Science, University of Virginia, Charlottesville, Virginia 22908, USA
| | - Ankit Malhotra
- Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, Virginia 22908, USA
- Department of Computer Science, University of Virginia, Charlottesville, Virginia 22908, USA
| | - Anindya Dutta
- Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, Virginia 22908, USA
- Corresponding author.E-mail ; fax (434) 924-5069
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Ghosh M, Liu G, Randall G, Bevington J, Leffak M. Transcription factor binding and induced transcription alter chromosomal c-myc replicator activity. Mol Cell Biol 2005; 24:10193-207. [PMID: 15542830 PMCID: PMC529035 DOI: 10.1128/mcb.24.23.10193-10207.2004] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The observation that transcriptionally active genes generally replicate early in S phase and observations of the interaction between transcription factors and replication proteins support the thesis that promoter elements may have a role in DNA replication. To test the relationship between transcription and replication we constructed HeLa cell lines in which inducible green fluorescent protein (GFP)-encoding genes replaced the proximal approximately 820-bp promoter region of the c-myc gene. Without the presence of an inducer, basal expression occurred from the GFP gene in either orientation and origin activity was restored to the mutant c-myc replicator. In contrast, replication initiation was repressed upon induction of transcription. When basal or induced transcription complexes were slowed by the presence of alpha-amanitin, origin activity depended on the orientation of the transcription unit. To test mechanistically whether basal transcription or transcription factor binding was sufficient for replication rescue by the uninduced GFP genes, a GAL4p binding cassette was used to replace all regulatory sequences within approximately 1,400 bp 5' to the c-myc gene. In these cells, expression of a CREB-GAL4 fusion protein restored replication origin activity. These results suggest that transcription factor binding can enhance replication origin activity and that high levels of expression or the persistence of transcription complexes can repress it.
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Affiliation(s)
- M Ghosh
- Department of Biochemistry and Molecular Biology, Wright State University School of Medicine, 3640 Colonel Glenn Highway, Dayton, OH 45435, USA
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Yusa K, Takeda J, Horie K. Enhancement of Sleeping Beauty transposition by CpG methylation: possible role of heterochromatin formation. Mol Cell Biol 2004; 24:4004-18. [PMID: 15082793 PMCID: PMC387741 DOI: 10.1128/mcb.24.9.4004-4018.2004] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The Sleeping Beauty (SB) transposase is the most active transposase in vertebrate cells, and the SB transposon system has been used as a tool for insertional mutagenesis and gene delivery. Previous studies have indicated that the frequency of chromosomal transposition is considerably higher in mouse germ cells than in mouse embryonic stem cells, suggesting the existence of unknown mechanisms that regulate SB transposition. Here, we demonstrated that CpG methylation of the transposon region enhances SB transposition. The transposition efficiencies of a methylated transposon and an unmethylated transposon which had been targeted in the same genomic loci by recombination-mediated cassette exchange in mouse erythroleukemia cells were compared, and at least a 100-fold increase was observed in the methylated transposon. CpG methylation also enhanced transposition from plasmids into the genome. Chromatin immunoprecipitation assays revealed that histone H3 methylated at lysine-9, a hallmark of condensed heterochromatin, was enriched at the methylated transposon, whereas the unmethylated transposon formed a relaxed euchromatin structure, as evidenced by enrichment of acetylated histone H3 and reporter gene expression. Possible roles of heterochromatin formation in the transposition reaction are discussed. Our findings indicate a novel relationship between CpG methylation and transposon mobilization.
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Affiliation(s)
- Kosuke Yusa
- Department of Social and Environmental Medicine, Graduate School of Medicine, Osaka University, Suita, Osaka 565-0871, Japan
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Abstract
Locus control regions (LCRs) are operationally defined by their ability to enhance the expression of linked genes to physiological levels in a tissue-specific and copy number-dependent manner at ectopic chromatin sites. Although their composition and locations relative to their cognate genes are different, LCRs have been described in a broad spectrum of mammalian gene systems, suggesting that they play an important role in the control of eukaryotic gene expression. The discovery of the LCR in the beta-globin locus and the characterization of LCRs in other loci reinforces the concept that developmental and cell lineage-specific regulation of gene expression relies not on gene-proximal elements such as promoters, enhancers, and silencers exclusively, but also on long-range interactions of various cis regulatory elements and dynamic chromatin alterations.
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Affiliation(s)
- Qiliang Li
- Division of Medical Genetics, Department of Genome Sciences, University of Washington, 1705 NE Pacific Street, Seattle, WA 98195, USA
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Kim SM, Huberman JA. Regulation of replication timing in fission yeast. EMBO J 2001; 20:6115-26. [PMID: 11689451 PMCID: PMC125695 DOI: 10.1093/emboj/20.21.6115] [Citation(s) in RCA: 109] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2001] [Revised: 09/06/2001] [Accepted: 09/06/2001] [Indexed: 11/12/2022] Open
Abstract
Here we report the first characterization of replication timing and its regulation in the fission yeast Schizosaccharomyces pombe. We used three different synchronization methods: centrifugal elutriation, cdc10 temperature-shift and release, and starvation for deoxyribonucleoside triphosphates (dNTPs) by treatment with hydroxyurea (HU) followed by removal of HU, to study the times when specific autonomously replicating sequence elements (ARS elements; potential replication origins) replicate during S phase. We found that individual ARS elements replicate at characteristic times, some early and some late, independently of synchronization method. In wild-type cells treated with HU, early ARS elements replicated but late ones did not. However, in HU-treated mutant cells lacking the Rad3 (similar to human ATR and ATM) or Cds1 (similar to human CHK2) checkpoint kinase, both early and late ARS elements were able to replicate. Thus under conditions of dNTP starvation the Rad3 and Cds1 kinases are needed to suppress the replication of normally late-replicating regions.
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Affiliation(s)
- Soo-Mi Kim
- Department of Cancer Genetics, Roswell Park Cancer Institute, Elm & Carlton Streets, Buffalo, NY 14263, USA Present address: Department of Biology, California Institute of Technology, Pasadena, CA 91125, USA Corresponding author e-mail:
| | - Joel A. Huberman
- Department of Cancer Genetics, Roswell Park Cancer Institute, Elm & Carlton Streets, Buffalo, NY 14263, USA Present address: Department of Biology, California Institute of Technology, Pasadena, CA 91125, USA Corresponding author e-mail:
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Simon I, Tenzen T, Mostoslavsky R, Fibach E, Lande L, Milot E, Gribnau J, Grosveld F, Fraser P, Cedar H. Developmental regulation of DNA replication timing at the human beta globin locus. EMBO J 2001; 20:6150-7. [PMID: 11689454 PMCID: PMC125288 DOI: 10.1093/emboj/20.21.6150] [Citation(s) in RCA: 73] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
The human beta globin locus replicates late in most cell types, but becomes early replicating in erythroid cells. Using FISH to map DNA replication timing around the endogenous beta globin locus and by applying a genetic approach in transgenic mice, we have demonstrated that both the late and early replication states are controlled by regulatory elements within the locus control region. These results also show that the pattern of replication timing is set up by mechanisms that work independently of gene transcription.
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Affiliation(s)
- Itamar Simon
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Toyoaki Tenzen
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Raul Mostoslavsky
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Eitan Fibach
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Laura Lande
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Eric Milot
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Joost Gribnau
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Frank Grosveld
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Peter Fraser
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
| | - Howard Cedar
- Department of Cellular Biochemistry and Department of Hematology, Hebrew University Medical School, Jerusalem, Israel 91120, Department of Evolutionary Genetics, National Institute of Genetics, Mishima, Shizuoka-ken, Japan 411-8540 and
MGC Department of Cell Biology and Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands Present address: Laboratory of Chromatin and Gene Expression, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Corresponding author e-mail: I.Simon and T.Tenzen contributed equally to this work
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Li F, Chen J, Izumi M, Butler MC, Keezer SM, Gilbert DM. The replication timing program of the Chinese hamster beta-globin locus is established coincident with its repositioning near peripheral heterochromatin in early G1 phase. J Cell Biol 2001; 154:283-92. [PMID: 11470818 PMCID: PMC1255917 DOI: 10.1083/jcb.200104043] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We have examined the dynamics of nuclear repositioning and the establishment of a replication timing program for the actively transcribed dihydrofolate reductase (DHFR) locus and the silent beta-globin gene locus in Chinese hamster ovary cells. The DHFR locus was internally localized and replicated early, whereas the beta-globin locus was localized adjacent to the nuclear periphery and replicated during the middle of S phase, coincident with replication of peripheral heterochromatin. Nuclei were prepared from cells synchronized at various times during early G1 phase and stimulated to enter S phase by introduction into Xenopus egg extracts, and the timing of DHFR and beta-globin replication was evaluated in vitro. With nuclei isolated 1 h after mitosis, neither locus was preferentially replicated before the other. However, with nuclei isolated 2 or 3 h after mitosis, there was a strong preference for replication of DHFR before beta-globin. Measurements of the distance of DHFR and beta-globin to the nuclear periphery revealed that the repositioning of the beta-globin locus adjacent to peripheral heterochromatin also took place between 1 and 2 h after mitosis. These results suggest that the CHO beta-globin locus acquires the replication timing program of peripheral heterochromatin upon association with the peripheral subnuclear compartment during early G1 phase.
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Affiliation(s)
- F Li
- Department of Biochemistry and Molecular Biology, State University of New York Upstate Medical University, Syracuse, NY 13210, USA
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10
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Kamath S, Leffak M. Multiple sites of replication initiation in the human beta-globin gene locus. Nucleic Acids Res 2001; 29:809-17. [PMID: 11160905 PMCID: PMC30394 DOI: 10.1093/nar/29.3.809] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The cell cycle-dependent, ordered assembly of protein prereplicative complexes suggests that eukaryotic replication origins determine when genomic replication initiates. By comparison, the factors that determine where replication initiates relative to the sites of prereplicative complex formation are not known. In the human globin gene locus previous work showed that replication initiates at a single site 5' to the ss-globin gene when protein synthesis is inhibited by emetine. The present study has examined the pattern of initiation around the genetically defined ss-globin replicator in logarithmically growing HeLa cells, using two PCR-based nascent strand assays. In contrast to the pattern of initiation detected in emetine-treated cells, analysis of the short nascent strands at five positions spanning a 40 kb globin gene region shows that replication initiates at more than one site in non-drug-treated cells. Quantitation of nascent DNA chains confirmed that replication begins at several locations in this domain, including one near the initiation region (IR) identified in emetine-treated cells. However, the abundance of short nascent strands at another initiation site approximately 20 kb upstream is approximately 4-fold as great as that at the IR. The latter site abuts an early S phase replicating fragment previously defined at low resolution in logarithmically dividing cells.
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Affiliation(s)
- S Kamath
- Department of Biochemistry and Molecular Biology, Wright State University, Dayton, OH 45435, USA
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11
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Cimbora DM, Schübeler D, Reik A, Hamilton J, Francastel C, Epner EM, Groudine M. Long-distance control of origin choice and replication timing in the human beta-globin locus are independent of the locus control region. Mol Cell Biol 2000; 20:5581-91. [PMID: 10891496 PMCID: PMC86017 DOI: 10.1128/mcb.20.15.5581-5591.2000] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
DNA replication in the human beta-globin locus is subject to long-distance regulation. In murine and human erythroid cells, the human locus replicates in early S phase from a bidirectional origin located near the beta-globin gene. This Hispanic thalassemia deletion removes regulatory sequences located over 52 kb from the origin, resulting in replication of the locus from a different origin, a shift in replication timing to late S phase, adoption of a closed chromatin conformation, and silencing of globin gene expression in murine erythroid cells. The sequences deleted include nuclease-hypersensitive sites 2 to 5 (5'HS2-5) of the locus control region (LCR) plus an additional 27-kb upstream region. We tested a targeted deletion of 5'HS2-5 in the normal chromosomal context of the human beta-globin locus to determine the role of these elements in replication origin choice and replication timing. We demonstrate that the 5'HS2-5-deleted locus initiates replication at the appropriate origin and with normal timing in murine erythroid cells, and therefore we conclude that 5'HS2-5 in the classically defined LCR do not control replication in the human beta-globin locus. Recent studies also show that targeted deletion of 5'HS2-5 results in a locus that lacks globin gene expression yet retains an open chromatin conformation. Thus, the replication timing of the locus is closely correlated with nuclease sensitivity but not globin gene expression.
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Affiliation(s)
- D M Cimbora
- Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, USA
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12
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Ma H, Samarabandu J, Devdhar RS, Acharya R, Cheng PC, Meng C, Berezney R. Spatial and temporal dynamics of DNA replication sites in mammalian cells. J Biophys Biochem Cytol 1998; 143:1415-25. [PMID: 9852140 PMCID: PMC2132991 DOI: 10.1083/jcb.143.6.1415] [Citation(s) in RCA: 235] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Fluorescence microscopic analysis of newly replicated DNA has revealed discrete granular sites of replication (RS). The average size and number of replication sites from early to mid S-phase suggest that each RS contains numerous replicons clustered together. We are using fluorescence laser scanning confocal microscopy in conjunction with multidimensional image analysis to gain more precise information about RS and their spatial-temporal dynamics. Using a newly improved imaging segmentation program, we report an average of approximately 1,100 RS after a 5-min pulse labeling of 3T3 mouse fibroblast cells in early S-phase. Pulse-chase-pulse double labeling experiments reveal that RS take approximately 45 min to complete replication. Appropriate calculations suggest that each RS contains an average of 1 mbp of DNA or approximately 6 average-sized replicons. Double pulse-double chase experiments demonstrate that the DNA sequences replicated at individual RS are precisely maintained temporally and spatially as the cell progresses through the cell cycle and into subsequent generations. By labeling replicated DNA at the G1/S borders for two consecutive cell generations, we show that the DNA synthesized at early S-phase is replicated at the same time and sites in the next round of replication.
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Affiliation(s)
- H Ma
- Department of Biological Sciences, State University of New York at Buffalo, Buffalo, New York 14260, USA
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13
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Strehl S, LaSalle JM, Lalande M. High-resolution analysis of DNA replication domain organization across an R/G-band boundary. Mol Cell Biol 1997; 17:6157-66. [PMID: 9315676 PMCID: PMC232466 DOI: 10.1128/mcb.17.10.6157] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Establishing how mammalian chromosome replication is regulated and how groups of replication origins are organized into replication bands will significantly increase our understanding of chromosome organization. Replication time bands in mammalian chromosomes show overall congruency with structural R- and G-banding patterns as revealed by different chromosome banding techniques. Thus, chromosome bands reflect variations in the longitudinal structure and function of the chromosome, but little is known about the structural basis of the metaphase chromosome banding pattern. At the microscopic level, both structural R and G bands and replication bands occupy discrete domains along chromosomes, suggesting separation by distinct boundaries. The purpose of this study was to determine replication timing differences encompassing a boundary between differentially replicating chromosomal bands. Using competitive PCR on replicated DNA from flow-sorted cell cycle fractions, we have analyzed the replication timing of markers spanning roughly 5 Mb of human chromosome 13q14.3/q21.1. This is only the second report of high-resolution analysis of replication timing differences across an R/G-band boundary. In contrast to previous work, however, we find that band boundaries are defined by a gradient in replication timing rather than by a sharp boundary separating R and G bands into functionally distinct chromatin compartments. These findings indicate that topographical band boundaries are not defined by specific sequences or structures.
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Affiliation(s)
- S Strehl
- Genetics Division, Children's Hospital and Harvard Medical School, Boston, Massachusetts 02115, USA
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14
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A mammalian origin of bidirectional DNA replication within the Chinese hamster RPS14 locus. Mol Cell Biol 1994. [PMID: 8065299 DOI: 10.1128/mcb.14.9.5628] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
Two complementary experimental approaches have been used to identify a chromosomal origin of bidirectional DNA replication within or immediately downstream of the Chinese hamster ribosomal protein S14 gene (RPS14). The replication origin, designated oriS14, maps within a 1.6- to 2.0-kbp region of RPS14 that includes the gene's third and fourth introns, exons IV plus V, and approximately 500 bp of proximal downstream flanking DNA. The nucleic acid sequence encoding oriS14 closely resembles the other mammalian chromosomal replication origins whose primary structures are known. It contains DNA binding sites for a large number of transcription factors, replication proteins, and mammalian oncogenes as well as several dinucleotide repeat motifs, an AT-rich region, and a sequence that is likely to bend the DNA. In contrast to the other well-characterized mammalian replication origins, which are autosomal and therefore carried as two copies per somatic cell, oriS14 is encoded by single-copy DNA within a hemizygous segment of chromosome 2q in CHO-K1 cells. Also, other known mammalian replication origins are situated in nontranscribed, intergenic DNA, whereas the DNA sequence encoding oriS14 substantially overlaps the transcribed portion of a constitutively expressed housekeeping gene.
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15
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Tasheva ES, Roufa DJ. A mammalian origin of bidirectional DNA replication within the Chinese hamster RPS14 locus. Mol Cell Biol 1994; 14:5628-35. [PMID: 8065299 PMCID: PMC359087 DOI: 10.1128/mcb.14.9.5628-5635.1994] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Two complementary experimental approaches have been used to identify a chromosomal origin of bidirectional DNA replication within or immediately downstream of the Chinese hamster ribosomal protein S14 gene (RPS14). The replication origin, designated oriS14, maps within a 1.6- to 2.0-kbp region of RPS14 that includes the gene's third and fourth introns, exons IV plus V, and approximately 500 bp of proximal downstream flanking DNA. The nucleic acid sequence encoding oriS14 closely resembles the other mammalian chromosomal replication origins whose primary structures are known. It contains DNA binding sites for a large number of transcription factors, replication proteins, and mammalian oncogenes as well as several dinucleotide repeat motifs, an AT-rich region, and a sequence that is likely to bend the DNA. In contrast to the other well-characterized mammalian replication origins, which are autosomal and therefore carried as two copies per somatic cell, oriS14 is encoded by single-copy DNA within a hemizygous segment of chromosome 2q in CHO-K1 cells. Also, other known mammalian replication origins are situated in nontranscribed, intergenic DNA, whereas the DNA sequence encoding oriS14 substantially overlaps the transcribed portion of a constitutively expressed housekeeping gene.
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Affiliation(s)
- E S Tasheva
- Division of Biology, Kansas State University, Manhattan 66506
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16
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Two alleles of a developmentally regulated alpha-tubulin locus in Physarum polycephalum replicate on different schedules. Mol Cell Biol 1993. [PMID: 8093328 DOI: 10.1128/mcb.13.1.449] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The replication timing of a pair of natural alleles was compared at two alpha-tubulin loci of the Physarum plasmodium. Taking advantage of the naturally synchronous cell cycle of nuclei within the syncytial plasmodium, we analyzed the replication schedule of specific DNA fragments to a resolution of 10-min intervals within a 3-h S phase. At this level of resolution, differences in replication timing between polymorphic alleles at the same locus can be detected in a heterozygote. Specifically, the 3' region of the altA1 allele completes replication at between 20 and 40 min of S phase. The same region of the altA2 allele completes replication at between 40 and 80 min of S phase. In contrast, both alleles at the altB locus replicate concurrently within the first 10 to 15 min of S phase. Previous studies showed that both altA and altB are expressed in the plasmodium, their message levels peaking at mitosis, just minutes before the onset of S phase. However, altB message is detected at substantially higher levels than altA message on Northern (RNA) blots. The temporal windows over which the altA alleles each replicate are very broad in comparison with the levels of mitotic synchrony and altB replication synchrony in a single plasmodium. The allele-specific replication schedule of the altA locus demonstrates that the temporal organization of replicons is not strictly conserved between homologous chromosomes.
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17
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Cunningham DB, Dove WF. Two alleles of a developmentally regulated alpha-tubulin locus in Physarum polycephalum replicate on different schedules. Mol Cell Biol 1993; 13:449-61. [PMID: 8093328 PMCID: PMC358925 DOI: 10.1128/mcb.13.1.449-461.1993] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
The replication timing of a pair of natural alleles was compared at two alpha-tubulin loci of the Physarum plasmodium. Taking advantage of the naturally synchronous cell cycle of nuclei within the syncytial plasmodium, we analyzed the replication schedule of specific DNA fragments to a resolution of 10-min intervals within a 3-h S phase. At this level of resolution, differences in replication timing between polymorphic alleles at the same locus can be detected in a heterozygote. Specifically, the 3' region of the altA1 allele completes replication at between 20 and 40 min of S phase. The same region of the altA2 allele completes replication at between 40 and 80 min of S phase. In contrast, both alleles at the altB locus replicate concurrently within the first 10 to 15 min of S phase. Previous studies showed that both altA and altB are expressed in the plasmodium, their message levels peaking at mitosis, just minutes before the onset of S phase. However, altB message is detected at substantially higher levels than altA message on Northern (RNA) blots. The temporal windows over which the altA alleles each replicate are very broad in comparison with the levels of mitotic synchrony and altB replication synchrony in a single plasmodium. The allele-specific replication schedule of the altA locus demonstrates that the temporal organization of replicons is not strictly conserved between homologous chromosomes.
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Affiliation(s)
- D B Cunningham
- McArdle Laboratory for Cancer Research University of Wisconsin, Madison 53706
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18
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The gene for a novel human lamin maps at a highly transcribed locus of chromosome 19 which replicates at the onset of S-phase. Mol Cell Biol 1992. [PMID: 1630457 DOI: 10.1128/mcb.12.8.3499] [Citation(s) in RCA: 84] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
A previously described human DNA fragment which is replicated early in S-phase of HL-60 cell DNA (C. Tribioli, G. Biamonti, M. Giacca, M. Colonna, S. Riva, and A. Falaschi, Nucleic Acids Res. 15:10211-10232, 1987) was used to screen a genomic library in lambda Ch28. A clone which contained a 13.7-kb insert (L30E) found to code for several transcripts was isolated. The transcription of L30E DNA exhibited a complex pattern and a tissue-specific and proliferation-dependent type of regulation. The data were consistent with two tandemly arranged transcription units, the 3' end of one separated from the 5' end of the other by a sequence of about 600 bp containing an active promoter. The isolation and sequencing of L30E-specific cDNAs permitted identification of two genes, one of which encoded a B-type human lamin (analogous to mouse lamin B2). L30E DNA was mapped by in situ hybridization at the G-negative subtelomeric band p13.3 of chromosome 19. Interestingly, in synchronized HL-60 cells, L30E DNA is replicated in the first minute of S-phase. Replication of the lamin gene early in S-phase may reflect a coupling between early replication and transcription of genes for S-phase-specific proteins such as lamins.
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19
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Biamonti G, Giacca M, Perini G, Contreas G, Zentilin L, Weighardt F, Guerra M, Della Valle G, Saccone S, Riva S. The gene for a novel human lamin maps at a highly transcribed locus of chromosome 19 which replicates at the onset of S-phase. Mol Cell Biol 1992; 12:3499-506. [PMID: 1630457 PMCID: PMC364599 DOI: 10.1128/mcb.12.8.3499-3506.1992] [Citation(s) in RCA: 57] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
A previously described human DNA fragment which is replicated early in S-phase of HL-60 cell DNA (C. Tribioli, G. Biamonti, M. Giacca, M. Colonna, S. Riva, and A. Falaschi, Nucleic Acids Res. 15:10211-10232, 1987) was used to screen a genomic library in lambda Ch28. A clone which contained a 13.7-kb insert (L30E) found to code for several transcripts was isolated. The transcription of L30E DNA exhibited a complex pattern and a tissue-specific and proliferation-dependent type of regulation. The data were consistent with two tandemly arranged transcription units, the 3' end of one separated from the 5' end of the other by a sequence of about 600 bp containing an active promoter. The isolation and sequencing of L30E-specific cDNAs permitted identification of two genes, one of which encoded a B-type human lamin (analogous to mouse lamin B2). L30E DNA was mapped by in situ hybridization at the G-negative subtelomeric band p13.3 of chromosome 19. Interestingly, in synchronized HL-60 cells, L30E DNA is replicated in the first minute of S-phase. Replication of the lamin gene early in S-phase may reflect a coupling between early replication and transcription of genes for S-phase-specific proteins such as lamins.
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Affiliation(s)
- G Biamonti
- Istituto di Genetica Biochimica ed Evoluzionistica del Consiglio Nazionale delle Ricerche, Pavia, Italy
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20
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O'Keefe RT, Henderson SC, Spector DL. Dynamic organization of DNA replication in mammalian cell nuclei: spatially and temporally defined replication of chromosome-specific alpha-satellite DNA sequences. J Biophys Biochem Cytol 1992; 116:1095-110. [PMID: 1740468 PMCID: PMC2289349 DOI: 10.1083/jcb.116.5.1095] [Citation(s) in RCA: 366] [Impact Index Per Article: 11.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
Five distinct patterns of DNA replication have been identified during S-phase in asynchronous and synchronous cultures of mammalian cells by conventional fluorescence microscopy, confocal laser scanning microscopy, and immunoelectron microscopy. During early S-phase, replicating DNA (as identified by 5-bromodeoxyuridine incorporation) appears to be distributed at sites throughout the nucleoplasm, excluding the nucleolus. In CHO cells, this pattern of replication peaks at 30 min into S-phase and is consistent with the localization of euchromatin. As S-phase continues, replication of euchromatin decreases and the peripheral regions of heterochromatin begin to replicate. This pattern of replication peaks at 2 h into S-phase. At 5 h, perinucleolar chromatin as well as peripheral areas of heterochromatin peak in replication. 7 h into S-phase interconnecting patches of electron-dense chromatin replicate. At the end of S-phase (9 h), replication occurs at a few large regions of electron-dense chromatin. Similar or identical patterns have been identified in a variety of mammalian cell types. The replication of specific chromosomal regions within the context of the BrdU-labeling patterns has been examined on an hourly basis in synchronized HeLa cells. Double labeling of DNA replication sites and chromosome-specific alpha-satellite DNA sequences indicates that the alpha-satellite DNA replicates during mid S-phase (characterized by the third pattern of replication) in a variety of human cell types. Our data demonstrates that specific DNA sequences replicate at spatially and temporally defined points during the cell cycle and supports a spatially dynamic model of DNA replication.
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Affiliation(s)
- R T O'Keefe
- Cold Spring Harbor Laboratory, New York 11724-2217
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21
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The mouse immunoglobulin kappa light-chain genes are located in early- and late-replicating regions of chromosome 6. Mol Cell Biol 1990. [PMID: 2115125 DOI: 10.1128/mcb.10.8.4314] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The murine immunoglobulin kappa (kappa) light-chain multigene family includes the constant region (C kappa), joining-region genes, and approximately 30 kappa-variable (V kappa) region families. The entire region occupies an estimated 1,000 to 3,000 kilobases, and some V kappa families have been linked by recombinant inbred mapping. The C kappa gene and 14 V kappa families replicated differently among cell lines of lymphoid and nonlymphoid origin. In nonlymphoid cells, the C kappa gene replicated earlier than the V kappa families. A transition from replication during the second third of S phase for the C kappa gene to later replication during S for V kappa families was observed. The V kappa family (V kappa 21) that maps closest to the C kappa gene, replicated during the first half of the S phase; most of the other V kappa families replicated during the second half of S, and some replicated during the last quarter of the S phase. In lymphoid cells, the kappa locus replicated earlier in the pre-B than in the B-cell lines. In one pre-B-cell line, 22D6, the kappa genes examined replicated at the beginning of the S phase. In the B-cell lines, the EcoRI segment containing the transcribed gene replicated near the beginning of the S phase. Other V kappa families replicated within the first two-thirds of S phase. Some linked V kappa families replicated at similar times. In the B-cell lines, a transition from replication at the beginning of S for the transcribed C kappa and V kappa genes and surrounding DNA sequences to later replication for the other V kappa families was observed. However, in contrast to the non-lymphoid cell lines, the replication of this locus occurred predominantly during the first half of S. The kappa locus contains both early- and late-replicating genes, and early replication is usually associated with transcriptional activity. The results are discussed with respect to the organization of transcriptionally active chromatin domains.
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Position effects on the timing of replication of chromosomally integrated simian virus 40 molecules in Chinese hamster cells. Mol Cell Biol 1990. [PMID: 2164638 DOI: 10.1128/mcb.10.8.4345] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Simian virus 40 (SV40) DNA molecules chromosomally integrated at different sites in three Chinese hamster lung fibroblast lines replicated during the middle portion of S phase but not precisely at the same time in all three cell lines. The time of replication was unrelated to the presence of T antigen or to its relative activity in promoting SV40 replication. SV40 sequences and chromosomal DNA sequences adjacent to the SV40 insert in one cell line expressing a temperature-sensitive T antigen showed a T-antigen-independent difference in replication timing from the homologous, allelic locus not linked to SV40. Our results indicate that the timing of replication of these integrated SV40 molecules is dependent upon the site of integration and is not determined by the level of T antigen replication-promoting activity.
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23
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Hatton KS, Schildkraut CL. The mouse immunoglobulin kappa light-chain genes are located in early- and late-replicating regions of chromosome 6. Mol Cell Biol 1990; 10:4314-23. [PMID: 2115125 PMCID: PMC360978 DOI: 10.1128/mcb.10.8.4314-4323.1990] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
The murine immunoglobulin kappa (kappa) light-chain multigene family includes the constant region (C kappa), joining-region genes, and approximately 30 kappa-variable (V kappa) region families. The entire region occupies an estimated 1,000 to 3,000 kilobases, and some V kappa families have been linked by recombinant inbred mapping. The C kappa gene and 14 V kappa families replicated differently among cell lines of lymphoid and nonlymphoid origin. In nonlymphoid cells, the C kappa gene replicated earlier than the V kappa families. A transition from replication during the second third of S phase for the C kappa gene to later replication during S for V kappa families was observed. The V kappa family (V kappa 21) that maps closest to the C kappa gene, replicated during the first half of the S phase; most of the other V kappa families replicated during the second half of S, and some replicated during the last quarter of the S phase. In lymphoid cells, the kappa locus replicated earlier in the pre-B than in the B-cell lines. In one pre-B-cell line, 22D6, the kappa genes examined replicated at the beginning of the S phase. In the B-cell lines, the EcoRI segment containing the transcribed gene replicated near the beginning of the S phase. Other V kappa families replicated within the first two-thirds of S phase. Some linked V kappa families replicated at similar times. In the B-cell lines, a transition from replication at the beginning of S for the transcribed C kappa and V kappa genes and surrounding DNA sequences to later replication for the other V kappa families was observed. However, in contrast to the non-lymphoid cell lines, the replication of this locus occurred predominantly during the first half of S. The kappa locus contains both early- and late-replicating genes, and early replication is usually associated with transcriptional activity. The results are discussed with respect to the organization of transcriptionally active chromatin domains.
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Affiliation(s)
- K S Hatton
- Department of Cell Biology, Albert Einstein College of Medicine, Bronx, New York 10461
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24
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Gilbert DM, Cohen SN. Position effects on the timing of replication of chromosomally integrated simian virus 40 molecules in Chinese hamster cells. Mol Cell Biol 1990; 10:4345-55. [PMID: 2164638 PMCID: PMC360984 DOI: 10.1128/mcb.10.8.4345-4355.1990] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
Simian virus 40 (SV40) DNA molecules chromosomally integrated at different sites in three Chinese hamster lung fibroblast lines replicated during the middle portion of S phase but not precisely at the same time in all three cell lines. The time of replication was unrelated to the presence of T antigen or to its relative activity in promoting SV40 replication. SV40 sequences and chromosomal DNA sequences adjacent to the SV40 insert in one cell line expressing a temperature-sensitive T antigen showed a T-antigen-independent difference in replication timing from the homologous, allelic locus not linked to SV40. Our results indicate that the timing of replication of these integrated SV40 molecules is dependent upon the site of integration and is not determined by the level of T antigen replication-promoting activity.
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Affiliation(s)
- D M Gilbert
- Department of Genetics, Stanford University School of Medicine, California 94305
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