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Irwin NAT, Pittis AA, Mathur V, Howe LJ, Keeling PJ, Lynn DH, Bourland WA. The Function and Evolution of Motile DNA Replication Systems in Ciliates. Curr Biol 2020; 31:66-76.e6. [PMID: 33125869 DOI: 10.1016/j.cub.2020.09.077] [Citation(s) in RCA: 54] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Revised: 09/09/2020] [Accepted: 09/25/2020] [Indexed: 01/31/2023]
Abstract
DNA replication is a ubiquitous and conserved cellular process. However, regulation of DNA replication is only understood in a small fraction of organisms that poorly represent the diversity of genetic systems in nature. Here we used computational and experimental approaches to examine the function and evolution of one such system, the replication band (RB) in spirotrich ciliates, which is a localized, motile hub that traverses the macronucleus while replicating DNA. We show that the RB can take unique forms in different species, from polar bands to a "replication envelope," where replication initiates at the nuclear periphery before advancing inward. Furthermore, we identify genes involved in cellular transport, including calcium transporters and cytoskeletal regulators, that are associated with the RB and may be involved in its function and translocation. These findings highlight the evolution and diversity of DNA replication systems and provide insights into the regulation of nuclear organization and processes.
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Affiliation(s)
- Nicholas A T Irwin
- Department of Botany, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
| | - Alexandros A Pittis
- Department of Botany, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
| | - Varsha Mathur
- Department of Botany, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
| | - LeAnn J Howe
- Department of Biochemistry and Molecular Biology, Life Sciences Institute, University of British Columbia, Vancouver, BC V6T 1Z3, Canada
| | - Patrick J Keeling
- Department of Botany, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
| | - Denis H Lynn
- Department of Zoology, University of British Columbia, Vancouver, BC V6T 1Z4, Canada; Department of Integrative Biology, University of Guelph, Guelph, ON N1G 2W1, Canada
| | - William A Bourland
- Department of Biological Sciences, Boise State University, Boise, ID 83725, USA.
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2
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Jin X, Hapsari ND, Lee S, Jo K. DNA binding fluorescent proteins as single-molecule probes. Analyst 2020; 145:4079-4095. [DOI: 10.1039/d0an00218f] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
DNA binding fluorescent proteins are useful probes for a broad range of biological applications.
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Affiliation(s)
- Xuelin Jin
- Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology
- Sogang University
- Seoul
- Republic of Korea
| | - Natalia Diyah Hapsari
- Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology
- Sogang University
- Seoul
- Republic of Korea
- Chemistry Education Program
| | - Seonghyun Lee
- Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology
- Sogang University
- Seoul
- Republic of Korea
| | - Kyubong Jo
- Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology
- Sogang University
- Seoul
- Republic of Korea
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3
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Grattepanche J, Walker LM, Ott BM, Paim Pinto DL, Delwiche CF, Lane CE, Katz LA. Microbial Diversity in the Eukaryotic SAR Clade: Illuminating the Darkness Between Morphology and Molecular Data. Bioessays 2018; 40:e1700198. [DOI: 10.1002/bies.201700198] [Citation(s) in RCA: 27] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2017] [Revised: 01/16/2018] [Indexed: 01/09/2023]
Affiliation(s)
| | - Laura M. Walker
- Department of Biological Sciences, Smith CollegeNorthamptonMA 01063USA
| | - Brittany M. Ott
- Department of Cell Biology and Molecular Genetics, University of MarylandCollege ParkMD 20742USA
| | | | - Charles F. Delwiche
- Department of Cell Biology and Molecular Genetics, University of MarylandCollege ParkMD 20742USA
| | - Christopher E. Lane
- Department of Biological SciencesUniversity of Rhode IslandKingstonRI 02881USA
| | - Laura A. Katz
- Department of Biological Sciences, Smith CollegeNorthamptonMA 01063USA
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4
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Current Perspectives of Telomerase Structure and Function in Eukaryotes with Emerging Views on Telomerase in Human Parasites. Int J Mol Sci 2018; 19:ijms19020333. [PMID: 29364142 PMCID: PMC5855555 DOI: 10.3390/ijms19020333] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2017] [Revised: 01/10/2018] [Accepted: 01/17/2018] [Indexed: 12/11/2022] Open
Abstract
Replicative capacity of a cell is strongly correlated with telomere length regulation. Aberrant lengthening or reduction in the length of telomeres can lead to health anomalies, such as cancer or premature aging. Telomerase is a master regulator for maintaining replicative potential in most eukaryotic cells. It does so by controlling telomere length at chromosome ends. Akin to cancer cells, most single-cell eukaryotic pathogens are highly proliferative and require persistent telomerase activity to maintain constant length of telomere and propagation within their host. Although telomerase is key to unlimited cellular proliferation in both cases, not much was known about the role of telomerase in human parasites (malaria, Trypanosoma, etc.) until recently. Since telomerase regulation is mediated via its own structural components, interactions with catalytic reverse transcriptase and several factors that can recruit and assemble telomerase to telomeres in a cell cycle-dependent manner, we compare and discuss here recent findings in telomerase biology in cancer, aging and parasitic diseases to give a broader perspective of telomerase function in human diseases.
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5
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Abstract
In this Hypothesis, Greider describes a new model for telomere length regulation, which links DNA replication and telomere elongation. Telomere length is regulated around an equilibrium set point. Telomeres shorten during replication and are lengthened by telomerase. Disruption of the length equilibrium leads to disease; thus, it is important to understand the mechanisms that regulate length at the molecular level. The prevailing protein-counting model for regulating telomerase access to elongate the telomere does not explain accumulating evidence of a role of DNA replication in telomere length regulation. Here I present an alternative model: the replication fork model that can explain how passage of a replication fork and regulation of origin firing affect telomere length.
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Affiliation(s)
- Carol W Greider
- Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA; Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218, USA
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6
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Abstract
Chromosome stability requires a dynamic balance of DNA loss and gain in each terminal tract of telomeric repeats. Repeat addition by a specialized reverse transcriptase, telomerase, has an important role in maintaining this equilibrium. Insights that have been gained into the cellular pathways for biogenesis and regulation of telomerase ribonucleoproteins raise new questions, particularly concerning the dynamic nature of this unique polymerase.
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Affiliation(s)
- Kathleen Collins
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-3204, USA.
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7
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Nakamura M, Nabetani A, Mizuno T, Hanaoka F, Ishikawa F. Alterations of DNA and chromatin structures at telomeres and genetic instability in mouse cells defective in DNA polymerase alpha. Mol Cell Biol 2006; 25:11073-88. [PMID: 16314528 PMCID: PMC1316980 DOI: 10.1128/mcb.25.24.11073-11088.2005] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Telomere length is controlled by a homeostatic mechanism that involves telomerase, telomere-associated proteins, and conventional replication machinery. Specifically, the coordinated actions of the lagging strand synthesis and telomerase have been argued. Although DNA polymerase alpha, an enzyme important for the lagging strand synthesis, has been indicated to function in telomere metabolism in yeasts and ciliates, it has not been characterized in higher eukaryotes. Here, we investigated the impact of compromised polymerase alpha activity on telomeres, using tsFT20 mouse mutant cells harboring a temperature-sensitive polymerase alpha mutant allele. When polymerase alpha was temperature-inducibly inactivated, we observed sequential events that included an initial extension of the G-tail followed by a marked increase in the overall telomere length occurring in telomerase-independent and -dependent manners, respectively. These alterations of telomeric DNA were accompanied by alterations of telomeric chromatin structures as revealed by quantitative chromatin immunoprecipitation and immunofluorescence analyses of TRF1 and POT1. Unexpectedly, polymerase alpha inhibition resulted in a significantly high incidence of Robertsonian chromosome fusions without noticeable increases in other types of chromosomal aberrations. These results indicate that although DNA polymerase alpha is essential for genome-wide DNA replication, hypomorphic activity leads to a rather specific spectrum of chromosomal abnormality.
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Affiliation(s)
- Mirai Nakamura
- Laboratory of Cell Cycle Regulation, Department of Gene Mechanisms, Graduate School of Biostudies, Kyoto University, Kitashirakawa-Oiwake-cho, Japan
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8
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Tomlinson RL, Ziegler TD, Supakorndej T, Terns RM, Terns MP. Cell cycle-regulated trafficking of human telomerase to telomeres. Mol Biol Cell 2005; 17:955-65. [PMID: 16339074 PMCID: PMC1356603 DOI: 10.1091/mbc.e05-09-0903] [Citation(s) in RCA: 209] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022] Open
Abstract
Telomerase synthesizes telomeres at the ends of human chromosomes during S phase. The results presented here suggest that telomerase activity may be regulated by intranuclear trafficking of the key components of the enzyme in human cells. We examined the subcellular localization of endogenous human telomerase RNA (hTR) and telomerase reverse transcriptase (hTERT) in HeLa cervical carcinoma cells. Throughout most of the cell cycle, we found that the two essential components of telomerase accumulate at intranuclear sites separate from telomeres. However, during S phase, both hTR and hTERT are specifically recruited to subsets of telomeres. The localization of telomerase to telomeres is dynamic, peaking at mid-S phase. We also found complex associations of both hTR and hTERT with nucleoli and Cajal bodies during S phase, implicating both structures in the biogenesis and trafficking of telomerase. Our results mark the first observation of human telomerase at telomeres and provide a mechanism for the cell cycle-dependent regulation of telomere synthesis in human cells.
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Affiliation(s)
- Rebecca L Tomlinson
- Departments of Biochemistry and Molecular Biology and Genetics, University of Georgia, Athens, GA 30602, USA
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9
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Abstract
The Cajal body is an intriguing nuclear structure present in a great variety of plant, animal, and some fungal cells. Recent work on the ribonucleoprotein enzyme telomerase has indicated an unanticipated degree of intranuclear dynamics of both its RNA and protein subunits. In this issue, Jady et al. place the Cajal body on the intranuclear traffic route of telomerase RNA (Jady et al., 2004).
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Affiliation(s)
- Thoru Pederson
- Program in Cell Dynamics and Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
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10
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Witkin KL, Collins K. Holoenzyme proteins required for the physiological assembly and activity of telomerase. Genes Dev 2004; 18:1107-18. [PMID: 15131081 PMCID: PMC415636 DOI: 10.1101/gad.1201704] [Citation(s) in RCA: 109] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2004] [Accepted: 04/05/2004] [Indexed: 11/25/2022]
Abstract
Many proteins have been implicated in the physiological function of telomerase, but specific roles of telomerase-associated proteins other than telomerase reverse transcriptase (TERT) remain ambiguous. To gain a more comprehensive understanding of catalytically active enzyme composition, we performed affinity purification of epitope-tagged, endogenously assembled Tetrahymena telomerase. We identified and cloned genes encoding four telomerase proteins in addition to TERT. We demonstrate that both of the two new proteins characterized in detail, p65 and p45, have essential roles in the maintenance of telomere length as part of a ciliate telomerase holoenzyme. The p65 subunit contains an La motif characteristic of a family of direct RNA-binding proteins. We find that p65 in cell extract is associated specifically with telomerase RNA, and that genetic depletion of p65 reduces telomerase RNA accumulation in vivo. These findings demonstrate that telomerase holoenzyme proteins other than TERT play critical roles in RNP biogenesis and function.
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Affiliation(s)
- Keren L Witkin
- Department of Molecular and Cell Biology, University of California at Berkeley, Berkeley, California 94720-3204, USA
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11
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Zhu Y, Tomlinson RL, Lukowiak AA, Terns RM, Terns MP. Telomerase RNA accumulates in Cajal bodies in human cancer cells. Mol Biol Cell 2003; 15:81-90. [PMID: 14528011 PMCID: PMC307529 DOI: 10.1091/mbc.e03-07-0525] [Citation(s) in RCA: 157] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Telomerase synthesizes telomeric DNA repeats at the ends of eukaryotic chromosomes. The RNA component of the enzyme (hTR) provides the template for telomere synthesis, which is catalyzed by telomerase reverse transcriptase (hTERT). Little is known regarding the subcellular localization of hTR and hTERT and the pathway by which telomerase is assembled. Here we report the first glimpse of the detailed subcellular localization of endogenous hTR in human cells, which we obtained by fluorescence in situ hybridization (FISH). Our studies have revealed a distinctive hTR localization pattern in cancer cells. We have found that hTR accumulates within intranuclear foci called Cajal bodies in all typical tumor-derived cell lines examined (in which telomerase is active), but not in primary or ALT cells (where little or no hTERT is present). Accumulation of hTR in the Cajal bodies of primary cells is induced when hTERT is ectopically expressed. Moreover, we report that hTERT is also found in Cajal bodies. Our data suggest that Cajal bodies are involved in the assembly and/or function of human telomerase.
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Affiliation(s)
- Yusheng Zhu
- Departments of Biochemistry and Molecular Biology, and Genetics, University of Georgia, Athens, Georgia 30602, USA
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12
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Jönsson F, Postberg J, Schaffitzel C, Lipps HJ. Organization of the macronuclear gene-sized pieces of stichotrichous ciliates into a higher order structure via telomere-matrix interactions. Chromosome Res 2003; 10:445-53. [PMID: 12489827 DOI: 10.1023/a:1021065512933] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Macronuclear DNA of stichotrichous ciliates occurs in small 'gene-sized' molecules with sizes of about 0.5 to 40 kb. Each of these molecules is terminated by telomeric sequences of defined length. A single macronucleus contains up to 10(8) DNA molecules; due to the high concentration of telomeric sequences in this nucleus it is an attractive model to study telomere behaviour. We recently provided evidence that macronuclear telomeres are attached to the nuclear matrix and that this interaction is mediated by the telomere binding protein (TeBP). Using various experimental approaches, we now demonstrate that telomeres as well as both subunits of the telomere binding protein are associated with the nuclear matrix. However, there is no direct binding of telomeric DNA to the matrix but telomere matrix interaction is exclusively mediated by the TeBP. In addition, we show that telomeric sequences adopt in vivo the antiparallel G-quartet structure when bound to the nuclear matrix. These data not only allow us to propose a model for macronuclear architecture but may also be relevant for further analysis of telomere-matrix interactions in higher eukaryotes.
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Affiliation(s)
- Franziska Jönsson
- Institute of Cell Biology, University Witten/Herdecke, Stockumer Str. 10, D-58448 Witten, Germany
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13
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Aigner S, Postberg J, Lipps HJ, Cech TR. The Euplotes La motif protein p43 has properties of a telomerase-specific subunit. Biochemistry 2003; 42:5736-47. [PMID: 12741831 DOI: 10.1021/bi034121y] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Telomerase is a specialized reverse transcriptase synthesizing DNA repeats at telomeres. In addition to the RNA and catalytic protein components, telomerase from the ciliate Euplotes aediculatus contains the subunit p43. This protein is homologous to the La autoantigen, functioning in maturation of RNA polymerase III transcripts. Here we provide evidence that p43 is primarily associated with the telomerase ribonucleoprotein in vivo. Recombinant p43 binds telomerase RNA with low-nanomolar affinity in vitro, recognizing stem I and adjacent nucleotides or structures in the core of the RNA. Unlike authentic La proteins, p43 does not bind strongly to RNA polymerase III precursor transcripts and does not exhibit a marked binding preference for 3'-terminal oligouridylate residues. In isolated macronuclei, p43 largely colocalizes with telomerase RNA in discrete foci. These findings suggest that p43 is not the Euplotes La protein but instead plays a dedicated role in telomerase assembly and/or function. Thus, p43 joins the telomerase reverse transcriptase and the yeast proteins Est1p and Est3p as the only telomerase-specific proteins identified so far.
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Affiliation(s)
- Stefan Aigner
- Department of Chemistry and Biochemistry and Howard Hughes Medical Institute, University of Colorado, Boulder, Colorado 80309-0215, USA
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14
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Möllenbeck M, Postberg J, Paeschke K, Rossbach M, Jönsson F, Lipps HJ. The telomerase-associated protein p43 is involved in anchoring telomerase in the nucleus. J Cell Sci 2003; 116:1757-61. [PMID: 12665556 DOI: 10.1242/jcs.00351] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Telomere replication of eukaryotic chromosomes is achieved by a specialized enzyme, the telomerase. Although the biochemistry of end-replication is well understood, little is known about the organization of the end-replication machinery, its regulation throughout the cell cycle or the biological function of the telomerase-associated proteins. Here we investigate the function of the telomerase-associated protein p43 within the macronucleus of the ciliated protozoa Euplotes. It has been shown that p43 binds in vitro to the RNA subunit of telomerase and shares homology with the La autoantigen family. It therefore has been suggested that it is involved in the assembly and/or nuclear retention of telomerase. We show that the p43-telomerase complex is bound to a subnuclear structure in vivo and is resistant to electroelution. Upon inhibition of p43 or telomerase expression by RNAi, which in this study was used for the first time in spirotrichs, this complex is no longer retained in the nucleus. Further analysis revealed that the p43-telomerase complex is bound to the nuclear matrix in vivo and that after inhibition of p43 expression, telomerase is released from this structure, strongly suggesting that p43 is involved in anchoring of telomerase in the nucleus. This is the first in vivo demonstration of the biological function of this telomerase-associated component involved in telomere replication and allows us to propose a model for the organization of the end-replication machinery in the eukaryotic cell.
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Affiliation(s)
- Matthias Möllenbeck
- Institute of Cell Biology, University Witten/Herdecke, Stockumer Strasse 10, D-58453 Witten, Germany
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15
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Lerch-Gaggl A, Haque J, Li J, Ning G, Traktman P, Duncan SA. Pescadillo is essential for nucleolar assembly, ribosome biogenesis, and mammalian cell proliferation. J Biol Chem 2002; 277:45347-55. [PMID: 12237316 DOI: 10.1074/jbc.m208338200] [Citation(s) in RCA: 98] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Mutation of the zebrafish pescadillo gene blocks expansion of a number of tissues in the developing embryo, suggesting roles for its gene product in controlling cell proliferation. We report that levels of the pescadillo protein increase in rodent hepatocytes as they enter the cell cycle. Pescadillo protein localizes to distinct substructures of the interphase nucleus including nucleoli, the site of ribosome biogenesis. During mitosis pescadillo closely associates with the periphery of metaphase chromosomes and by late anaphase is associated with nucleolus-derived foci and prenucleolar bodies. Blastomeres in mouse embryos lacking pescadillo arrest at morula stages of development, the nucleoli fail to differentiate and accumulation of ribosomes is inhibited. We propose that in mammalian cells pescadillo is essential for ribosome biogenesis and nucleologenesis and that disruption to its function results in cell cycle arrest.
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Affiliation(s)
- Alexandra Lerch-Gaggl
- Department of Cell Biology, Neurobiology, and Anatomy, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, USA
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16
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Ray S, Karamysheva Z, Wang L, Shippen DE, Price CM. Interactions between telomerase and primase physically link the telomere and chromosome replication machinery. Mol Cell Biol 2002; 22:5859-68. [PMID: 12138196 PMCID: PMC133977 DOI: 10.1128/mcb.22.16.5859-5868.2002] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
In the ciliate Euplotes crassus, millions of new telomeres are synthesized by telomerase and polymerase alpha-primase during macronuclear development in mated cells. Concomitant with de novo telomere formation, telomerase assembles into higher-order complexes of 550 kDa, 1,600 kDa, and 5 MDa. We show here that telomerase is physically associated with the lagging-strand replication machinery in these complexes. Antibodies against DNA primase precipitated telomerase activity from all three complexes from mated cells but not the 280-kDa telomerase complex from vegetatively growing cells. Moreover, when telomerase was affinity purified, primase copurified with enzyme from mated cells but not with the 280-kDa vegetative complex. Thus, the association of telomerase and primase is developmentally regulated. Intriguingly, PCNA (proliferating cell nuclear antigen) was also found in the 5-MDa complex from mated cells. We therefore speculate that this complex is a complete telomere synthesis machine, while the smaller complexes are assembly intermediates. The physical association of telomerase and primase explains the coordinate regulation of telomeric G- and C-strand synthesis and the efficiency of telomere addition in E. crassus.
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Affiliation(s)
- Saugata Ray
- Department of Pediatrics, University of California, San Diego School of Medicine, La Jolla, California 92093-0627, USA
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17
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Moy TI, Boettner D, Rhodes JC, Silver PA, Askew DS. Identification of a role for Saccharomyces cerevisiae Cgr1p in pre-rRNA processing and 60S ribosome subunit synthesis. MICROBIOLOGY (READING, ENGLAND) 2002; 148:1081-1090. [PMID: 11932453 DOI: 10.1099/00221287-148-4-1081] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
Saccharomyces cerevisiae CGR1 encodes a conserved fungal protein that localizes to the nucleolus. To determine if this localization reflects a role for Cgr1p in ribosome biogenesis two yeast cgr1 mutants were examined for defects in ribosome synthesis: a conditional depletion strain in which CGR1 is under the control of a tetracycline-repressible promoter and a mutant strain in which a C-terminal truncated Cgr1p is expressed. Both strains had impaired growth rates and were hypersensitive to the aminoglycosides paromomycin and hygromycin. Polysome analyses of the mutants revealed increased levels of free 40S subunits relative to 60S subunits, a decrease in 80S monosomes and accumulation of half-mer polysomes. Pulse-chase labelling demonstrated that pre-rRNA processing was defective in the mutants, resulting in accumulation of the 35S, 27S and 7S pre-rRNAs and delayed production of the mature 25S and 5 small middle dot8S rRNAs. The synthesis of the 18S and 5S rRNAs was unaffected. Loss of Cgr1 function also caused a partial delocalization of the 5'-ITS1 RNA and the nucleolar protein Nop1p into the nucleoplasm, suggesting that Cgr1p contributes to compartmentalization of nucleolar constituents. Together these findings establish a role for Cgr1p in ribosome biogenesis.
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Affiliation(s)
- Terence I Moy
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School and Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, MA 02115, USA2
| | - Douglas Boettner
- University of Cincinnati College of Medicine, Department of Pathology & Laboratory Medicine, Cincinnati, OH 45267-0529, USA1
| | - Judith C Rhodes
- University of Cincinnati College of Medicine, Department of Pathology & Laboratory Medicine, Cincinnati, OH 45267-0529, USA1
| | - Pamela A Silver
- Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School and Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, MA 02115, USA2
| | - David S Askew
- University of Cincinnati College of Medicine, Department of Pathology & Laboratory Medicine, Cincinnati, OH 45267-0529, USA1
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18
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Lukowiak AA, Narayanan A, Li ZH, Terns RM, Terns MP. The snoRNA domain of vertebrate telomerase RNA functions to localize the RNA within the nucleus. RNA (NEW YORK, N.Y.) 2001; 7:1833-1844. [PMID: 11780638 PMCID: PMC1370221] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
Telomerase RNA is an essential component of the ribonucleoprotein enzyme involved in telomere length maintenance, a process implicated in cellular senescence and cancer. Vertebrate telomerase RNAs contain a box H/ACA snoRNA motif that is not required for telomerase activity in vitro but is essential in vivo. Using the Xenopus oocyte system, we have found that the box H/ACA motif functions in the subcellular localization of telomerase RNA. We have characterized the transport and biogenesis of telomerase RNA by injecting labeled wild-type and variant RNAs into Xenopus oocytes and assaying nucleocytoplasmic distribution, intranuclear localization, modification, and protein binding. Although yeast telomerase RNA shares characteristics of spliceosomal snRNAs, we show that human telomerase RNA is not associated with Sm proteins or efficiently imported into the nucleus. In contrast, the transport properties of vertebrate telomerase RNA resemble those of snoRNAs; telomerase RNA is retained in the nucleus and targeted to nucleoli. Furthermore, both nuclear retention and nucleolar localization depend on the box H/ACA motif. Our findings suggest that the H/ACA motif confers functional localization of vertebrate telomerase RNAs to the nucleus, the compartment where telomeres are synthesized. We have also found that telomerase RNA localizes to Cajal bodies, intranuclear structures where it is thought that assembly of various cellular RNPs takes place. Our results identify the Cajal body as a potential site of telomerase RNP biogenesis.
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Affiliation(s)
- A A Lukowiak
- Department of Biochemistry and Molecular Biology, University of Georgia, Athens 30602, USA
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19
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Kuramoto M, Ohsumi K, Kishimoto T, Ishikawa F. Identification and analyses of the Xenopus TERT gene that encodes the catalytic subunit of telomerase. Gene 2001; 277:101-10. [PMID: 11602347 DOI: 10.1016/s0378-1119(01)00684-9] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The Xenopus telomerase catalytic component gene, xTERT (Xenopus telomerase reverse transcriptase), has been cloned. The production of xTERT recombinant protein together with the proposed Xenopus telomerase RNA (xTR) (Chen et al., 2000. Cell 100, 503-514) in a rabbit reticulocyte lysate system led to the reconstitution of active telomerase, indicating that both products are functional telomerase components. Both xTERT expression and telomerase activity are high from the early to the late blastula stage. However, they are decreased at the gastrula stage and thereafter, suggesting that the xTERT expression level is the primary mechanism for regulating telomerase activity in Xenopus development. This is the first report of a non-mammalian vertebrate TERT gene. Sequence comparison of xTERT with human and mouse TERTs has uncovered four regions conserved in the amino-terminal halves of vertebrate TERT proteins, the functions of which will be discussed herein.
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Affiliation(s)
- M Kuramoto
- Laboratory of Molecular and Cellular Assembly, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
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Schaffitzel C, Berger I, Postberg J, Hanes J, Lipps HJ, Plückthun A. In vitro generated antibodies specific for telomeric guanine-quadruplex DNA react with Stylonychia lemnae macronuclei. Proc Natl Acad Sci U S A 2001; 98:8572-7. [PMID: 11438689 PMCID: PMC37477 DOI: 10.1073/pnas.141229498] [Citation(s) in RCA: 476] [Impact Index Per Article: 20.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Most eukaryotic telomeres contain a repeating motif with stretches of guanine residues that form a 3'-terminal overhang extending beyond the telomeric duplex region. The telomeric repeat of hypotrichous ciliates, d(T(4)G(4)), forms a 16-nucleotide 3'-overhang. Such sequences can adopt parallel-stranded as well as antiparallel-stranded quadruplex conformations in vitro. Although it has been proposed that guanine-quadruplex conformations may have important cellular roles including telomere function, recombination, and transcription, evidence for the existence of this DNA structure in vivo has been elusive to date. We have generated high-affinity single-chain antibody fragment (scFv) probes for the guanine-quadruplex formed by the Stylonychia telomeric repeat, by ribosome display from the Human Combinatorial Antibody Library. Of the scFvs selected, one (Sty3) had an affinity of K(d) = 125 pM for the parallel-stranded guanine-quadruplex and could discriminate with at least 1,000-fold specificity between parallel or antiparallel quadruplex conformations formed by the same sequence motif. A second scFv (Sty49) bound both the parallel and antiparallel quadruplex with similar (K(d) = 3--5 nM) affinity. Indirect immunofluorescence studies show that Sty49 reacts specifically with the macronucleus but not the micronucleus of Stylonychia lemnae. The replication band, the region where replication and telomere elongation take place, was also not stained, suggesting that the guanine-quadruplex is resolved during replication. Our results provide experimental evidence that the telomeres of Stylonychia macronuclei adopt in vivo a guanine-quadruplex structure, indicating that this structure may have an important role for telomere functioning.
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Affiliation(s)
- C Schaffitzel
- Biochemisches Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
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21
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Postberg J, Juranek SA, Feiler S, Kortwig H, Jönsson F, Lipps HJ. Association of the telomere-telomere-binding protein complex of hypotrichous ciliates with the nuclear matrix and dissociation during replication. J Cell Sci 2001; 114:1861-6. [PMID: 11329372 DOI: 10.1242/jcs.114.10.1861] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Telomeric interactions with the nuclear matrix have been described in a variety of eukaryotic cells and seem to be essential for specific nuclear localization. Macronuclear DNA of hypotrichous ciliates occurs in small gene-sized DNA molecules, each being terminated by telomeres. Each macronucleus contains over 10(8)individual DNA molecules. Owing to the high number of telomeres present in this nucleus it provides an excellent model to study telomere behaviour throughout the cell cycle. In this study we provide experimental evidence that the telomere-telomere-binding protein (TEBP) complex specifically interacts with components of the nuclear matrix in vivo. In the course of replication the specific interaction of the TEBP with components of the nuclear matrix is resolved and an attachment of the telomeres to the matrix no longer occurs.
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Affiliation(s)
- J Postberg
- Institute for Cell Biology, University Witten/Herdecke, Stockumer Str. 10, D-58448 Witten, Germany
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22
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Abstract
Telomere elongation by telomerase balances the progressive shortening of chromosome ends due to the succession of replication cycles [1] [2]. Telomerase activity is regulated in vivo at its site of action by the telomere itself. In yeast and human cells, the mean telomere length is maintained at a constant value through a cis-inhibition of telomerase by factors specifically bound to the telomeric DNA [3] [4] [5] [6] [7]. Here, we address an unexplored aspect of telomerase regulation by testing the link between telomere dynamics and cell cycle progression in the budding yeast Saccharomyces cerevisiae. We followed the elongation of an abnormally shortened telomere and observed that, like telomere shortening in the absence of telomerase, telomere elongation is linked to the succession of cell divisions. In cells progressing synchronously through the cell cycle, telomere elongation coincided with the time of telomere replication. On a minichromosome, a replication defect partially suppressed telomere elongation, suggesting a coupling between in vivo telomerase activity and conventional DNA replication.
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Affiliation(s)
- S Marcand
- CEA/Saclay, Laboratoire de Biologie Moléculaire et Cellulaire, UMR5665 CNRS/ENSL, Service de Biochimie et de Génétique Moléculaire, Ecole Normale Supérieure de Lyon, France.
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23
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Ahmed S, Hodgkin J. MRT-2 checkpoint protein is required for germline immortality and telomere replication in C. elegans. Nature 2000; 403:159-64. [PMID: 10646593 DOI: 10.1038/35003120] [Citation(s) in RCA: 215] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
The germ line is an immortal cell lineage that is passed indefinitely from one generation to the next. To identify the genes that are required for germline immortality, we isolated Caenorhabditis elegans mutants with mortal germ lines--worms that can reproduce for several healthy generations but eventually become sterile. One of these mortal germline (mrt) mutants, mrt-2, exhibits progressive telomere shortening and accumulates end-to-end chromosome fusions in later generations, indicating that the MRT-2 protein is required for telomere replication. In addition, the germ line of mrt-2 is hypersensitive to X-rays and to transposon activity. Therefore, mrt-2 has defects in responding both to damaged DNA and to normal double-strand breaks present at telomeres. mrt-2 encodes a homologue of a checkpoint gene that is required to sense DNA damage in yeast. These results indicate that telomeres may be identified as a type of DNA damage and then repaired by the telomere-replication enzyme telomerase.
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Affiliation(s)
- S Ahmed
- MRC Laboratory of Molecular Biology, Cambridge, UK.
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Dionne I, Wellinger RJ. Processing of telomeric DNA ends requires the passage of a replication fork. Nucleic Acids Res 1998; 26:5365-71. [PMID: 9826760 PMCID: PMC148004 DOI: 10.1093/nar/26.23.5365] [Citation(s) in RCA: 71] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
During telomere replication in yeast, chromosome ends acquire a long single-stranded extension of the strand making the 3' end. Previous work showed that these 3' tails are generated late in S-phase, when conventional replication is virtually complete. In addition, the extensions were also observed in cells that lacked telomerase. Therefore, a model was proposed that predicted an activity that recessed the 5' ends at yeast telomeres after conventional replication was complete. Here, we demonstrate that this processing activity is dependent on the passage of a replication fork through yeast telomeres. A non-replicating linear plasmid with telomeres at each end does not acquire single-stranded extensions, while an identical construct containing an origin of replication does. Thus, the processing activity could be associated with the enzymes at the replication fork itself, or the passage of the fork through the telomeric sequences allows a transient access for the activity to the telomeres. We therefore propose that there is a mechanistic link between the conventional replication machinery and telomere maintenance.
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Affiliation(s)
- I Dionne
- Département de Microbiologie et Infectiologie, Faculté de Médecine, Université de Sherbrooke, 3001 12ème Avenue Nord, Sherbrooke, Québec J1H 5N4, Canada
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25
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Abstract
The nucleolus of eukaryotic cells was first described in the early 19th century and was discovered in the 1960s to be the seat of ribosome synthesis. Although rRNA transcription, rRNA processing and ribosome assembly have been clearly established as major functions of the nucleolus, recent studies suggest that the nucleolus participates in many other aspects of gene expression as well. Thus, the nucleolus has been implicated in the processing or nuclear export of certain mRNAs. In addition, new results indicate that biosyntheses of signal recognition particle RNA and telomerase RNA involve a nucleolar stage and that the nucleolus is also involved in processing of U6 RNA, one of the spliceosomal small nuclear RNAs. Interestingly, these three nucleolus-associated small nuclear RNAs (signal recognition particle RNA, telomerase RNA and U6 RNA) are components of catalytic ribonucleoprotein machines. Finally, recent work has also suggested that some transfer RNA precursors are processed in the nucleolus. The nucleolus may have evolutionarily descended from a proto-eukaryotic minimal genome that was spatially linked to vicinal RNA processing and ribonucleoprotein assembly events involved in gene read-out. The nucleolus of today's eukaryotes, now surrounded by the chromatin of over 2 billion years of genome expansion, may still perform these ancient functions, in addition to ribosome biosynthesis. The plurifunctional nucleolus concept has a strong footing in contemporary data and adds a new perspective to our current picture of the spatial-functional design of the cell nucleus.
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Affiliation(s)
- T Pederson
- Worcester Foundation for Biomedical Research and Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, Worcester Foundation Campus, 222 Maple Avenue, Shrewsbury, MA 01545, USA.
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26
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Froelich-Ammon SJ, Dickinson BA, Bevilacqua JM, Schultz SC, Cech TR. Modulation of telomerase activity by telomere DNA-binding proteins in Oxytricha. Genes Dev 1998; 12:1504-14. [PMID: 9585510 PMCID: PMC316840 DOI: 10.1101/gad.12.10.1504] [Citation(s) in RCA: 65] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/1998] [Accepted: 03/10/1998] [Indexed: 02/07/2023]
Abstract
Telomere proteins protect the chromosomal terminus from nucleolytic degradation and end-to-end fusion, and they may contribute to telomere length control and the regulation of telomerase. The current studies investigate the effect of Oxytricha single-stranded telomere DNA-binding protein subunits alpha and beta on telomerase elongation of telomeric DNA. A native agarose gel system was used to evaluate telomere DNA-binding protein complex composition, and the ability of telomerase to use these complexes as substrates was characterized. Efficient elongation occurred in the presence of the alpha subunit. Moreover, the alpha-DNA cross-linked complex was a substrate for telomerase. At higher alpha concentrations, two alpha subunits bound to the 16-nucleotide single-stranded DNA substrate and rendered it inaccessible to telomerase. The formation of this alpha . DNA . alpha complex may contribute to regulation of telomere length. The alpha . beta . DNA ternary complex was not a substrate for telomerase. Even when telomerase was prebound to telomeric DNA, the addition of alpha and beta inhibited elongation, suggesting that these telomere protein subunits have a greater affinity for the DNA and are able to displace telomerase. In addition, the ternary complex was not a substrate for terminal deoxynucleotidyltransferase. We conclude that the telomere protein inhibits telomerase by rendering the telomeric DNA inaccessible, thereby helping to maintain telomere length.
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Affiliation(s)
- S J Froelich-Ammon
- Department of Chemistry and Biochemistry and Howard Hughes Medical Institute, University of Colorado, Boulder, Colorado 80309-0215, USA.
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27
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Cech TR, Lingner J. Telomerase and the chromosome end replication problem. CIBA FOUNDATION SYMPOSIUM 1998; 211:20-8; discussion 28-34. [PMID: 9524749 DOI: 10.1002/9780470515433.ch3] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Telomerase, the enzyme that extends chromosomal DNA ends in most eukaryotes, contains essential RNA and protein subunits. We have been studying telomere replication in hypotrichous ciliates such as Euplotes aediculatus, which have numerous short macronuclear DNA molecules and therefore are highly enriched in telomeres and in telomerase. Cloning and sequencing genes for the RNA subunits from several ciliates revealed that telomerase RNAs with insignificant nucleotide sequence homology nevertheless form a common secondary structure. Affinity chromatography based on the sequence of the RNA subunit was used to purify the Euplotes telomerase as an active ribonucleoprotein enzyme. Two protein subunits, 123 kDa and 43 kDa, were identified. The finding of a yeast homologue to the 123 kDa subunit suggests that telomerase protein components may be much more highly conserved in evolution than the RNA subunits. The purified Euplotes telomerase has no activity with blunt-ended DNA primers, but instead requires a four to six nucleotide single-stranded 3' tail. This result supports a model for telomere replication in which other activities such as helicases or nucleases activate replicated DNA for extension by telomerase, a model that may be applicable to telomere replication in diverse eukaryotes.
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Affiliation(s)
- T R Cech
- Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado, Boulder 80309-0215, USA
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28
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Hammond PW, Lively TN, Cech TR. The anchor site of telomerase from Euplotes aediculatus revealed by photo-cross-linking to single- and double-stranded DNA primers. Mol Cell Biol 1997; 17:296-308. [PMID: 8972210 PMCID: PMC231754 DOI: 10.1128/mcb.17.1.296] [Citation(s) in RCA: 82] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Telomerase is a ribonucleoprotein enzyme that adds telomeric sequence repeats to the ends of linear chromosomes. In vitro, telomerase has been observed to add repeats to a DNA oligonucleotide primer in a processive manner, leading to the postulation of a DNA anchor site separate from the catalytic site of the enzyme. We have substituted photoreactive 5-iododeoxypyrimidines into the DNA oligonucleotide primer d(T4G4T4G4T4G2) and, upon irradiation, obtained cross-links with the anchor site of telomerase from Euplotes aediculatus nuclear extract. No cross-linking occurred with a primer having the same 5' end and a nontelomeric 3' end. These cross-links were shown to be between the DNA primer and (i) a protein moiety of approximately 130 kDa and (ii) U51-U52 of the telomerase RNA. The cross-linked primer could be extended by telomerase in the presence of [alpha-32P]dGTP, thus indicating that the 3' end was bound in the enzyme active site. The locations of the cross-links within the single-stranded primers were 20 to 22 nucleotides upstream of the 3' end, providing a measure of the length of DNA required to span the telomerase active and anchor sites. When the single-stranded primers are aligned with the G-rich strand of a Euplotes telomere, the cross-linked nucleotides correspond to the duplex region. Consistent with this finding, a cross-link to telomerase was obtained by substitution of 5-iododeoxycytidine into the CA strand of the duplex region of telomere analogs. We conclude that the anchor site in the approximately 130-kDa protein can bind duplex as well as single-stranded DNA, which may be critical for its function at chromosome ends. Quantitation of the processivity with single-stranded DNA primers and double-stranded primers with 3' tails showed that only 60% of the primer remains bound after each repeat addition.
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Affiliation(s)
- P W Hammond
- Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado, Boulder 80309-0215, USA
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29
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Lingner J, Cech TR. Purification of telomerase from Euplotes aediculatus: requirement of a primer 3' overhang. Proc Natl Acad Sci U S A 1996; 93:10712-7. [PMID: 8855245 PMCID: PMC38220 DOI: 10.1073/pnas.93.20.10712] [Citation(s) in RCA: 202] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Telomerase is a ribonucleoprotein enzyme that uses its internal RNA moiety as a template for synthesis of telomeric repeats at chromosome ends. Here we report the purification of telomerase from Euplotes aediculatus by affinity chromatography with antisense 2'-O-methyl oligonucleotides, a method that was developed for small nuclear ribonucleoprotein particles (snRNPs). Elution of bound ribonucleoprotein from the antisense oligonucleotide under nondenaturing conditions was achieved by a novel approach, using a displacement oligonucleotide. Polypeptides of 120 kDa and 43 kDa (a doublet) copurify with the active telomerase and appear stoichiometric with telomerase RNA. A simple model for DNA end replication predicts that after semiconservative DNA replication, telomerase will extend the newly synthesized, blunt-ended leading strand. We show that purified Euplotes telomerase has no activity with blunt-ended primers. Instead, efficient extension requires 4 to 6 single-stranded nucleotides at the 3' end. Therefore, this model predicts the existence of other activities such as helicases or nucleases that generate a single-stranded 3' end from a blunt end, thus activating the end for telomerase extension.
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Affiliation(s)
- J Lingner
- Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado 80309-0215, USA
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30
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Skopp R, Wang W, Price C. rTP: a candidate telomere protein that is associated with DNA replication. Chromosoma 1996; 105:82-91. [PMID: 8753697 DOI: 10.1007/bf02509517] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
In this paper we describe the isolation and characterization of rTP, the replication Telomere Protein, formerly known as the telomere protein homolog. The rTP was initially identified because of its homology to the gene for the Oxytricha telomere-binding protein alpha-subunit. The protein encoded by the rTP gene has extensive amino acid sequence identity to the DNA-binding domain of the telomere-binding proteins from both Euplotes crassus and Oxytricha nova. We have now identified the protein encoded by the rTP gene and have shown that it differs from the telomere-binding protein in its abundance, solubility and intracellular location. To learn more about the function of rTP, we determined when during the Euplotes life cycle the gene is transcribed. The transcript was detectable only in nonstarved vegetative cells and during the final stages of macronuclear development. Since the peak transcript level coincided with the rounds of replication that take place toward the end of macronuclear development, it appeared that rTP might be involved in DNA replication. Immunolocalization experiments provided support for this hypothesis as antibodies to rTP specifically stain the replication bands. Replication bands are the sites of DNA replication in Euplotes macronuclei. Our results suggest that rTP may be a new telomere replication factor.
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Affiliation(s)
- R Skopp
- Department of Chemistry, University of Nebraska, Lincoln, NE 68588, USA
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31
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Abstract
The study of eukaryotic telomeres at the molecular level began with the discovery of short, tandem repeats at Tetrahymena chromosome ends. In the following two decades, major insights about telomere structure and function have come from investigations of telomerase, the DNA polymerase that synthesizes these repeats. In the past year, three areas of telomerase research have been particularly intense: assays of telomerase activity, isolation of telomerase components, and studies of the regulation of telomerase and telomere length in vivo.
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Affiliation(s)
- K Collins
- Department of Molecular and Cell Biology, University of California, Berkeley 94720-3204, USA.
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