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Abstract
The leptotene/zygotene transition of meiosis, as defined by classical cytological studies, is the period when homologous chromosomes, already being discernible individualized entities, begin to be close together or touching over portions of their lengths. This period also includes the bouquet stage: Chromosome ends, which have already become integral components of the inner nuclear membrane, move into a polarized configuration, along with other nuclear envelope components. Chromosome movements, active or passive, also occur. The detailed nature of interhomologue interactions during this period, with special emphasis on the involvement of chromosome ends, and the overall role for meiosis and recombination of chromosome movement and, especially, the bouquet stage are discussed.
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Affiliation(s)
- D Zickler
- Institut de Génétique et Microbiologie, Université Paris-Sud, Orsay, France.
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3
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Abstract
Telomeres of most investigated species terminate with short repeats and are elongated by telomerase. Short repeats have never been detected in dipteran species which have found other solutions to end a chromosome. Whereas in Drosophila melanogaster retroelements are added onto the termini, chironomids have long complex repeats at their chromosome ends. We review evidence that these units are terminal and probably have evolved from short telomeric repeats. In Chironomus pallidivittatus the units have been shown to belong to different subfamilies which have specific inter- and intrachromosomal distribution, the most terminal subfamily of repeats being characterized by pronounced secondary structures for the single strand. The complex repeats are efficiently homogenized both within and between different chromosome ends. Gene conversion is probably an important component in the coordinate evolution of the repeats but it is not known whether it is used for net synthesis of DNA. RNA is used as an intermediate in telomere elongation both by organisms having chromosomes terminating with short repeats and by D. melanogaster. It is therefore interesting that the terminal repeats in chironomids are transcribed.
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Affiliation(s)
- I Kamnert
- Department of Genetics, University of Lund, Sweden
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4
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Zhimulev IF. Polytene chromosomes, heterochromatin, and position effect variegation. ADVANCES IN GENETICS 1997; 37:1-566. [PMID: 9352629 DOI: 10.1016/s0065-2660(08)60341-7] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- I F Zhimulev
- Institute of Cytology and Genetics, Siberian Division of the Russian Academy of Sciences, Novosibirsk, Russia
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5
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Roth CW, Kobeski F, Walter MF, Biessmann H. Chromosome end elongation by recombination in the mosquito Anopheles gambiae. Mol Cell Biol 1997; 17:5176-83. [PMID: 9271395 PMCID: PMC232368 DOI: 10.1128/mcb.17.9.5176] [Citation(s) in RCA: 70] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
One of the functions of telomeres is to counteract the terminal nucleotide loss associated with DNA replication. While the vast majority of eukaryotic organisms maintain their chromosome ends via telomerase, an enzyme system that generates short, tandem repeats on the ends of chromosomes, other mechanisms such as the transposition of retrotransposons or recombination can also be used in some species. Chromosome end regression and extension were studied in a medically important mosquito, the malaria vector Anopheles gambiae, to determine how this dipteran insect maintains its chromosome ends. The insertion of a transgenic pUChsneo plasmid at the left end of chromosome 2 provided a unique marker for measuring the dynamics of the 2L telomere over a period of about 3 years. The terminal length was relatively uniform in the 1993 population with the chromosomes ending within the white gene sequence of the inserted transgene. Cloned terminal chromosome fragments did not end in short repeat sequences that could have been synthesized by telomerase. By late 1995, the chromosome ends had become heterogeneous: some had further shortened while other chromosomes had been elongated by regenerating part of the integrated pUChsneo plasmid. A model is presented for extension of the 2L chromosome by recombination between homologous 2L chromosome ends by using the partial plasmid duplication generated during its original integration. It is postulated that this mechanism is also important in wild-type telomere elongation.
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Affiliation(s)
- C W Roth
- Developmental Biology Center, University of California, Irvine 92697, USA
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6
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Kirk KE, Blackburn EH. An unusual sequence arrangement in the telomeres of the germ-line micronucleus in Tetrahymena thermophila. Genes Dev 1995; 9:59-71. [PMID: 7828852 DOI: 10.1101/gad.9.1.59] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The ciliated protozoan Tetrahymena thermophila contains two nuclei that differ dramatically in function, chromosome size and number, chromatin structure, and mode of division. It is possible that the telomeres of the two nuclei have different functions. Although macronuclear telomeric DNA has been well characterized and consists of tandem G4T2/C4A2 repeats that are synthesized by the enzyme telomerase, micronuclear telomeres have not been isolated previously. Here, we report the identification and cloning of micronuclear telomeres and demonstrate that although they contain the same terminal tandem G4T2 repeats as macronuclear telomeres, they are strikingly different in three respects. First, the tracts of G/C-rich telomeric repeats are approximately seven times longer in the micronucleus than in the macronucleus (approximately 2.0-3.4 vs. approximately 0.3-0.5 kb, respectively) from the same cell population. Second, the immediate telomere-associated sequences (TASs) from six different micronuclear chromosome ends have an unusually high G/C content and degree of homology to one another, unlike macronuclear TASs. The TAS from at least one micronuclear chromosome is unique to micronuclear telomeres and is not present in the macronucleus. Finally, and unexpectedly, all micronuclear telomere clones contain an inner homogeneous tract of a variant G4T3 repeat adjacent to the distal tract of G4T2 repeats. The native micronuclear telomeric DNA is composed of approximately 30% G4T3 repeats, corresponding to 0.6-1.0 kb per average telomere, positioned centromere-proximally to most or all of the G4T2 repeats. Neither the G4T3 sequence nor any other variant repeat is found in macronuclear telomeres. Furthermore, such a homogeneous tract of a variant repeat has not been found in the telomeres of any eukaryote.
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Affiliation(s)
- K E Kirk
- Department of Microbiology, University of California at San Francisco 94122
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7
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Fang G, Gray JT, Cech TR. Oxytricha telomere-binding protein: separable DNA-binding and dimerization domains of the alpha-subunit. Genes Dev 1993; 7:870-82. [PMID: 8491383 DOI: 10.1101/gad.7.5.870] [Citation(s) in RCA: 57] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
A telomere-binding protein heterodimer of 56-kD (alpha) and 41-kD (beta) subunits binds to the single-stranded (T4G4)2 terminus of each Oxytricha nova macronuclear DNA molecule. The alpha-subunit by itself binds to telomeric DNA. The beta-subunit alone does not bind to DNA specifically but interacts with the alpha-subunit to form a very stable ternary complex. We show that the formation of alpha-beta-DNA ternary complex is extremely cooperative. Furthermore, the binary complex (alpha-DNA) has a dissociation half-life of much less than 1 min; addition of the beta-subunit increases the half-life to approximately 100 hrs. Libraries of plasmids with random deletions of the open reading frame for the alpha-subunit were introduced into Escherichia coli, and extracts were subsequently checked for both protein expression and DNA-binding activity with or without added beta-subunit. The alpha-subunit was found to contain two structurally separable domains with distinct functions. The amino-terminal two-thirds is necessary and sufficient for sequence-specific DNA binding. The carboxy-terminal one-third is responsible for alpha/beta-subunit interactions. When expressed separately in E. coli, purified, and mixed together, these two domains reconstitute the activity of the wild-type alpha-subunit (trans-complementation in vitro). The amino-terminal two-thirds of the beta-subunit is necessary and sufficient both for alpha/beta-subunit interactions and for ternary complex formation. We conclude that the alpha-subunit of the telomere-binding protein, like many transcription factors, has separable DNA-binding and protein-protein interaction domains.
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Affiliation(s)
- G Fang
- Department of Chemistry and Biochemistry, Howard Hughes Medical Institute, University of Colorado, Boulder 80309-0215
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8
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Runge KW, Zakian VA. Saccharomyces cerevisiae linear chromosome stability (lcs) mutants increase the loss rate of artificial and natural linear chromosomes. Chromosoma 1993; 102:207-17. [PMID: 8458255 DOI: 10.1007/bf00387735] [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: 01/30/2023]
Abstract
We isolated mutants of Saccharomyces cerevisiae that lose a 100 kb linear yeast artificial chromosome (YAC) at elevated rates. Mutations in two of these LCS (linear chromosome stability) genes had little or no effect on the loss rate of a circular YAC that had the same centromere and origin of replication as present on the linear YAC. Moreover, mutations in these LCS genes also increased the loss rate of an authentic linear yeast chromosome, chromosome III, but had only small effects on the loss rate of a circular derivative of chromosome III. As these mutants preferentially destabilize linear chromosomes, they may affect chromosome stability through interactions at telomeres. Telomeres are thought to be essential for the protection and complete replication of chromosome ends. The cytological properties of telomeres suggest that these structures may play additional roles in chromosome function. The lengths of the terminal C1-3A repeats at the ends of yeast chromosomes were unaltered in the linear preferential lcs mutants, suggesting that these mutants do not affect the replication or protection of telomeric DNA. Thus, the linear-preferential lcs mutants may identify a role for telomeres in chromosome stability that is distinct from their function in the replication and protection of chromosomal termini.
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Affiliation(s)
- K W Runge
- Fred Hutchinson Cancer Research Center, Seattle, WA 98104
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9
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Hiraoka Y, Dernburg AF, Parmelee SJ, Rykowski MC, Agard DA, Sedat JW. The onset of homologous chromosome pairing during Drosophila melanogaster embryogenesis. J Biophys Biochem Cytol 1993; 120:591-600. [PMID: 8425892 PMCID: PMC2119536 DOI: 10.1083/jcb.120.3.591] [Citation(s) in RCA: 154] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
We have determined the position within the nucleus of homologous sites of the histone gene cluster in Drosophila melanogaster using in situ hybridization and high-resolution, three-dimensional wide field fluorescence microscopy. A 4.8-kb biotinylated probe for the histone gene repeat, located approximately midway along the short arm of chromosome 2, was hybridized to whole-mount embryos in late syncytial and early cellular blastoderm stages. Our results show that the two homologous histone loci are distinct and separate through all stages of the cell cycle up to nuclear cycle 13. By dramatic contrast, the two homologous clusters were found to colocalize with high frequency during interphase of cycle 14. Concomitant with homolog pairing at cycle 14, both histone loci were also found to move from their position near the midline of the nucleus toward the apical side. This result suggests that coincident with the initiation of zygotic transcription, there is dramatic chromosome and nuclear reorganization between nuclear cycles 13 and 14.
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Affiliation(s)
- Y Hiraoka
- Department of Biochemistry and Biophysics, University of California, San Francisco 94143-0554
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10
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Affiliation(s)
- H Biessmann
- Developmental Biology Center, University of California, Irvine 92717
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11
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Gray JT, Celander DW, Price CM, Cech TR. Cloning and expression of genes for the Oxytricha telomere-binding protein: specific subunit interactions in the telomeric complex. Cell 1991; 67:807-14. [PMID: 1840510 DOI: 10.1016/0092-8674(91)90075-a] [Citation(s) in RCA: 128] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Telomeres of Oxytricha nova macronuclear chromosomes consist of a repeated T4G4 sequence, single-stranded at the 3' terminus, bound by a heterodimeric protein. The cloning of genes for the two polypeptides and their separate expression in E. coli have enabled evaluation of their individual contributions to DNA binding. The 56 kd alpha subunit binds single-stranded DNA by itself, one polypeptide per T4G4 block; multiple subunits can coat a (T4G4)n multimer. The derived amino acid sequence of alpha does not reveal any known DNA-binding motif, so it appears to represent a novel type of DNA-binding protein. The previously cloned 41 kd beta subunit does not by itself protect DNA from methylation, but is required along with alpha to recreate the pattern of methylation protection indicative of telomeres in vivo. The unusual ability of the protein to engage in two different interactions with the same telomeric DNA sequence might provide the versatility necessary for diverse telomere functions.
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Affiliation(s)
- J T Gray
- Department of Chemistry and Biochemistry, Howard Hughes Medical Institute, University of Colorado, Boulder 80309
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12
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Kojis TL, Gatti RA, Sparkes RS. The cytogenetics of ataxia telangiectasia. CANCER GENETICS AND CYTOGENETICS 1991; 56:143-56. [PMID: 1756458 DOI: 10.1016/0165-4608(91)90164-p] [Citation(s) in RCA: 104] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Ataxia-telangiectasia (AT) is a heterogeneous autosomal recessive disorder marked by cerebellar ataxia, oculocutaneous telangiectases, hypersensitivity to ionizing radiation, immunodeficiency, and cancer susceptibility. AT is also a spontaneous chromosomal breakage syndrome, notable for tissue-specific cytogenetic changes and telomeric fusions. Molecular characterization of rearrangements specific to T-lymphocytes suggests that a DNA repair/processing defect is potentially responsible for the diverse array of chromosomal abnormalities observed in a variety of AT cell types.
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Affiliation(s)
- T L Kojis
- Jules Stein Eye Institute, Los Angeles, CA 90024-7008
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13
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Hiraoka Y, Agard DA, Sedat JW. Temporal and spatial coordination of chromosome movement, spindle formation, and nuclear envelope breakdown during prometaphase in Drosophila melanogaster embryos. J Cell Biol 1990; 111:2815-28. [PMID: 2125300 PMCID: PMC2116368 DOI: 10.1083/jcb.111.6.2815] [Citation(s) in RCA: 81] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022] Open
Abstract
The spatial and temporal dynamics of diploid chromosome organization, microtubule arrangement, and the state of the nuclear envelope have been analyzed in syncytial blastoderm embryos of Drosophila melanogaster during the transition from prophase to metaphase, by three-dimensional optical sectioning microscopy. Time-lapse, three-dimensional data recorded in living embryos revealed that congression of chromosomes (the process whereby chromosomes move to form the metaphase plate) at prometaphase occurs as a wave, starting at the top of the nucleus near the embryo surface and proceeding through the nucleus to the bottom. The time-lapse analysis was augmented by a high-resolution analysis of fixed embryos where it was possible to unambiguously trace the three-dimensional paths of individual chromosomes. In prophase, the centromeres were found to be clustered at the top of the nucleus while the telomeres were situated at the bottom of the nucleus or towards the embryo interior. This polarized centromere-telomere orientation, perpendicular to the embryo surface, was preserved during the process of prometaphase chromosome congression. Correspondingly, breakdown of the nuclear envelope started at the top of the nucleus with the mitotic spindle being formed at the positions of the partial breakdown of the nuclear envelope. Our observation provide an example in which nuclear structures are spatially organized and their functions are locally and coordinately controlled in three dimensions.
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Affiliation(s)
- Y Hiraoka
- Department of Biochemistry and Biophysics, University of California, San Francisco 94143-0554
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14
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Brown WR, MacKinnon PJ, Villasanté A, Spurr N, Buckle VJ, Dobson MJ. Structure and polymorphism of human telomere-associated DNA. Cell 1990; 63:119-32. [PMID: 2208276 DOI: 10.1016/0092-8674(90)90293-n] [Citation(s) in RCA: 248] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
We have analyzed the DNA sequences associated with four different human telomeres. Two are members of distinct repeated sequence families which are located mainly but not exclusively at telomeres. Two are unique in the genome, one deriving from the long arm telomere of chromosome 7 and the other from the pseudoautosomal telomere. One telomere-associated repeated sequence has a polymorphic distribution among the chromosome ends, being present at a different combination of ends in different individuals. These data thus identify a new source of human genetic variation and indicate that the canonical features of the organization of telomere-associated DNA are widely conserved in evolution.
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Affiliation(s)
- W R Brown
- Biochemistry Department, Oxford University, England
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15
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Abstract
Acentric extrachromosomal elements, such as submicroscopic autonomously replicating circular molecules (episomes) and double minute chromosomes, are common early, and in some cases initial, intermediates of gene amplification in many drug-resistant and tumor cell lines. In order to gain a more complete understanding of the amplification process, we investigated the molecular mechanisms by which such extrachromosomal elements are generated and we traced the fate of these amplification intermediates over time. The model system consists of a Chinese hamster cell line (L46) created by gene transfer in which the initial amplification product was shown previously to be an unstable extrachromosomal element containing an inverted duplication spanning more than 160 kilobases (J. C. Ruiz and G. M. Wahl, Mol. Cell. Biol. 8:4302-4313, 1988). In this study, we show that these molecules were formed by a process involving chromosomal deletion. Fluorescence in situ hybridization was performed at multiple time points on cells with amplified sequences. These studies reveal that the extrachromosomal molecules rapidly integrate into chromosomes, often near or at telomeres, and once integrated, the amplified sequences are themselves unstable. These data provide a molecular and cytogenetic chronology for gene amplification in this model system; an early event involves deletion to generate extrachromosomal elements, and subsequent integration of these elements precipitates a cascade of chromosome instability.
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16
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Abstract
Acentric extrachromosomal elements, such as submicroscopic autonomously replicating circular molecules (episomes) and double minute chromosomes, are common early, and in some cases initial, intermediates of gene amplification in many drug-resistant and tumor cell lines. In order to gain a more complete understanding of the amplification process, we investigated the molecular mechanisms by which such extrachromosomal elements are generated and we traced the fate of these amplification intermediates over time. The model system consists of a Chinese hamster cell line (L46) created by gene transfer in which the initial amplification product was shown previously to be an unstable extrachromosomal element containing an inverted duplication spanning more than 160 kilobases (J. C. Ruiz and G. M. Wahl, Mol. Cell. Biol. 8:4302-4313, 1988). In this study, we show that these molecules were formed by a process involving chromosomal deletion. Fluorescence in situ hybridization was performed at multiple time points on cells with amplified sequences. These studies reveal that the extrachromosomal molecules rapidly integrate into chromosomes, often near or at telomeres, and once integrated, the amplified sequences are themselves unstable. These data provide a molecular and cytogenetic chronology for gene amplification in this model system; an early event involves deletion to generate extrachromosomal elements, and subsequent integration of these elements precipitates a cascade of chromosome instability.
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Affiliation(s)
- J C Ruiz
- Gene Expression Laboratory, Salk Institute, La Jolla, California 92037
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17
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Biessmann H, Mason JM, Ferry K, d'Hulst M, Valgeirsdottir K, Traverse KL, Pardue ML. Addition of telomere-associated HeT DNA sequences "heals" broken chromosome ends in Drosophila. Cell 1990; 61:663-73. [PMID: 2111731 DOI: 10.1016/0092-8674(90)90478-w] [Citation(s) in RCA: 203] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Stocks of D. melanogaster X chromosomes carrying terminal deletions (RT chromosomes) have been maintained for several years. Some of the chromosomes are slowly losing DNA from the broken ends (as expected if replication is incomplete) and show no telomere-associated DNA added to the receding ends. Two stocks carry chromosomes that have become "healed" and are no longer losing DNA. In both stocks the broken chromosome end has acquired a segment of HeT DNA, a family of complex repeats found only at telomeres and in pericentric heterochromatin. Although the HeT family is complex, the HeT sequence joined to the broken chromosome end is the same in both stocks. In contrast, the two chromosomes are broken in different places and have no detectable sequence similarity at the junction with the new DNA. Sequence analysis suggests that the new telomere sequences have been added by a specific mechanism that does not involve homologous recombination.
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Affiliation(s)
- H Biessmann
- Developmental Biology Center, University of California, Irvine 92717
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18
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Hicke BJ, Celander DW, MacDonald GH, Price CM, Cech TR. Two versions of the gene encoding the 41-kilodalton subunit of the telomere binding protein of Oxytricha nova. Proc Natl Acad Sci U S A 1990; 87:1481-5. [PMID: 1689486 PMCID: PMC53499 DOI: 10.1073/pnas.87.4.1481] [Citation(s) in RCA: 55] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
Macronuclear chromosomes of the ciliated protozoan Oxytricha nova terminate with a single-stranded (T4G4)2 overhang. The (T4G4)2 telomeric overhang is tenaciously bound by a protein heterodimer. We have cloned and sequenced the gene encoding the 41-kDa subunit of this telomere binding protein. The predicted amino acid sequence comprises two distinct regions, a carboxyl-terminal two-thirds that is 23% lysine and bears similarity to histone H1 and an amino-terminal one-third containing a hydrophobic stretch of about 15 amino acids. Two macronuclear versions of the gene differ in nucleotide sequence at several positions, but the derived polypeptides differ only at a single position, Ser-110 or Ala-110. Both versions harbor a small intron. The existence of this intron demonstrates that, despite the elimination of 95% of the micronuclear genome from the developing macronucleus, at least some noncoding DNA is retained during macronuclear development of hypotrichous ciliates.
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Affiliation(s)
- B J Hicke
- Department of Molecular, Cellular and Developmental Biology, Howard Hughes Medical Institute, University of Colorado, Boulder 80309
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19
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Abstract
Two types of specific telomeric protein-DNA complex are reconstituted upon incubation of purified Oxytricha telomere protein with (T4G4)4, an oligodeoxynucleotide of telomeric sequence. The complexes differ in electrophoretic mobility, in protein-DNA contacts, and in the rate of DNA exchange. The patterns of protein-DNA interaction determined by modification interference suggest a model in which the protein can bind either to the two T4G4 repeats at the 3' end or to two internal repeats; in the latter case, it can make a different set of contacts with the terminal repeat to form the more stable complex. Native telomeric chromatin isolated from Oxytricha contains both types of complexes. The reconstituted monomeric complexes associate to give a high molecular weight form that has an altered chemical footprint. Such interactions may mediate the association of chromosomal telomeres in vivo.
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Affiliation(s)
- M K Raghuraman
- Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder 80309
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20
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Morin GB. The human telomere terminal transferase enzyme is a ribonucleoprotein that synthesizes TTAGGG repeats. Cell 1989; 59:521-9. [PMID: 2805070 DOI: 10.1016/0092-8674(89)90035-4] [Citation(s) in RCA: 1100] [Impact Index Per Article: 31.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
I have identified an activity in crude HeLa cell extracts that satisfies the requirements for a human telomere terminal transferase or telomerase. It catalyzes the addition of a 6 nucleotide repeating pattern to oligonucleotide primers containing human or nonhuman telomeric repeat sequences. Direct sequence analyses of reaction products reveal the added sequence to be TTAGGG in all cases. Under optimal conditions 65-70 repeats can be synthesized. The enzyme has the properties of a ribonucleoprotein. Telomerase has previously been observed only in ciliated protozoans, which possess 10(4) - 10(7) macronuclear telomeres. The identification of telomerase in HeLa cells with only approximately 100 telomeres indicates that telomerase-mediated telomere maintenance is conserved throughout eukaryotes.
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Affiliation(s)
- G B Morin
- Department of Molecular Biophysics and Biochemistry, Howard Hughes Medical Institute, Yale University, New Haven, Connecticut 06510
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21
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Taylor AM, Metcalfe JA, McConville C. Increased radiosensitivity and the basic defect in ataxia telangiectasia. Int J Radiat Biol 1989; 56:677-84. [PMID: 2573663 DOI: 10.1080/09553008914551901] [Citation(s) in RCA: 32] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Various cellular defects have been found in ataxia telangiectasia (A-T) cells including increased radiosensitivity, increased sensitivity to various chemical agents, a probable DNA repair defect and a defect in DNA synthesis. How these different features are related to each other is at present unknown. It has been suggested that there is a defect in A-T that acts in tissue differentiation as well as during growth and in the mature adult. This hypothesis is supported by the observations, for example, of an immature thymus present in patients, the production of alpha-fetoprotein, which results in a high serum level, and ovarian dysgenesis. A gene for A-T has recently been localized to chromosome region 11q22-23, a site involved in chromosomes translocations in some non-lymphoid leukaemias. At the chromosomal level the spontaneous abnormalities in A-T include, first, an increased frequency of cells showing chromosome translocations involving immune system genes that normally undergo rearrangement to form a functional product; secondly, the formation of telometric dicentrics in both lymphocytes and fibroblasts; and thirdly formation of long-lived chromosome damage following exposure to ionizing radiation and radiomimetic drugs. The gene defect underlying this disorder is unknown and distinguishing between primary and secondary effects of the mutant gene is difficult. We consider alternative models for retention of translocation T cells. First, it is possible that there is a defect in recognition of site-specific damage leading to retention of translocation cells that might otherwise be removed. Secondly, a feature common to the production of illegitimate T-cell receptor gene rearrangements and to formation of telomeric dicentric chromosomes in A-T cells is an increased period of time available for chromosome interchange, possibly due to a site-specific defect in strand break repair. It is possible that this defect may also prevent chromosome restitution following exposure of cells to ionizing radiation.
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Affiliation(s)
- A M Taylor
- Department of Cancer Studies, Medical School, University of Birmingham, U.K
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22
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Abstract
A method is described that allows cloning of human telomeres in S. cerevisiae by joining human telomeric restriction fragments to yeast artificial chromosome halves. The resulting chimeric yeast-human chromosomes propagate as true linear chromosomes, demonstrating that the human telomere structure is capable of functioning in yeast and suggesting that telomere functions are evolutionarily conserved between yeast and human. One cloned human telomere, yHT1, contains 4 kb of human genomic DNA sequence next to the tandemly repeating TTAGGG hexanucleotide. Genomic hybridizations using both cloned DNA and TTAGGG repeats have revealed a common structural organization of human telomeres. This 4 kb of genomic DNA sequence is present in most, but not all, human telomeres, suggesting that the region is not involved in crucial chromosome-specific functions. However, the extent of common features among the human telomeres and possible similarities in organization with yeast telomeres suggest that this region may play a role in general chromosome behavior such as telomere-telomere interactions. Unlike the simple telomeric TTAGGG repeats, our cloned human genomic DNA sequence does not cross-hybridize with rodent DNA. Thus, this clone allows the identifications of the terminal restriction fragments of specific human chromosomes in human-rodent hybrid cells.
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Affiliation(s)
- J F Cheng
- Department of Genetics, College of Physicians and Surgeons, Columbia University, New York, NY 10032
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23
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Petkovic I, Agbor P, Hittelman WN. Simultaneous cell type identification and premature chromosome condensation analysis in a case of multiple myeloma. CANCER GENETICS AND CYTOGENETICS 1989; 39:203-17. [PMID: 2502306 DOI: 10.1016/0165-4608(89)90187-8] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
The technique of premature chromosome condensation was combined with immunocytochemical techniques to determine the karyotype of the plasma cells in a patient with multiple myeloma. Although the patient's myeloma cells had a diploid DNA content, the mean chromosome number was 39. Multiple chromosome rearrangements were documented in the G- and C-banded G1 and G2 prematurely condensed chromosomes, and several of these involved telomeric regions resulting in dicentric chromosomes. That the aberrant karyotype was present in the kappa light chain positive plasma cells was proved by simultaneous chromosome analysis and immunocytochemical examination of the fused cells. Thus the combination of premature chromosome condensation and immunocytochemistry proved a powerful tool for cytogenetic analysis of a slow-growing, heterogeneous cell population.
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Affiliation(s)
- I Petkovic
- Department of Medical Oncology, University of Texas M.D. Anderson Cancer Center, Houston 77030
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24
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Abstract
We describe a general assay designed to detect mutants of yeast that are defective for any of several aspects of telomere function. Using this assay, we have isolated a mutant that displays a progressive decrease in telomere length as well as an increased frequency of chromosome loss. This mutation defines a new gene, designated EST1 (for ever shorter telomeres). Null alleles of EST1 are not immediately inviable; instead, they have a senescence phenotype, due to the gradual loss of sequences essential for telomere function, leading to a progressive decrease in chromosomal stability and subsequent cell death.
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Affiliation(s)
- V Lundblad
- Department of Genetics, Harvard Medical School, Boston, Massachusetts
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25
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Abstract
Short stretches of cloned telomeric sequences are necessary and sufficient for telomere formation in yeast as long as the sequences are present in the same orientation as they are found in vivo. During telomere formation, DNA termini usually undergo RAD52-independent recombination with other DNA termini as would be predicted by models of recombination-mediated telomere replication.
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Affiliation(s)
- A F Pluta
- Fred Hutchinson Cancer Research Center, Seattle, Washington 98104
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26
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Radford IR, Hodgson GS, Matthews JP. Critical DNA target size model of ionizing radiation-induced mammalian cell death. Int J Radiat Biol 1988; 54:63-79. [PMID: 2899615 DOI: 10.1080/09553008814551511] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
A new model of mammalian cell killing by ionizing radiation is presented. This model, termed the critical DNA target size model, postulates that DNA double-strand breakage is the critical radiation-induced lesion and that the dose-response for such breakage can be non-linear due to the action of a saturable chemical repair process. DNA double-strand breakage occurring within critical targets (proto-oncogene- or common fragile site-associated sequences) is postulated to initiate recombination events with undamaged sequences, leading to chromosomal aberrations. The subsequent loss of acentric fragments at mitosis is postulated to prevent the continuity of the genome and to produce cell death by the induction of chromatin structural changes. Experimental evidence contrary to other radiation action models is examined, and the hypotheses of the model are justified.
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Affiliation(s)
- I R Radford
- Molecular Science Group, Peter MacCallum Cancer Institute, Melbourne, Victoria, Australia
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27
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Abstract
The structural elements required for chromosome replication, segregation, and stability are replication origins, centromeres, and telomeres. DNA sequences capable of organizing these three elements have been isolated from yeast chromosomal DNA by means of recombinant DNA techniques and yeast cell transformation. It is now possible to combine these sequences into "artificial" chromosomes for yeast cells to obtain more insight into chromosome structure and function. Evidence is presented that the construction of artificial chromosomes functional in higher eukaryotes will be possible in the near future.
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28
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Chandley AC, Hargreave TB, McBeath S, Mitchell AR, Speed RM. Ring XY bivalent: a new phenomenon at metaphase I of meiosis in man. J Med Genet 1987; 24:101-6. [PMID: 3560166 PMCID: PMC1049898 DOI: 10.1136/jmg.24.2.101] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
The unusual appearance of a ring XY bivalent at metaphase I of meiosis is reported in some cells of an oligospermic human male. Higher than usual frequencies of ring configuration in the XY pair were also observed during prophase I. The defect could be attributable to loss of some DNA sequences from the distal heterochromatic tip of the Y chromosome long arm.
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29
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Potashkin JA, Huberman JA. Characterization of DNA sequences associated with residual nuclei of Saccharomyces cerevisiae. Exp Cell Res 1986; 165:29-40. [PMID: 3519258 DOI: 10.1016/0014-4827(86)90530-6] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
We have used two different approaches to determine whether particular DNA sequences are specifically associated with high-salt-treated residual nuclei of Saccharomyces cerevisiae. First, libraries of yeast DNA in phage lambda were probed with nick-translated total nuclear or residual nuclear DNA from unsynchronized yeast cells. None of the plaques gave a significantly stronger or weaker signal with the residual nuclear probe than with the total nuclear probe. Second, DNA was purified from whole nuclei or residual nuclei which had been isolated from cells in G1, G1/S, early S, or nuclear division. This DNA was "dot-blotted" and then probed with specific yeast DNA sequences. Ribosomal DNA was 2- to 3-fold enriched in residual nuclei in late G1, G1/S, and early S, and 2 microns plasmid DNA sequences were 3- to 5-fold depleted during nuclear division and early G1. However, ARS1, TRP1, CEN6, and a telomere sequence were neither enriched nor depleted at any time during the cell cycle.
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30
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31
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Abstract
Telomeres, the ends of eukaryotic chromosomes, have long been recognized as specialized structures. Their stability compared with broken ends of chromosomes suggested that they have properties which protect them from fusion, degradation or recombination. Furthermore, a linear DNA molecule such as that of a eukaryotic chromosome must have a structure at its ends which allows its complete replication, as no known DNA polymerase can initiate synthesis without a primer. At the ends of the relatively short, multi-copy linear DNA molecules found naturally in the nuclei of several lower eukaryotes, there are simple tandemly repeated sequences with, in the cases analysed, a specific array of single-strand breaks, on both DNA strands, in the distal portion of the block of repeats. In general, however, direct analysis of chromosomal termini presents problems because of their very low abundance in nuclei. To circumvent this problem, we have previously cloned a chromosomal telomere of the yeast Saccharomyces cerevisiae on a linear DNA vector molecule. Here we show that yeast chromosomal telomeres terminate in a DNA sequence consisting of tandem irregular repeats of the general form C1-3A. The same repeat units are added to the ends of Tetrahymena telomeres, in an apparently non-template-directed manner, during their replication on linear plasmids in yeast. Such DNA addition may have a fundamental role in telomere replication.
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32
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Abstract
We have shown that the DNA sequences adjacent to the telomeres of Saccharomyces cerevisiae chromosomes are highly conserved and contain a high density of replication origins. The salient features of these telomeres can be summarized as follows. There are three moderately repetitive elements present at the telomeres: the 131 sequence (1 to 1.5 kb), the highly conserved Y sequence (5.2 kb), and the less conserved X sequence (0.3 to 3.75 kb). There is a high density of replication origins spaced about 6.7 kb apart at the telomeres. These replication origins are part of the X or the Y sequences. Some of the 131-Y repetitive units are tandemly arranged. The terminal sequence T (about 0.33 to 0.6 kb) is different from the 131, X, or Y sequences and is heterogeneous in length. The order of these sequences from the telomeric end towards the centromere is T-(Y-131)n-X-, where n ranges from 1 to no more than 4. Although these telomeric sequences are conserved among S. cerevisiae strains, they show striking divergence in certain closely related yeast species.
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33
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Bergold PJ, Campbell GR, Littau VC, Johnson EM. Sequence and hairpin structure of an inverted repeat series at termini of the Physarum extrachromosomal rDNA molecule. Cell 1983; 32:1287-99. [PMID: 6301691 DOI: 10.1016/0092-8674(83)90310-0] [Citation(s) in RCA: 28] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
The termini of the 61 kb palindromic rDNA molecules of Physarum polycephalum possess a series of multiple inverted repeats in which are located specific single-strand gaps and tightly attached protein. After treating rDNA with S1 nuclease, we have cloned several 5 kb Eco RI terminal restriction fragments. Sequencing of more than 800 nucleotides from the end of one such clone reveals the presence of six to ten tandemly repeated units averaging 140 +/- 4 bp in length and flanked by Hae III sites. Each 140 nucleotide repeat unit can form thermodynamically stable hairpin structures based on complex internal palindromic components. When the specific gap sequence CCCTA is present, it is located near the apex of a hairpin component. These secondary structures are formed in growing plasmodia, as seen in electron micrographs of native rDNA molecules, which also reveal apparent recombination forms involving rDNA ends and noncontiguous DNA segments. Recombination initiated at terminal single-strand hairpin loops can result in genetic exchange of ribosomal gene sequences and can lead to completion of 5' nucleotide sequences at ends of newly replicated rDNA molecules.
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34
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Abstract
We have constructed a linear yeast plasmid by joining fragments from the termini of Tetrahymena ribosomal DNA to a yeast vector. Structural features of the terminus region of the Tetrahymena rDNA plasmid maintained in the yeast linear plasmid include a set of specifically placed single-strand interruptions within the cluster of hexanucleotide (C4A2) repeat units. An artificially constructed hairpin terminus was unable to stabilize a linear plasmid in yeast. The fact that yeast can recognize and use DNA ends from the distantly related organism Tetrahymena suggests that the structural features required for telomere replication and resolution have been highly conserved in evolution. The linear plasmid was used as a vector to clone chromosomal telomeres from yeast. One Tetrahymena end was removed by restriction digestion, and yeast fragments that could function as an end on a linear plasmid were selected. Restriction mapping and hybridization analysis demonstrated that these fragments were yeast telomeres, and suggested that all yeast chromosomes might have a common telomere sequence. Yeast telomeres appear to be similar in structure to the rDNA of Tetrahymena, in which specific nicks or gaps are present within a simple repeated sequence near the terminus of the DNA.
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35
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Bellett AJ. Models for the replication of linear DNA must account for the completion of both molecular ends. J Theor Biol 1980; 85:549-51. [PMID: 7442278 DOI: 10.1016/0022-5193(80)90327-6] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
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