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Cranz-Mileva S, Reilly E, Chalhoub N, Patel R, Atanassova T, Cao W, Ellison C, Zaratiegui M. Transposon Removal Reveals Their Adaptive Fitness Contribution. Genome Biol Evol 2024; 16:evae010. [PMID: 38245838 PMCID: PMC10836971 DOI: 10.1093/gbe/evae010] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2023] [Revised: 01/04/2024] [Accepted: 01/12/2024] [Indexed: 01/22/2024] Open
Abstract
Transposable elements are molecular parasites that persist in their host genome by generating new copies to outpace natural selection. Transposable elements exert a large influence on host genome evolution, in some cases providing adaptive changes. Here we measure the fitness effect of the transposable element insertions in the fission yeast Schizosaccharomyces pombe type strain by removing all insertions of its only native transposable element family, the long terminal repeat retrotransposon Tf2. We show that Tf2 elements provide a positive fitness contribution to its host. Tf2 ablation results in changes to the regulation of a mitochondrial gene and, consistently, the fitness effect are sensitive to growth conditions. We propose that Tf2 influences host fitness in a directed manner by dynamically rewiring the transcriptional response to metabolic stress.
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Affiliation(s)
- Susanne Cranz-Mileva
- Department of Molecular Biology and Biochemistry, Rutgers, the State University of New Jersey, Piscataway, NJ, USA
| | - Eve Reilly
- Department of Molecular Biology and Biochemistry, Rutgers, the State University of New Jersey, Piscataway, NJ, USA
| | - Noor Chalhoub
- Department of Molecular Biology and Biochemistry, Rutgers, the State University of New Jersey, Piscataway, NJ, USA
| | - Rohan Patel
- Department of Molecular Biology and Biochemistry, Rutgers, the State University of New Jersey, Piscataway, NJ, USA
| | - Tania Atanassova
- Department of Molecular Biology and Biochemistry, Rutgers, the State University of New Jersey, Piscataway, NJ, USA
| | - Weihuan Cao
- Department of Genetics, Rutgers, the State University of New Jersey, Piscataway, NJ, USA
| | - Christopher Ellison
- Department of Genetics, Rutgers, the State University of New Jersey, Piscataway, NJ, USA
| | - Mikel Zaratiegui
- Department of Molecular Biology and Biochemistry, Rutgers, the State University of New Jersey, Piscataway, NJ, USA
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2
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Cao J, Yu T, Xu B, Hu Z, Zhang XO, Theurkauf W, Weng Z. Epigenetic and chromosomal features drive transposon insertion in Drosophila melanogaster. Nucleic Acids Res 2023; 51:2066-2086. [PMID: 36762470 PMCID: PMC10018349 DOI: 10.1093/nar/gkad054] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2022] [Revised: 01/12/2023] [Accepted: 02/07/2023] [Indexed: 02/11/2023] Open
Abstract
Transposons are mobile genetic elements prevalent in the genomes of most species. The distribution of transposons within a genome reflects the actions of two opposing processes: initial insertion site selection, and selective pressure from the host. By analyzing whole-genome sequencing data from transposon-activated Drosophila melanogaster, we identified 43 316 de novo and 237 germline insertions from four long-terminal-repeat (LTR) transposons, one LINE transposon (I-element), and one DNA transposon (P-element). We found that all transposon types favored insertion into promoters de novo, but otherwise displayed distinct insertion patterns. De novo and germline P-element insertions preferred replication origins, often landing in a narrow region around transcription start sites and in regions of high chromatin accessibility. De novo LTR transposon insertions preferred regions with high H3K36me3, promoters and exons of active genes; within genes, LTR insertion frequency correlated with gene expression. De novo I-element insertion density increased with distance from the centromere. Germline I-element and LTR transposon insertions were depleted in promoters and exons, suggesting strong selective pressure to remove transposons from functional elements. Transposon movement is associated with genome evolution and disease; therefore, our results can improve our understanding of genome and disease biology.
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Affiliation(s)
| | - Tianxiong Yu
- Correspondence may also be addressed to Tianxiong Yu. Tel: +1 774 641 0409; Fax: +1 508 856 0017;
| | - Bo Xu
- The School of Life Sciences and Technology, Tongji University, Shanghai 200092, China
| | - Zhongren Hu
- The School of Life Sciences and Technology, Tongji University, Shanghai 200092, China
| | - Xiao-ou Zhang
- The School of Life Sciences and Technology, Tongji University, Shanghai 200092, China
| | - William E Theurkauf
- Program in Molecular Medicine, University of Massachusetts Chan Medical School, Worcester, MA, USA
| | - Zhiping Weng
- To whom correspondence should be addressed. Tel: +1 508 856 8866; Fax: +1 508 856 0017;
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3
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Barkova A, Adhya I, Conesa C, Asif-Laidin A, Bonnet A, Rabut E, Chagneau C, Lesage P, Acker J. A proteomic screen of Ty1 integrase partners identifies the protein kinase CK2 as a regulator of Ty1 retrotransposition. Mob DNA 2022; 13:26. [PMCID: PMC9673352 DOI: 10.1186/s13100-022-00284-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2022] [Accepted: 10/13/2022] [Indexed: 11/19/2022] Open
Abstract
Abstract
Background
Transposable elements are ubiquitous and play a fundamental role in shaping genomes during evolution. Since excessive transposition can be mutagenic, mechanisms exist in the cells to keep these mobile elements under control. Although many cellular factors regulating the mobility of the retrovirus-like transposon Ty1 in Saccharomyces cerevisiae have been identified in genetic screens, only very few of them interact physically with Ty1 integrase (IN).
Results
Here, we perform a proteomic screen to establish Ty1 IN interactome. Among the 265 potential interacting partners, we focus our study on the conserved CK2 kinase. We confirm the interaction between IN and CK2, demonstrate that IN is a substrate of CK2 in vitro and identify the modified residues. We find that Ty1 IN is phosphorylated in vivo and that these modifications are dependent in part on CK2. No significant change in Ty1 retromobility could be observed when we introduce phospho-ablative mutations that prevent IN phosphorylation by CK2 in vitro. However, the absence of CK2 holoenzyme results in a strong stimulation of Ty1 retrotransposition, characterized by an increase in Ty1 mRNA and protein levels and a high accumulation of cDNA.
Conclusion
Our study shows that Ty1 IN is phosphorylated, as observed for retroviral INs and highlights an important role of CK2 in the regulation of Ty1 retrotransposition. In addition, the proteomic approach enabled the identification of many new Ty1 IN interacting partners, whose potential role in the control of Ty1 mobility will be interesting to study.
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4
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Li F, Lee M, Esnault C, Wendover K, Guo Y, Atkins P, Zaratiegui M, Levin HL. Identification of an integrase-independent pathway of retrotransposition. SCIENCE ADVANCES 2022; 8:eabm9390. [PMID: 35767609 DOI: 10.1126/sciadv.abm9390] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Retroviruses and long terminal repeat retrotransposons rely on integrase (IN) to insert their complementary DNA (cDNA) into the genome of host cells. Nevertheless, in the absence of IN, retroelements can retain notable levels of insertion activity. We have characterized the IN-independent pathway of Tf1 and found that insertion sites had homology to the primers of reverse transcription, which form single-stranded DNAs at the termini of the cDNA. In the absence of IN activity, a similar bias was observed with HIV-1. Our studies showed that the Tf1 insertions result from single-strand annealing, a noncanonical form of homologous recombination mediated by Rad52. By expanding our analysis of insertions to include repeat sequences, we found most formed tandem elements by inserting at preexisting copies of a related transposable element. Unexpectedly, we found that wild-type Tf1 uses the IN-independent pathway as an alternative mode of insertion.
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Affiliation(s)
- Feng Li
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Michael Lee
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Caroline Esnault
- Bioinformatics and Scientific Programming Core, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Katie Wendover
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Yabin Guo
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Paul Atkins
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
| | - Mikel Zaratiegui
- Department of Molecular Biology and Biochemistry, Rutgers, The State University of New Jersey, Nelson Biological Laboratories A133, 604 Allison Rd., Piscataway, NJ 08854, USA
| | - Henry L Levin
- Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA
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5
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Cui M, Bai Y, Li K, Rong YS. Taming active transposons at Drosophila telomeres: The interconnection between HipHop's roles in capping and transcriptional silencing. PLoS Genet 2021; 17:e1009925. [PMID: 34813587 PMCID: PMC8651111 DOI: 10.1371/journal.pgen.1009925] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2021] [Revised: 12/07/2021] [Accepted: 11/03/2021] [Indexed: 11/19/2022] Open
Abstract
Drosophila chromosomes are elongated by retrotransposon attachment, a process poorly understood. Here we characterized a mutation affecting the HipHop telomere-capping protein. In mutant ovaries and the embryos that they produce, telomere retrotransposons are activated and transposon RNP accumulates. Genetic results are consistent with that this hiphop mutation weakens the efficacy of HP1-mediated silencing while leaving piRNA-based mechanisms largely intact. Remarkably, mutant females display normal fecundity suggesting that telomere de-silencing is compatible with germline development. Moreover, unlike prior mutants with overactive telomeres, the hiphop stock does not over-accumulate transposons for hundreds of generations. This is likely due to the loss of HipHop’s abilities both to silence transcription and to recruit transposons to telomeres in the mutant. Furthermore, embryos produced by mutant mothers experience a checkpoint activation, and a further loss of maternal HipHop leads to end-to-end fusion and embryonic arrest. Telomeric retroelements fulfill an essential function yet maintain a potentially conflicting relationship with their Drosophila host. Our study thus showcases a possible intermediate in this arm race in which the host is adapting to over-activated transposons while maintaining genome stability. Our results suggest that the collapse of such a relationship might only occur when the selfish element acquires the ability to target non-telomeric regions of the genome. HipHop is likely part of this machinery restricting the elements to the gene-poor region of telomeres. Lastly, our hiphop mutation behaves as a recessive suppressor of PEV that is mediated by centric heterochromatin, suggesting its broader effect on chromatin not limited to telomeres. Transposons are selfish elements that multiply by inserting extra copies of themselves into the host genome. Active transposons thus threaten the stability of the host genome, while the host responses by transcriptionally silencing the selfish elements or targeting their insertions towards gene-poor regions of the genome. Chromosome ends (telomeres) in the fruit fly Drosophila are elongated by active transposition of retrotransposons. Although much is known about how these elements are silenced, little is known about the remarkable accuracy by which they are targeted to telomeres. Prime candidates through which the host mounts such defenses are members of the protein complexes that protect telomeres. Here we characterized a hypomorphic mutation of the HipHop protein, and showed that active telomeric transcription in the mutant germline persists for generations without leading to runaway telomere elongation, that embryos laid by the mutant female suffer rampant end-to-end fusions, and that telomeric targeting of the transposon machinery is defective in the mutant soma. Collectively our data suggest that HipHop is essential for preventing telomere fusions, silencing telomeric transposons, and recruiting transposon machinery to telomeres. Our study thus identifies a factor essential for the host control over active transposons and a paradigm for studying such control mechanisms.
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Affiliation(s)
- Min Cui
- School of Life Sciences, Sun Yat-sen University, Guangzhou, China
- Hengyang College of Medicine, University of South China, Hengyang, China
| | - Yaofu Bai
- School of Life Sciences, Sun Yat-sen University, Guangzhou, China
- Hengyang College of Medicine, University of South China, Hengyang, China
| | - Kaili Li
- School of Life Sciences, Sun Yat-sen University, Guangzhou, China
- Hengyang College of Medicine, University of South China, Hengyang, China
| | - Yikang S. Rong
- Hengyang College of Medicine, University of South China, Hengyang, China
- * E-mail:
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6
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Correa M, Lerat E, Birmelé E, Samson F, Bouillon B, Normand K, Rizzon C. The Transposable Element Environment of Human Genes Differs According to Their Duplication Status and Essentiality. Genome Biol Evol 2021; 13:6273345. [PMID: 33973013 PMCID: PMC8155550 DOI: 10.1093/gbe/evab062] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/17/2021] [Indexed: 12/13/2022] Open
Abstract
Transposable elements (TEs) are major components of eukaryotic genomes and represent approximately 45% of the human genome. TEs can be important sources of novelty in genomes and there is increasing evidence that TEs contribute to the evolution of gene regulation in mammals. Gene duplication is an evolutionary mechanism that also provides new genetic material and opportunities to acquire new functions. To investigate how duplicated genes are maintained in genomes, here, we explored the TE environment of duplicated and singleton genes. We found that singleton genes have more short-interspersed nuclear elements and DNA transposons in their vicinity than duplicated genes, whereas long-interspersed nuclear elements and long-terminal repeat retrotransposons have accumulated more near duplicated genes. We also discovered that this result is highly associated with the degree of essentiality of the genes with an unexpected accumulation of short-interspersed nuclear elements and DNA transposons around the more-essential genes. Our results underline the importance of taking into account the TE environment of genes to better understand how duplicated genes are maintained in genomes.
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Affiliation(s)
- Margot Correa
- Laboratoire de Mathématiques et Modélisation d'Evry (LaMME), UMR CNRS 8071, ENSIIE, USC INRA, Université d'Evry Val d'Essonne, Evry, France
| | - Emmanuelle Lerat
- Laboratoire de Biométrie et Biologie Evolutive, UMR 5558, Université de Lyon, Université Lyon 1, CNRS, Villeurbanne, France
| | - Etienne Birmelé
- Laboratoire MAP5 UMR 8145, Université de Paris, Paris, France
| | - Franck Samson
- Laboratoire de Mathématiques et Modélisation d'Evry (LaMME), UMR CNRS 8071, ENSIIE, USC INRA, Université d'Evry Val d'Essonne, Evry, France
| | - Bérengère Bouillon
- Laboratoire de Mathématiques et Modélisation d'Evry (LaMME), UMR CNRS 8071, ENSIIE, USC INRA, Université d'Evry Val d'Essonne, Evry, France
| | - Kévin Normand
- Laboratoire de Mathématiques et Modélisation d'Evry (LaMME), UMR CNRS 8071, ENSIIE, USC INRA, Université d'Evry Val d'Essonne, Evry, France
| | - Carène Rizzon
- Laboratoire de Mathématiques et Modélisation d'Evry (LaMME), UMR CNRS 8071, ENSIIE, USC INRA, Université d'Evry Val d'Essonne, Evry, France
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7
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Bonnet A, Lesage P. Light and shadow on the mechanisms of integration site selection in yeast Ty retrotransposon families. Curr Genet 2021; 67:347-357. [PMID: 33590295 DOI: 10.1007/s00294-021-01154-7] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2020] [Revised: 01/04/2021] [Accepted: 01/07/2021] [Indexed: 12/21/2022]
Abstract
Transposable elements are ubiquitous in genomes. Their successful expansion depends in part on their sites of integration in their host genome. In Saccharomyces cerevisiae, evolution has selected various strategies to target the five Ty LTR-retrotransposon families into gene-poor regions in a genome, where coding sequences occupy 70% of the DNA. The integration of Ty1/Ty2/Ty4 and Ty3 occurs upstream and at the transcription start site of the genes transcribed by RNA polymerase III, respectively. Ty5 has completely different integration site preferences, targeting heterochromatin regions. Here, we review the history that led to the identification of the cellular tethering factors that play a major role in anchoring Ty retrotransposons to their preferred sites. We also question the involvement of additional factors in the fine-tuning of the integration site selection, with several studies converging towards an importance of the structure and organization of the chromatin.
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Affiliation(s)
- Amandine Bonnet
- INSERM U944, CNRS UMR 7212, Genomes and Cell Biology of Disease Unit, Institut de Recherche Saint-Louis, Université de Paris, Hôpital Saint-Louis, Paris, France
| | - Pascale Lesage
- INSERM U944, CNRS UMR 7212, Genomes and Cell Biology of Disease Unit, Institut de Recherche Saint-Louis, Université de Paris, Hôpital Saint-Louis, Paris, France.
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8
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Asif‐Laidin A, Conesa C, Bonnet A, Grison C, Adhya I, Menouni R, Fayol H, Palmic N, Acker J, Lesage P. A small targeting domain in Ty1 integrase is sufficient to direct retrotransposon integration upstream of tRNA genes. EMBO J 2020; 39:e104337. [PMID: 32677087 PMCID: PMC7459421 DOI: 10.15252/embj.2019104337] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2020] [Revised: 06/09/2020] [Accepted: 06/18/2020] [Indexed: 12/25/2022] Open
Abstract
Integration of transposable elements into the genome is mutagenic. Mechanisms targeting integrations into relatively safe locations, hence minimizing deleterious consequences for cell fitness, have emerged during evolution. In budding yeast, integration of the Ty1 LTR retrotransposon upstream of RNA polymerase III (Pol III)-transcribed genes requires interaction between Ty1 integrase (IN1) and AC40, a subunit common to Pol I and Pol III. Here, we identify the Ty1 targeting domain of IN1 that ensures (i) IN1 binding to Pol I and Pol III through AC40, (ii) IN1 genome-wide recruitment to Pol I- and Pol III-transcribed genes, and (iii) Ty1 integration only at Pol III-transcribed genes, while IN1 recruitment by AC40 is insufficient to target Ty1 integration into Pol I-transcribed genes. Swapping the targeting domains between Ty5 and Ty1 integrases causes Ty5 integration at Pol III-transcribed genes, indicating that the targeting domain of IN1 alone confers Ty1 integration site specificity.
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Affiliation(s)
- Amna Asif‐Laidin
- INSERM U944, CNRS UMR 7212Genomes& Cell Biology of Disease UnitInstitut de Recherche Saint‐LouisHôpital Saint‐LouisUniversité de ParisParisFrance
| | - Christine Conesa
- CEACNRSInstitute for Integrative Biology of the Cell (I2BC)Université Paris‐SaclayGif‐sur‐YvetteFrance
| | - Amandine Bonnet
- INSERM U944, CNRS UMR 7212Genomes& Cell Biology of Disease UnitInstitut de Recherche Saint‐LouisHôpital Saint‐LouisUniversité de ParisParisFrance
| | - Camille Grison
- INSERM U944, CNRS UMR 7212Genomes& Cell Biology of Disease UnitInstitut de Recherche Saint‐LouisHôpital Saint‐LouisUniversité de ParisParisFrance
| | - Indranil Adhya
- CEACNRSInstitute for Integrative Biology of the Cell (I2BC)Université Paris‐SaclayGif‐sur‐YvetteFrance
| | - Rachid Menouni
- INSERM U944, CNRS UMR 7212Genomes& Cell Biology of Disease UnitInstitut de Recherche Saint‐LouisHôpital Saint‐LouisUniversité de ParisParisFrance
| | - Hélène Fayol
- INSERM U944, CNRS UMR 7212Genomes& Cell Biology of Disease UnitInstitut de Recherche Saint‐LouisHôpital Saint‐LouisUniversité de ParisParisFrance
| | - Noé Palmic
- INSERM U944, CNRS UMR 7212Genomes& Cell Biology of Disease UnitInstitut de Recherche Saint‐LouisHôpital Saint‐LouisUniversité de ParisParisFrance
| | - Joël Acker
- CEACNRSInstitute for Integrative Biology of the Cell (I2BC)Université Paris‐SaclayGif‐sur‐YvetteFrance
| | - Pascale Lesage
- INSERM U944, CNRS UMR 7212Genomes& Cell Biology of Disease UnitInstitut de Recherche Saint‐LouisHôpital Saint‐LouisUniversité de ParisParisFrance
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Zhang X, Zhao M, McCarty DR, Lisch D. Transposable elements employ distinct integration strategies with respect to transcriptional landscapes in eukaryotic genomes. Nucleic Acids Res 2020; 48:6685-6698. [PMID: 32442316 PMCID: PMC7337890 DOI: 10.1093/nar/gkaa370] [Citation(s) in RCA: 23] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2019] [Revised: 04/24/2020] [Accepted: 04/29/2020] [Indexed: 12/27/2022] Open
Abstract
Transposable elements (TEs) are ubiquitous DNA segments capable of moving from one site to another within host genomes. The extant distributions of TEs in eukaryotic genomes have been shaped by both bona fide TE integration preferences in eukaryotic genomes and by selection following integration. Here, we compare TE target site distribution in host genomes using multiple de novo transposon insertion datasets in both plants and animals and compare them in the context of genome-wide transcriptional landscapes. We showcase two distinct types of transcription-associated TE targeting strategies that suggest a process of convergent evolution among eukaryotic TE families. The integration of two precision-targeting elements are specifically associated with initiation of RNA Polymerase II transcription of highly expressed genes, suggesting the existence of novel mechanisms of precision TE targeting in addition to passive targeting of open chromatin. We also highlight two features that can facilitate TE survival and rapid proliferation: tissue-specific transposition and minimization of negative impacts on nearby gene function due to precision targeting.
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Affiliation(s)
- Xinyan Zhang
- Shenzhen Branch, Guangdong Laboratory for Lingnan Modern Agriculture, Genome Analysis Laboratory of the Ministry of Agriculture, Agricultural Genomics Institute at Shenzhen, Chinese Academy of Agricultural Sciences, Shenzhen 518120, China
- Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907, USA
| | - Meixia Zhao
- Department of Biology, Miami University, Oxford, OH 45056, USA
| | - Donald R McCarty
- Horticultural Sciences Department, University of Florida, Gainesville, FL 32611, USA
| | - Damon Lisch
- Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907, USA
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10
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Maxwell PH. Diverse transposable element landscapes in pathogenic and nonpathogenic yeast models: the value of a comparative perspective. Mob DNA 2020; 11:16. [PMID: 32336995 PMCID: PMC7175516 DOI: 10.1186/s13100-020-00215-x] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2020] [Accepted: 04/16/2020] [Indexed: 12/14/2022] Open
Abstract
Genomics and other large-scale analyses have drawn increasing attention to the potential impacts of transposable elements (TEs) on their host genomes. However, it remains challenging to transition from identifying potential roles to clearly demonstrating the level of impact TEs have on genome evolution and possible functions that they contribute to their host organisms. I summarize TE content and distribution in four well-characterized yeast model systems in this review: the pathogens Candida albicans and Cryptococcus neoformans, and the nonpathogenic species Saccharomyces cerevisiae and Schizosaccharomyces pombe. I compare and contrast their TE landscapes to their lifecycles, genomic features, as well as the presence and nature of RNA interference pathways in each species to highlight the valuable diversity represented by these models for functional studies of TEs. I then review the regulation and impacts of the Ty1 and Ty3 retrotransposons from Saccharomyces cerevisiae and Tf1 and Tf2 retrotransposons from Schizosaccharomyces pombe to emphasize parallels and distinctions between these well-studied elements. I propose that further characterization of TEs in the pathogenic yeasts would enable this set of four yeast species to become an excellent set of models for comparative functional studies to address outstanding questions about TE-host relationships.
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11
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Dawes JC, Webster P, Iadarola B, Garcia-Diaz C, Dore M, Bolt BJ, Dewchand H, Dharmalingam G, McLatchie AP, Kaczor J, Caceres JJ, Paccanaro A, Game L, Parrinello S, Uren AG. LUMI-PCR: an Illumina platform ligation-mediated PCR protocol for integration site cloning, provides molecular quantitation of integration sites. Mob DNA 2020; 11:7. [PMID: 32042315 PMCID: PMC7001329 DOI: 10.1186/s13100-020-0201-4] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2019] [Accepted: 01/08/2020] [Indexed: 12/21/2022] Open
Abstract
BACKGROUND Ligation-mediated PCR protocols have diverse uses including the identification of integration sites of insertional mutagens, integrating vectors and naturally occurring mobile genetic elements. For approaches that employ NGS sequencing, the relative abundance of integrations within a complex mixture is typically determined through the use of read counts or unique fragment lengths from a ligation of sheared DNA; however, these estimates may be skewed by PCR amplification biases and saturation of sequencing coverage. RESULTS Here we describe a modification of our previous splinkerette based ligation-mediated PCR using a novel Illumina-compatible adapter design that prevents amplification of non-target DNA and incorporates unique molecular identifiers. This design reduces the number of PCR cycles required and improves relative quantitation of integration abundance for saturating sequencing coverage. By inverting the forked adapter strands from a standard orientation, the integration-genome junction can be sequenced without affecting the sequence diversity required for cluster generation on the flow cell. Replicate libraries of murine leukemia virus-infected spleen samples yielded highly reproducible quantitation of clonal integrations as well as a deep coverage of subclonal integrations. A dilution series of DNAs bearing integrations of MuLV or piggyBac transposon shows linearity of the quantitation over a range of concentrations. CONCLUSIONS Merging ligation and library generation steps can reduce total PCR amplification cycles without sacrificing coverage or fidelity. The protocol is robust enough for use in a 96 well format using an automated liquid handler and we include programs for use of a Beckman Biomek liquid handling workstation. We also include an informatics pipeline that maps reads, builds integration contigs and quantitates integration abundance using both fragment lengths and unique molecular identifiers. Suggestions for optimizing the protocol to other target DNA sequences are included. The reproducible distinction of clonal and subclonal integration sites from each other allows for analysis of populations of cells undergoing selection, such as those found in insertional mutagenesis screens.
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Affiliation(s)
- Joanna C. Dawes
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
| | - Philip Webster
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
- Imperial College Healthcare NHS Trust, London, UK
| | - Barbara Iadarola
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
| | - Claudia Garcia-Diaz
- Samantha Dickson Brain Cancer Unit, UCL Cancer Institute, WC1E 6DD, London, UK
| | - Marian Dore
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
| | - Bruce J. Bolt
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
| | - Hamlata Dewchand
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
| | - Gopuraja Dharmalingam
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
| | | | - Jakub Kaczor
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
| | - Juan J. Caceres
- Centre for Systems and Synthetic Biology, Department of Computer Science, Royal Holloway, University of London, London, UK
| | - Alberto Paccanaro
- Centre for Systems and Synthetic Biology, Department of Computer Science, Royal Holloway, University of London, London, UK
| | - Laurence Game
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
| | - Simona Parrinello
- Samantha Dickson Brain Cancer Unit, UCL Cancer Institute, WC1E 6DD, London, UK
| | - Anthony G. Uren
- MRC London Institute of Medical Sciences (LMS), Du Cane Road, London, W12 0NN UK
- Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, Du Cane Road, London, UK
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12
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Esnault C, Lee M, Ham C, Levin HL. Transposable element insertions in fission yeast drive adaptation to environmental stress. Genome Res 2018; 29:85-95. [PMID: 30541785 PMCID: PMC6314160 DOI: 10.1101/gr.239699.118] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2018] [Accepted: 11/15/2018] [Indexed: 01/06/2023]
Abstract
Cells are regularly exposed to a range of naturally occurring stress that can restrict growth or cause lethality. In response, cells activate expression networks with hundreds of genes that together increase resistance to common environmental insults. However, stress response networks can be insufficient to ensure survival, which raises the question of whether cells possess genetic programs that can promote adaptation to novel forms of stress. We found transposable element (TE) mobility in Schizosaccharomyces pombe was greatly increased when cells were exposed to unusual forms of stress such as heavy metals, caffeine, and the plasticizer phthalate. By subjecting TE-tagged cells to CoCl2, we found the TE integration provided the major path to resistance. Groups of insertions that provided resistance were linked to TOR regulation and metal response genes. We extended our study of adaptation by analyzing TE positions in 57 genetically distinct wild strains. The genomic positions of 1048 polymorphic LTRs were strongly associated with a range of stress response genes, indicating TE integration promotes adaptation in natural conditions. These data provide strong support for the idea, first proposed by Barbara McClintock, that TEs provide a system to modify the genome in response to stress.
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Affiliation(s)
- Caroline Esnault
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA
| | - Michael Lee
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA
| | - Chloe Ham
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA
| | - Henry L Levin
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA
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13
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Barkova A, Asif-Laidin A, Lesage P. Genome-Wide Mapping of Yeast Retrotransposon Integration Target Sites. Methods Enzymol 2018; 612:197-223. [PMID: 30502942 DOI: 10.1016/bs.mie.2018.08.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 03/29/2023]
Abstract
Transposable elements (TEs) are present in virtually all organisms. TE integration into genomes contributes to their structure and evolution, but can also be harmful in some cases. Deciphering where and how TE integration is targeted is fundamental to understand their intricate relationship with their host and explore the outcome of TE mobility on genome evolution and cell fitness. In general, TEs display integration site preference, which differs between elements. High-throughput mapping of de novo insertions by deep sequencing has recently allowed identifying genome-wide integration preferences of several TEs. These studies have provided invaluable clues to address the molecular determinants of integration site preference. Here, we provide a step-by-step methodology to generate massive de novo insertion events and prepare a library of genomic DNA for next-generation sequencing. We also describe a primary bioinformatic procedure to map these insertions in the genome. The whole procedure comes from our recent work on the integration of Ty1 in Saccharomyces cerevisiae, but could be easily adapted to the study of other TEs.
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Affiliation(s)
- Anastasia Barkova
- INSERM U944, CNRS UMR 7212, Université Paris Diderot, Sorbonne Paris Cité, Institut Universitaire d'Hématologie, Hôpital St. Louis, Paris Cedex 10, France
| | - Amna Asif-Laidin
- INSERM U944, CNRS UMR 7212, Université Paris Diderot, Sorbonne Paris Cité, Institut Universitaire d'Hématologie, Hôpital St. Louis, Paris Cedex 10, France
| | - Pascale Lesage
- INSERM U944, CNRS UMR 7212, Université Paris Diderot, Sorbonne Paris Cité, Institut Universitaire d'Hématologie, Hôpital St. Louis, Paris Cedex 10, France.
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14
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Raimondi C, Jagla B, Proux C, Waxin H, Gangloff S, Arcangioli B. Molecular signature of the imprintosome complex at the mating-type locus in fission yeast. MICROBIAL CELL 2018; 5:169-183. [PMID: 29610759 PMCID: PMC5878685 DOI: 10.15698/mic2018.04.623] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
Genetic and molecular studies have indicated that an epigenetic imprint at mat1, the sexual locus of fission yeast, initiates mating type switching. The polar DNA replication of mat1 generates an imprint on the Watson strand. The process by which the imprint is formed and maintained through the cell cycle remains unclear. To understand better the mechanism of imprint formation and stability, we characterized the recruitment of early players of mating type switching at the mat1 region. We found that the switch activating protein 1 (Sap1) is preferentially recruited inside the mat1M allele on a sequence (SS13) that enhances the imprint. The lysine specific demethylases, Lsd1/2, that control the replication fork pause at MPS1 and the formation of the imprint are specifically drafted inside of mat1, regardless of the allele. The CENP-B homolog, Abp1, is highly enriched next to mat1 but it is not required in the process. Additionally, we established the computational signature of the imprint. Using this signature, we show that both sides of the imprinted molecule are bound by Lsd1/2 and Sap1, suggesting a nucleoprotein protective structure defined as imprintosome.
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Affiliation(s)
- Célia Raimondi
- Genomes and Genetics department, Genome Dynamics Unit, UMR 3525 CNRS, Institut Pasteur, 25-28 rue du docteur Roux, Paris, France. Sorbonne Universités, Université Pierre et Marie Curie, Institut de Formation Doctorale, 75252 Paris Cedex 05, France
| | - Bernd Jagla
- Center for Human Immunology, CRT & Hub de Bioinformatique et Biostatistiques, C3BI, Institut Pasteur, 25-28 rue du Docteur Roux, Paris, France
| | - Caroline Proux
- Genomes and Genetics department, Plate-forme Transcriptome & Epigenome, Biomics, Centre d'Innovation et Recherche Technologique (Citech), Institut Pasteur, 25-28 rue du Docteur Roux, Paris, France
| | - Hervé Waxin
- Enseignement, Institut Pasteur, 25-28 rue du Docteur Roux, Paris, France
| | - Serge Gangloff
- Genomes and Genetics department, Genome Dynamics Unit, UMR 3525 CNRS, Institut Pasteur, 25-28 rue du docteur Roux, Paris, France. Sorbonne Universités, Université Pierre et Marie Curie, Institut de Formation Doctorale, 75252 Paris Cedex 05, France
| | - Benoit Arcangioli
- Genomes and Genetics department, Genome Dynamics Unit, UMR 3525 CNRS, Institut Pasteur, 25-28 rue du docteur Roux, Paris, France. Sorbonne Universités, Université Pierre et Marie Curie, Institut de Formation Doctorale, 75252 Paris Cedex 05, France
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15
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Rai SK, Sangesland M, Lee M, Esnault C, Cui Y, Chatterjee AG, Levin HL. Host factors that promote retrotransposon integration are similar in distantly related eukaryotes. PLoS Genet 2017; 13:e1006775. [PMID: 29232693 PMCID: PMC5741268 DOI: 10.1371/journal.pgen.1006775] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2016] [Revised: 12/22/2017] [Accepted: 11/07/2017] [Indexed: 12/14/2022] Open
Abstract
Retroviruses and Long Terminal Repeat (LTR)-retrotransposons have distinct patterns of integration sites. The oncogenic potential of retrovirus-based vectors used in gene therapy is dependent on the selection of integration sites associated with promoters. The LTR-retrotransposon Tf1 of Schizosaccharomyces pombe is studied as a model for oncogenic retroviruses because it integrates into the promoters of stress response genes. Although integrases (INs) encoded by retroviruses and LTR-retrotransposons are responsible for catalyzing the insertion of cDNA into the host genome, it is thought that distinct host factors are required for the efficiency and specificity of integration. We tested this hypothesis with a genome-wide screen of host factors that promote Tf1 integration. By combining an assay for transposition with a genetic assay that measures cDNA recombination we could identify factors that contribute differentially to integration. We utilized this assay to test a collection of 3,004 S. pombe strains with single gene deletions. Using these screens and immunoblot measures of Tf1 proteins, we identified a total of 61 genes that promote integration. The candidate integration factors participate in a range of processes including nuclear transport, transcription, mRNA processing, vesicle transport, chromatin structure and DNA repair. Two candidates, Rhp18 and the NineTeen complex were tested in two-hybrid assays and were found to interact with Tf1 IN. Surprisingly, a number of pathways we identified were found previously to promote integration of the LTR-retrotransposons Ty1 and Ty3 in Saccharomyces cerevisiae, indicating the contribution of host factors to integration are common in distantly related organisms. The DNA repair factors are of particular interest because they may identify the pathways that repair the single stranded gaps flanking the sites of strand transfer following integration of LTR retroelements. Retroviruses and retrotransposons are genetic elements that propagate by integrating into chromosomes of eukaryotic cells. Genetic disorders are being treated with retrovirus-based vectors that integrate corrective genes into the chromosomes of patients. Unfortunately, the vectors can alter expression of adjacent genes and depending on the position of integration, cancer genes can be induced. It is therefore essential that we understand how integration sites are selected. Interestingly, different retroviruses and retrotransposons have different profiles of integration sites. While specific proteins have been identified that select target sites, it’s not known what other cellular factors promote integration. In this paper, we report a comprehensive screen of host factors that promote LTR-retrotransposon integration in the widely-studied yeast, Schizosaccharomyces pombe. Unexpectedly, we found a wide range of pathways and host factors participate in integration. And importantly, we found the cellular processes that promote integration relative to recombination in S. pombe are the same that drive integration of LTR-retrotransposons in the distantly related yeast Saccharomyces cerevisiae. This suggests a specific set of cellular pathways are responsible for integration in a wide range of eukaryotic hosts.
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Affiliation(s)
- Sudhir Kumar Rai
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health (NIH), Bethesda, Maryland, United States of America
| | - Maya Sangesland
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health (NIH), Bethesda, Maryland, United States of America
| | - Michael Lee
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health (NIH), Bethesda, Maryland, United States of America
| | - Caroline Esnault
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health (NIH), Bethesda, Maryland, United States of America
| | - Yujin Cui
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health (NIH), Bethesda, Maryland, United States of America
| | - Atreyi Ghatak Chatterjee
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health (NIH), Bethesda, Maryland, United States of America
| | - Henry L. Levin
- Section on Eukaryotic Transposable Elements, Division of Molecular and Cellular Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health (NIH), Bethesda, Maryland, United States of America
- * E-mail:
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16
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Cross-Regulation between Transposable Elements and Host DNA Replication. Viruses 2017; 9:v9030057. [PMID: 28335567 PMCID: PMC5371812 DOI: 10.3390/v9030057] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2017] [Revised: 03/13/2017] [Accepted: 03/15/2017] [Indexed: 12/27/2022] Open
Abstract
Transposable elements subvert host cellular functions to ensure their survival. Their interaction with the host DNA replication machinery indicates that selective pressures lead them to develop ancestral and convergent evolutionary adaptations aimed at conserved features of this fundamental process. These interactions can shape the co-evolution of the transposons and their hosts.
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17
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Integration site selection by retroviruses and transposable elements in eukaryotes. Nat Rev Genet 2017; 18:292-308. [PMID: 28286338 DOI: 10.1038/nrg.2017.7] [Citation(s) in RCA: 145] [Impact Index Per Article: 20.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Abstract
Transposable elements and retroviruses are found in most genomes, can be pathogenic and are widely used as gene-delivery and functional genomics tools. Exploring whether these genetic elements target specific genomic sites for integration and how this preference is achieved is crucial to our understanding of genome evolution, somatic genome plasticity in cancer and ageing, host-parasite interactions and genome engineering applications. High-throughput profiling of integration sites by next-generation sequencing, combined with large-scale genomic data mining and cellular or biochemical approaches, has revealed that the insertions are usually non-random. The DNA sequence, chromatin and nuclear context, and cellular proteins cooperate in guiding integration in eukaryotic genomes, leading to a remarkable diversity of insertion site distribution and evolutionary strategies.
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18
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Persson J, Steglich B, Smialowska A, Boyd M, Bornholdt J, Andersson R, Schurra C, Arcangioli B, Sandelin A, Nielsen O, Ekwall K. Regulating retrotransposon activity through the use of alternative transcription start sites. EMBO Rep 2016; 17:753-68. [PMID: 26902262 PMCID: PMC5341516 DOI: 10.15252/embr.201541866] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2015] [Accepted: 01/13/2016] [Indexed: 01/22/2023] Open
Abstract
Retrotransposons, the ancestors of retroviruses, have the potential for gene disruption and genomic takeover if not kept in check. Paradoxically, although host cells repress these elements by multiple mechanisms, they are transcribed and are even activated under stress conditions. Here, we describe a new mechanism of retrotransposon regulation through transcription start site (TSS) selection by altered nucleosome occupancy. We show that Fun30 chromatin remodelers cooperate to maintain a high level of nucleosome occupancy at retrotransposon-flanking long terminal repeat (LTR) elements. This enforces the use of a downstream TSS and the production of a truncated RNA incapable of reverse transcription and retrotransposition. However, in stressed cells, nucleosome occupancy at LTR elements is reduced, and the TSS shifts to allow for productive transcription. We propose that controlled retrotransposon transcription from a nonproductive TSS allows for rapid stress-induced activation, while preventing uncontrolled transposon activity in the genome.
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Affiliation(s)
- Jenna Persson
- Department of Biosciences and Nutrition, Karolinska Institutet, Huddinge, Sweden
| | - Babett Steglich
- Department of Biosciences and Nutrition, Karolinska Institutet, Huddinge, Sweden
| | - Agata Smialowska
- Department of Biosciences and Nutrition, Karolinska Institutet, Huddinge, Sweden
| | - Mette Boyd
- Department of Biology, The Bioinformatics Centre, University of Copenhagen, Copenhagen, Denmark Biotech Research and Innovation Centre, University of Copenhagen, Copenhagen, Denmark
| | - Jette Bornholdt
- Department of Biology, The Bioinformatics Centre, University of Copenhagen, Copenhagen, Denmark Biotech Research and Innovation Centre, University of Copenhagen, Copenhagen, Denmark
| | - Robin Andersson
- Department of Biology, The Bioinformatics Centre, University of Copenhagen, Copenhagen, Denmark
| | - Catherine Schurra
- Unite Dynamique du Génome, Département Génomes et Génétique, Pasteur Institute, Paris, France
| | - Benoit Arcangioli
- Unite Dynamique du Génome, Département Génomes et Génétique, Pasteur Institute, Paris, France
| | - Albin Sandelin
- Department of Biology, The Bioinformatics Centre, University of Copenhagen, Copenhagen, Denmark Biotech Research and Innovation Centre, University of Copenhagen, Copenhagen, Denmark
| | - Olaf Nielsen
- Department of Biology, Cell Cycle and Genome Stability Group, University of Copenhagen, Copenhagen, Denmark
| | - Karl Ekwall
- Department of Biosciences and Nutrition, Karolinska Institutet, Huddinge, Sweden
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