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Kojima KK. Diversity and Evolution of DNA Transposons Targeting Multicopy Small RNA Genes from Actinopterygian Fish. BIOLOGY 2022; 11:biology11020166. [PMID: 35205033 PMCID: PMC8869645 DOI: 10.3390/biology11020166] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/22/2021] [Revised: 01/14/2022] [Accepted: 01/18/2022] [Indexed: 11/16/2022]
Abstract
Simple Summary DNA transposons are parasitic DNA segments that can move or duplicate themselves from one site to another in the genome. Dada is a unique group of DNA transposons, which specifically insert themselves into multicopy RNA genes such as transfer RNA (tRNA) genes or small nuclear RNA (snRNA) genes to avoid the disruption of single-copy functional genes. However, only a few Dada families have been characterized along with their target sequences. Here, vertebrate genomes were surveyed to characterize new Dada transposons, and over 120 Dada families were characterized from diverse fishes. They were classified into 12 groups with confirmed target specificities. Various tRNA genes, as well as 5S ribosomal RNA (rRNA) genes were inserted by Dada transposons. Phylogenetic analysis revealed that Dada transposons inserted in the same RNA genes are closely related. Phylogenetically related Dada transposons inserted in different RNA genes show the sequence similarity around their insertion sites, indicating Dada proteins recognize DNA nucleotide sequences to find their targets. Understanding how Dada discovers the targets would help develop target-specific insertions of foreign DNA segments. Abstract Dada is a unique superfamily of DNA transposons, inserted specifically in multicopy RNA genes. The zebrafish genome harbors five families of Dada transposons, whose targets are U6 and U1 snRNA genes, and tRNA-Ala and tRNA-Leu genes. Dada-U6, which is inserted specifically in U6 snRNA genes, is found in four animal phyla, but other target-specific lineages have been reported only from one or two species. Here, vertebrate genomes and transcriptomes were surveyed to characterize Dada families with new target specificities, and over 120 Dada families were characterized from the genomes of actinopterygian fish. They were classified into 12 groups with confirmed target specificities. Newly characterized Dada families target tRNA genes for Asp, Asn, Arg, Gly, Lys, Ser, Tyr, and Val, and 5S rRNA genes. Targeted positions inside of tRNA genes are concentrated in two regions: around the anticodon and the A box of RNA polymerase III promoter. Phylogenetic analysis revealed the relationships among actinopterygian Dada families, and one domestication event in the common ancestor of carps and minnows belonging to Cyprinoidei, Cypriniformes. Sequences targeted by phylogenetically related Dada families show sequence similarities, indicating that the target specificity of Dada is accomplished through the recognition of primary nucleotide sequences.
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Affiliation(s)
- Kenji K Kojima
- Genetic Information Research Institute, Cupertino, CA 95014, USA
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Moura Gama J, Ludwig A, Gazolla CB, Guizelini D, Recco-Pimentel SM, Bruschi DP. A genomic survey of LINE elements in Pipidae aquatic frogs shed light on Rex-elements evolution in these genomes. Mol Phylogenet Evol 2022; 168:107393. [PMID: 35051593 DOI: 10.1016/j.ympev.2022.107393] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2021] [Revised: 11/09/2021] [Accepted: 12/25/2021] [Indexed: 11/19/2022]
Abstract
The transposable elements (TE) represent a large portion of anuran genomes that act as components of genetic diversification. The LINE order of retrotransposons is among the most representative and diverse TEs and is poorly investigated in anurans. Here we explored the LINE diversity with an emphasis on the elements generically called Rex in Pipidae species, more specifically, in the genomes ofXenopus tropicalis, used as a model genome in the study of anurans,the allotetraploid sister species Xenopus laevis and theAmerican species Pipa carvalhoi. We were able to identify a great diversity of LINEs from five clades, Rex1, L2, CR1, L1 and Tx1, in these three species, and the RTE clade was lost in X. tropicalis. It is clear that elements classified as Rex are distributed in distinct clades. The evolutionary pattern of Rex1 elements denote a complex evolution with independent losses of families and some horizontal transfer events between fishes and amphibians which were supported not only by the phylogenetic inconsistencies but also by the very low Ks values found for the TE sequences. The data obtained here update the knowledge of the LINEs diversity in X. laevis and represent the first study of TEs in P. carvalhoi.
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Affiliation(s)
- Joana Moura Gama
- Programa de Pós-Graduação em Genética (PPG-GEN), Universidade Federal do Paraná (UFPR), Curitiba, Brazil; Laboratório de Citogenética evolutiva e Conservação Animal (LabCeca), Departamento de Genética, Universidade Federal do Paraná (UFPR), Brazil
| | - Adriana Ludwig
- Laboratório de Ciências e Tecnologias Aplicadas em Saúde (LaCTAS), Instituto Carlos Chagas, Fiocruz-PR, Brazil.
| | - Camilla Borges Gazolla
- Programa de Pós-Graduação em Genética (PPG-GEN), Universidade Federal do Paraná (UFPR), Curitiba, Brazil; Laboratório de Citogenética evolutiva e Conservação Animal (LabCeca), Departamento de Genética, Universidade Federal do Paraná (UFPR), Brazil
| | - Dieval Guizelini
- Programa de Pós-Graduação em Bioinformática, Universidade Federal do Paraná, Curitiba, Brazil
| | | | - Daniel Pacheco Bruschi
- Programa de Pós-Graduação em Genética (PPG-GEN), Universidade Federal do Paraná (UFPR), Curitiba, Brazil; Laboratório de Citogenética evolutiva e Conservação Animal (LabCeca), Departamento de Genética, Universidade Federal do Paraná (UFPR), Brazil.
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Abstract
Transposable elements (TEs) are mobile DNA sequences that propagate within genomes. Through diverse invasion strategies, TEs have come to occupy a substantial fraction of nearly all eukaryotic genomes, and they represent a major source of genetic variation and novelty. Here we review the defining features of each major group of eukaryotic TEs and explore their evolutionary origins and relationships. We discuss how the unique biology of different TEs influences their propagation and distribution within and across genomes. Environmental and genetic factors acting at the level of the host species further modulate the activity, diversification, and fate of TEs, producing the dramatic variation in TE content observed across eukaryotes. We argue that cataloging TE diversity and dissecting the idiosyncratic behavior of individual elements are crucial to expanding our comprehension of their impact on the biology of genomes and the evolution of species.
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Affiliation(s)
- Jonathan N Wells
- Department of Molecular Biology and Genetics, Cornell University, Ithaca, New York 14850; ,
| | - Cédric Feschotte
- Department of Molecular Biology and Genetics, Cornell University, Ithaca, New York 14850; ,
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Abstract
Although most of non-long terminal repeat (non-LTR) retrotransposons are incorporated in the host genome almost randomly, some non-LTR retrotransposons are incorporated into specific sequences within a target site. On the basis of structural and phylogenetic features, non-LTR retrotransposons are classified into two large groups, restriction enzyme-like endonuclease (RLE)-encoding elements and apurinic/apyrimidinic endonuclease (APE)-encoding elements. All clades of RLE-encoding non-LTR retrotransposons include site-specific elements. However, only two of more than 20 APE-encoding clades, Tx1 and R1, contain site-specific non-LTR elements. Site-specific non-LTR retrotransposons usually target within multi-copy RNA genes, such as rRNA gene (rDNA) clusters, or repetitive genomic sequences, such as telomeric repeats; this behavior may be a symbiotic strategy to reduce the damage to the host genome. Site- and sequence-specificity are variable even among closely related non-LTR elements and appeared to have changed during evolution. In the APE-encoding elements, the primary determinant of the sequence- specific integration is APE itself, which nicks one strand of the target DNA during the initiation of target primed reverse transcription (TPRT). However, other factors, such as interaction between mRNA and the target DNA, and access to the target region in the nuclei also affect the sequence-specificity. In contrast, in the RLE-encoding elements, DNA-binding motifs appear to affect their sequence-specificity, rather than the RLE domain itself. Highly specific integration properties of these site-specific non-LTR elements make them ideal alternative tools for sequence-specific gene delivery, particularly for therapeutic purposes in human diseases.
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Abstract
Eukaryotic genomes are colonized by various transposons including short interspersed elements (SINEs). The 5' region (head) of the majority of SINEs is derived from one of the three types of RNA genes--7SL RNA, transfer RNA (tRNA), or 5S ribosomal RNA (rRNA)--and the internal promoter inside the head promotes the transcription of the entire SINEs. Here I report a new group of SINEs whose heads originate from either the U1 or U2 small nuclear RNA gene. These SINEs, named SINEU, are distributed among crocodilians and classified into three families. The structures of the SINEU-1 subfamilies indicate the recurrent addition of a U1- or U2-derived sequence onto the 5' end of SINEU-1 elements. SINEU-1 and SINEU-3 are ancient and shared among alligators, crocodiles, and gharials, while SINEU-2 is absent in the alligator genome. SINEU-2 is the only SINE family that was active after the split of crocodiles and gharials. All SINEU families, especially SINEU-3, are preferentially inserted into a family of Mariner DNA transposon, Mariner-N4_AMi. A group of Tx1 non-long terminal repeat retrotransposons designated Tx1-Mar also show target preference for Mariner-N4_AMi, indicating that SINEU was mobilized by Tx1-Mar.
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Repanas K, Fuentes G, Cohen SX, Bonvin AMJJ, Perrakis A. Insights into the DNA cleavage mechanism of human LINE-1 retrotransposon endonuclease. Proteins 2009; 74:917-28. [PMID: 18767160 DOI: 10.1002/prot.22201] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
The human LINE-1 endonuclease (L1-EN) contributes in defining the genomic integration sites of the abundant human L1 and Alu retrotransposons. LINEs have been considered as possible vehicles for gene delivery and understanding the mechanism of L1-EN could help engineering them as genetic tools. We tested the in vitro activity of point mutants in three L1-EN residues--Asp145, Arg155, Ile204--that are key for DNA cleavage, and determined their crystal structures. The L1-EN structure remains overall unaffected by the mutations, which change the enzyme activity but leave DNA cleavage sequence specificity mostly unaffected. To better understand the mechanism of L1-EN, we performed molecular dynamics simulations using as model the structures of wild type EN-L1, of two betaB6-betaB5 loop exchange mutants we have described previously to be important for DNA recognition, of the R155A mutant from this study, and of the homologous TRAS1 endonuclease: all confirm a rigid scaffold. The simulations crucially indicate that the betaB6-betaB5 loop shows an anticorrelated motion with the surface loops betaA6-betaA5 and betaB3-alphaB1. The latter loop harbors N118, a residue that alters DNA cleavage specificity in homologous endonucleases, and implies that the plasticity and correlated motion of these loops has a functional importance in DNA recognition and binding. To further explore how these loops are possibly involved in DNA binding, we docked computationally two DNA substrates to our structure, one involving a flipped-out nucleotide downstream the scissile phosphodiester; and one not. The models for both scenarios are feasible and agree with the hypotheses derived from the dynamic simulations. The reduced cleavage activity we have observed for the I204Y mutant above however, favors the flipped out nucleotide model.
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Affiliation(s)
- Kostas Repanas
- Division of Molecular Cancer Biology, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands
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Eickbush TH, Jamburuthugoda VK. The diversity of retrotransposons and the properties of their reverse transcriptases. Virus Res 2008; 134:221-34. [PMID: 18261821 DOI: 10.1016/j.virusres.2007.12.010] [Citation(s) in RCA: 171] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2007] [Revised: 12/14/2007] [Accepted: 12/14/2007] [Indexed: 11/30/2022]
Abstract
A number of abundant mobile genetic elements called retrotransposons reverse transcribe RNA to generate DNA for insertion into eukaryotic genomes. Four major classes of retrotransposons are described here. First, the long-terminal-repeat (LTR) retrotransposons have similar structures and mechanisms to those of the vertebrate retroviruses. Genes that may enable these retrotransposons to leave a cell have been acquired by these elements in a number of animal and plant lineages. Second, the tyrosine recombinase retrotransposons are similar to the LTR retrotransposons except that they have substituted a recombinase for the integrase and recombine into the host chromosomes. Third, the non-LTR retrotransposons use a cleaved chromosomal target site generated by an encoded endonuclease to prime reverse transcription. Finally, the Penelope-like retrotransposons are not well understood but appear to also use cleaved DNA or the ends of chromosomes as primer for reverse transcription. Described in the second part of this review are the enzymatic properties of the reverse transcriptases (RTs) encoded by retrotransposons. The RTs of the LTR retrotransposons are highly divergent in sequence but have similar enzymatic activities to those of retroviruses. The RTs of the non-LTR retrotransposons have several unique properties reflecting their adaptation to a different mechanism of retrotransposition.
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Affiliation(s)
- Thomas H Eickbush
- Department of Biology, University of Rochester, Rochester, NY 14627, USA.
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Repanas K, Zingler N, Layer LE, Schumann GG, Perrakis A, Weichenrieder O. Determinants for DNA target structure selectivity of the human LINE-1 retrotransposon endonuclease. Nucleic Acids Res 2007; 35:4914-26. [PMID: 17626046 PMCID: PMC1950540 DOI: 10.1093/nar/gkm516] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
The human LINE-1 endonuclease (L1-EN) is the targeting endonuclease encoded by the human LINE-1 (L1) retrotransposon. L1-EN guides the genomic integration of new L1 and Alu elements that presently account for ∼28% of the human genome. L1-EN bears considerable technological interest, because its target selectivity may ultimately be engineered to allow the site-specific integration of DNA into defined genomic locations. Based on the crystal structure, we generated L1-EN mutants to analyze and manipulate DNA target site recognition. Crystal structures and their dynamic and functional analysis show entire loop grafts to be feasible, resulting in altered specificity, while individual point mutations do not change the nicking pattern of L1-EN. Structural parameters of the DNA target seem more important for recognition than the nucleotide sequence, and nicking profiles on DNA oligonucleotides in vitro are less well defined than the respective integration site consensus in vivo. This suggests that additional factors other than the DNA nicking specificity of L1-EN contribute to the targeted integration of non-LTR retrotransposons.
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Affiliation(s)
- Kostas Repanas
- Division of Molecular Carcinogenesis, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands and Paul-Ehrlich-Institut, Section PR2/Retroelements, 63225 Langen, Germany
| | - Nora Zingler
- Division of Molecular Carcinogenesis, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands and Paul-Ehrlich-Institut, Section PR2/Retroelements, 63225 Langen, Germany
| | - Liliana E. Layer
- Division of Molecular Carcinogenesis, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands and Paul-Ehrlich-Institut, Section PR2/Retroelements, 63225 Langen, Germany
| | - Gerald G. Schumann
- Division of Molecular Carcinogenesis, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands and Paul-Ehrlich-Institut, Section PR2/Retroelements, 63225 Langen, Germany
| | - Anastassis Perrakis
- Division of Molecular Carcinogenesis, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands and Paul-Ehrlich-Institut, Section PR2/Retroelements, 63225 Langen, Germany
| | - Oliver Weichenrieder
- Division of Molecular Carcinogenesis, The Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands and Paul-Ehrlich-Institut, Section PR2/Retroelements, 63225 Langen, Germany
- *To whom correspondence should be addressed.+4970716011358+4970716011353
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Maita N, Aoyagi H, Osanai M, Shirakawa M, Fujiwara H. Characterization of the sequence specificity of the R1Bm endonuclease domain by structural and biochemical studies. Nucleic Acids Res 2007; 35:3918-27. [PMID: 17537809 PMCID: PMC1919474 DOI: 10.1093/nar/gkm397] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2006] [Revised: 04/25/2007] [Accepted: 05/01/2007] [Indexed: 11/30/2022] Open
Abstract
R1Bm is a long interspersed element (LINE) inserted into a specific sequence within 28S rDNA of the silkworm genome. Of two open reading frames (ORFs) of R1Bm, ORF2 encodes a reverse transcriptase (RT) and an endonuclease (EN) domain which digests specifically both top and bottom strand of the target sequence in 28S rDNA. To elucidate the sequence specificity of EN domain of R1Bm (R1Bm EN), we examined the cleavage tendency for the target sequences, and found that 5'-A(G/C)(A/T)!(A/G)T-3' is the consensus sequence (! = cleavage site). We also determined the crystal structure of R1Bm EN at 2.0 A resolution. Its structure was basically similar to AP endonuclease family, but had a special beta-hairpin at the edge of the DNA binding surface, which is a common feature among EN of LINEs. Point-mutations on the DNA binding surface of R1Bm EN significantly decreased the cleavage activities, but did not affect the sequence recognition in most residues. However, two mutants Y98A and N180A had altered cleavage patterns, suggesting an important role of these residues (Y98 and N180) for the sequence recognition of R1Bm EN. In addition, Y98A mutant showed another cleavage pattern, that implies de novo design of novel sequence-specific EN.
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Affiliation(s)
- Nobuo Maita
- Graduate School of Systems Life Sciences, Kyushu University, Fukuoka 812-8582, Japan, Graduate School of Integrated Science, Yokohama City University, Yokohama 230-0045, Japan, Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bioscience Building 501, Kashiwa, Chiba 277-8562, Japan, Graduate School of Engineering Kyoto University, Kyoto 615-8510, Japan and CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan
| | - Hideyuki Aoyagi
- Graduate School of Systems Life Sciences, Kyushu University, Fukuoka 812-8582, Japan, Graduate School of Integrated Science, Yokohama City University, Yokohama 230-0045, Japan, Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bioscience Building 501, Kashiwa, Chiba 277-8562, Japan, Graduate School of Engineering Kyoto University, Kyoto 615-8510, Japan and CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan
| | - Mizuko Osanai
- Graduate School of Systems Life Sciences, Kyushu University, Fukuoka 812-8582, Japan, Graduate School of Integrated Science, Yokohama City University, Yokohama 230-0045, Japan, Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bioscience Building 501, Kashiwa, Chiba 277-8562, Japan, Graduate School of Engineering Kyoto University, Kyoto 615-8510, Japan and CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan
| | - Masahiro Shirakawa
- Graduate School of Systems Life Sciences, Kyushu University, Fukuoka 812-8582, Japan, Graduate School of Integrated Science, Yokohama City University, Yokohama 230-0045, Japan, Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bioscience Building 501, Kashiwa, Chiba 277-8562, Japan, Graduate School of Engineering Kyoto University, Kyoto 615-8510, Japan and CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan
| | - Haruhiko Fujiwara
- Graduate School of Systems Life Sciences, Kyushu University, Fukuoka 812-8582, Japan, Graduate School of Integrated Science, Yokohama City University, Yokohama 230-0045, Japan, Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bioscience Building 501, Kashiwa, Chiba 277-8562, Japan, Graduate School of Engineering Kyoto University, Kyoto 615-8510, Japan and CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan
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Mandal PK, Rawal K, Ramaswamy R, Bhattacharya A, Bhattacharya S. Identification of insertion hot spots for non-LTR retrotransposons: computational and biochemical application to Entamoeba histolytica. Nucleic Acids Res 2006; 34:5752-63. [PMID: 17040894 PMCID: PMC1635306 DOI: 10.1093/nar/gkl710] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The genome of the human pathogen Entamoeba histolytica contains non-long terminal repeat (LTR) retrotransposons, the EhLINEs and EhSINEs, which lack targeted insertion. We investigated the importance of local DNA structure, and sequence preference of the element-encoded endonuclease (EN) in selecting target sites for retrotransposon insertion. Pre-insertion loci were tested computationally to detect unique features based on DNA structure, thermodynamic considerations and protein interaction measures. Target sites could readily be distinguished from other genomic sites based on these criteria. The contribution of the EhLINE1-encoded EN in target site selection was investigated biochemically. The sequence-specificity of the EN was tested in vitro with a variety of mutated substrates. It was possible to assign a consensus sequence, 5′-GCATT-3′, which was efficiently nicked between A-T and T-T. The upstream G residue enhanced EN activity, possibly serving to limit retrotransposition in the A+T-rich E.histolytica genome. Mutated substrates with poor EN activity showed structural differences compared with normal substrates. Analysis of retrotransposon insertion sites from a variety of organisms showed that, in general, regions of favorable DNA structure were recognized for retrotransposition. A combination of favorable DNA structure and preferred EN nicking sequence in the vicinity of this structure may determine the genomic hotspots for retrotransposition.
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Affiliation(s)
- Prabhat K. Mandal
- School of Life Sciences, Jawaharlal Nehru UniversityNew Delhi 110 067, India
| | - Kamal Rawal
- School of Information Technology, Jawaharlal Nehru UniversityNew Delhi 110 067, India
| | - Ram Ramaswamy
- School of Information Technology, Jawaharlal Nehru UniversityNew Delhi 110 067, India
- School of Physical Sciences, Jawaharlal Nehru UniversityNew Delhi 110 067, India
| | - Alok Bhattacharya
- School of Information Technology, Jawaharlal Nehru UniversityNew Delhi 110 067, India
- School of Life Sciences, Jawaharlal Nehru UniversityNew Delhi 110 067, India
| | - Sudha Bhattacharya
- To whom correspondence should be addressed. Tel: +91 11 26704308; Fax: +91 11 26172438;
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Fujiwara H, Osanai M, Matsumoto T, Kojima KK. Telomere-specific non-LTR retrotransposons and telomere maintenance in the silkworm, Bombyx mori. Chromosome Res 2005; 13:455-67. [PMID: 16132811 DOI: 10.1007/s10577-005-0990-9] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
Most insects have telomeres that consist of pentanucleotide (TTAGG) telomeric repeats, which are synthesized by telomerase. However, all species in Diptera so far examined and several species in other orders of insect have lost the (TTAGG)n repeats, suggesting that some of them recruit telomerase-independent telomere maintenance. The silkworm, Bombyx mori, retains the TTAGG motifs in the chromosomal ends but expresses quite a low level of telomerase activity in all stages of various tissues. Just proximal to a 6-8-kb stretch of the TTAGG repeats in B. mori, more than 1000 copies of non-LTR retrotransposons, designated TRAS and SART families, occur among the telomeric repeats and accumulate. TRAS and SART are abundantly transcribed and actively retrotransposed into TTAGG telomeric repeats in a highly sequence-specific manner. They have three possible mechanisms to ensure specific integration into the telomeric repeats. This article focuses on the telomere structure and telomere-specific non-LTR retrotransposons in B. mori and discusses the mechanisms for telomere maintenance in this insect.
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Affiliation(s)
- Haruhiko Fujiwara
- Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bioscience Building 501, Kashiwano-ha, 277-8562 Kashiwa, Japan.
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Zingler N, Weichenrieder O, Schumann GG. APE-type non-LTR retrotransposons: determinants involved in target site recognition. Cytogenet Genome Res 2005; 110:250-68. [PMID: 16093679 DOI: 10.1159/000084959] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2003] [Accepted: 02/05/2004] [Indexed: 10/25/2022] Open
Abstract
Non-long terminal repeat (Non-LTR) retrotransposons represent a diverse and widely distributed group of transposable elements and an almost ubiquitous component of eukaryotic genomes that has a major impact on evolution. Their copy number can range from a few to several million and they often make up a significant fraction of the genomes. The members of the dominating subtype of non-LTR retrotransposons code for an endonuclease with homology to apurinic/apyrimidinic endonucleases (APE), and are thus termed APE-type non-LTR retrotransposons. In the last decade both the number of identified non-LTR retrotransposons and our knowledge of biology and evolution of APE-type non-LTR retrotransposons has increased tremendously.
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Affiliation(s)
- N Zingler
- Paul-Ehrlich-Institut, Langen, Germany
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Christensen SM, Eickbush TH. R2 target-primed reverse transcription: ordered cleavage and polymerization steps by protein subunits asymmetrically bound to the target DNA. Mol Cell Biol 2005; 25:6617-28. [PMID: 16024797 PMCID: PMC1190342 DOI: 10.1128/mcb.25.15.6617-6628.2005] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
R2 elements are non-long terminal repeat retrotransposons that specifically insert into 28S rRNA genes of many animal groups. These elements encode a single protein with reverse transcriptase and endonuclease activities as well as specific DNA and RNA binding properties. In this report, gel shift experiments were conducted to investigate the stoichiometry of the DNA, RNA, and protein components of the integration reaction. The enzymatic functions associated with each of the protein complexes were also determined, and DNase I digests were used to footprint the protein onto the target DNA. Additionally, a short polypeptide containing the N-terminal putative DNA-binding motifs was footprinted on the DNA target site. These combined findings revealed that one protein subunit binds the R2 RNA template and the DNA 10 to 40 bp upstream of the insertion site. This subunit cleaves the first DNA strand and uses that cleavage to prime reverse transcription of the R2 RNA transcript. Another protein subunit(s) uses the N-terminal DNA binding motifs to bind to the 18 bp of target DNA downstream of the insertion site and is responsible for cleavage of the second DNA strand. A complete model for the R2 integration reaction is presented, which with minor modifications is adaptable to other non-LTR retrotransposons.
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Kojima KK, Fujiwara H. Long-term inheritance of the 28S rDNA-specific retrotransposon R2. Mol Biol Evol 2005; 22:2157-65. [PMID: 16014872 DOI: 10.1093/molbev/msi210] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
R2 is a non-long-terminal-repeat (LTR) retrotransposon that inserts specifically into 28S rDNA. R2 has been identified in many species of arthropods and three species of chordates. R2 may be even more widely distributed in animals, and its origin may be traceable to early animal evolution. In this study, we identified R2 elements in medaka fish, White Cloud Mountain minnow, Reeves' turtle, hagfish, sea lilies, and some arthropod species, using degenerate polymerase chain reaction methods. We also identified two R2 elements from the public genomic sequence database of the bloodfluke Schistosoma mansoni. One of the two bloodfluke R2 elements has two zinc-finger motifs at the N-terminus; this differs from other known R2 elements, which have one or three zinc-finger motifs. Phylogenetic analysis revealed that the whole phylogeny of R2 can be divided into 11 parts (subclades), in which the local R2 phylogeny and the corresponding host phylogeny are consistent. Divergence-versus-age analysis revealed that there is no reliable evidence for the horizontal transfer of R2 but supports the proposition that R2 has been vertically transferred since before the divergence of the deuterostomes and protostomes. The seeming inconsistency between the R2 phylogeny and the phylogeny of their hosts is due to the existence of paralogous lineages. The number of N-terminal zinc-finger motifs is consistent with the deep phylogeny of R2 and indicates that the common ancestor of R2 had three zinc-finger motifs at the N-terminus. This study revealed the long-term vertical inheritance and the ancient origin of sequence specificity of R2, both of which seem applicable to some other non-LTR retrotransposons.
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Affiliation(s)
- Kenji K Kojima
- Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Chiba, Japan
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16
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Mandal PK, Bagchi A, Bhattacharya A, Bhattacharya S. An Entamoeba histolytica LINE/SINE pair inserts at common target sites cleaved by the restriction enzyme-like LINE-encoded endonuclease. EUKARYOTIC CELL 2004; 3:170-9. [PMID: 14871947 PMCID: PMC329514 DOI: 10.1128/ec.3.1.170-179.2004] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The non-long-terminal-repeat (non-LTR) retrotransposons (also called long interspersed repetitive elements [LINEs]) are among the oldest retroelements. Here we describe the properties of such an element from a primitive protozoan parasite, Entamoeba histolytica, that infects the human gut. This 4.8-kb element, called EhLINE1, is present in about 140 copies dispersed throughout the genome. The element belongs to the R4 clade of non-LTR elements. It has a centrally located reverse transcriptase domain and a restriction enzyme-like endonuclease (EN) domain at the carboxy terminus. We have cloned and expressed a 794-bp fragment containing the EN domain in Escherichia coli. The purified protein could nick supercoiled pBluescript DNA to yield open circular and linear DNAs. The conserved PDX(12-14)D motif was required for activity. Genomic sequences flanking the sites of insertion of EhLINE1 and the putative partner short interspersed repetitive element (SINE), EhSINE1, were analyzed. Both elements resulted in short target site duplications (TSD) upon insertion. A common feature was the presence of a short T-rich stretch just upstream of the TSD in most insertion sites. By sequence analysis an empty target site in the E. histolytica genome, known to be occupied by EhSINE1, was identified. When a 176-bp fragment containing the empty site was used as a substrate for EN, it was prominently nicked on the bottom strand at the precise point of insertion of EhSINE1, showing that this SINE could use the LINE-encoded endonuclease for its insertion. The nick on the bottom strand was toward the right of the TSD, which is uncommon. The lack of strict target site-specificity of the restriction enzyme-like EN encoded by EhLINE1 is also exceptional. A model for retrotransposition of EhLINE1/SINE1 is presented.
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Affiliation(s)
- Prabhat K Mandal
- School of Life Sciences, Jawaharlal Nehru University, New Delhi 110067, India
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17
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Christensen S, Eickbush TH. Footprint of the retrotransposon R2Bm protein on its target site before and after cleavage. J Mol Biol 2004; 336:1035-45. [PMID: 15037067 DOI: 10.1016/j.jmb.2003.12.077] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/22/2003] [Revised: 12/23/2003] [Accepted: 12/24/2003] [Indexed: 10/26/2022]
Abstract
R2 elements are non-long terminal repeat (non-LTR) retrotransposons that specifically integrate into the 28 S rRNA genes of their host. These elements encode a single open reading frame with a genome-specific endonuclease and a reverse transcriptase that uses the cleaved chromosomal target site to prime reverse transcription. Cleavage of the DNA strand that is used to prime reverse transcription is an efficient process that occurs in the presence or absence of RNA. Cleavage of the second DNA strand is much less efficient and requires RNA. Reverse transcription occurs before second strand cleavage and only if the RNA bound to the protein contains the 3' untranslated region of the R2 element. Thus a complex series of protein interactions with the DNA and conformational changes in the protein are likely to occur during this retrotransposition reaction. Here, we conduct electrophoretic mobility-shift assays and DNase I footprint studies on the binding of the R2 protein to the DNA target in the presence and absence of RNA both before and after first strand cleavage. While the total expanse of the protein footprint on the DNA eventually covers five helical turns, before cleavage the footprint only extends from 17 bp to 40 bp upstream of the cleavage site. This footprint is the same in the presence and absence of RNA. We hypothesize that the active site of the endonuclease domain is analogous to type IIS restriction enzymes in that it is located on a flexible domain that is not tightly bound to the cleavage site. After first strand cleavage the protein footprint extends beyond the cleavage site. We suggest that this increased protection after cleavage is the RT domain that is positioned over the free DNA end to begin reverse transcription on the nicked DNA substrate.
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Affiliation(s)
- Shawn Christensen
- Department of Biology, University of Rochester, Hutchinson Hall 334, Rochester, NY 14627-0211, USA
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18
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Fujimoto H, Hirukawa Y, Tani H, Matsuura Y, Hashido K, Tsuchida K, Takada N, Kobayashi M, Maekawa H. Integration of the 5' end of the retrotransposon, R2Bm, can be complemented by homologous recombination. Nucleic Acids Res 2004; 32:1555-65. [PMID: 14999096 PMCID: PMC390292 DOI: 10.1093/nar/gkh304] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
R2Bm is a non-long-terminal-repeat (non-LTR) retrotransposon that was identified at a specific target site in the 28S rRNA genes of the silkworm, Bombyx mori. Although in vitro analysis has revealed that the 3' end of R2Bm is integrated into the target site by means of target-primed reverse transcription (TPRT), the mechanism of the 5' end integration is not well understood. We established a novel in vivo system to assay the insertion mechanism of R2Bm using a cultured cell line, C65, and a baculovirus, AcNPV, as host and vector, respectively. The 3' end of R2Bm integrated at the target site in the rRNA genes of C65 cells when an AcNPV containing both the full-length 3' UTR and the entire open reading frame (ORF) of R2Bm was introduced while the 5' end integration was incorrect. The 5' end of R2Bm was integrated, however, when the 28S gene sequence upstream of the R2Bm target site was added to the R2Bm sequence. Thus, in our assay, homologous sequences were likely essential for the successful integration of the entire R2Bm into the host cell genome. We also demonstrated that the failure to integrate caused by a frame-shifted ORF was rescued by co-infection with a helper virus that contained only the R2Bm ORF. This indicates that R2 retrotransposition can be complemented in trans. These findings suggest that the host's mechanism for DNA repair may be necessary for the integration of the 5' end of R2Bm and that R2Bm protein may only have the ability to integrate the 3' end of the element by TPRT.
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Affiliation(s)
- Hirofumi Fujimoto
- Division of Radiological Protection and Biology, National Institute of Infectious Diseases, Toyama 1-23-1, Shinjuku-ku, Tokyo 162-8640, Japan
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Matsumoto T, Takahashi H, Fujiwara H. Targeted nuclear import of open reading frame 1 protein is required for in vivo retrotransposition of a telomere-specific non-long terminal repeat retrotransposon, SART1. Mol Cell Biol 2004; 24:105-22. [PMID: 14673147 PMCID: PMC303349 DOI: 10.1128/mcb.24.1.105-122.2004] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Non-long terminal repeat (non-LTR) retrotransposons, most of which carry two open reading frames (ORFs), are abundant mobile elements that are distributed widely among eukaryotes. ORF2 encodes enzymatic domains, such as reverse transcriptase, that are conserved in all retroelements, but the functional roles of ORF1 in vivo are little understood. We show with green fluorescent protein-ORF1 fusion proteins that the ORF1 proteins of SART1, a telomeric repeat-specific non-LTR retrotransposon in Bombyx mori, are transported into the nucleus to produce a dotted localization pattern. Nuclear localization signals N1 (RRKR) and N2 (PSKRGRG) at the N terminus and a highly basic region in the center of SART1 ORF1 are involved in nuclear import and the dotted localization pattern in the nucleus, respectively. An in vivo retrotransposition assay clarified that at least three ORF1 domains, N1/N2, the central basic domain, and CCHC zinc fingers are required for SART1 retrotransposition. The nuclear import activity of SART1 ORF1 makes it clear that the ORF1 proteins of non-LTR retrotransposons work mainly in the nucleus, in contrast to the cytoplasmic action of Gag proteins of LTR elements. The functional domains found here in SART1 ORF1 will be useful for developing a more efficient and target-specific LINE-based gene delivery vector.
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Affiliation(s)
- Takumi Matsumoto
- Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Kashiwa, Chiba 277-8562, Japan
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20
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Jiang N, Bao Z, Temnykh S, Cheng Z, Jiang J, Wing RA, McCouch SR, Wessler SR. Dasheng: a recently amplified nonautonomous long terminal repeat element that is a major component of pericentromeric regions in rice. Genetics 2002; 161:1293-305. [PMID: 12136031 PMCID: PMC1462185 DOI: 10.1093/genetics/161.3.1293] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
A new and unusual family of LTR elements, Dasheng, has been discovered in the genome of Oryza sativa following database searches of approximately 100 Mb of rice genomic sequence and 78 Mb of BAC-end sequence information. With all of the cis-elements but none of the coding domains normally associated with retrotransposons (e.g., gag, pol), Dasheng is a novel nonautonomous LTR element with high copy number. Over half of the approximately 1000 Dasheng elements in the rice genome are full length (5.6-8.6 kb), and 60% are estimated to have amplified in the past 500,000 years. Using a modified AFLP technique called transposon display, 215 elements were mapped to all 12 rice chromosomes. Interestingly, more than half of the mapped elements are clustered in the heterochromatic regions around centromeres. The distribution pattern was further confirmed by FISH analysis. Despite clustering in heterochromatin, Dasheng elements are not nested, suggesting their potential value as molecular markers for these marker-poor regions. Taken together, Dasheng is one of the highest-copy-number LTR elements and one of the most recent elements to amplify in the rice genome.
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Affiliation(s)
- Ning Jiang
- Departments of Plant Biology and Genetics, University of Georgia, Athens, GA 30602, USA
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McClure MA, Donaldson E, Corro S. Potential multiple endonuclease functions and a ribonuclease H encoded in retroposon genomes. Virology 2002; 296:147-58. [PMID: 12036326 DOI: 10.1006/viro.2002.1392] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Among the retroposons, the source of the endonuclease activity is known to be variable and can be provided as either a retroviral-like integrase or a protein similar to the cellular apurinic-apyrimidinic endonuclease. It has also been reported that other retroposon and retrointron sequences have limited similarity to various eubacterial endonucleases. We investigated whether any retroposon genomes possibly encode multiple endonuclease functions. Amino acid alignments were generated and analyzed for the presence of the characterized ordered-series-of-motifs (OSM) representative of four different endonuclease functions. The results indicate that SLACS, CZAR, CRE1, CRE2, and some Trypanosoma brucei retroposon sequences encode multiple putative endonuclease functions. Interestingly, one of the endonuclease functions is embedded within the potential ribonuclease H sequence found in SLACS, CZAR, CRE1, CRE2, and R2BM retroposons.
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Affiliation(s)
- Marcella A McClure
- Department of Microbiology, Montana State University, Bozeman, Montana 59717, USA.
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22
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Takahashi H, Fujiwara H. Transplantation of target site specificity by swapping the endonuclease domains of two LINEs. EMBO J 2002; 21:408-17. [PMID: 11823433 PMCID: PMC125841 DOI: 10.1093/emboj/21.3.408] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Long interspersed elements (LINEs) are ubiquitous genomic elements in higher eukaryotes. Here we develop a novel assay to analyze in vivo LINE retrotransposition using the telomeric repeat-specific elements SART1 and TRAS1. We demonstrate by PCR that silkworm SART1, which is expressed from a recombinant baculovirus, transposes in Sf9 cells into the chromosomal (TTAGG)n sequences, at the same specific nucleotide position as in the silkworm genome. Thus authentic retrotransposition by complete reverse transcription of the entire RNA transcription unit and occasional 5' truncation is observed. The retrotransposition requires conserved domains in both open reading frames (ORFs), including the ORF1 cysteine- histidine motifs. In contrast to human L1, recognition of the 3' untranslated region sequence is crucial for SART1 retrotransposition, which results in efficient trans-complementation. Swapping the endonuclease domain from TRAS1 into SART1 converts insertion specificity to that of TRAS1. Thus the primary determinant of in vivo target selection is the endonuclease domain, suggesting that modified LINEs could be used as gene therapy vectors, which deliver only genes of interest but not retrotransposons themselves in trans to specific genomic locations.
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Affiliation(s)
- Hidekazu Takahashi
- Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bioscience Building 501, Kashiwa, Chiba 277-8562, Japan
Present address: Department of Molecular Biology and Genetics, Johns Hopkins School of Medicine, Baltimore, MD 21205, USA Corresponding author e-mail:
| | - Haruhiko Fujiwara
- Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bioscience Building 501, Kashiwa, Chiba 277-8562, Japan
Present address: Department of Molecular Biology and Genetics, Johns Hopkins School of Medicine, Baltimore, MD 21205, USA Corresponding author e-mail:
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Kubo Y, Okazaki S, Anzai T, Fujiwara H. Structural and phylogenetic analysis of TRAS, telomeric repeat-specific non-LTR retrotransposon families in Lepidopteran insects. Mol Biol Evol 2001; 18:848-57. [PMID: 11319268 DOI: 10.1093/oxfordjournals.molbev.a003866] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
TRAS1 is a non-LTR retrotransposon inserted specifically into the telomeric repeat (TTAGG)(n) in the silkworm, Bombyx mori. To characterize the evolutionary origin of TRAS-like elements, we identified seven TRAS families (TRAS3, TRAS4, TRAS5, TRAS6, TRASY, TRASZ, and TRASW) from B. mori and four elements from two Lepidoptera, Dictyoploca japonica (TRASDJ) and Samia cynthia ricini (TRASSC3, TRASSC4, and TRASSC9). More than 2,000 copies of various Bombyx TRAS elements accumulated within (TTAGG)(n) sequences as unusual but orderly tandem repeats. The 5' and 3' regions were highly conserved within each class of Bombyx TRAS elements without truncation. This suggests that distinct classes of TRAS have been maintained independently by retrotransposition into (TTAGG)(n). The phylogenetic tree of site-specific retroelements showed that nine TRAS families in Lepidoptera constitute a single phylogenetic group that is closely related to the R1 family that inserts specifically into arthropod 28S rDNA. The higher amino acid sequence identity from endonuclease (EN) to reverse transcriptase (RT) domains between TRAS groups (about 37%-70%) than among TRAS elements and R1Bm (about 25%-30%), may reflect the presence of some DNA structure responsible for their target specificity. Sequence comparison from EN to RT domains among non-LTR elements revealed several regions conserved only within TRAS elements. We found a highly conserved region that resembles the Myb-like DNA-binding structure, between the EN and RT domains. These regions may be involved in site-specific integration of TRAS elements into the (TTAGG)(n) telomeric repeats.
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Affiliation(s)
- Y Kubo
- Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Tokyo, Japan
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24
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Wei W, Gilbert N, Ooi SL, Lawler JF, Ostertag EM, Kazazian HH, Boeke JD, Moran JV. Human L1 retrotransposition: cis preference versus trans complementation. Mol Cell Biol 2001; 21:1429-39. [PMID: 11158327 PMCID: PMC99594 DOI: 10.1128/mcb.21.4.1429-1439.2001] [Citation(s) in RCA: 477] [Impact Index Per Article: 20.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Long interspersed nuclear elements (LINEs or L1s) comprise approximately 17% of human DNA; however, only about 60 of the approximately 400,000 L1s are mobile. Using a retrotransposition assay in cultured human cells, we demonstrate that L1-encoded proteins predominantly mobilize the RNA that encodes them. At much lower levels, L1-encoded proteins can act in trans to promote retrotransposition of mutant L1s and other cellular mRNAs, creating processed pseudogenes. Mutant L1 RNAs are mobilized at 0.2 to 0.9% of the retrotransposition frequency of wild-type L1s, whereas cellular RNAs are mobilized at much lower frequencies (ca. 0.01 to 0.05% of wild-type levels). Thus, we conclude that L1-encoded proteins demonstrate a profound cis preference for their encoding RNA. This mechanism could enable L1 to remain retrotransposition competent in the presence of the overwhelming number of nonfunctional L1s present in human DNA.
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Affiliation(s)
- W Wei
- Department of Human Genetics, The University of Michigan Medical School, Ann Arbor, Michigan 48109, USA
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Anzai T, Takahashi H, Fujiwara H. Sequence-specific recognition and cleavage of telomeric repeat (TTAGG)(n) by endonuclease of non-long terminal repeat retrotransposon TRAS1. Mol Cell Biol 2001; 21:100-8. [PMID: 11113185 PMCID: PMC88784 DOI: 10.1128/mcb.21.1.100-108.2001] [Citation(s) in RCA: 44] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The telomere of the silkworm Bombyx mori consists of (TTAGG/CCTAA)(n) repeats and harbors a large number of telomeric repeat-specific non-long terminal repeat retrotransposons, such as TRAS1 and SART1. To understand how these retrotransposons recognize and integrate into the telomeric repeat in a sequence-specific manner, we expressed the apurinic-apryrimidinic endonuclease-like endonuclease domain of TRAS1 (TRAS1 EN), which is supposed to digest the target DNA, and characterized its enzymatic properties. Purified TRAS1 EN could generate specific nicks on both strands of the telomeric repeat sequence between T and A of the (TTAGG)(n) strand (bottom strand) and between C and T of the (CCTAA)(n) strand (top strand). These sites are consistent with insertion sites expected from the genomic structure of boundary regions of TRAS1. Time course studies of nicking activities on both strands revealed that the cleavages on the bottom strand preceded those on the top strand, supporting the target-primed reverse transcription model. TRAS1 EN could cleave the telomeric repeats specifically even if it was flanked by longer tracts of nontelomeric sequence, indicating that the target site specificity of the TRAS1 element was mainly determined by its EN domain. Based on mutation analyses, TRAS1 EN recognizes less than 10 bp around the initial cleavage site (upstream 7 bp and downstream 3 bp), and the GTTAG sequence especially is essential for the cleavage reaction on the bottom strand (5'. TTAGGTT downward arrow AGG. 3'). TRAS1 EN, the first identified endonuclease digesting telomeric repeats, may be used as a genetic tool to shorten the telomere in insects and some other organisms.
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Affiliation(s)
- T Anzai
- Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
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