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LTR retrotransposon

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A. Genetic structure of LTR-retrotransposons (gypsy-type). B. Mechanism of retrotransposition, occurring inside viral-like particles in the cytoplasm. Reverse-transcription initiates at a host tRNA primer binding site (PBS) located immediately downstream of the 5’LTR. The newly synthesized minus-strand cDNA copy of the 5’LTR is then transferred to the 3’LTR and used as a primer for reverse-transcription of the entire minus-strand sequence. An RNase H-resistant polypurine tract then serves as a primer for plus-strand synthesis of the 3’LTR and complementary PBS. The newly-synthesized plus-strand PBS then associates with the already-synthesized minus-strand PBS, and double-stranded cDNA is finally produced. Double-stranded cDNA is then transferred to the nucleus by integrase proteins, and a new copy is integrated into the genome.

LTR retrotransposons are class I transposable element characterized by the presence of Long Terminal Repeats (LTRs) directly flanking an internal coding region. As retrotransposons, they mobilize through reverse-transcription of their mRNA and integration of the newly created cDNA into another location. Their mechanism of retrotransposition is shared with retroviruses, with the difference that most LTR-retrotransposons do not form infectious particles that leave the cells and therefore only replicate inside their genome of origin.

Their size ranges from a few hundred base pairs to 25kb, like the Ogre retrotransposon in the Pea genome

In plant genomes, LTR retrotransposons are the major repetitive sequence class, e.g. able to constitute more than 75% of the maize genome.[1]

Structure and propagation

LTR retrotransposons have direct long terminal repeat that range from ~100 bp to over 5 kb in size. LTR retrotransposons are further sub-classified into the Ty1-copia-like (Pseudoviridae), Ty3-gypsy-like (Metaviridae), and BEL-Pao-like groups based on both their degree of sequence similarity and the order of encoded gene products. Ty1-copia and Ty3-gypsy groups of retrotransposons are commonly found in high copy number (up to a few million copies per haploid nucleus) in animals, fungi, protista, and plants genomes. BEL-Pao like elements have so far only been found in animals.[2][3]

All functional LTR-retrotransposons encode a minimum of two genes, gag and pol, that are sufficient for their replication. Gag encodes a polyprotein with a capsid and a nucleocapsid domain.[4] Gag proteins form virus-like particles in the cytoplasm inside which reverse-transcription occurs. The Pol gene produces three proteins: a protease (PR), a reverse-transcriptase endowed with an RT (reverse-transcriptase) and an RNAse H domains, and an integrase (IN).[5]

Typically, LTR-retrotransposon mRNAs are produced by the host RNA pol II acting on a promoter located in their 5’ LTR. The Gag and Pol genes are encoded in the same mRNA. Depending on the host species, two different strategies can be used to express the two polyproteins: a fusion into a single open reading frame (ORF) that is then cleaved or the introduction of a frameshift between the two ORFs.[6] Occasional ribosomal frameshifting allows the production of both proteins, while ensuring that much more Gag protein is produced to form virus-like particles.

Reverse-transcription usually initiates at a short sequence located immediately downstream of the 5’-LTR and termed the primer binding site (PBS). Specific host tRNAs bind to the PBS and act as primers for reverse-transcription, which occurs in a complex and multi-step process, ultimately producing a double- stranded cDNA molecule. The cDNA is finally integrated into a new location, creating short TSDs and adding a new copy in the host genome

Types

Ty1-copia retrotransposons

Ty1-copia retrotransposons are abundant in species ranging from single-cell algae to bryophytes, gymnosperms, and angiosperms. They encode four protein domains in the following order: protease, integrase, reverse transcriptase, and ribonuclease H.

At least two classification systems exist for the subdivision of Ty1-copia retrotransposons into five lineages:[7][8] Sireviruses/Maximus, Oryco/Ivana, Retrofit/Ale, TORK (subdivided in Angela/Sto, TAR/Fourf, GMR/Tork), and Bianca.

Sireviruses/Maximus retrotransposons contain an additional putative envelope gene. This lineage is named for the founder element SIRE1 in the Glycine max genome,[9] and was later described in many species such as Zea mays,[10] Arabidopsis thaliana,[11] Beta vulgaris,[12] and Pinus pinaster.[13] Plant Sireviruses of many sequenced plant genomes are summarized at the MASIVEdb Sirevirus database.[14]

Ty3-gypsy retrotransposons

Ty3-gypsy retrotransposons (Metaviridae) are widely distributed in the plant kingdom, including both gymnosperms and angiosperms. They encode at least four protein domains in the order: protease, reverse transcriptase, ribonuclease H, and integrase. Based on structure, presence/absence of specific protein domains, and conserved protein sequence motifs, they can be subdivided into several lineages:

Errantiviruses contain an additional defective envelope ORF with similarities to the retroviral envelope gene. First described as Athila-elements in Arabidopsis thaliana,[15][16] they have been later identified in many species, such as Glycine max[17] and Beta vulgaris.[18]

Chromoviruses contain an additional chromodomain (chromatin organization modifier domain) at the C-terminus of their integrase protein.[19][20] They are widespread in plants and fungi, probably retaining protein domains during evolution of these two kingdoms.[21] It is thought that the chromodomain directs retrotransposon integration to specific target sites.[22] According to sequence and structure of the chromodomain, chromoviruses are subdivided into the four clades CRM, Tekay, Reina and Galadriel. Chromoviruses from each clade show distinctive integration patterns, e.g. into centromeres or into the rRNA genes.[23][24]

Ogre-elements are gigantic Ty3-gypsy retrotransposons reaching lengths up to 25 kb.[25] Ogre elements have been first described in Pisum sativum.[26]

Metaviruses describe conventional Ty3-gypsy retrotransposons that do not contain additional domains or ORFs.

Endogenous retroviruses (ERV)

Main article: Endogenous retrovirus

Although retroviruses are often classified separately, they share many features with LTR retrotransposons. A major difference with Ty1-copia and Ty3-gypsy retrotransposons is that retroviruses have an envelope protein (ENV). A retrovirus can be transformed into an LTR retrotransposon through inactivation or deletion of the domains that enable extracellular mobility. If such a retrovirus infects and subsequently inserts itself in the genome in germ line cells, it may become transmitted vertically and become an Endogenous Retrovirus.[3]

Endogenous retroviruses are the most important LTR retrotransposons in mammals, making up about 8% of the human genome and approximately 10% of the mouse genome.[27]

References

  1. ^ Baucom, RS; Estill, JC; Chaparro, C; Upshaw, N; Jogi, A; Deragon, JM; Westerman, RP; Sanmiguel, PJ; Bennetzen, JL (November 2009). "Exceptional diversity, non-random distribution, and rapid evolution of retroelements in the B73 maize genome". PLoS Genetics. 5 (11): e1000732. doi:10.1371/journal.pgen.1000732. PMC 2774510. PMID 19936065.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  2. ^ Copeland CS, Mann VH, Morales ME, Kalinna BH, Brindley PJ (2005). "The Sinbad retrotransposon from the genome of the human blood fluke, Schistosoma mansoni, and the distribution of related Pao-like elements". BMC Evol. Biol. 5 (1): 20. doi:10.1186/1471-2148-5-20. PMC 554778. PMID 15725362.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  3. ^ a b Wicker T, Sabot F, Hua-Van A, et al. (December 2007). "A unified classification system for eukaryotic transposable elements". Nat. Rev. Genet. 8 (12): 973–82. doi:10.1038/nrg2165. PMID 17984973.
  4. ^ Sandmeyer, Suzanne B; Clemens, Kristina A (2010). "Function of a retrotransposon nucleocapsid protein". RNA Biology. 7 (6): 642–654. doi:10.4161/rna.7.6.14117. ISSN 1547-6286. PMC 3073325. PMID 21189452.
  5. ^ Wicker, Thomas; Sabot, François; Hua-Van, Aurélie; Bennetzen, Jeffrey L.; Capy, Pierre; Chalhoub, Boulos; Flavell, Andrew; Leroy, Philippe; Morgante, Michele (December 2007). "A unified classification system for eukaryotic transposable elements". Nature Reviews. Genetics. 8 (12): 973–982. doi:10.1038/nrg2165. ISSN 1471-0064. PMID 17984973.
  6. ^ GAO, XIANG; HAVECKER, ERICKA R.; BARANOV, PAVEL V.; ATKINS, JOHN F.; VOYTAS, DANIEL F. (December 2003). "Translational recoding signals between gag and pol in diverse LTR retrotransposons". RNA. 9 (12): 1422–1430. doi:10.1261/rna.5105503. ISSN 1355-8382. PMC 1370496. PMID 14623998.
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  9. ^ Laten, HM; Majumdar, A; Gaucher, EA (9 June 1998). "SIRE-1, a copia/Ty1-like retroelement from soybean, encodes a retroviral envelope-like protein". Proceedings of the National Academy of Sciences of the United States of America. 95 (12): 6897–902. doi:10.1073/pnas.95.12.6897. PMC 22677. PMID 9618510.
  10. ^ Bousios, A; Kourmpetis, YA; Pavlidis, P; Minga, E; Tsaftaris, A; Darzentas, N (February 2012). "The turbulent life of Sirevirus retrotransposons and the evolution of the maize genome: more than ten thousand elements tell the story". The Plant Journal. 69 (3): 475–88. doi:10.1111/j.1365-313x.2011.04806.x. PMID 21967390.
  11. ^ Kapitonov, VV; Jurka, J (1999). "Molecular paleontology of transposable elements from Arabidopsis thaliana". Genetica. 107 (1–3): 27–37. doi:10.1023/a:1004030922447. PMID 10952195.
  12. ^ Weber, B; Wenke, T; Frömmel, U; Schmidt, T; Heitkam, T (February 2010). "The Ty1-copia families SALIRE and Cotzilla populating the Beta vulgaris genome show remarkable differences in abundance, chromosomal distribution, and age". Chromosome Research. 18 (2): 247–63. doi:10.1007/s10577-009-9104-4. PMID 20039119.
  13. ^ Miguel, C; Simões, M; Oliveira, MM; Rocheta, M (November 2008). "Envelope-like retrotransposons in the plant kingdom: evidence of their presence in gymnosperms (Pinus pinaster)". Journal of Molecular Evolution. 67 (5): 517–25. doi:10.1007/s00239-008-9168-3. PMID 18925379.
  14. ^ Bousios, A; Minga, E; Kalitsou, N; Pantermali, M; Tsaballa, A; Darzentas, N (30 April 2012). "MASiVEdb: the Sirevirus Plant Retrotransposon Database". BMC Genomics. 13: 158. doi:10.1186/1471-2164-13-158. PMC 3414828. PMID 22545773.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  15. ^ Pélissier, T; Tutois, S; Deragon, JM; Tourmente, S; Genestier, S; Picard, G (November 1995). "Athila, a new retroelement from Arabidopsis thaliana". Plant Molecular Biology. 29 (3): 441–52. doi:10.1007/bf00020976. PMID 8534844.
  16. ^ Wright, DA; Voytas, DF (June 1998). "Potential retroviruses in plants: Tat1 is related to a group of Arabidopsis thaliana Ty3/gypsy retrotransposons that encode envelope-like proteins". Genetics. 149 (2): 703–15. PMC 1460185. PMID 9611185.
  17. ^ Wright, DA; Voytas, DF (January 2002). "Athila4 of Arabidopsis and Calypso of soybean define a lineage of endogenous plant retroviruses". Genome Research. 12 (1): 122–31. doi:10.1101/gr.196001. PMC 155253. PMID 11779837.
  18. ^ Wollrab, C; Heitkam, T; Holtgräwe, D; Weisshaar, B; Minoche, AE; Dohm, JC; Himmelbauer, H; Schmidt, T (November 2012). "Evolutionary reshuffling in the Errantivirus lineage Elbe within the Beta vulgaris genome". The Plant Journal. 72 (4): 636–51. doi:10.1111/j.1365-313x.2012.05107.x. PMID 22804913.
  19. ^ Marín, I; Lloréns, C (July 2000). "Ty3/Gypsy retrotransposons: description of new Arabidopsis thaliana elements and evolutionary perspectives derived from comparative genomic data". Molecular Biology and Evolution. 17 (7): 1040–9. doi:10.1093/oxfordjournals.molbev.a026385. PMID 10889217.
  20. ^ Gorinsek, B; Gubensek, F; Kordis, D (May 2004). "Evolutionary genomics of chromoviruses in eukaryotes". Molecular Biology and Evolution. 21 (5): 781–98. doi:10.1093/molbev/msh057. PMID 14739248.
  21. ^ Novikova, O; Smyshlyaev, G; Blinov, A (8 April 2010). "Evolutionary genomics revealed interkingdom distribution of Tcn1-like chromodomain-containing Gypsy LTR retrotransposons among fungi and plants". BMC Genomics. 11: 231. doi:10.1186/1471-2164-11-231. PMC 2864245. PMID 20377908.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  22. ^ Gao, X; Hou, Y; Ebina, H; Levin, HL; Voytas, DF (March 2008). "Chromodomains direct integration of retrotransposons to heterochromatin". Genome Research. 18 (3): 359–69. doi:10.1101/gr.7146408. PMC 2259100. PMID 18256242.
  23. ^ Neumann, P; Navrátilová, A; Koblížková, A; Kejnovský, E; Hřibová, E; Hobza, R; Widmer, A; Doležel, J; Macas, J (3 March 2011). "Plant centromeric retrotransposons: a structural and cytogenetic perspective". Mobile DNA. 2 (1): 4. doi:10.1186/1759-8753-2-4. PMC 3059260. PMID 21371312.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  24. ^ Weber, B; Heitkam, T; Holtgräwe, D; Weisshaar, B; Minoche, AE; Dohm, JC; Himmelbauer, H; Schmidt, T (1 March 2013). "Highly diverse chromoviruses of Beta vulgaris are classified by chromodomains and chromosomal integration". Mobile DNA. 4 (1): 8. doi:10.1186/1759-8753-4-8. PMC 3605345. PMID 23448600.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  25. ^ Macas, J; Neumann, P (1 April 2007). "Ogre elements--a distinct group of plant Ty3/gypsy-like retrotransposons". Gene. 390 (1–2): 108–16. doi:10.1016/j.gene.2006.08.007. PMID 17052864.
  26. ^ Neumann, P; Pozárková, D; Macas, J (October 2003). "Highly abundant pea LTR retrotransposon Ogre is constitutively transcribed and partially spliced". Plant Molecular Biology. 53 (3): 399–410. doi:10.1023/b:plan.0000006945.77043.ce. PMID 14750527.
  27. ^ McCarthy EM, McDonald JF (2004). "Long terminal repeat retrotransposons of Mus musculus". Genome Biol. 5 (3): R14. doi:10.1186/gb-2004-5-3-r14. PMC 395764. PMID 15003117.{{cite journal}}: CS1 maint: unflagged free DOI (link)
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