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Ty1-copia group retrotransposons are ubiquitous and heterogeneous in higher plants.

We have used the polymerase chain reaction to isolate fragments of Ty1-copia group retrotransposons from a wide variety of members of the higher plant kingdom. 56 out of 57 species tested generate an amplified fragment of the size expected for reverse transcriptase fragments of Ty1-copia group retrotransposons. Sequence analysis of subclones shows that the PCR fragments display varying degrees of sequence heterogeneity. Sequence heterogeneity therefore seems a general property of Ty1-copia group retrotransposons of higher plants, in contrast to the limited diversity seen in retrotransposons of Saccharomyces cerevisiae and Drosophila melanogaster. Phylogenetic analysis of all these sequences shows, with some significant exceptions, that the degree of sequence divergence in the retrotransposon populations between any pair of species is proportional to the evolutionary distance between those species. This implies that sequence divergence during vertical transmission of Ty1-copia group retrotransposons within plant lineages has been a major factor in the evolution of Ty1-copia group retrotransposons in higher plants. Additionally, we suggest that horizontal transmission of this transposon group between different species has also played a role in this process.

Amino Acid Sequence↗

A retrotransposon of the non-long terminal repeat class from the human blood fluke Schistosoma mansoni. Similarities to the chicken-repeat-1-like elements of vertebrates.

The genomes of representative species of fishes, amphibians, and reptiles contain non-long-terminal-repeat (non-LTR) retrotransposons showing strong sequence identity to the chicken repeat 1 (CR1) non-LTR retrotransposon from birds. These nonavian retroelements have been termed CR1-like elements. We have isolated sequences of a non-LTR retrotransposon from the human blood fluke Schistosoma mansoni. These schistosome sequences, which we have termed the SR1 family of non-LTR retrotransposons, contain regions of deduced amino acids characteristic of the CR1-like elements. SR1 elements possess atypical 3' termini consisting of the tandem repeat (AACCATTTG)2 which are similar in structure to the imperfect tandem repeat of the 3' termini of CR1. There are at least 200 copies of SR1 interspersed through the genome of S. mansoni. The structural and amino acid sequence similarities of SR1 with members of the CR1-like elements suggest that the SR1 family belongs to the CR1-like category of non-LTR retrotransposons. Although other non-LTR retrotransposons have been described in invertebrates, this is the first CR1-like element reported from a nonvertebrate taxon, suggesting that the phylogenetic distribution of CR1-like retrotransposons is not restricted to vertebrates.

Amino Acid Sequence↗

Transcriptional activation of mouse retrotransposons in vivo: specific expression in steroidogenic cells in response to trophic hormones.

Transcription of cellular retrotransposons is induced by a variety of physiological stimuli. We have used in situ hybridization analysis to determine the cell types in which mouse retrotransposons are transcriptionally activated in vivo under physiological conditions. Here, we report that VL30 retrotransposons are specifically expressed in steroidogenic cells within all four endocrine tissues engaged in synthesis of steroid hormones in response to the respective pituitary-derived trophic hormones. These tissues include ovarian steroidogenic theca cells and lutein cells of the corpus luteum, testosterone-producing Leydig cells of the testis, steroidogenic cells confined to the zona reticularis of the adrenal cortex, and progesterone-producing cells of the placenta. In the course of preovulatory follicular development and maturation, the profile of cells expressing the retrotransposon shifted in parallel to the changing profiles of the leutinizing hormone (LH)-induced steroidogenic output of the respective cells. Expression of VL30 in both male and female gonads was shown to be greatly stimulated by external administration of gonadotropins. In vitro studies using a LH-responsive Leydig cell line have confirmed that expression of the resident retrotransposons is gonadotropin dependent. Run-off transcription assays have indicated that activation is at the transcriptional level. To allow molecular access to gonadotropin-activated transcription units, the long terminal repeat (LTR) regulatory domains were cloned from VL30 cDNAs of LH-induced ovaries. Through the use of reporter gene constructs and transfection experiments it was shown that expression of these elements in steroidogenic cells is LH dependent. Furthermore, cAMP, a known mediator of trophic hormone responses, could replace the hormone for inducibility. Transfection studies have also shown that the retrotransposon LTRs may function as hormone-activated enhancers conferring a LH-dependent phenotype on a surrogate transcription unit. These studies have thus demonstrated that the transcriptional activation of resident retrotransposons in vivo is a dynamic process that can be modulated by gonadotropins and have the potential of imposing this phenotype on adjacent cellular genes.

Animals↗

Analyses of LTR-retrotransposon structures reveal recent and rapid genomic DNA loss in rice.

We initially analyzed 11 families of low- and middle-copy-number long terminal repeat (LTR) retrotransposons in rice to determine how their structures have diverged from their predicted ancestral forms. These elements, many highly fragmented, were identified on the basis of sequence homology and structural characteristics. The 11 families, totaling 1000 elements, have copy numbers ranging from 1 to 278. Less than one-quarter of these elements are intact, whereas the remaining are solo LTRs and variously truncated fragments. We also analyzed two highly repetitive families (Osr8 and Osr30) of LTR retrotransposons and observed the same results. Our data indicate that unequal homologous recombination and illegitimate recombination are primarily responsible for LTR-retrotransposon removal. Further analysis suggests that most of the detectable LTR retrotransposons in rice inserted less than 8 million years ago, and have now lost over two-thirds of their encoded sequences. Hence, we predict that the half-life of LTR-retrotransposon sequences in rice is less than 6 million years. Moreover, our data demonstrate that at least 22% (97 Mb) of the current rice genome is comprised of LTR-retrotransposon sequences, and that more than 190 Mb of LTR-retrotransposon sequences have been deleted from the rice genome in the last 8 million years.

Base Sequence↗

Genomic evolution of the long terminal repeat retrotransposons in hemiascomycetous yeasts.

We identified putative long terminal repeat- (LTR) retrotransposon sequences among the 50,000 random sequence tags (RSTs) obtained by the Génolevures project from genomic libraries of 13 Hemiascomycetes species. In most cases additional sequencing enabled us to assemble the whole sequences of these retrotransposons. These approaches identified 17 distinct families, 10 of which are defined by full-length elements. We also identified five families of solo LTRs that were not associated with retrotransposons. Ty1-like retrotransposons were found in four of five species that are phylogenetically related to Saccharomyces cerevisiae (S. uvarum, S. exiguus, S. servazzii, and S. kluyveri but not Zygosaccharomyces rouxii), and in two of three Kluyveromyces species (K. lactis and K. marxianus but not K. thermotolerans). Only multiply crippled elements could be identified in the K. lactis and S. servazzii strains analyzed, and only solo LTRs could be identified in S. uvarum. Ty4-like elements were only detected in S. uvarum, indicating that these elements appeared recently before speciation of the Saccharomyces sensu stricto species. Ty5-like elements were detected in S. exiguus, Pichia angusta, and Debaryomyces hansenii. A retrotransposon homologous with Tca2 from Candida albicans, an element absent from S. cerevisiae, was detected in the closely related species D. hansenii. A complete Ty3/gypsy element was present in S. exiguus, whereas only partial, often degenerate, sequences resembling this element were found in S. servazzii, Z. rouxii, S. kluyveri, C. tropicalis, and Yarrowica lipolytica. P. farinosa (syn. P. sorbitophila) is currently the only yeast species in which no LTR retrotransposons or remnants have been found. Thorough analysis of protein sequences, structural characteristics of the elements, and phylogenetic relationships deduced from these data allowed us to propose a classification for the Ty1/copia elements of hemiascomycetous yeasts and a model of LTR-retrotransposon evolution in yeasts.

Amino Acid Sequence↗

A family of neofunctionalized Ty3/gypsy retrotransposon genes in mammalian genomes.

A family of at least eleven genes called Mar related to long terminal repeat retrotransposons from the Ty3/gypsy group, including two genes previously identified as such, is present in human and mouse genomes. Single orthologous copies were identified for most Mar genes in different mammals. All of them have lost essential structural features necessary for autonomous retrotransposition before divergence between mouse and human. Three Mar genes also have introns at identical positions in human and mouse. Hence, Mar genes do not correspond to functional retrotransposons. Mar genes evolved under purifying selection, strongly suggesting that they are not pseudogenic relics but rather neofunctionalized retrotransposon genes. All putative Mar proteins display sequence similarity to the capsid-like domain of the Gag protein of Tf1/Sushi retrotransposons. In addition, three Mar proteins have conserved the Gag CCHC zinc finger motif, suggesting a role in nucleic acid binding. Some Mar genes have also retained from their retrotransposon origin a -1 ribosomal frameshifting between the gag-related open reading frame and a region encoding a putative aspartyl protease domain. EST analysis revealed that the majority of Mar genes are expressed in brain as well as in other tissues and organs. Some Mar proteins might function as transcription factors or be involved in the control of cell proliferation and apoptosis. Strikingly, as many as eight Mar genes are located on the X chromosome in human, mouse and other mammals, and at least two of the autosomal genes are subject to imprinting. We suggest that retrotransposons might be a source for epigenetically regulated genes. Epigenetic regulation of these neogenes might be derived from the cellular defense mechanisms having controlled their retrotransposon ancestor.

Animals↗

The Sinbad retrotransposon from the genome of the human blood fluke, Schistosoma mansoni, and the distribution of related Pao-like elements.

BACKGROUND: Of the major families of long terminal repeat (LTR) retrotransposons, the Pao/BEL family is probably the least well studied. It is becoming apparent that numerous LTR retrotransposons and other mobile genetic elements have colonized the genome of the human blood fluke, Schistosoma mansoni. RESULTS: A proviral form of Sinbad, a new LTR retrotransposon, was identified in the genome of S. mansoni. Phylogenetic analysis indicated that Sinbad belongs to one of five discreet subfamilies of Pao/BEL like elements. BLAST searches of whole genomes and EST databases indicated that members of this clade occurred in species of the Insecta, Nematoda, Echinodermata and Chordata, as well as Platyhelminthes, but were absent from all plants, fungi and lower eukaryotes examined. Among the deuterostomes examined, only aquatic species harbored these types of elements. All four species of nematode examined were positive for Sinbad sequences, although among insect and vertebrate genomes, some were positive and some negative. The full length, consensus Sinbad retrotransposon was 6,287 bp long and was flanked at its 5'- and 3'-ends by identical LTRs of 386 bp. Sinbad displayed a triple Cys-His RNA binding motif characteristic of Gag of Pao/BEL-like elements, followed by the enzymatic domains of protease, reverse transcriptase (RT), RNAseH, and integrase, in that order. A phylogenetic tree of deduced RT sequences from 26 elements revealed that Sinbad was most closely related to an unnamed element from the zebrafish Danio rerio and to Saci-1, also from S. mansoni. It was also closely related to Pao from Bombyx mori and to Ninja of Drosophila simulans. Sinbad was only distantly related to the other schistosome LTR retrotransposons Boudicca, Gulliver, Saci-2, Saci-3, and Fugitive, which are gypsy-like. Southern hybridization and bioinformatics analyses indicated that there were about 50 copies of Sinbad in the S. mansoni genome. The presence of ESTs representing transcripts of Sinbad in numerous developmental stages of S. mansoni along with the identical 5'- and 3'-LTR sequences suggests that Sinbad is an active retrotransposon. CONCLUSION: Sinbad is a Pao/BEL type retrotransposon from the genome of S. mansoni. The Pao/BEL group appears to be comprised of at least five discrete subfamilies, which tend to cluster with host species phylogeny. Pao/BEL type elements appear to have colonized only the genomes of the Animalia. The distribution of these elements in the Ecdysozoa, Deuterostomia, and Lophotrochozoa is discontinuous, suggesting horizontal transmission and/or efficient elimination of Pao-like mobile genetic elements from some genomes.

Amino Acid Motifs↗

[Polymorphism of virus-like particles of retrotransposons in Drosophila and yeast cells].

Data on the molecular arrangement of viruslike particles (VLPs) of yeast and Drosophila retrotransposons are presented. Two methods for identifying VLPs from specific retrotransposon families have been offered. The first method is based on VLPs fractionation by electrophoresis in agarose gel under strictly controlled conditions. VLPs of the Drosophila melanogaster retrotransposon families copia and gypsy and D. virilis retrotransposon Tv1 were identified by this method. The method based on heterologous induction of retrotransposons in cells of the mutant spt3 strain of Saccharomyces cerevisiae was used to identify VLPs of yeast retrotransposon Tyl and D. melanogaster gypsy retrotransposon.

Animals↗

Do the integrases of LTR-retrotransposons and class II element transposases have a common ancestor?

The integrases of retrotransposons (class I) and retroviruses and the transposases of bacterial type elements (class II) were compared. The DDE signature that is crucial for the integration of these elements is present in most of them, except for the non-LTR retrotransposons and members of the hAT and P super-families. Alignment of this region was used to infer the relationships between class II elements, retrotransposons, and retroviruses. The mariner-Tc1 and the Pogo-Fot1 super-families were found to be closely related and probably monophyletic, as were LTR retrotransposons and retroviruses. The IS elements of bacteria were clustered in several families, some of them being closely related to the transposase of the mariner-Tc1 super-family or to the LTR retrotransposon and retrovirus integrases. These results plus that of Xiong and Eickbush (1990) were used to develop an evolutionary history suggesting a common ancestral origin(s) for the integrases and transposases containing the DDE signature. The position of the telomeric elements (Het-A and TART) was assessed by comparing their gag and reverse transcriptase domains (when present) to those of group II introns and non-LTR retrotransposons. This preliminary analysis suggests that telomeric elements may be derived from non-LTR retrotransposons.

Amino Acid Sequence↗

The Ty1-copia group of retrotransposons in plants: genomic organisation, evolution, and use as molecular markers.

The genomic organisation and diversity of the Ty1-copia group retrotransposons has been investigated in several crop plants and their relatives from both dicotyledonous and monocotyledonous families, including potato (Solanum tuberosum), faba beans (Vicia faba), Vicia melanops, Vicia sativa, barley (Hordeum vulgare), rye (Secale cereale), and onion (Allium cepa). Extreme heterogeneity in the sequence of the Ty1-copia retrotransposons from all these plants was revealed following sequence analysis of reverse transcriptase fragments. The estimated copy numbers of the Ty1-copia group retrotransposons for the genomes of S. tuberosum, L. esculentum, A. cepa, S. cereale, and V. faba is highly variable, ranging from a few hundred to approximately a million copies per genome. In situ hybridisation data from metaphase and prophase chromosomes of V. faba, S. cereale, and H. vulgare suggest that retrotransposon sequences are dispersed throughout the euchromatic regions of the genome but are almost undetectable in most heterochromatic regions. In contrast, similar data from metaphase chromosomes of A. cepa suggests that although retrotransposon sequences are dispersed throughout the euchromatic regions of the genome, they are predominantly concentrated in the terminal heterochromatin. These results are discussed in the context of the role played by the Ty1-copia group retrotransposons in the evolution of the plant genome. Lastly, the application of retrotransposon sequences as genetic markers for mapping genomes and for studying genetic biodiversity in plants is presented.

Chromosome Mapping↗

Activation of tobacco retrotransposons during tissue culture.

Sequences of at least three new families of retrotransposons (Tto1-Tto3) were amplified by PCR from cDNA prepared from protoplasts of an established tobacco cell line, based on the fact that certain amino acids are highly conserved in the reverse transcriptases encoded by retrotransposons. Structural analysis indicates that Tto1 is 5.5 kb long and has features typical of retrotransposons. Transcription of Tto1 starting in the long terminal repeat was active only in cultured cells. Protoplast formation enhanced the transcription. The copy number of Tto1 increased 10-fold in established cell lines; it also increased in plants regenerated from tissue cultures and in transgenic plants. These results indicate that Tto1 is activated during tissue culture. This is the first demonstration of activation of a plant retrotransposon by tissue culture. The copy number of Tto2 and a previously isolated transposon, Tnt1, also increased in established cell lines, indicating that these two retrotransposons may also be activated by tissue culture. These three retrotransposons are cryptic in normally propagated plants: no difference in the copy number was observed between individuals of the same cultivars or even between different cultivars.

Amino Acid Sequence↗

Phylogeny and transpositional activity of Ty1-copia group retrotransposons in cereal genomes.

The Ty1-copia group retrotransposon populations of barley (Hordeum vulgare) and bread wheat (Triticum aestivum) have been characterised by degenerate PCR and sequence analysis of fragments of the reverse transcriptase genes. The barley population is comprised of a highly heterogeneous set of retrotransposons, together with a collection of sequences that are closely related to the BARE-1 element. Wheat also contains a highly diverse Ty1-copia retrotransposon population, together with a less prominent BARE-1 subgroup. These data have been combined with previously published Gramineae sequences to construct a composite phylogenetic tree for this class of retrotransposons in cereal grasses. The analysis indicates that the ancestral Gramineae genome contained a heterogeneous population of Ty1-copia group retrotransposons, the descendants of which have proliferated to differing degrees in present-day species. Lastly, the level of recent transpositional activity of two Ty1-copia elements has been estimated by measuring their insertional polymorphism within species. Both transposons are highly polymorphic within all species tested. These data suggest that transposition proficiency may be a common and evolutionarily stable feature of the Ty1-copia group retrotransposons of cereal grasses.

Base Sequence↗

TRIM retrotransposons occur in apple and are polymorphic between varieties but not sports.

Retrotransposon markers have been demonstrated to be powerful tools for investigating linkage, evolution and genetics diversity in plants. In the present study, we identified and cloned three full-size TRIM (terminal-repeat retrotransposon in miniature) group retrotransposon elements from apple (Malus domestica) cv. 'Antonovka', the first from the Rosaceae. To investigate their utility as markers, we designed primers to match the long terminal repeats (LTRs) of the apple TRIM sequences. We found that PCR reactions with even a single primer produced multiple bands, suggesting that the copy number of these TRIM elements is relatively high, and that they may be locally clustered or nested in the genome. Furthermore, the apple TRIM primers employed in IRAP (inter-retrotransposon amplified polymorphism) or REMAP (retrotransposon-microsatellite amplified polymorphism) analyses produced unique, reproducible profiles for 12 standard apple cultivars. On the other hand, all seven of the sport mutations in this study were identical to their mother cultivar. Genetic similarity values calculated from the IRAP/REMAP analyses or the STMS (sequence tagged microsatellite sites) analysis were generally comparable. PAUP cluster analysis based on IRAP and REMAP markers in apple and Japanese quince generated an NJ tree that is in good accordance with both a tree based on SMTS markers and the origin of the studied samples. Our results demonstrate that, although they do not encode the proteins necessary to carry out a life cycle and are thereby non-autonomous, TRIMs are at least as polymorphic in their insertion patterns as conventional complete retrotransposons.

DNA, Plant↗

A long terminal repeat retrotransposon Cgret from the phytopathogenic fungus Colletotrichum gloeosporioides on cranberry.

A repetitive DNA element cloned from the cranberry fruit rot pathogen Colletotrichum gloeosporioides has been characterized. Sequence data indicate that it is a long terminal repeat (LTR) retrotransposon of 7,916 base pairs. LTR of 544 base pairs occur at either end of an internal region of 6,828 base pairs. This element, designated Cgret (C. gloeosporioides retrotransposon), encodes two putative polypeptides which have high homology to the gag and pol genes of other fungal retrotransposons. The sequence and structure suggest that Cgret is a member of the gypsy group of LTR retrotransposons. The Cgret retrotransposon was present in all of the cranberry isolates of the fungus C. gloeosporioides from New Jersey and Massachusetts, but not in the cranberry isolates from Wisconsin or Chile. Polymorphisms were detected among field isolates of C. gloeosporioides from various hosts, using hybridization probes derived from the LTR and the reverse transcriptase domain of Cgret. The structural integrity of Cgret suggests that it is still a functional retrotransposon and may be used as a molecular marker to study the genetic diversity distribution of this fungal pathogen.

Chile↗

Comparison of the utility of barley retrotransposon families for genetic analysis by molecular marker techniques.

The Sequence-Specific Amplification Polymorphism (S-SAP) method, and the related molecular marker techniques IRAP (inter-retrotransposon amplified polymorphism) and REMAP (retrotransposon-microsatellite amplified polymorphism), are based on retrotransposon activity, and are increasingly widely used. However, there have been no systematic analyses of the parameters of these methods or of the utility of different retrotransposon families in producing polymorphic, scorable fingerprints. We have generated S-SAP, IRAP, and REMAP data for three barley (Hordeum vulgare L.) varieties using primers based on sequences from six retrotransposon families (BARE-1, BAGY-1, BAGY-2, Sabrina, Nikita and Sukkula). The effect of the number of selective bases on the S-SAP profiles has been examined and the profiles obtained with eight MseI+3 selective primers compared for all the elements. Polymorphisms detected in the insertion pattern of all the families show that each can be used for S-SAP. The uniqueness of each transposition event and differences in the historic activity of each family suggest that the use of multiple retrotransposon families for genetic analysis will find applications in mapping, fingerprinting, and marker-assisted selection and evolutionary studies, not only in barley and other Hordeum species and related taxa, but also more generally.

DNA Fingerprinting↗

Retrotransposon populations of Vicia species with varying genome size.

The (non-LTR) LINE and Ty3-gypsy-type LTR retrotransposon populations of three Vicia species that differ in genome size (Vicia faba, Vicia melanops and Vicia sativa) have been characterised. In each species the LINE retrotransposons comprise a complex, very heterogeneous set of sequences, while the Ty3-gypsy elements are much more homogeneous. Copy numbers of all three retrotransposon groups (Ty1-copia, Ty3-gypsy and LINE) in these species have been estimated by random genomic sequencing and Southern hybridisation analysis. The Ty3-gypsy elements are extremely numerous in all species, accounting for 18-35% of their genomes. The Ty1-copia group elements are somewhat less abundant and LINE elements are present in still lower amounts. Collectively, 20-45% of the genomes of these three Vicia species are comprised of retrotransposons. These data show that the three retrotransposon groups have proliferated to different extents in members of the Vicia genus and high proliferation has been associated with homogenisation of the retrotransposon population.

Blotting, Southern↗

Molecular analysis of the yeast Ty4 element: homology with Ty1, copia, and plant retrotransposons.

The element; Ty4 is a retrotransposon present in low copy number in the genome of Saccharomyces cerevisiae [Stucka et al., Nucleic Acids Res. 17 (1989) 4993-5001]. We have determined the complete nucleotide sequence of one such element from a particular strain and compared it to the other two elements occurring in this strain. The genomic organization of Ty4 is homologous to that found in other retrotransposons of the Ty1-copia group. The internal part of the element contains two large open reading frames (TY4A and TY4B) overlapping by 226 bp in a + 1 mode. TY4A reveals characteristics of the gag portion of retrotransposons and retroviruses, while TY4B consists of a protease, an integrase, a reverse transcriptase, and an RNase H domain (in that order). Our analyses suggest that only one of these copies might be transpositionally active. Sequence comparisons at the amino acid level show that the domains in Ty4 diverge considerably from those of other retrotransposons. The greatest similarity is seen between the reverse transcriptases (50%), the proteases (40%), and the integrases (30%) of Ty4, Ty1/2 and copia, respectively, whereas the degree of similarity for all other entities of these elements is much lower. Considering evolutionary aspects of the retrotransposons, we have to conclude that Ty4 has diverged at an early stage from the progenitors of other known retroelements and represents a novel and independent subgroup of the Ty1-copia class of retrotransposons.

Amino Acid Sequence↗

Identification of 18 new transcribed retrotransposons in Schistosoma mansoni.

This work describes 18 new transcribed retrotransposons of the blood fluke Schistosoma mansoni. Among them, 9 were LTR, 8 non-LTR, and 1 Penelope-like element (PLE) retrotransposon. Sequences were generated by in silico reconstruction using S. mansoni ESTs and transcripts obtained by rapid amplification of cDNA ends, complemented in some cases by sequencing of genomic clones amplified by PCR. A novel element from the ancient R2/R4/CRE transposon group is described for the first time in S. mansoni. In addition, one non-LTR retrotransposon family displays long (40-450 bp) 3'-UTR with at least six different transcribed sequences among the copies, five LTR retrotransposons have abundantly transcribed incomplete copies lacking the sequence segment coding for the reverse transcriptase domain, and four non-LTR retrotransposons code for DNA-binding PHD domains that may give them a differential targeting. These results allow for a comprehensive description of the transcribed retrotransposon diversity of this complex human parasite.

Animals↗