Modeling early germline immunization after horizontal transfer of transposable elements reveals internal piRNA cluster heterogeneity.

BACKGROUND: A fraction of all genomes is composed of transposable elements (TEs) whose mobility needs to be carefully controlled. In gonads, TE activity is repressed by PIWI-interacting RNAs (piRNAs), a class of small RNAs synthesized by heterochromatic loci enriched in TE fragments, called piRNA clusters. Maintenance of active piRNA clusters across generations is secured by maternal piRNA inheritance providing the memory for TE repression. On rare occasions, genomes encounter horizontal transfer (HT) of new TEs with no piRNA targeting them, threatening the host genome integrity. Naïve genomes can eventually start to produce new piRNAs against these genomic invaders, but the timing of their emergence remains elusive. RESULTS: Using a set of TE-derived transgenes inserted in different germline piRNA clusters and functional assays, we have modeled a TE HT in Drosophila melanogaster. We have found that the complete co-option of these transgenes by a germline piRNA cluster can occur within four generations associated with the production of new piRNAs all along the transgenes and the germline silencing of piRNA sensors. Synthesis of new transgenic TE piRNAs is linked to piRNA cluster transcription dependent on Moonshiner and heterochromatin mark deposition that propagates more efficiently on short sequences. Moreover, we found that sequences located within piRNA clusters can have different piRNA profiles and can influence transcript accumulation of nearby sequences. CONCLUSIONS: Our study reveals that genetic and epigenetic properties, such as transcription, piRNA profiles, heterochromatin, and conversion efficiency along piRNA clusters, could be heterogeneous depending on the sequences that compose them. These findings suggest that the capacity of transcriptional signal erasure induced by the chromatin complex specific of the piRNA cluster can be incomplete through the piRNA cluster loci. Finally, these results have revealed an unexpected level of complexity that highlights a new magnitude of piRNA cluster plasticity fundamental for the maintenance of genome integrity.

The histone demethylase Kdm3 prevents auto-immune piRNAs production in Drosophila.

Genome integrity of the animal germline is protected from transposable element activity by PIWI-interacting RNAs (piRNAs). While piRNA biogenesis is intensively explored, little is known about the genetical determination of piRNA clusters, the genomic sources of piRNAs. Using a bimodal epigenetic state piRNA cluster (BX2), we identified the histone demethylase Kdm3 as being able to prevent a cryptic piRNA production. In the absence of Kdm3, dozens of coding gene-containing regions become genuine germline dual-strand piRNA clusters. Eggs laid by Kdm3 mutant females show developmental defects phenocopying loss of function of genes embedded into the additional piRNA clusters, suggesting an inheritance of functional ovarian "auto-immune" piRNAs. Antagonizing piRNA cluster determination through chromatin modifications appears crucial to prevent auto-immune genic piRNAs production.

The ribose methylation enzyme FTSJ1 has a conserved role in neuron morphology and learning performance.

FTSJ1 is a conserved human 2'-O-methyltransferase (Nm-MTase) that modifies several tRNAs at position 32 and the wobble position 34 in the anticodon loop. Its loss of function has been linked to X-linked intellectual disability (XLID), and more recently to cancers. However, the molecular mechanisms underlying these pathologies are currently unclear. Here, we report a novel FTSJ1 pathogenic variant from an X-linked intellectual disability patient. Using blood cells derived from this patient and other affected individuals carrying FTSJ1 mutations, we performed an unbiased and comprehensive RiboMethSeq analysis to map the ribose methylation on all human tRNAs and identify novel targets. In addition, we performed a transcriptome analysis in these cells and found that several genes previously associated with intellectual disability and cancers were deregulated. We also found changes in the miRNA population that suggest potential cross-regulation of some miRNAs with these key mRNA targets. Finally, we show that differentiation of FTSJ1-depleted human neural progenitor cells into neurons displays long and thin spine neurites compared with control cells. These defects are also observed in Drosophila and are associated with long-term memory deficits. Altogether, our study adds insight into FTSJ1 pathologies in humans and flies by the identification of novel FTSJ1 targets and the defect in neuron morphology.

Comparative genomic and transcriptomic analyses of transposable elements in polychaetous annelids highlight LTR retrotransposon diversity and evolution.

BACKGROUND: With the expansion of high throughput sequencing, we now have access to a larger number of genome-wide studies analyzing the Transposable elements (TEs) composition in a wide variety of organisms. However, genomic analyses often remain too limited in number and diversity of species investigated to study in depth the dynamics and evolutionary success of the different types of TEs among metazoans. Therefore, we chose to investigate the use of transcriptomes to describe the diversity of TEs in phylogenetically related species by conducting the first comparative analysis of TEs in two groups of polychaetes and evaluate the diversity of TEs that might impact genomic evolution as a result of their mobility. RESULTS: We present a detailed analysis of TEs distribution in transcriptomes extracted from 15 polychaetes depending on the number of reads used during assembly, and also compare these results with additional TE scans on associated low-coverage genomes. We then characterized the clades defined by 1021 LTR-retrotransposon families identified in 26 species. Clade richness was highly dependent on the considered superfamily. Copia elements appear rare and are equally distributed in only three clades, GalEa, Hydra and CoMol. Among the eight BEL/Pao clades identified in annelids, two small clades within the Sailor lineage are new for science. We characterized 17 Gypsy clades of which only 4 are new; the C-clade largely dominates with a quarter of the families. Finally, all species also expressed for the majority two distinct transcripts encoding PIWI proteins, known to be involved in control of TEs mobilities. CONCLUSIONS: This study shows that the use of transcriptomes assembled from 40 million reads was sufficient to access to the diversity and proportion of the transposable elements compared to those obtained by low coverage sequencing. Among LTR-retrotransposons Gypsy elements were unequivocally dominant but results suggest that the number of Gypsy clades, although high, may be more limited than previously thought in metazoans. For BEL/Pao elements, the organization of clades within the Sailor lineage appears more difficult to establish clearly. The Copia elements remain rare and result from the evolutionary consistent success of the same three clades.

tRNA 2'-O-methylation by a duo of TRM7/FTSJ1 proteins modulates small RNA silencing in Drosophila.

2'-O-Methylation (Nm) represents one of the most common RNA modifications. Nm affects RNA structure and function with crucial roles in various RNA-mediated processes ranging from RNA silencing, translation, self versus non-self recognition to viral defense mechanisms. Here, we identify two Nm methyltransferases (Nm-MTases) in Drosophila melanogaster (CG7009 and CG5220) as functional orthologs of yeast TRM7 and human FTSJ1. Genetic knockout studies together with MALDI-TOF mass spectrometry and RiboMethSeq mapping revealed that CG7009 is responsible for methylating the wobble position in tRNAPhe, tRNATrp and tRNALeu, while CG5220 methylates position C32 in the same tRNAs and also targets additional tRNAs. CG7009 or CG5220 mutant animals were viable and fertile but exhibited various phenotypes such as lifespan reduction, small RNA pathways dysfunction and increased sensitivity to RNA virus infections. Our results provide the first detailed characterization of two TRM7 family members in Drosophila and uncover a molecular link between enzymes catalyzing Nm at specific tRNAs and small RNA-induced gene silencing pathways.

Environmentally-Induced Transgenerational Epigenetic Inheritance: Implication of PIWI Interacting RNAs.

Environmentally-induced transgenerational epigenetic inheritance is an emerging field. The understanding of associated epigenetic mechanisms is currently in progress with open questions still remaining. In this review, we present an overview of the knowledge of environmentally-induced transgenerational inheritance and associated epigenetic mechanisms, mainly in animals. The second part focuses on the role of PIWI-interacting RNAs (piRNAs), a class of small RNAs involved in the maintenance of the germline genome, in epigenetic memory to put into perspective cases of environmentally-induced transgenerational inheritance involving piRNA production. Finally, the last part addresses how genomes are facing production of new piRNAs, and from a broader perspective, how this process might have consequences on evolution and on sporadic disease development.

Environmentally-induced epigenetic conversion of a piRNA cluster.

Transposable element (TE) activity is repressed in animal gonads by PIWI-interacting RNAs (piRNAs) produced by piRNA clusters. Current models in flies propose that germinal piRNA clusters are functionally defined by the maternal inheritance of piRNAs produced during the previous generation. Taking advantage of an inactive, but ready to go, cluster of P-element derived transgene insertions in Drosophila melanogaster, we show here that raising flies at high temperature (29°C) instead of 25°C triggers the stable conversion of this locus from inactive into actively producing functional piRNAs. The increase of antisense transcripts from the cluster at 29°C combined with the requirement of transcription of euchromatic homologous sequences, suggests a role of double stranded RNA in the production of de novo piRNAs. This report describes the first case of the establishment of an active piRNA cluster by environmental changes in the absence of maternal inheritance of homologous piRNAs. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

RNA 2'-O-Methylation (Nm) Modification in Human Diseases.

Nm (2'-O-methylation) is one of the most common modifications in the RNA world. It has the potential to influence the RNA molecules in multiple ways, such as structure, stability, and interactions, and to play a role in various cellular processes from epigenetic gene regulation, through translation to self versus non-self recognition. Yet, building scientific knowledge on the Nm matter has been hampered for a long time by the challenges in detecting and mapping this modification. Today, with the latest advancements in the area, more and more Nm sites are discovered on RNAs (tRNA, rRNA, mRNA, and small non-coding RNA) and linked to normal or pathological conditions. This review aims to synthesize the Nm-associated human diseases known to date and to tackle potential indirect links to some other biological defects.

Short and long-term evolutionary dynamics of subtelomeric piRNA clusters in Drosophila.

Two Telomeric Associated Sequences, TAS-R and TAS-L, form the principal subtelomeric repeat families identified in Drosophila melanogaster. They are PIWI-interacting RNA (piRNA) clusters involved in repression of Transposable Elements. In this study, we revisited TAS structural and functional dynamics in D. melanogaster and in related species. In silico analysis revealed that TAS-R family members are composed of previously uncharacterized domains. This analysis also showed that TAS-L repeats are composed of arrays of a region we have named "TAS-L like" (TLL) identified specifically in one TAS-R family member, X-TAS. TLL were also present in other species of the melanogaster subgroup. Therefore, it is possible that TLL represents an ancestral subtelomeric piRNA core-cluster. Furthermore, all D. melanogaster genomes tested possessed at least one TAS-R locus, whereas TAS-L can be absent. A screen of 110 D. melanogaster lines showed that X-TAS is always present in flies living in the wild, but often absent in long-term laboratory stocks and that natural populations frequently lost their X-TAS within 2 years upon lab conditioning. Therefore, the unexpected structural and temporal dynamics of subtelomeric piRNA clusters demonstrated here suggests that genome organization is subjected to distinct selective pressures in the wild and upon domestication in the laboratory.

Paramutation in Drosophila Requires Both Nuclear and Cytoplasmic Actors of the piRNA Pathway and Induces Cis-spreading of piRNA Production.

Transposable element activity is repressed in the germline in animals by PIWI-interacting RNAs (piRNAs), a class of small RNAs produced by genomic loci mostly composed of TE sequences. The mechanism of induction of piRNA production by these loci is still enigmatic. We have shown that, in Drosophila melanogaster, a cluster of tandemly repeated P-lacZ-white transgenes can be activated for piRNA production by maternal inheritance of a cytoplasm containing homologous piRNAs. This activated state is stably transmitted over generations and allows trans-silencing of a homologous transgenic target in the female germline. Such an epigenetic conversion displays the functional characteristics of a paramutation, i.e., a heritable epigenetic modification of one allele by the other. We report here that piRNA production and trans-silencing capacities of the paramutated cluster depend on the function of the rhino, cutoff, and zucchini genes involved in primary piRNA biogenesis in the germline, as well as on that of the aubergine gene implicated in the ping-pong piRNA amplification step. The 21-nt RNAs, which are produced by the paramutated cluster, in addition to 23- to 28-nt piRNAs are not necessary for paramutation to occur. Production of these 21-nt RNAs requires Dicer-2 but also all the piRNA genes tested. Moreover, cytoplasmic transmission of piRNAs homologous to only a subregion of the transgenic locus can generate a strong paramutated locus that produces piRNAs along the whole length of the transgenes. Finally, we observed that maternally inherited transgenic small RNAs can also impact transgene expression in the soma. In conclusion, paramutation involves both nuclear (Rhino, Cutoff) and cytoplasmic (Aubergine, Zucchini) actors of the piRNA pathway. In addition, since it is observed between nonfully homologous loci located on different chromosomes, paramutation may play a crucial role in epigenome shaping in Drosophila natural populations.

piRNAs and epigenetic conversion in Drosophila.

Transposable element (TE) activity is repressed in the Drosophila germline by Piwi-Interacting RNAs (piRNAs), a class of small non-coding RNAs. These piRNAs are produced by discrete genomic loci containing TE fragments. In a recent publication, we tested for the existence of a strict epigenetic induction of piRNA production capacity by a locus in the D. melanogaster genome. We used 2 lines carrying a transgenic 7-copy tandem cluster (P-lacZ-white) at the same genomic site. This cluster generates in both lines a local heterochromatic sector. One line (T-1) produces high levels of ovarian piRNAs homologous to the P-lacZ-white transgenes and shows a strong capacity to repress homologous sequences in trans, whereas the other line (BX2) is devoid of both of these capacities. The properties of these 2 lines are perfectly stable over generations. We have shown that the maternal transmission of a cytoplasm carrying piRNAs from the first line can confer to the inert transgenic locus of the second, a totally de novo capacity to produce high levels of piRNAs as well as the ability to induce homology-dependent silencing in trans. These new properties are stably inherited over generations (n>50). Furthermore, the converted locus has itself become able to convert an inert transgenic locus via cytoplasmic maternal inheritance. This results in a stable epigenetic conversion process, which can be performed recurrently--a phenomenon termed paramutation and discovered in Maize 60 y ago. Paramutation in Drosophila corresponds to the first stable paramutation in animals and provides a model system to investigate the epigenetically induced emergence of a piRNA-producing locus, a crucial step in epigenome shaping. In this Extra View, we discuss some additional functional aspects and the possible molecular mechanism of this piRNA-linked paramutation.

Profiles of piRNA abundances at emerging or established piRNA loci are determined by local DNA sequences.

Piwi-interacting RNAs (piRNAs) ensure transposable element silencing in Drosophila, thereby preserving genome integrity across generations. Primary piRNAs arise from the processing of long RNA transcripts produced in the germ line by a limited number of telomeric and pericentromeric loci. Primary piRNAs bound to the Argonaute protein Aubergine then drive the production of secondary piRNAs through the "ping-pong" amplification mechanism that involves an interplay with piRNAs bound to the Argonaute protein Argonaute-3. We recently discovered that clusters of P-element-derived transgenes produce piRNAs and mediate silencing of homologous target transgenes in the female germ line. We also demonstrated that some clusters are able to convert other homologous inactive transgene clusters into piRNA-producing loci, which then transmit their acquired silencing capacity over generations. This paramutation phenomenon is mediated by maternal inheritance of piRNAs homologous to the transgenes. Here we further mined our piRNA sequencing data sets generated from various strains carrying transgenes with partial sequence homology at distinct genomic sites. This analysis revealed that same sequences in different genomic contexts generate highly similar profiles of piRNA abundances. The strong tendency of piRNAs for bearing a U at their 5' end has long been recognized. Our observations support the notion that, in addition, the relative frequencies of Drosophila piRNAs are locally determined by the DNA sequence of piRNA loci.

Paramutation in Drosophila linked to emergence of a piRNA-producing locus.

A paramutation is an epigenetic interaction between two alleles of a locus, through which one allele induces a heritable modification in the other allele without modifying the DNA sequence. The paramutated allele itself becomes paramutagenic, that is, capable of epigenetically converting a new paramutable allele. Here we describe a case of paramutation in animals showing long-term transmission over generations. We previously characterized a homology-dependent silencing mechanism referred to as the trans-silencing effect (TSE), involved in P-transposable-element repression in the germ line. We now show that clusters of P-element-derived transgenes that induce strong TSE can convert other homologous transgene clusters incapable of TSE into strong silencers, which transmit the acquired silencing capacity through 50 generations. The paramutation occurs without any need for chromosome pairing between the paramutagenic and the paramutated loci, and is mediated by maternal inheritance of cytoplasm carrying Piwi-interacting RNAs (piRNAs) homologous to the transgenes. The repression capacity of the paramutated locus is abolished by a loss-of-function mutation of the aubergine gene involved in piRNA biogenesis, but not by a loss-of-function mutation of the Dicer-2 gene involved in siRNA production. The paramutated cluster, previously producing barely detectable levels of piRNAs, is converted into a stable, strong piRNA-producing locus by the paramutation and becomes fully paramutagenic itself. Our work provides a genetic model for the emergence of piRNA loci, as well as for RNA-mediated trans-generational repression of transposable elements.

Homology-dependent silencing by an exogenous sequence in the Drosophila germline.

The study of P transposable element repression in Drosophila melanogaster led to the discovery of the trans-silencing effect (TSE), a homology-dependent repression mechanism by which a P-transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequences) represses in trans, in the female germline, a homologous P-lacZ transgene inserted in euchromatin. TSE shows variegation in ovaries and displays a maternal effect as well as epigenetic transmission through meiosis. In addition, TSE is highly sensitive to mutations affecting heterochromatin components (including HP1) and the Piwi-interacting RNA silencing pathway (piRNA), a homology-dependent silencing mechanism that functions in the germline. TSE appears thus to involve the piRNA-based silencing proposed to play a major role in P repression. Under this hypothesis, TSE may also be established when homology between the telomeric and target loci involves sequences other than P elements, including sequences exogenous to the D. melanogaster genome. We have tested whether TSE can be induced via lacZ sequence homology. We generated a piggyBac-otu-lacZ transgene in which lacZ is under the control of the germline ovarian tumor promoter, resulting in strong expression in nurse cells and the oocyte. We show that all piggyBac-otu-lacZ transgene insertions are strongly repressed by maternally inherited telomeric P-lacZ transgenes. This repression shows variegation between egg chambers when it is incomplete and presents a maternal effect, two of the signatures of TSE. Finally, this repression is sensitive to mutations affecting aubergine, a key player of the piRNA pathway. These data show that TSE can occur when silencer and target loci share solely a sequence exogenous to the D. melanogaster genome. This functionally supports the hypothesis that TSE represents a general repression mechanism which can be co-opted by new transposable elements to regulate their activity after a transfer to the D. melanogaster genome.

The epigenetic trans-silencing effect in Drosophila involves maternally-transmitted small RNAs whose production depends on the piRNA pathway and HP1.

BACKGROUND: The study of P transposable element repression in Drosophila melanogaster led to the discovery of the Trans-Silencing Effect (TSE), a homology-dependent repression mechanism by which a P-transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequences, "TAS") has the capacity to repress in trans, in the female germline, a homologous P-lacZ transgene located in euchromatin. Phenotypic and genetic analysis have shown that TSE exhibits variegation in ovaries, displays a maternal effect as well as epigenetic transmission through meiosis and involves heterochromatin (including HP1) and RNA silencing. PRINCIPAL FINDINGS: Here, we show that mutations in squash and zucchini, which are involved in the piwi-interacting RNA (piRNA) silencing pathway, strongly affect TSE. In addition, we carried out a molecular analysis of TSE and show that silencing is correlated to the accumulation of lacZ small RNAs in ovaries. Finally, we show that the production of these small RNAs is sensitive to mutations affecting squash and zucchini, as well as to the dose of HP1. CONCLUSIONS AND SIGNIFICANCE: Thus, our results indicate that the TSE represents a bona fide piRNA-based repression. In addition, the sensitivity of TSE to HP1 dose suggests that in Drosophila, as previously shown in Schizosaccharomyces pombe, a RNA silencing pathway can depend on heterochromatin components.

Telomeric trans-silencing in Drosophila melanogaster: tissue specificity, development and functional interactions between non-homologous telomeres.

BACKGROUND: The study of P element repression in Drosophila melanogaster led to the discovery of the telomeric Trans-Silencing Effect (TSE), a homology-dependent repression mechanism by which a P-transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequences, "TAS") has the capacity to repress in trans, in the female germline, a homologous P-lacZ transgene located in euchromatin. TSE can show variegation in ovaries, displays a maternal effect as well as an epigenetic transmission through meiosis and involves heterochromatin and RNA silencing pathways. PRINCIPAL FINDINGS: Here, we analyze phenotypic and genetic properties of TSE. We report that TSE does not occur in the soma at the adult stage, but appears restricted to the female germline. It is detectable during development at the third instar larvae where it presents the same tissue specificity and maternal effect as in adults. Transgenes located in TAS at the telomeres of the main chromosomes can be silencers which in each case show the maternal effect. Silencers located at non-homologous telomeres functionally interact since they stimulate each other via the maternally-transmitted component. All germinally-expressed euchromatic transgenes tested, located on all major chromosomes, were found to be repressed by a telomeric silencer: thus we detected no TSE escaper. The presence of the euchromatic target transgene is not necessary to establish the maternal inheritance of TSE, responsible for its epigenetic behavior. A single telomeric silencer locus can simultaneously repress two P-lacZ targets located on different chromosomal arms. CONCLUSIONS AND SIGNIFICANCE: Therefore TSE appears to be a widespread phenomenon which can involve different telomeres and work across the genome. It can explain the P cytotype establishment by telomeric P elements in natural Drosophila populations.

Telomeric trans-silencing: an epigenetic repression combining RNA silencing and heterochromatin formation.

The study of P-element repression in Drosophila melanogaster led to the discovery of the telomeric Trans-Silencing Effect (TSE), a repression mechanism by which a transposon or a transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequence or TAS) has the capacity to repress in trans in the female germline, a homologous transposon, or transgene located in euchromatin. TSE shows variegation among egg chambers in ovaries when silencing is incomplete. Here, we report that TSE displays an epigenetic transmission through meiosis, which involves an extrachromosomal maternally transmitted factor. We show that this silencing is highly sensitive to mutations affecting both heterochromatin formation (Su(var)205 encoding Heterochromatin Protein 1 and Su(var)3-7) and the repeat-associated small interfering RNA (or rasiRNA) silencing pathway (aubergine, homeless, armitage, and piwi). In contrast, TSE is not sensitive to mutations affecting r2d2, which is involved in the small interfering RNA (or siRNA) silencing pathway, nor is it sensitive to a mutation in loquacious, which is involved in the micro RNA (or miRNA) silencing pathway. These results, taken together with the recent discovery of TAS homologous small RNAs associated to PIWI proteins, support the proposition that TSE involves a repeat-associated small interfering RNA pathway linked to heterochromatin formation, which was co-opted by the P element to establish repression of its own transposition after its recent invasion of the D. melanogaster genome. Therefore, the study of TSE provides insight into the genetic properties of a germline-specific small RNA silencing pathway.

Transposition of a fungal miniature inverted-repeat transposable element through the action of a Tc1-like transposase.

The mimp1 element previously identified in the ascomycete fungus Fusarium oxysporum has hallmarks of miniature inverted-repeat transposable elements (MITEs): short size, terminal inverted repeats (TIRs), structural homogeneity, and a stable secondary structure. Since mimp1 has no coding capacity, its mobilization requires a transposase-encoding element. On the basis of the similarity of TIRs and target-site preference with the autonomous Tc1-like element impala, together with a correlated distribution of both elements among the Fusarium genus, we investigated the ability of mimp1 to jump upon expression of the impala transposase provided in trans. Under these conditions, we present evidence that mimp1 transposes by a cut-and-paste mechanism into TA dinucleotides, which are duplicated upon insertion. Our results also show that mimp1 reinserts very frequently in genic regions for at least one-third of the cases. We also show that the mimp1/impala double-component system is fully functional in the heterologous species F. graminearum, allowing the development of a highly efficient tool for gene tagging in filamentous fungi.

A long terminal repeat-containing retrotransposon of Schizosaccharomyces pombe expresses a Gag-like protein that assembles into virus-like particles which mediate reverse transcription.

The Tf1 element of Schizosaccharomyces pombe is a long terminal repeat-containing retrotransposon that encodes functional protease, reverse transcriptase, and integrase proteins. Although these proteins are known to be necessary for protein processing, reverse transcription, and integration, respectively, the function of the protein thought to be Gag has not been determined. We present here the first electron microscopy of Tf1 particles. We tested whether the putative Gag of Tf1 was required for particle formation, packaging of RNA, and reverse transcription. We generated deletions of 10 amino acids in each of the four hydrophilic domains of the protein and found that all four mutations reduced transposition activity. The N-terminal deletion removed a nuclear localization signal and inhibited nuclear import of the transposon. The two mutations in the center of Gag destabilized the protein and resulted in no virus-like particles. The C-terminal deletion caused a defect in RNA packaging and, as a result, low levels of cDNA. The electron microscopy of cells expressing a truncated Tf1 showed that Gag alone was sufficient for the formation of virus-like particles. Taken together, these results indicate that Tf1 encodes a Gag protein that is a functional equivalent of the Gag proteins of retroviruses.