What Have We Learned in 30 Years of Investigations on Bari Transposons?

Transposable elements (TEs) have been historically depicted as detrimental genetic entities that selfishly aim at perpetuating themselves, invading genomes, and destroying genes. Scientists often co-opt "special" TEs to develop new and powerful genetic tools, that will hopefully aid in changing the future of the human being. However, many TEs are gentle, rarely unleash themselves to harm the genome, and bashfully contribute to generating diversity and novelty in the genomes they have colonized, yet they offer the opportunity to develop new molecular tools. In this review we summarize 30 years of research focused on the Bari transposons. Bari is a "normal" transposon family that has colonized the genomes of several Drosophila species and introduced genomic novelties in the melanogaster species. We discuss how these results have contributed to advance the field of TE research and what future studies can still add to the current knowledge.

Evidence of the Physical Interaction between Rpl22 and the Transposable Element Doc5, a Heterochromatic Transposon of Drosophila melanogaster.

Chromatin is a highly dynamic biological entity that allows for both the control of gene expression and the stabilization of chromosomal domains. Given the high degree of plasticity observed in model and non-model organisms, it is not surprising that new chromatin components are frequently described. In this work, we tested the hypothesis that the remnants of the Doc5 transposable element, which retains a heterochromatin insertion pattern in the melanogaster species complex, can be bound by chromatin proteins, and thus be involved in the organization of heterochromatic domains. Using the Yeast One Hybrid approach, we found Rpl22 as a potential interacting protein of Doc5. We further tested in vitro the observed interaction through Electrophoretic Mobility Shift Assay, uncovering that the N-terminal portion of the protein is sufficient to interact with Doc5. However, in situ localization of the native protein failed to detect Rpl22 association with chromatin. The results obtained are discussed in the light of the current knowledge on the extra-ribosomal role of ribosomal protein in eukaryotes, which suggests a possible role of Rpl22 in the determination of the heterochromatin in Drosophila.

The mitochondrial aspartate/glutamate carrier (AGC or Aralar1) isoforms in D. melanogaster: biochemical characterization, gene structure, and evolutionary analysis.

BACKGROUND: In man two mitochondrial aspartate/glutamate carrier (AGC) isoforms, known as aralar and citrin, are required to accomplish several metabolic pathways. In order to fill the existing gap of knowledge in Drosophila melanogaster, we have studied aralar1 gene, orthologue of human AGC-encoding genes in this organism. METHODS: The blastp algorithm and the "reciprocal best hit" approach have been used to identify the human orthologue of AGCs in Drosophilidae and non-Drosophilidae. Aralar1 proteins have been overexpressed in Escherichia coli and functionally reconstituted into liposomes for transport assays. RESULTS: The transcriptional organization of aralar1 comprises six isoforms, three constitutively expressed (aralar1-RA, RD and RF), and the remaining three distributed during the development or in different tissues (aralar1-RB, RC and RE). Aralar1-PA and Aralar1-PE, representative of all isoforms, have been biochemically characterized. Recombinant Aralar1-PA and Aralar1-PE proteins share similar efficiency to exchange glutamate against aspartate, and same substrate affinities than the human isoforms. Interestingly, although Aralar1-PA and Aralar1-PE diverge only in their EF-hand 8, they greatly differ in their specific activities and substrate specificity. CONCLUSIONS: The tight regulation of aralar1 transcripts expression and the high request of aspartate and glutamate during early embryogenesis suggest a crucial role of Aralar1 in this Drosophila developmental stage. Furthermore, biochemical characterization and calcium sensitivity have identified Aralar1-PA and Aralar1-PE as the human aralar and citrin counterparts, respectively. GENERAL SIGNIFICANCE: The functional characterization of the fruit fly mitochondrial AGC transporter represents a crucial step toward a complete understanding of the metabolic events acting during early embryogenesis.

"What You Need, Baby, I Got It": Transposable Elements as Suppliers of Cis-Operating Sequences in Drosophila.

Transposable elements (TEs) are constitutive components of both eukaryotic and prokaryotic genomes. The role of TEs in the evolution of genes and genomes has been widely assessed over the past years in a variety of model and non-model organisms. Drosophila is undoubtedly among the most powerful model organisms used for the purpose of studying the role of transposons and their effects on the stability and evolution of genes and genomes. Besides their most intuitive role as insertional mutagens, TEs can modify the transcriptional pattern of host genes by juxtaposing new cis-regulatory sequences. A key element of TE biology is that they carry transcriptional control elements that fine-tune the transcription of their own genes, but that can also perturb the transcriptional activity of neighboring host genes. From this perspective, the transposition-mediated modulation of gene expression is an important issue for the short-term adaptation of physiological functions to the environmental changes, and for long-term evolutionary changes. Here, we review the current literature concerning the regulatory and structural elements operating in cis provided by TEs in Drosophila. Furthermore, we highlight that, besides their influence on both TEs and host genes expression, they can affect the chromatin structure and epigenetic status as well as both the chromosome's structure and stability. It emerges that Drosophila is a good model organism to study the effect of TE-linked regulatory sequences, and it could help future studies on TE-host interactions in any complex eukaryotic genome.

Comparative Genomic Analyses Provide New Insights into the Evolutionary Dynamics of Heterochromatin in Drosophila.

The term heterochromatin has been long considered synonymous with gene silencing, but it is now clear that the presence of transcribed genes embedded in pericentromeric heterochromatin is a conserved feature in the evolution of eukaryotic genomes. Several studies have addressed the epigenetic changes that enable the expression of genes in pericentric heterochromatin, yet little is known about the evolutionary processes through which this has occurred. By combining genome annotation analysis and high-resolution cytology, we have identified and mapped 53 orthologs of D. melanogaster heterochromatic genes in the genomes of two evolutionarily distant species, D. pseudoobscura and D. virilis. Our results show that the orthologs of the D. melanogaster heterochromatic genes are clustered at three main genomic regions in D. virilis and D. pseudoobscura. In D. virilis, the clusters lie in the middle of euchromatin, while those in D. pseudoobscura are located in the proximal portion of the chromosome arms. Some orthologs map to the corresponding Muller C element in D. pseudoobscura and D. virilis, while others localize on the Muller B element, suggesting that chromosomal rearrangements that have been instrumental in the fusion of two separate elements involved the progenitors of genes currently located in D. melanogaster heterochromatin. These results demonstrate an evolutionary repositioning of gene clusters from ancestral locations in euchromatin to the pericentromeric heterochromatin of descendent D. melanogaster chromosomes. Remarkably, in both D. virilis and D. pseudoobscura the gene clusters show a conserved association with the HP1a protein, one of the most highly evolutionarily conserved epigenetic marks. In light of these results, we suggest a new scenario whereby ancestral HP1-like proteins (and possibly other epigenetic marks) may have contributed to the evolutionary repositioning of gene clusters into heterochromatin.

On the evolution of Yeti, a Drosophila melanogaster heterochromatin gene.

Constitutive heterochromatin is a ubiquitous and still unveiled component of eukaryotic genomes, within which it comprises large portions. Although constitutive heterochromatin is generally considered to be transcriptionally silent, it contains a significant variety of sequences that are expressed, among which about 300 single-copy coding genes have been identified by genetic and genomic analyses in the last decades. Here, we report the results of the evolutionary analysis of Yeti, an essential gene of Drosophila melanogaster located in the deep pericentromeric region of chromosome 2R. By FISH, we showed that Yeti maintains a heterochromatin location in both D. simulans and D. sechellia species, closely related to D. melanogaster, while in the more distant species e.g., D. pseudoobscura and D. virilis, it is found within euchromatin, in the syntenic chromosome Muller C, that corresponds to the 2R arm of D. melanogaster chromosome 2. Thus, over evolutionary time, Yeti has been resident on the same chromosomal element, but it progressively moved closer to the pericentric regions. Moreover, in silico reconstruction of the Yeti gene structure in 19 Drosophila species and in 5 non-drosophilid dipterans shows a rather stable organization during evolution. Accordingly, by PCR analysis and sequencing, we found that the single intron of Yeti does not undergo major intraspecies or interspecies size changes, unlike the introns of other essential Drosophila heterochromatin genes, such as light and Dbp80. This implicates diverse evolutionary forces in shaping the structural organization of genes found within heterochromatin. Finally, the results of dS - dN tests show that Yeti is under negative selection both in heterochromatin and euchromatin, and indicate that the change in genomic location did not affected significantly the molecular evolution of the gene. Together, the results of this work contribute to our understanding of the evolutionary dynamics of constitutive heterochromatin in the genomes of higher eukaryotes.

The Drosophila mojavensis Bari3 transposon: distribution and functional characterization.

BACKGROUND: Bari-like transposons belong to the Tc1-mariner superfamily, and they have been identified in several genomes of the Drosophila genus. This transposon's family has been used as paradigm to investigate the complex dynamics underlying the persistence and structural evolution of transposable elements (TEs) within a genome. Three structural Bari variants have been identified so far and can be distinguished based on the organization of their terminal inverted repeats. Bari3 is the last discovered member of this family identified in Drosophila mojavensis, a recently emerged species of the Repleta group of the genus Drosophila. RESULTS: We studied the insertion pattern of Bari3 in different D. mojavensis populations and found evidence of recent transposition activity. Analysis of the transposase domains unveiled the presence of a functional nuclear localization signal, as well as a functional binding domain. Using luciferase-based assays, we investigated the promoter activity of Bari3 as well as the interaction of its transposase with its left terminus. The results suggest that Bari3 is transposition-competent. Finally we demonstrated transposase transcript processing when the transposase gene is overexpressed in vivo and in vitro. CONCLUSIONS: Bari3 displays very similar structural and functional features with its close relative, Bari1. Our results strongly suggest that Bari3 is an independent element that has generated genomic diversity in D. mojavensis. It can autonomously transcribe its transposase gene, which in turn can localize in the nucleus and bind the terminal inverted repeats of the transposon. Nevertheless, the identification of an unpredicted spliced form of the Bari3 transposase transcript allows us to hypothesize a control mechanism of its mobility based on mRNA processing. These results will aid the studies on the Bari family of transposons, which is intriguing for its widespread diffusion in Drosophilids coupled with a structural diversity generated during the evolution of Bari-like elements in their host genomes.

Yeti, an essential Drosophila melanogaster gene, encodes a protein required for chromatin organization.

The evolutionarily conserved family of Bucentaur (BCNT) proteins exhibits a widespread distribution in animal and plants, yet its biological role remains largely unknown. Using Drosophila melanogaster as a model organism, we investigated the in vivo role of the Drosophila BCNT member called YETI. We report that loss of YETI causes lethality before pupation and defects in higher-order chromatin organization, as evidenced by severe impairment in the association of histone H2A.V, nucleosomal histones and epigenetic marks with polytene chromosomes. We also find that YETI binds to polytene chromosomes through its conserved BCNT domain and interacts with the histone variant H2A.V, HP1a and Domino-A (DOM-A), the ATPase subunit of the DOM/Tip60 chromatin remodeling complex. Furthermore, we identify YETI as a downstream target of the Drosophila DOM-A. On the basis of these results, we propose that YETI interacts with H2A.V-exchanging machinery, as a chaperone or as a new subunit of the DOM/Tip60 remodeling complex, and acts to regulate the accumulation of H2A.V at chromatin sites. Overall, our findings suggest an unanticipated role of YETI protein in chromatin organization and provide, for the first time, mechanistic clues on how BCNT proteins control development in multicellular organisms.

Genomic instability of I elements of Drosophila melanogaster in absence of dysgenic crosses.

Retrotranspostion of I factors in the female germline of Drosophila melanogaster is responsible for the so called I-R hybrid dysgenesis, a phenomenon that produces a broad spectrum of genetic abnormalities including reduced fertility, increased frequency of mutations and chromosome loss. Transposition of I factor depends on cellular conditions that are established in the oocytes of the reactive females and transmitted to their daughters. The so-called reactivity is a cellular state that may exhibit variable levels of expression and represents a permissive condition for I transposition at high levels. Defective I elements have been proposed to be the genetic determinants of reactivity and, through their differential expression, to modulate transposition of active copies in somatic and/or germ line cells. Recently, control of transposable element activity in the germ line has been found to depend on pi-RNAs, small repressive RNAs interacting with Piwi-family proteins and derived from larger transposable elements (TE)-derived primary transcripts. In particular, maternally transmitted I-element piRNAs originating from the 42AB region of polytene chromosomes were found to be involved in control of I element mobility. In the present work, we use a combination of cytological and molecular approaches to study the activity of I elements in three sublines of the inducer y; cn bw; sp isogenic strain and in dysgenic and non-dysgenic genetic backgrounds. Overall, the results of FISH and Southern blotting experiments clearly show that I elements are highly unstable in the Montpellier subline in the absence of classical dysgenic conditions. Such instability appears to be correlated to the amount of 5' and 3' I element transcripts detected by quantitative and real-time RT-PCR. The results of this study indicate that I elements can be highly active in the absence of a dysgenic crosses. Moreover, in the light of our results caution should be taken to assimilate the genomic annotation data on transposable elements to all y; cn bw sp sublines.

The paracentric inversion In(2Rh)PL alters the centromeric organization of chromosome 2 in Drosophila melanogaster.

Centromeres are complex structures involved in an evolutionarily conserved function, the correct segregation of chromosomes and chromatids during meiosis and mitosis. The centromere is determined by epigenetic processes that result in a particular nucleosome organization (CEN chromatin) that differs from the rest of the chromatin including the heterochromatin that normally surrounds the centromere in higher organisms. Many of the current models of centromere origin and organization rely on the molecular and cytological characterization of minichromosomes and their derivatives, and on studies on the origin and maintenance of neocentromeres. Here, we describe the peculiar centromere organization observed in In(2Rh)PL, a paracentric D. melanogaster inversion in which the centromere is maintained in its natural context but is directly flanked by a euchromatic domain as a result of the rearrangement. We have identified the breakpoints of the inversion and show that the proximal one is within the centromere region. The data presented suggest that, notwithstanding the loss of all the pericentric 2Rh heterochromatin, the centromere of the In(2Rh)PL chromosome is still active but presents a nucleosomal organization quite different from the organization usually observed in the centromeric region.

Conserved motifs and dynamic aspects of the terminal inverted repeat organization within Bari-like transposons.

In this work the structural variations of Terminal Inverted Repeats (TIR) of Bari like transposons in Drosophila species has been studied. The aim is to try and assess the relevance of different variants in the evolutionary distribution of Bari elements. Bari is a member of the widespread Tc1 superfamily of transposable elements that has colonized most species of the Drosophila genus. We previously reported the structure of two related elements that differ in their TIR organization: Bari1 harbouring 26-bp TIR (short TIRs) and Bari2 with about 250-bp TIR (long TIIR). While elements with short TIRs are complete and potentially autonomous, long ones are invariably composed of defective copies. The results show that in D. pseudobscura, D. persimilis and D. mojavensis, there is a third class of Bari elements, Bari3, that exhibit a long TIR structure and are not defective. Phylogenetic relationships among reconstructed transposases are consistent with the three subfamilies sharing a common origin. However, the final TIR organization into long or short structure is not related by descent but appears to be lineage-specific. Furthermore, we show that, independently of origin and organization, within the 250-bp terminal sequences there are three regions that are conserved in both sequence and position suggesting they are under functional constraint.

Cytogenetic and molecular characterization of heterochromatin gene models in Drosophila melanogaster.

In the past decade, genome-sequencing projects have yielded a great amount of information on DNA sequences in several organisms. The release of the Drosophila melanogaster heterochromatin sequence by the Drosophila Heterochromatin Genome Project (DHGP) has greatly facilitated studies of mapping, molecular organization, and function of genes located in pericentromeric heterochromatin. Surprisingly, genome annotation has predicted at least 450 heterochromatic gene models, a figure 10-fold above that defined by genetic analysis. To gain further insight into the locations and functions of D. melanogaster heterochromatic genes and genome organization, we have FISH mapped 41 gene models relative to the stained bands of mitotic chromosomes and the proximal divisions of polytene chromosomes. These genes are contained in eight large scaffolds, which together account for approximately 1.4 Mb of heterochromatic DNA sequence. Moreover, developmental Northern analysis showed that the expression of 15 heterochromatic gene models tested is similar to that of the vital heterochromatic gene Nipped-A, in that it is not limited to specific stages, but is present throughout all development, despite its location in a supposedly "silent" region of the genome. This result is consistent with the idea that genes resident in heterochromatin can encode essential functions.

Evidence for a functional interaction between the Bari1 transposable element and the cytochrome P450 cyp12a4 gene in Drosophila melanogaster.

Previous studies of the genomic distribution of the transposon Bari1 in Drosophila melanogaster have revealed an element which is fixed at division 91F in over 90 lab and natural populations. Here we report about the structural and transcriptional features of the insertion site which was studied in sublines isolated from an exceptional Drosophila line polymorphic for the presence/absence of Bari1 at 91F. The insert is located at the 3' end of the cyp12a4 gene that belongs to the cytochrome P450 family. In flies with the insert the transcript of this gene encompasses 18 nucleotides of the transposon, it is shorter and is about tenfold more abundant compared to flies devoid of it. Although the hypothetical selective agent remains unknown, these data are suggestive of a selective advantage brought about by the Bari1 insert and are reminiscent of recent evidence for functional mutagenesis of cyp6g1, another P450 gene, brought about by Accord and Doc transposable elements in D. melanogaster and Drosophila simulans.

A comparative study of the porin genes encoding VDAC, a voltage-dependent anion channel protein, in Anopheles gambiae and Drosophila melanogaster.

The protein called voltage-dependent anion-selective channel (VDAC), or mitochondrial porin, forms channels that provide the major pathway for small metabolites across the mitochondrial outer membrane. We have identified and sequenced agporin, a gene of the malaria vector mosquito Anopheles gambiae that conceptually encodes a protein with 73% identity to the VDAC protein encoded by the porin gene in Drosophila melanogaster. By in situ hybridization, we have localized agporin at region 35D on the right arm of A. gambiae chromosome 3, which is homologous to the 2L chromosomal arm of D. melanogaster where the porin gene resides. The comparison of agporin with its putative Drosophila counterpart revealed that both the nucleotide sequence and the structural organization of the two genes are strikingly conserved even though the ancestral lines of A. gambiae and D. melanogaster are thought to have diverged about 250 million years ago. Our results suggest that, while in yeast, plants, and mammals, VDAC isoforms are encoded by small multigene families and are able to compensate for each other at least partially, in A. gambiae a single gene encodes the VDAC protein.

Organization and possible origin of the Bari-1 cluster in the heterochromatic h39 region of Drosophila melanogaster.

The molecular organization of the heterochromatic h39 region of the Drosophila melanogaster second chromosome has been investigated by studying two BAC clones identified both by Southern blotting and by FISH experiments as containing tandem arrays of Bari1, a transposable element present only in this region. Such BAC clones appear to contain different portions of the h39 region since they differ in the DNA sequences flanking the Bari1 repeats on both sides. Thus, the 80 Bari1 copies estimated to be present in the h39 region are split into at least two separated subregions. On the basis of the analysis of the flanking sequences a possible mechanism depending on an aberrant activity of the Bari1 transposase is proposed for the genesis of the heterochromatic tandem arrays of the element.

A survey of the DNA sequences surrounding the Bari1 repeats in the pericentromeric h39 region of Drosophila melanogaster.

In Drosophila melanogaster, clustered copies of the Bari1 transposon are only present in the pericentromeric h39 region of the second chromosome, where other clusters of repetitive elements, either found organized in large tandem arrays only in the h39 region (Responder, PortoI), or both in the h39 region and in other heterochromatic regions (Hoppel), are also observed. The topological relationship among the repetitive sequences of the h39 region and the nature of the sequences separating its large repeat clusters are at present largely unknown. To get new insights on the sequence composition of the heterochromatin and on the forces governing its origin and maintenance, we have cloned and analyzed part of the DNA sequences flanking the h39 Bari1 repeats. In a region spanning 3 and 9 kb, respectively, from the ends of a Bari1 array we found only single copies of the PortoI and Hoppel transposable elements, and five copies of a variant form of the Responder repeats. No large tandem arrays of any repeated element were present. In addition, a highly conserved 596 bp sequence, that may have a functional role, is present on both sides of the Bari1 repeats. We suggest that the current organization of the h39 heterochromatin implies some topological or functional constraint that prevents the formation of further arrays of repetitive elements in the region.