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issn:2159-256
1.  LINE-1 activation and epigenetic silencing of suppressor genes in cancer 
Mobile Genetic Elements  2013;3(5):e26832.
The ability of active retrotransposon elements to move within the host genome and alter gene expression with subsequent phenotypic variation led to their initial discovery. In recent years it has become apparent that these elements can also modulate host gene expression independently of their transposition activity. Many retrotransposons maintain endogenous promoter motifs that can potentially drive expression of adjacent DNA modules. Similarly to transposition dependent dysregulation, these proto-promoters can progress disease states when active. Indeed aberrant activation of retrotransposon derived promoters in cancer can lead to transcription of oncogenic isoforms of cellular genes. Here we propose that activation of promoters of transposable elements in cancer can also drive transcription of long non-coding RNAs whose expression leads to silencing of linked tumor suppressor genes. Such transcription driven by aberrantly active transposable elements in cancer can lead to a characteristic reprogramming of epigenetic profiles, thus extending the potential molecular mechanisms whereby retrotransposons can directly contribute to cancer development and subsequent progression.
doi:10.4161/mge.26832
PMCID: PMC3827066  PMID: 24251074
LINE-1 chimeric transcripts; epigenetic silencing; transposable elements; cancer; antisense RNA
2.  Genome variation in the hyperthermophilic archaeon Aeropyrum 
Mobile Genetic Elements  2013;3(5):e26833.
Aeropyrum spp are aerobic, heterotrophic, and hyperthermophilic marine archaea. There are two closely related Aeropyrum species, Aeropyrum camini and Aeropyrum pernix, which are isolated from geographically distinct locations. Recently, we compared their genome sequences to determine their genomic variation. They possess highly conserved small genomes, reflecting their close relationship. The entire genome similarity may result from their survival strategies in adapting to extreme environmental conditions. Meanwhile, synteny disruptions were observed in some regions including clustered regularly interspaced short palindromic repeats elements. Further, the largest portion of their non-orthologous genes were genes in the two proviral regions of A. pernix (Aeropyrum pernix spindle-shaped virus 1 and Aeropyrum pernix ovoid virus 1) or ORFans considered to be derived from viruses. Our data shows that genomic diversification of Aeropyrum spp may be substantially induced by viruses. This suggests that Aeropyrum spp may have a large pan-genome that can be extended by viruses, while each of the species shares a highly conserved small genome specializing for extreme environments.
doi:10.4161/mge.26833
PMCID: PMC3827072  PMID: 24251075
hyperthermophilic archaea; genome synteny; virus; CRISPR; pan-genome
3.  Horizontal transfer of potential mobile units in phytoplasmas 
Mobile Genetic Elements  2013;3(5):e26145.
Phytoplasmas are uncultivated phytopathogenic bacteria that cause diseases in a wide range of economically important plants. Through secretion of effector proteins, they are able to manipulate their plant hosts to facilitate their multiplication and dispersal by insect vectors. The genome sequences of several phytoplasmas have been characterized to date and a group of putative composite transposons called potential mobile units (PMUs) are found in these highly reduced genomes. Recently, our team reported the genome sequence and comparative analysis of a peanut witches’ broom (PnWB) phytoplasma, the first representative of the phytoplasma 16SrII group. Comparisons between the species phylogeny and the phylogenies of the PMU genes revealed that the PnWB PMU is likely to have been transferred from the 16SrI group. This indicates that PMUs are not only the DNA unit for transposition within a genome, but also for horizontal transfer among divergent phytoplasma lineages. Given the association of PMUs with effector genes, the mobility of PMUs across genomes has important implications for phytoplasma ecology and evolution.
doi:10.4161/mge.26145
PMCID: PMC3827095  PMID: 24251068
composite transposon; effector; horizontal gene transfer; Mollicutes; phytoplasma; potential mobile unit
4.  Domesticated transposable element gene products in human cancer 
Mobile Genetic Elements  2013;3(5):e26693.
The adaptation of transposable elements inserted within the genome to serve novel functions in a host cell, a process known as molecular domestication, is a widespread phenomenon in nature. Around fifty protein-coding genes in humans have arisen through this mechanism. Functional characterization of these domesticated genes has revealed involvement in a multitude of diverse cellular processes. Some of these functions are related to cellular activities and pathways known to be involved in cancer development. In this mini-review we discuss such roles of domesticated genes that may be aberrantly regulated in human cancer, as well as studies that have identified disrupted expression in tumors. We also describe studies that have provided definitive experimental evidence for transposable element-derived gene products in promoting tumorigenesis.
doi:10.4161/mge.26693
PMCID: PMC3827096  PMID: 24251072
domesticated genes; cancer; transposons; exaptation; Rtl1; LDOC1; LDOC1L
5.  Chromosomal targeting by CRISPR-Cas systems can contribute to genome plasticity in bacteria 
Mobile Genetic Elements  2013;3(5):e26831.
The clustered regularly interspaced short palindromic repeats (CRISPR) and their associated (Cas) proteins form adaptive immune systems in bacteria to combat phage and other foreign genetic elements. Typically, short spacer sequences are acquired from the invader DNA and incorporated into CRISPR arrays in the bacterial genome. Small RNAs are generated that contain these spacer sequences and enable sequence-specific destruction of the foreign nucleic acids. Occasionally, spacers are acquired from the chromosome, which instead leads to targeting of the host genome. Chromosomal targeting is highly toxic to the bacterium, providing a strong selective pressure for a variety of evolutionary routes that enable host cell survival. Mutations that inactivate the CRISPR-Cas functionality, such as within the cas genes, CRISPR repeat, protospacer adjacent motifs (PAM), and target sequence, mediate escape from toxicity. This self-targeting might provide some explanation for the incomplete distribution of CRISPR-Cas systems in less than half of sequenced bacterial genomes. More importantly, self-genome targeting can cause large-scale genomic alterations, including remodeling or deletion of pathogenicity islands and other non-mobile chromosomal regions. While control of horizontal gene transfer is perceived as their main function, our recent work illuminates an alternative role of CRISPR-Cas systems in causing host genomic changes and influencing bacterial evolution.
doi:10.4161/mge.26831
PMCID: PMC3827097  PMID: 24251073
CRISPR; Cas; chromosomal targeting; bacterial evolution; genomic islands; plasmids; horizontal gene transfer; bacteriophages; integrative and conjugative elements
6.  Enzyme evolution beyond gene duplication 
Mobile Genetic Elements  2013;3(5):e26439.
Understanding the evolution of enzyme function after gene duplication has been a major goal of molecular biologists, biochemists and evolutionary biologists alike, for almost half a century. In contrast, the impact that horizontal gene transfer (HGT) has had on the evolution of enzyme specialization and the assembly of metabolic networks has just started to being investigated. Traditionally, evolutionary studies of enzymes have been limited to either the function of enzymes in vitro, or to sequence variability at the population level, where in almost all cases the starting conceptual framework embraces gene duplication as the mechanism responsible for the appearance of genetic redundancy. Very recently, we merged comparative phylogenomics, detection of selection signals, enzyme kinetics, X-ray crystallography and computational molecular dynamics, to characterize the sub-functionalization process of an amino acid biosynthetic enzyme prompted by an episode of HGT in bacteria. Some of the evolutionary implications of these functional studies, including a proposed model of enzyme specialization independent of gene duplication, are developed in this commentary.
doi:10.4161/mge.26439
PMCID: PMC3827091  PMID: 24251070
enzyme evolution; horizontal gene transfer; substrate specificity; subHisA and PriA; tryptophan and histidine biosynthesis
7.  Novel strategies for gene trapping and insertional mutagenesis mediated by Sleeping Beauty transposon 
Mobile Genetic Elements  2013;3(5):e26499.
Gene and poly(A) trappings are high-throughput approaches to capture and interrupt the expression of endogenous genes within a target genome. Although a number of trapping vectors have been developed for investigation of gene functions in cells and vertebrate models, there is still room for the improvement of their efficiency and sensitivity. Recently, two novel trapping vectors mediated by Sleeping Beauty (SB) transposon have been generated by the combination of three functional cassettes that are required for finding endogenous genes, disrupting the expression of trapped genes, and inducing the excision of integrated traps from their original insertion sites and then inserting into another gene. In addition, several other strategies are utilized to improve the activities of two trapping vectors. First, activities of all components were examined in vitro before the generation of two vectors. Second, the inducible promoter from the tilapia Hsp70 gene was used to drive the expression of SB gene, which can mediate the excision of integrated transposons upon induction at 37 °C. Third, the Cre/LoxP system was introduced to delete the SB expression cassette for stabilization of gene interruption and bio-safety. Fourth, three stop codons in different reading frames were introduced downstream of a strong splice acceptor (SA) in the gene trapping vector to effectively terminate the translation of trapped endogenous genes. Fifth, the strong splicing donor (SD) and AU-rich RNA-destabilizing element exhibited no obvious insertion bias and markedly reduced SD read-through events, and the combination of an enhanced SA, a poly(A) signal and a transcript terminator in the poly(A) trapping vector efficiently disrupted the transcription of trapped genes. Thus, these two trapping vectors are alternative and effective tools for large-scale identification and disruption of endogenous genes in vertebrate cells and animals.
doi:10.4161/mge.26499
PMCID: PMC3827069  PMID: 24251071
insertional mutagenesis; transposon; Sleeping Beauty; gene trapping; poly(A) trapping
8.  Interspecific hybridization as a genomic stressor inducing mobilization of transposable elements in Drosophila 
Mobile Genetic Elements  2014;4:e34394.
Transposable elements (TEs) are DNA sequences able to be mobilized in host genomes. They are currently recognized as the major mutation inducers because of their insertion in the target, their effect on neighboring regions, or their ectopic recombination. A large number of factors including chemical and physical factors as well as intraspecific crosses have traditionally been identified as inducers of transposition. Besides environmental factors, interspecific crosses have also been proposed as promoters of transposition of particular TEs in plants and different animals. Our previous published work includes a genome-wide survey with the set of genomic TEs and shows that interspecific hybridization between the species Drosophila buzzatii and Drosophila koepferae induces genomic instability by transposition bursts. A high percentage of this instability corresponds to TEs belonging to classes I and II. The detailed study of three TEs (Osvaldo, Helena, and Galileo), representative of the different TE families, shows an increase of transposition in hybrids compared with parental species, that varies depending on the element. This study suggests ample variation in TE regulation mechanisms and the question is why this variation occurs. Interspecific hybridization is a genomic stressor that disrupts the stability of TEs probably contributing to a relaxation of the mechanisms controlling TEs in the Drosophila genome. In this commentary paper we will discuss these results and the molecular mechanisms that could explain these increases of transposition rates observed in interspecific Drosophila hybrids.
doi:10.4161/mge.34394
PMCID: PMC4132227  PMID: 25136509
Drosophila; transposable elements; interspecific hybridization; transposition; genomic instability; epigenetic factors
9.  SVA retrotransposons as modulators of gene expression 
Mobile Genetic Elements  2014;4:e32102.
Endogenous mobile genetic elements can give rise to de novo germline or somatic mutations that can have dramatic consequences for genome regulation both local and possibly more globally based on the site of integration. However if we consider them as “normal genetic” components of the reference genome then they are likely to modify local chromatin structure which would have an effect on gene regulation irrelevant of their ability to further transpose. As such they can be treated as any other domain involved in a gene × environment interaction. Similarly their evolutionary appearance in the reference genome would supply a driver for species specific responses/traits. Our recent data would suggest the hominid specific subset of retrotransposons, SINE-VNTR-Alu (SVA), can function as transcriptional regulatory domains both in vivo and in vitro when analyzed in reporter gene constructs. Of particular interest in the SVA element, were the variable number tandem repeat (VNTR) domains which as their name suggests can be polymorphic. We and others have previously shown that VNTRs can be both differential regulators and biomarkers of disease based on the genotype of the repeat. Here, we provide an overview of why polymorphism in the SVA elements, in particular the VNTRs, could alter gene expression patterns that could be mechanistically associated with different traits in evolution or disease progression in humans.
doi:10.4161/mge.32102
PMCID: PMC4114917  PMID: 25077041
SVA; VNTR; gene expression; polymorphism; retrotransposon
10.  Differential expression of a retrotransposable element, Rex6, in Colossoma macropomum fish from different Amazonian environments 
Mobile Genetic Elements  2014;4:e30003.
Transposable elements (TEs) are DNA sequences that have the ability to move and replicate within the genomes. TEs can be classified according to their intermediates of transposition, RNA (retrotransposons) or DNA. In some aquatic organisms, it has been observed that environmental factors such as pH, temperature and pollution may stimulate differential transcription and mobilization of retrotransposons. In light of this information, the present study sought to evaluate the expression of Rex6 TE transcripts in Colossoma macropomum, which is a very commercially exploited fish in Brazil. In order to establish a comparative analysis using real-time PCR, the samples were collected from Amazonian rivers with different physical and chemical characteristics (distinguished by clear water and black water). Quantitative RT-PCR analyses revealed a differential pattern of expression between tissues collected from different types of water (clear and black waters). When it came to the hepatic and muscle tissues sampled, the levels of Rex6 transcripts were significantly different between the two Amazonian water types. These results suggest that environmental conditions operate differently in the regulation of Rex6 transcription in C. macropomum, results which have implications in the reshaping of the genome against environmental variations.
doi:10.4161/mge.30003
PMCID: PMC4113519  PMID: 25089227
transposable elements; quantitative RT-PCR; differential expression; transcript tissue distribution
11.  The Ellis Island Effect 
Mobile Genetic Elements  2014;4:e29801.
Objectives: Bacteroides fragilis, a Gram-negative anaerobic bacterium, is alternately a gut commensal or virulent pathogen and is an important reservoir for horizontal gene transfer (HGT) of bacterial resistance and virulence genes in the human gastrointestinal tract. We identified a unique conjugative transposon (CTn) in a multidrug resistant clinical isolate of B. fragilis (BF-HMW615); we named this element CTnHyb because it included a hybrid mosaic of foreign elements. This study reports the characterization of CTnHyb and discusses the potential impact on horizontal spread of resistance genes.
Results: CTnHyb contains several efflux pump genes and several genes that confer or may confer antibiotic resistance to tetracycline, kanamycin, metronidazole and spectinomycin (truncated gene). CTnHyb also contains a mosaic of mobile elements from Gram-positive organisms. CTnHyb is easily transferred from BF-HMW615 (the original isolate) to BF638R (lab strain) and integrated into the BF638R chromosome. The “foreign” (from Gram-positive bacteria) nucleotide sequences within CTnHyb were > 99% preserved indicating that the gene acquisition from the Gram-positive bacteria was very recent.
Conclusion: CTnHyb is a novel CTn residing in a multidrug resistant strain of B. fragilis. The global nature and wide phylogenetic reach of HGT means that any gene in any bacterium can potentially be mobilized. Understanding the mechanisms that drive the formation and transfer of these elements and, potentially, ways to limit the transfer are necessary to prevent a devastating spread of resistance elements.
doi:10.4161/mge.29801
PMCID: PMC4145004  PMID: 25165618
bacteroides; horizontal gene transfer; conjugative transposon; integrative conjugal element; antimicrobial resistance
12.  Harnessing mobile genetic elements to explore gene regulation 
Mobile Genetic Elements  2014;4:e29759.
Sequences that regulate expression of a gene in cis but are located at large distances along the DNA from the gene, as found with most developmentally regulated genes in higher vertebrates, are difficult to identify if those sequences are not conserved across species. Mutating suspected gene-regulatory sequences to alter expression then becomes a hit-or-miss affair. The relaxed specificity of transposon insertions offers an opportunity to develop alternate strategies, to scan in an unbiased manner, pieces of chromosomal DNA cloned in BACs for transcription enhancing elements. This article illustrates how insertions of Tn10 with enhancer-traps into BAC DNA containing the gene, and its germ-line expression in zebrafish, have identified distal regulatory elements functionally. Transposition of Tn10 first introduces the enhancer-trap with a loxP site randomly into BAC DNA. Cre-recombination between the inserted loxP and the loxP endogenous to a BAC-end positions the enhancer-trap to the newly created truncated end of BAC DNA. The procedure generates a library of integration-ready enhancer-trap BACs with progressive truncations from an end in a single experiment. Individual enhancer-trap BACs from the library can be evaluated functionally in zebrafish or mice. Furthermore, the ability to readily alter sequences in a small transposon plasmid containing a regulatory domain of the gene allows re-introduction of altered parts of a BAC back into itself. It serves as a useful strategy to functionally dissect multiple discontinuous regulatory domains of a gene quickly. These methodologies have been successfully used in identifying novel regulatory domains of the Amyloid Precursor Protein (appb) gene in zebrafish, and provided important clues for regulation of the gene in humans.
doi:10.4161/mge.29759
PMCID: PMC4092005  PMID: 25054085
amyloid precursor protein gene regulation; enhancer-trap BACs; gene regulation by discontinuous DNA domains; long-range gene regulation by enhancer-trap BACs; new technology for BAC modification; regulation of appb gene in zebrafish; regulation of APP gene in humans; zebrafish model for gene regulation in humans; zebrafish transgenesis
13.  Something new to explore 
Mobile Genetic Elements  2014;4:e29782.
Functional genomics in plants has been facilitated greatly by the use of plant viruses to carry segments of host genes that can then promote the silencing of the RNAs expressed from the corresponding host genes; a process called virus-induced gene silencing (VIGS). The silencing of genes in filamentous fungi is either technically more problematic or labor-intensive, especially if transgenic plants need to be generated first. However, a recent paper from our team demonstrated that a plant virus could infect three related fungal species, as well as express a reporter gene ectopically, and also silence the correspondingly expressed reporter transgene. The gene expression and RNA silencing of the reporter gene was maintained for six passages in culture and also persisted in plants infected by the virus-infected fungus. Here, we consider how the virus can enter and migrate within the fungus, whether the virus can move back and forth between the fungus and the plant and the ramifications of this, the prospects for VIGS being used to silence fungal endogenes and possible biotechnological or therapeutic applications of using plant viruses for expressing foreign proteins in fungi or silencing fungal endogenes.
PMCID: PMC4091559  PMID: 25057444
Fungal transfection; host species jump; plant pathogen adaptation; protein overexpression; virus-induced gene silencing
14.  Driving DNA transposition by lentiviral protein transduction 
Mobile Genetic Elements  2014;4:e29591.
Gene vectors derived from DNA transposable elements have become powerful molecular tools in biomedical research and are slowly moving into the clinic as carriers of therapeutic genes. Conventional uses of DNA transposon-based gene vehicles rely on the intracellular production of the transposase protein from transfected nucleic acids. The transposase mediates mobilization of the DNA transposon, which is typically provided in the context of plasmid DNA. In recent work, we established lentiviral protein transduction from Gag precursors as a new strategy for direct delivery of the transposase protein. Inspired by the natural properties of infecting viruses to carry their own enzymes, we loaded lentivirus-derived particles not only with vector genomes carrying the DNA transposon vector but also with hundreds of transposase subunits. Such particles were found to drive efficient transposition of the piggyBac transposable element in a range of different cell types, including primary cells, and offer a new transposase delivery approach that guarantees short-term activity and limits potential cytotoxicity. DNA transposon vectors, originally developed and launched as a non-viral alternative to viral integrating vectors, have truly become viral. Here, we briefly review our findings and speculate on the perspectives and potential advantages of transposase delivery by lentiviral protein transduction.
doi:10.4161/mge.29591
PMCID: PMC4092313  PMID: 25057443
DNA transposition; protein transduction; piggyBac; Sleeping Beauty; lentiviral vector; IDLV
15.  An intronic open reading frame was released from one of group II introns in the mitochondrial genome of the haptophyte Chrysochromulina sp. NIES-1333 
Mobile Genetic Elements  2014;4:e29384.
Mitochondrial (mt) genome sequences, which often bear introns, have been sampled from phylogenetically diverse eukaryotes. Thus, we can anticipate novel insights into intron evolution from previously unstudied mt genomes. We here investigated the origins and evolution of three introns in the mt genome of the haptophyte Chrysochromulina sp. NIES-1333, which was sequenced completely in this study. All the three introns were characterized as group II, on the basis of predicted secondary structure, and the conserved sequence motifs at the 5′ and 3′ termini. Our comparative studies on diverse mt genomes prompt us to propose that the Chrysochromulina mt genome laterally acquired the introns from mt genomes in distantly related eukaryotes. Many group II introns harbor intronic open reading frames for the proteins (intron-encoded proteins or IEPs), which likely facilitate the splicing of their host introns. However, we propose that a “free-standing,” IEP-like protein, which is not encoded within any introns in the Chrysochromulina mt genome, is involved in the splicing of the first cox1 intron that lacks any open reading frames.
doi:10.4161/mge.29384
PMCID: PMC4091101  PMID: 25054084
group II intron; mitochondrial genome; intron encoded protein; lateral intron transfer; Chrysochromulina sp
16.  The taming of the shrew 
Mobile Genetic Elements  2014;4:e29383.
Transposons are mobile genetic elements that can be harmful for the host when mobilized. However, they are also genomic reservoirs for novel genes that can be evolutionarily beneficial. There are many examples of domesticated transposases, which play important roles in the hosts. In most cases domesticated transposases have lost their endonuclease activities and the hosts utilize their DNA-binding properties. However, some other domesticated transposases perform endonuclease activities for host biological processes. Because such a catalytically active transposase is potentially harmful for the integrity of the host genome, its activity should be tightly regulated. The catalytically active domesticated piggyBac transposase Tpb2p catalyzes programmed DNA elimination in the ciliate Tetrahymena. Here, we discuss the regulatory mechanism that prevents unintended DNA cleavage by Tpb2p and compare it to another well-studied catalytically active domesticated transposase, the RAG recombinase in V(D)J recombination. The regulatory mechanisms involve the temporarily regulated expression of the transposases, the target sequence preference of the endonuclease, and the recruitment of the transposases to locally restricted chromatin environments.
doi:10.4161/mge.29383
PMCID: PMC4091102  PMID: 25054083
piggyBac; DNA elimination; Tetrahymena; RAG; V(D)J recombination
17.  Expression and detection of LINE-1 ORF-encoded proteins 
Mobile Genetic Elements  2014;4:e29319.
LINE-1 (L1) elements are endogenous retrotransposons active in mammalian genomes. The L1 RNA is bicistronic, encoding two non-overlapping open reading frames, ORF1 and ORF2, whose protein products (ORF1p and ORF2p) bind the L1 RNA to form a ribonucleoprotein (RNP) complex that is presumed to be a critical retrotransposition intermediate. However, ORF2p is expressed at a significantly lower level than ORF1p; these differences are thought to be controlled at the level of translation, due to a low frequency ribosome reinitiation mechanism controlling ORF2 expression. As a result, while ORF1p is readily detectable, ORF2p has previously been very challenging to detect in vitro and in vivo. To address this, we recently tested several epitope tags fused to the N- or C-termini of the ORF proteins in an effort to enable robust detection and affinity purification from native (L1RP) and synthetic (ORFeus-Hs) L1 constructs. An analysis of tagged RNPs from both L1RP and ORFeus-Hs showed similar host-cell-derived protein interactors. Our observations also revealed that the tag sequences affected the retrotransposition competency of native and synthetic L1s differently although they encode identical ORF proteins. Unexpectedly, we observed apparently stochastic expression of ORF2p within seemingly homogenous L1-expressing cell populations.
doi:10.4161/mge.29319
PMCID: PMC4091050  PMID: 25054082
LINE-1; mass spectrometry; I-DIRT; synthetic transposon; epitope tag
18.  Burgeoning evidence indicates that microRNAs were initially formed from transposable element sequences 
Mobile Genetic Elements  2014;4:e29255.
MicroRNAs (miRNAs) constitute a recently discovered class of noncoding RNAs that play key roles in the regulation of gene expression. Despite being only ~20 nucleotides in length, these highly versatile molecules have been shown to play pivotal roles in development, basic cellular metabolism, apoptosis, and disease. While over 24,000 miRNAs have been characterized since they were first isolated in mammals in 2001, the functions of the majority of these miRNAs remain largely undescribed. That said, many now suggest that characterization of the relationships between miRNAs and transposable elements (TEs) can help elucidate miRNA functionality. Strikingly, over 20 publications have now reported the initial formation of thousands of miRNA loci from TE sequences. In this review we chronicle the findings of these reports, discuss the evolution of the field along with future directions, and examine how this information can be used to ascertain insights into miRNA transcriptional regulation and how it can be exploited to facilitate miRNA target prediction.
doi:10.4161/mge.29255
PMCID: PMC4091103  PMID: 25054081
microRNA; miR; miRNA; repetitive; retrotransposon; transposable; transposon
19.  A case of horizontal gene transfer from Wolbachia to Aedes albopictus C6/36 cell line 
Mobile Genetic Elements  2014;4:e28914.
Horizontal gene transfer plays an essential role in evolution and ecological adaptation, yet this phenomenon has remained controversial, particularly where it occurs between prokaryotes and eukaryotes. There are a handful of reported examples of horizontal gene transfer occurring between prokaryotes and eukaryotes in the literature, with most of these documented cases pertaining to invertebrates and endosymbionts. However, the vast majority of these horizontally transferred genes were either eventually excluded or rapidly became nonfunctional in the recipient genome. In this study, we report the discovery of a horizontal gene transfer from the endosymbiont Wolbachia in the C6/36 cell line derived from the mosquito Aedes albopictus. Moreover, we report that this horizontally transferred gene displayed high transcription level. This finding and the results of further experimentation strongly suggest this gene is functional and has been expressed and translated into a protein in the mosquito host cells.
doi:10.4161/mge.28914
PMCID: PMC4013104  PMID: 24812591
horizontal gene transfer; endosymbiont; Wolbachia; mosquito; Aedes Albopictus; C6/36 cell line
20.  L1 retrotransposition 
Mobile Genetic Elements  2014;4:e28907.
LINE-1 (L1) elements are the only active and autonomous transposable elements in humans. The core retrotransposition machinery is a ribonucleoprotein particle (RNP) containing the L1 mRNA, with endonuclease and reverse transcriptase activities. It initiates reverse transcription directly at genomic target sites upon endonuclease cleavage. Recently, using a direct L1 extension assay (DLEA), we systematically tested the ability of native L1 RNPs to extend DNA substrates of various sequences and structures. We deduced from these experiments the general rules guiding the initiation of L1 reverse transcription, referred to as the snap-velcro model. In this model, L1 target choice is not only mediated by the sequence specificity of the endonuclease, but also through base-pairing between the L1 mRNA and the target site, which permits the subsequent L1 reverse transcription step. In addition, L1 reverse transcriptase efficiently primes L1 DNA synthesis only when the 3′ end of the DNA substrate is single-stranded, suggesting so-far unrecognized DNA processing steps at the integration site.
doi:10.4161/mge.28907
PMCID: PMC4014453  PMID: 24818067
reverse transcription; reverse transcriptase; target-primed reverse transcription; TPRT; non-LTR retrotransposon; LINE-1; L1; poly(A) tail; endonuclease
21.  One to rule them all 
Mobile Genetic Elements  2014;4:e28807.
The development of transposon-based genome manipulation tools can benefit greatly from understanding transposons’ inherent regulatory mechanisms. The Tc1-mariner transposons, which are being widely used in biotechnological applications, are subject to a self-inhibitory mechanism whereby increasing transposase expression beyond a certain point decreases the rate of transposition. In a recent paper, Liu and Chalmers performed saturating mutagenesis on the highly conserved WVPHEL motif in the mariner-family transposase from the Hsmar1 element. Curiously, they found that the majority of all possible single mutations were hyperactive. Biochemical characterizations of the mutants revealed that the hyperactivity is due to a defect in communication between transposase subunits, which normally regulates transposition by reducing the rate of synapsis. This provides important clues for improving transposon-based tools. However, some WVPHEL mutants also showed features that would be undesirable for most biotechnological applications: they showed uncontrolled DNA cleavage activities and defects in the coordination of cleavage between the two transposon ends. The study illustrates how the knowledge of inhibitory mechanisms can help improve transposon tools but also highlights an important challenge, which is to specifically target a regulatory mechanism without affecting other important functions of the transposase.
doi:10.4161/mge.28807
PMCID: PMC4013102  PMID: 24812590
transposition; DNA recombination; human genome; gene therapy; genetic engineering; hyperactive transposase; conformational change; overproduction inhibition
22.  Control of mammalian retrotransposons by cellular RNA processing activities 
Mobile Genetic Elements  2014;4:e28439.
Retrotransposons make up roughly 50% of the mammalian genome and have played an important role in genome evolution. A small fraction of non-LTR retrotransposons, LINE-1 and SINE elements, is currently active in the human genome. These elements move in our genome using an intermediate RNA and a reverse transcriptase activity by a copy and paste mechanism. Their ongoing mobilization can impact the human genome leading to several human disorders. However, how the cell controls the activity of these elements minimizing their mutagenic effect is not fully understood. Recent studies have highlighted that the intermediate RNA of retrotransposons is a target of different mechanisms that limit the mobilization of endogenous retrotransposons in mammals. Here, we provide an overview of recent discoveries that show how RNA processing events can act to control the activity of mammalian retrotransposons and discuss several arising questions that remain to be answered.
doi:10.4161/mge.28439
PMCID: PMC4203495  PMID: 25346866
LINE-1; SINE-1; retrotransposon; Microprocessor; small RNAs; DGCR8; Drosha; transposable elements; Dicer; microRNAs
23.  The role of horizontal gene transfer in kleptoplastidy and the establishment of photosynthesis in the eukaryotes 
Mobile Genetic Elements  2013;3(2):e24773.
Found in different eukaryotic lineages, kleptoplastidy is the ability to sequester chloroplasts from algal preys that are ingested and partially digested. While most of the genetic information required for the activity and maintenance of the kleptoplastids disappeared with the digestion of the algal nuclei, the photosynthetic organelles remain active during extended period of time. Many different hypotheses have been proposed to explain the longevity of the kleptoplastids within their host. The most popular one involves Horizontal Gene Transfer (HGT) from the algal genome to the host nucleus. In order to test this hypothesis, transcriptome-based analyses have been performed on different kleptoplastidic organisms during the past few years. However, the variability of the results obtained does not allow drawing a convincing conclusion regarding the precise role of HGT in kleptoplastidy. Understanding the mechanism that allow persistence of the plastids is crucial, not only for the characterization of kleptoplastidy, but also for important evolutionary questions surrounding endosymbiotic events and the emergence and spread of photosynthesis in the eukaryotes. Here, I discuss alternative theories that could explain the longevity of sequestered plastids in their host, with special focus on the simplest chloroplast stability hypothesis.
doi:10.4161/mge.24773
PMCID: PMC3681741  PMID: 23914312
photosynthesis; endosymbiosis; photosymbiosis; kleptoplastidy; horizontal gene transfer; foraminifera; diatom; EST; plastid
24.  Composition of the DNA-uptake complex of Vibrio cholerae 
Mobile Genetic Elements  2014;4:e28142.
Natural competence for transformation is a developmental program that allows certain bacteria to take up free extracellular DNA from the environment and integrate this DNA into their genome. Thereby, natural transformation acts as mode of horizontal gene transfer and impacts bacterial evolution. The number of genes induced upon competence induction varies significantly between organisms. However, all of the naturally competent bacteria possess competence genes that encode so-called DNA-uptake machineries. Some components of these multi-protein complexes resemble subunits of type IV pili and type II secretion systems. However, knowledge on the mechanistic aspects of such DNA-uptake complexes is still very limited. Here, we discuss some new findings regarding the DNA-uptake machinery of the naturally transformable human pathogen Vibrio cholerae. The potential of this organism to initiate the competence program was discovered less than a decade ago. However, recent studies have provided new insight into both the regulatory pathways of competence induction and into the DNA uptake dynamics.
doi:10.4161/mge.28142
PMCID: PMC3919817  PMID: 24558639
horizontal gene transfer; V. cholerae; DNA uptake machinery; type IV pilus; natural transformation
25.  Quadruplex-forming DNA sequences spread by retrotransposons may serve as genome regulators 
Mobile Genetic Elements  2014;4:e28084.
Transposable elements (TEs) are ubiquitous genome inhabitants in eukaryotes. Increasing evidence shows that TEs are involved in regulatory networks of eukaryotic cells and contribute to genome evolution. Recently, we reported that many plant long-terminal repeat (LTR) retrotransposons contain DNA quadruplex-forming sequences at precise positions inside their LTRs and that quadruplexes are better preserved in evolutionary younger elements. As quadruplexes can modulate molecular processes, quadruplexes found at specific distances upstream and downstream from the endogenous TE promoter can affect transcription of the element. Moreover, quadruplexes found in solo LTRs, as well as in 3′ ends of 5′-truncated copies of LINE-1 elements, can affect expression of neighboring genes. Here, we propose that this way retrotransposons can serve as vehicles for spread of DNA quadruplexes. Quadruplexes can thus fulfill a dual regulatory role—to influence both the retrotransposons carrying them and the neighboring host genes, e.g., by direct effect on transcription or by modifying the local chromatin state. Additionally, four-stranded DNA structures may serve as hotspots for recombination-based genome rearrangements.
doi:10.4161/mge.28084
PMCID: PMC3933402  PMID: 24616836
LTR retrotransposons; DNA quadruplexes; TE transcription

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