Interplasmid transposition demonstrates piggyBac mobility in vertebrate species

Development of Transformation for Genome Editing of an Emerging Model Organism

With the advances in genomic sequencing, many organisms with novel biological properties are ripe for use as emerging model organisms. However, to make full use of them, transformation methods need to be developed to permit genome editing. Here, we present the development of transformation for the fungus fly Bradysia (Sciara) coprophila; this may serve as a paradigm for the development of transformation for other emerging systems, especially insects. Bradysia (Sciara) has a variety of unique biological features, including locus-specific developmentally regulated DNA amplification, chromosome imprinting, a monopolar spindle in male meiosis I, non-disjunction of the X chromosome in male meiosis II, X chromosome elimination in early embryogenesis, germ-line-limited (L) chromosomes and high resistance to radiation. Mining the unique biology of Bradysia (Sciara) requires a transformation system to test mutations of DNA sequences that may play roles for these features. We describe a Bradysia (Sciara) transformation system using a modified piggyBac transformation vector and detailed protocols we have developed to accommodate Bradysia (Sciara) specific requirements. This advance will provide a platform for us and others in the growing Bradysia (Sciara) community to take advantage of this unique biological system. In addition, the versatile piggyBac vectors described here and transformation methods will be useful for other emerging model systems.

Sequence-specific DNA binding activity of the cross-brace zinc finger motif of the piggyBac transposase

The piggyBac transposase (PB) is distinguished by its activity and utility in genome engineering, especially in humans where it has highly promising therapeutic potential. Little is known, however, about the structure-function relationships of the different domains of PB. Here, we demonstrate in vitro and in vivo that its C-terminal Cysteine-Rich Domain (CRD) is essential for DNA breakage, joining and transposition and that it binds to specific DNA sequences in the left and right transposon ends, and to an additional unexpectedly internal site at the left end. Using NMR, we show that the CRD adopts the specific fold of the cross-brace zinc finger protein family. We determine the interaction interfaces between the CRD and its target, the 5'-TGCGT-3'/3'-ACGCA-5' motifs found in the left, left internal and right transposon ends, and use NMR results to propose docking models for the complex, which are consistent with our site-directed mutagenesis data. Our results provide support for a model of the PB/DNA interactions in the context of the transpososome, which will be useful for the rational design of PB mutants with increased activity.

A universal vector concept for a direct genotyping of transgenic organisms and a systematic creation of homozygous lines

Diploid transgenic organisms are either hemi- or homozygous. Genetic assays are, therefore, required to identify the genotype. Our AGameOfClones vector concept uses two clearly distinguishable transformation markers embedded in interweaved, but incompatible Lox site pairs. Cre-mediated recombination leads to hemizygous individuals that carry only one marker. In the following generation, heterozygous descendants are identified by the presence of both markers and produce homozygous progeny that are selected by the lack of one marker. We prove our concept in Tribolium castaneum by systematically creating multiple functional homozygous transgenic lines suitable for long-term fluorescence live imaging. Our approach saves resources and simplifies transgenic organism handling. Since the concept relies on the universal Cre-Lox system, it is expected to work in all diploid model organisms, for example, insects, zebrafish, rodents and plants. With appropriate adaptions, it can be used in knock-out assays to preselect homozygous individuals and thus minimize the number of wasted animals.

Vast potential for using the piggyBac transposon to engineer transgenic plants at specific genomic locations

The acceptance of bioengineered plants by some nations is hampered by a number of factors, including the random insertion of a transgene into the host genome. Emerging technologies, such as site-specific nucleases, are enabling plant scientists to promote recombination or mutations at specific plant loci. Off target activity of these nucleases may limit widespread use. Insertion of transgenes by transposases engineered with a specific DNA binding domain has been accomplished in a number of organisms, but not in plants. The piggyBac transposon system, originally isolated from an insect, has been utilized to transform a variety of organisms. The piggyBac transposase is amendable to structural modifications, and was able to insert a transgene at a specific human locus through fusion of a DNA binding domain to its N-terminus. Recent developments demonstrating the activity of piggyBac transposase in plants is an important first step toward the potential use of engineered versions of piggyBac transposase for site-specific transgene insertion in plants.

Sleeping Beauty transposition: from biology to applications

Sleeping Beauty (SB) is the first synthetic DNA transposon that was shown to be active in a wide variety of species. Here, we review studies from the last two decades addressing both basic biology and applications of this transposon. We discuss how host-transposon interaction modulates transposition at different steps of the transposition reaction. We also discuss how the transposon was translated for gene delivery and gene discovery purposes. We critically review the system in clinical, pre-clinical and non-clinical settings as a non-viral gene delivery tool in comparison with viral technologies. We also discuss emerging SB-based hybrid vectors aimed at combining the attractive safety features of the transposon with effective viral delivery. The success of the SB-based technology can be fundamentally attributed to being able to insert fairly randomly into genomic regions that allow stable long-term expression of the delivered transgene cassette. SB has emerged as an efficient and economical toolkit for safe and efficient gene delivery for medical applications.

piggyBac Transposon

The piggyBac transposon was originally isolated from the cabbage looper moth, Trichoplusia ni, in the 1980s. Despite its early discovery and dissimilarity to the other DNA transposon families, the piggyBac transposon was not recognized as a member of a large transposon superfamily for a long time. Initially, the piggyBac transposon was thought to be a rare transposon. This view, however, has now been completely revised as a number of fully sequenced genomes have revealed the presence of piggyBac-like repetitive elements. The isolation of active copies of the piggyBac-like elements from several distinct species further supported this revision. This includes the first isolation of an active mammalian DNA transposon identified in the bat genome. To date, the piggyBac transposon has been deeply characterized and it represents a number of unique characteristics. In general, all members of the piggyBac superfamily use TTAA as their integration target sites. In addition, the piggyBac transposon shows precise excision, i.e., restoring the sequence to its preintegration state, and can transpose in a variety of organisms such as yeasts, malaria parasites, insects, mammals, and even in plants. Biochemical analysis of the chemical steps of transposition revealed that piggyBac does not require DNA synthesis during the actual transposition event. The broad host range has attracted researchers from many different fields, and the piggyBac transposon is currently the most widely used transposon system for genetic manipulations.

A non-autonomous insect piggyBac transposable element is mobile in tobacco

The piggyBac transposable element, originally isolated from a virus in an insect cell line, is a valuable molecular tool for transgenesis and mutagenesis of invertebrates. For heterologous transgenesis in a variety of mammals, transfer of the piggyBac transposable element from an ectopic plasmid only requires expression of piggyBac transposase. To determine if piggyBac could function in dicotyledonous plants, a two-element system was developed in tobacco (Nicotiana tabacum) to test for transposable element excision and insertion. The first transgenic line constitutively expressed piggyBac transposase, while the second transgenic line contained at least two non-autonomous piggyBac transposable elements. Progeny from crosses of the two transgenic lines was analyzed for piggyBac excision and transposition. Several progeny displayed excision events, and all the sequenced excision sites exhibited evidence of the precise excision mechanism characteristic of piggyBac transposase. Two unique transposition insertion events were identified that each included diagnostic duplication of the target site. These data indicate that piggyBac transposase is active in a dicotyledonous plant, although at a low frequency.

Suicidal autointegration of sleeping beauty and piggyBac transposons in eukaryotic cells

Transposons are discrete segments of DNA that have the distinctive ability to move and replicate within genomes across the tree of life. 'Cut and paste' DNA transposition involves excision from a donor locus and reintegration into a new locus in the genome. We studied molecular events following the excision steps of two eukaryotic DNA transposons, Sleeping Beauty (SB) and piggyBac (PB) that are widely used for genome manipulation in vertebrate species. SB originates from fish and PB from insects; thus, by introducing these transposons to human cells we aimed to monitor the process of establishing a transposon-host relationship in a naïve cellular environment. Similarly to retroviruses, neither SB nor PB is capable of self-avoidance because a significant portion of the excised transposons integrated back into its own genome in a suicidal process called autointegration. Barrier-to-autointegration factor (BANF1), a cellular co-factor of certain retroviruses, inhibited transposon autointegration, and was detected in higher-order protein complexes containing the SB transposase. Increasing size sensitized transposition for autointegration, consistent with elevated vulnerability of larger transposons. Both SB and PB were affected similarly by the size of the transposon in three different assays: excision, autointegration and productive transposition. Prior to reintegration, SB is completely separated from the donor molecule and followed an unbiased autointegration pattern, not associated with local hopping. Self-disruptive autointegration occurred at similar frequency for both transposons, while aberrant, pseudo-transposition events were more frequently observed for PB.

Moving "Forward" in Plasmodium Genetics through a Transposon-Based Approach

The genome sequence of the human malaria parasite, Plasmodium falciparum, was released almost a decade ago. A majority of the Plasmodium genome, however, remains annotated to code for hypothetical proteins with unknown functions. The introduction of forward genetics has provided novel means to gain a better understanding of gene functions and their associated phenotypes in Plasmodium. Even with certain limitations, the technique has already shown significant promise to increase our understanding of parasite biology needed for rationalized drug and vaccine design. Further improvements to the mutagenesis technique and the design of novel genetic screens should lead us to some exciting discoveries about the critical weaknesses of Plasmodium, and greatly aid in the development of new disease intervention strategies.

"Calling cards" for DNA-binding proteins in mammalian cells

The ability to chronicle transcription-factor binding events throughout the development of an organism would facilitate mapping of transcriptional networks that control cell-fate decisions. We describe a method for permanently recording protein-DNA interactions in mammalian cells. We endow transcription factors with the ability to deposit a transposon into the genome near to where they bind. The transposon becomes a "calling card" that the transcription factor leaves behind to record its visit to the genome. The locations of the calling cards can be determined by massively parallel DNA sequencing. We show that the transcription factor SP1 fused to the piggyBac transposase directs insertion of the piggyBac transposon near SP1 binding sites. The locations of transposon insertions are highly reproducible and agree with sites of SP1-binding determined by ChIP-seq. Genes bound by SP1 are more likely to be expressed in the HCT116 cell line we used, and SP1-bound CpG islands show a strong preference to be unmethylated. This method has the potential to trace transcription-factor binding throughout cellular and organismal development in a way that has heretofore not been possible.

A gene delivery system for human cells mediated by both a cell-penetrating peptide and a piggyBac transposase

The piggyBac (PB) transposable element has recently accumulated enormous attention as a tool for the transgenesis in various eukaryotic organisms. Arginine-rich cell-penetrating peptides (CPPs) are protein transduction domains containing a large amount of basic amino acids that were found to be capable of delivering biologically active macromolecules into living cells. In this study, we demonstrate a strategy, which we called "transposoduction", which is a one-plasmid gene delivery system mediated by the nontoxic CPP-piggyBac transposase (CPP-PBase) fusion protein to accomplish both protein transduction and transposition. CPPs were proven to be able to synchronously deliver covalently linked PBase and noncovalently linked a cis plasmid into human cells. The expression of promoterless reporter genes coding for red (dTomato) and yellow (mOrange) fluorescent proteins (RFP and YFP) with PB elements could be detected in cells treated with the PBase-expressing plasmid after 3 days indicating transposition of coding regions to downstream of endogenous promoter sequences. An enhanced green fluorescent protein (EGFP) plasmid-based excision assay further confirmed the efficiency of the bifunctional CPP-PBase fusion protein. In conclusion, this strategy representing a combinational concept of both protein transduction and mobile transposition may provide tremendous potential for safe and efficient cell line transformation, gene therapy and functional genomics.

Pogostick: a new versatile piggyBac vector for inducible gene over-expression and down-regulation in emerging model systems

Background

Non-traditional model systems need new tools that will enable them to enter the field of functional genetics. These tools should enable the exploration of gene function, via knock-downs of endogenous genes, as well as over-expression and ectopic expression of transgenes.

Methodology

We constructed a new vector called Pogostick that can be used to over-express or down-regulate genes in organisms amenable to germ line transformation by the piggyBac transposable element. Pogostick can be found at www.addgene.org, a non-profit plasmid repository. The vector currently uses the heat-shock promoter Hsp70 from Drosophila to drive transgene expression and, as such, will have immediate applicability to organisms that can correctly interpret this promotor sequence. We detail how to clone candidate genes into this vector and test its functionality in Drosophila by targeting a gene coding for the fluorescent protein DsRed. By cloning a single DsRed copy into the vector, and generating transgenic lines, we show that DsRed mRNA and protein levels are elevated following heat-shock. When cloning a second copy of DsRed in reverse orientation into a flanking site, and transforming flies constitutively expressing DsRed in the eyes, we show that endogenous mRNA and protein levels drop following heat-shock. We then test the over-expression vector, containing the complete cDNA of Ultrabithorax (Ubx) gene, in an emerging model system, Bicyclus anynana. We produce a transgenic line and show that levels of Ubx mRNA expression rise significantly following a heat-shock. Finally, we show how to obtain genomic sequence adjacent to the Pogostick insertion site and to estimate transgene copy number in genomes of transformed individuals.

Significance

This new vector will allow emerging model systems to enter the field of functional genetics with few hurdles.

Time-lapse imaging of neural development: zebrafish lead the way into the fourth dimension

Time-lapse imaging is often the only way to appreciate fully the many dynamic cell movements critical to neural development. Zebrafish possess many advantages that make them the best vertebrate model organism for live imaging of dynamic development events. This review will discuss technical considerations of time-lapse imaging experiments in zebrafish, describe selected examples of imaging studies in zebrafish that revealed new features or principles of neural development, and consider the promise and challenges of future time-lapse studies of neural development in zebrafish embryos and adults.

Development of the piggyBac transposable system for Plasmodium berghei and its application for random mutagenesis in malaria parasites

BACKGROUND

The genome of a number of species of malaria parasites (Plasmodium spp.) has been sequenced in the hope of identifying new drug and vaccine targets. However, almost one-half of predicted Plasmodium genes are annotated as hypothetical and are difficult to analyse in bulk due to the inefficiency of current reverse genetic methodologies for Plasmodium. Recently, it has been shown that the transposase piggyBac integrates at random into the genome of the human malaria parasite P. falciparum offering the possibility to develop forward genetic screens to analyse Plasmodium gene function. This study reports the development and application of the piggyBac transposition system for the rodent malaria parasite P. berghei and the evaluation of its potential as a tool in forward genetic studies. P. berghei is the most frequently used malaria parasite model in gene function analysis since phenotype screens throughout the complete Plasmodium life cycle are possible both in vitro and in vivo.

RESULTS

We demonstrate that piggyBac based gene inactivation and promoter-trapping is both easier and more efficient in P. berghei than in the human malaria parasite, P. falciparum. Random piggyBac-mediated insertion into genes was achieved after parasites were transfected with the piggyBac donor plasmid either when transposase was expressed either from a helper plasmid or a stably integrated gene in the genome. Characterization of more than 120 insertion sites demonstrated that more than 70 most likely affect gene expression classifying their protein products as non-essential for asexual blood stage development. The non-essential nature of two of these genes was confirmed by targeted gene deletion one of which encodes P41, an ortholog of a human malaria vaccine candidate. Importantly for future development of whole genome phenotypic screens the remobilization of the piggyBac element in parasites that stably express transposase was demonstrated.

CONCLUSION

These data demonstrate that piggyBac behaved as an efficient and random transposon in P. berghei. Remobilization of piggyBac element shows that with further development the piggyBac system can be an effective tool to generate random genome-wide mutation parasite libraries, for use in large-scale phenotype screens in vitro and in vivo.

A piggyBac transposon-based mutagenesis system for the fission yeast Schizosaccharomyces pombe

The TTAA-specific transposon piggyBac (PB), originally isolated from the cabbage looper moth, Trichoplusia ni, has been utilized as an insertional mutagenesis tool in various eukaryotic organisms. Here, we show that PB transposes in the fission yeast Schizosaccharomyces pombe and leaves almost no footprints. We developed a PB-based mutagenesis system for S. pombe by constructing a strain with a selectable transposon excision marker and an integrated transposase gene. PB transposition in this strain has low chromosomal distribution bias as shown by deep sequencing-based insertion site mapping. Using this system, we obtained loss-of-function alleles of klp5 and klp6, and a gain-of-function allele of dam1 from a screen for mutants resistant to the microtubule-destabilizing drug thiabendazole. From another screen for cdc25-22 suppressors, we obtained multiple alleles of wee1 as expected. The success of these two screens demonstrated the usefulness of this PB-mediated mutagenesis tool for fission yeast.

piggyBac is an effective tool for functional analysis of the Plasmodium falciparum genome

BACKGROUND

Much of the Plasmodium falciparum genome encodes hypothetical proteins with limited homology to other organisms. A lack of robust tools for genetic manipulation of the parasite limits functional analysis of these hypothetical proteins and other aspects of the Plasmodium genome. Transposon mutagenesis has been used widely to identify gene functions in many organisms and would be extremely valuable for functional analysis of the Plasmodium genome.

RESULTS

In this study, we investigated the lepidopteran transposon, piggyBac, as a molecular genetic tool for functional characterization of the Plasmodium falciparum genome. Through multiple transfections, we generated 177 unique P. falciparum mutant clones with mostly single piggyBac insertions in their genomes. Analysis of piggyBac insertion sites revealed random insertions into the P. falciparum genome, in regards to gene expression in parasite life cycle stages and functional categories. We further explored the possibility of forward genetic studies in P. falciparum with a phenotypic screen for attenuated growth, which identified several parasite genes and pathways critical for intra-erythrocytic development.

CONCLUSION

Our results clearly demonstrate that piggyBac is a novel, indispensable tool for forward functional genomics in P. falciparum that will help better understand parasite biology and accelerate drug and vaccine development.

Transposon tools hopping in vertebrates

In the past decade, tools derived from DNA transposons have made major contributions to vertebrate genetic studies from gene delivery to gene discovery. Multiple, highly complementary systems have been developed, and many more are in the pipeline. Judging which DNA transposon element will work the best in diverse uses from zebrafish genetic manipulation to human gene therapy is currently a complex task. We have summarized the major transposon vector systems active in vertebrates, comparing and contrasting known critical biochemical and in vivo properties, for future tool design and new genetic applications.

Mutational analysis of highly conserved aspartate residues essential to the catalytic core of the piggyBac transposase

Background

The piggyBac mobile element is quickly gaining popularity as a tool for the transgenesis of many eukaryotic organisms. By studying the transposase which catalyzes the movement of piggyBac, we may be able to modify this vector system to make it a more effective transgenesis tool. In a previous publication, Sarkar A, Sim C, Hong YS, Hogan JR, Fraser MJ, Robertson HM, and Collins FH have proposed the presence of the widespread 'DDE/DDD' motif for piggyBac at amino acid positions D268, D346, and D447.

Results

This study utilizes directed mutagenesis and plasmid-based mobility assays to assess the importance of these residues as the catalytic core of the piggyBac transposase. We have functionally analyzed individual point-mutations with respect to charge and physical size in all three proposed residues of the 'DDD' motif as well as another nearby, highly conserved aspartate at D450. All of our mutations had a significant effect on excision frequency in S2 cell cultures. We have also aligned the piggyBac transposase to other close family members, both functional and non-functional, in an attempt to identify the most highly conserved regions and position a number of interesting features.

Conclusion

We found all the designated DDD aspartates reside in clusters of amino acids that conserved among piggyBac family transposase members. Our results indicate that all four aspartates are necessary, to one degree or another, for excision to occur in a cellular environment, but D450 seems to have a tolerance for a glutamate substitution. All mutants tested significantly decreased excision frequency in cell cultures when compared with the wild-type transposase.

Analysis of the piggyBac transposase reveals a functional nuclear targeting signal in the 94 c-terminal residues

Background

The piggyBac transposable element is a popular tool for germ-line transgenesis of eukaryotes. Despite this, little is known about the mechanism of transposition or the transposase (TPase) itself. A thorough understanding of just how piggyBac works may lead to more effective use of this important mobile element. A PSORTII analysis of the TPase amino acid sequence predicts a bipartite nuclear localization signal (NLS) near the c-terminus, just upstream of a putative ZnF (ZnF).

Results

We fused the piggyBac TPase upstream of and in-frame with the enhanced yellow fluorescent protein (EYFP) in the Drosophila melanogaster inducible metallothionein protein. Using Drosophila Schneider 2 (S2) cells and the deep red fluorescent nuclear stain Draq5, we were able to track the pattern of piggyBac localization with a scanning confocal microscope 48 hours after induction with copper sulphate.

Conclusion

Through n and c-terminal truncations, targeted internal deletions, and specific amino acid mutations of the piggyBac TPase open reading frame, we found that not only is the PSORTII-predicted NLS required for the TPase to enter the nucleus of S2 cells, but there are additional requirements for negatively charged amino acids a short length upstream of this region for nuclear localization.

Gal4/UAS transgenic tools and their application to zebrafish

The ability to regulate gene expression in a cell-specific and temporally restricted manner provides a powerful means to test gene function, bypass the action of lethal genes, label subsets of cells for developmental studies, monitor subcellular structures, and target tissues for selective ablation or physiological analyses. The galactose-inducible system of yeast, mediated by the transcriptional activator Gal4 and its consensus UAS binding site, has proven to be a highly successful and versatile system for controlling transcriptional activation in Drosophila. It has also been used effectively, albeit in a more limited manner, in the mouse. While zebrafish has lagged behind other model systems in the widespread application of Gal4 transgenic approaches to modulate gene activity during development, recent technological advances are permitting rapid progress. Here we review Gal4-regulated genetic tools and discuss how they have been used in zebrafish as well as their potential drawbacks. We describe some exciting new directions, in large part afforded by the Tol2 transposition system, that are generating valuable new Gal4/UAS reagents for zebrafish research.

An abundant evolutionarily conserved CSB-PiggyBac fusion protein expressed in Cockayne syndrome

Cockayne syndrome (CS) is a devastating progeria most often caused by mutations in the CSB gene encoding a SWI/SNF family chromatin remodeling protein. Although all CSB mutations that cause CS are recessive, the complete absence of CSB protein does not cause CS. In addition, most CSB mutations are located beyond exon 5 and are thought to generate only C-terminally truncated protein fragments. We now show that a domesticated PiggyBac-like transposon PGBD3, residing within intron 5 of the CSB gene, functions as an alternative 3′ terminal exon. The alternatively spliced mRNA encodes a novel chimeric protein in which CSB exons 1–5 are joined in frame to the PiggyBac transposase. The resulting CSB-transposase fusion protein is as abundant as CSB protein itself in a variety of human cell lines, and continues to be expressed by primary CS cells in which functional CSB is lost due to mutations beyond exon 5. The CSB-transposase fusion protein has been highly conserved for at least 43 Myr since the divergence of humans and marmoset, and appears to be subject to selective pressure. The human genome contains over 600 nonautonomous PGBD3-related MER85 elements that were dispersed when the PGBD3 transposase was last active at least 37 Mya. Many of these MER85 elements are associated with genes which are involved in neuronal development, and are known to be regulated by CSB. We speculate that the CSB-transposase fusion protein has been conserved for host antitransposon defense, or to modulate gene regulation by MER85 elements, but may cause CS in the absence of functional CSB protein.

For reasons that are still unclear, genetic defects in DNA repair can cause diseases that resemble aspects of premature ageing (“segmental progerias”). Cockayne syndrome (CS) is a particularly devastating progeria most commonly caused by mutations in the CSB chromatin remodeling gene. About 43 million years ago, before humans diverged from marmosets, one of the last PiggyBac transposable elements to invade the human lineage landed within intron 5 of the 21 exon CSB gene. As a result, the CSB locus now encodes two equally abundant proteins generated by alternative mRNA splicing: the original full length CSB protein, and a novel CSB-PiggyBac fusion protein in which the N-terminus of CSB is fused to the complete PiggyBac transposase. Conservation of the CSB-PiggyBac fusion protein since marmoset suggests that it is normally beneficial, demonstrating once again that “selfish” transposable elements can be exploited or “domesticated” by the host. More importantly, almost all CSB mutations that cause CS continue to make the CSB-PiggyBac fusion protein, whereas a mutation that compromises both does not cause CS. Thus the fusion protein which is beneficial in the presence of functional CSB may be harmful in its absence. This may help clarify the cause of CS and other progerias.

Active integration: new strategies for transgenesis

This paper presents novel methods for producing transgenic animals, with a further emphasis on how these techniques may someday be applied in gene therapy. There are several passive methods for transgenesis, such as pronuclear microinjection (PNI) and Intracytoplasmic Sperm Injection-Mediated Transgenesis (ICSI-Tr), which rely on the repair mechanisms of the host for transgene (tg) insertion. ICSI-Tr has been shown to be an effective means of creating transgenic animals with a transfection efficiency of approximately 45% of animals born. Furthermore, because this involves the injection of the transgene into the cytoplasm of oocytes during fertilization, limited mosaicism has traditionally occurred using this technique. Current active transgenesis techniques involve the use of viruses, such as disarmed retroviruses which can insert genes into the host genome. However, these methods are limited by the size of the sequence that can be inserted, high embryo mortality, and randomness of insertion. A novel active method has been developed which combines ICSI-Tr with recombinases or transposases to increase transfection efficiency. This technique has been termed "Active Transgenesis" to imply that the tg is inserted into the host genome by enzymes supplied into the oocyte during tg introduction. DNA based methods alleviate many of the costs and time associated with purifying enzyme. Further studies have shown that RNA can be used for the transposase source. Using RNA may prevent problems with continued transposase activity that can occur if a DNA transposase is integrated into the host genome. At present piggyBac is the most effective transposon for stable integration in mammalian systems and as further studies are done to elucidate modifications which improve piggyBac's specificity and efficacy, efficiency in creating transgenic animals should improve further. Subsequently, these methods may someday be used for gene therapy in humans.

Functional genomics of Plasmodium falciparum through transposon-mediated mutagenesis

Plasmodium falciparum is the causative agent for the most lethal form of human malaria, killing millions annually. Genetic analyses of P. falciparum have been relatively limited due to the lack of robust techniques to manipulate this parasite. Development of transfection technologies and whole genome analyses have helped in understanding the complex biology of this parasite. Even with this wealth of information functional genomics approaches are still very limited in P. falciparum due to the cumbersome and inefficient methods of genetic manipulation. This review focuses on a recently developed, highly efficient method for transposon-based mutagenesis and transgene expression in P. falciparum that will allow functional genomics studies to be performed proficiently on this deadly malaria parasite. By using a piggyBac-based transposition system, multiple random integrations have been obtained into the genome of the parasite. This technique could hence be employed to set up several biological screens in this lethal protozoan parasite that may lead to identification of novel drug targets and vaccine candidates.