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.

Post-integration stability of piggyBac in Aedes aegypti

The post-integration activity of piggyBac transposable element gene vectors in Aedes aegypti mosquitoes was tested under a variety of conditions. The embryos from five independent transgenic lines of Ae. aegypti, each with a single integrated non-autonomous piggyBac transposable element gene vector, were injected with plasmids containing the piggyBac transposase open-reading frame under the regulatory control of the Drosophila melanogaster hsp70 promoter. No evidence for somatic remobilization was detected in the subsequent adults whereas somatic remobilization was readily detected when similar lines of transgenic D. melanogaster were injected with the same piggyBac transposase-expressing plasmid. Ae. aegypti heterozygotes of piggyBac reporter-containing transgenes and piggyBac transposase-expressing transgenes showed no evidence of somatic and germ-line remobilization based on phenotypic and molecular detection methods. The post-integration mobility properties of piggyBac in Ae. aegypti enhance the utility of this gene vector for certain applications, particularly those where any level of vector remobilization is unacceptable.

High efficiency germ-line transformation of mosquitoes

The ability to manipulate the mosquito genome through germ-line transformation provides us with a powerful tool for investigating gene structure and function. It is also a valuable method for the development of novel approaches to combating the spread of mosquito-vectored diseases. To date, germ-line transformation has been demonstrated in several mosquito species. Transgenes are introduced into pre-blastocyst mosquito embryos using microinjection techniques that take a few hours, and progeny are screened for the presence of a marker gene. The microinjection protocol presented here can be applied to most mosquitoes and contains several improvements over other published methods that increase the survival of injected embryos and, therefore, the number of transformants. Transgenic lines can be established in approximately 1 month using this technique.

piggyBac transposon mediated transgenesis of the human blood fluke, Schistosoma mansoni

The transposon piggyBac from the genome of the cabbage looper moth Trichoplusia ni has been observed in the laboratory to jump into the genomes of key model and pathogenic eukaryote organisms including mosquitoes, planarians, human and other mammalian cells, and the malaria parasite Plasmodium falciparum. Introduction of exogenous transposons into schistosomes has not been reported but transposon-mediated transgenesis of schistosomes might supersede current methods for functional genomics of this important human pathogen. In the present study we examined whether the piggyBac transposon could deliver reporter transgenes into the genome of Schistosoma mansoni parasites. A piggyBac donor plasmid modified to encode firefly luciferase under control of schistosome gene promoters was introduced along with 7-methylguanosine capped RNAs encoding piggyBac transposase into cultured schistosomula by square wave electroporation. The activity of the helper transposase mRNA was confirmed by Southern hybridization analysis of genomic DNA from the transformed schistosomes, and hybridization signals indicated that the piggyBac transposon had integrated into numerous sites within the parasite chromosomes. piggyBac integrations were recovered by retrotransposon-anchored PCR, revealing characteristic piggyBac TTAA footprints in the vicinity of the endogenous schistosome retrotransposons Boudicca, SR1, and SR2. This is the first report of chromosomal integration of a transgene and somatic transgenesis of this important human pathogen, in this instance accomplished by mobilization of the piggyBac transposon.

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.

Construction and characterization of new piggyBac vectors for constitutive or inducible expression of heterologous gene pairs and the identification of a previously unrecognized activator sequence in piggyBac

BACKGROUND

We constructed and characterized several new piggyBac vectors to provide transposition of constitutively- or inducibly-expressible heterologous gene pairs. The dual constitutive control element consists of back-to-back copies of a baculovirus immediate early (ie1) promoter separated by a baculovirus enhancer (hr5). The dual inducible control element consists of back-to-back copies of a minimal cytomegalovirus (CMVmin) promoter separated by a synthetic operator (TetO7), which drives transcription in the presence of a mutant transcriptional repressor plus tetracycline.

RESULTS

Characterization of these vectors revealed an unexpected position effect, in which heterologous genes adjacent to the 3'- terminal region ("rightward" genes) were consistently expressed at higher levels than those adjacent to the 5'-terminal region ("leftward" genes) of the piggyBac element. This position effect was observed with all six heterologous genes examined and with both transcriptional control elements. Further analysis demonstrated that this position effect resulted from stimulation of rightward gene expression by the internal domain sequence of the 3'-terminal region of piggyBac. Inserting a copy of this sequence into the 5'- terminal repeat region of our new piggyBac vectors in either orientation stimulated leftward gene expression. Representative piggyBac vectors designed for constitutive or inducible expression of heterologous gene pairs were shown to be functional as insect transformation vectors.

CONCLUSION

This study is significant because (a) it demonstrates the utility of a strategy for the construction of piggyBac vectors that can provide constitutive or inducible heterologous gene pair expression and (b) it reveals the presence of a previously unrecognized transcriptional activator in piggyBac, which is an important and increasingly utilized transposable element.

Interplasmid transposition demonstrates piggyBac mobility in vertebrate species

The piggyBac transposon is an extremely versatile helper-dependent vector for gene transfer and germ line transformation in a wide range of invertebrate species. Analyses of genome sequencing databases have identified piggyBac homologues among several sequenced animal genomes, including the human genome. In this report we demonstrate that this insect transposon is capable of transposition in primate cells and embryos of the zebrafish, Danio rerio. piggyBac mobility was demonstrated using an interplasmid transposition assay that has consistently predicted the germ line transformation capabilities of this mobile element in several other species. Both transfected COS-7 primate cells and injected zebrafish embryos supported the helper-dependent movement of tagged piggyBac element between plasmids in the characteristic cut-and-paste, TTAA target-site specific manner. These results validate piggyBac as a valuable tool for genetic analysis of vertebrates.

High-efficiency transformation of Plasmodium falciparum by the lepidopteran transposable element piggyBac

Functional analysis of the Plasmodium falciparum genome is restricted because of the limited ability to genetically manipulate this important human pathogen. We have developed an efficient transposon-mediated insertional mutagenesis method much needed for high-throughput functional genomics of malaria parasites. A drug-selectable marker, human dihydrofolate reductase, added to the lepidopteran transposon piggyBac, transformed parasites by integration into the P. falciparum genome in the presence of a transposase-expressing helper plasmid. Multiple integrations occurred at the expected TTAA target sites throughout the genome of the parasite. We were able to transform P. falciparum with this piggyBac element at high frequencies, in the range of 10(-3), and obtain stable clones of insertional mutants in a few weeks instead of 6-12 months. Our results show that the piggyBac transposition system can be used as an efficient, random integration tool needed for large-scale, whole-genome mutagenesis of malaria parasites. The availability of such an adaptable genetic tool opens the way for much needed forward genetic approaches to study this lethal human parasite.

piggyBac internal sequences are necessary for efficient transformation of target genomes

A previously reported piggyBac minimal sequence cartridge, which is capable of efficient transposition in embryo interplasmid transposition assays, failed to produce transformants at a significant frequency in Drosophila melanogaster compared with full-length or less extensive internal deletion constructs. We have re-examined the importance of these internal domain (ID) sequences for germline transformation using a PCR strategy that effectively adds increasing lengths of ID sequences to each terminus. A series of these piggyBac ID synthetic deletion plasmids containing the 3xP3-ECFP marker gene are compared for germline transformation of D. melanogaster. Our analyses identify a minimal sequence configuration that is sufficient for movement of piggyBac vectored sequences from plasmids into the insect genome. Southern hybridizations confirm the presence of the piggyBac transposon sequences, and insertion site analyses confirm these integrations target TTAA sites. The results verify that ID sequences adjacent to the 5' and 3' terminal repeat domains are crucial for effective germline transformation with piggyBac even though they are not required for excision or interplasmid transposition. Using this information we reconstructed an inverted repeat cartridge, ITR1.1k, and a minimal piggyBac transposon vector, pXL-BacII-ECFP, each of which contains these identified ID sequences in addition to the terminal repeat configuration previously described as essential for mobility. We confirm in independent experiments that these new minimal constructs yield transformation frequencies similar to the control piggyBac vector. Sequencing analyses of our constructs verify the position and the source of a point mutation within the 3' internal repeat sequence of our vectors that has no apparent effect on transformation efficiency.

An investigation of a possible role for mitochondrial nuclease in apoptosis

Since mitochondrial factors have been implicated in apoptosis, experiments were designed to assess whether or not the potent mitochondrial nuclease could be one of these factors. Nuclei isolated by two different methods were found to contain mitochondrial nuclease in masked form. This nuclease was released by treatment with the non-ionic detergent NP-40 and rendered trypsin-sensitive. It was not removed appreciably from the nuclei by washing and sedimentation of the nuclei through a sucrose cushion. Levels of the mitochondrial nuclease were followed during drug-induced apoptosis. Time courses of apoptosis in cultures of HL-60 cells were monitored by flow cytometry of propidium iodide-stained cells and by agarose gel electrophoresis of extracted DNA. Changes in the inner mitochondrial transmembrane potential were monitored by flow cytometry of chloromethyl-X-Rosamine-stained cells. Apoptosis was induced by treatment with either the chemotherapeutic agent etoposide (VP-16 at 10 microM) over an 8 h period or with the anti-rheumatic agent hydroxychloroquine (HCQ at 0.28 mM) over a 24 h period. These two drugs likely act in different pathways of apoptosis. VP-16 caused loss of the mitochondrial transmembrane potential 1.0-1.5 h before apoptosis was detected. On the other hand, treatment with HCQ caused these processes to occur in parallel possibly indicating that the mitochondrial changes are secondary events. No losses of masked mitochondrial nuclease were detected with either drug treatment during the course of apoptosis. HL-60 mitochondrial DNA was also not degraded during apoptosis induced by either agent. These observations likely explain why the mitochondrial DNA is not degraded and make it unlikely that mitochondrial nuclease plays any role in vivo in chromatin DNA fragmentation.

Molecular evolutionary analysis of the widespread piggyBac transposon family and related "domesticated" sequences

piggyBac is a short inverted-repeat-type DNA transposable element originally isolated from the genome of the moth Trichoplusia ni. It is currently the gene vector of choice for the transformation of various insect species. A few sequences with similarity to piggyBac have previously been identified from organisms such as humans ( Looper), the pufferfish Takifugu rubripes ( Pigibaku), Xenopus ( Tx), Daphnia ( Pokey), and the Oriental fruit fly Bactrocera dorsalis. We have now identified 50 piggyBac-like sequences from publicly available genome sequences and expressed sequence tags (ESTs). This survey allows the first comparative examination of the distinctive piggyBac transposase, suggesting that it might contain a highly divergent DDD domain, comparable to the widespread DDE domain found in many DNA transposases and retroviral integrases which consists of two absolutely conserved aspartic acids separated by about 70 amino acids with a highly conserved glutamic acid about 35 amino acids further away. Many piggyBac-like sequences were found in the genomes of a phylogenetically diverse range of organisms including fungi, plants, insects, crustaceans, urochordates, amphibians, fishes and mammals. Also, several instances of "domestication" of the piggyBac transposase sequence by the host genome for cellular functions were identified. Novel members of the piggyBac family may be useful in genetic engineering of many organisms.

Germ line transformation of the yellow fever mosquito, Aedes aegypti, mediated by transpositional insertion of a piggyBac vector

Mosquito-vectored diseases such as yellow fever and dengue fever continue to have a substantial impact on human populations world-wide. Novel strategies for control of these mosquito vectored diseases can arise through the development of reliable systems for genetic manipulation of the insect vector. A piggyBac vector marked with the Drosophila melanogaster cinnabar (cn) gene was used to transform the white-eyed khw strain of Aedes aegypti. Microinjection of preblastoderm embryos resulted in four families of cinnabar transformed insects. An overall transformation frequency of 4%, with a range of 0% to as high as 13% for individual experiments, was achieved when using a heat-shock induced transposase providing helper plasmid. Southern hybridizations indicated multiple insertion events in three of four transgenic lines, while the presence of duplicated target TTAA sites at either ends of individual insertions confirmed characteristic piggyBac transposition events in these three transgenic lines. The transgenic phenotype has remained stable for more than twenty generations. The transformations effected using the piggyBac element establish the potential of this element as a germ-line transformation vector for Aedine mosquitoes.

The minimum internal and external sequence requirements for transposition of the eukaryotic transformation vector piggyBac

The piggyBac element from Trichoplusia ni is recognized as a useful vector for transgenesis of a wide variety of species. This transposable element is 2472 bp in length, and has a complex repeat configuration consisting of an internal repeat (IR), spacer, and terminal repeat (TR) at both ends, and a single ORF encoding the transposase. Excision assays performed in microinjected T. ni embryos using plasmids deleted for progressively larger portions of the piggyBac internal sequence reveal that the 5' and 3' IR, spacer, and TR configuration is sufficient for precise excision of piggyBac when transposase is provided in trans. Interplasmid transposition assays using plasmids carrying varying lengths of intervening sequence between the piggyBac termini in T. ni demonstrate that a minimum of 55 bp of intervening sequence is required for optimal transposition, while lengths less than 40 bp result in a dramatic decrease in transposition frequency. These results suggest that the piggyBac transposase may bind both termini simultaneously before cleavage can occur, and/or that the formation of a transposition complex requires DNA bending between the two termini. Based on these results we constructed a 702-bp cartridge with minimal piggyBac 5' and 3' terminal regions separated by an intervening sequence of optimal length. Interplasmid transposition assays demonstrate that the minimal terminal configuration is sufficient to mediate transposition, and also verify that simply inserting this cartridge into an existing plasmid converts that plasmid into a non-autonomous piggyBac transposon. We also constructed a minimal piggyBac vector, pXL-Bac, that contains an internal multiple cloning site sequence between the minimal terminal regions. These vectors should greatly facilitate the utilization of the piggyBac transposon in a wide range of hosts.

The piggyBac element is capable of precise excision and transposition in cells and embryos of the mosquito, Anopheles gambiae

The piggyBac transposable element was tested for transposition activity in plasmid-based excision and inter-plasmid transposition assays to determine if this element would function in Anopheles gambiae cells and embryos. In the Mos55 cell line, precise excision of the piggyBac element was observed only in the presence of a helper plasmid. Excision occurred at a rate of 1 event per 1000 donor plasmids screened. Precise excision of the piggyBac element was also observed in injected An. gambiae embryos, but at a lower rate of 1 excision per 5000 donor plasmids. Transposition of the marked piggyBac element into a target plasmid occurred in An. gambiae cells at a rate of 1 transposition event per 24,000 donor plasmids. The piggyBac element transposed in a precise manner, with the TTAA target site being duplicated upon insertion, in 56% of transpositions observed, and only in the presence of the piggyBac helper. The remaining transpositions resulted in a deletion of target sequence, a novel observation for the phenomenon of piggyBac element insertion. 'Hot spots' for insertion into the target plasmid were observed, with 25 of 34 events involving one particular site. These results are the first demonstration of the precise mobility of piggyBac in this malaria vector and suggest that the lepidopteran piggyBac transposon is a candidate element for germline transformation of anopheline mosquitoes.

Caspase-3 activates endo-exonuclease: further evidence for a role of the nuclease in apoptosis

Single-strand DNase and poly rAase, activities characteristic of endo-exonuclease, were co-activated in nuclear fractions of HL-60 cells by caspase-3. Activation was accompanied by cleavages of large soluble polypeptides (130-185 kDa) and a 65 kDa inactive chromatin-associated polypeptide related to the endo-exonuclease of Neurospora crassa as detected on immunoblots. The major products seen in vitro were a 77 kDa soluble polypeptide and an active chromatin-associated 34 kDa polypeptide. When HL-60 cells were induced to undergo apoptosis by treating with 50 microM etoposide (VP-16) for 4 hours, 77 kDa and 40 kDa polypeptides accumulated in nuclear fractions. Chromatin DNA fragmentation activity was also activated in cytosol and nuclear extract either by pre-treating the cells in vivo with VP-16 or by treating the cytosol in vitro with caspase-3 or dATP and cytochrome c. Endo-exonuclease activated by caspase-3 in cytosol-derived fractions augmented chromatin DNA fragmentation activity in vitro. Endo-exonuclease is proposed to act in vivo in conjunction with the caspase-activated DNase (CAD) to degrade chromatin DNA during apoptosis of HL-60 cells.

Caspase-3-dependent and caspase-3-independent pathways leading to chromatin DNA fragmentation in HL-60 cells

Apoptosis induced by etoposide (VP-16) in HL-60 cells was confirmed to be caspase-dependent. It was fully inhibited by the broad-spectrum caspase inhibitor Z-VAD-fmk. However, the caspase-3-specific inhibitor Z-DEVDfmk only partially inhibited apoptosis. This indicated that a second caspase is required in vivo for full activation of the apoptotic nucease CAD. Aurin tricarboxylic acid (ATA) did not inhibit VP-16-induced apoptosis. In contrast, apoptosis induced by hydroxychloroquine (HCQ) in HL-60 cells was caspase-3 independent and was fully inhibited by ATA. Thus, CAD does not appear to be involved in chromatin DNA degradation in this case. A second apoptotic nuclease is postulated to degrade the DNA, likely endo- exonuclease, an abundant nuclear enzyme that acts on both DNA and RNA and is present in latent form. HCQ, but not VP-16, stimulated DNA degradation ("laddering") in isolated nuclei. This indicates that the drug can act directly in the nuclei to trigger activation of the second latent apoptotic nuclease.

Transposition of the piggyBac element in embryos of Drosophila melanogaster, Aedes aegypti and Trichoplusia ni

The Lepidopteran transposable element piggyBac is being recognized as a useful vector for genetic engineering in a variety of insect species. This transposon can mediate transformation in the Dipteran species Ceratitis capitata, and can potentially serve as a versatile vector for transformation of a wide variety of insect species. Using a plasmid-based interplasmid transposition assay, we have demonstrated that this transposon, of the short inverted terminal repeat type, is capable of transposition in embryos of three different insect species, Drosophila melanogaster, the yellow fever mosquito Aedes aegypti, and its host of origin, Trichoplusia ni. This assay can confirm the potential utility of piggyBac as a gene transfer tool in a given insect species, and provides an experimental model for assessing molecular mechanisms of transposon movement.

Proteins from nuclear extracts of two lepidopteran cell lines recognize the ends of TTAA-specific transposons piggyBac and tagalong

The transposons piggyBac and tagalong are Lepidopteran transposons that exhibit extreme site-specificity for the tetranucleotide TTAA upon insertion and excise in a characteristic precise fashion, regenerating a single TTAA target site. The precise excision of both piggyBac and tagalong can occur in the absence of factors encoded by either transposon, possibly through the recruitment of host proteins and/or cross-mobilizing transposons. In this report, we utilize mobility shift assays and exonuclease III protection analyses to identify DNA binding activities from IPLB-SF21AE and TN-368 cells that are specific for piggyBac and tagalong-terminal repeats.

The lepidopteran transposon vector, piggyBac, mediates germ-line transformation in the Mediterranean fruit fly

The piggyBac (IFP2) short inverted terminal repeat transposable element from the cabbage looper Trichoplusia ni was tested for gene transfer vector function as part of a bipartite vector-helper system in the Mediterranean fruit fly Ceratitis capitata. A piggyBac vector marked with the medfly white gene was tested with a normally regulated piggyBac transposase helper at two different concentrations in a white eye host strain. Both experiments yielded transformants at an approximate frequency of 3-5%, with a total of six lines isolated having pigmented eyes with various levels of coloration. G1 transformant siblings from each line shared at least one common integration, with several sublines having an additional second integration. For the first transformant line isolated, two integrations were determined to be stable for 15 generations. For five of the lines, a piggyBac-mediated transposition was verified by sequencing the insertion site junctions isolated by inverse PCR that identified a characteristic piggyBac TTAA target site duplication. The efficient and stable transformation of the medfly with a lepidopteran vector represents transposon function over a relatively large evolutionary distance and suggests that the piggyBac system will be functional in a broad range of insects.

Isolation and characterization of LMC, a novel lymphocyte and monocyte chemoattractant human CC chemokine, with myelosuppressive activity

By searching the Expressed Sequence Tag (EST) data base, we identified a partial cDNA sequence encoding a novel human CC chemokine. The entire cDNA sequence was determined and revealed a CC chemokine whose mature protein consisted of 100 amino acids with predicted molecular weight of 11 kd. The chemokine preferantially chemoattracted lymphocytes and monocytes but not neutrophils. It was, therefore, named LMC (Lymphocyte and Monocyte Chemoattractant). LMC exhibited potent myelosuppressive activity, which was comparable to that of MIP-1alpha. We identified several bacterial artificial clones (BAC) containing the LMC gene along with two human CC chemokine subfamily members; leukotactin-1 (Lkn-1) and CKbeta8-1/CKbeta8. This data suggests that the LMC gene is located at human chromosome 17q which encompasses a human CC chemokine gene cluster.

Analysis of the cis-acting DNA elements required for piggyBac transposable element excision

The terminal DNA sequence requirements for piggyBac transposable element excision were explored using a plasmid-based assay in transfected, cultured insect cells. A donor plasmid containing duplicate 3' piggyBac terminal inverted repeats was constructed that allowed individual nucleotides or groups of nucleotides within one of the 3' repeats to be mutated. The relative extent of excision using the mutated end versus the wild-type end was then assayed. Removal of even one of the terminal 3' G nucleotides from the piggyBac inverted repeat, or removal of the dinucleotide AA from the flanking TTAA target site prevents excision of piggyBac at the mutated terminus. Incorporation of an asymmetric TTAC target site at the 3' end does not prevent excision from the mutated end. Thus, both piggyBac DNA and flanking host DNA appear to play crucial roles in the excision process.

Endo-exonuclease of human leukaemic cells: evidence for a role in apoptosis

Inactive forms of endo-exonuclease, activated in vitro by treatment with trypsin, have been identified in human leukaemic CEM and MOLT-4 cells. They comprise over 95% of the total single-strand DNase activity in nuclei and are mainly bound to chromatin and the nuclear matrix. The activated enzyme had Mg2+(Mn2+)-dependent, Ca(2+)-stimulated activities with single- and double-strand DNAs and RNA (polyriboadenylic acid) and other properties characteristic of endo-exonucleases previously described. At least twice as much inactive endo-exonuclease has also been localised in extranuclear compartments of CEM and MOLT-4 cells, 85% bound to the membranes of the endoplasmic reticulum and 15% free in the cytosol. The soluble cytosolic trypsin-activatable endo-exonuclease was immunoprecipitated by antibodies raised independently to both Neurospora and monkey CV-1 cell endo-exonucleases. The free and bound enzymes of both nuclear and extranuclear compartments also cross-reacted on immunoblots with the antibody raised to Neurospora endo-exonuclease to reveal multiple polypeptides ranging in size from 18 to 145 kDa, many of which exhibited activity on DNA gels. The major species bound to the chromatin/matrix were in the 55–63 kDa range. Limited proteolysis of the large polypeptides to those of 18 to 46 kDa accompanied spontaneous chromatin DNA fragmentation to form DNA “ladders' in an isolated nuclei/cytosol system. When the leukaemic cells were treated in culture with either etoposide or podophyllotoxin to induce apoptosis, the largest polypeptides disappeared and smaller endo-exonuclease-related polypeptides of 18 to 46 kDa were detected in the nuclear extracts. The appearance of these polypeptides also correlated with extensive chromatin DNA fragmentation. In addition, there were correlations between the depletion of the major 55–63 kDa species bound to the membranes of the endoplasmic reticulum, depletion of the extranuclear trypsin-activatable activity and the onset and extent of chromatin DNA fragmentation in both cell lines. The extranuclear 55–63 kDa species may be precursors of the chromatin/matrix bound endo-exonuclease. The results indicate that endo-exonuclease plays a role in chromatin DNA degradation in mammalian cells during apoptosis.

Excision of the piggyBac transposable element in vitro is a precise event that is enhanced by the expression of its encoded transposase

The piggyBac Lepidopteran transposable element moves from the cellular genome into infecting baculovirus genomes during passage of the virus in cultured TN-368 cells. We have constructed genetically tagged piggyBac elements that permit analysis of excision when transiently introduced on plasmids into the piggyBac-deficient Spodoptera frugiperda IPLB-SF21AE cell line. Precise excision of the element from these plasmids occurs at a higher frequency in the presence of a helper plasmid that presumably supplies the piggyBac transposase. The results suggest that the piggyBac transposon encodes a protein that functions to facilitate not only insertion, but precise excision as well. This is the first demonstration of piggyBac mobility from plasmid sources in uninfected Lepidopteran cells.

Precise excision of TTAA-specific lepidopteran transposons piggyBac (IFP2) and tagalong (TFP3) from the baculovirus genome in cell lines from two species of Lepidoptera

Transposon mutagenesis of baculoviruses provides an ideal experimental system for analysis of the movement of a unique family of mobile element identified from lepidopteran genomes. Members of this family of short-inverted-repeat elements are characterized by their extreme specificity for TTAA target sites. This report describes the analysis of excision events for two representatives of this family, tagalong (formerly TFP3) and piggyBac (formerly IFP2). These elements were tagged with a polyhedrin/lacZ reporter gene and inserted back into the virus genome either by homologous recombination or by transposition. Revertants were selected based on a white plaque phenotype. Both elements excise in a precise fashion from their positions in the baculovirus genome in either TN-368 cells or IPLB-SF21 AE cells. The precise excision of these elements in infected IPLB-SF21 AE cells occurs in the absence of either tagalong or piggyBac element encoded functions. The common characteristics of both insertion and excision for these elements provides further validation for their inclusion in a single family of unique transposons.