Visualization of site-specific recombination catalyzed by a recombinase from Zygosaccharomyces rouxii in Arabidopsis thaliana

Production of selectable marker gene-free Cavendish banana (Musa spp.) using a steroid-inducible recombinase platform

Genetic improvement of commercially accepted banana cultivars is strongly reliant on the ability to introduce genes that encode important agro-traits such as disease resistance. In most cases this can only be achieved using a transgenic approach. Public and regulatory acceptance of these events would greatly increase with “clean” single copy integration events free of the selectable marker gene and extraneous vector backbone. This would also allow for the successive addition of new genes and traits as they become available. In this study, we used the pMarker Free 1 (pMF1) vector containing the green fluorescent protein (gfp) reporter gene to assess the effectiveness of steroid-inducible recombination and positive/negative dual selection to regenerate transgenic Cavendish banana plants that were potentially free of the selectable marker gene. By examining the interaction of two different Agrobacterium strains with two different cultivars of Cavendish banana, namely Williams and Grand Naine, we describe a transformation and regeneration strategy that successfully produced marker-free, single transgene copy, gfp-expressing events. The system will provide a useful means of serially improving banana into the future.

Production of Marker-Free Apple Plants Expressing the Supersweet Protein Gene Driven by Plant Promoter

The presence of antibiotic resistance and other marker genes in genetically modified plants causes concern in society because of perceived risks for the environment and human health. The creation of transgenic plants that do not contain foreign genetic material, especially that of bacterial and viral origin, largely alleviates the tension and makes the plants potentially more attractive for consumers. To produce marker-free transgenic apple plants, we used the pMF1 vector, which combines Zygosaccharomyces rouxii recombinaseR and a CodA-nptII bifunctional selectable gene. The thaumatin II gene from the tropical plant Thaumatococcus daniellii, which is under the control of the plant E8 gene (a predominantly fruit-specific promoter) and rbsS3A terminator, was taken as the gene of interest for modification of the fruit taste and enhancing its sweetness. Exploitation of this gene in our laboratory has allowed enhancing the sweetness, as well as improving the taste characteristics, of fruits and vegetables of plants such as strawberry, carrot, tomato and pear. We have obtained three independent transgenic apple lines that have been analyzed by PCR and Southern blot analyses for the presence of T-DNA sequences. Two of them contained a partial sequence of the T-DNA. With one line containing the full insert we then used a delayed strategy for the selection of marker-free plants. After induction of recombinase activity in leaf explants on selective media with 5-fluorocytosine (5-FC) we obtained more than 30 sublines, most of which lost their resistance to kanamycin. Most of the apple sublines showed the expression of the supersweet protein gene in a wide range of levels as detected by RNA accumulation. The plants from the group with the highest transcript level were propagated and grafted onto dwarf rootstocks for early fruit production for future estimates of protein levels and organoleptic analyses. Thus, we developed a protocol that allowed the production of marker-free apple plants expressing the supersweet protein.

Purple chromoprotein gene serves as a new selection marker for transgenesis of the microalga Nannochloropsis oculata

Among the methods used to screen transgenic microalgae, antibiotics selection has raised environmental and food safety concerns, while the observation of fluorescence proteins could be influenced by the endogenous fluorescence of host chloroplasts. As an alternative, this study isolated the purple chromoprotein (CP) from Stichodacyla haddoni (shCP). A plasmid in which shCP cDNA is driven by a heat-inducible promoter was linearized and electroporated into 2.5×10⁸ protoplasts of Nannochloropsis oculata. Following regeneration and cultivation on an f/2 medium plate for two weeks, we observed 26 colonies that displayed a slightly dark green coloration. After individually subculturing and performing five hours of heat shock at 42°C, a dark brown color was mosaically displayed in five of these colonies, indicating that both untransformed and transformed cells were mixed together in each colony. To obtain a uniform expression of shCP throughout the whole colony, we continuously isolated each transformed cell that exhibited brown coloration and subcultured it on a fresh plate, resulting in the generation of five transgenic lines of N. oculata which stably harbored the shCP gene for at least 22 months, as confirmed by PCR detection and observation by the naked eye. As shown by Western blot, exogenous shCP protein was expressed in these transgenic microalgae. Since shCP protein is biodegradable and originates from a marine organism, both environmental and food safety concerns have been eliminated, making this novel shCP reporter gene a simple, but effective and ecologically safe, marker for screening and isolating transgenic microalgae.

Development of selectable marker free, insect resistant, transgenic mustard (Brassica juncea) plants using Cre/lox mediated recombination

Background

Antibiotic/ herbicide resistant marker genes have been proven to be very useful in plant transformation for the initial selection of desired transgenic events. However, presence of these genes in the genetically modified crops may render the crop less acceptable to the consumers. Among several different approaches, the effectiveness of Cre/lox mediated recombination strategy for selectable marker gene (SMG) elimination has previously been demonstrated by different groups in several plants including Brassica. In the present study exploiting Cre/lox mediated recombination strategy, attempt has been made for selectable marker gene elimination from Allium sativum leaf agglutinin (ASAL) expressing Brassica plants with hemipteran insect resistant phenotype.

Results

Allium sativum leaf agglutinin (ASAL) linked with lox flanked hygromycin resistant (hpt) gene was introduced in mustard. Cre recombinase gene cassette was also integrated in separate event. A Cre/lox mediated recombination using crossing strategy was adopted to remove the hpt gene from the subsequent generation of selected hybrid events. Reciprocal crosses were made between T₁ASAL-lox-hpt-lox and cre-bar plants. Marker gene elimination was confirmed in the resulting F₁ hybrid progenies by PCR analysis, using hpt, cre and ASAL specific primers followed by Southern hybridization. In marker free plants, expression of ASAL was also confirmed by western blotting and ELISA analysis. Retention of functionality of expressed ASAL was investigated by agglutination assay using rabbit erythrocytes. Expressed ASAL was also found to be thermo-sensitive. In planta insect bioassay on F₁ hybrid progenies exhibited detrimental effect on the performance of devastating target pest, Lipaphis erysimi. The F₁ hybrid hpt negative, ASAL positive plants were allowed to self- fertilize to obtain F₂ progeny plants. In some of these plants cre gene was found to be segregated out of the ASAL gene by genetic segregation yielding completely marker free plants.

Conclusions

The present study establishes the efficient expression of the newly introduced insect resistant ASAL gene even after Cre/lox mediated recombination resulting in elimination of selectable marker gene.

Gene use restriction technologies for transgenic plant bioconfinement

The advances of modern plant technologies, especially genetically modified crops, are considered to be a substantial benefit to agriculture and society. However, so-called transgene escape remains and is of environmental and regulatory concern. Genetic use restriction technologies (GURTs) provide a possible solution to prevent transgene dispersal. Although GURTs were originally developed as a way for intellectual property protection (IPP), we believe their maximum benefit could be in the prevention of gene flow, that is, bioconfinement. This review describes the underlying signal transduction and components necessary to implement any GURT system. Furthermore, we review the similarities and differences between IPP- and bioconfinement-oriented GURTs, discuss the GURTs' design for impeding transgene escape and summarize recent advances. Lastly, we go beyond the state of the science to speculate on regulatory and ecological effects of implementing GURTs for bioconfinement.

Site-specific T-DNA integration in Arabidopsis thaliana mediated by the combined action of CRE recombinase and ϕC31 integrase

Random T-DNA integration into the plant host genome can be problematic for a variety of reasons, including potentially variable transgene expression as a result of different integration positions and multiple T-DNA copies, the risk of mutating the host genome and the difficulty of stacking well-defined traits. Therefore, recombination systems have been proposed to integrate the T-DNA at a pre-selected site in the host genome. Here, we demonstrate the capacity of the ϕC31 integrase (INT) for efficient targeted T-DNA integration. Moreover, we show that the iterative site-specific integration system (ISSI), which combines the activities of the CRE recombinase and INT, enables the targeting of genes to a pre-selected site with the concomitant removal of the resident selectable marker. To begin, plants expressing both the CRE and INT recombinase and containing the target attP site were constructed. These plants were supertransformed with a T-DNA vector harboring the loxP site, the attB sites, a selectable marker and an expression cassette encoding a reporter protein. Three out of the 35 transformants obtained (9%) showed transgenerational site-specific integration (SSI) of this T-DNA and removal of the resident selectable marker, as demonstrated by PCR, Southern blot and segregation analysis. In conclusion, our results show the applicability of the ISSI system for precise and targeted Agrobacterium-mediated integration, allowing the serial integration of transgenic DNA sequences in plants.

Genetically modified plants: public and scientific perceptions

The potential of genetically modified plants to meet the requirements of growing population is not being recognized at present. This is a consequence of concerns raised by the public and the critics about their applications and release into the environment. These include effect on human health and environment, biosafety, world trade monopolies, trustworthiness of public institutions, integrity of regulatory agencies, loss of individual choice, and ethics as well as skepticism about the real potential of the genetically modified plants, and so on. Such concerns are enormous and prevalent even today. However, it should be acknowledged that most of them are not specific for genetically modified plants, and the public should not forget that the conventionally bred plants consumed by them are also associated with similar risks where no information about the gene(s) transfer is available. Moreover, most of the concerns are hypothetical and lack scientific background. Though a few concerns are still to be disproved, it is viewed that, with proper management, these genetically modified plants have immense potential for the betterment of mankind. In the present paper, an overview of the raised concerns and wherever possible reasons assigned to explain their intensity or unsuitability are reviewed.

Transgene excision in pollen using a codon optimized serine resolvase CinH-RS2 site-specific recombination system

Transgene escape, a major environmental and regulatory concern in transgenic crop cultivation, could be alleviated by removing transgenes from pollen, the most frequent vector for transgene flow. A transgene excision vector containing a codon optimized serine resolvase CinH recombinase (CinH) and its recognition sites RS2 were constructed and transformed into tobacco (Nicotiana tabacum cv. Xanthi). CinH recombinase recognized 119 bp of nucleic acid sequences, RS2, in pollen and excised the transgene flanked by the RS2 sites. In this system, the pollen-specific LAT52 promoter from tomato was employed to control the expression of CinH recombinase. Loss of expression of a green fluorescent protein (GFP) gene under the control of the LAT59 promoter from tomato was used as an indicator of transgene excision. Efficiency of transgene excision from pollen was determined by flow cytometry (FCM)-based pollen screening. While a transgenic event in the absence of CinH recombinase contained about 70% of GFP-synthesizing pollen, three single-copy transgene events contained less than 1% of GFP-synthesizing pollen based on 30,000 pollen grains analyzed per event. This suggests that CinH-RS2 recombination system could be effectively utilized for transgene biocontainment.

Auto-excision of selectable marker genes from transgenic tobacco via a stress inducible FLP/FRT site-specific recombination system

Antibiotic resistance marker genes are powerful selection tools for use in plant transformation processes. However, once transformation is accomplished, the presence of these resistance genes is no longer necessary and can even be undesirable. We herein describe the successful excision of antibiotic resistance genes from transgenic plants via the use of an oxidative stress-inducible FLP gene. FLP encodes a recombinase that can eliminate FLP and hpt selection genes flanked by two FRT sites. During a transformation procedure in tobacco, transformants were obtained by selection on hygromycin media. Regenerants of the initial transformants were screened for selective marker excision in hydrogen peroxide supplemented media and both the FLP and hpt genes were found to have been eliminated. About 13-41% of regenerated shoots on hydrogen peroxide media were marker-free. This auto-excision system, mediated by the oxidative stress-inducible FLP/FRT system to eliminate a selectable marker gene can be very readily adopted and used to efficiently generate marker-free transgenic plants.

Cre/lox system to develop selectable marker free transgenic tobacco plants conferring resistance against sap sucking homopteran insect

A binary expression vector was constructed containing the insecticidal gene Allium sativum leaf agglutinin (ASAL), and a selectable nptII marker gene cassette, flanked by lox sites. Similarly, another binary vector was developed with the chimeric cre gene construct. Transformed tobacco plants were generated with these two independent vectors. Each of the T(0) lox plants was crossed with T(0) Cre plants. PCR analyses followed by the sequencing of the target T-DNA part of the hybrid T(1) plants demonstrated the excision of the nptII gene in highly precised manner in certain percentage of the T(1) hybrid lines. The frequency of such marker gene excision was calculated to be 19.2% in the hybrids. Marker free plants were able to express ASAL efficiently and reduce the survivability of Myzus persiceae, the deadly pest of tobacco significantly, compared to the control tobacco plants. Results of PCR and Southern blot analyses of some of the T(2) plants detected the absence of cre as well as nptII genes. Thus, the crossing strategy involving Cre/lox system for the excision of marker genes appears to be very effective and easy to execute. Documentation of such marker excision phenomenon in the transgenic plants expressing the important insecticidal protein for the first time has a great significance from agricultural and biotechnological points of view.

Generation of marker- and backbone-free transgenic potatoes by site-specific recombination and a bi-functional marker gene in a non-regular one-border agrobacterium transformation vector

A binary vector, designated PROGMO, was constructed to assess the potential of the Zygosaccharomyces rouxii R/Rs recombination system for generating marker- and backbone-free transgenic potato (Solanum tuberosum) plants with high transgene expression and low copy number insertion. The PROGMO vector utilises a constitutively expressed plant-adapted R recombinase and a codA-nptII bi-functional, positive/negative selectable marker gene. It carries only the right border (RB) of T-DNA and consequently the whole plasmid will be inserted as one long T-DNA into the plant genome. The recognition sites (Rs) are located at such positions that recombinase enzyme activity will recombine and delete both the bi-functional marker genes as well as the backbone of the binary vector, leaving only the gene of interest flanked by a copy of Rs and RB. Efficiency of PROGMO transformation was tested by introduction of the GUS reporter gene into potato. It was shown that after 21 days of positive selection and using 300 mgl(-1 )5-fluorocytosine for negative selection, 29% of regenerated shoots carried only the GUS gene flanked by a copy of Rs and RB. The PROGMO vector approach is simple and might be widely applicable for the production of marker- and backbone-free transgenic plants of many crop species.

Towards the ideal GMP: homologous recombination and marker gene excision

A mayor aim of biotechnology is the establishment of techniques for the precise manipulation of plant genomes. Two major efforts have been undertaken over the last dozen years, one to set up techniques for site-specific alteration of the plant genome via homologous recombination ("gene targeting") and the other for the removal of selectable marker genes from transgenic plants. Unfortunately, despite multiple promising approaches that will be shortly described in this review no feasible gene targeting technique has been developed till now for crop plants. In contrast, several alternative procedures have been established successfully to remove selectable markers from plant genomes. Intriguingly besides techniques relying on transposons and site-specific recombinases, recent results indicate that homologous recombination might be a valuable alternative for the excision of marker genes.

Visualization of somatic deletions mediated by R/RS site-specific recombination and induction of germinal deletions caused by callus differentiation and regeneration in rice

A transgenic rice plant expressing the recombinase of Zygosaccharomyces rouxii under the control of the CaMV 35S promoter was crossed with a transgenic plant carrying a cryptic (beta-glucuronidase) GUS reporter gene, which was activated by recombinase-mediated deletions between two specific recombination sites ( RSs). In F(1) plants, GUS activity was observed as blue spots and stripes in vascular bundles in several parts of the leaves. GUS expression was detected in all of the calli induced from F(1) seeds and throughout the regenerated plants. DNA analysis using the polymerase chain reaction and Southern blotting showed that R/ RS-mediated deletions occurred in all of the cells of the regenerated plants. Stable GUS expression was confirmed in the progeny resulting from self-pollination. Thus, the deletions obtained in the regenerated plants were genetically equivalent to the germinal deletions. These results indicate that the induction of callus differentiation and shoot regeneration is an effective manner to activate the R/ RS system and to produce plants with chromosomal deletions.

The ASYMMETRIC LEAVES2 gene of Arabidopsis thaliana, required for formation of a symmetric flat leaf lamina, encodes a member of a novel family of proteins characterized by cysteine repeats and a leucine zipper

The ASYMMETRIC LEAVES2 (AS2) gene of Arabidopsis thaliana is involved in the establishment of the leaf venation system, which includes the prominent midvein, as well as in the development of a symmetric lamina. The gene product also represses the expression of class 1 knox homeobox genes in leaves. We have characterized the AS2 gene, which appears to encode a novel protein with cysteine repeats (designated the C-motif) and a leucine-zipper-like sequence in the amino-terminal half of the primary sequence. The Arabidopsis genome contains 42 putative genes that potentially encode proteins with conserved amino acid sequences that include the C-motif and the leucine-zipper-like sequence in the amino-terminal half. Thus, the AS2 protein belongs to a novel family of proteins that we have designated the AS2 family. Members of this family except AS2 also have been designated ASLs (AS2-like proteins). Transcripts of AS2 were detected mainly in adaxial domains of cotyledonary primordia. Green fluorescent protein-fused AS2 was concentrated in plant cell nuclei. Overexpression of AS2 cDNA in transgenic Arabidopsis plants resulted in upwardly curled leaves, which differed markedly from the downwardly curled leaves generated by loss-of-function mutation of AS2. Our results suggest that AS2 functions in the transcription of a certain gene(s) in plant nuclei and thereby controls the formation of a symmetric flat leaf lamina and the establishment of a prominent midvein and other patterns of venation.

FLP/FRT-mediated restoration of normal phenotypes and clonal sectors formation in rolC transgenic tobacco

Site-specific recombination systems have been shown to excise transgene DNA sequences positioned between their cognate target sites, and thus be used to generate clonal sectors in transgenic plants. Here we characterized clonal sectors derived from genetic reversion of rolC (A. rhizogenes)--induced vegetative and reproductive phenotypes, mediated by FLP recombinase from S. cerevisiae, in tobacco. The constitutive expression of rolC induces pleiotropic effects including reduced apical dominance and plant height, lanceolate and pale green leaves and small, male-sterile flowers. Two transgenic male-sterile tobacco lines (N. tabacum, Samsun NN) expressing a 35sP-rolC gene construct flanked by two FRT (FLP recombinase target) sites, were cross-pollinated with pollen from a constitutive 35sP-FLP expressing line. Three main phenotypes were generated in result of recombinase-mediated excision of the 35sP-rolC locus in the F1 (FLP x FRT-35sP-rolC-FRT) hybrid progenies: (a) restoration of male fertility, associated with reversion to normal leaf phenotypes prior to flower bud formation, (b) development of normal and fertile lateral shoot sectors on the background of rolC-type plants, (c) restoration of partially fertile flowers, associated with display of peripheral normal leaf sectors surrounding rolC-type inner-leaf tissues, consistent with periclinal chimeras. These results, supported by DNA molecular analysis, indicate that site-specific recombination might be used as a relatively efficient tool for generation of transgenic periclinal chimeric plants.

The ASYMMETRIC LEAVES2 gene of Arabidopsis thaliana regulates formation of a symmetric lamina, establishment of venation and repression of meristem-related homeobox genes in leaves

The asymmetric leaves2 (as2) mutant of Arabidopsis thaliana generated leaf lobes and leaflet-like structures from the petioles of leaves in a bilaterally asymmetric manner. Both the delayed formation of the primary vein and the asymmetric formation of secondary veins were apparent in leaf primordia of as2 plants. A distinct midvein, which is the thickest vein and is located in the longitudinal center of the leaf lamina of wild-type plants, was often rudimentary even in mature as2 leaves. However, several parallel veins of very similar thickness were evident in such leaves. The complexity of venation patterns in all leaf-like organs of as2 plants was reduced. The malformed veins were visible before the development of asymmetry of the leaf lamina and were maintained in mature as2 leaves. In vitro culture on phytohormone-free medium of leaf sections from as2 mutants and from the asymmetric leaves1 (as1) mutant, which has a phenotype similar to that of as2, revealed an elevated potential in both cases for regeneration of shoots from leaf cells. Analysis by the reverse transcription-polymerase chain reaction showed that transcripts of the KNAT1, KNAT2 and KNAT6 (a recently identified member of the class 1 knox family) genes accumulated in the leaves of both as2 and as1 plants but not of wild type. Transcripts of the STM gene also accumulated in as1 leaves. These findings suggest that, in leaves, the AS2 and AS1 genes repress the expression of these homeobox genes, which are thought to maintain the indeterminate cell state in the shoot apical meristem. Taken together, our results suggest that AS2 and AS1 might be involved in establishment of a prominent midvein and of networks of other veins as well as in the formation of the symmetric leaf lamina, which might be related to repression of class 1 knox homeobox genes in leaves.

The transposition pattern of the Ac element in tobacco cultured cells

We investigated physical distances and directions of transposition of the maize transposable element Ac in tobacco cultured cells. We introduced a T-DNA construct that carried a non-autonomous derivative of Ac (designated dAc-I-RS) that included sites for cleavage by restriction endonuclease MluI. Another cleavage site was also introduced into the T-DNA region outside of the dAc-I-RS transposable element. The tobacco cultured cell line BY-2 was transformed with the T-DNA and several transformed lines that had a single copy of the T-DNA at a different chromosomal location were isolated. These lines were co-cultured with Agrobacterium tumefaciens cells that carried a cDNA for the Ac transposase gene under the control of various promoters. Sublines of cultured cells in which dAc-I-RS had been transposed, were isolated. The genomic DNAs of these sublines were isolated and digested with MluI. Sizes of DNA segments generated by digestion were determined by pulse-field gel electrophoresis. Our results showed that 20 to 70% of transposition events had occurred within several hundreds kilo-base pairs (kb) on the same chromosome. These results demonstrate that the Ac-Ds element preferentially transposed to regions near the original site in a tobacco chromosome. In addition, the present results are an example of asymmetric transposition as demonstrated by the distance of transposition on the chromosome.

Elimination of selection markers from transgenic plants

Selection markers, which were necessary for the isolation of transgenic plants, are no longer required in mature plants, especially when they are grown in fields. Regimes to achieve their efficient elimination, mostly through site-specific recombination or transposition, are being developed.

The NPK1 mitogen-activated protein kinase kinase kinase is a regulator of cell-plate formation in plant cytokinesis

Mitogen-activated protein kinase (MAPK) cascades play important roles not only in the transduction of extracellular signals but in the progression of the cell cycle. However, evidence for their role in cytokinesis is limited. Here, we show that a tobacco MAPK kinase kinase (MAPKKK), designated NPK1, is required for cytokinesis. The activity of NPK1 increases in the late M phase of the tobacco cell cycle. During expansion of a new cross-wall (cell plate) toward the cell cortex, NPK1 is consistently localized to the equatorial zone of the phragmoplast, the cytokinetic apparatus where the cell plate is formed. Expression of a kinase-negative mutant of NPK1 results in the generation of multinucleate cells with incomplete cell plates. Phragmoplasts can be formed, but its expansion toward the cell cortex is also blocked. Thus, our results indicate that the NPK1 MAPKKK is essential for the formation of the cell plate, especially for its lateral growth.

Genomic exploration of the hemiascomycetous yeasts: 8. Zygosaccharomyces rouxii

This paper reports the genomic analysis of strain CBS732 of Zygosaccharomyces rouxii, a homothallic diploid yeast. We explored the sequences of 4934 random sequencing tags of about 1 kb in size and compared them to the Saccharomyces cerevisiae gene products. Approximately 2250 nuclear genes, 57 tRNAs, the rDNA locus, the endogenous pSR1 plasmid and 15 mitochondrial genes were identified. According to 18S and 25S rRNA cladograms and to synteny analysis, Z. rouxii could be placed among the S. cerevisiae sensu lato yeasts.

A transformation vector for the production of marker-free transgenic plants containing a single copy transgene at high frequency

We represent here the GST-MAT vector system. The R recombinase gene of the site-specific recombination system R/RS from Zygosaccharomyces rouxii was fused to the chemical inducible promoter of the glutathione-S-transferase (GST-II-27) gene from Zea mays. Upon excision, the isopentenyltransferase (ipt) gene that is used as a selectable marker gene is removed. When the cauliflower mosaic virus 35S promoter (CaMV 35S) was used to express R recombinase, 67% of the marker-free transgenic plants had more than three transgene copies. Because the CaMV 35S promoter transiently and efficiently excised the ipt gene before callus and adventitious bud formation, the frequency of emergence of the ipt-shooty explants with a single T-DNA copy might be reduced. In this study we show that the GST-MAT vector efficiently produced transgenic ipt-shooty explants from 37 (88%) out of 42 differentiated adventitious buds and marker-free transgenic plants containing the GUS gene from five (14%) out of 37 ipt-shooty lines. Furthermore, the GST-MAT vector also induced two marker-free transgenic plants without the production of ipt-shooty intermediates. Southern blot analysis showed that six (86%) out of seven marker-free transgenic plants had a single GUS gene. This result suggests that the GST-MAT vector is useful to generate high frequency, marker-free transgenic plants containing a single transgene.

Genetic recombination in plants

Meiotic recombination generates novel allelic arrays on chromosomes. Recent experiments have revealed an extraordinarily nonrandom distribution of recombination breakpoints along the lengths of plant chromosomes; for example, recombination breakpoints often resolve within genic sequences, and thereby generate novel alleles. The mechanism by which recombination breakpoints are determined is an area of active investigation. In addition, recent developments are providing recombination-based technologies for creating targeted alterations in the architecture of plant genomes.

Characterization of the transposition pattern of the Ac element in Arabidopsis thaliana using endonuclease I-SceI

We have investigated physical distances and directions of transposition of the maize transposable element Ac in Arabidopsis thaliana. We prepared a transferred DNA (T-DNA) construct that carried a non-autonomous derivative of Ac with a site for cleavage by endonuclease I-SceI (designated dAc-I-RS element). Another cleavage site was also introduced into the T-DNA region outside dAc-I-RS. Three transgenic Arabidopsis plants were generated, each of which had a single copy of the T-DNA at a different chromosomal location. These transgenic plants were crossed with the Arabidopsis that carried the gene for Ac transposase and progeny in which dAc-I-RS had been transposed were isolated. After digestion of the genomic DNA of these progeny with endonuclease I-SceI, sizes of segment of DNA were determined by pulse-field gel electrophoresis. We also performed linkage analysis for the transposed elements and sites of mutations near the elements. Our results showed that 50% of all transposition events had occurred within 1,700 kb on the same chromosome, with 35% within 200 kb, and that the elements transposed in both directions on the chromosome with roughly equal probability. The data thus indicate that the Ac-Ds system is most useful for tagging of genes that are present within 200 kb of the chromosomal site of Ac in Arabidopsis. In addition, determination of the precise localization of the transposed dAc-I-RS element should definitely assist in map-based cloning of genes around insertion sites.