Activity of the yeast FLP recombinase in Arabidopsis

FLP recombinase-mediated site-specific recombination in silkworm, Bombyx mori

A comprehensive understanding of gene function and the production of site-specific genetically modified mutants are two major goals of genetic engineering in the post-genomic era. Although site-specific recombination systems have been powerful tools for genome manipulation of many organisms, they have not yet been established for use in the manipulation of the silkworm Bombyx mori genome. In this study, we achieved site-specific excision of a target gene at predefined chromosomal sites in the silkworm using a FLP/FRT site-specific recombination system. We first constructed two stable transgenic target silkworm strains that both contain a single copy of the transgene construct comprising a target gene expression cassette flanked by FRT sites. Using pre-blastoderm microinjection of a FLP recombinase helper expression vector, 32 G3 site-specific recombinant transgenic individuals were isolated from five of 143 broods. The average frequency of FLP recombinase-mediated site-specific excision in the two target strains genome was approximately 3.5%. This study shows that it is feasible to achieve site-specific recombination in silkworms using the FLP/FRT system. We conclude that the FLP/FRT system is a useful tool for genome manipulation in the silkworm. Furthermore, this is the first reported use of the FLP/FRT system for the genetic manipulation of a lepidopteran genome and thus provides a useful reference for the establishment of genome manipulation technologies in other lepidopteran species.

A Cre::FLP fusion protein recombines FRT or loxP sites in transgenic maize plants

The coding sequences of Cre (site-specific recombinase from bacteriophage P1) and FLP (yeast 2-microm plasmid site-specific recombinase) were fused in frame to produce a novel, dual-function, site-specific recombinase gene. Transgenic maize plants containing the Cre::FLP fusion expression vector were crossed to transgenic plants containing either the loxP or FRT excision substrate. Complete and precise excisions of chromosomal fragments flanked by the respective target sites were observed in the F1 and F2 progeny plants. The episomal DNA recombination products were frequently lost. Non-recombined FRT substrates found in the F1 plants were recovered in the F2 generation after the Cre::FLP gene segregated out. They produced the recombination products in the F3 generation when crossed back to the FLP-expressing plants. These observations may indicate that the efficiency of site-specific recombination is affected by the plant developmental stage, with site-specific recombination being more prevalent in developing embryos. The Cre::FLP fusion protein was also tested for excisions catalysed by Cre. Excisions were identified in the F1 plants and verified in the F2 plants by polymerase chain reaction and Southern blotting. Both components of the fusion protein (FLP and Cre) were functional and acted with similar efficiency. The crossing strategy proved to be suitable for the genetic engineering of maize using the FLP or Cre site-specific recombination system.

FLP recombinase-mediated site-specific recombination in rice

The feasibility of using the FLP/FRT site-specific recombination system in rice for genome engineering was evaluated. Transgenic rice plants expressing the FLP recombinase were crossed with plants harbouring the kanamycin resistance gene (neomycin phosphotransferase II, nptII) flanked by FRT sites, which also served to separate the corn ubiquitin promoter from a promoterless gusA. Hybrid progeny were tested for excision of the nptII gene and the positioning of the ubiquitin promoter proximal to gusA. While the hybrid progeny from various crosses exhibited beta-glucuronidase (GUS) expression, the progeny of selfed parental rice plants did not show detectable GUS activity. Despite the variable GUS expression and incomplete recombination displayed in hybrids from some crosses, uniform GUS staining and complete recombination were observed in hybrids from other crosses. The recombined locus was shown to be stably inherited by the progeny. These data demonstrate the operation of FLP recombinase in catalysing excisional DNA recombination in rice, and confirm that the FLP/FRT recombination system functions effectively in the cereal crop rice. Transgenic rice lines expressing active FLP recombinase generated in this study provide foundational stock material, thus facilitating the future application and development of the FLP/FRT system in rice genetic improvement.

FLP-mediated site-specific recombination for genome modification in turfgrass

To develop molecular strategies for gene containment in genetically modified (GM) turfgrass, we have studied the feasibility of using the FLP/FRT site-specific DNA recombination system from yeast for controlled genome modification in turfgrass. Suspension cell cultures of creeping bentgrass (Agrostis stolonifera L.) and Kentucky bluegrass (Poa pratensis) were co-transformed with a FLP recombinase expression vector and a recombination-reporter test plasmid containing beta-glucuronidase (gusA) gene which was separated from the maize ubiquitin (ubi) promoter by an FRT-flanked blocking DNA sequence to prevent its transcription. GUS activity was observed in co-transformed cells, in which molecular analyses indicated that FLP-mediated excision of the blocking sequence had brought into proximity the upstream promoter and the downstream reporter gene, resulting in GUS expression. Functional evaluation of the FLP/FRT system using transgenic creeping bentgrass stably expressing FLP recombinase confirmed the observation in suspension cell culture. Our results indicate that FLP/FRT system is a useful tool for genetic manipulation of turfgrass, pointing to the great potential of exploiting the system to develop molecular strategies for transgene containment in perennials.

Site-specific recombination in Zea mays

The elimination of marker genes after selection is recommended for the commercial use of genetically modified plants. We compared the applicability of the two site-specific recombination systems Cre/lox and Flp/FRT for marker gene elimination in maize plants. The selection marker gene pat surrounded by two identically directed lox or FRT sites was introduced into maize. Sexual crossing with plants harboring the corresponding constitutively expressed recombinase led to the precise and complete excision of the lox-flanked marker gene in the F1 progeny, whereas Flp-mediated recombination of FRT sequences occurred rarely. Further examination of site-specific integration was done by biolistic bombardment of immature embryos harboring only one lox site with a lox.uidA sequence with results indicating directed integration.

Utility of the FLP-FRT recombination system for genetic manipulation of rice

To develop an FLP-FRT recombination system- (derived from 2 mu plasmid of Saccharomyces cerevisiae) based marker gene removal application for rice, we introduced the gene for FLP recombinase, under the control of the maize ubiquitin-1 promoter, into the rice genome. FLP activity was monitored in callus and regenerated plants by an assay based on the deletion of the FRT-flanked DNA fragment, leading to the activation of the beta-glucuronidase gene. FLP activity was detected both in the callus and leaves of some of the transgenic lines. Based on our comparison of the recombination efficiency of the FLP-FRT system expressed in the transgenic lines with that of the widely used Cre-lox system (derived from bacteriophage P1), we suggest that the FLP-FRT system is a useful tool for the genetic manipulation of rice.

Cre/lox-mediated marker gene excision in transgenic maize (Zea mays L.) plants

After the initial transformation and tissue culture process is complete, selectable marker genes, which are used in virtually all transformation approaches, are not required for the expression of the gene of interest in the transgenic plants. There are several advantages to removing the selectable marker gene after it is no longer needed, such as enabling the reuse of selectable markers and simplifying transgene arrays. We have tested the Cre/ lox system from bacteriophage P1 for its ability to precisely excise stably integrated marker genes from chromosomes in transgenic maize plants. Two strategies, crossing and autoexcision, have been tested and demonstrated. In the crossing strategy, plants expressing the Cre recombinase are crossed with plants bearing a transgene construct in which the selectable marker gene is flanked by directly repeated lox sites. Unlike previous reports in which incomplete somatic and germline excision were common, in our experiments complete somatic and germline marker gene excision occurred in the F(1) plants from most crosses with multiple independent Cre and lox lines. In the autoexcision strategy, the cre gene, under the control of a heat shock-inducible promoter, is excised along with the nptII marker gene. Our results show that a transient heat shock treatment of primary transgenic callus is sufficient for inducing cre and excising the cre and nptII genes. Genetic segregation and molecular analysis confirmed that marker gene removal is precise, complete and stable. The autoexcision strategy provides a way of removing the selectable marker gene from callus or other tissues such as embryos and kernels.

FLP-mediated recombination for use in hybrid plant production

We have studied the feasibility in Arabidopsis of using a site-specific recombination system FLP/FRT, from the 2 microm plasmid of yeast, for making plant hybrids. Initially, Arabidopsis plants expressing the FLP site-specific recombinase were crossed with plants transformed with a vector containing kanamycin-resistance gene (npt) flanked by FRT sites, which also served to separate the CaMV35S promoter from a promoterless gusA. Hybrid progeny were tested for excision of the npt gene and the positioning of 35S promoter proximal to gusA. GUS activity was observed in the progeny of all crosses, but not in the progeny derived from the self-pollinated homozygous parents. We then induced male sterility in Arabidopsis plants using the antisense expression of a pollen- and tapetum-specific gene, bcp1, flanked by FRT sites. Upon cross-pollination of flowers on the same male-sterile plants with pollen from FLP-containing plants, viable seeds were produced and the progeny hybrid plants developed normally. Molecular analyses revealed that the antisense expression cassette of bcp1 had been excised in these plants. These results show for the first time that a site-specific recombinase can be used to restore fertility in male-sterile plants, providing an alternative method for the production of hybrid seeds and plants.

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.

Modification of plant components

With respect to plant biotechnology, 1995 and 1996 will be marked by the commercialization of the first genetically engineered plant oil and a number of ground-breaking publications. The modification of plant components using transgenic technology is not just 'switching' phenotypes from one host to another, rather, it is a means for producing valuable novel products that are normally not found (or are difficult to find) in plants. Active research is being carried out with similar schemes in both academic laboratories and biotechnology companies. As a result, the traditional line that separates the 'basic' research of universities and the 'practical' work of industry is becoming fuzzy. Although many roadblocks remain, judging from the progress made in the past two years, the genetic engineering of plant components is heading towards a bright and exciting future.

FLP-mediated recombination of FRT sites in the maize genome

Molecular evidence is provided for genomic recombinations in maize cells induced by the yeast FLP/FRT site-specific recombination system. The FLP protein recombined FRT sites previously integrated into the maize genome leading to excision of a selectable marker, the neo gene. NPTII activity was not observed after the successful recombination process; instead, the gusA gene was activated by the removal of the blocking DNA fragment. Genomic sequencing in the region of the FRT site (following the recombination reaction) indicated that a precise rearrangement of genomic DNA sequences had taken place. The functional FLP gene could be either expressed transiently or after stable integration into the maize genome. The efficiency of genomic recombinations was high enough that a selection for recombination products, or for FLP expression, was not required. The results presented here establish the FLP/FRT site-specific recombination system as an important tool for controlled modifications of maize genomic DNA.