Intermolecular ligation mediates efficient cotransformation in Phytophthora infestans

Defining Transgene Insertion Sites and Off-Target Effects of Homology-Based Gene Silencing Informs the Application of Functional Genomics Tools in Phytophthora infestans

DNA transformation and homology-based transcriptional silencing are frequently used to assess gene function in Phytophthora spp. Since unplanned side-effects of these tools are not well-characterized, we used P. infestans to study plasmid integration sites and whether knockdowns caused by homology-dependent silencing spread to other genes. Insertions occurred both in gene-dense and gene-sparse regions but disproportionately near the 5' ends of genes, which disrupted native coding sequences. Microhomology at the recombination site between plasmid and chromosome was common. Studies of transformants silenced for 12 different gene targets indicated that neighbors within 500 nt were often cosilenced, regardless of whether hairpin or sense constructs were employed and the direction of transcription of the target. However, this cis spreading of silencing did not occur in all transformants obtained with the same plasmid. Genome-wide studies indicated that unlinked genes with partial complementarity with the silencing-inducing transgene were not usually down-regulated. We learned that hairpin or sense transgenes were not cosilenced with the target in all transformants, which informs how screens for silencing should be performed. We conclude that transformation and gene silencing can be reliable tools for functional genomics in Phytophthora spp. but must be used carefully, especially by testing for the spread of silencing to genes flanking the target.

Illuminating Phytophthora Biology with Fluorescent Protein Tags

Phytophthora species cause diseases that threaten agricultural, ornamental, and forest plants worldwide. Explorations of the biology of these pathogens have been aided by the availability of genome sequences, but much work remains to decipher the roles of their proteins. Insight into protein function can be obtained by visualizing them within cells, which has been facilitated by recent improvements in fluorescent protein and microscope technologies. Here, we describe strategies to permit investigators to generate strains of Phytophthora that express fluorescently tagged proteins and study their localization during growth in artificial media and during plant infection.

Comparisons of Ribosomal Protein Gene Promoters Indicate Superiority of Heterologous Regulatory Sequences for Expressing Transgenes in Phytophthora infestans

Molecular genetics approaches in Phytophthora research can be hampered by the limited number of known constitutive promoters for expressing transgenes and the instability of transgene activity. We have therefore characterized genes encoding the cytoplasmic ribosomal proteins of Phytophthora and studied their suitability for expressing transgenes in P. infestans. Phytophthora spp. encode a standard complement of 79 cytoplasmic ribosomal proteins. Several genes are duplicated, and two appear to be pseudogenes. Half of the genes are expressed at similar levels during all stages of asexual development, and we discovered that the majority share a novel promoter motif named the PhRiboBox. This sequence is enriched in genes associated with transcription, translation, and DNA replication, including tRNA and rRNA biogenesis. Promoters from the three P. infestans genes encoding ribosomal proteins S9, L10, and L23 and their orthologs from P. capsici were tested for their ability to drive transgenes in stable transformants of P. infestans. Five of the six promoters yielded strong expression of a GUS reporter, but the stability of expression was higher using the P. capsici promoters. With the RPS9 and RPL10 promoters of P. infestans, about half of transformants stopped making GUS over two years of culture, while their P. capsici orthologs conferred stable expression. Since cross-talk between native and transgene loci may trigger gene silencing, we encourage the use of heterologous promoters in transformation studies.

Development of a bipartite ecdysone-responsive gene switch for the oomycete Phytophthora infestans and its use to manipulate transcription during axenic culture and plant infection

Conditional expression systems have been proven to be useful tools for the elucidation of gene function in many taxa. Here, we report the development of the first useful inducible promoter system for an oomycete, based on an ecdysone receptor (EcR) and the ecdysone analogue methoxyfenozide. In Phytophthora infestans, the potato late blight pathogen, a monopartite transactivator containing the VP16 activation domain from herpes simplex virus, the GAL4 DNA-binding domain from yeast and the EcR receptor domain from the spruce budworm enabled high levels of expression of a β-glucuronidase (GUS) reporter gene, but unacceptable basal activity in the absence of the methoxyfenozide inducer. Greatly improved performance was obtained using a bipartite system in which transcription is activated by a heterodimer between a chimera of VP16 and the migratory locust retinoid X receptor, and a separate EcR-DNA-binding domain chimera. Transformants were obtained that exhibited >100-fold activation of the reporter by methoxyfenozide, with low basal levels of expression and induced activity approaching that of the strong ham34 promoter. Performance varied between transformants, probably as a result of position effects. The addition of methoxyfenozide enabled strong induction during hyphal growth, zoosporogenesis and colonization of tomato. No significant effects of the inducer or transactivators on growth, development or pathogenicity were observed. The technology should therefore be a useful addition to the arsenal of methods for the study of oomycete plant pathogens.

Myb transcription factors and light regulate sporulation in the oomycete Phytophthora infestans

Life cycle progression in eukaryotic microbes is often influenced by environment. In the oomycete Phytophthora infestans, which causes late blight on potato and tomato, sporangia have been reported to form mostly at night. By growing P. infestans under different light regimes at constant temperature and humidity, we show that light contributes to the natural pattern of sporulation by delaying sporulation until the following dark period. However, illumination does not permanently block sporulation or strongly affect the total number of sporangia that ultimately form. Based on measurements of sporulation-induced genes such as those encoding protein kinase Pks1 and Myb transcription factors Myb2R1 and Myb2R3, it appears that most spore-associated transcripts start to rise four to eight hours before sporangia appear. Their mRNA levels oscillate with the light/dark cycle and increase with the amount of sporangia. An exception to this pattern of expression is Myb2R4, which is induced several hours before the other genes and declines after cultures start to sporulate. Transformants over-expressing Myb2R4 produce twice the number of sporangia and ten-fold higher levels of Myb2R1 mRNA than wild-type, and chromatin immunoprecipitation showed that Myb2R4 binds the Myb2R1 promoter in vivo. Myb2R4 thus appears to be an early regulator of sporulation. We attempted to silence eight Myb genes by DNA-directed RNAi, but succeeded only with Myb2R3, which resulted in suppressed sporulation. Ectopic expression studies of seven Myb genes revealed that over-expression frequently impaired vegetative growth, and in the case of Myb3R6 interfered with sporangia dormancy. We observed that the degree of silencing induced by a hairpin construct was correlated with its copy number, and ectopic expression was often unstable due to epigenetic silencing and transgene excision.

New role for Cdc14 phosphatase: localization to basal bodies in the oomycete phytophthora and its evolutionary coinheritance with eukaryotic flagella

Cdc14 protein phosphatases are well known for regulating the eukaryotic cell cycle, particularly during mitosis. Here we reveal a distinctly new role for Cdc14 based on studies of the microbial eukaryote Phytophthora infestans, the Irish potato famine agent. While Cdc14 is transcribed constitutively in yeast and animal cells, the P. infestans ortholog is expressed exclusively in spore stages of the life cycle and not in vegetative hyphae where the bulk of mitosis takes place. PiCdc14 expression is first detected in nuclei at sporulation, and during zoospore formation the protein accumulates at the basal body, which is the site from which flagella develop. The association of PiCdc14 with basal bodies was supported by co-localization studies with the DIP13 basal body protein and flagellar β-tubulin, and by demonstrating the enrichment of PiCdc14 in purified flagella-basal body complexes. Overexpressing PiCdc14 did not cause defects in growth or mitosis in hyphae, but interfered with cytoplasmic partitioning during zoosporogenesis. This cytokinetic defect might relate to its ability to bind microtubules, which was shown using an in vitro cosedimentation assay. The use of gene silencing to reveal the precise function of PiCdc14 in flagella is not possible since we showed previously that silencing prevents the formation of the precursor stage, sporangia. Nevertheless, the association of Cdc14 with flagella and basal bodies is consistent with their phylogenetic distribution in eukaryotes, as species that lack the ability to produce flagella generally also lack Cdc14. An ancestral role of Cdc14 in the flagellar stage of eukaryotes is thereby proposed.

A motif within a complex promoter from the oomycete Phytophthora infestans determines transcription during an intermediate stage of sporulation

Sporulation in Phytophthora infestans is associated with a major remodeling of the transcriptome. To better understand promoter structure and how sporulation-specific expression is determined in this organism, the Pks1 gene was analyzed. Pks1 encodes a protein kinase that is induced at an intermediate stage of sporulation, prior to sporangium maturation. Major and minor transcription start sites mapped throughout the promoter, which contains many T-rich stretches and Inr-like elements. Within the T-rich region are several motifs which bound nuclear proteins in EMSA. Tests of modified promoters in transformants implicated a CCGTTG located 110-nt upstream of the transcription start point as a major regulator of sporulation-specific transcription. The motif also bound a sporulation-specific nuclear protein complex. A bioinformatics analysis indicated that the motif is highly over-represented within co-expressed promoters, in which it predominantly resides 100-300-nt upstream of transcription start sites. Other sequences, such as a CATTTGTT motif, also bound nuclear proteins but did not play an essential role in spore-specific expression.

Exploration of the late stages of the tomato-Phytophthora parasitica interactions through histological analysis and generation of expressed sequence tags

The oomycete Phytophthora parasitica is a soilborne pathogen infecting numerous plants. The infection process includes an initial biotrophic stage, followed by a necrotrophic stage. The aim here was to identify genes that are involved in the late stages of infection. Using the host tomato and a transformed strain of P. parasitica expressing the green fluorescent protein (GFP), the various infection steps from recognition of the host to the colonization of plant tissues were studied. This late stage was selected to generate 4000 ESTs (expressed sequence tags), among which approx. 80% were from the pathogen. Comparison with an EST data set created previously from in vitro growth of P. parasitica allowed the identification of several genes, the expression of which might be regulated during late stages of infection. Changes in gene expression of several candidate genes predicted from in silico analysis were validated by quantitative RT-PCR experiments. These results give insights into the molecular bases of the necrotrophic stage of an oomycete pathogen.

Performance of a tetracycline-responsive transactivator system for regulating transgenes in the oomycete Phytophthora infestans

The oomycete genus Phytophthora includes many important plant pathogens for which extensive genome data exist, but lacking is an inducible expression system to study contributions of their genes to growth and pathogenicity. Here the adaptation of the reverse tetracycline transactivator (rtTA) system to P. infestans is described. Vectors were developed containing rtTA expressed from an oomycete promoter, and beta-glucuronidase (GUS) controlled by TetR binding sites fused to a minimal oomycete promoter. Transformants were obtained in which GUS was expressed in a dose-dependent manner by the rtTA inducer doxycycline, indicating that the gene switch functions in P. infestans. However, toxicity of rtTA hindered the isolation of transformants if expressed on the same plasmid as the nptII selection marker. Better results were obtained by cotransforming those genes on separate plasmids, with 92% of transformants acquiring both DNAs although only 4% expressed rtTA at detectable levels. Low levels of reporter activity were measured in such transformants, suggesting that rtTA activated transcription weakly. Also, significant variation in the sensitivity of isolates to doxycycline and tetracycline was observed. These results are useful both in terms of developing tools for functional genomics and understanding the fate of DNA during Phytophthora transformation.

Genomics of the plant pathogenic oomycete Phytophthora: insights into biology and evolution

The genus Phytophthora includes many destructive pathogens of plants. Although having "fungus-like" appearances, Phytophthora species reside in a eukaryotic kingdom separate from that of true fungi. Distinct strategies are therefore required to study and defend against Phytophthora. Large sequence databases have recently been developed for several species, and tools for functional genomics have been enhanced. This chapter will review current progress in understanding the genome and transcriptome of Phytophthora, and provide examples of how genomics resources are advancing molecular studies of pathogenesis, development, transcription, and evolution. A better understanding of these remarkable pathogens should lead to new approaches for managing their diseases.

Quantification of induced resistance against Phytophthora species expressing GFP as a vital marker: beta-aminobutyric acid but not BTH protects potato and Arabidopsis from infection

SUMMARY Induced resistance was studied in the model pathosystem Arabidopsis-Phytophthora brassicae (formerly P. porri) in comparison with the agronomically important late blight disease of potato caused by Phytophthora infestans. For the quantification of disease progress, both Phytophthora species were transformed with the vector p34GFN carrying the selectable marker gene neomycine phosphotransferase (nptII) and the reporter gene green fluorescent protein (gfp). Eighty five per cent of the transformants of P. brassicae and P. infestans constitutively expressed GFP at high levels at all developmental stages both in vitro and in planta. Transformants with high GFP expression and normal in vitro growth and virulence were selected to quantify pathogen growth by measuring the in planta emitted GFP fluorescence. This non-destructive monitoring of the infection process was applied to analyse the efficacy of two chemical inducers of disease resistance, a functional SA-analogue, benzothiadiazole (BTH), and beta-aminobutyric acid (BABA) which is involved in priming mechanisms of unknown nature. BABA pre-treatment (300 microm) via soil drench applied 24 h before inoculation completely protected the susceptible Arabidopsis accession Landsberg erecta (Ler) from infection with P. brassicae. A similar treatment with BTH (330 microm) did not induce resistance. Spraying the susceptible potato cultivar Bintje with BABA (1 mm) 2 days before inoculation resulted in a phenocopy of the incompatible interaction shown by the resistant potato cultivar Matilda while BTH (1.5 mm) did not protect Bintje from severe infection. Thus, in both pathosystems, the mechanisms of induced resistance appeared to be similar, suggesting that the Arabidopsis-P. brassicae pathosystem is a promising model for the molecular analysis of induced resistance mechanisms of potato against the late blight disease.

A gene expressed during sexual and asexual sporulation in Phytophthora infestans is a member of the Puf family of translational regulators

A gene from Phytophthora infestans that was previously identified as being induced during the development of sexual spores was also found to be active during asexual sporulation. The gene, M90, was expressed as a 3.1-kb primary transcript containing two introns and was predicted to encode a member of the Puf family of translational regulators. The protein showed up to 51% amino acid identity to other Puf proteins within its 353-amino-acid RNA-binding domain. Little similarity extended beyond this region, as noted for other members of the family. Expression of M90 was measured by using RNA blots and transformants of P. infestans expressing a fusion between the M90 promoter and the beta-glucuronidase (GUS) gene. A 1.3-kb promoter fragment conferred the normal M90 pattern of expression to the GUS reporter in transformants. In matings, expression was first detected in male and female gametangial initials and persisted in mature oospores. Expression was also observed in hyphal tips just prior to asexual sporulation, in sporangiophores, in mature sporangia, and in zoospores. The signal quickly disappeared once spores made the transition to hyphae after germination. Nutrient limitation did not induce the gene. Potential roles for a translational regulator during both sexual development and asexual sporulation are discussed.

Novel protein kinase induced during sporangial cleavage in the oomycete Phytophthora infestans

A study of the effect of inhibitors on zoospore development in Phytophthora infestans demonstrated the involvement of protein kinases and calcium and led to the discovery of a gene induced during zoosporogenesis that encoded a protein resembling Ca2+- and calmodulin-regulated serine/threonine protein kinases. The calcium channel blocker verapamil and the calmodulin antagonist trifluoroperazine inhibited zoosporogenesis and encystment. The protein kinase inhibitors K-252a and KN-93 inhibited zoospore release, encystment, and cyst germination, and K-252a reduced zoospore viability. In contrast, the inhibitors had minor or no effects on sporangia directly germinating in media. Spurred by these findings, a survey of putative protein kinase genes was performed to identify any that were up-regulated during zoosporogenesis. A kinase-encoding gene was identified for which mRNA accumulation was first detected soon after chilling sporangia in water, conditions that induce sporangial cytoplasm to cleave and release zoospores. The transcript persisted in motile zoospores and in germinated cysts but was not detected in other tissues, including hyphae, hyphae placed in water, or directly germinating sporangia. The structure of the predicted protein was novel, as its C-terminal region, which binds calmodulin in related proteins, was unusually short. Concentrations of actinomycin D previously used in experiments that suggested that de novo transcription was not needed for zoosporogenesis or encystment only partially inhibited transcription of the kinase gene, probably due to poor uptake into sporangia.

Phytophthora infestans enters the genomics era

summary Phytophthora infestans, cause of late-blight, is the most devastating disease of potato world-wide. Recent years have seen a dramatic intensification in molecular biological studies of P. infestans, including the development of novel tools for transformation and gene silencing and the resources for genetical, transcriptional and physical mapping of the genome. This review will focus on the increasing efforts to use these resources to discover the genetic bases of pathogenicity, avirulence and host-specificity.

TAXONOMY

Phytophthora infestans (Mont.) de Bary-Kingdom Chromista, Phylum Oomycota, Order Peronosporales, Family Peronosporaceae, Genus Phytophthora, of which it is the type species.

HOST RANGE

Infects a wide range of solanaceous species. Economically important hosts are potato, tomato, eggplant and some other South American hosts (tree tomato and pear melon) on which it causes late blight. Disease symptoms: Infected foliage is initially yellow, becomes water soaked and eventually blackens. Leaf symptoms comprise purple-black or brown-black lesions at the leaf tip, later spreading across the leaf to the stem. Whitish masses of sporangia develop on the underside of the leaf. Tubers become infected later in the season and, in the early stages, consist of slightly brown or purple blotches on the skin. In damp soils the tuber decays rapidly before harvest. Tuber infection is quickly followed by secondary fungal or bacterial infection known as 'wet rot'. Useful web sites:http://www.ncgr.org/pgc/; http://www.oardc.ohio-state.edu/phytophthora/.

Cellular and molecular mechanisms of sexual incompatibility in plants and fungi

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

Green fluorescent protein (GFP) as gene expression reporter and vital marker for studying development and microbe-plant interaction in the tobacco pathogen Phythphthora parasitica var. nicotianae

PEG-mediated transformation of protoplasts in the presence of lipofectin was achieved in Phytophthora parasitica var. nicotianae, an oomycete pathogen of tobacco. Using oomycete promoter and terminator sequences, a plant-adapted green fluorescent protein (GFP) was introduced into the microorganism. The data show for the first time that this eukaryotic gene reporter can be used in an oomycete, both as a quantitative reporter of gene induction and as a vital marker allowing the study of development of Phytophthora in vitro and in the host plant.

Heterologous expression of a basic elicitin from Phytophthora cryptogea in Phytophthora infestans increases its ability to cause leaf necrosis in tobacco

The cry-b sequence, encoding a basic elicitin (cryptogein B) from Phytophthora cryptogea, was co-transformed into Phytophthora infestans. The copy number of the cry-b sequence varied in co-transformants. Nevertheless, in all cases the alien elicitin gene was transcribed, translated and the protein secreted in vitro from such transformants. Moreover, the secreted cryptogein B from P. infestans co-transformants increased their ability to cause a hypersensitive-response-like necrosis of tobacco leaves. It was thus concluded that the transfer of a single gene encoding a basic elicitin from one Phytophthora species to another can dramatically alter the phenotypic interaction of the transformed species with tobacco.

The ipiO gene of Phytophthora infestans is highly expressed in invading hyphae during infection

The expression of the in planta-induced gene ipiO of the potato late blight pathogen Phytophthora infestans was analyzed during various developmental stages of its life cycle. ipiO mRNA was detected in zoospores, cysts, germinating cysts, and young mycelia, but not in sporangia or in old mycelia grown in vitro. ipiO is not only expressed in stages prior to infection but also during colonization of potato and tomato leaves. In disease lesions, ipiO mRNA was detected in the water-soaked area and the healthy-looking plant tissue surrounding it. In contrast, ipiO mRNA was not found in necrotized tissue or in sporulating areas of a lesion. To determine more precisely the location and time of ipiO gene expression in planta, cytological assays were performed using a P. infestans transformant expressing a transcriptional fusion between the ipiO1 promoter and the beta-glucuronidase (GUS) reporter gene. GUS staining was found specifically in the subapical and vacuolated area of tips of invading hyphae. The histochemical GUS assays demonstrate that ipiO is expressed during biotrophic stages of the disease cycle.

The genetics and biology of Phytophthora infestans: modern approaches to a historical challenge

The oomyceteous fungus Phytophthora infestans, which causes the late blight diseases of potato and tomato, has a history that is closely associated with that of mycology and plant pathology. Nevertheless, P. infestans and other oomycetes remain poorly understood relative to fungi in other groups. A resurgence in the worldwide impact of late blight has recently increased interest in the species. Fortunately, over the past decade improved tools for laboratory analysis have been developed which provide an opportunity to advance our understanding of this important pathogen. Since oomycetes do not have a close taxonomic affinity with well-characterized organisms such as ascomycetes and basidiomycetes, it is likely that studies of P. infestans will yield novel biological findings. This review provides an update on the status of research into the fundamental aspects of the biology, genetics, and pathology of P. infestans and describes prospects for future advances.

Chromosomal heteromorphism linked to the mating type locus of the oomycete Phytophthora infestans

The mating type locus of the oomycete, Phytophthora infestans, is embedded in a region of DNA that displays distorted and non-Mendelian segregation. By using DNA probes linked to the mating type locus to genetically and physically characterize that region, a large zone of chromosomal heteromorphism was detected. Locus S1 was shown to represent a tandemly repeated array of DNA that was typically present in a hemizygous state in A1 isolates while being absent from A2 isolates. The analysis of the parents and progeny of seven crosses indicated that the tandem array was linked in cis to the A1-determining allele of the mating type locus. A worldwide survey of genotypically diverse field isolates of P. infestans indicated that S1 was present in each of 48 isolates of the A1 mating type that were tested, but was absent in 46 of 47 A2 strains. Physical analysis of S1 indicated that the tandemly repeated DNA sequence spanned about 300 kb and had evolved from a 1.35-kb monomer. Internal deletions occurred within S1 during sexual propagation. This and other mutations apparently contributed to a high degree of polymorphism within the S1 array.

An autonomously replicating plasmid transforms Botrytis cinerea to phleomycin resistance

A transformation system has been developed for the pathogen fungus Botrytis cinerea, based on the utilization of the wide host plasmid pUT737 that contains the Sh ble gene, conferring resistance to phleomycin. Transformed protoplasts were regenerated at 10-25 micrograms ml(-1) of phleomycin, at a frequency of 25-40 transformants per microgram of DNA, and they were resistant up to 50 micrograms ml(-1). Southern hybridization using undigested and digested total DNA showed the presence of circular autonomously replicating plasmid pUT737 in the transformants. Reisolated plasmid from transformed fungus transformed E. coli and rescued plasmid was identified as PUT737. Transformants were grown for four generations under non-selective conditions and replicative plasmids were still detected. Plasmids present in all transformants at this stage had been modified from native pUT737 and showed the same size and configuration indicating that selection through stabilizing plasmid forms has happened.

Transformation of the cultivated mushroom, Agaricus bisporus, to hygromycin B resistance

Application of biotechnology to the cultivated mushroom, Agaricus bisporus, has been hampered thus far by the lack of a transformation system. Here, transformation of both a homo- and a heterokaryotic strain of A. bisporus to hygromycin B resistance is described. Transforming DNA was integrated into the A. bisporus genome and stably maintained throughout vegetative growth. Transformants of the heterokaryotic strain formed transgenic fruiting bodies. Promoters derived from the unrelated ascomycete Aspergillus nidulans and from A. bisporus itself, were able to drive expression of the hygromycin B resistance gene. Expression controlled by a fragment of 265 bp from the A. bisporus GPD promoter was sufficient to generate transformants. However, transformation efficiency was not enhanced by using this homologous promoter.

Inactivation of transgenes in Phytophthora infestans is not associated with their deletion, methylation, or mutation

The mitotic and meiotic stabilities of transgenes were evaluated in the oomycete, Phytophthora infestans. Genes encoding beta-glucuronidase (GUS), neomycin phosphotransferase (NPT) and hygromycin phosphotransferase (HPT), fused to one of six promoters from P. infestans or other oomycetes, were usually stably expressed during continued asexual culture and transmitted to progeny. However, the activity of these genes became undetectable in many strains during asexual or sexual propagation. Over 33 months of growth, transgene expression stopped each month in 1-3% of the transformants. Silencing of the genes was not associated with their deletion, mutation, or hypermethylation. The conformation of the integrated sequences was similar in strains destined to continue or terminate expression of the transgenes. Expression of the genes was not associated with a loss of fitness during growth in vitro and in planta, which might otherwise have selected for silencing events.

Cointegration of transforming DNAs in Aspergillus nidulans: a model using autonomously-replicating plasmids

Transforming DNAs form cointegrates in Aspergillus nidulans by homologous and non-homologous recombination as well as by end-to-end ligation of linear fragments. This process has been studied by means of a model in which the linkage of a marker gene to the origin of autonomous replication AMA1 was selected for. Recombinant plasmids were rescued into Escherichia coli and subjected to restriction mapping and sequence analysis. It was shown that circular DNA molecules recombined predominantly within homologous fragments. Linear DNA fragments integrated into circular plasmids by invasion of their ends into random non-homologous sites, but exhibited some bias in choice of a target sequence. Cointegrates of multiple plasmid copies were often observed. In some of the plasmids analysed, short duplications of the target sequence flanking an inserted linear DNA fragment have been revealed.