Transformation of Penicillium roqueforti to phleomycin- and to hygromycin B-resistance

Geneticin (G418) resistance and electroporation-mediated transformation of Fusarium graminearum and F. culmorum

Fusarium graminearum and F. culmorum are phytopathogenic species causing scab and root rot diseases in all small grain cereals worldwide including Turkey. In this study, resistance levels to geneticin (G418) of 14 F. graminearum and 24 F. culmorum isolates collected from cereals were determined. Fungal cultures were grown on potato dextrose agar medium supplemented with 0, 25, 50, 75 and 100 µg/mL of G418. Minimum inhibitory concentration was determined as 25 µg/mL. As a result, it was concluded that all isolates were highly sensitive to G418. Plasmid pFA6-kanmx4 containing geneticin resistance gene (kanmx) was introduced singly or co-electroporated with pEGFP75 plasmid, containing GFP gene, into fungal protoplast cultures obtained with lytic enzyme. Transformants were grown in media including 25 µg/mL G418. Transformation frequencies were 2.8 and 1.8 transformant per µg plasmid for F. graminearum and F. culmorum isolates, respectively. Transformation process was also confirmed by spectrofluorimetric assay. Relative fluorescence unit values in co-transformants were calculated as 1.87 ± 0.04 for F. graminearum and 2.26 ± 0.08 for F. culmorum. The results obtained from the study gave information about antibiotic resistance levels of two Fusarium species in Turkey. Moreover, it was shown that pFA6-kanmx4 plasmid was a suitable vector, which can be used in genetic manipulation studies of these two fungal species in particular suppression of endogenous and/or the expression of exogenous genes.

Gene replacement in Penicillium roqueforti

Most cheese-making filamentous fungi lack suitable molecular tools to improve their biotechnology potential. Penicillium roqueforti, a species of high industrial importance, would benefit from functional data yielded by molecular genetic approaches. This work provides the first example of gene replacement by homologous recombination in P. roqueforti, demonstrating that knockout experiments can be performed in this fungus. To do so, we improved the existing transformation method to integrate transgenes into P. roqueforti genome. In the meantime, we cloned the PrNiaD gene, which encodes a NADPH-dependent nitrate reductase that reduces nitrate to nitrite. Then, we performed a deletion of the PrNiaD gene from P. roqueforti strain AGO. The ΔPrNiaD mutant strain is more resistant to chlorate-containing medium than the wild-type strain, but did not grow on nitrate-containing medium. Because genomic data are now available, we believe that generating selective deletions of candidate genes will be a key step to open the way for a comprehensive exploration of gene function in P. roqueforti.

Horizontal transfer of supernumerary chromosomes in fungi

Several species of filamentous fungi contain so-called dispensable or supernumerary chromosomes. These chromosomes are dispensable for the fungus to survive, but may carry genes required for specialized functions, such as infection of a host plant. It has been shown that at least some dispensable chromosomes are able to transfer horizontally (i.e., in the absence of a sexual cycle) from one fungal strain to another. In this paper, we describe a method by which this can be shown. Horizontal chromosome transfer (HCT) occurs during co-incubation of two strains. To document the actual occurrence of HCT, it is necessary to select for HCT progeny. This is accomplished by transforming two different drug-resistance genes into the two parent strains before their co-incubation. In one of the strains (the "donor"), a drug-resistance gene should be integrated in a chromosome of which the propensity for HCT is under investigation. In the "tester" or "recipient" strain, another drug-resistance gene should be integrated somewhere in the core genome. In this way, after co-incubation, HCT progeny can be selected on plates containing both drugs. HCT can be initiated with equal amounts of asexual spores of both strains, plated on regular growth medium for the particular fungus, followed by incubation until new asexual spores are formed. The new asexual spores are then harvested and plated on plates containing both drugs. Double drug-resistant colonies that appear should carry at least one chromosome from each parental strain. Finally, double drug-resistant strains need to be analysed to assess whether HCT has actually occurred. This can be done by various genome mapping methods, like CHEF-gels, AFLP, RFLP, PCR markers, optical maps, or even complete genome sequencing.

Rock phosphate solubilization and colonization of maize rhizosphere by wild and genetically modified strains of Penicillium rugulosum

Maize root colonization and phosphate solubilizing activity of the fungus Penicillium rugulosum were assessed in a greenhouse trial using soil-plant microcosms. The bacterial gene hph conferring resistance to hygromicin B was introduced by electroporation in the wild-type strain IR-94MF1 of P. rugulosum and one transformant, w-T3, was selected. Maize plants were grown for 5 weeks in a P-poor soil and fertilized with a Florida apatite mineral, with Navay, an apatite rock deposit from Venezuela, or with simple superphosphate. Inoculation treatments included strain IR-94MF1, transformant w-T3 and two IR-94MF1 UV-induced mutants with enhanced (Mps++) or reduced (Mps-) in vitro mineral phosphate solubilizing activity. In the absence of P fertilization, inoculation with any P. rugulosum isolate significantly reduced the size of the total and P-solubilizing bacterial community present in maize rhizosphere. The bacterial community significantly increased in maize inoculated with IR-94MF1 and w-T3 when P was added as apatites Navay or Florida. All P. rugulosum strains were able to stimulate the growth of maize plants as indicated by 3.6 to 28.6% increases in dry matter yields. In the presence of rock phosphate, P uptake by maize plants inoculated with the two mutants Mps++ and Mps- was not always in agreement with their P-solubilizing phenotypes. Strain IR-94MF1 and transformant w-T3 increased P assimilation by the plants fertilized with Navay rock phosphate by 26 and 38%, respectively. In this treatment, w-T3 showed its highest significant maize rhizosphere colonization. With the simple superphosphate treatment, w-T3 increased P uptake in plants by 8% over the uninoculated control and also decreased significantly the community size of total bacteria, total fungi, and P-solubilizing fungi in the rhizosphere.

AAL-Toxin-Deficient Mutants of Alternaria alternata Tomato Pathotype by Restriction Enzyme-Mediated Integration

ABSTRACT Host-specific toxins are produced by three pathotypes of Alternaria alternata: AM-toxin, which affects apple; AK-toxin, which affects Japanese pear; and AAL-toxin, which affects tomato. Each toxin has a role in pathogenesis. To facilitate molecular genetic analysis of toxin production, isolation of toxin-deficient mutants utilizing ectopic integration of plasmid DNA has been attempted. However, the transformation frequency was low, and integration events in most transformants were complicated. Addition of a restriction enzyme during transformation has been reported to increase transformation frequencies significantly and results in simple plasmid integration events. We have, therefore, optimized this technique, known as restriction enzyme-mediated integration (REMI), for A. alternata pathotypes. Plasmid pAN7-1, conferring resistance to hygromycin B, with no detectable homology to the fungal genome was used as the transforming DNA. Among the three restriction enzymes examined, HindIII was most effective, as it increased transformation frequency two-to 10-fold depending on the pathotype, facilitating generation of several hundred transformants with a 1-day protocol. BamHI and XbaI had no significant effect on transformation frequencies in A. alternata pathotypes. Furthermore, the transforming plasmid tended to integrate as a single copy at single sites in the genome, compared with trials without addition of enzyme. Libraries of plasmid-tagged transformants obtained with and without addition of restriction enzyme were constructed for the tomato pathotype of A. alternata and were screened for toxin production. Three AAL-toxin-deficient mutants were isolated from a library of transformants obtained with addition of enzyme. These mutants did not cause symptoms on susceptible tomato, indicating that the toxin is required for pathogenicity of the fungus. Characterization of the plasmid integration sites and rescue of flanking sequences are in progress.

Transformation of Penicillium freii and a rapid PCR screening procedure for cotransformation events

A strain of Penicillium freii was transformed to hygromycin B resistance using the pAN7-1 plasmid. The transformation frequency was approximately 100 transformants per microgram DNA per 1 x 10(7) protoplasts. Southern blotting revealed different types of integration into the genome of P.freii. When non-selectable plasmid DNA was added together with pAN7-1, this resulted in a cotransformation frequency of more than 60%. The cotransformation frequency was estimated using a facile DNA extraction method together with PCR.

Transformations of Penicillium islandicum and Penicillium frequentans that produce anthraquinone-related compounds

Wild-type strains of Penicillium islandicum and Penicillium frequentans, which produce anthraquinone and related compounds, were transformed to benomyl and hygromycin B resistance. Plasmids pSV50 and pBT6, with benomyl-resistant beta-tublin genes, and plasmids pAN7-1 and pDH25, with a bacterial hygromycin phosphotransferase gene under the control of Aspergillus nidulans sequences, were used respectively. Transformation frequencies with these plasmids were 10-20 transformants per micrograms of DNA per 4-8 x 10(7) viable protoplasts. Integration of plasmid DNAs into chromosomal DNAs was confirmed by Southern-blot analysis. Copy numbers and sites of integration varied among transformants. The integrated plasmid DNAs conferring a drug-resistant phenotype were mitotically stable with or without selection. The demonstration of such transformation systems in the essential first step in the application of recombinant DNA technology to study the biosynthetic genes of anthraquinone and related compounds in P. islandicum and P. frequentans.

Characterization of the "promoter region" of the enolase-encoding gene enol from the anaerobic fungus Neocallimastix frontalis: sequence and promoter analysis

The sequence of the Neocallimastix frontalis enolase gene promoter was determined up to 1800 nucleotides 5' to the major transcriptional start point. The base composition of the enolase upstream sequence revealed a very A + T-rich profile (13.5% G + C) leading to many putative hairpin structures. The functional organization of the N. frontalis enolase promoter was investigated by heterologous transient-expression assays. DNA fragments obtained by the sequential removal of sequences upstream of the translation start codon were fused to the Escherichia coli lacZ gene and the resulting plasmids were used to transform the ascomycetes Aspergillus nidulans and Penicillium roqueforti and the oomycete Saprolegnia monoica. Transient expression of the lacZ reporter gene was observed in regenerating proteoplasts of S. monoica when using the 0.3 kb or 1 kb upstream of the enolase coding region. In contrast no beta-galactosidase activity was detected in ascomycete protoplasts. DNA hybridization analysis revealed the integration of vector DNA in the genomic DNA of S. monoica and the presence of free copies of the transformation plasmid which could be rescued in E. coli. Our results indicate that the transcriptional machinery of the anaerobic chytrid N. frontalis may differ significantly from that of ascomycetes but that enough conservation exists within the lower fungi to allow a transient-driven expression of a reporter gene in an oomycete fungus.

Heterologous and homologous plasmid integration at a spore-pigment locus in Penicillium paxilli generates large deletions

Mutations in a spore pigmentation locus (brs; brown spore) in Penicillium paxilli were isolated at a relatively-high frequency (0.17%) following integrative transformation of the hygromycin-resistance plasmid pAN7-1. A molecular analysis of four independently-isolated Brs- mutants showed that all contained pAN7-1 integrated at a single-site that was unique for each mutant. A previously-described Brs- mutant, YI-34 (Itoh et al. 1994), was a two-site integration. Three of the mutants had multiple copies of pAN7-1 arranged in head-to-tail tandem arrays. A 9.6-kb BamHI junction fragment was cloned from one of these, YI-33, by plasmid rescue and used to isolate two overlapping lambda clones, lambda WB33-1 and lambda WB33-2, that span about 30 kb in the region of the wild-type locus. When genomic digests of the five Brs- mutants were probed with these lambda clones all of them were found to contain an extensive deletion through a common region of the P. paxilli genome. Subsequent attempts to generate one-step gene replacements within a 4.5-kb EcoRI fragment at the wild-type locus resulted in the isolation of Brs- mutants at a frequency of 1.6%, but all mutants with this phenotype were also found to contain an extensive genomic deletion. Therefore, a common outcome of both heterologous and homologous plasmid integration at this locus is deletion formation.

Integrative transformation of the mycotoxin-producing fungus, Penicillium paxilli

A high frequency transformation system has been developed for Penicillium paxilli using pAN7-1. Up to 44% of the primary transformants were heterokaryons. Loss of hygromycin resistance was observed in primary transformants that were sub-cultured on non-selective media, but single spores of these primary transformants were mitotically stable on both selective and non-selective media. A molecular analysis of the transformants generated showed that 78% had single-site integrations, with half of these containing a single copy of pAN7-1. CHEF-gel electrophoresis showed that P. paxilli has at least six chromosomes with a total genome size of about 23.4 Mb.

Randomly amplified polymorphic DNAs assess recombination following an induced parasexual cycle in Penicillium roqueforti

Random amplified polymorphic DNAs (RAPDs) were used as a genetic marker system to characterize recombinant strains following the parasexual cycle of Penicillium roqueforti. After protoplast fusion and haploidization of diploid hybrids, segregants characterized by a reassortment of the parental genetic markers displayed specific RAPD fingerprints. The appearance or the loss of RAPD fragments demonstrate that these markers provide an efficient method to analyze recombination and to characterize somatic hybrids.

Targeted integration into the Acremonium chrysogenum genome: disruption of the pcbC gene

The cephalosporin C-producing fungus Acremonium chrysogenum was transformed to hygromycin B resistance using different vector constructs. These constructs contain sequences of the pcbC gene from A. chrysogenum, encoding isopenicillin N synthetase. Detailed analysis of transformants, including pulsed-field gel electrophoresis (PFGE), suggests that integration of multiple vector copies takes place predominantly via non-homologous integration. By increasing the length of vector-DNA homologous to genomic DNA, integration occurs more frequently into chromosome VI, carrying the endogenous pcbC gene copy. In gene disruption experiments, the length of vector homology required to obtain cephalosporin C-minus transformants was investigated. Inactivation of the pcbC gene was observed only when homologous fragments of more than 3.0 kb were used on both sites of the resistance cassette. Southern analysis indicated homologous, as well as heterologous, integration of recombinant DNA. The integration of multiple vector copies leads to the appearance of truncated pcbC transcripts.

Development of a transformation system for the filamentous, ML-236B (compactin)--producing fungus Penicillium citrinum

We present here the first report of a transformation system developed for the filamentous, ML-236B (compactin)-producing fungus Penicillium citrinum. Hygromycin B-resistant colonies were obtained after treatment of protoplasts with a vector containing an Escherichia coli hygromycin B phosphotransferase gene fused to a 3-phosphoglycerate kinase promoter from Aspergillus nidulans. The transformation rate was 194 transformants per microgram circular DNA per 4 x 10(5) viable protoplasts under optimized transformation conditions. Transformation took place via the integration of plasmid DNA into the fungal chromosomal DNA. Most of the integration events appeared to produce tandemly iterated arrays of plasmid molecules at different sites in the chromosome. The transformed, drug-resistant, phenotype and the integrated plasmids were mitotically stable with or without selection in a majority of cases. The demonstration of such a transformation system is an essential first step in the application of recombinant DNA technology to strain improvement and for the production of novel ML-236B derivatives.

Transformation of Gibberella fujikuroi: effect of the Aspergillus nidulans AMA1 sequence on frequency and integration

A stable and reproducible transformation selection system for Gibberella fujikuroi protoplasts based on the Aspergillus nidulans arg B gene, encoding ornithine transcarbamylase, has been developed. Inclusion into the vector of the A. nidulans DNA fragment (AMA1), which permits plasmid autonomous replication in A. nidulans, A. niger and A. oryzae, appeared to permit autonomous replication of G. fujikuroi although the transformation frequency was increased by only two-fold. Transformation was also achieved using the bacterial hygromycin B resistance gene under the control of G. pulicaris and A. nidulans promoters.

Transformation of Cochliobolus lunatus with pUT 720 changes the steroid hydroxylating ability of the fungus

The filamentous fungus Cochliobolus lunatus, a known 11 beta-hydroxylator of steroids, was transformed to bleomycin resistance using the heterologous plasmid pUT 720. This plasmid contains the Sh ble gene expressed under the control of the Aspergillus nidulans gpd and trpC expression signals. The bleomycin-resistant colonies appeared with a frequency of six per microgram of DNA. All colonies were real transformants and no "abortive" growth was observed. In all transformants tested the plasmid molecules became stably integrated into the genome of the host, and one of the plasmid molecules integrated in a site-specific manner. Transformants retained the ability to hydroxylate the steroid ring, but the hydroxy group was inserted at the 15 alpha position.