Transformation

Agrobacterium tumefaciens-mediated transformation of filamentous fungi

Agrobacterium tumefaciens transfers part of its Ti plasmid, the T-DNA, to plant cells during tumorigenesis. It is routinely used for the genetic modification of a wide range of plant species. We report that A. tumefaciens can also transfer its T-DNA efficiently to the filamentous fungus Aspergillus awamori, demonstrating DNA transfer between a prokaryote and a filamentous fungus. We transformed both protoplasts and conidia with frequencies that were improved up to 600-fold as compared with conventional techniques for transformation of A. awamori protoplasts. The majority of the A. awamori transformants contained a single T-DNA copy randomly integrated at a chromosomal locus. The T-DNA integrated into the A. awamori genome in a manner similar to that described for plants. We also transformed a variety of other filamentous fungi, including Aspergillus niger, Fusarium venenatum, Trichoderma reesei, Colletotrichum gloeosporioides, Neurospora crassa, and the mushroom Agaricus bisporus, demonstrating that transformation using A. tumefaciens is generally applicable to filamentous fungi.

Genetic transformation of the pathogenic fungus Wangiella dermatitidis

Genetic transformation of Wangiella dermatitidis was studied using three plasmid vectors (pAN7-1, pWU44, and pKK5) and both electroporation and polyethyleneglycol-mediated methods. pAN7-1 contains the E. coli hygromycin B (HmB) phosphotransferase (hph) gene. Expression of the hph gene confers resistance to antibiotic HmB. Selection for resistance, indicative of transformation, resulted in 10 203 HmB-resistant colonies/micrograms pAN7-1 on medium containing 100 micrograms HmB/ml. Strains of W. dermatitidis used in this study have innate sensitivity to HmB at a critical inhibitory concentration of 20-40 micrograms/ml. Vectors pWU44 and pKK5 contain a URA5 gene from Podospora anserina. A ura5 auxotroph of W. dermatitidis was transformed to prototrophy with pWU44 or pKK5 by complementation. Transformation frequencies for these two plasmids were between 17-50 transformants/micrograms vector DNA. Southern blotting analysis and polymerase chain reaction detection of DNA from putative transformants confirmed transformation.

DNA-mediated transformation of a fungus employing a vector devoid of bacterial DNA sequences

Acremonium chrysogenum, a producer of cephalosporin C, was subjected to DNA-mediated transformations using a vector without bacterial DNA sequences. Recombinant fungal strains were generated with a gel-purified DNA fragment, carrying only the mutated beta-tubulin gene from A. chrysogenum. The lack of any bacterial DNA was verified by Southern hybridization analysis and polymerase chain reaction amplifications to detect even residual DNA sequences. This procedure can be referred to as a self-cloning experiment for which less restricted working regulations are needed. Finally, the transfer of a synthetic hirudin gene by cotransformation demonstrated that any DNA molecule can be introduced into the A. chrysogenum genome without bacterial marker genes. This seems to be highly relevant for biotechnical processes in which safe recombinant producer strains are required to satisfy governmental restrictions.

The nitrate reductase gene from a shoyu koji mold, Aspergillus oryzae KBN616

A niaD gene encoding nitrate reductase was isolated from Aspergillus oryzae KBN616 and sequenced. The structural gene comprises 2973 bp and 868 amino acids, which showed a high degree of similarity to nitrate reductases from other filamentous fungi. The coding sequence is interrupted by six introns varying in size from 48 to 98 bp. The intron positions are all conserved among the niaD genes from A. oryzae, Aspergillus nidulans, and Aspergillus niger. A homologous transformation system was developed for an industrial shoyu koji mold, A. oryzae KBN616, based on the nitrate reductase (niaD) of the nitrate assimilation pathway.

The Thom Award address. Industrial mycology and the new genetics

The genetic investigation of fungi has been extended substantially by DNA-mediated transformation, providing a supplement to more conventional genetic approaches based upon sexual and parasexual processes. Initial transformation studies with the yeast Saccharomyces cerevisiae provided the model for transformation systems in other fungi with regard to methodology, vector construction and selection strategies. There are, however, certain differences between S. cerevisiae and filamentous fungi with regard to type of genomic insertion and the availability of shuttle vectors. Single-site linked insertions are common in yeast due to the high level of homology required for recombination between vectored and genomic sequences, whereas mycelial fungi often show a high frequency of heterologous and unlinked insertions, often in the form of random and multiple-site integrations. While extrachromosomally-maintained or replicative vectors are readily available for use with yeasts, such vectors have been difficult to construct for use with filamentous fungi. The development of vectors for replicative transformation with these fungi awaits further study. It is proposed that replicative vectors may be inherently less efficient for use with mycelial fungi relative to yeasts, since the mycelium, as an extended and semicontinuous network of cells, may delimit an adequate diffusion of the vector carrying the selectable gene, thus leading to a high frequency of abortive or unstable transformants.

Construction by one-step gene replacement of Trichoderma reesei strains that produce the glucoamylase P of Hormoconis resinae

Two one-step gene replacement vectors containing either the Hormoconis resinae glucoamylase P (gamP) genomic gene or the corresponding cDNA, each under the control of the promoter of the Trichoderma reesei cellobiohydrolase 1 gene (cbh1), were constructed and used to replace the cbh1 gene in a T. reesei strain. In both vectors the cbh1 promoter is precisely fused to the gamP protein coding region. Both the gamP cDNA and the genomic gene direct the secretion of the active glucoamylase P (GAMP) enzyme from T. reesei, which indicates that the intron sequences in the genomic gamP gene are processed in T. reesei. According to the results, a T. reesei transformant strain, in which the cbh1 gene has been replaced by a single copy of the gamP genomic gene, secretes more active GAMP than does a transformant strain having three copies of the cDNA clone in tandem orientation at the cbh1 locus.

Biolistic transformation of conidia of Botryotinia fuckeliana

Botryotinia fuckeliana, the causal agent of grey mould, was biolistically transformed to hygromycin B resistance using a plasmid (pOHT) containing a bacterial hygromycin phosphotransferase gene fused to regulatory sequences from Aspergillus nidulans. Multiple copies of the plasmid, precipitated onto tungsten particles, were delivered into the conidia by a helium-driven gene gun. Southern analysis showed that the plasmid was integrated into the fungal genome at one single locus. After five subsequent transfers on selective medium, all transformants were mitotically stable. When propagated on non-selective medium, four out of eight transformants retained their resistance to hygromycin B. Southern analysis of the fifth generation of transformants showed that no genetic rearrangements occurred during vegetative growth of stable transformants.

Development of an homologous transformation system for Acremonium chrysogenum based on the beta-tubulin gene

The beta-tubulin gene was isolated from the filamentous fungus Acremonium chrysogenum using a heterologous gene probe to screen an A. chrysogenum lambda library. Sequencing of the A. chrysogenum gene revealed a mosaic gene which contains five exons and four intervening sequences. The exons encode for a polypeptide of 447 amino-acid residues which showed a high degree of similarity when compared with amino-acid sequences from beta-tubulins of other eukaryotes. The introns are characterized by typical consensus sequences found in intervening sequences from other filamentous fungi. In-vitro mutagenesis of codon 167 of the beta-tubulin gene resulted in the substitution of a phenylalanine by a tyrosine in the corresponding polypeptide sequence. The mutated gene was used successfully in the transformation and co-transformation of A. chrysogenum to benomyl resistance. The molecular analysis of transformants provided evidence that they contain the mutated beta-tubulin gene in addition to the wild-type gene, as was proved by Southern-hybridization analysis and direct sequencing of PCR amplification products.

High frequency one-step gene replacement in Trichoderma reesei. II. Effects of deletions of individual cellulase genes

Four cellulase genes of Trichoderma reesei, cbh1, cbh2, egl1 and egl2, have been replaced by the amdS marker gene. When linear DNA fragments and flanking regions of the corresponding cellulase locus of more than 1 kb were used, the replacement frequencies were high, ranging from 32 to 52%. Deletion of the major cellobiohydrolase 1 gene led to a 2-fold increase in the production of cellobiohydrolase II; however, replacement of the cbh2 gene did not affect the final cellulase levels and deletion of egl1 or egl2 slightly increased production of both cellobiohydrolases. Based on our results, endoglucanase II accounts for most of the endoglucanase activity produced by the hypercellulolytic host strain. Furthermore, loss of the egl2 gene causes a significant drop in the filter paper-hydrolysing activity, indicating that endoglucanase II has an important role in the total hydrolysis of cellulose.

A nucleotide sequence involved in replicative transformation of a filamentous fungus

Replicative plasmids generated through in-vivo recombination have been identified among transformants of the fungus Pleurotus ostreatus. In addition to sequences from a standard selection vector (pAN7-1), these recombinant plasmids contain recombined sequences of chromosomal origin conferring replicative potential upon the vector. One such recombined sequence, an 1148-bp insert into plasmid pP01, has been characterized. This sequence has been analyzed for secondary structural features as well as for consensus sites affiliated with origins of replication (ori) in other eukaryotic systems. The 1148-bp insert lacks an ORF and does not contain an acceptable match to the commonly identified 11-bp ars consensus sequence (A/TTTTATA/GTTTA/T) for autonomous replication in the yeast Saccharomyces cerevisiae. The analysis, however, revealed a cluster of three hairpin-loop-forming subsequences with individual delta G25 degrees C free energy values of -7.6, -6.4 and -5.2 kcal mol-1. Also found were two 7-bp analogues to centromere-affiliated sequences recognized in other fungi, as well as several putative gyrase recognition sites comparable to the 9-bp S. cerevisiae/E. coli gyrase-binding consensus sequence. Sequences comparable to the ori of the yeast 2-microns plasmid or to various sequences associated with ori of yeast/fungal mitochondrial DNAs (mtDNA) were not present in the 1148-bp insert. Replication of pP01 appears rather to involve a replication of chromosomal derivation devoid of an ars-type consensus.