Co-transformation with autonomous replicating and integrative plasmids in Penicillium chrysogenum is highly efficient and leads in some cases to rescue of the intact integrative plasmid

Penicillium chrysogenum: Beyond the penicillin

Almost one century after the Sir Alexander Fleming's fortuitous discovery of penicillin and the identification of the fungal producer as Penicillium notatum, later Penicillium chrysogenum (currently reidentified as Penicillium rubens), the molecular mechanisms behind the massive production of penicillin titers by industrial strains could be considered almost fully characterized. However, this filamentous fungus is not only circumscribed to penicillin, and instead, it seems to be full of surprises, thereby producing important metabolites and providing expanded biotechnological applications. This review, in addition to summarizing the classical role of P. chrysogenum as penicillin producer, highlights its ability to generate an array of additional bioactive secondary metabolites and enzymes, together with the use of this microorganism in relevant biotechnological processes, such as bioremediation, biocontrol, production of bioactive nanoparticles and compounds with pharmaceutical interest, revalorization of agricultural and food-derived wastes or the enhancement of food industrial processes and the agricultural production.

Strong preference for the integration of transforming DNA via homologous recombination in Trichoderma atroviride

Trichoderma species play important roles in nature as plant growth promotors and antagonists of phytopathogenic fungi, and are used as models to study photomorphogenesis. Molecular tools have been implemented to manipulate and improve these fungi. However, instability of transformants or very low frequency of homologous recombination has been reported. Here, we report the fate of transforming DNA, demonstrating that it can follow two different fates. When a vector contains sequences also present in the Trichodermaatroviride genome, it mainly integrates by homologous recombination generating stable recombinant strains. In contrast, vectors with no sequence homology to the T. atroviride genome generate unstable transformants, losing the transforming DNA in the first generation of conidia produced without selection where, surprisingly, the vector behaves as autoreplicative. Integration by homologous recombination was demonstrated when transformants were generated with a truncated version of the blr2 gene, resulting in insertional mutants with phenotypes identical to those of knockout mutants. Our results indicate that T. atroviride is highly efficient in integrating DNA by homologous recombination and that plasmid vectors with no sequence homology to the genome are maintained for several generations in T. atroviride if kept under selective pressure even though they lacked fungal autonomous replication sequences.

Transformation of Penicillium rubens 212 and Expression of GFP and DsRED Coding Genes for Visualization of Plant-Biocontrol Agent Interaction

Strain 212 of Penicillium rubens (PO212) is an effective fungal biological control agent against a broad spectrum of diseases of horticultural plants. A pyrimidine auxotrophic isolate of PO212, PO212_18.2, carrying an inactive pyrG gene, has been used as host for transformation by positive selection of vectors containing the gene complementing the pyrG1 mutation. Both integrative and autonomously replicating plasmids transformed PO212_18.2 with high efficiency. Novel PO212-derived strains expressed green (sGFP) and red (Ds-Red Express) fluorescent reporter proteins, driven by the A. nidulans gpdA promoter. Fluorescence microscopy revealed constitutive expression of the sGFP and Ds-Red Express proteins, homogenously distributed across fungal cells. Transformation with either type of plasmid, did not affect the growth and morphological culture characteristics, and the biocontrol efficacy of either transformed strains compared to the wild-type, PO212. Fluorescent transformants pointed the capacity of PO212 to colonize tomato roots without invading plant root tissues. This work demonstrates susceptibility of the biocontrol agent PO212 to be transformed, showing that the use of GFP and DsRed as markers for PO212 is a useful, fast, reliable and effective approach for studying plant-fungus interactions and tomato root colonization.

Phleomycin increases transformation efficiency and promotes single integrations in Schizophyllum commune

Phleomycin is mutagenic by introducing double-strand breaks in DNA. The ble gene of Streptoalloteychus hindustanus, which confers resistance to this substance, is widely used as a selection marker for transformation. Schizophyllum commune grows on 25 microg of phleomycin ml(-1) after introduction of a resistance cassette based on the ble gene. However, we here report that growth of resistant colonies on this concentration of phleomycin resulted in aberrant colony morphologies. Apparently, phleomycin was mutagenic despite acquired resistance. Therefore, a new selection system was developed based on resistance to the antibiotic nourseothricin. However, the transformation efficiency was tenfold lower than that obtained with phleomycin as a selection agent. This low transformation efficiency could be rescued by addition of a nonselective concentration of phleomycin during protoplast regeneration. This was accompanied by a higher incidence of single-copy integrations and with an increase of expression of key genes involved in double-strand break repair. Taken together, we conclude that the effect of a nonselective concentration of phleomycin strongly resembles the effect of restriction enzyme-mediated integration (REMI) but, unlike REMI, it does not depend on the presence of a target restriction site.

Improvements to gene deletion in the fungal pathogen Cryptococcus neoformans: absence of Ku proteins increases homologous recombination, and co-transformation of independent DNA molecules allows rapid complementation of deletion phenotypes

Cryptococcus neoformans is a pathogenic fungus that is relatively amenable to molecular genetic analysis, including gene deletion. However, rates of homologous recombination can be low, so obtaining specific gene deletion transformants is challenging. We have utilized two new technologies, cku deletion strains to improve the efficiency of gene deletions in this organism, and co-transformations. The Ku70-Ku80 heterodimer is predicted to be an essential part of the non-homologous end-joining process in C. neoformans. Here we show that a deletion in one or both of these proteins results in an increase in the rates of homologous recombination. Importantly, we demonstrate that after generation of a strain with a particular deletion of interest, the cku deletion can be removed by mating and segregation. We also utilize co-transformation of wild-type genes and selectable markers on separate linear DNA molecules to complement a deletion event. We show that co-transformation results in the successful restoration of wild-type phenotype, though variations in this phenotype often occur.

The potential of Pseudozyma yeastlike epiphytes for the production of heterologous recombinant proteins

Although Basidiomycetes represent the most evolved class of fungi, they have been neglected with regard to recombinant gene expression. In this work, basidiomycetous yeasts belonging to Pseudozyma spp. were studied with respect to their amenability to heterologous protein production. Single plasmid or cotransformation experiments routinely afforded 100 to 200 independent transformants for the two tested species of Pseudozyma. Green fluorescent protein (GFP) was expressed in the correctly folded conformation, as demonstrated by fluorescence microscopy, and hen egg white lysozyme (HEWL) was expressed in its active form, as revealed by its lytic activity on Micrococcus lysodeikticus cells. Protease analysis established that Pseudozyma spp. contained equivalent or less extracellular protease activity than yeasts and far less protease activity than ascomycetous filamentous fungi in similar culture conditions. This proteolytic activity was inhibited by over 97% with a combination of PMSF and Pepstatin A. N-glycosylation patterns of native Pseudozyma flocculosa secreted proteins were comprised of one or a few short glycan chains that possess a classic eukaryotic structure typical of higher fungi and animal cells. This is the first report of a Basidiomycete that possesses multiple intrinsic characteristics necessary for use as a heterologous gene expression system.

Agrobacterium tumefaciens-mediated transformation of the antitumor clavaric acid-producing basidiomycete Hypholoma sublateritium

The basidiomycete Hypholoma sublateritium produces clavaric acid, an antitumor isoprenoid compound. Arthrospores of this fungus were transformed by Agrobacterium tumefaciens-mediated conjugation. Five plasmids carrying different regulatory sequences to drive expression of the hph (hygromycin phosphotransferase) gene were tested. The promoter used was critically important in order to express heterologous genes in H. sublateritium. Constructions carrying the Agaricus bisporus glyceraldehyde-3-phosphate dehydrogenase promoter (P gpd) showed a good transformation efficiency, whereas constructions with the gpd promoter from ascomycetes were ineffective. Transformant clones showed a random integration pattern of plasmid DNA. Most transformants showed a single integrated copy of the transforming plasmid, but about 1.5% showed double or multiple integrations. All the analyzed transformants were mitotically stable and maintained the integrated exogenous DNA in the absence of antibiotic. The green fluorescent protein gene was expressed from the A. bisporus gpd promoter, as shown by RT-PCR studies, but no significant fluorescence was observed. Transformation of H. sublateritium opens the way for the genetic manipulation of clavaric acid biosynthesis in this fungus.