Minos as a novel Tc1/mariner-type transposable element for functional genomic analysis in Aspergillus nidulans

Minos and Restless transposon insertion mutagenesis of psychrotrophic fungus for red pigment synthesis adaptive to normal temperature

The polar psychrotrophic fungus Geomyces sp. WNF-15A can produce high-quality natural red pigment for the potential use as edible pigment. However, it shows low-temperature-dependent synthesis of red pigment, which limits its large-scale industrial applications due to the difficult and high-cost bioprocess control. This study aims to develop transposon-mediated mutagenesis methods to generate mutants that are able to synthesize red pigment at normal temperature. Four transposable systems, including single and dual transposable systems, were established in this fungus based on the Minos from Drosophila hydei and the Restless from Tolypocladium inflatum. A total of 23 production-dominant mutants and 12 growth-dominant mutants were thus obtained by constructed transposable systems. At 14 °C and 20 °C, the MPS1 mutant strain achieved the highest level of red pigment (OD520 of 43.3 and 29.7, respectively), which was increased by 78.4% and 128.7% compared to the wild-type, respectively. Of note, 4 mutants (MPS1, MPS3, MPS4 and MPD1) successfully synthesized red pigment (OD520 of 5.0, 5.3, 4.7 and 4.9, respectively) at 25 °C, which broke the limit of the wild-type production under normal temperature. Generally, the dual transposable systems of Minos and Restless were more efficient than their single transposable systems for mutagenesis in this fungus. However, the positive mutation ratios were similar between the dual and single transposable systems for either Minos or Restless. This study provides alternative tools for genetic mutagenesis breeding of fungi from extreme environments.

Horizontal Gene Transfer of Triazole Resistance in Aspergillus fumigatus

Aspergillus fumigatus is the primary mold pathogen in humans. It can cause a wide range of diseases in humans, with high mortality rates in immunocompromised patients. The first-line treatments for invasive A. fumigatus infections are the triazole antifungals that inhibit Cyp51 lanosterol demethylase activity, blocking ergosterol biosynthesis. However, triazole-resistant strains of A. fumigatus are increasingly encountered, leading to increased mortality. The most common triazole resistance mechanisms in A. fumigatus are alterations in the cyp51A gene or promoter. We tested the hypothesis that A. fumigatus can acquire triazole resistance by horizontal gene transfer (HGT) of resistance-conferring gene cyp51A. HGT has not been experimentally analyzed in filamentous fungi. Therefore, we developed an HGT assay containing donor A. fumigatus strains carrying resistance-conferring mutated cyp51A, either in its chromosomal locus or in a self-replicating plasmid, and recipient strains that were hygromycin resistant and triazole sensitive. Donor and recipient A. fumigatus strains were cocultured and transferred to selective conditions, and the recipient strain tested for transferred triazole resistance. We found that chromosomal transfer of triazole resistance required selection under both voriconazole and hygromycin, resulting in diploid formation. Notably, plasmid-mediated transfer was also activated by voriconazole or hypoxic stress alone, suggesting a possible route to HGT of antifungal resistance in A. fumigatus, both in the environment and during host infection. This study provides, for the first time, preliminary experimental evidence for HGT mediating antifungal resistance in a pathogenic fungus. IMPORTANCE It is well known that bacteria can transfer antibiotic resistance from one strain to another by horizontal gene transfer (HGT), leading to the current worldwide crisis of rapidly emerging antibiotic-resistant bacteria. However, in fungi, HGT events have only been indirectly documented by whole-genome sequencing. This study directly examined fungal HGT of antibiotic resistance in a laboratory setting. We show that HGT of antifungal triazole resistance occurs in the important human fungal pathogen Aspergillus fumigatus. Importantly, we show a plasmid-mediated transfer of triazole resistance occurs under conditions likely to prevail in the environment and in infected patients. This study provides an experimental foundation for future work identifying the drivers and mechanistic underpinnings of HGT in fungi.

The crustacean model Parhyale hawaiensis

Arthropods are the most abundant and diverse animals on earth. Among them, pancrustaceans are an ancient and morphologically diverse group, comprising a wide range of aquatic and semi-aquatic crustaceans as well as the insects, which emerged from crustacean ancestors to colonize most terrestrial habitats. Within insects, Drosophila stands out as one of the most powerful animal models, making major contributions to our understanding of development, physiology and behavior. Given these attributes, crustaceans provide a fertile ground for exploring biological diversity through comparative studies. However, beyond insects, few crustaceans are developed sufficiently as experimental models to enable such studies. The marine amphipod Parhyale hawaiensis is currently the best established crustacean system, offering year-round accessibility to developmental stages, transgenic tools, genomic resources, and established genetics and imaging approaches. The Parhyale research community is small but diverse, investigating the evolution of development, regeneration, aspects of sensory biology, chronobiology, bioprocessing and ecotoxicology.

Hydroxytyrosol (HT) Analogs Act as Potent Antifungals by Direct Disruption of the Fungal Cell Membrane

Fungal infections constitute an emerging threat and a prevalent health problem due to increasing number of immunocompromised people and pharmacological or other treatments aiming at viral infections, cancer or allergies. Currently used antifungals suffer from inefficiency, toxic side effects and developing drug-resistance. Additionally, over the last two decades no new classes of antifungals have been approved, emphasizing the urgent need for developing a novel generation of antifungals. Here, we investigate the antifungal activity of a series of chemically synthesized Hydroxytyrosol (HT) analogs. HT is one of the major phenolic compounds in olive oil, shown to possess radical-scavenging antioxidant, antiproliferative, proapoptotic and anti-inflammatory activities. No previous report has studied HT analogs as antifungals. We show that specific analogs have broad and strong antifungal activity, significantly stronger than the parent compound HT. Using Aspergillus nidulans as an in vivo cellular model system, we show that antifungal HT analogs have an unprecedented efficiency in fungal plasma membrane destruction. Importantly, antifungal HT analogs did not show toxicity in a mammalian cell line, whereas no resistance to HT analogs was obtained by standard mutagenesis. Our results open the way for the development of a novel, efficient and safer class of antifungals.

NmeA, a novel efflux transporter specific for nucleobases and nucleosides, contributes to metal resistance in Aspergillus nidulans

Through Minos transposon mutagenesis we obtained A. nidulans mutants resistant to 5-fluorouracil due to insertions into the upstream region of the uncharacterized gene nmeA, encoding a Major Facilitator Superfamily (MFS) transporter. Minos transpositions increased nmeA transcription, which is otherwise extremely low under all conditions tested. To dissect the function of NmeA we used strains overexpressing or genetically lacking the nmeA gene. Strains overexpressing NmeA are resistant to toxic purine analogues, but also, to cadmium, zinc and borate, whereas an isogenic nmeAΔ null mutant exhibits increased sensitivity to these compounds. We provide direct evidence that nmeA overexpression leads to efflux of adenine, xanthine, uric acid and allantoin, the latter two being intermediate metabolites of purine catabolism that are toxic when accumulated cytoplasmically due to relevant genetic lesions. By using a functional GFP-tagged version we show that NmeA is a plasma membrane transporter. Homology modeling and docking approaches identified a single purine binding site and a tentative substrate translocation trajectory in NmeA. Orthologues of NmeA are present in all Aspergilli and other Eurotiomycetes, but are absent from other fungi or non-fungal organisms. NmeA is thus the founding member of a new class of transporters essential for fungal success under specific toxic conditions.

An inducible tool for random mutagenesis in Aspergillus niger based on the transposon Vader

The ascomycete Aspergillus niger is widely used in the biotechnology, for instance in producing most of the world's citric acid. It is also known as a major food and feed contaminant. While generation of gene knockouts for functional genomics has become feasible in ku70 mutants, analyzing gene functions or metabolic pathways remains a laborious task. An unbiased transposon-based mutagenesis approach may aid this process of analyzing gene functions by providing mutant libraries in a short time. The Vader transposon is a non-autonomous DNA-transposon, which is activated by the homologous tan1-transposase. However, in the most commonly used lab strain of A. niger (N400 strain and derivatives), we found that the transposase, encoded by the tan1 gene, is mutated and inactive. To establish a Vader transposon-based mutagenesis system in the N400 background, we expressed the functional transposase of A. niger strain CBS 513.88 under the control of an inducible promoter based on the Tet-on system, which is activated in the presence of the antibiotic doxycycline (DOX). Increasing amounts of doxycycline lead to higher Vader excision frequencies, whereas little to none activity of Vader was observed without addition of doxycycline. Hence, this system appears to be suitable for producing stable mutants in the A. niger N400 background.