A Versatile in Vivo DNA Assembly Toolbox for Fungal Strain Engineering

A CRISPR Cas12a/Cpf1 strategy to facilitate robust multiplex gene editing in Aspergillus Niger

BACKGROUND

CRISPR technologies have revolutionized strain engineering of Aspergillus species, and drastically increased the ease and speed at which genomic modifications can be performed. One of the advantages of CRISPR technologies is the possibility of rapid strain engineering using multiplex experiments. This can be achieved by using a set of different guiding RNA molecules (gRNA) to target multiple loci in the same experiment. Two major challenges in such experiments are firstly, the delivery of multiple guides simultaneously, and secondly, ensuring that each target locus is cut efficiently by the CRISPR nuclease. The CRISPR nuclease Cas12a, also known as Cpf1, presents a unique advantage to bypass this challenge. Specifically, and unlike Cas9, Cpf1 is able to release several gRNAs from a common precursor RNA molecule through its own RNase activity, eliminating the need for elements such as ribozymes or tRNA machinery for gRNA maturation. This feature sets the stage for much more straightforward construction of vectors for the delivery of many gRNAs, which in turn allows each locus to be targeted by multiple gRNAs to increase the odds of successfully inducing a break in the DNA.

RESULTS

Here we present a toolbox that can be used to assemble plasmids containing a gRNA multiplex expression cassette, which is able to express a multi gRNA precursor. The precursor can be processed via Cpf1 RNase activity to produce multiple functional gRNAs in vivo. Using our setup, we have constructed plasmids that are able to deliver up to ten gRNAs. In addition, we show that three simultaneous deletions can be introduced robustly in Aspergillus niger by targeting each gene with several gRNAs, without prior gRNA validation or the use of genomically integrated selection markers.

CONCLUSION

In this study we have established an efficient system for the construction of CRISPR-Cpf1 vectors that are able to deliver a large number of gRNAs for multiplex genome editing in Aspergillus species. Our strategy allows multiple specific genomic modifications to be performed in a time frame of less than two weeks, and we envision this will be able to speed up cell factory construction efforts significantly.

Synthetic Biology in Natural Product Biosynthesis

Synthetic biology has played an important role in the renaissance of natural products research during the post-genomics era. The development and integration of new tools have transformed the workflow of natural product discovery and engineering, generating multidisciplinary interest in the field. In this review, we summarize recent developments in natural product biosynthesis from three different aspects. First, advances in bioinformatics, experimental, and analytical tools to identify natural products associated with predicted biosynthetic gene clusters (BGCs) will be covered. This will be followed by an extensive review on the heterologous expression of natural products in bacterial, fungal and plant organisms. The native host-independent paradigm to natural product identification, pathway characterization, and enzyme discovery is where synthetic biology has played the most prominent role. Lastly, strategies to engineer biosynthetic pathways for structural diversification and complexity generation will be discussed, including recent advances in assembly-line megasynthase engineering, precursor-directed structural modification, and combinatorial biosynthesis.

CRISPR/Cas9-assisted gene editing reveals that EgPKS, a polyketide synthase, is required for the biosynthesis of preussomerins in Edenia gomezpompae SV2

Edenia gomezpompae, an endophytic fungus derived from plants, produced a diverse array of preussomerins, a type of spirobisnaphthalenes featuring two spiroketal groups, which exhibited significant antibacterial, antifungal, and cytotoxic activities. Structurally, the biosynthesis of preussomerins might be related to the biosynthesis of 1,8-dihydroxynaphthalene (DHN), a precursor of DHN-melanin. However, the absence of efficient gene-editing tools for E. gomezpompae has hindered the biosynthetic study of preussomerins. In this study, we developed a CRISPR/Cas9-based gene editing system for E. gomezpompae SV2 that was isolated from the stem of Setaria viridis, by utilizing the endogenous U6 snRNA promoter to drive sgRNA expression. Using this system, we successfully disrupted the polyketide synthase (PKS)-encoding gene, Egpks, a putative 1,3,6,8-tetrahydroxynaphthalene synthase gene involved in the biosynthesis of DHN-melanin, with an editing efficiency up to 92% and a knockout efficiency of 71% when employing the U6 snRNA-3 promoter. Furthermore, the disrupted mutant (∆Egpks) displayed white hyphae and lost the ability to produce preussomerins. These results provided a foundational tool for genetic manipulation in E. gomezpompae and revealed the role of EgPKS in the biosynthesis of preussomerin-type spirobisnaphthalenes.

Metarhizium anisopliae engineering mediated by a CRISPR/Cas9 recyclable system

The advent of CRISPR/Cas technology has revolutionized genome editing, offering simplicity, precision, and cost-effectiveness. While its application in biological control fungi has been limited, including the cosmopolitan fungus Metarhizium anisopliae, recent advancements show promise. However, integrating cas9 and selection-marker genes into fungal genomes poses challenges, including reduced efficiency, toxicity, and off-target effects. Besides, marker-free genetic engineering through a CRISPR recyclable system presents a viable solution, enabling efficient mutant generation without compromising fitness and virulence. This study pioneers the construction of marker-free strains of M. anisopliae using a CRISPR/Cas9 recyclable system. Precise deletion of albA and ku70, alongside gfp cassette insertion, confirms the system efficiency. This innovative approach holds significant potential for facilitating in-depth molecular studies, understanding their ecological roles in agricultural systems, and enhancing biocontrol efficacy against insect pests through genetic improvement.

Simultaneous In Vivo Assembly and Targeted Genome Integration of Gene Clusters in Trichoderma reesei

The saprophytic filamentous fungus Trichoderma reesei represents one of the most prolific cellulase producers and also has the potential to be developed into a tractable fungal host for biosynthesizing secondary metabolite products. To expedite the genetic engineering of filamentous fungi, efficient DNA assembly processes that can facilitate the transfer of large-sized DNA to fungal hosts, including T. reesei, are still in demand. Here, we developed a method for the simultaneous in vivo assembly and targeted genome integration of multiple DNA fragments (SATIMD) in T. reesei. While efficient orderly DNA end fusions were achieved by homologous recombination (HR) with various lengths of sequence overlaps (100-500 bp), the assembled DNA was also precisely integrated into a specific locus when combined with CRISPR/Cas9-mediated genome cutting. Specifically, we have used this method to achieve the assembly and functional expression of T. reesei key transcriptional activator Xyr1 for cellulase genes. Moreover, fusions and targeted integration of up to 10 different DNA fragments comprising the 32.7 kb sorbicillinoids biosynthetic gene cluster via a single-step transformation was demonstrated. We envision that SATIMD is a powerful tool not only useful for direct large heterologous gene cluster assembly in T. reesei but also can facilitate large-scale fungal strain genetic engineering.

Methodology comparison of environmental sediment fungal community analysis

Fungi play important roles in ecosystems. Analyzing fungal communities in environments has long been a challenge due to the large difference in compositions retrieved using different methods or sequencing regions, obscuring the true abundance and species information. Our study aimed to compare and determine more accurate approach for evaluating fungal populations in river sediment. To achieve this, different primer sets in the internal transcribed spacer (ITS) (ITS5/ITS1R, ITS1F/ITS2), 18S rRNA gene (0817F/1196R) for High-throughput sequencing (HTS), metagenomic shotgun sequencing (MS) directly from environmental samples, and HTS using ITS primers for the fungal samples collected from plate cultivation were used to characterize the fungal communities. We calculated diversity index and used FungalTraits to analyze methods preferences for fungal species. The study revealed that when analyzing the fungal species directly from environmental samples, amplification and sequencing of ITS region demonstrated more accuracy than MS and 18S rRNA gene sequencing methods, but displayed significant primer preference. Over 30 % fungal species from HTS after plate cultivation were not present in HTS from the environmental samples. NMDS analysis demonstrated significant disparities in species diversity among different methods, suggesting potential complementarity between them. Over 85% species identified by HTS using ITS primers belonged to filamentous fungi, while the MS mostly identified yeast (62%). Therefore, to get more accurate fungal community information in sediment, multiple methods were recommended by using cultivation, molecular biological methods dependent on PCR techniques like ITS1F/ITS2 primer for HTS and PCR independent method such as metagenomic shotgun sequencing techniques.

Gene expression screening and cell factory engineering for enhancing echinocandin B production in Aspergillus nidulans NRRL8112

BACKGROUND

Echinocandin B (ECB) is a key precursor of the antifungal drug anidulafungin and its biosynthesis occurs via ani gene cluster in Aspergillus nidulans NRRL8112. Strain improvement for industrial ECB production has mainly relied on mutation breeding due to the lack of genetic tools.

RESULTS

Here, a CRISPR-base-editing tool was developed in A. nidulans NRRL8112 for simultaneous inactivation of the nkuA gene and two marker genes, pryoA and riboB, which enabled efficient genetic manipulation. Then, in-vivo plasmid assembly was harnessed for ani gene expression screening, identifying the rate-limiting enzyme AniA and a pathway-specific transcription factor AniJ. Stepwise titer enhancement was achieved by overexpressing aniA and/or aniJ, and ECB production reached 1.5 g/L during 5-L fed-batch fermentation, an increase of ~ 30-fold compared with the parent strain.

CONCLUSION

This study, for the first time, revealed the regulatory mechanism of ECB biosynthesis and harnessed genetic engineering for the development of an efficient ECB-producing strain.

Unlocking Fungal Potential: The CRISPR-Cas System as a Strategy for Secondary Metabolite Discovery

Natural products (NPs) are crucial for the development of novel antibiotics, anticancer agents, and immunosuppressants. To highlight the ability of fungi to produce structurally diverse NPs, this article focuses on the impact of genome mining and CRISPR-Cas9 technology in uncovering and manipulating the biosynthetic gene clusters (BGCs) responsible for NP synthesis. The CRISPR-Cas9 system, originally identified as a bacterial adaptive immune mechanism, has been adapted for precise genome editing in fungi, enabling targeted modifications, such as gene deletions, insertions, and transcription modulation, without altering the genomic sequence. This review elaborates on various CRISPR-Cas9 systems used in fungi, notably the Streptococcus pyogenes type II Cas9 system, and explores advancements in different Cas proteins for fungal genome editing. This review discusses the methodologies employed in CRISPR-Cas9 genome editing of fungi, including guide RNA design, delivery methods, and verification of edited strains. The application of CRISPR-Cas9 has led to enhanced production of secondary metabolites in filamentous fungi, showcasing the potential of this system in biotechnology, medical mycology, and plant pathology. Moreover, this article emphasizes the integration of multi-omics data (genomics, transcriptomics, proteomics, and metabolomics) to validate CRISPR-Cas9 editing effects in fungi. This comprehensive approach aids in understanding molecular changes, identifying off-target effects, and optimizing the editing protocols. Statistical and machine learning techniques are also crucial for analyzing multi-omics data, enabling the development of predictive models and identification of key molecular pathways affected by CRISPR-Cas9 editing. In conclusion, CRISPR-Cas9 technology is a powerful tool for exploring fungal NPs with the potential to accelerate the discovery of novel bioactive compounds. The integration of CRISPR-Cas9 with multi-omics approaches significantly enhances our ability to understand and manipulate fungal genomes for the production of valuable secondary metabolites and for promising new applications in medicine and industry.

Next-generation AMA1-based plasmids for enhanced heterologous expression in filamentous fungi

Episomal AMA1-based plasmids are increasingly used for expressing biosynthetic pathways and CRISPR/Cas systems in filamentous fungi cell factories due to their high transformation efficiency and multicopy nature. However, the gene expression from AMA1 plasmids has been observed to be highly heterogeneous in growing mycelia. To overcome this limitation, here we developed next-generation AMA1-based plasmids that ensure homogeneous and strong expression. We achieved this by evaluating various degradation tags fused to the auxotrophic marker gene on the AMA1 plasmid, which introduces a more stringent selection pressure throughout multicellular fungal growth. With these improved plasmids, we observed in Aspergillus nidulans a 5-fold increase in the expression of a fluorescent reporter, a doubling in the efficiency of a CRISPRa system for genome mining, and a up to a 10-fold increase in the production of heterologous natural product metabolites. This strategy has the potential to be applied to diverse filamentous fungi.

Reconstitution of the Early Stage of Chetomin Biosynthesis in Aspergillus fumigatus Leads to the Production of Epipolythiodioxopiperazines

Using CRISPR-Cas9 technology and a microhomology-mediated end-joining repair system, we substituted genes of the gliotoxin pathway in Aspergillus fumigatus with genes responsible for chetomin biosynthesis from Chaetomium cochliodes, leading to the production of three new epipolythiodioxopiperazines (ETPs). This work represents the first successful endeavor to produce ETPs in a non-native host. Additionally, the simultaneous disruption of five genes in a single transformation marks the most extensive gene knockout event in filamentous fungi to date.

Edible mycelium bioengineered for enhanced nutritional value and sensory appeal using a modular synthetic biology toolkit

Filamentous fungi are critical in the transition to a more sustainable food system. While genetic modification of these organisms has promise for enhancing the nutritional value, sensory appeal, and scalability of fungal foods, genetic tools and demonstrated use cases for bioengineered food production by edible strains are lacking. Here, we develop a modular synthetic biology toolkit for Aspergillus oryzae, an edible fungus used in fermented foods, protein production, and meat alternatives. Our toolkit includes a CRISPR-Cas9 method for gene integration, neutral loci, and tunable promoters. We use these tools to elevate intracellular levels of the nutraceutical ergothioneine and the flavor-and color molecule heme in the edible biomass. The strain overproducing heme is red in color and is readily formulated into imitation meat patties with minimal processing. These findings highlight the promise of synthetic biology to enhance fungal foods and provide useful genetic tools for applications in food production and beyond.

The spatial organization of sphingofungin biosynthesis in Aspergillus fumigatus and its cross-interaction with sphingolipid metabolism

Sphingofungins are sphinganine analog mycotoxins acting as inhibitors of serine palmitoyl transferases, enzymes responsible for the first step in the sphingolipid biosynthesis. Eukaryotic cells are highly organized with various structures and organelles to facilitate cellular processes and chemical reactions, including the ones occurring as part of the secondary metabolism. We studied how sphingofungin biosynthesis is compartmentalized in the human-pathogenic fungus Aspergillus fumigatus, and we observed that it takes place in the endoplasmic reticulum (ER), ER-derived vesicles, and the cytosol. This implies that sphingofungin and sphingolipid biosynthesis colocalize to some extent. Automated analysis of confocal microscopy images confirmed the colocalization of the fluorescent proteins. Moreover, we demonstrated that the cluster-associated aminotransferase (SphA) and 3-ketoreductase (SphF) play a bifunctional role, supporting sphingolipid biosynthesis, and thereby antagonizing the toxic effects caused by sphingofungin production.IMPORTANCEA balanced sphingolipid homeostasis is critical for the proper functioning of eukaryotic cells. To this end, sphingolipid inhibitors have therapeutic potential against diseases related to the deregulation of sphingolipid balance. In addition, some of them have significant antifungal activity, suggesting that sphingolipid inhibitors-producing fungi have evolved mechanisms to escape self-poisoning. Here, we propose a novel self-defense mechanism, with cluster-associated genes coding for enzymes that play a dual role, being involved in both sphingofungin and sphingolipid production.

A genetic tool to express long fungal biosynthetic genes

BACKGROUND

Secondary metabolites (SMs) from mushroom-forming fungi (Basidiomycota) and early diverging fungi (EDF) such as Mucoromycota are scarcely investigated. In many cases, production of SMs is induced by unknown stress factors or is accompanied by seasonable developmental changes on fungal morphology. Moreover, many of these fungi are considered as non-culturable under laboratory conditions which impedes investigation into SM. In the post-genomic era, numerous novel SM genes have been identified especially from EDF. As most of them encode multi-module enzymes, these genes are usually long which limits cloning and heterologous expression in traditional hosts.

RESULTS

An expression system in Aspergillus niger is presented that is suitable for the production of SMs from both Basidiomycota and EDF. The akuB gene was deleted in the expression host A. niger ATNT∆pyrG, resulting in a deficient nonhomologous end-joining repair mechanism which in turn facilitates the targeted gene deletion via homologous recombination. The ∆akuB mutant tLK01 served as a platform to integrate overlapping DNA fragments of long SM genes into the fwnA locus required for the black pigmentation of conidia. This enables an easy discrimination of correct transformants by screening the transformation plates for fawn-colored colonies. Expression of the gene of interest (GOI) is induced dose-dependently by addition of doxycycline and is enhanced by the dual TetON/terrein synthase promoter system (ATNT) from Aspergillus terreus. We show that the 8 kb polyketide synthase gene lpaA from the basidiomycete Laetiporus sulphureus is correctly assembled from five overlapping DNA fragments and laetiporic acids are produced. In a second approach, we expressed the yet uncharacterized > 20 kb nonribosomal peptide synthetase gene calA from the EDF Mortierella alpina. Gene expression and subsequent LC-MS/MS analysis of mycelial extracts revealed the production of the antimycobacterial compound calpinactam. This is the first report on the heterologous production of a full-length SM multidomain enzyme from EDF.

CONCLUSIONS

The system allows the assembly, targeted integration and expression of genes of > 20 kb size in A. niger in one single step. The system is suitable for evolutionary distantly related SM genes from both Basidiomycota and EDF. This uncovers new SM resources including genetically intractable or non-culturable fungi.

A Mad7 System for Genetic Engineering of Filamentous Fungi

The introduction of CRISPR technologies has revolutionized strain engineering in filamentous fungi. However, its use in commercial applications has been hampered by concerns over intellectual property (IP) ownership, and there is a need for implementing Cas nucleases that are not limited by complex IP constraints. One promising candidate in this context is the Mad7 enzyme, and we here present a versatile Mad7-CRISPR vector-set that can be efficiently used for the genetic engineering of four different Aspergillus species: Aspergillus nidulans, A. niger, A. oryzae and A. campestris, the latter being a species that has never previously been genetically engineered. We successfully used Mad7 to introduce unspecific as well as specific template-directed mutations including gene disruptions, gene insertions and gene deletions. Moreover, we demonstrate that both single-stranded oligonucleotides and PCR fragments equipped with short and long targeting sequences can be used for efficient marker-free gene editing. Importantly, our CRISPR/Mad7 system was functional in both non-homologous end-joining (NHEJ) proficient and deficient strains. Therefore, the newly implemented CRISPR/Mad7 was efficient to promote gene deletions and integrations using different types of DNA repair in four different Aspergillus species, resulting in the expansion of CRISPR toolboxes in fungal cell factories.