Acurin A, a novel hybrid compound, biosynthesized by individually translated PKS- and NRPS-encoding genes in Aspergillus aculeatus.

This work presents the identification and proposed biosynthetic pathway for a compound of mixed polyketide-nonribosomal peptide origin that we named acurin A. The compound was isolated from an extract of the filamentous fungus Aspergillus aculeatus, and its core structure resemble that of the mycotoxin fusarin C produced by several Fusarium species. Based on bioinformatics in combination with RT-qPCR experiments and gene-deletion analysis, we identified a biosynthetic gene cluster (BGC) in A. aculeatus responsible for the biosynthesis of acurin A. Moreover, we were able to show that a polyketide synthase (PKS) and a nonribosomal peptide synthetase (NRPS) enzyme separately encoded by this BGC are responsible for the synthesis of the PK-NRP compound, acurin A, core structure. In comparison, the production of fusarin C is reported to be facilitated by a linked PKS-NRPS hybrid enzyme. Phylogenetic analyses suggest the PKS and NRPS in A. aculeatus resulted from a recent fission of an ancestral hybrid enzyme followed by gene duplication. In addition to the PKS- and NRPS-encoding genes of acurin A, we show that six other genes are influencing the biosynthesis including a regulatory transcription factor. Altogether, we have demonstrated the involvement of eight genes in the biosynthesis of acurin A, including an in-cluster transcription factor. This study highlights the biosynthetic capacity of A. aculeatus and serves as an example of how the CRISPR/Cas9 system can be exploited for the construction of fungal strains that can be readily engineered.

Efficient oligo nucleotide mediated CRISPR-Cas9 gene editing in Aspergilli.

CRISPR-Cas9 technologies are revolutionizing fungal gene editing. Here we show that survival of specific Cas9/sgRNA mediated DNA double strand breaks (DSBs) depends on the non-homologous end-joining, NHEJ, DNA repair pathway and we use this observation to develop a tool, TAPE, to assess protospacer efficiency in Aspergillus nidulans. Moreover, we show that in NHEJ deficient strains, highly efficient marker-free gene targeting can be performed. Indeed, we show that even single-stranded oligo nucleotides efficiently work as repair templates of specific Cas9/sgRNA induced DNA DSBs in A. nidulans, A. niger, and in A. oryzae indicating that this type of repair may be wide-spread in filamentous fungi. Importantly, we demonstrate that by using single-stranded oligo nucleotides for CRISPR-Cas9 mediated gene editing it is possible to introduce specific point mutations as well gene deletions at efficiencies approaching 100%. The efficiency of the system invites for multiplexing and we have designed a vector system with the capacity of delivering Cas9 and multiple sgRNAs based on polymerase III promoters and tRNA spacers. We show that it is possible to introduce two point mutations and one gene insertion in one transformation experiment with a very high efficiency. Our system is compatible with future high-throughput gene-editing experiments.

DIVERSIFY: A Fungal Multispecies Gene Expression Platform.

Recent sequencing of numerous fungal species revealed large repertoires of putative biotechnologically relevant genes and secondary metabolite gene clusters. However, often the commercial potential of these species is impeded by difficulties to predict host physiological and metabolic compatibility with a given product, and lack of adequate genetic tools. Consequently, most heterologous production is performed in standard hosts where genetic tools and experience are in place. However, these species may not be suitable for all products. To increase chances of successful heterologous production, we have created a flexible platform, DIVERSIFY, for multispecies heterologous gene expression. This reduces the workload to construction of a single gene expression cassette, used to transform all DIVERSIFY strains in order to identify the optimal cell factory host. As proof of principle of the DIVERSIFY concept, we present the first version of our platform, DIVERSIFY 1.0, which we have successfully used for the production of three proteins and a metabolite in four different Aspergilli species, and for the identification of the best producer for each of the products. Moreover, we show that DIVERSIFY 1.0 is compatible with marker-free gene targeting induced by the CRISPR nucleases Cas9 and MAD7.

Genome Editing: CRISPR-Cas9.

In the present chapter, we present the protocols and guidelines to facilitate implementation of CRISPR-Cas9 technology in fungi where few or no genetic tools are in place. Hence, we firstly explain how to identify dominant markers for genetic transformation. Secondly, we provide a guide for construction of Cas9/sgRNA episomal expression vectors. Thirdly, we present how to mutagenize reporter genes to explore the efficiency of CRISPR-Cas9 in the relevant fungus and to ease subsequent CRISPR-mediated genetic engineering. Lastly, we describe how to make CRISPR-mediated marker-dependent and marker-free gene targeting.

Novofumigatonin biosynthesis involves a non-heme iron-dependent endoperoxide isomerase for orthoester formation.

Novofumigatonin (1), isolated from the fungus Aspergillus novofumigatus, is a heavily oxygenated meroterpenoid containing a unique orthoester moiety. Despite the wide distribution of orthoesters in nature and their biological importance, little is known about the biogenesis of orthoesters. Here we show the elucidation of the biosynthetic pathway of 1 and the identification of key enzymes for the orthoester formation by a series of CRISPR-Cas9-based gene-deletion experiments and in vivo and in vitro reconstitutions of the biosynthesis. The novofumigatonin pathway involves endoperoxy compounds as key precursors for the orthoester synthesis, in which the Fe(II)/α-ketoglutarate-dependent enzyme NvfI performs the endoperoxidation. NvfE, the enzyme catalyzing the orthoester synthesis, is an Fe(II)-dependent, but cosubstrate-free, endoperoxide isomerase, despite the fact that NvfE shares sequence homology with the known Fe(II)/α-ketoglutarate-dependent dioxygenases. NvfE thus belongs to a class of enzymes that gained an isomerase activity by losing the α-ketoglutarate-binding ability.

Functional Reconstitution of a Fungal Natural Product Gene Cluster by Advanced Genome Editing.

Filamentous fungi produce varieties of natural products even in a strain dependent manner. However, the genetic basis of chemical speciation between strains is still widely unknown. One example is trypacidin, a natural product of the opportunistic human pathogen Aspergillus fumigatus, which is not produced among different isolates. Combining computational analysis with targeted gene editing, we could link a single nucleotide insertion in the polyketide synthase of the trypacidin biosynthetic pathway and reconstitute its production in a nonproducing strain. Thus, we present a CRISPR/Cas9-based tool for advanced molecular genetic studies in filamentous fungi, exploiting selectable markers separated from the edited locus.

A CRISPR-Cas9 System for Genetic Engineering of Filamentous Fungi.

The number of fully sequenced fungal genomes is rapidly increasing. Since genetic tools are poorly developed for most filamentous fungi, it is currently difficult to employ genetic engineering for understanding the biology of these fungi and to fully exploit them industrially. For that reason there is a demand for developing versatile methods that can be used to genetically manipulate non-model filamentous fungi. To facilitate this, we have developed a CRISPR-Cas9 based system adapted for use in filamentous fungi. The system is simple and versatile, as RNA guided mutagenesis can be achieved by transforming a target fungus with a single plasmid. The system currently contains four CRISPR-Cas9 vectors, which are equipped with commonly used fungal markers allowing for selection in a broad range of fungi. Moreover, we have developed a script that allows identification of protospacers that target gene homologs in multiple species to facilitate introduction of common mutations in different filamentous fungi. With these tools we have performed RNA-guided mutagenesis in six species of which one has not previously been genetically engineered. Moreover, for a wild-type Aspergillus aculeatus strain, we have used our CRISPR Cas9 system to generate a strain that contains an AACU_pyrG marker and demonstrated that the resulting strain can be used for iterative gene targeting.