Biosynthesis of the Isocoumarin Derivatives Fusamarins is Mediated by the PKS8 Gene Cluster in Fusarium

Unveiling the fungal color palette: pigment analysis of Fusarium solani species complex and Curvularia verruculosa clinical isolates

Fungal species in the Nectriaceae, such as Fusarium spp. (Hypocreales: Nectriaceae), are etiologic agents of hyalohyphomycosis capable of producing violaceous or yellowish pigments under certain conditions, while Curvularia spp. (Pleosporales: Pleosporaceae) are agents of phaeohyphomycosis and typically produce melanin in their cell walls. In nectriaceous and pleosporaceous fungi, these pigments are mainly constituted by polyketides (e.g., azaphilones, naphthoquinones, and hydroxyanthraquinones). Considering the importance of pigments synthesized by these genera, this work focused on the selective extraction of pigments produced by eight Fusarium solani species complex and one Curvularia verruculosa isolate recovered from dermatomycosis specimens, their separation, purification, and posterior chemical analysis. The pigments were characterized through spectral and acid-base analysis, and their maximum production time was determined. Moreover, spectral identification of isolates was carried out to approach the taxonomic specificity of pigment production. Herein we describe the isolation and characterization of three acidic pigments, yellowish and pinkish azaphilones (i.e., coaherin A and sclerotiorin), and a purplish xanthone, reported for the first time in the Nectriaceae and Pleosporaceae, which appear to be synthesized in a species-independent manner, in the case of fusaria.

Comparative genomic analysis of Sanghuangporus sanghuang with other Hymenochaetaceae species

Sanghuangporus sanghuang is a medicinal macrofungus with antioxidant and antitumor activities, and it is enriched with secondary metabolites such as polysaccharides, terpenes, polyphenols, and styrylpyrone compounds. To explore the putative core genes and gene clusters involved in sanghuang biosynthesis, we sequenced and assembled a 40.5-Mb genome of S. sanghuang (SH1 strain). Using antiSMASH, local BLAST, and NCBI comparison, 12 terpene synthases (TPSs), 1 non-ribosomal peptide synthase, and five polyketide synthases (PKSs) were identified in SH1. Combining the transcriptome analysis with liquid chromatography mass spectrometry-ion trap-time of flight analysis, we determined that ShPKS1, one phenylalanine aminolyase (ShPAL), and one P450 monooxygenase (ShC4H1) were associated with hispidin biosynthesis. Structural domain comparison indicated that ShPKS2 and ShPKS3 are involved in the biosynthesis of orsellinic acid and 2-hydroxy-6-methylbenzoic acid, respectively. Furthermore, comparative genomic analysis of SH1 with 14 other fungi from the Hymenochaetaceae family showed variation in the number of TPSs among different genomes, with Coniferiporia weirii exhibiting only 9 TPSs and Inonotus obliquus having 20. The number of TPSs also differed among the genomes of three strains of S. sanghuang, namely Kangneng (16), MS2 (9), and SH1 (12). The type and number of PKSs also varied among species and even strains, ranging from two PKSs in Pyrrhoderma noxium to five PKSs in S. sanghuang SH1. Among the three strains of S. sanghuang, both the structural domains and the number of PKSs in strains MS2 and SH1 were consistent, whereas strain Kangneng exhibited only four PKSs and lacked the PKS with the structural domain KS-AT-DH-KR-ACP. Additionally, Sanghuangporus species exhibited more similar PKSs to Inonotus, with higher gene similarity around five PKSs, while showing differences from those of other fungi in the same family, including Phellinus lamaoensis. This result supports the independent taxonomic significance of the genus Sanghuangporus to some extent.

Activation of a Silent Polyketide Synthase SlPKS4 Encoding the C7-Methylated Isocoumarin in a Marine-Derived Fungus Simplicillium lamellicola HDN13-430

Coumarins, isocoumarins and their derivatives are polyketides abundant in fungal metabolites. Although they were first discovered over 50 years ago, the biosynthetic process is still not entirely understood. Herein, we report the activation of a silent nonreducing polyketide synthase that encodes a C7-methylated isocoumarin, similanpyrone B (1), in a marine-derived fungus Simplicillium lamellicola HDN13-430 by heterologous expression. Feeding studies revealed the host enzymes can change 1 into its hydroxylated derivatives pestapyrone A (2). Compounds 1 and 2 showed moderate radical scavenging activities with ED50 values of 67.4 µM and 104.2 µM. Our discovery fills the gap in the enzymatic elucidation of naturally occurring C7-methylated isocoumarin derivatives.

Genome analysis of Cephalotrichum gorgonifer and identification of the biosynthetic pathway for rasfonin, an inhibitor of KRAS dependent cancer

BACKGROUND

Fungi are important sources for bioactive compounds that find their applications in many important sectors like in the pharma-, food- or agricultural industries. In an environmental monitoring project for fungi involved in soil nitrogen cycling we also isolated Cephalotrichum gorgonifer (strain NG_p51). In the course of strain characterisation work we found that this strain is able to naturally produce high amounts of rasfonin, a polyketide inducing autophagy, apoptosis, necroptosis in human cell lines and showing anti-tumor activity in KRAS-dependent cancer cells.

RESULTS

In order to elucidate the biosynthetic pathway of rasfonin, the strain was genome sequenced, annotated, submitted to transcriptome analysis and genetic transformation was established. Biosynthetic gene cluster (BGC) prediction revealed the existence of 22 BGCs of which the majority was not expressed under our experimental conditions. In silico prediction revealed two BGCs with a suite of enzymes possibly involved in rasfonin biosynthesis. Experimental verification by gene-knock out of the key enzyme genes showed that one of the predicted BGCs is indeed responsible for rasfonin biosynthesis.

CONCLUSIONS

This study identified a biosynthetic gene cluster containing a key-gene responsible for rasfonin production. Additionally, molecular tools were established for the non-model fungus Cephalotrichum gorgonifer which allows strain engineering and heterologous expression of the BGC for high rasfonin producing strains and the biosynthesis of rasfonin derivates for diverse applications.