Biosynthetic Pathways of Dimeric Natural Products Containing Bisanthraquinone and Related Xanthones

Antimicrobial Potential of Secalonic Acids from Arctic-Derived Penicillium chrysogenum INA 01369

In this study, two compounds have been isolated from the Arctic-derived fungus Penicillium chrysogenum INA 13460. Structural elucidation, performed using 2D NMR and HR-ESIMS data, has identified the compounds as stereoisomers of secalonic acids, dimeric tetrahydroxanthones. The absolute configurations of these stereoisomers have been determined through conformational NMR analysis and circular dichroism spectroscopy. The antimicrobial activity of secalonic acids D and F has been evaluated against a diverse range of microorganisms, including Gram-positive multidrug-resistant Staphylococcus aureus, Gram-negative Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, the phytopathogen Pectobacterium carotovorum VKM-B1247, and the fungi Fusarium oxysporum VKPM F 890, Aspergillus fumigatus VKM F-37, and A. niger ATCC 16404. Genomic and chemical analyses further support P. chrysogenum INA 13460 as a promising natural source for antimicrobial drug discovery and biological control applications.

In silico prediction of polyketide biosynthetic gene clusters in the genomes of Hypericum-borne endophytic fungi

BACKGROUND

The search for new bioactive natural compounds with anticancer activity is still of great importance. Even though their potential for diagnostics and treatment of cancer has already been proved, the availability is still limited. Hypericin, a naphthodianthrone isolated essentially from plant source Hypericum perforatum L. along with other related anthraquinones and bisanthraquinones belongs to this group of compounds. Although it has been proven that hypericin is synthesized by the polyketide pathway in plants, none of the candidate genes coding for key enzymes has been experimentally validated yet. Despite the rare occurrence of anthraquinones in plants, their presence in microorganisms, including endophytic fungi, is quite common. Unlike plants, several biosynthetic genes grouped into clusters (BGCs) in fungal endophytes have already been characterized.

RESULTS

The aim of this work was to predict, identify and characterize the anthraquinone BGCs in de novo assembled and functionally annotated genomes of selected endophytic fungal isolates (Fusarium oxysporum, Plectosphaerella cucumerina, Scedosporium apiospermum, Diaporthe eres, Canariomyces subthermophilus) obtained from different tissues of Hypericum spp. The number of predicted type I polyketide synthase (PKS) BGCs in the studied genomes varied. The non-reducing type I PKS lacking thioesterase domain and adjacent discrete gene encoding protein with product release function were identified only in the genomes of C. subthermophilus and D. eres. A candidate bisanthraquinone BGC was predicted in C. subthermophilus genome and comprised genes coding the enzymes that catalyze formation of the basic anthraquinone skeleton (PKS, metallo-beta-lactamase, decarboxylase, anthrone oxygenase), putative dimerization enzyme (cytochrome P450 monooxygenase), other tailoring enzymes (oxidoreductase, dehydrogenase/reductase), and non-catalytic proteins (fungal transcription factor, transporter protein).

CONCLUSIONS

The results provide an insight into genetic background of anthraquinone biosynthesis in Hypericum-borne endophytes. The predicted bisanthraquinone gene cluster represents a basis for functional validation of the candidate biosynthetic genes in a simple eukaryotic system as a prospective biotechnological alternative for production of hypericin and related bioactive anthraquinones.

Discovery of a Fungal P450 with an Unusual Two-Step Mechanism for Constructing a Bicyclo[3.2.2]nonane Skeleton

The successful biomimetic or chemoenzymatic synthesis of target natural products (NPs) and their derivatives relies on enzyme discovery. Herein, we discover a fungal P450 BTG5 that can catalyze the formation of a bicyclo[3.2.2]nonane structure through an unusual two-step mechanism of dimerization and cyclization in the biosynthesis of beticolin 1, whose bicyclo[3.2.2]nonane skeleton connects an anthraquinone moiety and a xanthone moiety. Further investigation reveals that BTG5-T318 not only determines the substrate selectivity but also alters the catalytic reactions, which allows the separation of the reaction to two individual steps, thereby understanding its catalytic mechanism. It reveals that the first heterodimerization undergoes the common oxidation process for P450s, while the second uncommon formal redox-neutral cyclization step is proved as a redox-mediated reaction, which has never been reported. Therefore, this work advances our understanding of P450-catalyzed reactions and paves the way for expansion of the diversity of this class of NPs through synthetic biology.