Bioinformatics-Guided Reconstitution of Biosynthetic Machineries of Fungal Eremophilane Sesquiterpenes.

Eremophilanes exhibit diverse biological activities and chemical structures. This study reports the bioinformatics-guided reconstitution of the biosynthetic machinery of fungal eremophilanes, eremofortin C and sporogen-AO1, to elucidate their biosynthetic pathways. Their biosyntheses include P450-catalyzed multistep oxidation and enzyme-catalyzed isomerization by the DUF3237 family protein. Successful characterization of six P450s enabled us to discuss the functions of eremophilane P450s in putative eremophilane biosynthetic gene clusters, providing opportunities to understand the oxidative modification pathways of fungal eremophilanes.

A fungal sesquiterpene biosynthesis gene cluster critical for mutualist-pathogen transition in Colletotrichum tofieldiae.

Plant-associated fungi show diverse lifestyles from pathogenic to mutualistic to the host; however, the principles and mechanisms through which they shift the lifestyles require elucidation. The root fungus Colletotrichum tofieldiae (Ct) promotes Arabidopsis thaliana growth under phosphate limiting conditions. Here we describe a Ct strain, designated Ct3, that severely inhibits plant growth. Ct3 pathogenesis occurs through activation of host abscisic acid pathways via a fungal secondary metabolism gene cluster related to the biosynthesis of sesquiterpene metabolites, including botrydial. Cluster activation during root infection suppresses host nutrient uptake-related genes and changes mineral contents, suggesting a role in manipulating host nutrition state. Conversely, disruption or environmental suppression of the cluster renders Ct3 beneficial for plant growth, in a manner dependent on host phosphate starvation response regulators. Our findings indicate that a fungal metabolism cluster provides a means by which infectious fungi modulate lifestyles along the parasitic-mutualistic continuum in fluctuating environments.

Elucidation of Late-Stage Biosynthesis of Phomoidride: Proposal of Cyclization Mechanism Affording Characteristic Nine-Membered Ring of Fungal Dimeric Anhydride.

Antihypercholesterolemic agent phomoidride (PMD) B has a highly elaborated bicyclo[4.3.1]deca-1,6-diene core scaffold derived from dimeric anhydride with a nine-membered ring. This report elucidated the late stage transformation from an anhydride monomer to PMD B through the heterologous expression of three enzyme genes, TstC, TstK, and TstE. Additional in vitro studies of TstK and TstE provided evidence on the formation of PMD via dimerization, three-step oxidation, and unusual methylation-triggered bicyclic ketal formation. Elucidation of the function of cyclase TstC prompts us to examine the cyclization mechanism of TstC by using a computational approach. Computational analytical data on PMD and structurally related glaucanic acid indicated that the initial decarboxylation of monomer results in enolate and subsequent double Michael reactions of another monomer, followed by an optional aldol reaction proceeding in an endo-selective manner to give cycloadducts, supporting the fact that the starting orientation of two monomers is directly transferred to the product configurations.

Heterologous expression of a polyketide synthase ACRTS2 in Aspergillus oryzae produces host-selective ACR toxins: coproduction of minor metabolites.

Previously, we succeeded to produce the core structure of the host-selective ACR toxin (1) on brown leaf spot on rough lemon when the polyketide synthase ACRTS2 gene was heterologously expressed in Aspergillus oryzae (AO). To confirm the production of 1 in AO, the detection limit and suppressing decarboxylation were improved, and these efforts led us to conclude the direct production of 1 instead of its decarboxylation product. During this examination, minor ACR-toxin-related metabolites were found. Their structure determination enabled us to propose a decarboxylation mechanism and a novel branching route forming byproducts from the coupling of the dihydropyrone moiety of 1 with the acetaldehyde and kojic acid abundant in AO. The involvement of putative cyclase ACRTS3 in the chain release of linear polyketide was excluded by the coexpression analysis of ACRTS2 and ACRTS3. Taken together, we concluded that the production of 1 in AO is solely responsible for ACRTS2.

Biochemistry-Guided Prediction of the Absolute Configuration of Fungal Reduced Polyketides.

Highly reducing polyketide synthases (HR-PKSs) produce structurally diverse polyketides (PKs). The PK diversity is constructed by a variety of factors, including the ß-keto processing, chain length, methylation pattern, and relative and absolute configurations of the substituents. We examined the stereochemical course of the PK processing for the synthesis of polyhydroxy PKs such as phialotides, phomenoic acid, and ACR-toxin. Heterologous expression of a HR-PKS gene, a trans-acting enoylreductase gene, and a truncated non-ribosomal peptide synthetase gene resulted in the formation of a linear PK with multiple stereogenic centers. The absolute configurations of the stereogenic centers were determined by chemical degradation followed by comparison of the degradation products with synthetic standards. A stereochemical rule was proposed to explain the absolute configurations of other reduced PKs and highlights an error in the absolute configurations of a reported structure. The present work demonstrates that focused functional analysis of functionally related HR-PKSs leads to a better understanding of the stereochemical course.

Elucidation of biosynthetic pathway of a plant hormone abscisic acid in phytopathogenic fungi.

Abscisic acid (ABA) is one of the plant hormones that regulates physiological functions in various organisms, including plants, sponges, and humans. The biosynthetic machinery in plants is firmly established, while that in fungi is still unclear. Here, we elucidated the functions of the four biosynthetic genes, bcABA1-bcABA4, found in Botrytis cinerea by performing biotransformation experiments and in vitro enzymatic reactions with putative biosynthetic intermediates. The first-committed step is the cyclization of farnesyl diphosphate to give α-ionylideneethane catalyzed by a novel sesquiterpene synthase, BcABA3, which exhibits low amino acid sequence identities with sesquiterpene synthases. Subsequently, two cytochrome P450s, BcABA1 and BcABA2, mediate oxidative modifications of the cyclized product to afford 1',4'-trans-dihydroxy-α-ionylideneacetic acid, which undergoes alcohol oxidation to furnish ABA. Our results demonstrated that production of ABA does not depend on the nucleotide sequence of bcABA genes. The present study set the stage to investigate the role of ABA in infections.

Unveiling Biosynthesis of the Phytohormone Abscisic Acid in Fungi: Unprecedented Mechanism of Core Scaffold Formation Catalyzed by an Unusual Sesquiterpene Synthase.

Abscisic acid (ABA) is a well-known phytohormone that regulates abiotic stresses. ABA produced by fungi is also proposed to be a virulence factor of fungal pathogens. Although its biosynthetic pathway in fungi was proposed by a series of feeding experiments, the enzyme catalyzing the reaction from farnesyl diphosphate to α-ionylideneethane remains to be identified. In this work, we identified the novel type of sesquiterpene synthase BcABA3 and its unprecedented three-step reaction mechanism involving two neutral intermediates, ß-farnesene and allofarnesene. Database searches showed that BcABA3 has no homology with typical sesquiterpene synthases and that the homologous enzyme genes are found in more than 100 bacteria, suggesting that these enzymes form a new family of sesquiterpene synthases.