7,8- and 5,8-Linoleate diol synthases support the heterolytic scission of oxygen-oxygen bonds by different amide residues

Fatty acid dioxygenase-cytochrome P450 fusion enzymes of filamentous fungal pathogens

Oxylipins designate oxygenated unsaturated C₁₈ fatty acids. Many filamentous fungi pathogens contain dioxygenases (DOX) in oxylipin biosynthesis with homology to human cyclooxygenases. They contain a DOX domain, which is often fused to a functional cytochrome P450 at the C-terminal end. A Tyr radical in the DOX domain initiates dioxygenation of linoleic acid by hydrogen abstraction with formation of 8-, 9-, or 10-hydroperoxy metabolites. The P450 domains can catalyze heterolytic cleavage of 8- and 10-hydroperoxides with oxidation of the heme thiolate iron for hydroxylation at C-5, C-7, C-9, or C-11 and for epoxidation of the 12Z double bond; thus displaying linoleate diol synthase (LDS) and epoxy alcohol synthase (EAS) activities. LSD activities are present in the rice blast pathogen Magnaporthe oryzae, Botrytis cinerea causing grey mold and the black scurf pathogen Rhizoctonia solani. 10R-DOX-EAS has been found in M. oryzae and Fusarium oxysporum. The P450 domains may also catalyze homolytic cleavage of 8- and 9-hydroperoxy fatty acids and dehydration to produce epoxides with an adjacent double bond, i.e., allene oxides, thus displaying 8- and 9-DOX-allene oxide synthases (AOS). F. oxysporum, F. graminearum, and R. solani express 9S-DOX-AOS and Zymoseptoria tritici 8S-and 9R-DOX-AOS. Homologues are present in endemic human-pathogenic fungi with extensive studies in Aspergillus fumigatus, A. flavus (also a plant pathogen) as well as the genetic model A. nidulans. 8R-and 10R-DOX appear to bind fatty acids "headfirst" in the active site, whereas 9S-DOX binds them "tail first" in analogy with cyclooxygenases. The biological relevance of 8R-DOX-5,8-LDS (also designated PpoA) was first discovered in relation to sporulation of A. nidulans and recently for development and programmed hyphal branching of A. fumigatus. Gene deletion DOX-AOS homologues in F. verticillioides, A. flavus, and A. nidulans alters, inter alia, mycotoxin production, sporulation, and gene expression.

WITHDRAWN: Fatty acid dioxygenase-cytochrome P450 fusion enzymes of the top 10 fungal pathogens in molecular plant pathology and human-pathogenic fungi

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Transcriptome of different fruiting stages in the cultivated mushroom Cyclocybe aegerita suggests a complex regulation of fruiting and reveals enzymes putatively involved in fungal oxylipin biosynthesis

BACKGROUND

Cyclocybe aegerita (syn. Agrocybe aegerita) is a commercially cultivated mushroom. Its archetypal agaric morphology and its ability to undergo its whole life cycle under laboratory conditions makes this fungus a well-suited model for studying fruiting body (basidiome, basidiocarp) development. To elucidate the so far barely understood biosynthesis of fungal volatiles, alterations in the transcriptome during different developmental stages of C. aegerita were analyzed and combined with changes in the volatile profile during its different fruiting stages.

RESULTS

A transcriptomic study at seven points in time during fruiting body development of C. aegerita with seven mycelial and five fruiting body stages was conducted. Differential gene expression was observed for genes involved in fungal fruiting body formation showing interesting transcriptional patterns and correlations of these fruiting-related genes with the developmental stages. Combining transcriptome and volatilome data, enzymes putatively involved in the biosynthesis of C8 oxylipins in C. aegerita including lipoxygenases (LOXs), dioxygenases (DOXs), hydroperoxide lyases (HPLs), alcohol dehydrogenases (ADHs) and ene-reductases could be identified. Furthermore, we were able to localize the mycelium as the main source for sesquiterpenes predominant during sporulation in the headspace of C. aegerita cultures. In contrast, changes in the C8 profile detected in late stages of development are probably due to the activity of enzymes located in the fruiting bodies.

CONCLUSIONS

In this study, the combination of volatilome and transcriptome data of C. aegerita revealed interesting candidates both for functional genetics-based analysis of fruiting-related genes and for prospective enzyme characterization studies to further elucidate the so far barely understood biosynthesis of fungal C8 oxylipins.

Molecular characterization of Penicillium oxalicum 6R,8R-linoleate diol synthase with new regiospecificity

Diol synthase-derived metabolites are involved in the sexual and asexual life cycles of fungi. A putative diol synthase from Penicillium oxalicum was found to convert palmitoleic acid (16:1n-7), oleic acid (18:1n-9), linoleic acid (18:2n-6), and α-linolenic acid (18:3n-3) to 6S,8R-dihydroxy-9(Z)-hexadecenoic acid, 6R,8R-dihydroxy-9(Z)-octadecenoic acid, 6R,8R-dihydroxy-9,12(Z,Z)-octadecadienoic acid, and 6S,8R-dihydroxy-9,12,15(Z,Z,Z)-octadecatrienoic acid, respectively, which were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and nuclear magnetic resonance (NMR) spectroscopy analyses. The specific activity and catalytic efficiency of P. oxalicum 6,8-diol synthase were the highest for 18:2n-6, indicating that the enzyme is a 6R,8R-linoleate diol synthase (6R,8R-LDS) with new regiospecificity. This is the first report of a 6R,8R-LDS. LDS is a fusion protein consisting of a dioxygenase domain at the N-terminus and a cytochrome P450/hydroperoxide isomerase (P450/HPI) domain at the C-terminus. The putative active-site residues in the C-terminal domain of P. oxalicum 6R,8R-LDS were proposed based on a substrate-docking homology model. The results of the site-directed mutagenesis within C-terminal P450 domain suggested that Asn⁸⁸⁶, Arg⁷⁰⁷, and Arg⁹³⁴, are catalytic importance and belong to the catalytic groove. Phe⁷⁹⁴ and Gln⁸⁸⁹ were found to be involved in the regiospecific rearrangement of hydroperoxide, while the F794E and Q889A variants of P. oxalicum 6,8-LDS acted as 7,8- and 8,11-LDSs, respectively. All these mutations critically affected the HPI activity of P. oxalicum 6R,8R-LDS.

Biosynthesis of Oxylipins by Rhizoctonia solani with Allene Oxide and Oleate 8S,9S-Diol Synthase Activities

Oxylipin biosynthesis by fungi is catalyzed by both the lipoxygenase (LOX) family and the linoleate diol synthase (LDS) family of the peroxidase-cyclooxygenase superfamily. Rhizoctonia solani, a pathogenic fungus, infects staple crops such as potato and rice. The genome predicts three genes with 9-13 introns, which code for tentative dioxygenase (DOX)-cytochrome P450 fusion enzymes of the LDS family, and one gene, which might code for a 13-LOX. The objective was to determine whether mycelia or nitrogen powder of mycelia oxidized unsaturated C₁₈ fatty acids to LDS- or LOX-related metabolites. Mycelia converted 18:2n-6 to 8R-hydroxy-9Z,12Z-octadecadienoic acid and to an α-ketol, 9S-hydroxy-10-oxo-12Z-octadecenoic acid. In addition to these metabolites, nitrogen powder of mycelia oxidized 18:2n-6 to 9S-hydroperoxy-10E, 12Z-octadecadienoic, and 13S-hydroperoxy-9Z,11E-octadecadienoic acids; the latter was likely formed by the predicted 13-LOX. 18:1n-9 was transformed into 8S-hydroperoxy-9Z-octadecenoic and into 8S,9S-dihydroxy-10E-octadecenoic acids, indicating the expression of 8,9-diol synthase. The allene oxide, 9S(10)epoxy-10,12Z-octadecadienoic acid, is unstable and decomposes rapidly to the α-ketol above, indicating biosynthesis by 9S-DOX-allene oxide synthase. This allene oxide and α-ketol are also formed by potato stolons, which illustrates catalytic similarities between the plant host and fungal pathogen.

Linoleic acid isomerase gene FgLAI12 affects sensitivity to salicylic acid, mycelial growth and virulence of Fusarium graminearum

Fusarium graminearum is the major causal agent of fusarium head blight in wheat, a serious disease worldwide. Linoleic acid isomerase (LAI) catalyses the transformation of linoleic acid (LA) to conjugated linoleic acid (CLA), which is beneficial for human health. We characterised a cis-12 LAI gene of F. graminearum (FGSG_02668; FgLAI12), which was downregulated by salicylic acid (SA), a plant defence hormone. Disruption of FgLAI12 in F. graminearum resulted in decreased accumulation of cis-9,trans-11 CLA, enhanced sensitivity to SA, and increased accumulation of LA and SA in wheat spikes during infection. In addition, mycelial growth, accumulation of deoxynivalenol, and pathogenicity in wheat spikes were reduced. Re-introduction of a functional FgLAI12 gene into ΔFgLAI12 recovered the wild-type phenotype. Fluorescent microscopic analysis showed that FgLAI12 protein was usually expressed in the septa zone of conidia and the vacuole of hyphae, but was expressed in the cell membrane of hyphae in response to exogenous LA, which may be an element of LA metabolism during infection by F. graminearum. The cis-12 LAI enzyme encoded by FgLAI12 is critical for fungal response to SA, mycelial growth and virulence in wheat. The gene FgLAI12 is potentially valuable for biotechnological synthesis of cis-9,trans-11 CLA.

A new class of fatty acid allene oxide formed by the DOX-P450 fusion proteins of human and plant pathogenic fungi, C. immitis and Z. tritici

Linoleate dioxygenase-cytochrome P450 (DOX-CYP) fusion enzymes are common in pathogenic fungi. The DOX domains form hydroperoxy metabolites of 18:2n-6, which can be transformed by the CYP domains to 1,2- or 1,4-diols, epoxy alcohols, or to allene oxides. We have characterized two novel allene oxide synthases (AOSs), namely, recombinant 8R-DOX-AOS of Coccidioides immitis (causing valley fever) and 8S-DOX-AOS of Zymoseptoria tritici (causing septoria tritici blotch of wheat). The 8R-DOX-AOS oxidized 18:2n-6 sequentially to 8R-hydroperoxy-9Z,12Z-octadecadienoic acid (8R-HPODE) and to an allene oxide, 8R(9)-epoxy-9,12Z-octadecadienoic acid, as judged from the accumulation of the α-ketol, 8S-hydroxy-9-oxo-12Z-octadecenoic acid. The 8S-DOX-AOS of Z. tritici transformed 18:2n-6 sequentially to 8S-HPODE and to an α-ketol, 8R-hydroxy-9-oxo-12Z-octadecenoic acid, likely formed by hydrolysis of 8S(9)-epoxy-9,12Z-octadecadienoic acid. The 8S-DOX-AOS oxidized [8R-(2)H]18:2n-6 to 8S-HPODE with retention of the (2)H-label, suggesting suprafacial hydrogen abstraction and oxygenation in contrast to 8R-DOX-AOS. Both enzymes oxidized 18:1n-9 and 18:3n-3 to α-ketols, but the catalysis of the 8R- and 8S-AOS domains differed. 8R-DOX-AOS transformed 9R-HPODE to epoxy alcohols, but 8S-DOX-AOS converted 9S-HPODE to an α-ketol (9-hydroxy-10-oxo-12Z-octadecenoic acid) and epoxy alcohols in a ratio of ∼1:2. Whereas all fatty acid allene oxides described so far have a conjugated diene impinging on the epoxide, the allene oxides formed by 8-DOX-AOS are unconjugated.

Characterization of a novel 8R,11S-linoleate diol synthase from Penicillium chrysogenum by identification of its enzymatic products

To identify novel fatty acid diol synthases, putative candidate sequences from Penicillium species were analyzed, and hydroxy fatty acid production by crude Penicillium enzyme extracts was assessed. Penicillium chrysogenum was found to produce an unknown dihydroxy fatty acid, a candidate gene implicated in this production was cloned and expressed, and the expressed enzyme was purified. The product obtained by the reaction of the purified enzyme with linoleic acid was identified as 8R,11S-dihydroxy-9,12(Z,Z)-octadecadienoic acid (8R,11S-DiHODE). The catalytic efficiency of this enzyme toward linoleic acid was the highest among the unsaturated fatty acids tested, indicating that this enzyme was a novel 8R,11S-linoleate diol synthase (8R,11S-LDS). A sexual stage in the life cycle of P. chrysogenum has recently been discovered, and 8R,11S-DiHODE produced by 8R,11S-LDS may constitute a precocious sexual inducer factor, responsible for regulating the sexual and asexual cycles of this fungus.

Discovery of a Novel Linoleate Dioxygenase of Fusarium oxysporum and Linoleate Diol Synthase of Colletotrichum graminicola

Fungal pathogens constitute serious threats for many forms of life. The pathogenic fungi Fusarium and Colletotrichum and their formae speciales (f. spp.) infect many types of crops with severe consequences and Fusarium oxysporum can also induce keratitis and allergic conditions in humans. These fungi code for homologues of dioxygenase-cytochrome P450 (DOX-CYP) fusion proteins of the animal heme peroxidase (cyclooxygenase) superfamily. The objective was to characterize the enzymatic activities of the DOX-CYP homologue of Colletotrichum graminicola (EFQ34869) and the DOX homologue of F. oxysporum (EGU79548). The former oxidized oleic and linoleic acids in analogy with 7,8-linoleate diol synthases (LDSs), but with the additional biosynthesis of 8,11-dihydroxylinoleic acid. The latter metabolized fatty acids to hydroperoxides with broad substrate specificity. It oxidized 20:4n-6 and 18:2n-6 to hydroperoxides with an R configuration at the (n-10) positions, and other n-6 fatty acids in the same way. [11S-(2)H]18:2n-6 was oxidized with retention and [11R-(2)H]18:2n-6 with loss of deuterium, suggesting suprafacial hydrogen abstraction and oxygen insertion. Fatty acids of the n-3 series were oxidized less efficiently and often to hydroperoxides with an R configuration at both (n-10) and (n-7) positions. The enzyme spans 1426 amino acids with about 825 residues in the N-terminal domain with DOX homology and 600 residues at the C-terminal domain without homology to other enzymes. We conclude that fungal oxylipins can be formed by two novel subfamilies of cyclooxygenase-related DOX.

Epoxy alcohol synthase of the rice blast fungus represents a novel subfamily of dioxygenase-cytochrome P450 fusion enzymes

The genome of the rice blast fungus Magnaporthe oryzae codes for two proteins with N-terminal dioxygenase (DOX) and C-terminal cytochrome P450 (CYP) domains, respectively. One of them, MGG_13239, was confirmed as 7,8-linoleate diol synthase by prokaryotic expression. The other recombinant protein (MGG_10859) possessed prominent 10R-DOX and epoxy alcohol synthase (EAS) activities. This enzyme, 10R-DOX-EAS, transformed 18:2n-6 sequentially to 10(R)-hydroperoxy-8(E),12(Z)-octadecadienoic acid (10R-HPODE) and to 12S(13R)-epoxy-10(R)-hydroxy-8(E)-octadecenoic acid as the end product. Oxygenation at C-10 occurred by retention of the pro-R hydrogen of C-8 of 18:2n-6, suggesting antarafacial hydrogen abstraction and oxygenation. Experiments with (18)O2 and (16)O2 gas confirmed that the epoxy alcohol was formed from 10R-HPODE, likely by heterolytic cleavage of the dioxygen bond with formation of P450 compound I, and subsequent intramolecular epoxidation of the 12(Z) double bond. Site-directed mutagenesis demonstrated that the cysteinyl heme ligand of the P450 domain was required for the EAS activity. Replacement of Asn(965) with Val in the conserved AsnGlnXaaGln sequence revealed that Asn(965) supported formation of the epoxy alcohol. 10R-DOX-EAS is the first member of a novel subfamily of DOX-CYP fusion proteins of devastating plant pathogens.

Discovery of a linoleate 9S-dioxygenase and an allene oxide synthase in a fusion protein of Fusarium oxysporum

Fusarium oxysporum is a devastating plant pathogen that oxidizes C₁₈ fatty acids sequentially to jasmonates. The genome codes for putative dioxygenase (DOX)-cytochrome P450 (CYP) fusion proteins homologous to linoleate diol synthases (LDSs) and the allene oxide synthase (AOS) of Aspergillus terreus, e.g., FOXB_01332. Recombinant FOXB_01332 oxidized 18:2n-6 to 9S-hydroperoxy-10(E),12(Z)-octadecadienoic acid by hydrogen abstraction and antarafacial insertion of molecular oxygen and sequentially to an allene oxide, 9S(10)-epoxy-10,12(Z)-octadecadienoic acid, as judged from nonenzymatic hydrolysis products (α- and γ-ketols). The enzyme was therefore designated 9S-DOX-AOS. The 9S-DOX activity oxidized C₁₈ and C₂₀ fatty acids of the n-6 and n-3 series to hydroperoxides at the n-9 and n-7 positions, and the n-9 hydroperoxides could be sequentially transformed to allene oxides with only a few exceptions. The AOS activity was stereospecific for 9- and 11-hydroperoxides with S configurations. FOXB_01332 has acidic and alcoholic residues, Glu⁹⁴⁶-Val-Leu-Ser⁹⁴⁹, at positions of crucial Asn and Gln residues (Asn-Xaa-Xaa-Gln) of the AOS and LDS. Site-directed mutagenesis studies revealed that FOXB_01332 and AOS of A. terreus differ in catalytically important residues suggesting that AOS of A. terreus and F. oxysporum belong to different subfamilies. FOXB_01332 is the first linoleate 9-DOX with homology to animal heme peroxidases and the first 9-DOX-AOS fusion protein.