genetic methodology in the filamentous fungus Aspergillus
nidulans allowed exchange of the starter unit ACP transacylase
in the nonreduced polyketide synthase (NR-PKS) AfoE of the asperfuranone
pathway with the SAT domains from 10 other NR-PKSs. The newly created
hybrid with the NR-PKS AN3386 is able to accept a longer starter unit
in place of the native substrate to create a novel aromatic polyketide in vivo.
A perplexing aspect of fungal secondary metabolite gene clusters is that most clusters remain ‘silent’ under common laboratory growth conditions where activation is obtained through gene manipulation or encounters with environmental signals. Few proteins have been found involved in repression of silent clusters. Through multicopy suppressor mutagenesis, we have identified a novel cluster suppressor in Aspergillus nidulans, MvlA (modulator of veA
loss). Genetic assessment of MvlA mutants revealed the role of both itself and VeA (but not the VeA partner LaeA) in the suppression of the cryptic ors gene cluster producing orsellinic acid and its F9775 derivatives. Loss of veA upregulates F9775A and F9775B production and this increase is reduced 4~5 fold when an overexpression mvlA (OE::mvlA) allele is introduced into the ΔveA background. Previous studies have implicated a positive role for GcnE (H3K9 acetyltransferase of the SAGA/ADA complex) in ors cluster expression and here we find expression of gcnE is upregulated in ΔveA and suppressed by OE::mvlA in the ΔveA background. H3K9 acetylation levels of ors cluster genes correlated with gcnE expression and F9775 production in ΔveA and OE::mvlAΔveA strains. Finally, deletion of gcnE in the ΔveA background abolishes ors cluster activation and F9775 production. Together, this work supports a role for VeA and MvlA in modifying SAGA/ADA complex activity.
LaeA; Velvet Complex; F9775A; Urc4; suppressor; CclA; SAGA/ADA
Nonribosomal peptides (NRPs) are natural products biosynthesized by NRP synthetases. A kusA-, pyrG- mutant strain of Aspergillus terreus NIH 2624 was developed that greatly facilitated the gene targeting efficiency in this organism. Application of this tool allowed us to link four major types of NRP related secondary metabolites to their responsible genes in A. terreus. In addition, an NRP affecting melanin synthesis was also identified in this species.
Aspernidine A is a prenylated isoindolinone alkaloid isolated from the model fungus Aspergillus nidulans. A genome-wide kinase knock out library of A. nidulans was examined and it was found that a mitogen-activated protein kinase gene, mpkA, deletion strain produces aspernidine A. Targeted gene deletions were performed in the kinase deletion background to identify the gene cluster for aspernidine A biosynthesis. Intermediates were isolated from mutant strains which provided information about the aspernidine A biosynthesis pathway.
Fungal secondary metabolites (SMs) are an important source of medically valuable compounds. Genome projects have revealed that fungi have many SM biosynthetic gene clusters that are not normally expressed. To access these potentially valuable, cryptic clusters, we have developed a heterologous expression system in Aspergillus nidulans. We have developed an efficient system for amplifying genes from a target fungus, placing them under control of a regulatable promoter, transferring them into A. nidulans and expressing them. We have validated this system by expressing non-reducing polyketide synthases of Aspergillus terreus and additional genes required for compound production and release. We have obtained compound production and release from six of these NR-PKSs and have identified the products. To demonstrate that the procedure allows transfer and expression of entire secondary metabolite biosynthetic pathways, we have expressed all the genes of a silent A. terreus cluster and demonstrate that it produces asperfuranone. Further, by expressing the genes of this pathway in various combinations, we have clarified the asperfuranone biosynthetic pathway. We have also developed procedures for deleting entire A. nidulans SM clusters. This allows us to remove clusters that might interfere with analyses of heterologously expressed genes and to eliminate unwanted toxins.
Epipolythiodioxopiperazines (ETPs) are a class of fungal secondary metabolites derived from diketopiperazines. Acetylaranotin belongs to one structural subgroup of ETPs characterized by the presence of a seven-membered 4,5-dihydrooxepine ring. Defining the genes involved in acetylaranotin biosynthesis should provide a means to increase production of these compounds and facilitate the engineering of second-generation molecules. The filamentous fungus Aspergillus terreus produces acetylaranotin and related natural products. Using targeted gene deletions, we have identified a cluster of nine genes including one nonribosomal peptide synthetase gene, ataP, which is required for acetylaranotin biosynthesis. Chemical analysis of the wild type and mutant strains enabled us to isolate seventeen natural products from the acetylaranotin biosynthesis pathway. Nine of the compounds identified in this study are previously not reported natural products. Our data allow us to propose a biosynthetic pathway for acetylaranotin and related natural products.
The gliotoxin, a member of the epipolythiodioxopiperazine (ETP), has received considerable attention from the scientific community for its wide range of biological activity. Despite the identification of gliotoxin cluster, however, the sequence of steps in the gliotoxin biosynthesis has remained elusive. As an alternative to the gene knock out and biochemical approaches used so far, here we report using a heterologous expression approach to determine the sequence of the early steps of gliotoxin biosynthesis in A. nidulans. We identified the GliC, a monooxygenases that involved in the second step of gliotoxin biosynthesis pathway through the catalyzing the hydroxylation at the α-position of l-Phe.
Biosynthesis; Gliotoxin; Heterologous expression
We re-annotated the A. niger NR-PKS gene, e_gw1_19.204 and its downstream R domain gene, est_GWPlus_C_190476 as a single gene which we name dtbA. Heterologous expression of dtbA in A. nidulans demonstrated that DtbA protein produces two polyketides, 2,4-dihydroxy-3,5,6-trimethylbenzaldehyde 1 and 6-ethyl-2,4-dihydroxy-3,5-dimethylbenzaldehyde 2. Generation of DtbAΔR+TE chimeric PKSs by swapping the DtbA R domain with the AusA (austinol biosynthesis) or ANID_06448 TE domain enabled the production of two metabolites with carboxylic acids replacing the corresponding aldehydes.
The eukaryotic basic leucine zipper (bZIP) transcription factors play critical roles in the organismal response to the environment. Recently, a novel YAP-like bZIP, restorer of secondary metabolism A (RsmA), was found in a suppressor screen of an Aspergillus nidulans secondary metabolism (SM) mutant in which overexpression of rsmA was found to partially remediate loss of SM in Velvet Complex mutants. The Velvet Complex is a conserved fungal transcriptional heteromer that couples SM with sexual development in fungi. Here we characterized and contrasted SM in mutants of RsmA and four other A. nidulans bZIP proteins (NapA, ZipA, ZipB and ZipC) with predicted DNA binding motifs similar to RsmA. Only two overexpression mutants exhibited both SM and sexual abnormalities that were noteworthy: OE : : rsmA resulted in a 100-fold increase in sterigmatocystin and a near loss of meiotic spore production. OE : : napA displayed decreased production of sterigmatocystin, emericellin, asperthecin, shamixanthone and epishamixanthone, coupled with a shift from sexual to asexual development. Quantification of bZIP homodimer and heterodimer formation using fluorescence resonance energy transfer (FRET) suggested that these proteins preferentially self-associate.
Meroterpenoids are natural products produced from polyketide and terpenoid precursors. A gene targeting system for A. terreus NIH2624 was developed and a gene cluster for terretonin biosynthesis was characterized. The intermediates and shunt products were isolated from the mutant strains and a pathway for terretonin biosynthesis is proposed. Analysis of two meroterpenoid pathways corresponding to terretonin in A. terreus and austinol in A. nidulans reveals that they are closely related evolutionarily.
Genome sequencing of Aspergillus species including A. nidulans has revealed that there are far more secondary metabolite biosynthetic gene clusters than secondary metabolites isolated from these organisms. This implies that these organisms can produce additional secondary metabolites have not yet been elucidated. The A. nidulans genome contains twelve nonribosomal peptide synthetase (NRPS), one hybrid polyketide synthase/nonribosomal peptide synthetase (PKS/NRPS), and fourteen NRPS-like genes. The only NRPS-like gene in A. nidulans with a known product is tdiA which is involved in terrequinone A biosynthesis. To attempt to identify the products of these NRPS-like genes, we replaced the native promoters of the NRPS-like genes with the inducible alcohol dehydrogenase (alcA) promoter. Our results demonstrated that induction of the single NRPS-like gene AN3396.4 led to the enhanced production of microperfuranone. Furthermore, heterologous expression of AN3396.4 in A. niger confirmed that only one NRPS-like gene, AN3396.4, is necessary for the production of microperfuranone.
Aspergillus nidulans; nonribosomal peptide synthetase-like; microperfuranone; biosynthesis
Regulation of secondary metabolite (SM) gene clusters in Aspergillus nidulans has been shown to occur through cluster specific transcription factors or through global regulators of chromatin structure such as histone methyltransferases, histone deacetylases, or the putative methyltransferase LaeA. A multi-copy suppressor screen for genes capable of returning SM production to the SM deficient ΔlaeA mutant resulted in identification of the essential histone acetyltransferase EsaA, able to complement an esa1 deletion in Saccharomyces cereviseae. Here we report that EsaA plays a novel role in SM cluster activation through histone 4 lysine 12 (H4K12) acetylation in four examined SM gene clusters (sterigmatocystin, penicillin, terrequinone, and orsellinic acid), in contrast to no increase in H4K12 acetylation of the housekeeping tubA promoter. This augmented SM cluster acetylation requires LaeA for full effect and correlates with both increased transcript levels and metabolite production relative to wild type. H4K12 levels may thus represent a unique indicator of relative production potential, notably of SMs.
Norsolorinic acid, isolated from the Aspergillus nidulans, was investigated for its anti-proliferative activity in human breast adenocarcinoma MCF-7 cells. To identity the anticancer mechanism of norsolorinic acid, we assayed its effect on apoptosis, cell cycle distribution, and levels of p53, p21/WAF1, Fas/APO-1 receptor, and Fas ligand. The results showed that norsolorinic acid induced apoptosis of MCF-7 cells without mediation of p53 and p21/WAF1. We suggest that Fas/Fas ligand apoptotic system is the main pathway of norsolorinic acid-mediated apoptosis of MCF-7 cells. Our study reports here for the first time that the activity of the Fas/Fas ligand apoptotic system may participate in the anti-proliferative activity of norsolorinic acid in MCF-7 cells.
norsolorinic acid; breast cancer; p53; Fas/APO-1; Fas ligand; apoptosis
Genome sequencing has revealed that fungi have the ability to synthesize many more natural products (NPs) than are currently known, but methods for obtaining suitable expression of NPs have been inadequate. We have developed a successful strategy that bypasses normal regulatory mechanisms. By efficient gene targeting, we have replaced, en masse, the promoters of non-reducing polyketide synthase (NR-PKS) genes, key genes in NP biosynthetic pathways and other genes necessary for NR-PKS product formation or release. This has allowed us to determine the products of eight NR-PKSs of A. nidulans, including seven novel compounds, as well as the NR-PKS genes required for the synthesis of the toxins, alternariol (8) and cichorine (19).
Hepatic stellate cells (HSCs) undergo myofibroblastic trans-differentiation (activation) to participate in liver fibrosis, and identification of molecular targets for this cell fate regulation is essential for development of efficacious therapeutic modalities for the disease. Peroxisomal proliferator-activated receptor γ (PPARγ) is required for differentiation of HSCs and its epigenetic repression underlies HSC activation. The herbal prescription Yang-Gan-Wan (YGW) prevents liver fibrosis, but its active ingredients and molecular mechanisms are unknown. Here we demonstrate YGW prevents and reverses HSC activation via epigenetic de-repression of Pparγ involving reductions in MeCP2 expression and its recruitment to Pparγ promoter, suppressed expression of PRC2 methyltrasferase EZH2 and consequent reduction of H2K27di-methylation at the 3’ exon. HPLC/MS and NMR analyses identify polyphenolic rosmarinic acid (RA) and baicalin (BC) as active phytocompounds. RA and BC suppress the expression and signaling by canonical Wnts, which are implicated in the aforementioned Pparγ epigenetic repression. RA treatment in mice with existing cholestatic liver fibrosis inhibits HSC activation and progression of liver fibrosis. In conclusion, these results demonstrate a therapeutic potential of YGW and its active component RA and BC for liver fibrosis via Pparγ de-repression mediated by suppression of canonical Wnt signaling in HSCs.
liver fibrosis; MeCP2; EZH2; Wnt; H3K27-methylation
Meroterpenoids are a class of fungal natural products that are produced from polyketide and terpenoid precursors. An understanding of meroterpenoid biosynthesis at the genetic level should facilitate engineering of second-generation molecules and increasing production of first-generation compounds. The filamentous fungus Aspergillus nidulans has previously been found to produce two meroterpenoids, austinol and dehydroaustinol. Using targeted deletions that we created, we have determined that, surprisingly, two separate gene clusters are required for meroterpenoid biosynthesis. One is a cluster of four genes including a polyketide synthase gene, ausA. The second is a cluster of ten additional genes including a prenyltransferase gene, ausN, located on a separate chromosome. Chemical analysis of mutant extracts enabled us to isolate 3,5-dimethylorsellinic acid and ten additional meroterpenoids that are either intermediates or shunt products from the biosynthetic pathway. Six of them were identified as novel meroterpenoids in this study. Our data, in aggregate, allow us to propose a complete biosynthetic pathway for the A. nidulans meroterpenoids.
The eukaryotic bZIP transcription factors are critical players in organismal response to environmental challenges. In fungi, the production of secondary metabolites (SMs) is hypothesized as one of the responses to environmental insults, e.g. attack by fungivorous insects, yet little data to support this hypothesis exists. Here we establish a mechanism of bZIP regulation of SMs through RsmA, a recently discovered YAP-like bZIP protein. RsmA greatly increases SM production by binding to two sites in the A. nidulans AflR promoter region, a C6 transcription factor known for activating production of the carcinogenic and anti-predation SM, sterigmatocystin (ST). Deletion of aflR in an overexpression rsmA (OE::rsmA) background not only eliminates ST production but also significantly reduces asperthecin synthesis. Furthermore, the fungivore, Folsomia candida, exhibited a distinct preference for feeding on wild type rather than an OE::rsmA strain. RsmA may thus have a critical function in mediating direct chemical resistance against predation. Taken together, these results suggest RsmA represents a bZIP pathway hardwired for defensive SM production.
sterigmatocystin; asperthecin; fungivory; Yap; rsmA
Sclerotiorin, an azaphilone polyketide, is a bioactive natural product known to inhibit 15-lipoxygenase and many other biological targets. To readily access sclerotiorin and analogs, we developed a 2–3 step semisynthetic route to produce a variety of azaphilones starting from an advanced, putative azaphilone intermediate (5) over-produced by an engineered strain of Aspergillus nidulans. The inhibitory activities of the semisynthetic azaphilones against 15-lipoxygenase were evaluated with several compounds displaying low micromolar potency.
A StcA-AfoE hybrid PKS, generated from swapping the AfoE (asperfuranone biosynthesis) SAT domain with the StcA (sterigmatocystin biosynthesis) SAT domian, produced a major new metabolite with the same chain length as the native AfoE product. Structure elucidation allowed us to propose a likely pathway and feeding studies supported the hypothesis that the chain length of PKS metabolites may be under precise control of KS and PT domains.
The genome sequencing of the fungus Aspergillus niger uncovered a large cache of genes encoding enzymes thought to be involved in the production of secondary metabolites yet to be identified. Identification and structural characterization of many of these predicted secondary metabolites are hampered by their low concentration relative to the known A. niger metabolites such as the naphtho-γ-pyrone family of polyketides. We deleted a nonreducing PKS gene in A. niger strain ATCC 11414, a daughter strain of A. niger ATCC strain 1015 whose genome was sequenced by the DOE Joint Genome Institute. This PKS encoding gene we name albA is a predicted ortholog of alb1 from Aspergillus fumigatus which is responsible for production of the naphtho-γ-pyrone precursor for the 1,8-dihydroxynaphthalene (DHN) melanin/spore pigment. Our results show that the A. nigeralbA PKS is responsible for both the production of the spore pigment precursor and a family of naphtho-γ-pyrones commonly found in significant quantity in A. niger culture extracts. The generation of an A. niger strain devoid of naphtho-γ-pyrones will greatly facilitate the elucidation of cryptic biosynthetic pathways in this organism.
Secondary Metabolism; Aspergillus niger; Natural Products; Genomics; Naphtho-γ-pyrone; Polyketides
Xanthones are a class of molecules that bind to a number of drug targets and possess a myriad of biological properties. An understanding of xanthone biosynthesis at the genetic level should facilitate engineering of second-generation molecules and increasing production of first-generation compounds. The filamentous fungus Aspergillus nidulans has been found to produce two prenylated xanthones, shamixanthone and emericellin, and we report the discovery of two more, variecoxanthone A and epishamixanthone. Using targeted deletions that we created, we determined that a cluster of 10 genes including a polyketide synthase gene, mdpG, is required for prenyl xanthone biosynthesis. mdpG was shown to be required for the synthesis of the anthraquinone emodin, monodictyphenone, and related compounds, and our data indicate that emodin and monodictyphenone are precursors of prenyl xanthones. Isolation of intermediate compounds from the deletion strains provided valuable clues as to the biosynthetic pathway, but no genes accounting for the prenylations were located within the cluster. To find the genes responsible for prenylation, we identified and deleted seven putative prenyltransferases in the A. nidulans genome. We found that two prenyltransferase genes, distant from the cluster, were necessary for prenyl xanthone synthesis. These genes belong to the fungal indole prenyltransferase family that had previously been shown to be responsible for the prenylation of amino acid derivatives. In addition, another prenyl xanthone biosynthesis gene is proximal to one of the prenyltransferase genes. Our data, in aggregate, allow us to propose a complete biosynthetic pathway for the A. nidulans xanthones.
Secondary metabolites from microorganisms have a broad spectrum of applications, particularly in therapeutics. The growing number of sequenced microbial genomes has revealed a remarkably large number of natural product biosynthetic clusters for which the products are still unknown. These cryptic clusters are potentially a treasure house of medically useful compounds. The recent development of new methodologies has made it possible to begin unlock this treasure house, to discover new natural products and determine their biosynthesis pathways. This review will highlight some of the most recent strategies to activate silent biosynthetic gene clusters and to elucidate of their corresponding products and pathways.
Asperfuranone, a novel compound of genomic mining in Aspergillus nidulans, was investigated for its anti-proliferative activity in human non-small cell lung cancer A549 cells. To identity the anti-cancer mechanism of asperfuranone, we assayed its effect on apoptosis, cell cycle distribution, and levels of p53, p21 Waf1/Cip1, Fas/APO-1 receptor and Fas ligand. Enzyme-linked immunosorbent assay showed that the G0/G1 phase arrest might be due to p53-dependent induction of p21 Waf1/Cip1. An enhancement in Fas/APO-1 and its two form ligands, membrane-bound Fas ligand (mFasL) and soluble Fas ligand (sFasL), might be responsible for the apoptotic effect induced by asperfuranone. Our study reports here for the first time that the induction of p53 and the activity of Fas/Fas ligand apoptotic system may participate in the anti-proliferative activity of asperfuranone in A549 cells.
Aspergillus species have the ability to produce a wide range of secondary metabolites including polyketides that are generated by multi-domain polyketide synthases (PKSs). Recent biochemical studies using dissected single or multiple domains from PKSs have provided deep insight into how these PKSs control the structural outcome. Moreover, the recent genome sequencing of several species has greatly facilitated the understanding of the biosynthetic pathways for these secondary metabolites. In this review, we will highlight the current knowledge regarding polyketide biosynthesis in Aspergillus based on the domain architecture of non-reducing, highly reducing, and partially reducing PKSs, and PKS-non-ribosomal peptide synthetases.
Secondary metabolites; Fungi; Polyketide; Nonribosomal peptides
Recent published sequencing of fungal genomes has revealed that these microorganisms have a surprisingly large number of secondary metabolite pathways that can serve as potential sources for new and useful natural products. Most of the secondary metabolites and their biosynthesis pathways are currently unknown, possibly because they are produced in very small amounts and are thus difficult to detect or are produced only under specific conditions. Elucidating these fungal metabolites will require new molecular genetic tools, better understanding of the regulation of secondary metabolism, and state of the art analytical methods. This review describes recent strategies to mine the cryptic natural products and their biosynthetic pathways in fungi.
natural products biosynthesis; genomic mining; polyketide synthase; nonribosomal peptide synthetase