Cytotoxic Pheofungins from an Engineered Fungus Impaired in Posttranslational Protein Modification

Cu-Catalyzed Three-Component Cascade Synthesis of 1,3-Benzothiazines from ortho-Aminohydrazones and Bromodifluoroacetamides

An efficient and convenient synthesis of benzo[d][1,3]thiazine has been developed by employing a copper-catalyzed transformation of readily available ketone-derived hydrazones with elemental sulfur and bromodifluoroalkylative reagents. The strategy involves an S₈-catalyzed selective triple-cleavage of bromodifluoroacetamides, which acts as a C1 synthon at the 2-position of benzo[d][1,3]thiazine. A mechanism proceeding through a Cu-carbene intermediate is proposed for the C-S bond formation.

Efficient synthesis of 4-sulfanylcoumarins from 3-bromo-coumarins via a highly selective DABCO-mediated one-pot thia-Michael addition/elimination process

A facile and efficient protocol for the highly selective direct sulfanylation of 3-bromocoumarins under DABCO promotion, was developed. The transformation took place with aromatic and aliphatic thiols as well as with α,ω-dithiols, affording the expected products in very good to excellent yields. Simple and convenient ways to access 4-((ω-mercaptoalkyl) thio)coumarins and the dimeric 4,4′-(alkane-1,4-diylbis(sulfanediyl))bis(coumarins) were also devised with the use of α,ω-alkanedithiols in different ratios with regards to the starting 3-bromocoumarin. The transformation seems to proceed through the DABCO-mediated thia-Michael stereoselective addition of the thiolate anion to the α,β-unsaturated carbonyl system of the coumarin, followed by a DABCO-assisted stereoselective dehydrobromination of the resulting α-bromo carbonyl intermediate.

A facile, simple and metal-free protocol for the 4-sulfanylation of 3-bromocoumarins was developed. It involves a thia-Michael addition and a dehydrobromination under DABCO assistance.

Harnessing diverse transcriptional regulators for natural product discovery in fungi

This review covers diverse transcriptional regulators for the activation of secondary metabolism and novel natural product discovery in fungi.

On water: iodine-mediated direct construction of 1,3-benzothiazines from ortho-alkynylanilines by regioselective 6-exo-dig cyclization

Herein, we report the 6-exo-dig ring closure of ortho-alkynylanilines with readily available aroyl isothiocyanate. An environmentally benign, metal- and base-free, iodine promoted cascade synthesis of highly functionalized (benzo[1,3]thiazin-2-yl)benzimidic acids has been accomplished via in situ generated ortho-alkynylthiourea. The established methodology employs the abundant chemical feedstock of ortho-alkynylanilines and aroyl isothiocyanates and could be applied in the late-stage synthesis of pharmaceutically active 1,3-benzothiazine containing molecules. Furthermore, the discovered protocol exclusively delivers bis (benzo[1,3]thiazin-2-yl)dibenzimidic acid products and preserves the iodo-olefin substitution pattern which can be exploited by further derivatization.

A metal- and base-free domino protocol for the synthesis of 1,3-benzoselenazines, 1,3-benzothiazines and related scaffolds

Efficient protocols have been described for the synthesis of 1,3-benzoselenazines, 1,3-benzothiazines, 2-aryl thiazin-4-ones and diaryl[b,f][1,5]diazocine-6,12(5H,11H)-diones. These transformations were successfully driven towards the product formation under mild acid catalyzed reaction conditions at room temperature using 2-amino aryl/hetero-aryl alkyl alcohols and amides as substrates. The merits of the present methods also rely on the easy access of rarely explored bioactive scaffolds like 1,3-benzoselenazine derivatives, for which well-documented methods are rarely known in the literature. A broad range of substrates with both electron-rich and electron-deficient groups were well-tolerated under the developed conditions to furnish the desired products in yields up to 98%. The scope of the devised method is not only restricted to the synthesis of 1,3-benzoselenazines, but it was also further extended towards the synthesis of 1,3-benzothiazines, 1,3-benzothiazinones and the corresponding eight membered N-heterocycles such as diaryl[b,f][1,5]diazocine-6,12(5H,11H)-diones.

Epigenetic and Posttranslational Modifications in Regulating the Biology of Aspergillus Species

Epigenetic and posttranslational modifications have been proved to participate in multiple cellular processes and suggested to be an important regulatory mechanism on transcription of genes in eukaryotes. However, our knowledge about epigenetic and posttranslational modifications mainly comes from the studies of yeasts, plants, and animals. Recently, epigenetic and posttranslational modifications have also raised concern for the relevance of regulating fungal biology in Aspergillus. Emerging evidence indicates that these modifications could be a connection between genetic elements and environmental factors, and their combined effects may finally lead to fungal phenotypical changes. This article describes the advances in typical DNA and protein modifications in the genus Aspergillus, focusing on methylation, acetylation, phosphorylation, ubiquitination, sumoylation, and neddylation.

Eco-friendly reactions in PEG-400: a highly efficient and green approach for stereoselective access to multisubstituted 3,4-dihydro-2(1H)-quinazolines using 2-aminobenzylamines

An efficient and stereoselective synthesis of novel 3,4-dihydro-2(1H)-quinazolines has been developed through cyclization reactions of 2-aminobenzylamines with α-oxoketene dithioacetals using PEG-400 as an inexpensive, easy to handle, non-toxic and recyclable reaction medium. The developed protocol is operationally simple and tolerates various substrates having different functionalities. This protocol features several attributes such as excellent yields, no work up, green reaction conditions, and being environmentally benign. The attractive feature of this new strategy is that all the reported final compounds have been isolated as single (E)-stereoisomeric forms, which was confirmed by 1HNMR and X-ray crystallographic studies.

An unprecedented tandem synthesis of fluorescent coumarin-fused pyrimidines via copper-catalyzed cross-dehydrogenative C(sp3)–N bond coupling

A Cu-catalyzed tandem strategy for the synthesis of coumarin-fused pyrimidin-5-ones involving in situ intramolecular cross-dehydrogenative C(sp³)–N bond formation.

Enzymatic Carbon-Sulfur Bond Formation in Natural Product Biosynthesis

Sulfur plays a critical role for the development and maintenance of life on earth, which is reflected by the wealth of primary metabolites, macromolecules, and cofactors bearing this element. Whereas a large body of knowledge has existed for sulfur trafficking in primary metabolism, the secondary metabolism involving sulfur has long been neglected. Yet, diverse sulfur functionalities have a major impact on the biological activities of natural products. Recent research at the genetic, biochemical, and chemical levels has unearthed a broad range of enzymes, sulfur shuttles, and chemical mechanisms for generating carbon-sulfur bonds. This Review will give the first systematic overview on enzymes catalyzing the formation of organosulfur natural products.

Identification and biological evaluation of secondary metabolites from the endolichenic fungus Aspergillus versicolor

A chemical investigation of the endolichenic fungus Aspergillus versicolor (125a), which was found in the lichen Lobaria quercizans, resulted in the isolation of four novel diphenyl ethers, named diorcinols F-H (1-3, resp.) and 3-methoxyviolaceol-II (4), eight new bisabolane sesquiterpenoids, named (-)-(R)-cyclo-hydroxysydonic acid (5), (-)-(7S,8R)-8-hydroxysydowic acid (6), (-)-(7R,10S)-10-hydroxysydowic acid (7), (-)-(7R,10R)-iso-10-hydroxysydowic acid (8), (-)-12-acetoxy-1-deoxysydonic acid (9), (-)-12-acetoxysydonic acid (10), (-)-12-hydroxysydonic acid (11), and (-)-(R)-11-dehydrosydonic acid (12), two new tris(pyrogallol ethers), named sydowiols D (13) and E (14), and fifteen known compounds, 15-29. All of the structures were determined by spectroscopic analyses, and a number of them were further identified through chemical transformations and electronic circular dichroism (ECD) calculations. Preliminary bioassays of these isolates for the determination of their inhibitory activities against the fungus Candida albicans, and their cytotoxicities against the human cancer cell lines PC3, A549, A2780, MDA-MB-231, and HEPG2 were also evaluated.

Genetic regulation and manipulation for natural product discovery

Natural products are an important source of modern medical development, e.g., antibiotics, anticancers, immune modulators, etc. and will continue to be a powerful driving force for the discovery of novel potential drugs. In the heterologous hosts, natural products are biosynthesized using dedicated metabolic networks. By gene engineering, pathway reconstructing, and enzyme engineering, metabolic networks can be modified to synthesize novel compounds containing enhanced structural feature or produce a large quantity of known valuable bioactive compounds. The review introduces some important technical platforms and relevant examples of genetic regulation and manipulation to improve natural product titers or drive novel secondary metabolite discoveries.

Microbial communication leading to the activation of silent fungal secondary metabolite gene clusters

Microorganisms form diverse multispecies communities in various ecosystems. The high abundance of fungal and bacterial species in these consortia results in specific communication between the microorganisms. A key role in this communication is played by secondary metabolites (SMs), which are also called natural products. Recently, it was shown that interspecies “talk” between microorganisms represents a physiological trigger to activate silent gene clusters leading to the formation of novel SMs by the involved species. This review focuses on mixed microbial cultivation, mainly between bacteria and fungi, with a special emphasis on the induced formation of fungal SMs in co-cultures. In addition, the role of chromatin remodeling in the induction is examined, and methodical perspectives for the analysis of natural products are presented. As an example for an intermicrobial interaction elucidated at the molecular level, we discuss the specific interaction between the filamentous fungi Aspergillus nidulans and Aspergillus fumigatus with the soil bacterium Streptomyces rapamycinicus, which provides an excellent model system to enlighten molecular concepts behind regulatory mechanisms and will pave the way to a novel avenue of drug discovery through targeted activation of silent SM gene clusters through co-cultivations of microorganisms.

Pd-catalyzed Heck-conjoined amidation and concomitant chemoselective Michael-addition: an efficient tandem approach to highly functionalized tetrahydroquinazolines from o-haloanilines

Efficient palladium-catalyzed tandem approach for the synthesis of highly functionalized tetrahydroquinazolines from o-haloanilines with acrylates and isothiocyanates/isocyanates via Heck-conjoined amidation/thioamidation and concomitant chemoselective Michael-addition is described.

Activation of dormant secondary metabolite production by introducing neomycin resistance into the deep-sea fungus, Aspergillus versicolor ZBY-3

A new ultrasound-mediated approach has been developed to introduce neomycin-resistance to activate silent pathways for secondary metabolite production in a bio-inactive, deep-sea fungus, Aspergillus versicolor ZBY-3. Upon treatment of the ZBY-3 spores with a high concentration of neomycin by proper ultrasound irradiation, a total of 30 mutants were obtained by single colony isolation. The acquired resistance of the mutants to neomycin was confirmed by a resistance test. In contrast to the ZBY-3 strain, the EtOAc extracts of 22 of the 30 mutants inhibited the human cancer K562 cells, indicating that these mutants acquired a capability to produce antitumor metabolites. HPLC-photodiode array detector (PDAD)-UV and HPLC-electron spray ionization (ESI)-MS analyses of the EtOAc extracts of seven bioactive mutants and the ZBY-3 strain indicated that diverse secondary metabolites have been newly produced in the mutant extracts in contrast to the ZBY-3 extract. The followed isolation and characterization demonstrated that six metabolites, cyclo(d-Pro-d-Phe) (1), cyclo(d-Tyr-d-Pro) (2), phenethyl 5-oxo-l-prolinate (3), cyclo(l-Ile-l-Pro) (4), cyclo(l-Leu-l-Pro) (5) and 3β,5α,9α-trihydroxy-(22E,24R)-ergosta-7,22-dien-6-one (6), were newly produced by the mutant u2n2h3-3 compared to the parent ZBY-3 strain. Compound 3 was a new compound; 2 was isolated from a natural source for the first time, and all of these compounds were also not yet found in the metabolites of other A. versicolor strains. Compounds 1–6 inhibited the K562 cells, with inhibition rates of 54.6% (1), 72.9% (2), 23.5% (3), 29.6% (4), 30.9% (5) and 51.1% (6) at 100 μg/mL, and inhibited also other human cancer HL-60, BGC-823 and HeLa cells, to some extent. The present study demonstrated the effectiveness of the ultrasound-mediated approach to activate silent metabolite production in fungi by introducing acquired resistance to aminoglycosides and its potential for discovering new compounds from silent fungal metabolic pathways. This approach could be applied to elicit the metabolic potentials of other fungal isolates to discover new compounds from cryptic secondary metabolites.

A practical strategy to discover new antitumor compounds by activating silent metabolite production in fungi by diethyl sulphate mutagenesis

Many fungal biosynthetic pathways are silent in standard culture conditions, and activation of the silent pathways may enable access to new metabolites with antitumor activities. The aim of the present study was to develop a practical strategy for microbial chemists to access silent metabolites in fungi. We demonstrated this strategy using a marine-derived fungus Penicillium purpurogenum G59 and a modified diethyl sulphate mutagenesis procedure. Using this strategy, we discovered four new antitumor compounds named penicimutanolone (1), penicimutanin A (2), penicimutanin B (3), and penicimutatin (4). Structures of the new compounds were elucidated by spectroscopic methods, especially extensive 2D NMR analysis. Antitumor activities were assayed by the MTT method using human cancer cell lines. Bioassays and HPLC-photodiode array detector (PDAD)-UV and HPLC-electron spray ionization (ESI)-MS analyses were used to estimate the activated secondary metabolite production. Compounds 2 and 3 had novel structures, and 1 was a new compound belonging to a class of very rare natural products from which only four members are so far known. Compounds 1–3 inhibited several human cancer cell lines with IC₅₀ values lower than 20 μM, and 4 inhibited the cell lines to some extent. These results demonstrated the effectiveness of this strategy to discover new compounds by activating silent fungal metabolic pathways. These discoveries provide rationale for the increased use of chemical mutagenesis strategies in silent fungal metabolite studies.

Distinct amino acids of histone H3 control secondary metabolism in Aspergillus nidulans

Chromatin remodelling events play an important role in the secondary metabolism of filamentous fungi. Previously, we showed that a bacterium, Streptomyces rapamycinicus, is able to reprogram the histone-modifying Spt-Ada-Gcn5-acetyltransferase/ADA (SAGA/ADA) complex of the model fungus Aspergillus nidulans. Consequently, the histone H3 amino acids lysine 9 and lysine 14 at distinct secondary metabolism genes were specifically acetylated during the bacterial fungal interaction, which, furthermore, was associated with the activation of the otherwise silent orsellinic acid gene cluster. To investigate the importance of the histone modifications for distinct gene expression profiles in fungal secondary metabolism, we exchanged several amino acids of histone H3 of A. nidulans. These amino acids included lysine residues 9, 14, 18, and 23 as well as serine 10 and threonine 11. Lysine residues were replaced by arginine or glutamine residues, and serine/threonine residues were replaced by alanine. All generated mutant strains were viable, allowing direct analysis of the consequences of missing posttranslational histone modifications. In the mutant strains, major changes in the expression patterns at both the transcriptional and metabolite levels of the penicillin, sterigmatocystin, and orsellinic acid biosynthesis gene clusters were detected. These effects were due mainly to the substitution of the acetylatable lysine 14 of histone H3 and were enhanced in a lysine 14/lysine 9 double mutant of histone H3. Taken together, our findings show a causal linkage between the acetylation of lysine residue 14 of histone H3 and the transcription and product formation of secondary metabolite gene clusters.

Bacterium induces cryptic meroterpenoid pathway in the pathogenic fungus Aspergillus fumigatus

Stimulating encounter: The intimate, physical interaction between the soil-derived bacterium Streptomyces rapamycinicus and the human pathogenic fungus Aspergillus fumigatus led to the activation of an otherwise silent polyketide synthase (PKS) gene cluster coding for an unusual prenylated polyphenol (fumicycline A). The meroterpenoid pathway is regulated by a pathway-specific activator gene as well as by epigenetic factors.

Regulatory cross talk and microbial induction of fungal secondary metabolite gene clusters

Filamentous fungi are well-known producers of a wealth of secondary metabolites with various biological activities. Many of these compounds such as penicillin, cyclosporine, or lovastatin are of great importance for human health. Genome sequences of filamentous fungi revealed that the encoded potential to produce secondary metabolites is much higher than the actual number of compounds produced during cultivation in the laboratory. This finding encouraged research groups to develop new methods to exploit the silent reservoir of secondary metabolites. In this chapter, we present three successful strategies to induce the expression of secondary metabolite gene clusters. They are based on the manipulation of the molecular processes controlling the biosynthesis of secondary metabolites and the simulation of stimulating environmental conditions leading to altered metabolic profiles. The presented methods were successfully applied to identify novel metabolites. They can be also used to significantly increase product yields.

Red hair benzothiazines and benzothiazoles: mutation-inspired chemistry in the quest for functionality

Nature provides a primary source of leads for the design of π-conjugated organic chromophores and other functional molecular systems useful for molecular recognition, light harvesting, photoconversion, and other technological applications. In this Account, we draw attention to a unique group of naturally occurring heterocyclic compounds, the 2H-1,4-benzothiazines and related benzothiazole derivatives. Derived from tyrosine and cysteine, these molecules arise from a mutation-induced deviation of the melanin pathway to provide the core structure of the red human hair pigments pheomelanins. Since the elucidation of the biosynthetic pathway of pheomelanins in the 1960s, researchers have focused on 1,4-benzothiazines and red hair pigments. Not only do these molecules have interesting photochemical and molecular recognition properties, they also have compelling biomedical significance. Numerous studies have linked higher levels of pheomelanins and mutations in the pathways that produce these pigments in individuals with red hair and fair skin with an increased sensitivity to UV light and a higher susceptibility to melanoma and other skin cancers. Prompted by new data about the structure and photochemistry of the bibenzothiazine system, this Account highlights the chemistry of benzothiazines in red-haired individuals as a novel source of inspiration in the quest for innovative scaffolds and biomimetic functional systems. Model studies have gradually shed light on a number of remarkable physical and chemical properties of benzothiazine-based systems. Bibenzothiazine is a robust visible chromophore that combines photochromism and acidichromism. Benzothiazine-based polymers (synthetic pheomelanins) show remarkable photochemical, paramagnetic, and redox cycling properties. Biomimetic or synthetic manipulations of the benzothiazine systems, through decarboxylation pathways controlled by metal ions or unusually facile ring-contraction processes, can produce a diverse set of molecular scaffolds.

Regulation of fungal secondary metabolism

Fungi produce a multitude of low-molecular-mass compounds known as secondary metabolites, which have roles in a range of cellular processes such as transcription, development and intercellular communication. In addition, many of these compounds now have important applications, for instance, as antibiotics or immunosuppressants. Genome mining efforts indicate that the capability of fungi to produce secondary metabolites has been substantially underestimated because many of the fungal secondary metabolite biosynthesis gene clusters are silent under standard cultivation conditions. In this Review, I describe our current understanding of the regulatory elements that modulate the transcription of genes involved in secondary metabolism. I also discuss how an improved knowledge of these regulatory elements will ultimately lead to a better understanding of the physiological and ecological functions of these important compounds and will pave the way for a novel avenue to drug discovery through targeted activation of silent gene clusters.

Breaking the silence: protein stabilization uncovers silenced biosynthetic gene clusters in the fungus Aspergillus nidulans

The genomes of filamentous fungi comprise numerous putative gene clusters coding for the biosynthesis of chemically and structurally diverse secondary metabolites (SMs), which are rarely expressed under laboratory conditions. Previous approaches to activate these genes were based primarily on artificially targeting the cellular protein synthesis apparatus. Here, we applied an alternative approach of genetically impairing the protein degradation apparatus of the model fungus Aspergillus nidulans by deleting the conserved eukaryotic csnE/CSN5 deneddylase subunit of the COP9 signalosome. This defect in protein degradation results in the activation of a previously silenced gene cluster comprising a polyketide synthase gene producing the antibiotic 2,4-dihydroxy-3-methyl-6-(2-oxopropyl)benzaldehyde (DHMBA). The csnE/CSN5 gene is highly conserved in fungi, and therefore, the deletion is a feasible approach for the identification of new SMs.

4-Amino-8-cyclo-pent-yloxy-7-meth-oxy-2H-chromen-2-one monohydrate

The asymmetric unit of the title compound, C₁₅H₁₇NO₄·H₂O, contains two organic mol­ecules with marginal differences between them and two water molecules. The chromine rings in both mol­ecules are essentially planar, with maximum deviations of 0.012 (2) and 0.060 (2) Å. The five-membered cyclo­pentane rings adopt envelope conformations in both mol­ecules. In the crystal, the components are linked by N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds, resulting in a three-dimensional network.