Resorculins: hybrid polyketide macrolides from Streptomyces sp. MST-91080.

Fourteen-membered macrolides are a class of compounds with significant clinical value as antibacterial agents. As part of our ongoing investigation into the metabolites of Streptomyces sp. MST-91080, we report the discovery of resorculins A and B, unprecedented 3,5-dihydroxybenzoic acid (α-resorcylic acid)-containing 14-membered macrolides. We sequenced the genome of MST-91080 and identified the putative resorculin biosynthetic gene cluster (rsn BGC). The rsn BGC is hybrid of type I and type III polyketide synthases. Bioinformatic analysis revealed that the resorculins are relatives of known hybrid polyketides: kendomycin and venemycin. Resorculin A exhibited antibacterial activity against Bacillus subtilis (MIC 19.8 µg mL-1), while resorculin B showed cytotoxic activity against the NS-1 mouse myeloma cell line (IC50 3.6 µg mL-1).

Suertides A-C: selective antibacterial cyclic hexapeptides from Amycolatopsis sp. MST-135876v3.

Amycolatopsis sp. MST-135876 was isolated from soil collected from the riverbank of El Pont de Suert, Catalonia, Spain. Cultivation of MST-135876 on a range of media led to the discovery of a previously unreported dichlorinated cyclic hexapeptide, suertide A (D-Ser, 5-Cl-D-Trp, 6-Cl-D-Trp, L-Ile, D-Val, D-Glu), featuring an unprecedented pair of adjacent 5/6-chlorotryptophan residues. Supplementing the growth medium with KBr resulted in production of the mono- and dibrominated analogues suertides B and C, respectively. Suertides A-C displayed selective activity against Bacillus subtilis (MIC 1.6 µg ml-1) and Staphylococcus aureus (MIC 3.1, 6.3, and 12.5 µg ml-1, respectively), while suertides A and B showed appreciable activity against methicillin-resistant S. aureus (MIC 1.6 and 6.3 µg ml-1, respectively).

Recently Discovered Secondary Metabolites from Streptomyces Species.

The Streptomyces genus has been a rich source of bioactive natural products, medicinal chemicals, and novel drug leads for three-quarters of a century. Yet studies suggest that the genus is capable of making some 150,000 more bioactive compounds than all Streptomyces secondary metabolites reported to date. Researchers around the world continue to explore this enormous potential using a range of strategies including modification of culture conditions, bioinformatics and genome mining, heterologous expression, and other approaches to cryptic biosynthetic gene cluster activation. Our survey of the recent literature, with a particular focus on the year 2020, brings together more than 70 novel secondary metabolites from Streptomyces species, which are discussed in this review. This diverse array includes cyclic and linear peptides, peptide derivatives, polyketides, terpenoids, polyaromatics, macrocycles, and furans, the isolation, chemical structures, and bioactivity of which are appraised. The discovery of these many different compounds demonstrates the continued potential of Streptomyces as a source of new and interesting natural products and contributes further important pieces to the mostly unfinished puzzle of Earth's myriad microbes and their multifaceted chemical output.

Comprehensive chemotaxonomic and genomic profiling of a biosynthetically talented Australian fungus, Aspergillus burnettii sp. nov.

Aspergillus burnettii is a new species belonging to the A. alliaceus clade in Aspergillus subgenus Circumdati section Flavi isolated from peanut-growing properties in southern Queensland, Australia. A. burnettii is a fast-growing, floccose fungus with distinctive brown conidia and is a talented producer of biomass-degrading enzymes and secondary metabolites. Chemical profiling of A. burnettii revealed the metabolites ochratoxin A, kotanins, isokotanins, asperlicin E, anominine and paspalinine, which are common to subgenus Circumdati, together with burnettiene A, burnettramic acids, burnettides, and high levels of 14α-hydroxypaspalinine and hirsutide. The genome of A. burnettii was sequenced and an annotated draft genome is presented. A. burnettii is rich in secondary metabolite biosynthetic gene clusters, containing 51 polyketide synthases, 28 non-ribosomal peptide synthetases and 19 genes related to terpene biosynthesis. Functional annotation of digestive enzymes of A. burnettii and A. alliaceus revealed overlapping carbon utilisation profiles, consistent with a close phylogenetic relationship.

Conglobatins B-E: cytotoxic analogues of the C2-symmetric macrodiolide conglobatin.

Chemical investigation of a previously unreported indigenous Australian Streptomyces strain MST-91080 has identified six novel analogues related to the oxazole-pendanted macrodiolide, conglobatin. Phylogenetic analysis of the 16S rRNA gene sequence identified MST-91080 as a species of Streptomyces, distinct from reported conglobatin producer, Streptomyces conglobatus ATCC 31005. Conglobatins B-E diverge from conglobatin through differing patterns of methylation on the macrodiolide skeleton. The altered methyl positions suggest a deviation from the published biosynthetic pathway, which proposed three successive methylmalonyl-CoA extender unit additions to the conglobatin monomer. Conglobatins B1, C1 and C2 exhibited more potent cytotoxic activity selectively against the NS-1 myeloma cell line (IC50 0.084, 1.05 and 0.45 µg ml-1, respectively) compared with conglobatin (IC50 1.39 µg ml-1).

Biosynthesis of a New Benzazepine Alkaloid Nanangelenin A from Aspergillus nanangensis Involves an Unusual l-Kynurenine-Incorporating NRPS Catalyzing Regioselective Lactamization.

1-Benzazepine is a pharmaceutically important scaffold but is rare among natural products. Nanangelenin A (1), containing an unprecedented 3,4-dihydro-1-benzazepine-2,5-dione-N-prenyl-N-acetoxy-anthranilamide scaffold, was isolated from a novel species of Australian fungus, Aspergillus nanangensis. Genomic and retrobiosynthetic analyses identified a putative nonribosomal peptide synthetase (NRPS) gene cluster (nan). The detailed biosynthetic pathway to 1 was established by heterologous pathway reconstitution in A. nidulans, which led to biosynthesis of intermediates nanagelenin B-F (2-5 and 7). We demonstrated that the NRPS NanA incorporates anthranilic acid (Ant) and l-kynurenine (l-Kyn), which is supplied by a dedicated indoleamine-2,3-dioxygenase NanC encoded in the gene cluster. Using heterologous in vivo assays and mutagenesis, we demonstrated that the C-terminal condensation (CT) and thiolation (T3) domains of NanA are responsible for the regioselective cyclization of the tethered Ant-l-Kyn dipeptide to form the unusual benzazepine scaffold in 1. We also showed that NanA-CT catalyzes the regioselective cyclization of a surrogate synthetic substrate, Ant-l-Kyn-N-acetylcysteamine, to give the benzazepine scaffold, while spontaneous cyclization of the dipeptide yielded the alternative kinetically favored benzodiazepine scaffold. The discovery of 1 and the characterization of NanA have expanded the chemical and functional diversities of fungal NRPSs.

Nanangenines: drimane sesquiterpenoids as the dominant metabolite cohort of a novel Australian fungus, Aspergillus nanangensis.

Chemical investigation of an undescribed Australian fungus, Aspergillus nanangensis, led to the identification of the nanangenines - a family of seven new and three previously reported drimane sesquiterpenoids. The structures of the nanangenines were elucidated by detailed spectroscopic analysis supported by single crystal X-ray diffraction studies. The compounds were assayed for in vitro activity against bacteria, fungi, mammalian cells and plants. Bioinformatics analysis, including comparative analysis with other acyl drimenol-producing Aspergilli, led to the identification of a putative nanangenine biosynthetic gene cluster that corresponds to the proposed biosynthetic pathway for nanangenines.

Discovery and Heterologous Biosynthesis of the Burnettramic Acids: Rare PKS-NRPS-Derived Bolaamphiphilic Pyrrolizidinediones from an Australian Fungus, Aspergillus burnettii.

The burnettramic acids are a new class of antibiotics from an Australian fungus Aspergillus burnettii. The rare bolaamphiphilic scaffold consists of ß-d-mannose linked to a pyrrolizidinedione unit via a 26-carbon chain. The most abundant metabolite displayed potent in vitro antifungal activity. Comparative genomics identified the hybrid PKS-NRPS bua gene cluster, which was verified by heterologous pathway reconstitution in Aspergillus nidulans.

A study of the chemical diversity of macroalgae from South Eastern Australia.

Macroalgae are a rich source of biologically active chemical diversity for pharmaceutical and agrichemical discovery. However, the ability to understand the complexities of their chemical diversity will dictate whether these natural products have a place in modern discovery paradigms. In this study, we examined the relationship between secondary metabolite production and biological activity for a cohort of 127 macroalgae samples collected from various locations across South Eastern Australia. Approximately 20% of the macroalgae samples showed high levels of chemical diversity and productivity, which also correlated strongly with bioactivity. These "talented" species represent sustainable sources of metabolites that may be readily harvested for large-scale production. At a taxonomic level, significant differences in metabolite production and diversity were observed between Chlorophyta, Rhodophyta and Phaeophyta. For each talented species, the cometabolite pattern was unique to that species, with closely related species within the same genus displaying very different profiles. Despite over 50years of investigation, we estimate that more than two-thirds of the chemical diversity of macroalgae remains unknown to science. By understanding the physicochemical properties and distribution patterns of metabolites, it is possible to make reasoned judgements about sustainable sourcing of macroalgae for biodiscovery.