Biosynthesis, regulation, and engineering of a linear polyketide tautomycetin: a novel immunosuppressant in Streptomyces sp. CK4412

Streptomyces silvisoli sp. nov., a polyene producer, and Streptomyces tropicalis sp. nov., two novel actinobacterial species from peat swamp forests in Thailand

Two novel actinobacterial strains, designated RB6PN23T and K1PA1T, were isolated from peat swamp soil samples in Thailand and characterized using a polyphasic taxonomic approach. The strains were filamentous Gram-stain-positive bacteria containing ll-diaminopimelic acid in their whole-cell hydrolysates. Phylogenetic analysis of their 16S rRNA gene sequences revealed that strain RB6PN23T was most closely related to Streptomyces rubrisoli (99.1 % sequence similarity) and Streptomyces ferralitis (98.5%), while strain K1PA1T showed 98.8 and 98.7% sequence similarities to Streptomyces coacervatus and Streptomyces griseoruber, respectively. However, the average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values were below the species-level thresholds (95-96 % ANI and 70 % dDDH). The genomes of strains RB6PN23T and K1PA1T were estimated to be 7.88 Mbp and 7.39 Mbp in size, respectively, with DNA G+C contents of 70.2 and 73.2 mol%. Moreover, strains RB6PN23T and K1PA1T encode 37 and 24 putative biosynthetic gene clusters, respectively, and in silico analysis revealed that these new species have a high potential to produce unique natural products. Genotypic and phenotypic characteristics confirmed that strains RB6PN23T and K1PA1T represented two novel species in the genus Streptomyces. The names proposed for these strains are Streptomyces silvisoli sp. nov. (type strain RB6PN23T=TBRC 17040T=NBRC 116113T) and Streptomyces tropicalis sp. nov. (type strain K1PA1T=TBRC 17041T=NBRC 116114T). Additionally, a giant linear polyene compound, neotetrafibricin A, exhibiting antifungal activity in strain RB6PN23T, was identified through HPLC and quadrupole time-of-flight MS analysis. The crude extract from the culture broth of strain RB6PN23T exhibited strong antifungal activity against Fusarium verticillioides, Fusarium fujikuroi and Bipolaris zeicola. This finding suggests that strain RB6PN23T could be a promising candidate for biological control of fungal diseases.

Streptomyces: The biofactory of secondary metabolites

Natural products derived from microorganisms serve as a vital resource of valuable pharmaceuticals and therapeutic agents. Streptomyces is the most ubiquitous bacterial genus in the environments with prolific capability to produce diverse and valuable natural products with significant biological activities in medicine, environments, food industries, and agronomy sectors. However, many natural products remain unexplored among Streptomyces. It is exigent to develop novel antibiotics, agrochemicals, anticancer medicines, etc., due to the fast growth in resistance to antibiotics, cancer chemotherapeutics, and pesticides. This review article focused the natural products secreted by Streptomyces and their function and importance in curing diseases and agriculture. Moreover, it discussed genomic-driven drug discovery strategies and also gave a future perspective for drug development from the Streptomyces.

Screening and Isolation of a Novel Polyene-Producing Streptomyces Strain Inhibiting Phytopathogenic Fungi in the Soil Environment

Microbial-based eco-friendly biological substances are needed to protect crops from phytopathogenic fungi and replace toxic chemical fungicides that cause serious environmental issues. This study screened for soil antifungal Streptomyces strains, which produce rich, diverse, and valuable bioactive metabolites in the soil environment. Bioassay-based antifungal screening of approximately 2,400 Streptomyces strains led to the isolation of 149 strains as tentative antifungal producers. One Streptomyces strain showing the most potent antifungal activities against Candida albicans and Fusarium oxysporum was identified as a putative anti-phytopathogenic soil isolate that is highly homologous to Streptomyces rubrisoli (named S. rubrisoli Inha 501). An in vitro antifungal assay, pot-test, and field-test against various phytopathogenic fungi confirmed that S. rubrisoli Inha 501 is a potential novel phytopathogenic fungicide producer to protect various crops in the soil environment. Whole-genome sequencing of S. rubrisoli Inha 501 and an anti-SMASH genome mining approach revealed an approximately 150-kb polyene biosynthetic gene cluster (BGC) in the chromosome. The target compound isolation and its BGC analysis confirmed that the giant linear polyene compound exhibiting the anti-phytopathogenic activity in S. rubrisoli Inha 501 was highly homologous to the previously reported compound, neotetrafibricin A. These results suggest that a bioassay-based screening of a novel antifungal Streptomyces strain followed by its genome mining for target compound BGC characterization would be an efficient approach to isolating a novel candidate phytopathogenic fungicide that can protect crops in the soil environment.

Synthetic Studies toward the Total Synthesis of Tautomycetin

The studies culminating in the synthesis of two large subunits of tautomycetin are described. The first one, fragment C1-C12 that has an anti-1,3-dimethyl system and a terminal diene unit, was accomplished in 10 linear steps in 7.4% overall yield. The second one, fragment C13-C25 which bears the sensitive anhydride framework and the majority of the stereogenic centers, was prepared in 13 linear steps (longest sequence) in 8% overall yield. Among the key transformations used, a regioselective epoxide opening, a Pd-catalyzed addition of terminal alkyne to acceptor alkyne, a Mukaiyama aldol reaction, a Yamaguchi esterification, and a homemade mild di-esterification can be cited. The chosen strategies allowed good yields, stereoselectivity, reproducibility, and scalability for several important intermediates.

Diversification of polyketide structures via synthase engineering

Polyketide natural products possess diverse biological activities including antibiotic, anticancer, and immunosuppressive. Their equally varied and complex structures arise from head-to-tail condensation of simple carboxyacyl monomers. Since the seminal discovery that biosynthesis of polyketides such as the macrolide erythromycin is catalyzed by uncharacteristically large, multifunctional enzymes, termed modular type I polyketide synthases, chemists and biologists alike have been inspired to harness the apparent modularity of the synthases to further diversify polyketide structures. Yet, initial attempts to perform "combinatorial biosynthesis" failed due to challenges associated with maintaining the structural and catalytic integrity of large, chimeric synthases. Fast forward nearly 30 years, and advancements in our understanding of polyketide synthase structure and function have allowed the field to make significant progress toward effecting desired modifications to polyketide scaffolds in addition to engineering small, chiral fragments. This review highlights selected examples of polyketide diversification via control of monomer selection, oxidation state, stereochemistry, and cyclization. We conclude with a perspective on the present and future of polyketide structure diversification and hope that the examples presented here will encourage medicinal chemists to embrace polyketide synthetic biology as a means to revitalize polyketide drug discovery.

Why does tautomycetin thioesterase prefer hydrolysis to macrocyclization? Theoretical study on its catalytic mechanism

In this study, we attempted to uncover the reasons why Tautomycetin thioesterase (TMC TE) prefers hydrolysis rather than macrocyclization, and reveal the molecular basis of TE-catalyzed hydrolysis and macrocyclization.