Cross-Module Enoylreduction in the Azalomycin F Polyketide Synthase

Biosynthesis of Octacosamicin A: Uncommon Starter/extender Units and Product Releasing via Intermolecular Amidation

Octacosamicin A is an antifungal metabolite featuring a linear polyene-polyol chain flanked by N-hydroxyguanidine and glycine moieties. We report here that sub-inhibitory concentrations of streptomycin elicited the production of octacosamicin A in Amycolatopsis azurea DSM 43854T . We identified the biosynthetic gene cluster (oca BGC) that encodes a modular polyketide synthase (PKS) system for assembling the polyene-polyol chain of octacosamicin A. Our analysis suggested that the N-hydroxyguanidine unit originates from a 4-guanidinobutyryl-CoA starter unit, while the PKS incorporates an α-hydroxyketone moiety using a (2R)-hydroxymalonyl-CoA extender unit. The modular PKS system contains a non-canonical terminal module that lacks thioesterase (TE) and acyl carrier protein (ACP) domains, indicating the biosynthesis is likely to employ an unconventional and cryptic off-loading mechanism that attaches glycine to the polyene-polyol chain via an intermolecular amidation reaction.

Insights into azalomycin F assembly-line contribute to evolution-guided polyketide synthase engineering and identification of intermodular recognition

Modular polyketide synthase (PKS) is an ingenious core machine that catalyzes abundant polyketides in nature. Exploring interactions among modules in PKS is very important for understanding the overall biosynthetic process and for engineering PKS assembly-lines. Here, we show that intermodular recognition between the enoylreductase domain ER_1/2 inside module 1/2 and the ketosynthase domain KS₃ inside module 3 is required for the cross-module enoylreduction in azalomycin F (AZL) biosynthesis. We also show that KS₄ of module 4 acts as a gatekeeper facilitating cross-module enoylreduction. Additionally, evidence is provided that module 3 and module 6 in the AZL PKS are evolutionarily homologous, which makes evolution-oriented PKS engineering possible. These results reveal intermodular recognition, furthering understanding of the mechanism of the PKS assembly-line, thus providing different insights into PKS engineering. This also reveals that gene duplication/conversion and subsequent combinations may be a neofunctionalization process in modular PKS assembly-lines, hence providing a different case for supporting the investigation of modular PKS evolution.

New Glycosylated Polyene Macrolides: Refining the Ore from Genome Mining

Glycosylated polyene macrolides include effective antifungal agents, such as pimaricin, nystatin, candicidin, and amphotericin B. For the treatment of systemic mycoses, amphotericin B has been described as a gold-standard antibiotic because of its potent activity against a broad spectrum of fungal pathogens, which do not readily become resistant. However, amphotericin B has severe toxic side effects, and the development of safer alternatives remains an important objective. One approach towards obtaining such compounds is to discover new related natural products. Advances in next-generation sequencing have delivered a wealth of microbial genome sequences containing polyene biosynthetic gene clusters. These typically encode a modular polyketide synthase that catalyzes the assembly of the aglycone core, a cytochrome P450 that oxidizes a methyl branch to a carboxyl group, and additional enzymes for synthesis and attachment of a single mycosamine sugar residue. In some cases, further P450s catalyze epoxide formation or hydroxylation within the macrolactone. Bioinformatic analyses have identified over 250 of these clusters. Some are predicted to encode potentially valuable new polyenes that have not been uncovered by traditional screening methods. Recent experimental studies have characterized polyenes with new polyketide backbones, previously unknown late oxygenations, and additional sugar residues that increase water-solubility and reduce hemolytic activity. Here we review these studies and assess how this new knowledge can help to prioritize silent polyene clusters for further investigation. This approach should improve the chances of discovering better antifungal antibiotics.

Natural products from mangrove sediments-derived microbes: Structural diversity, bioactivities, biosynthesis, and total synthesis

The mangrove forests are a complex ecosystem, and the microbial communities in mangrove sediments play a critical role in the biogeochemical cycles of mangrove ecosystems. Mangrove sediments-derived microbes (MSM), as a rich reservoir of natural product diversity, could be utilized in the exploration of new antibiotics or drugs. To understand the structural diversity and bioactivities of the metabolites of MSM, this review for the first time provides a comprehensive overview of 519 natural products isolated from MSM with their bioactivities, up to 2021. Most of the structural types of these compounds are alkaloids, lactones, xanthones, quinones, terpenoids, and steroids. Among them, 210 compounds are obtained from bacteria, most of which are from Streptomyces, while 309 compounds are from fungus, especially genus Aspergillus and Penicillium. The pharmacological mechanisms of some representative lead compounds are well studied, revealing that they have important medicinal potentials, such as piericidins with anti-renal cell cancer effects, azalomycins with anti-MRSA activities, and ophiobolins as antineoplastic agents. The biosynthetic pathways of representative natural products from MSM have also been summarized, especially ikarugamycin, piericidins, divergolides, and azalomycins. In addition, the total synthetic strategies of representative secondary metabolites from MSM are also reviewed, such as piericidin A and borrelidin. This review provides an important reference for the research status of natural products isolated from MSM and the lead compounds worthy of further development, and reveals that MSM have important medicinal values and are worthy of further development.

Revised structural assignment of azalomycins based on genomic and chemical analysis

We clarified structural inconsistencies of four azalomycin derivatives (F4a, F4b, F5a and F5b) from Streptomyces sp. M56 by nuclear magnetic resonance, J-based configuration analyses, electronic circular dichroism and in silico genome studies.