Angucyclines and angucyclinones represent a class of natural compounds that belong to the group of aromatic polyketides. They exhibit a wide array of biological properties, such as antimicrobial, antiviral, and cytotoxic. Their considerable therapeutic potential and diverse scaffolds have attracted many synthetic chemists to devise novel strategies to construct their intricate molecular architecture. Over 300 class members have been isolated from natural sources, mainly from bacterial strains of Streptomyces species. This review highlights recent advancements in their synthesis, such as oxidative cyclization, photooxidation, and metal-catalyzed [4+2]-cycloaddition, which has facilitated the efficient and practical total syntheses of various angucyclines natural products.

Biosynthesis of atypical angucyclines involves unique oxidative B-ring cleavage and rearrangement reactions, which are catalyzed by AlpJ-family oxygenases, including AlpJ, JadG, and GilOII. Prior investigations established the essential requirement for FADH2/FMNH2 as cofactors when utilizing the quinone intermediate dehydrorabelomycin as a substrate. In this study, we unveil a previously unrecognized facet of these enzymes as cofactor-independent oxygenases when employing the hydroquinone intermediate CR1 as a substrate. The enzymes autonomously drive oxidative ring cleavage and rearrangement reactions of CR1, yielding products identical to those observed in cofactor-dependent reactions of AlpJ-family oxygenases. Furthermore, the AlpJ- and JadG-catalyzed reactions of CR1 could be quenched by superoxide dismutase, supporting a catalytic mechanism wherein the substrate CR1 reductively activates molecular oxygen, generating a substrate radical and the superoxide anion O2 •-. Our findings illuminate a substrate-controlled catalytic mechanism of AlpJ-family oxygenases, expanding the realm of cofactor-independent oxygenases. Notably, AlpJ-family oxygenases stand as a pioneering example of enzymes capable of catalyzing oxidative reactions in either an FADH2/FMNH2-dependent or cofactor-independent manner.

The one strain many compounds (OSMAC) strategy is an effective method for activating silent gene clusters by cultivating microorganisms under various conditions. The whole genome sequence of the marine-derived strain Streptomyces globisporus SCSIO LCY30 revealed that it contains 30 biosynthetic gene clusters (BGCs). By using the OSMAC strategy, three types of secondary metabolites were activated and identified, including three angucyclines, mayamycin A (1), mayamycin B (2), and rabolemycin (3); two streptophenazines (streptophenazin O (4) and M (5)); and a macrolide dimeric dinactin (6), respectively. The biosynthetic pathways of the secondary metabolites in these three families were proposed based on the gene function prediction and structural information. The bioactivity assays showed that angucycline compounds 1-3 exhibited potent antitumor activities against 11 human cancer cell lines and antibacterial activities against a series of Gram-positive bacteria. Mayamycin (1) selectively exhibited potent cytotoxicity activity against triple-negative breast cancer (TNBC) cell lines such as MDA-MB-231, MDA-MB-468, and Bt-549, with IC50 values of 0.60-2.22 μM.

Six angucyclines including three unreported compounds (1-3) were isolated from Streptomyces sp. XS-16 by overexpressing the native global regulator of SCrp (cyclic AMP receptor). The structures were characterized based on nuclear magnetic resonance (NMR) and spectrometry analysis and assisted by electronic circular dichroism (ECD) calculations. All compounds were tested for their antitumor and antimicrobial activities, and compound 1 showed different inhibitory activities against various tumor cell lines with IC50 values ranging from 0.32 to 5.33 μM.

Sch47554 and Sch47555 are two angucyclines with antifungal activities against various yeasts and dermatophytes from Streptomyces sp. SCC-2136. The sch gene cluster contains several putative regulatory genes. Both schA4 and schA21 were predicted as the TetR family transcriptional regulators, whereas schA16 shared significant similarity to the AraC family transcriptional regulators. Although Sch47554 is the major product of Streptomyces sp. SCC-2136, its titer is only 6.72 mg/L. This work aimed to increase the production of this promising antifungal compound by investigating and manipulating the regulatory genes in the Sch47554 biosynthetic pathway. Disruption of schA4 and schA16 led to a significant increase in the production of Sch47554, whereas the titer was dramatically decreased when schA21 was disrupted. Overexpression of these genes gave opposite results. The highest titer of Sch47554 was achieved in Streptomyces sp. SCC-2136/ΔschA4 (27.94 mg/L), which is significantly higher than the wild type. Our results indicate that SchA4 and SchA16 are repressors, whereas SchA21 acts as an activator. This work thus provides an initial understanding of functional roles of regulatory elements in the biosynthesis of Sch47554. Several efficient producing strains of Sch47554 were constructed by disrupting or overexpressing particular regulatory genes, which can be further engineered for industrial production of this medicinally important molecule.

Diazobenzofluorene-containing atypical angucyclines exhibit promising biological activities. Here we report the inactivation of an amidotransferase-encoding gene flsN3 in Micromonospora rosaria SCSIO N160, a producer of fluostatins. Bioinformatics analysis indicated that FlsN3 was involved in the diazo formation. Chemical investigation of the flsN3-inactivation mutant resulted in the isolation of a variety of angucycline aromatic polyketides, including four racemic aminobenzo[b]fluorenes stealthins D⁻G (9⁻12) harboring a stealthin C-like core skeleton with an acetone or butanone-like side chain. Their structures were elucidated on the basis of nuclear magnetic resonance (NMR) spectroscopic data and X-ray diffraction analysis. A plausible mechanism for the formation of stealthins D⁻G (9⁻12) was proposed. These results suggested a functional role of FlsN3 in the formation/modification of N⁻N bond-containing fluostatins.

This mini-review is centered on genetic aspects of biosynthesis of landomycins (La), a family of angucycline polyketides. From the very discovery in the 1990s, La were noted for unusual structure and potent anticancer properties. La are produced by a few actinobacteria that belong to genus Streptomyces. Biochemical logic behind the production of La aglycon and glycoside halves and effects of La on mammalian cells have been thoroughly reviewed in 2009-2012. Yet, the genetic diversity of La biosynthetic gene clusters (BGCs) and regulation of their production were not properly reviewed since discovery of La. Here, we aim to fill this gap by focusing on three interrelated topics. First, organization of known La BGCs is compared. Second, up-to-date scheme of biosynthetic pathway to landomycin A (LaA), the biggest (by molar weight) member of La family, is succinctly outlined. Third, we describe genetic and nutritional factors that influence La production and export. A summary of the practical utility of the gained knowledge and future directions to study La biosynthesis conclude this mini-review.

The biosynthetically well-studied landomycin A cluster has been used to demonstrate the unbalancing of gene transcription as an efficient method for the generation of new compounds. Landomycin A structural genes were decoupled from the native regulators LanI and LanK and placed under the control of a single synthetic promoter and expressed in a heterologous host Streptomyces albus J1074. In contrast to their native quantitative and temporal regulation, these genes were transcribed as a single polycistronic mRNA leading to the production of four novel and two known compounds. No glycosylated landomycins were detected though the entire biosynthetic cluster was transcribed, showing the crucial role of the balanced gene expression for the production of landomycin A. Two new compounds, fridamycin F and G, isolated in this study were shown to originate from the interplay between the expressed biosynthetic pathway and metabolic network of the heterologous host. Structure activity studies of the isolated compounds as well as results of transcriptome sequencing are discussed in this article.

Three regulators, Aur1P, Aur1R and a SARP-family Aur1PR3, have been previously found to control expression of the aur1 cluster for the angucycline antibiotic auricin in Streptomyces aureofaciens CCM 3239. Here, we describe an additional regulatory gene, aur1PR4, encoding a homologue from the SARP-family regulators. Its role in auricin regulation was confirmed by its disruption that dramatically affected auricin production. However, transcription from the aur1Ap promoter, directing expression of 22 auricin biosynthetic genes, was not substantially affected in the Δaur1PR4 mutant. A new promoter, sa13p, directing transcription of four putative auricin tailoring genes, was found to be dependent on aur1PR4. Moreover, analysis of the sa13p promoter region revealed the presence of three heptameric repeat sequences corresponding to putative SARP-binding sites. Expression of aur1PR4 is directed by a single promoter, aur1PR4p, which is induced after entry into stationary phase. Transcription from aur1PR4p was absent in a S. aureofaciens Δaur1P mutant strain, and Aur1P was shown to bind specifically to the aur1PR4p promoter. These results indicate a complex network of regulation of the auricin gene cluster. Both Aur1P and Aur1PR3 are involved in regulation of the core aur1A-U biosynthetic genes, and Aur1PR4 in regulation of putative auricin tailoring genes.