Abyssomicin biosynthesis: formation of an unusual polyketide, antibiotic-feeding studies and genetic analysis

TetR and OmpR family regulators in natural product biosynthesis and resistance

This article provides a comprehensive review and sequence-structure analysis of transcription regulator (TR) families, TetR and OmpR/PhoB, involved in specialized secondary metabolite (SSM) biosynthesis and resistance. Transcription regulation is a fundamental process, playing a crucial role in orchestrating gene expression to confer a survival advantage in response to frequent environmental stress conditions. This process, coupled with signal sensing, enables bacteria to respond to a diverse range of intra and extracellular signals. Thus, major bacterial signaling systems use a receptor domain to sense chemical stimuli along with an output domain responsible for transcription regulation through DNA-binding. Sensory and output domains on a single polypeptide chain (one component system, OCS) allow response to stimuli by allostery, that is, DNA-binding affinity modulation upon signal presence/absence. On the other hand, two component systems (TCSs) allow cross-talk between the sensory and output domains as they are disjoint and transmit information by phosphorelay to mount a response. In both cases, however, TRs play a central role. Biosynthesis of SSMs, which includes antibiotics, is heavily regulated by TRs as it diverts the cell's resources towards the production of these expendable compounds, which also have clinical applications. These TRs have evolved to relay information across specific signals and target genes, thus providing a rich source of unique mechanisms to explore towards addressing the rapid escalation in antimicrobial resistance (AMR). Here, we focus on the TetR and OmpR family TRs, which belong to OCS and TCS, respectively. These TR families are well-known examples of regulators in secondary metabolism and are ubiquitous across different bacteria, as they also participate in a myriad of cellular processes apart from SSM biosynthesis and resistance. As a result, these families exhibit higher sequence divergence, which is also evident from our bioinformatic analysis of 158 389 and 77 437 sequences from TetR and OmpR family TRs, respectively. The analysis of both sequence and structure allowed us to identify novel motifs in addition to the known motifs responsible for TR function and its structural integrity. Understanding the diverse mechanisms employed by these TRs is essential for unraveling the biosynthesis of SSMs. This can also help exploit their regulatory role in biosynthesis for significant pharmaceutical, agricultural, and industrial applications.

First total synthesis of type II abyssomicins: (±)-abyssomicin 2 and (±)-neoabyssomicin B

The intramolecular Diels-Alder reaction (IMDA) of a butenolide derivative, as an entry to the type II abyssomicin scaffold, and the total synthesis of (±)-abyssomicin 2 and (±)-neoabyssomicin B are reported for the first time. A facile route to the IMDA precursor, the formation of a type I intermediate and two paths to (±)-neoabyssomicin B are also discussed.

Biosynthesis of Tetronates by a Nonribosomal Peptide Synthetase-Polyketide Synthase System

A cryptic tetronate biosynthetic pathway was identified in Kitasatospora niigatensis DSM 44781 via heterologous expression. Distinct from the currently known biosynthetic pathways, this system utilizes a partially functional nonribosomal peptide synthetase and a broadly selective polyketide synthase to direct the assembly and lactonization of the tetronate scaffold. By employing a permissive crotonyl-CoA reductase/carboxylase to provide different extender units, seven new tetronates (kitaniitetronins A-G) were obtained via precursor-directed biosynthesis.

The Role of Cytochrome P450 AbyV in the Final Stages of Abyssomicin C Biosynthesis

Abyssomicin C and its atropisomer are potent inhibitors of bacterial folate metabolism. They possess complex polycyclic structures, and their biosynthesis has been shown to involve several unusual enzymatic transformations. Using a combination of synthesis and in vitro assays we reveal that AbyV, a cytochrome P450 enzyme from the aby gene cluster, catalyses a key late-stage epoxidation required for the installation of the characteristic ether-bridged core of abyssomicin C. The X-ray crystal structure of AbyV has been determined, which in combination with molecular dynamics simulations provides a structural framework for our functional data. This work demonstrates the power of combining selective carbon-13 labelling with NMR spectroscopy as a sensitive tool to interrogate enzyme-catalysed reactions in vitro with no need for purification.

The Role of Cytochrome P450 AbyV in the Final Stages of Abyssomicin C Biosynthesis

Abyssomicin C and its atropisomer are potent inhibitors of bacterial folate metabolism. They possess complex polycyclic structures, and their biosynthesis has been shown to involve several unusual enzymatic transformations. Using a combination of synthesis and in vitro assays we reveal that AbyV, a cytochrome P450 enzyme from the aby gene cluster, catalyses a key late-stage epoxidation required for the installation of the characteristic ether-bridged core of abyssomicin C. The X-ray crystal structure of AbyV has been determined, which in combination with molecular dynamics simulations provides a structural framework for our functional data. This work demonstrates the power of combining selective carbon-13 labelling with NMR spectroscopy as a sensitive tool to interrogate enzyme-catalysed reactions in vitro with no need for purification.

Chain release mechanisms in polyketide and non-ribosomal peptide biosynthesis

Review covering up to mid-2021The structure of polyketide and non-ribosomal peptide natural products is strongly influenced by how they are released from their biosynthetic enzymes. As such, Nature has evolved a diverse range of release mechanisms, leading to the formation of bioactive chemical scaffolds such as lactones, lactams, diketopiperazines, and tetronates. Here, we review the enzymes and mechanisms used for chain release in polyketide and non-ribosomal peptide biosynthesis, how these mechanisms affect natural product structure, and how they could be utilised to introduce structural diversity into the products of engineered biosynthetic pathways.

Abyssomicins-A 20-Year Retrospective View

Abyssomicins represent a new family of polycyclic macrolactones. The first described compounds of the abyssomicin family were abyssomicin B, C, atrop-C, and D, produced by the marine actinomycete strain Verrucosispora maris AB-18-032, which was isolated from a sediment collected in the Sea of Japan. Among the described abyssomicins, only abyssomicin C and atrop-abyssomicin C show a high antibiotic activity against Gram-positive bacteria, including multi-resistant and vancomycin-resistant strains. The inhibitory activity is caused by a selective inhibition of the enzyme 4-amino-4-deoxychorismate synthase, which catalyzes the transformation of chorismate to para-aminobenzoic acid, an intermediate in the folic acid pathway.

Abyssomicin derivatives from the rhizosphere soil actinomycete Microbispora rhizosphaerae sp. nov. TBRC6028

Three undescribed abyssomicin derivatives, including microbimisin, abyssomicins Z1, and Z2, were isolated from the soil actinomycete Microbispora rhizosphaerae sp. nov. TBRC6028. Chemical structures were determined by NMR spectroscopic data (¹H, ¹³C, COSY, HSQC, HMBC, and NOESY spectra) and the absolute configurations were verified by single-crystal X-ray diffraction analyses together with the ECD spectral data. Microbimisin and abyssomicin Z1 exhibited weak antibacterial activity against Bacillus cereus with MIC values of 25.0 and 50.0 μg/mL without cytotoxicity against MCF-7 and Vero cells at the concentration of 50 μg/mL.

Actinomadura decatromicini sp. nov., isolated from mountain soil in Thailand

A novel actinomycete strain CYP1-5T was isolated from the mountain soil sample collected from Chaiyaphum province, Thailand and its taxonomic position was clarified by using a polyphasic taxonomic approach. The chemotaxonomic properties of strain CYP1-5T were consistent within the genus Actinomadura. Cell-wall peptidoglycan of this strain contained meso-diaminopimelic acid. Galactose, madurose, and ribose were presented as the diagnostic sugars in whole-cell hydrolysates. The major menaquinone was MK-9(H6). Major cellular fatty acids were iso-C16:0 and C16:0. Phosphatidylglycerol, diphosphatidylglycerol, phosphatidylinositol, and phosphatidylinositol mannoside were observed as predominant phospholipids. Based on the results of phylogenetic analyses of 16S rRNA gene sequence, strain CYP1-5T was constituent with the genus Actinomadura and was closely related to Actinomadura syzygii GKU157T (99.5%) and Actinomadura chibensis IFM 10266T (= JCM 14158T) (98.2%). The draft genome size of strain CYP1-5T was 9.30 Mb with 72.2 mol% of G + C content. Strain CYP1-5T showed ANIb values of 94.9% with A. syzygii GKU157T and 93.2% with A. chibensis JCM 14158T. Phenotypic characteristics, phylogenetic analysis and genome data support that strain CYP1-5T could be discriminated from its closest relatives, representing a novel species of the genus Actinomadura, for which the name Actinomadura decatromicini sp. nov. is proposed. The type strain is CYP1-5T (= JCM 16996T = KCTC 19916T = TISTR 2901T).

Complete Genome of Nocamycin-Producing Strain Saccharothrix syringae NRRL B-16468 Reveals the Biosynthetic Potential for Secondary Metabolites

The bacterium Saccharothrix syringae NRRL B-16468 is the producer of nocamycin I and nocamycin II which feature tetramic acid and bicyclic ketal groups. In this study, we presented the complete genome of S. syringae NRRL B-16468 obtained from ARS Culture Collection. It contains a circular chromosome of 10,929,570 bp with an average GC content of 73.49%, 9316 genes, 12 rRNAs and 54 tRNAs. Bioinformatics analyses of the genome has demonstrated that it harbors 55 putative biosynthetic gene clusters (BGCs) involved in synthesizing diverse secondary metabolites. The backbones of the natural products synthesized by these BGCs encoding for type I polyketide synthase (PKS), non-ribosomal peptide synthetase (NRPS) and hybrid type I PKS-NRPS were analyzed, furthermore, the natural products synthesized by these BGCs with > 40% similarity to known BGCs were described in detail. The complete genome of S. syringae reveals its capacity in producing diverse bioactive natural products, and it will also shed lights on mining novel secondary metabolites from S. syringae through rational strategies.

Draft genome sequence of Actinomadura sp. K4S16 and elucidation of the nonthmicin biosynthetic pathway

Actinomadura sp. K4S16 (=NBRC 110471) is a producer of a novel tetronate polyether compound nonthmicin. Here, we report the draft genome sequence of this strain together with features of the organism and assembly, annotation and analysis of the genome sequence. The 9.6 Mb genome of Actinomadura sp. K4S16 encoded 9,004 putative ORFs, of which 7,701 were assigned with COG categories. The genome contained four type-I polyketide synthase (PKS) gene clusters, two type-II PKS gene clusters, and three nonribosomal peptide synthetase (NRPS) gene clusters. Among the type-I PKS gene (t1pks) clusters, a large t1pks cluster was annotated to be responsible for nonthmicin synthesis based on bioinformatic analyses. We also performed feeding experiments using labeled precursors and propose the biosynthetic pathway of nonthmicin.

Out of the Abyss: Genome and Metagenome Mining Reveals Unexpected Environmental Distribution of Abyssomicins

Natural products have traditionally been discovered through the screening of culturable microbial isolates from diverse environments. The sequencing revolution allowed the identification of dozens of biosynthetic gene clusters (BGCs) within single bacterial genomes, either from cultured or uncultured strains. However, we are still far from fully exploiting the microbial reservoir, as most of the species are non-model organisms with complex regulatory systems that can be recalcitrant to engineering approaches. Genomic and metagenomic data produced by laboratories worldwide covering the range of natural and artificial environments on Earth, are an invaluable source of raw information from which natural product biosynthesis can be accessed. In the present work, we describe the environmental distribution and evolution of the abyssomicin BGC through the analysis of publicly available genomic and metagenomic data. Our results demonstrate that the selection of a pathway-specific enzyme to direct genome mining is an excellent strategy; we identified 74 new Diels–Alderase homologs and unveiled a surprising prevalence of the abyssomicin BGC within terrestrial habitats, mainly soil and plant-associated. We also identified five complete and 12 partial new abyssomicin BGCs and 23 new potential abyssomicin BGCs. Our results strongly support the potential of genome and metagenome mining as a key preliminary tool to inform bioprospecting strategies aimed at the identification of new bioactive compounds such as -but not restricted to- abyssomicins.

Diversity of Bacterial Biosynthetic Genes in Maritime Antarctica

Bacterial natural products (NPs) are still a major source of new drug leads. Polyketides (PKs) and non-ribosomal peptides (NRP) are two pharmaceutically important families of NPs and recent studies have revealed Antarctica to harbor endemic polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) genes, likely to be involved in the production of novel metabolites. Despite this, the diversity of secondary metabolites genes in Antarctica is still poorly explored. In this study, a computational bioprospection approach was employed to study the diversity and identity of PKS and NRPS genes to one of the most biodiverse areas in maritime Antarctica—Maxwell Bay. Amplicon sequencing of soil samples targeting ketosynthase (KS) and adenylation (AD) domains of PKS and NRPS genes, respectively, revealed abundant and unexplored chemical diversity in this peninsula. About 20% of AD domain sequences were only distantly related to characterized biosynthetic genes. Several PKS and NRPS genes were found to be closely associated to recently described metabolites including those from uncultured and candidate phyla. The combination of new approaches in computational biology and new culture-dependent and -independent strategies is thus critical for the recovery of the potential novel chemistry encoded in Antarctica microorganisms.

A new abyssomicin polyketide with anti-influenza A virus activity from a marine-derived Verrucosispora sp. MS100137

Marine microorganisms live in dramatically different environments and have attracted much attention for their structurally unique natural products with potential strong biological activity. Based on the one strain-many compounds (OSMAC) strategy and liquid chromatography mass spectrometry (LC-MS) methods, our continuing efforts on the investigation of novel active compounds from marine Verrucosispora sp. MS100137 has led to the identification of a new polycyclic metabolite, abyssomicin Y (1), together with six known abyssomicin and proximicin analogs (2-7). Abyssomicin Y is a type I abyssomicin with an epoxide group at C-8 and C-9. Compounds 1-3 showed potent inhibitory effects against the influenza A virus; their observed inhibition rates were 97.9%, 98.3%, and 95.9%, respectively, at a concentration of 10 μM, and they displayed lower cytotoxicity than 4. The structures were determined by different NMR techniques and HRMS experiments. This investigation revealed that OSMAC could serve as a useful method for enabling the activation of the silent genes in the microorganism and for the formation of previously unreported active secondary metabolites.

Diversity of PKS and NRPS gene clusters between Streptomyces abyssomicinicus sp. nov. and its taxonomic neighbor

Streptomyces sp. CHI39, isolated from a rock soil sample, is a producer of abyssomicin I. The taxonomic status was clarified by a polyphasic approach. Phylogenetic analysis based on 16S rRNA gene sequences showed that the strain was closely related to Streptomyces fragilis, with similarity of 99.9%. Strain CHI39 comprised LL-diaminopimelic acid, glutamic acid, glycine, and alanine in its peptidoglycan. The predominant menaquinones were MK-9(H₆), and major fatty acids were anteiso-C_15:0, anteiso-C_17:0, and iso-C_16:0. The chemotaxonomic features matched those described for the genus Streptomyces. Genome sequencing was conducted for strain CHI39 and S. fragilis NBRC 12862^T. The results of digital DNA-DNA hybridization along with differences in phenotypic characteristics between the strains suggested strain CHI39 to be a novel species, for which Streptomyces abyssomicinicus sp. nov. is proposed; the type strain is CHI39^T (=NBRC 110469^T). Next, we surveyed polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) gene clusters in genomes of S. abyssomicinicus CHI39^T and S. fragilis NBRC 12862^T. These strains encoded 9 and 12 clusters, respectively, among which only four clusters were shared between them while the others are specific in each strain. This suggests that strains classified to distinct species each harbor many specific secondary metabolite-biosynthetic pathways even if the strains are taxonomically close.

Construction of Complex Bisether-Bridged Medium-Sized Cyclic Compounds from o-(1-(Acyloxy)propargyl)benzaldehydes under Base and Acid Catalysis

We report herein our serendipitous discovery of the rapid and straightforward accesses to unprecedented diverse complex molecular structures from readily available starting materials. Catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene under mild conditions, o-(1-(acyloxy)propargyl)benzaldehydes 1 underwent efficient and selective dimerization reactions to produce novel complex bisether-bridged tricyclic products 3 and 4. The reactions proceeded most probably through dimerizations between 3-methylene-3H-isochromene intermediate and its zwitterionic resonance structures which were generated from a concerted 6-π electrocyclic ring closure reaction from the initially formed (2-formylphenyl)allene intermediates derived directly from o-(1-(acyloxy)propargyl)benzaldehydes. Treatment of the resulting product simply with NaOEt in ethanol and aqueous HCl, respectively, enabled further development of complex molecular diversities.

3-Ketoacyl-ACP synthase (KAS) III homologues and their roles in natural product biosynthesis

The 3-ketoacyl-ACP synthase (KAS) III proteins are one of the most abundant enzymes in nature, as they are involved in the biosynthesis of fatty acids and natural products. KAS III enzymes catalyse a carbon-carbon bond formation reaction that involves the α-carbon of a thioester and the carbonyl carbon of another thioester. In addition to the typical KAS III enzymes involved in fatty acid and polyketide biosynthesis, there are proteins homologous to KAS III enzymes that catalyse reactions that are different from that of the traditional KAS III enzymes. Those include enzymes that are responsible for a head-to-head condensation reaction, the formation of acetoacetyl-CoA in mevalonate biosynthesis, tailoring processes via C-O bond formation or esterification, as well as amide formation. This review article highlights the diverse reactions catalysed by this class of enzymes and their role in natural product biosynthesis.

An Esterase-like Lyase Catalyzes Acetate Elimination in Spirotetronate/Spirotetramate Biosynthesis

Spirotetronate and spirotetramate natural products include a multitude of compounds with potent antimicrobial and antitumor activities. Their biosynthesis incorporates many unusual biocatalytic steps, including regio- and stereo-specific modifications, cyclizations promoted by Diels-Alderases, and acetylation-elimination reactions. Here we focus on the acetate elimination catalyzed by AbyA5, implicated in the formation of the key Diels-Alder substrate to give the spirocyclic system of the antibiotic abyssomicin C. Using synthetic substrate analogues, it is shown that AbyA5 catalyzes stereospecific acetate elimination, establishing the (R)-tetronate acetate as a biosynthetic intermediate. The X-ray crystal structure of AbyA5, the first of an acetate-eliminating enzyme, reveals a deviant acetyl esterase fold. Molecular dynamics simulations and enzyme assays show the use of a His-Ser dyad to catalyze either elimination or hydrolysis, via disparate mechanisms, under substrate control.

Review on Abyssomicins: Inhibitors of the Chorismate Pathway and Folate Biosynthesis

Antifolates targeting folate biosynthesis within the shikimate-chorismate-folate metabolic pathway are ideal and selective antimicrobials, since higher eukaryotes lack this pathway and rely on an exogenous source of folate. Resistance to the available antifolates, inhibiting the folate pathway, underlines the need for novel antibiotic scaffolds and molecular targets. While para-aminobenzoic acid synthesis within the chorismate pathway constitutes a novel molecular target for antifolates, abyssomicins are its first known natural inhibitors. This review describes the abyssomicin family, a novel spirotetronate polyketide Class I antimicrobial. It summarizes synthetic and biological studies, structural, biosynthetic, and biological properties of the abyssomicin family members. This paper aims to explain their molecular target, mechanism of action, structure⁻activity relationship, and to explore their biological and pharmacological potential. Thirty-two natural abyssomicins and numerous synthetic analogues have been reported. The biological activity of abyssomicins includes their antimicrobial activity against Gram-positive bacteria and mycobacteria, antitumor properties, latent human immunodeficiency virus (HIV) reactivator, anti-HIV and HIV replication inducer properties. Their antimalarial properties have not been explored yet. Future analoging programs using the structure⁻activity relationship data and synthetic approaches may provide a novel abyssomicin structure that is active and devoid of cytotoxicity. Abyssomicin J and atrop-o-benzyl-desmethylabyssomicin C constitute promising candidates for such programs.

Characterization and heterologous expression of the neoabyssomicin/abyssomicin biosynthetic gene cluster from Streptomyces koyangensis SCSIO 5802

Background

The deep-sea-derived microbe Streptomyces koyangensis SCSIO 5802 produces neoabyssomicins A–B (1–2) and abyssomicins 2 (3) and 4 (4). Neoabyssomicin A (1) augments human immunodeficiency virus-1 (HIV-1) replication whereas abyssomicin 2 (3) selectively reactivates latent HIV and is also active against Gram-positive pathogens including methicillin-resistant Staphylococcus aureus (MRSA). Structurally, neoabyssomicins A–B constitute a new subtype within the abyssomicin family and feature unique structural traits characteristic of extremely interesting biosynthetic transformations.

Results

In this work, the biosynthetic gene cluster (BGC) for the neoabyssomicins and abyssomicins, composed of 28 opening reading frames, was identified in S. koyangensis SCSIO 5802, and its role in neoabyssomicin/abyssomicin biosynthesis was confirmed via gene inactivation and heterologous expression experiments. Bioinformatics and genomics analyses enabled us to propose a biosynthetic pathway for neoabyssomicin/abyssomicin biosynthesis. Similarly, a protective export system by which both types of compounds are secreted from the S. koyangensis producer was identified, as was a four-component ABC transporter-based import system central to neoabyssomicin/abyssomicin biosynthesis. Furthermore, two regulatory genes, abmI and abmH, were unambiguously shown to be positive regulators of neoabyssomicin/abyssomicin biosynthesis. Consistent with their roles as positive regulatory genes, the overexpression of abmI and abmH (independent of each other) was shown to improve neoabyssomicin/abyssomicin titers.

Conclusions

These studies provide new insight into the biosynthesis of the abyssomicin class of natural products, and highlight important exploitable features of its BGC for future efforts. Elucidation of the neoabyssomicin/abyssomicin BGC now enables combinatorial biosynthetic initiatives aimed at improving both the titers and pharmaceutical properties of these important natural products-based drug leads.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0875-1) contains supplementary material, which is available to authorized users.

The Uncommon Enzymology of Cis-Acyltransferase Assembly Lines

The enzymology of 135 assembly lines containing primarily cis-acyltransferase modules is comprehensively analyzed, with greater attention paid to less common phenomena. Diverse online transformations, in which the substrate and/or product of the reaction is an acyl chain bound to an acyl carrier protein, are classified so that unusual reactions can be compared and underlying assembly-line logic can emerge. As a complement to the chemistry surrounding the loading, extension, and offloading of assembly lines that construct primarily polyketide products, structural aspects of the assembly-line machinery itself are considered. This review of assembly-line phenomena, covering the literature up to 2017, should thus be informative to the modular polyketide synthase novice and expert alike.

The Enzymology of Organic Transformations: A Survey of Name Reactions in Biological Systems

Chemical reactions that are named in honor of their true, or at least perceived, discoverers are known as "name reactions". This Review is a collection of biological representatives of named chemical reactions. Emphasis is placed on reaction types and catalytic mechanisms that showcase both the chemical diversity in natural product biosynthesis as well as the parallels with synthetic organic chemistry. An attempt has been made, whenever possible, to describe the enzymatic mechanisms of catalysis within the context of their synthetic counterparts and to discuss the mechanistic hypotheses for those reactions that are currently active areas of investigation. This Review has been categorized by reaction type, for example condensation, nucleophilic addition, reduction and oxidation, substitution, carboxylation, radical-mediated, and rearrangements, which are subdivided by name reactions.

Natural [4 + 2]-Cyclases

[4 + 2]-Cycloadditions are increasingly being recognized in the biosynthetic pathways of many structurally complex natural products. A relatively small collection of enzymes from these pathways have been demonstrated to increase rates of cyclization and impose stereochemical constraints on the reactions. While mechanistic investigation of these enzymes is just beginning, recent studies have provided new insights with implications for understanding their biosynthetic roles, mechanisms of catalysis, and evolutionary origin.

The cave microbiome as a source for drug discovery: Reality or pipe dream?

This review highlights cave habitats, cave microbiomes and their potential for drug discovery. Such studies face many challenges, including access to remote and pristine caves, and sample collection and transport. Inappropriate physical and chemical growth conditions in the laboratory for the isolation and cultivation of cave microorganisms pose many complications including length of cultivation; some cave microorganisms can take weeks and even months to grow. Additionally, DNA extraction from cave environmental samples may be difficult due to the high concentration of various minerals that are natural DNA blocking agents. Once cave microorganisms are grown in the lab, other problems often arise, such as maintenance of pure culture, consistency of antimicrobial activity and fermentation conditions for antimicrobial production. In this review, we suggest that, although based on what has been done in the field, there is potential in using cave microorganisms to produce antimicrobial agents, one needs to be highly committed and prepared.

Diversity of Culturable Thermophilic Actinobacteria in Hot Springs in Tengchong, China and Studies of their Biosynthetic Gene Profiles

The class Actinobacteria has been a goldmine for the discovery of antibiotics and has attracted interest from both academics and industries. However, an absence of novel approaches during the last few decades has limited the discovery of new microbial natural products useful for industries. Scientists are now focusing on the ecological aspects of diverse environments including unexplored or underexplored habitats and extreme environments in the search for new metabolites. This paper reports on the diversity of culturable actinobacteria associated with hot springs located in Tengchong County, Yunnan Province, southwestern China. A total of 58 thermophilic actinobacterial strains were isolated from the samples collected from ten hot springs distributed over three geothermal fields (e.g., Hehua, Rehai, and Ruidian). Phylogenetic positions and their biosynthetic profiles were analyzed by sequencing 16S rRNA gene and three biosynthetic gene clusters (KS domain of PKS-I, KSα domain of PKS-II and A domain of NRPS). On the basis of 16S rRNA gene phylogenetic analysis, the 58 strains were affiliated with 12 actinobacterial genera: Actinomadura Micromonospora, Microbispora, Micrococcus, Nocardiopsis, Nonomuraea, Promicromonospora, Pseudonocardia, Streptomyces, Thermoactinospora, Thermocatellispora, and Verrucosispora, of which the two novel genera Thermoactinospora and Thermocatellisopora were recently described from among these strains. Considering the biosynthetic potential of these actinobacterial strains, 22 were positive for PCR amplification of at least one of the three biosynthetic gene clusters (PKS-I, PKS-II, and NRPS). These actinobacteria were further subjected to antimicrobial assay against five opportunistic human pathogens (Acinetobacter baumannii, Escherichia coli, Micrococcus luteus, Staphylococcus aureus and Streptococcus faecalis). All of the 22 strains that were positive for PCR amplification of at least one of the biosynthetic gene domains exhibited antimicrobial activities against at least one of the five test organisms. Among the remaining 36 actinobacteria that are negative for PCR amplification of the domains for the biosynthetic genes, 33 strains showed antimicrobial activities against at least one of the five test pathogens. In summary, the findings presented in this study emphasized the importance of underexplored habitats such as Tengchong hot springs as potential sources for search of bioactive molecules.

Enzymatic [4+2] Cycloadditions in the Biosynthesis of Spirotetramates and Spirotetronates

The Diels-Alder reaction is a quintessential type of [4+2] cycloaddition that remains one of the most intriguing transformations in synthetic chemistry. This reaction has long been envisaged to participate in the biosynthesis of a number of cyclohexene-containing natural products, although the question of whether a bona fide Diels-Alderase exists remains unsolved. In nature, there are remarkably few enzymes known to have the activity of [4+2] cycloaddition. These enzymes are phylogenetically distinct and are often classified according to the specific chemical structures. The variation of protein ancestors and in many cases the instability/complexity of the substrates and products pose a significant challenge in identification of the [4+2] cycloaddition catalysts using general homology-based mining approaches. We here provide the detailed description of the multiple comparison-based strategy and methods for the characterization of two distinct types of dedicated [4+2] cyclases (eg, PyrE3 and PyrI4) in the biosynthesis of spirotetramates and spirotetronates, where they act in tandem for coordinated cross-bridging of a linear polyene intermediate into a enantiomerically pure pentacyclic core. The search of new protein scaffolds with the [4+2] cycloaddition activity could enrich the pool of the candidates for mechanistic examination of a true enzymatic Diels-Alder reaction. The protocols presented in this study would also be applicable to the study of other functionally similar but phylogenetically different proteins, eg, the spiroketal cyclases.

Anti-MRSA and anti-TB metabolites from marine-derived Verrucosispora sp. MS100047

Microbes belonging to the genus Verrucosispora possess significant chemical diversity and biological properties. They have attracted the interests of many researchers and are becoming promising resources in the marine natural product research field. A bioassay-guided isolation from the crude extract of Verrucosispora sp. strain MS100047, isolated from sediments collected from the South China Sea, has led to the identification of a new salicylic derivative, glycerol 1-hydroxy-2,5-dimethyl benzoate (1), along with three known compounds, brevianamide F (2), abyssomicin B (3), and proximicin B (4). Compound 1 showed selective activity against methicillin-resistant Staphylococcus aureus (MRSA) with a minimum inhibitory concentration (MIC) value of 12.5 μg/mL. Brevianamide F (2), which was isolated from actinomycete for the first time, showed a good anti-BCG activity with a MIC value of 12.5 μg/mL that has not been reported previously in literatures. Proximicin B (4) showed significant anti-MRSA (MIC = 3.125 μg/mL), anti-BCG (MIC = 6.25 μg/mL), and anti-tuberculosis (TB) (MIC = 25 μg/mL) activities. This is the first report on the anti-tubercular activities of proximicins. In addition, Verrucosispora sp. strain MS100047 was found to harbor 18 putative secondary metabolite gene clusters based on genomic sequence analysis. These include the biosynthetic loci encoding polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) consistent with abyssomicins and proximicins, respectively. The biosynthetic pathways of these isolated compounds have been proposed. These results indicate that MS100047 possesses a great potential as a source of active secondary metabolites.

Spirotetronate polyketides as leads in drug discovery

The discovery of chlorothricin (1) defined a new family of microbial metabolites with potent antitumor antibiotic properties collectively referred to as spirotetronate polyketides. These microbial metabolites are structurally distinguished by the presence of a spirotetronate motif embedded within a macrocyclic core. Glycosylation at the periphery of this core contributes to the structural complexity and bioactivity of this motif. The spirotetronate family displays impressive chemical structures, potent bioactivities, and significant pharmacological potential. This review groups the family members based on structural and biosynthetic considerations and summarizes synthetic and biological studies that aim to elucidate their mode of action and explore their pharmacological potential.

Biosynthesis of versipelostatin: identification of an enzyme-catalyzed [4+2]-cycloaddition required for macrocyclization of spirotetronate-containing polyketides

Versipelostatin (VST) is an unusual 17-membered macrocyclic polyketide product that contains a spirotetronate skeleton. In this study, the entire VST biosynthetic gene cluster (vst) spanning 108 kb from Streptomyces versipellis 4083-SVS6 was identified by heterologous expression using a bacterial artificial chromosome vector. Here, we demonstrate that an enzyme, VstJ, catalyzes the stereoselective [4+2]-cycloaddition between the conjugated diene and the exocyclic olefin of a newly identified tetronate-containing intermediate to form the spirotetronate skeleton during VST biosynthesis.

Abyssomicin 2 reactivates latent HIV-1 by a PKC- and HDAC-independent mechanism

Screening of a marine natural products library afforded three new analogues of the tetronic acid containing polyketide abyssomicin family and identified abyssomicin 2 as a selective reactivator of latent HIV virus. Examination of the mode of action of this new latent HIV reactivating agent demonstrated that it functions via a distinct mechanism compared to that of existing reactivating agents and is effective at reactivating latent virus in a subset of primary patient cell lines.

A twist of nature – the significance of atropisomers in biological systems

This review encompasses the synthesis and identification of recently detected natural atropisomers with potential therapeutic activity.

Marine Microbial Secondary Metabolites: Pathways, Evolution and Physiological Roles

Microbes produce a huge array of secondary metabolites endowed with important ecological functions. These molecules, which can be catalogued as natural products, have long been exploited in medical fields as antibiotics, anticancer and anti-infective agents. Recent years have seen considerable advances in elucidating natural-product biosynthesis and many drugs used today are natural products or natural-product derivatives. The major contribution to recent knowledge came from application of genomics to secondary metabolism and was facilitated by all relevant genes being organised in a contiguous DNA segment known as gene cluster. Clustering of genes regulating biosynthesis in bacteria is virtually universal. Modular gene clusters can be mixed and matched during evolution to generate structural diversity in natural products. Biosynthesis of many natural products requires the participation of complex molecular machines known as polyketide synthases and non-ribosomal peptide synthetases. Discovery of new evolutionary links between the polyketide synthase and fatty acid synthase pathways may help to understand the selective advantages that led to evolution of secondary-metabolite biosynthesis within bacteria. Secondary metabolites confer selective advantages, either as antibiotics or by providing a chemical language that allows communication among species, with other organisms and their environment. Herewith, we discuss these aspects focusing on the most clinically relevant bioactive molecules, the thiotemplated modular systems that include polyketide synthases, non-ribosomal peptide synthetases and fatty acid synthases. We begin by describing the evolutionary and physiological role of marine natural products, their structural/functional features, mechanisms of action and biosynthesis, then turn to genomic and metagenomic approaches, highlighting how the growing body of information on microbial natural products can be used to address fundamental problems in environmental evolution and biotechnology.