Distribution and functional analysis of the phosphopantetheinyl transferase superfamily in Actinomycetales microorganisms

Sorbremnoids A and B: NLRP3 Inflammasome Inhibitors Discovered from Spatially Restricted Crosstalk of Biosynthetic Pathways

Crosstalk-oriented chemical evolution of natural products (NPs) is an efficacious strategy for generating novel skeletons through coupling reactions between NP fragments. In this study, two NOD-like receptor protein 3 (NLRP3) inflammasome inhibitors, sorbremnoids A and B (1 and 2), with unprecedented chemical architectures were identified from a fungus Penicillium citrinum. Compounds 1 and 2 exemplify rare instances of hybrid NPs formed via a major facilitator superfamily (MFS)-like enzyme by coupling reactive intermediates from two separate biosynthetic gene clusters (BGCs), pcisor and pci56. Both sorbremnoids A and B are NLRP3 inflammasome inhibitors. Sorbremnoid A demonstrated strong inhibition of IL-1β by directly binding to the NLRP3 protein, inhibiting the assembly and activation of the NLRP3 inflammasome in vitro, with potential application in diabetic refractory wound healing through the suppression of excessive inflammatory responses. This research will inspire the development of anti-NLRP3 inflammasome agents as lead treatments and enhance knowledge pertaining to NPs derived from biosynthetic crosstalk.

Capability of a large bacterial artificial chromosome clone harboring multiple biosynthetic gene clusters for the production of diverse compounds

The biosynthetic gene clusters (BGCs) for the macrocyclic lactone-based polyketide compounds are extremely large-sized because the polyketide synthases that generate the polyketide chains of the basic backbone are of very high molecular weight. In developing a heterologous expression system for the large BGCs amenable to the production of such natural products, we selected concanamycin as an appropriate target. We obtained a bacterial artificial chromosome (BAC) clone with a 211-kb insert harboring the entire BGC responsible for the biosynthesis of concanamycin. Heterologous expression of this clone in a host strain, Streptomyces avermitilis SUKA32, permitted the production of concanamycin, as well as that of two additional aromatic polyketides. Structural elucidation identified these additional products as ent-gephyromycin and a novel compound that was designated JBIR-157. We describe herein sequencing and expression studies performed on these BGCs, demonstrating the utility of large BAC clones for the heterologous expression of cryptic or near-silent loci.

Actinobacterial chalkophores: the biosynthesis of hazimycins

Copper is a transition metal element with significant effects on the morphological development and secondary metabolism of actinobacteria. In some microorganisms, copper-binding natural products are employed to modulate copper homeostasis, although their significance in actinobacteria remains largely unknown. Here, we identified the biosynthetic genes of the diisocyanide natural product hazimycin in Kitasatospora purpeofusca HV058, through gene knock-out and heterologous expression. Biochemical analyses revealed that hazimycin A specifically binds to copper, which diminishes its antimicrobial activity. The presence of a set of putative importer/exporter genes surrounding the biosynthetic genes suggested that hazimycin is a chalkophore that modulates the intracellular copper level. A bioinformatic survey of homologous gene cassettes, as well as the identification of two previously unknown hazimycin-producing Streptomyces strains, indicated that the isocyanide-based mechanism of copper homeostasis is prevalent in actinobacteria.

Expression in CHO cells of a bacterial biosynthetic pathway producing a small non-ribosomal peptide aldehyde prevents proteolysis of recombinant proteins

A significant problem during recombinant protein production is proteolysis. One of the most common preventive strategies is the addition of protease inhibitors, which has drawbacks, such as their short half-life and high cost, and their limited prevention of extracellular proteolysis. Actinomycetes produce the most commonly used inhibitors, which are non-ribosomal small aldehydic peptides. Previously, an unprecedented biosynthetic route involving a condensation-minus non-ribosomal peptide synthetase (NRPSs) and a tRNA utilizing enzyme (tRUE) was shown to direct the synthesis of one of these inhibitor peptides, livipeptin. Here, we show that expression of the livipeptin biosynthetic pathway encoded by the lvp genes in CHO cells resulted in the production of this metabolite with cysteine protease inhibitory activity, implying that mammalian tRNAs were recruited by the lvp system. CHO cells transiently expressing the biosynthetic pathway produced livipeptin without affecting cell growth or viability. Expression of the lvp system in CHO cells producing two model proteins, secreted alkaline phosphatase (hSeAP) and a monoclonal antibody, resulted in higher specific productivity with reduced proteolysis. We show for the first time that the expression of a bacterial biosynthetic pathway is functional in CHO cells, resulting in the efficient, low-cost synthesis of a protease inhibitor without adverse effects on CHO cells. This expands the field of metabolic engineering of mammalian cells by expressing the overwhelming diversity of actinomycetes biosynthetic pathways and opens a new option for proteolysis inhibition in bioprocess engineering.

Discovery of type II polyketide synthase-like enzymes for the biosynthesis of cispentacin

Type II polyketide synthases (PKSs) normally synthesize polycyclic aromatic compounds in nature, and the potential to elaborate further diverse skeletons was recently revealed by the discovery of a polyene subgroup. Here, we show a type II PKS machinery for the biosynthesis of a five-membered nonaromatic skeleton contained in the nonproteinogenic amino acid cispentacin and the plant toxin coronatine. We successfully produce cispentacin in a heterologous host and reconstruct its biosynthesis using seven recombinant proteins in vitro. Biochemical analyses of each protein reveal the unique enzymatic reactions, indicating that a heterodimer of type II PKS-like enzymes (AmcF-AmcG) catalyzes a single C2 elongation as well as a subsequent cyclization on the acyl carrier protein (AmcB) to form a key intermediate with a five-membered ring. The subsequent reactions, which are catalyzed by a collection of type II PKS-like enzymes, are also peculiar. This work further expands the definition of type II PKS and illuminates an unexplored genetic resource for natural products.

Enhancement of milbemycins production by phosphopantetheinyl transferase and regulatory pathway engineering in Streptomyces bingchenggensis

Milbemycins (MILs), a group of 16-membered insecticidal macrocylic lactones, are widely used as the biological pesticide and the precursors of semi-synthetic veterinary drugs. Polyketide synthases (PKSs), which require phosphopantetheinyl transferases (PPTases) to activate their ACP domains from apo forms to holo forms, catalyze the backbone biosynthesis of MILs. Here we found there was a complex phosphopantetheinylation network mediated by five putative PPTases in Streptomyces bingchenggensis. Repression mutants of PpA27 and PpA62 via CRISPRi both produced significantly lower yields of MILs than that of the control strain. Repression mutant of PpA68 led to abolishment of the pigment production. MILs production was significantly enhanced by PpA27 overexpression, while not by the overexpression of other PPTases. PpA27 was thus proved a dedicated post-translational enzyme to activate PKSs involved in the MILs biosynthesis. MILs titer was further enhanced by co-overexpression of PpA27 and MilR, the pathway‑specific transcriptional activator of MIL biosynthetic gene cluster. When PpA27 and MilR were co-overexpressed in the industrial S. bingchenggensis HMB, MILs production was increased by 40.5%. These results indicated that tuning the antibiotic biosynthetic pathway by co-engineering transcriptional regulation network and post-translational phosphopantetheinylation network is an effective strategy for antibiotic production improvement.

Exploring a chemical scaffold for rapid and selective photoaffinity labelling of non-ribosomal peptide synthetases in living bacterial cells

Non-ribosomal peptide synthetases (NRPSs) biosynthesize many pharmaceuticals and virulence factors. The biosynthesis of these natural peptide products from biosynthetic gene clusters depends on complex regulations in bacteria. However, our current knowledge of NRPSs is based on enzymological studies using full NRPS systems and/or a single NRPS domain in heterologous hosts. Chemical and/or biochemical strategies to capture the endogenous activities of NRPSs facilitate studies on NRPS cell biology in bacterial cells. Here, we describe a chemical scaffold for the rapid and selective photoaffinity labelling of NRPSs in purified systems, crude biological samples and living bacterial cells. We synthesized photoaffinity labelling probes coupled with 5'-O-N-(phenylalanyl)sulfamoyladenosine with clickable alkyl diazirine or trifluoromethyl phenyl diazirine. We found that a trifluoromethyl phenyl diazirine-based probe cross-linked the Phe-activating domain of a GrsA-NRPS with high selectivity and sensitivity at shorter ultraviolet (UV) irradiation times (less than 5 min) relative to a prototypical benzophenone-based probe. Our results demonstrated that this quick labelling protocol can prevent damage to proteins and cells caused by long UV irradiation times, providing a mild photoaffinity labelling method for biological samples. This article is part of the theme issue 'Reactivity and mechanism in chemical and synthetic biology'.

An in-cluster Sfp-type phosphopantetheinyl transferase instead of the holo-ACP synthase activates the granaticin biosynthesis under natural physiological conditions

Bacterial aromatic polyketides are mainly biosynthesized by type II polyketide synthases (PKSs). The PKSs cannot be functional unless their acyl carrier proteins (ACPs) are phosphopantetheinylated by phosphopantetheinyl transferases (PPTases). Gra-ORF32 was identified as an in-cluster PPTase dedicated for granaticin biosynthesis in Streptomyces vietnamensis and the Arg- and Pro-rich N terminus was found to be crucial for catalytic activity. Overexpression of the encoding genes of the holo-ACP synthases of fatty acid synthases (FAS ACPSs) of both E. coli and S. vietnamensis could efficiently activate the production of granaticins in the Δgra-orf32 mutant, suggesting the ACP of granaticin (graACP) is an efficient substrate for FAS ACPSs. However, Gra-ORF32, the cognate PPTase of the graACP, could not compensate the conditional deficiency of ACPS in E. coli HT253, indicating that it has evolved to be functionally segregated from fatty acid biosynthesis. Nine out of eleven endogenous and all the tested exogenous non-cognate PPTases could activate the production of granaticins to varied extents when overexpressed in the Δgra-orf32 mutant, indicating that ACPs of type II PKSs could also be widely recognized as effective substrates by the Sfp-type PPTases. The exogenous PPTases of type II PKSs activated the production of granaticins with much higher efficiency, suggesting that the phylogenetically distant in-cluster PPTases of type II PKSs could share substrate preferences for the ACPs of type II PKSs. A significantly elevated production of granaticins was observed when the mutant Δgra-orf32 was cultivated on ISP2 plates, which was a consequence of crosstalk between the granaticin pathway and a kinamycin-like pathway as revealed by transcriptome analysis and pathway inactivations. Although the host FAS ACPS could efficiently activate the production of granaticins when overexpressed, only Gra-ORF32 activated the efficient production of granaticins under natural physiological conditions, indicating that the activity of the host FAS ACPS was strictly regulated, possibly by binding the FAS holo-ACP product with high affinity. Our findings would contribute to a more comprehensive understanding of how the ACPs of type II PKSs are activated and facilitate the future functional reconstitutions of type II PKSs in E. coli.

Enhanced Rishirilide Biosynthesis by a Rare In-Cluster Phosphopantetheinyl Transferase in Streptomyces xanthophaeus

Phosphopantetheinyl transferases (PPTases) play important roles in activating apo-acyl carrier proteins (apo-ACPs) and apo-peptidyl carrier proteins (apo-PCPs) in both primary and secondary metabolism. PPTases catalyze the posttranslational modifications of those carrier proteins by covalent attachment of the 4'-phosphopantetheine group to a conserved serine residue. The protein-protein interactions between a PPTase and a cognate acyl or peptidyl carrier protein have important regulatory functions in microbial biosynthesis, but the molecular mechanism underlying their specific recognition remains elusive. In this study, we identified a new rishirilide biosynthetic gene cluster with a rare in-cluster PPTase from Streptomyces xanthophaeus no2. The function of this Sfp-type PPTase, SxrX, in rishirilide production was confirmed using genetic mutagenesis and biochemical characterization. We applied molecular modeling and site-directed mutagenesis to identify key residues mediating the protein-protein interaction between SxrX and its cognate ACP. In addition, six natural products were isolated from wild-type S. xanthophaeus no2 and the ΔsxrX mutant, including rishirilide A and lupinacidin A, that exhibited antimicrobial and anticancer activities, respectively. SxrX is the first Sfp-type PPTase identified from an aromatic polyketide biosynthetic gene cluster and shown to be responsible for high-level production of rishirilide derivatives. IMPORTANCE Genome mining has been a vital means for natural product drug discovery in the postgenomic era. The rishirilide-type polyketides have attracted attention due to their potent bioactivity, but the poor robustness of production hosts has limited further research and development. This study not only identifies a hyperproducer of rishirilides but also reveals a rare, in-cluster PPTase SxrX that plays an important role in boosting rishirilide biosynthesis. Experimental and computational investigations revealed new insights on the protein-protein interaction between SxrX and its cognate ACP with wide implications for understanding polyketide biosynthesis.

Comparative and pangenomic analysis of the genus Streptomyces

Streptomycetes are highly metabolically gifted bacteria with the abilities to produce bioproducts that have profound economic and societal importance. These bioproducts are produced by metabolic pathways including those for the biosynthesis of secondary metabolites and catabolism of plant biomass constituents. Advancements in genome sequencing technologies have revealed a wealth of untapped metabolic potential from Streptomyces genomes. Here, we report the largest Streptomyces pangenome generated by using 205 complete genomes. Metabolic potentials of the pangenome and individual genomes were analyzed, revealing degrees of conservation of individual metabolic pathways and strains potentially suitable for metabolic engineering. Of them, Streptomyces bingchenggensis was identified as a potent degrader of plant biomass. Polyketide, non-ribosomal peptide, and gamma-butyrolactone biosynthetic enzymes are primarily strain specific while ectoine and some terpene biosynthetic pathways are highly conserved. A large number of transcription factors associated with secondary metabolism are strain-specific while those controlling basic biological processes are highly conserved. Although the majority of genes involved in morphological development are highly conserved, there are strain-specific varieties which may contribute to fine tuning the timing of cellular differentiation. Overall, these results provide insights into the metabolic potential, regulation and physiology of streptomycetes, which will facilitate further exploitation of these important bacteria.

Discovery of prescopranone, a key intermediate in scopranone biosynthesis

A key intermediate in scopranone biosynthesis, prescopranone, accumulated in the mycelium of Streptomyces avermitilis SUKA carrying the biosynthetic gene cluster for scopranone lacking the sprT encoding the monooxygenase. The structure of prescopranone was elucidated by NMR and other spectral data. Prescopranone consists of a 2-pyranone ring with two atypical scoop-like moieties (1-ethyl-1-propenyl and 2-ethylbutyl groups), which was deduced as a product of the modular polyketide syntheses encoded by sprA, sprB, and sprC. Prescopranone inhibited bone morphogenetic protein (BMP)-induced alkaline phosphatase activity in a BMP receptor mutant cell line.

Speed dating for enzymes! Finding the perfect phosphopantetheinyl transferase partner for your polyketide synthase

The biosynthetic pathways for the fungal polyketides bikaverin and bostrycoidin, from Fusarium verticillioides and Fusarium solani respectively, were reconstructed and heterologously expressed in S. cerevisiae alongside seven different phosphopantetheinyl transferases (PPTases) from a variety of origins spanning bacterial, yeast and fungal origins. In order to gauge the efficiency of the interaction between the ACP-domains of the polyketide synthases (PKS) and PPTases, each were co-expressed individually and the resulting production of target polyketides were determined after 48 h of growth. In co-expression with both biosynthetic pathways, the PPTase from Fusarium verticillioides (FvPPT1) proved most efficient at producing both bikaverin and bostrycoidin, at 1.4 mg/L and 5.9 mg/L respectively. Furthermore, the remaining PPTases showed the ability to interact with both PKS's, except for a single PKS-PPTase combination. The results indicate that it is possible to boost the production of a target polyketide, simply by utilizing a more optimal PPTase partner, instead of the commonly used PPTases; NpgA, Gsp and Sfp, from Aspergillus nidulans, Brevibacillus brevis and Bacillus subtilis respectively.

Engineering Aspergillus oryzae for the Heterologous Expression of a Bacterial Modular Polyketide Synthase

Microbial natural products have had phenomenal success in drug discovery and development yet form distinct classes based on the origin of their native producer. Methods that enable metabolic engineers to combine the most useful features of the different classes of natural products may lead to molecules with enhanced biological activities. In this study, we modified the metabolism of the fungus Aspergillus oryzae to enable the synthesis of triketide lactone (TKL), the product of the modular polyketide synthase DEBS1-TE engineered from bacteria. We established (2S)-methylmalonyl-CoA biosynthesis via introducing a propionyl-CoA carboxylase complex (PCC); reassembled the 11.2 kb DEBS1-TE coding region from synthetic codon-optimized gene fragments using yeast recombination; introduced bacterial phosphopantetheinyltransferase SePptII; investigated propionyl-CoA synthesis and degradation pathways; and developed improved delivery of exogenous propionate. Depending on the conditions used titers of TKL ranged from <0.01-7.4 mg/L. In conclusion, we have demonstrated that A. oryzae can be used as an alternative host for the synthesis of polyketides from bacteria, even those that require toxic or non-native substrates. Our metabolically engineered A. oryzae may offer advantages over current heterologous platforms for producing valuable and complex natural products.

Mining of Cyanobacterial Genomes Indicates Natural Product Biosynthetic Gene Clusters Located in Conjugative Plasmids

Microbial natural products are compounds with unique chemical structures and diverse biological activities. Cyanobacteria commonly possess a wide range of biosynthetic gene clusters (BGCs) to produce natural products. Although natural product BGCs have been found in almost all cyanobacterial genomes, little attention has been given in cyanobacterial research to the partitioning of these biosynthetic pathways in chromosomes and plasmids. Cyanobacterial plasmids are believed to disperse several natural product BGCs, such as toxins, by plasmids through horizontal gene transfer. Therefore, plasmids may confer the ability to produce toxins and may play a role in the evolution of diverse natural product BGCs from cyanobacteria. Here, we performed an analysis of the distribution of natural product BGCs in 185 genomes and mapped the presence of genes involved in the conjugation in plasmids. The 185 analyzed genomes revealed 1817 natural products BGCs. Individual genomes contained 1–42 biosynthetic pathways (mean 8), 95% of which were present in chromosomes and the remaining 5% in plasmids. Of the 424 analyzed cyanobacterial plasmids, 12% contained homologs of genes involved in conjugation and natural product biosynthetic pathways. Among the biosynthetic pathways in plasmids, manual curation identified those to produce aeruginosin, anabaenopeptin, ambiguine, cryptophycin, hassallidin, geosmin, and microcystin. These compounds are known toxins, protease inhibitors, odorous compounds, antimicrobials, and antitumorals. The present study provides in silico evidence using genome mining that plasmids may be involved in the distribution of natural product BGCs in cyanobacteria. Consequently, cyanobacterial plasmids have importance in the context of biotechnology, water management, and public health risk assessment. Future research should explore in vivo conjugation and the end products of natural product BGCs in plasmids via chemical analyses.

Escherichia coli Nissle 1917 secondary metabolism: aryl polyene biosynthesis and phosphopantetheinyl transferase crosstalk

Escherichia coli Nissle 1917 (EcN) is a Gram-negative bacterium that is used to treat inflammatory bowel diseases. The probiotic character of EcN is not well-understood, but its ability to produce secondary metabolites plays an important role in its activity. The EcN genome encodes for an aryl polyene (APE) biosynthetic gene cluster (BGC), and APE products have a role in biofilm formation. We show here that this unusual polyketide assembly line synthase produces four APE molecules which are likely cis/trans isomers. Within the APE BGC, two acyl carrier proteins are involved in biosynthesis. Acyl carrier proteins require activation by post-translational modification with a phosphopantetheinyl transferase (PPTase). Through analysis of single, double, and triple mutants of three PPTases, the PPTase-BGC crosstalk relationship in EcN was characterized. Understanding PPTase-BGC crosstalk is important for the engineering of secondary metabolite production hosts and for targeting of PPTases with new antibiotics. KEY POINTS: • Escherichia coli Nissle 1917 biosynthesizes four aryl polyene isoforms. • Phosphopantetheinyl transferase crosstalk is important for biosynthesis.

Reconstruction of a Genome-Scale Metabolic Model of Streptomyces albus J1074: Improved Engineering Strategies in Natural Product Synthesis

Streptomyces albus J1074 is recognized as an effective host for heterologous production of natural products. Its fast growth and efficient genetic toolbox due to a naturally minimized genome have contributed towards its advantage in expressing biosynthetic pathways for a diverse repertoire of products such as antibiotics and flavonoids. In order to develop precise model-driven engineering strategies for de novo production of natural products, a genome-scale metabolic model (GEM) was reconstructed for the microorganism based on protein homology to model species Streptomyces coelicolor while drawing annotated data from databases and literature for further curation. To demonstrate its capabilities, the Salb-GEM was used to predict overexpression targets for desirable compounds using flux scanning with enforced objective function (FSEOF). Salb-GEM was also utilized to investigate the effect of a minimized genome on metabolic gene essentialities in comparison to another Streptomyces species, S. coelicolor.

Hemiacetal-less rapamycin derivatives designed and produced by genetic engineering of a type I polyketide synthase

Engineering polyketide synthases is one of the most promising ways of producing a variety of polyketide derivatives. Exploring the undiscovered chemical space of this medicinally important class of middle molecular weight natural products will aid in the development of improved drugs in the future. In previous work, we established methodology designated 'module editing' to precisely manipulate polyketide synthase genes cloned in a bacterial artificial chromosome. Here, in the course of investigating the engineering capacity of the rapamycin PKS, novel rapamycin derivatives 1-4, which lack the hemiacetal moiety, were produced through the heterologous expression of engineered variants of the rapamycin PKS. Three kinds of module deletions in the polyketide synthase RapC were designed, and the genetically engineered vectors were prepared by the in vitro module editing technique. Streptomyces avermitilis SUKA34 transformed with these edited PKSs produced new rapamycin derivatives. The planar structures of 1-4 established based on 1D and 2D NMR, ESI-TOF-MS and UV spectra revealed that 2 and 3 had skeletons well-matched to the designs, but 1 and 4 did not. The observations provide important insights into the mechanisms of the later steps of rapamycin skeletal formation as well as the ketone-forming oxygenase RapJ.

Identification of functional cytochrome P450 and ferredoxin from Streptomyces sp. EAS-AB2608 by transcriptional analysis and their heterologous expression

Bioconversion using microorganisms and their enzymes is an important tool in many industrial fields. The discovery of useful new microbial enzymes contributes to the development of industries utilizing bioprocesses. Streptomyces sp. EAS-AB2608, isolated from a soil sample collected in Japan, can convert the tetrahydrobenzotriazole CPD-1 (a selective positive allosteric modulator of metabotropic glutamate receptor 5) to its hydroxylated form at the C4-(R) position. The current study was performed to identify the genes encoding the enzymes involved in CPD-1 bioconversion and to verify their function. To identify gene products responsible for the conversion of CPD-1, we used RNA sequencing to analyze EAS-AB2608; from its 8333 coding sequences, we selected two genes, one encoding cytochrome P450 (easab2608_00800) and the other encoding ferredoxin (easab2608_00799), as encoding desirable gene products involved in the bioconversion of CPD-1. The validity of this selection was tested by using a heterologous expression approach. A bioconversion assay using genetically engineered Streptomyces avermitilis SUKA24 ∆saverm3882 ∆saverm7246 co-expressing the two selected genes (strain ES_SUKA_63) confirmed that these gene products had hydroxylation activity with respect to CPD-1, indicating that they are responsible for the conversion of CPD-1. Strain ES_SUKA_63 also showed oxidative activity toward other compounds and therefore might be useful not only for bioconversion of CPD-1 but also as a tool for synthesis of drug metabolites and in optimization studies of various pharmaceutical lead compounds. We expect that this approach will be useful for bridging the gap between the latest enzyme optimization technologies and conventional enzyme screening using microorganisms. KEY POINTS: • Genes easab2608_00800 (cyp) and easab2608_00799 (fdx) were selected by RNA-Seq. • Selection validity was evaluated by an engineered S. avermitilis expression system. • Strain ES_SUKA_63 showed oxidative activity toward CPD-1 and other compounds.

A novel methymycin analog, 12-ketomethymycin N-oxide, produced by the heterologous expression of the large pikromycin/methymycin biosynthetic gene cluster of Streptomyces sp. AM4900

A novel methymycin analog, 12-ketomethymycin N-oxide, was produced by the heterologous expression of the pikromycin/methymycin biosynthetic gene cluster of Streptomyces sp. AM4900 together with 12-ketomethymycin, which was only isolated by the biotransformation of the synthetic intermediate before. Their structures were determined by the spectroscopic data and the chemical derivatization. 12-Ketomethymycin showed a weak cytotoxicity against SKOV-3 and Jurkat cells, although its N-oxide analog did not show any activity. Both showed no antibacterial activities against Escherichia coli and Micrococcus luteus.

Plant-Derived Cell-Free Biofactories for the Production of Secondary Metabolites

Cell-free expression systems enable the production of proteins and metabolites within a few hours or days. Removing the cellular context while maintaining the protein biosynthesis apparatus provides an open system that allows metabolic pathways to be installed and optimized by expressing different numbers and combinations of enzymes. This facilitates the synthesis of secondary metabolites that are difficult to produce in cell-based systems because they are toxic to the host cell or immediately converted into downstream products. Recently, we developed a cell-free lysate derived from tobacco BY-2 cell suspension cultures for the production of recombinant proteins. This system is remarkably productive, achieving yields of up to 3 mg/mL in a one-pot in vitro transcription-translation reaction and contains highly active energy and cofactor regeneration pathways. Here, we demonstrate for the first time that the BY-2 cell-free lysate also allows the efficient production of several classes of secondary metabolites. As case studies, we synthesized lycopene, indigoidine, betanin, and betaxanthins, which are useful in the food, cosmetic, textile, and pharmaceutical industries. Production was achieved by the co-expression of up to three metabolic enzymes. For all four products, we achieved medium to high yields. However, the yield of betanin (555 μg/mL) was outstanding, exceeding the level reported in yeast cells by a factor of more than 30. Our results show that the BY-2 cell-free lysate is suitable not only for the verification and optimization of metabolic pathways, but also for the efficient production of small to medium quantities of secondary metabolites.

Coordinating precursor supply for pharmaceutical polyketide production in Streptomyces

The widely used polyketide pharmaceuticals in medicine and agriculture are mainly produced by Streptomyces species. These compounds, as secondary metabolites, are not involved in essential cellular processes and are usually produced during the stationary phase of fermentation. Consequently, their yields and productivities are often low and frequently limited by the availability of the precursors. The precursor pathways, therefore, are key entities for synthetic biology-driven design and optimization. We discuss recent advances in precursor engineering, in both Streptomyces and other bacteria, focusing on the diverse native and heterologous precursor pathways that could be rewired for polyketide titer improvement. We also highlight the coordination of other required factors to direct the precursors towards polyketide biosynthesis. The precursor-supply enhancement tools and strategies covered in this review will facilitate the design and construction of synthetic Streptomyces 'cell-factories' for efficient polyketide production.

Modifications to central carbon metabolism in an engineered Streptomyces host to enhance secondary metabolite production

To improve the production of secondary metabolites by alternation of the carbon metabolic flux, two types of deletion mutants of the central metabolic pathway, the Embden-Meyerhof (EM) or pentose phosphate (PP) pathway, in the genetically engineered Streptomyces avermitilis were constructed. Double-deletion mutants of phosphofructokinase (ΔpfkA1ΔpfkA3) in the EM pathway carrying a gene cluster for chloramphenicol biosynthesis markedly increased chloramphenicol production synthesized through the shikimate pathway. Although the ΔpfkA1ΔpfkA3 double-deletion mutant grew more slowly, its specific productivity of chloramphenicol (per dry cell weight) was 2.0-fold higher than that of the engineered S. avermitilis strain. However, the productivity of chloramphenicol was lower by the double-deletion mutant of transaldolase in the PP pathway, which supplies the precursor of the shikimate pathway. A carbon-flux analysis of the EM and PP pathways using [1-¹³C] glucose revealed that carbon flux in the ΔpfkA1ΔpfkA3 double-deletion mutant increased through the PP pathway, which enhanced the production of chloramphenicol. These results suggest that a metabolic modification approach has the potential to increase the titers and yields of valuable secondary metabolites.

In vitro Cas9-assisted editing of modular polyketide synthase genes to produce desired natural product derivatives

One major bottleneck in natural product drug development is derivatization, which is pivotal for fine tuning lead compounds. A promising solution is modifying the biosynthetic machineries of middle molecules such as macrolides. Although intense studies have established various methodologies for protein engineering of type I modular polyketide synthase(s) (PKSs), the accurate targeting of desired regions in the PKS gene is still challenging due to the high sequence similarity between its modules. Here, we report an innovative technique that adapts in vitro Cas9 reaction and Gibson assembly to edit a target region of the type I modular PKS gene. Proof-of-concept experiments using rapamycin PKS as a template show that heterologous expression of edited biosynthetic gene clusters produced almost all the desired derivatives. Our results are consistent with the promiscuity of modular PKS and thus, our technique will provide a platform to generate rationally designed natural product derivatives for future drug development.

Several different genetic strategies have been reported for the modification of polyketide synthases but the highly repetitive modular structure makes this difficult. Here the authors report on an adapted Cas9 reaction and Gibson assembly to edit a target region of the polyketide synthases gene in vitro.

Identification, cloning and heterologous expression of biosynthetic gene cluster for desertomycin

From our in-house microbial genome database of secondary metabolite producers, we identified a candidate biosynthetic gene cluster for desertomycin from Streptomyces nobilis JCM4274. We report herein the cloning of the 127-kb entire gene cluster for desertomycin biosynthesis using bacterial artificial chromosome vector. The entire biosynthetic gene cluster for desertomycin was introduced in the heterologous host, Streptomyces lividans TK23, with an average yield of more than 130 mg l^-1.

An Unusual Extender Unit Is Incorporated into the Modular Polyketide Synthase of Scopranones Biosynthesis

Scopranones, produced by Streptomyces sp. BYK-11038, are the novel bone morphogenetic protein inhibitors characterized by atypical two scoop-like moieties and a 3-furanone moiety. Two scoop-like moieties connected to a 3-furanone have not previously been reported in natural products, and their biosynthesis must occur via a unique pathway. Feeding experiments using ¹³C-labeled precursors indicated that scopranones were synthesized from three acetates and three butyrates in polyketide-type biosynthesis. Genome mining of Streptomyces sp. BYK-11038 revealed that the candidate biosynthetic gene cluster contains 21 open reading frames (ORFs), including three modular polyketide synthases (PKSs; SprA, SprB, and SprC), which were composed of 4 modules with one loading module and 18 additional ORFs (SprD to SprU) spanning a distance of 55 kbp. The characterization of in-frame deletion mutants and feeding experiments with the predicted extender units indicated that two genes, sprP and sprR, encoding discrete 3-oxoacyl-ACP synthases, and a gene, sprO, encoding crotonyl-CoA reductase, were involved in assembling an unusual C₈ branched extender unit, 2-(2-ethylbutyl)malonyl-CoA. Additionally, three ORFs, sprM, sprN, and sprT, encoding cytochrome P450s and a monooxygenase, are important tailoring enzymes in post-PKS modification. SprT is an essential enzyme for decarboxylative ring contraction via oxidation, which converts the 2-pyranone to a 3-furanone.

Activation and Characterization of Cryptic Gene Cluster: Two Series of Aromatic Polyketides Biosynthesized by Divergent Pathways

One biosynthetic gene cluster (BGC) usually governs the biosynthesis of a series of compounds exhibiting either the same or similar molecular scaffolds. Reported here is a multiplex activation strategy to awaken a cryptic BGC associated with tetracycline polyketides, resulting in the discovery of compounds having different core structures. By constitutively expressing a positive regulator gene in tandem mode, a single BGC directed the biosynthesis of eight aromatic polyketides with two types of frameworks, two pentacyclic isomers and six glycosylated tetracyclines. The proposed biosynthetic pathway, based on systematic gene inactivation and identification of intermediates, employs two sets of tailoring enzymes with a branching point from the same intermediate. These findings not only provide new insights into the role of tailoring enzymes in the diversification of polyketides, but also highlight a reliable strategy for genome mining of natural products.

Functional Characterization of 3-Aminobenzoic Acid Adenylation Enzyme PctU and UDP-N-Acetyl-d-Glucosamine: 3-Aminobenzoyl-ACP Glycosyltransferase PctL in Pactamycin Biosynthesis

Pactamycin is an antibiotic produced by Streptomyces pactum with antitumor and antimalarial properties. Pactamycin has a unique aminocyclitol core that is decorated with 3-aminoacetophenone, 6-methylsaliciate, and an N,N-dimethylcarbamoyl group. Herein, we show that the adenylation enzyme PctU activates 3-aminobenzoic acid (3ABA) with adenosine triphosphate and ligates it to the holo form of the discrete acyl carrier protein PctK to yield 3ABA-PctK. Then, 3ABA-PctK is N-glycosylated with uridine diphosphate-N-acetyl-d-glucosamine (UDP-GlcNAc) by the glycosyltransferase PctL to yield GlcNAc-3ABA-PctK. Because 3ABA is known to be a precursor of the 3-aminoacetophenone moiety, PctU appears to be a gatekeeper that selects the appropriate 3-aminobenzoate starter unit. Overall, we propose that acyl carrier protein-bound glycosylated 3ABA derivatives are biosynthetic intermediates of pactamycin biosynthesis.

Leveraging synthetic biology for producing bioactive polyketides and non-ribosomal peptides in bacterial heterologous hosts

Bacteria have historically been a rich source of natural products (e.g. polyketides and non-ribosomal peptides) that possess medically-relevant activities. Despite extensive discovery programs in both industry and academia, a plethora of biosynthetic pathways remain uncharacterized and the corresponding molecular products untested for potential bioactivities. This knowledge gap comes in part from the fact that many putative natural product producers have not been cultured in conventional laboratory settings in which the corresponding products are produced at detectable levels. Next-generation sequencing technologies are further increasing the knowledge gap by obtaining metagenomic sequence information from complex communities where production of the desired compound cannot be isolated in the laboratory. For these reasons, many groups are turning to synthetic biology to produce putative natural products in heterologous hosts. This strategy depends on the ability to heterologously express putative biosynthetic gene clusters and produce relevant quantities of the corresponding products. Actinobacteria remain the most abundant source of natural products and the most promising heterologous hosts for natural product discovery and production. However, researchers are discovering more natural products from other groups of bacteria, such as myxobacteria and cyanobacteria. Therefore, phylogenetically similar heterologous hosts have become promising candidates for synthesizing these novel molecules. The downside of working with these microbes is the lack of well-characterized genetic tools for optimizing expression of gene clusters and product titers. This review examines heterologous expression of natural product gene clusters in terms of the motivations for this research, the traits desired in an ideal host, tools available to the field, and a survey of recent progress.