Bacterial hosts for natural product production

Natural products of pentacyclic triterpenoids: from discovery to heterologous biosynthesis

Covering: up to 2022Pentacyclic triterpenoids are important natural bioactive substances that are widely present in plants and fungi. They have significant medicinal efficacy, play an important role in reducing blood glucose and protecting the liver, and have anti-inflammatory, anti-oxidation, anti-fatigue, anti-viral, and anti-cancer activities. Pentacyclic triterpenoids are derived from the isoprenoid biosynthetic pathway, which generates common precursors of triterpenes and steroids, followed by cyclization with oxidosqualene cyclases (OSCs) and decoration via cytochrome P450 monooxygenases (CYP450s) and glycosyltransferases (GTs). Many biosynthetic pathways of triterpenoid saponins have been elucidated by studying their metabolic regulation network through the use of multiomics and identifying their functional genes. Unfortunately, natural resources of pentacyclic triterpenoids are limited due to their low content in plant tissues and the long growth cycle of plants. Based on the understanding of their biosynthetic pathway and transcriptional regulation, plant bioreactors and microbial cell factories are emerging as alternative means for the synthesis of desired triterpenoid saponins. The rapid development of synthetic biology, metabolic engineering, and fermentation technology has broadened channels for the accumulation of pentacyclic triterpenoid saponins. In this review, we summarize the classification, distribution, structural characteristics, and bioactivity of pentacyclic triterpenoids. We further discuss the biosynthetic pathways of pentacyclic triterpenoids and involved transcriptional regulation. Moreover, the recent progress and characteristics of heterologous biosynthesis in plants and microbial cell factories are discussed comparatively. Finally, we propose potential strategies to improve the accumulation of triterpenoid saponins, thereby providing a guide for their future biomanufacturing.

Marine Actinomycetes Associated with Stony Corals: A Potential Hotspot for Specialized Metabolites

Microbial secondary metabolites are an important source of antibiotics currently available for combating drug-resistant pathogens. These important secondary metabolites are produced by various microorganisms, including Actinobacteria. Actinobacteria have a colossal genome with a wide array of genes that code for several bioactive metabolites and enzymes. Numerous studies have reported the isolation and screening of millions of strains of actinomycetes from various habitats for specialized metabolites worldwide. Looking at the extent of the importance of actinomycetes in various fields, corals are highlighted as a potential hotspot for untapped secondary metabolites and new bioactive metabolites. Unfortunately, knowledge about the diversity, distribution and biochemistry of marine actinomycetes compared to hard corals is limited. In this review, we aim to summarize the recent knowledge on the isolation, diversity, distribution and discovery of natural compounds from marine actinomycetes associated with hard corals. A total of 11 new species of actinomycetes, representing nine different families of actinomycetes, were recovered from hard corals during the period from 2007 to 2022. In addition, this study examined a total of 13 new compounds produced by five genera of actinomycetes reported from 2017 to 2022 with antibacterial, antifungal and cytotoxic activities. Coral-derived actinomycetes have different mechanisms of action against their competitors.

Insights into evolution and coexistence of the colibactin- and yersiniabactin secondary metabolite determinants in enterobacterial populations

The bacterial genotoxin colibactin interferes with the eukaryotic cell cycle by causing dsDNA breaks. It has been linked to bacterially induced colorectal cancer in humans. Colibactin is encoded by a 54 kb genomic region in Enterobacteriaceae. The colibactin genes commonly co-occur with the yersiniabactin biosynthetic determinant. Investigating the prevalence and sequence diversity of the colibactin determinant and its linkage to the yersiniabactin operon in prokaryotic genomes, we discovered mainly species-specific lineages of the colibactin determinant and classified three main structural settings of the colibactin–yersiniabactin genomic region in Enterobacteriaceae. The colibactin gene cluster has a similar but not identical evolutionary track to that of the yersiniabactin operon. Both determinants could have been acquired on several occasions and/or exchanged independently between enterobacteria by horizontal gene transfer. Integrative and conjugative elements play(ed) a central role in the evolution and structural diversity of the colibactin–yersiniabactin genomic region. Addition of an activating and regulating module (clbAR) to the biosynthesis and transport module (clbB-S) represents the most recent step in the evolution of the colibactin determinant. In a first attempt to correlate colibactin expression with individual lineages of colibactin determinants and different bacterial genetic backgrounds, we compared colibactin expression of selected enterobacterial isolates in vitro. Colibactin production in the tested Klebsiella species and Citrobacter koseri strains was more homogeneous and generally higher than that in most of the Escherichia coli isolates studied. Our results improve the understanding of the diversity of colibactin determinants and its expression level, and may contribute to risk assessment of colibactin-producing enterobacteria.

Recent Advances in the Heterologous Biosynthesis of Natural Products from Streptomyces

Streptomyces is a significant source of natural products that are used as therapeutic antibiotics, anticancer and antitumor agents, pesticides, and dyes. Recently, with the advances in metabolite analysis, many new secondary metabolites have been characterized. Moreover, genome mining approaches demonstrate that many silent and cryptic biosynthetic gene clusters (BGCs) and many secondary metabolites are produced in very low amounts under laboratory conditions. One strain many compounds (OSMAC), overexpression/deletion of regulatory genes, ribosome engineering, and promoter replacement have been utilized to activate or enhance the production titer of target compounds. Hence, the heterologous expression of BGCs by transferring to a suitable production platform has been successfully employed for the detection, characterization, and yield quantity production of many secondary metabolites. In this review, we introduce the systematic approach for the heterologous production of secondary metabolites from Streptomyces in Streptomyces and other hosts, the genome analysis tools, the host selection, and the development of genetic control elements for heterologous expression and the production of secondary metabolites.

Recent Advances on Feasible Strategies for Monoterpenoid Production in Saccharomyces cerevisiae

Terpenoids are a large diverse group of natural products which play important roles in plant metabolic activities. Monoterpenoids are the main components of plant essential oils and the active components of some traditional Chinese medicinal herbs. Some monoterpenoids are widely used in medicine, cosmetics and other industries, and they are mainly obtained by plant biomass extraction methods. These plant extraction methods have some problems, such as low efficiency, unstable quality, and high cost. Moreover, the monoterpenoid production from plant cannot satisfy the growing monoterpenoids demand. The development of metabolic engineering, protein engineering and synthetic biology provides an opportunity to produce large amounts of monoterpenoids eco-friendly using microbial cell factories. This mini-review covers current monoterpenoids production using Saccharomyces cerevisiae. The monoterpenoids biosynthetic pathways, engineering of key monoterpenoids biosynthetic enzymes, and current monoterpenoids production using S. cerevisiae were summarized. In the future, metabolically engineered S. cerevisiae may provide one possible green and sustainable strategy for monoterpenoids supply.

Heterologous Production of 6-Deoxyerythronolide B in Escherichia coli through the Wood Werkman Cycle

Polyketides are a remarkable class of natural products with diverse functional and structural diversity. The class includes many medicinally important molecules with antiviral, antimicrobial, antifungal and anticancer properties. Native bacterial, fungal and plant hosts are often difficult to cultivate and coax into producing the desired product. As a result, Escherichia coli has been used for the heterologous production of polyketides, with the production of 6-deoxyerythronolide B (6-dEB) being the first example. Current strategies for production in E. coli require feeding of exogenous propionate as a source for the precursors propionyl-CoA and S-methylmalonyl-CoA. Here, we show that heterologous polyketide production is possible from glucose as the sole carbon source. The heterologous expression of eight genes from the Wood-Werkman cycle found in Propionibacteria, in combination with expression of the 6-dEB synthases DEBS1, DEBS2 and DEBS3 resulted in 6-dEB formation from glucose as the sole carbon source. Our results show that the Wood-Werkman cycle provides the required propionyl-CoA and the extender unit S-methylmalonyl-CoA to produce up to 0.81 mg/L of 6-dEB in a chemically defined media.

Modular engineering for microbial production of carotenoids

There is an increasing demand for carotenoids due to their applications in the food, flavor, pharmaceutical and feed industries, however, the extraction and synthesis of these compounds can be expensive and technically challenging. Microbial production of carotenoids provides an attractive alternative to the negative environmental impacts and cost of chemical synthesis or direct extraction from plants. Metabolic engineering and synthetic biology approaches have been widely utilized to reconstruct and optimize pathways for carotenoid overproduction in microorganisms. This review summarizes the current advances in microbial engineering for carotenoid production and divides the carotenoid biosynthesis building blocks into four distinct metabolic modules: 1) central carbon metabolism, 2) cofactor metabolism, 3) isoprene supplement metabolism and 4) carotenoid biosynthesis. These four modules focus on redirecting carbon flux and optimizing cofactor supplements for isoprene precursors needed for carotenoid synthesis. Future perspectives are also discussed to provide insights into microbial engineering principles for overproduction of carotenoids.

The Phylum Bryozoa: From Biology to Biomedical Potential

Less than one percent of marine natural products characterized since 1963 have been obtained from the phylum Bryozoa which, therefore, still represents a huge reservoir for the discovery of bioactive metabolites with its ~6000 described species. The current review is designed to highlight how bryozoans use sophisticated chemical defenses against their numerous predators and competitors, and which can be harbored for medicinal uses. This review collates all currently available chemoecological data about bryozoans and lists potential applications/benefits for human health. The core of the current review relates to the potential of bryozoan metabolites in human diseases with particular attention to viral, brain, and parasitic diseases. It additionally weighs the pros and cons of total syntheses of some bryozoan metabolites versus the synthesis of non-natural analogues, and explores the hopes put into the development of biotechnological approaches to provide sustainable amounts of bryozoan metabolites without harming the natural environment.

Two new compounds from Streptomyces sp. HS-NF-813

Two new derivatives of cytotoxic substance BE-52211, designed as BE-52211D (1) and BE-52211E (2), were isolated from the fermentation broth of the strain Streptomyces sp. HS-NF-813. Their structures were determined by 1D and 2D NMR techniques, ESI-MS and comparison with data from the literature. The absolute stereochemistry of 1 was elucidated by NMR data of the Mosher ester derivatives. Compounds 1 and 2 showed moderate cytotoxic activity against three human tumor cell lines.

Natural Products as a Foundation for Drug Discovery

Many natural products have been used as drugs for the treatment of diverse indications. Although most U.S. pharmaceutical companies have reduced or eliminated their in-house natural-product research over the years, new approaches for compound screening and chemical synthesis are resurrecting interest in exploring the therapeutic value of natural products. The aim of this commentary is to review emerging strategies and techniques that have made natural products a viable strategic choice for inclusion in drug discovery programs. Published 2019. U.S. Government.

Tailoring of microbes for the production of high value plant-derived compounds: From pathway engineering to fermentative production

Plant natural products have been an attracting platform for the isolation of various active drugs and other bioactives. However large-scale extraction of these compounds is affected by the difficulty in mass cultivation of these plants and absence of strategies for successful extraction. Even though, synthesis by chemical method is an alternative method; it is less efficient as their chemical structure is highly complex which involve enantio-selectivity. Thus an alternate bio-system for heterologous production of plant natural products using microbes has emerged. Advent of various omics, synthetic and metabolic engineering strategies revolutionised the field of heterologous plant metabolite production. In this context, various engineering methods taken to synthesise plant natural products are described with an additional focus to fermentation strategies.

Challenges and opportunities for natural product discovery, production, and engineering in native producers versus heterologous hosts

Recent advances and emerging technologies for metabolic pathway engineering and synthetic biology have transformed the field of natural product discovery, production, and engineering. Despite these advancements, there remain many challenges in understanding how biosynthetic gene clusters are silenced or activated, including changes in the transcription of key biosynthetic and regulatory genes. This knowledge gap is highlighted by the success and failed attempts of manipulating regulatory genes within biosynthetic gene clusters in both native producers and heterologous hosts. These complexities make the choice of native producers versus heterologous hosts, fermentation medium, and supply of precursors crucial factors in achieving the production of the target natural products and engineering designer analogs. Nature continues to serve as inspiration for filling the knowledge gaps and developing new research strategies. By exploiting the evolutionary power of nature, alternative producers, with the desired genetic amenability and higher titers of the target natural products, and new strains, harboring gene clusters that encode evolutionary optimized congeners of the targeted natural product scaffolds, can be discovered. These newly identified strains can serve as an outstanding biotechnology platform for the engineered production of sufficient quantities of the target natural products and their analogs, enabling biosynthetic studies and potential therapeutic applications. These challenges and opportunities are showcased herein using fredericamycin, iso-migrastatin, platencin and platensimycin, the enediynes of C-1027, tiancimycin, and yangpumicin, and the leinamycin family of natural products.

Activity-Based Protein Profiling of Non-ribosomal Peptide Synthetases

Non-ribosomal peptide (NRP) natural products are one of the most promising resources for drug discovery and development because of their wide-ranging of therapeutic potential, and their behavior as virulence factors and signaling molecules. The NRPs are biosynthesized independently of the ribosome by enzyme assembly lines known as the non-ribosomal peptide synthetase (NRPS) machinery. Genetic, biochemical, and bioinformatics analyses have provided a detailed understanding of the mechanism of NRPS catalysis. However, proteomic techniques for natural product biosynthesis remain a developing field. New strategies are needed to investigate the proteomes of diverse producer organisms and directly analyze the endogenous NRPS machinery. Advanced platforms should verify protein expression, protein folding, and activities and also enable the profiling of the NRPS machinery in biological samples from wild-type, heterologous, and engineered bacterial systems. Here, we focus on activity-based protein profiling strategies that have been recently developed for studies aimed at visualizing and monitoring the NRPS machinery and also for rapid labeling, identification, and biochemical analysis of NRPS enzyme family members as required for proteomic chemistry in natural product sciences.

Effect of Extracellular Tyrosinase on the Expression Level of P450, Fpr, and Fdx and Ortho-hydroxylation of Daidzein in Streptomyces avermitilis

We have reported that the expression of CYP105D7 in Streptomyces avermitilis produces 112.5 mg L^-1 of 7,3',4'-trihydroxyisoflavone (3'ODI) in 15 h of the reaction time, when 7,4'-dihydroxyisoflavone (daidzein) is used as a substrate. Although production is significant, rapid degradation of 3'ODI after 15 h was observed in a whole-cell biotransformation system, suggesting the further modification of 3'ODI by endogenous enzymes. In this present study, the effect of deletion of extracellular tyrosinase (melC2) in S. avermitilis for 3'ODI production as well as the expressions of CYP105D7, ferredoxin (Fdx), and ferredoxin reductase (Fpr) were investigated. The result revealed that daidzein hydroxylation activity in the ∆melC2 mutant decreased by 40% compared with wild-type S. avermitilis. Further, melC2 deletion significantly affects the messenger RNA (mRNA) expression profile of CYP105D7 and its electron transfer counterparts. Real-time PCR analysis of 9 Fdx, 6 Fpr, and CYP105D7 revealed a significant decrease in mRNA expression level compared to wild-type S. avermitilis. The result clearly shows that the decrease in daidzein hydroxylation activity is due to the lower expression level of CYP105D7 and its electron transfer counterpart in the ∆melC2 mutant. Furthermore, melC2 deletion prevents the degradation of 3'ODI.

De Novo Biosynthesis of Apigenin, Luteolin, and Eriodictyol in the Actinomycete Streptomyces albus and Production Improvement by Feeding and Spore Conditioning

Nutraceutical compounds as plant flavonoids play an important role in prevention and modulation of diverse heath conditions, as they exert interesting antifungal, antibacterial, antioxidant, and antitumor effects. They also possess anti-inflammatory activities in arthritis, cardiovascular disease or neurological diseases, as well as modulatory effects on the CYP450 activity on diverse drugs. Most flavonoids are bioactive molecules of plant origin, but their industrial production is sometimes hindered due to reasons as low concentration in the plant tissues, presence in only some species or as a complex mixture or inactive glycosides in plant vacuolae. In this work, we describe the de novo biosynthesis of two important flavones, apigenin and luteolin, and one known flavanone, eriodictyol. Their plant biosynthetic pathways have been reconstructed for heterologous expression in Streptomyces albus, an actinomycete bacterium manageable at industrial production level. Also, production levels for apigenin have been improved by feeding with naringenin precursor, and timing for settlement of secondary metabolism has been advanced by spore conditioning. In the cases of eriodictyol and luteolin, their production in this important type of biotechnology-prone bacteria, the actinomycetes, had not been described in the literature yet.

The antibiotic resistance crisis, with a focus on the United States

Beginning with the discovery of penicillin by Alexander Fleming in the late 1920s, antibiotics have revolutionized the field of medicine. They have saved millions of lives each year, alleviated pain and suffering, and have even been used prophylactically for the prevention of infectious diseases. However, we have now reached a crisis where many antibiotics are no longer effective against even the simplest infections. Such infections often result in an increased number of hospitalizations, more treatment failures and the persistence of drug-resistant pathogens. Of particular concern are organisms such as methicillin-resistant Staphylococcus aureus, Clostridium difficile, multidrug and extensively drug-resistant Mycobacterium tuberculosis, Neisseria gonorrhoeae, carbapenem-resistant Enterobacteriaceae and bacteria that produce extended spectrum β-lactamases, such as Escherichia coli. To make matters worse, there has been a steady decline in the discovery of new and effective antibiotics for a number of reasons. These include increased costs, lack of adequate support from the government, poor returns on investment, regulatory hurdles and pharmaceutical companies that have simply abandoned the antibacterial arena. Instead, many have chosen to focus on developing drugs that will be used on a chronic basis, which will offer a greater profit and more return on investment. Therefore, there is now an urgent need to develop new and useful antibiotics to avoid returning to the 'pre-antibiotic era'. Some potential opportunities for antibiotic discovery include better economic incentives, genome mining, rational metabolic engineering, combinatorial biosynthesis and further exploration of the earth's biodiversity.

Functional profiling of adenylation domains in nonribosomal peptide synthetases by competitive activity-based protein profiling

We describe competitive activity-based protein profiling (ABPP) to accelerate the functional prediction and assessment of adenylation (A) domains in nonribosomal peptide synthetases (NRPSs) in proteomic environments. Using a library of sulfamoyloxy-linked aminoacyl-AMP analogs, the competitive ABPP technique offers a simple and rapid assay system for adenylating enzymes and provides insight into enzyme substrate candidates and enzyme active-site architecture.

In vitro reconstitution guide for targeted synthetic metabolism of chemicals, nutraceuticals and drug precursors

With the developments in metabolic engineering and the emergence of synthetic biology, many breakthroughs in medicinal, biological and chemical products as well as biofuels have been achieved in recent decades. As an important barrier to traditional metabolic engineering, however, the identification of rate-limiting step(s) for the improvement of specific cellular functions is often difficult. Meanwhile, in the case of synthetic biology, more and more BioBricks could be constructed for targeted purposes, but the optimized assembly or engineering of these components for high-efficiency cell factories is still a challenge. Owing to the lack of steady-state kinetic data for overall flux, balancing many multistep biosynthetic pathways is time-consuming and needs vast resources of labor and materials. A strategy called targeted engineering is proposed in an effort to solve this problem. Briefly, a targeted biosynthetic pathway is to be reconstituted in vitro and then the contribution of cofactors, substrates and each enzyme will be analyzed systematically. Next is in vivo engineering or de novo pathway assembly with the guidance of information gained from in vitro assays. To demonstrate its practical application, biosynthesis pathways for the production of important products, e.g. chemicals, nutraceuticals and drug precursors, have been engineered in Escherichia coli and Saccharomyces cerevisiae. These cases can be regarded as concept proofs indicating targeted engineering might help to create high-efficiency cell factories based upon constructed biological components.

Discovery and resupply of pharmacologically active plant-derived natural products: A review

Medicinal plants have historically proven their value as a source of molecules with therapeutic potential, and nowadays still represent an important pool for the identification of novel drug leads. In the past decades, pharmaceutical industry focused mainly on libraries of synthetic compounds as drug discovery source. They are comparably easy to produce and resupply, and demonstrate good compatibility with established high throughput screening (HTS) platforms. However, at the same time there has been a declining trend in the number of new drugs reaching the market, raising renewed scientific interest in drug discovery from natural sources, despite of its known challenges. In this survey, a brief outline of historical development is provided together with a comprehensive overview of used approaches and recent developments relevant to plant-derived natural product drug discovery. Associated challenges and major strengths of natural product-based drug discovery are critically discussed. A snapshot of the advanced plant-derived natural products that are currently in actively recruiting clinical trials is also presented. Importantly, the transition of a natural compound from a "screening hit" through a "drug lead" to a "marketed drug" is associated with increasingly challenging demands for compound amount, which often cannot be met by re-isolation from the respective plant sources. In this regard, existing alternatives for resupply are also discussed, including different biotechnology approaches and total organic synthesis. While the intrinsic complexity of natural product-based drug discovery necessitates highly integrated interdisciplinary approaches, the reviewed scientific developments, recent technological advances, and research trends clearly indicate that natural products will be among the most important sources of new drugs also in the future.

Scale-up bioprocess development for production of the antibiotic valinomycin in Escherichia coli based on consistent fed-batch cultivations

Background

Heterologous production of natural products in Escherichia coli has emerged as an attractive strategy to obtain molecules of interest. Although technically feasible most of them are still constrained to laboratory scale production. Therefore, it is necessary to develop reasonable scale-up strategies for bioprocesses aiming at the overproduction of targeted natural products under industrial scale conditions. To this end, we used the production of the antibiotic valinomycin in E. coli as a model system for scalable bioprocess development based on consistent fed-batch cultivations.

Results

In this work, the glucose limited fed-batch strategy based on pure mineral salt medium was used throughout all scales for valinomycin production. The optimal glucose feed rate was initially detected by the use of a biocatalytically controlled glucose release (EnBase® technology) in parallel cultivations in 24-well plates with continuous monitoring of pH and dissolved oxygen. These results were confirmed in shake flasks, where the accumulation of valinomycin was highest when the specific growth rate decreased below 0.1 h⁻¹. This correlation was also observed for high cell density fed-batch cultivations in a lab-scale bioreactor. The bioreactor fermentation produced valinomycin with titers of more than 2 mg L⁻¹ based on the feeding of a concentrated glucose solution. Valinomycin production was not affected by oscillating conditions (i.e. glucose and oxygen) in a scale-down two-compartment reactor, which could mimic similar situations in industrial bioreactors, suggesting that the process is very robust and a scaling of the process to a larger industrial scale appears a realistic scenario.

Conclusions

Valinomycin production was scaled up from mL volumes to 10 L with consistent use of the fed-batch technology. This work presents a robust and reliable approach for scalable bioprocess development and represents an example for the consistent development of a process for a heterologously expressed natural product towards the industrial scale.

Opportunities for synthetic biology in antibiotics: expanding glycopeptide chemical diversity

Synthetic biology offers a new path for the exploitation and improvement of natural products to address the growing crisis in antibiotic resistance. All antibiotics in clinical use are facing eventual obsolesce as a result of the evolution and dissemination of resistance mechanisms, yet there are few new drug leads forthcoming from the pharmaceutical sector. Natural products of microbial origin have proven over the past 70 years to be the wellspring of antimicrobial drugs. Harnessing synthetic biology thinking and strategies can provide new molecules and expand chemical diversity of known antibiotic scaffolds to provide much needed new drug leads. The glycopeptide antibiotics offer paradigmatic scaffolds suitable for such an approach. We review these strategies here using the glycopeptides as an example and demonstrate how synthetic biology can expand antibiotic chemical diversity to help address the growing resistance crisis.

Effects of exogenous nutrients on polyketide biosynthesis in Escherichia coli

Heterologous hosts are important platforms for engineering natural product biosynthesis. Escherichia coli is such a host widely used for expression of various biosynthetic enzymes. While numerous studies have been focused on optimizing the expression conditions for desired functional proteins, this work describes how supplement of exogenous nutrients into the fermentation broth influences the formation of natural products in E. coli. A type III polyketide synthase gene stts from Streptomyces toxytricini NRRL 15443 was heterogeneously expressed in E. coli BL21(DE3). This enzyme uses five units of malonyl-CoA to generate a polyketide 1,3,6,8-tetrahydroxynaphthalene, which can be spontaneously oxidized into a red compound flaviolin. In this work, we manipulated the fermentation broth of E. coli BL21(DE3)/pET28a-stts by supplying different nutrients including glucose and sodium pyruvate at different concentrations, from which six flaviolin derivatives 1-6 were produced. While addition of glucose yielded the production of 1-4, supplement of sodium pyruvate into the induced broth of E. coli BL21(DE3)/pET28a-stts resulted in the synthesis of 5 and 6, suggesting that different nutrients may enable E. coli to generate different metabolites. These products were purified and structurally characterized based on the spectral data, among which 2-6 are novel compounds. These molecules were formed through addition of different moieties such as acetone and indole to the flaviolin scaffold. The concentrations of glucose and sodium pyruvate and incubation time affect the product profiles. This work demonstrates that supplement of nutrients can link certain intracellular metabolites to the engineered biosynthetic pathway to yield new products. It provides a new approach to biosynthesizing novel molecules in the commonly used heterologous host E. coli.

Deleting multiple lytic genes enhances biomass yield and production of recombinant proteins by Bacillus subtilis

Background

Bacillus subtilis is widely used in agriculture and industrial biotechnology; however, cell autolysis significantly decreases its yield in liquid cultures. Numerous factors mediate the lysis of B. subtilis, such as cannibalism factors, prophages, and peptidoglycan (PG) hydrolases. The aim of this work was to use molecular genetic techniques to develop a new strategy to prevent cell lysis and enhance biomass as well as the production of recombinant proteins.

Results

Five genes or genetic elements representing three different functional categories were studied as follows: lytC encoding PG hydrolases, the prophage genes xpf and yqxG-yqxH-cwlA (yGlA), and skfA and sdpC that encode cannibalism factors. Cell lysis was reduced and biomass was enhanced by deleting individually skfA, sdpC, xpf, and lytC. We constructed the multiple deletion mutant LM2531 (skfA sdpC lytC xpf) and found that after 4 h of culture, its biomass yield was significantly increased compared with that of prototypical B. subtilis 168 (wild-type) strain and that 15% and 92% of the cells were lysed in cultures of LM2531 and wild-type, respectively. Moreover, two expression vectors were constructed for producing recombinant proteins (β-galactosidase and nattokinase) under the control of the P43 promoter. Cultures of LM2531 and wild-type transformants produced 13741 U/ml and 7991 U/ml of intracellular β-galactosidase, respectively (1.72-fold increase). Further, the level of secreted nattokinase produced by strain LM2531 increased by 2.6-fold compared with wild-type (5226 IU/ml vs. 2028 IU/ml, respectively).

Conclusions

Our novel, systematic multigene deletion approach designed to inhibit cell lysis significantly increased the biomass yield and the production of recombinant proteins by B. subtilis. These findings show promise for guiding efforts to manipulate the genomes of other B. subtilis strains that are used for industrial purposes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0129-9) contains supplementary material, which is available to authorized users.

Escherichia coli as a cell factory for heterologous production of nonribosomal peptides and polyketides

Nonribosomal peptides (NRPs) and polyketides (PKs) are two classes of natural products with numerous bioactivities such as antiviral, antimicrobial and anticancer activity. However, pharmaceutical applications of these products are often impeded because many native producers are difficult to cultivate or show a low productivity. Over the last decade, with the development of synthetic biology and metabolic engineering, more and more bioactive natural products including NRPs and PKs have been heterologously produced using easy-to-handle surrogate microbes. In this process, the full biosynthetic pathway for the production of a target compound is first identified and isolated from the native producer, and then reconstituted in a well-characterized and easily culturable heterologous producer like Escherichia coli. Thereafter, the productivity could be rationally improved through multiple strategies from strain to bioprocess optimization. This review summarizes the endeavors and progresses made in the heterologous production of NRPs, PKs and NRP/PK hybrids using E. coli as a robust whole-cell factory in recent years.

Mitigation of ROS insults by Streptomyces secondary metabolites in primary cortical neurons

Oxidative stress is a common point in neurodegenerative diseases, widely connected with mitochondrial dysfunction. In this study, we screened seven natural products from Streptomyces sources against hydrogen peroxide insult in primary cortical neurons, an oxidative stress in vitro model. We showed the ability of these compounds to inhibit neuronal cytotoxicity and to reduce ROS release after 12 h treatment. Among the tested compounds, the quinone anhydroexfoliamycin and the red pyrrole-type pigment undecylprodigiosin stand out. These two compounds displayed the most complete protection against oxidative stress with mitochondrial function improvement, ROS production inhibition, and increase of antioxidant enzyme levels, glutathione and catalase. Further investigations confirmed that anhydroexfoliamycin acts over the Nrf2-ARE pathway, as a Nrf2 nuclear translocation inductor, and is able to strongly inhibit the effect of the mitochondrial uncoupler FCCP over cytosolic Ca(2+), pointing to mitochondria as a cellular target for this molecule. In addition, both compounds were able to reduce caspase-3 activity induced by the apoptotic enhancer staurosporine, but undecylprodigiosin failed to inhibit FCCP effects and it did not act over the Nrf2 pathway as was the case for anhydroexfoliamycin. These results show that Streptomyces metabolites could be useful for the development of new drugs for prevention of neurodegenerative disorders such as Parkinson's and Alzheimer's diseases and cerebral ischemia.

Distinct mechanisms for spiro-carbon formation reveal biosynthetic pathway crosstalk

Spirotryprostatins, an indole alkaloid class of nonribosomal peptides isolated from Aspergillus fumigatus, are known for their antimitotic activity in tumor cells. Because spirotryprostatins and many other chemically complex spiro-carbon-bearing natural products exhibit useful biological activities, identifying and understanding the mechanism of spiro-carbon biosynthesis is of great interest. Here we report a detailed study of spiro-ring formation in spirotryprostatins from tryprostatins derived from the fumitremorgin biosynthetic pathway, using reactants and products prepared with engineered yeast and fungal strains. Unexpectedly, FqzB, an FAD-dependent monooxygenase from the unrelated fumiquinazoline biosynthetic pathway, catalyzed spiro-carbon formation in spirotryprostatin A via an epoxidation route. Furthermore, FtmG, a cytochrome P450 from the fumitremorgin biosynthetic pathway, was determined to catalyze the spiro-ring formation in spirotryprostatin B. Our results highlight the versatile role of oxygenating enzymes in the biosynthesis of structurally complex natural products and indicate that cross-talk of different biosynthetic pathways allows product diversification in natural product biosynthesis.

Echinomycin biosynthesis

Echinomycin is an antitumor antibiotic secondary metabolite isolated from streptomycetes, whose core structure is biosynthesized by nonribosomal peptide synthetase (NRPS). The echinomycin biosynthetic pathway was successfully reconstituted in Escherichia coli. NRPS often contains a thioesterase domain at its C terminus for cyclorelease of the elongating peptide chain. Those thioesterase domains were shown to exhibit significant substrate tolerance. More recently, an oxidoreductase Ecm17, which forms the disulfide bridge in triostin A, was characterized. Surprisingly, an unrelated disulfide-forming enzyme GliT for gliotoxin biosynthesis was also able to catalyze the same reaction, providing another example of broad substrate specificity in secondary metabolite biosynthetic enzymes. Those promiscuous catalysts can be a valuable tool in generating diversity in natural products analogs we can produce heterologously.

Improved heterologous erythromycin A production through expression plasmid re-design

The production of complex compounds from technically convenient microorganisms is an emerging route to the chemical diversity found in the surrounding environment. In this study, the antibiotic compound erythromycin A is produced from Escherichia coli as an alternative to native production through the soil bacterium Saccharopolyspora erythraea. By doing so, there is an opportunity to apply and refine engineering strategies for the manipulation of the erythromycin biosynthetic pathway and for the overproduction of this and other complex natural compounds. Previously, E. coli-derived production was enabled by the introduction of the entire erythromycin pathway (20 genes total) using separately selectable expression plasmids which demonstrated negative effects on final biosynthesis through metabolic burden and plasmid instability. In this study, improvements to final production were made by altering the design of the expression plasmids needed for biosynthetic pathway introduction. Specifically, the total number of genes and plasmids was pruned to reduce both metabolic burden and plasmid instability. Further, a comparison was conducted between species-specific (E. coli vs. S. coelicolor) protein chaperonins. Results indicate improvements in growth and plasmid retention metrics. The newly designed expression platform also increased erythromycin A production levels 5-fold. In conclusion, the steps outlined in this report were designed to upgrade the E. coli erythromycin A production system, led to improved final compound titers, and suggest additional forms of pathway engineering to further improve results from heterologous production attempts.

Novel quorum-sensing peptides mediating interspecies bacterial cell death

Escherichia coli mazEF is a toxin-antitoxin stress-induced module mediating cell death. It requires the quorum-sensing signal (QS) “extracellular death factor” (EDF), the penta-peptide NNWNN (EcEDF), enhancing the endoribonucleolytic activity of E. coli toxin MazF. Here we discovered that E. coli mazEF-mediated cell death could be triggered by QS peptides from the supernatants (SN) of the Gram-positive bacterium Bacillus subtilis and the Gram-negative bacterium Pseudomonas aeruginosa. In the SN of B. subtilis, we found one EDF, the hexapeptide RGQQNE, called BsEDF. In the SN of P. aeruginosa, we found three EDFs: the nonapeptide INEQTVVTK, called PaEDF-1, and two hexadecapeptides, VEVSDDGSGGNTSLSQ, called PaEDF-2, and APKLSDGAAAGYVTKA, called PaEDF-3. When added to a diluted E. coli cultures, each of these peptides acted as an interspecies EDF that triggered mazEF-mediated death. Furthermore, though their sequences are very different, each of these EDFs amplified the endoribonucleolytic activity of E. coli MazF, probably by interacting with different sites on E. coli MazF. Finally, we suggest that EDFs may become the basis for a new class of antibiotics that trigger death from outside the bacterial cells.

Bacteria communicate with one another via quorum-sensing signal (QS) molecules. QS provides a mechanism for bacteria to monitor each other’s presence and to modulate gene expression in response to population density. Previously, we added E. coli EDF (EcEDF), the peptide NNWNN, to this list of QS molecules. Here we extended the group of QS peptides to several additional different peptides. The new EDFs are produced by two other bacteria, Bacillus subtilis and Pseudomonas aeruginosa. Thus, in this study we established a “new family of EDFs.” This family provides the first example of quorum-sensing molecules participating in interspecies bacterial cell death. Furthermore, each of these peptides provides the basis of a new class of antibiotics triggering death by acting from outside the cell.

TREX: a universal tool for the transfer and expression of biosynthetic pathways in bacteria

Secondary metabolites represent a virtually inexhaustible source of natural molecules exhibiting a high potential as pharmaceuticals or chemical building blocks. To gain broad access to these compounds, sophisticated expression systems are needed that facilitate the transfer and expression of large chromosomal regions, whose genes encode complex metabolic pathways. Here, we report on the development of the novel system for the transfer and expression of biosynthetic pathways (TREX), which comprises all functional elements necessary for the delivery and concerted expression of clustered pathway genes in different bacteria. TREX employs (i) conjugation for DNA transfer, (ii) randomized transposition for its chromosomal insertion, and (iii) T7 RNA polymerase for unimpeded bidirectional gene expression. The applicability of the TREX system was demonstrated by establishing the biosynthetic pathways of two pigmented secondary metabolites, zeaxanthin and prodigiosin, in bacteria with different metabolic capacities. Thus, TREX represents a valuable tool for accessing natural products by allowing comparative expression studies with clustered genes.

Metabolic engineering for the production of natural products

Natural products and their derivatives play an important role in modern healthcare as frontline treatments for many diseases and as inspiration for chemically synthesized therapeutics. With advances in sequencing and recombinant DNA technology, many of the biosynthetic pathways responsible for the production of these chemically complex yet valuable compounds have been elucidated. With an ever-expanding toolkit of biosynthetic components, metabolic engineering is an increasingly powerful method to improve natural product titers and generate novel compounds. Heterologous production platforms have enabled access to pathways from difficult to culture strains, systems biology and metabolic modeling tools have resulted in increasing predictive and analytic capabilities, advances in expression systems and regulation have enabled the fine-tuning of pathways for increased efficiency, and characterization of individual pathway components has facilitated the construction of hybrid pathways for the production of new compounds. These advances in the many aspects of metabolic engineering not only have yielded fascinating scientific discoveries but also make it an increasingly viable approach for the optimization of natural product biosynthesis.

Synthetic biology triggers new era of antibiotics development

As a discipline to design and construct organisms with desired properties, synthetic biology has generated rapid progresses in the last decade. Combined synthetic biology with the traditional process, a new universal workflow for drug development has been becoming more and more attractive. The new methodology exhibits more efficient and inexpensive comparing to traditional methods in every aspect, such as new compounds discovery & screening, process design & drug manufacturing. This article reviews the application of synthetic biology in antibiotics development, including new drug discovery and screening, combinatorial biosynthesis to generate more analogues and heterologous expression of biosynthetic gene clusters with systematic engineering the recombinant microbial systems for large scale production.

Multi-factorial engineering of heterologous polyketide production in Escherichia coli reveals complex pathway interactions

Polyketides represent a significant fraction of all natural products. Many possess pharmacological activity which makes them attractive drug candidates. The production of the parent macrocyclic aglycones is catalyzed by multi-modular polyketide synthases utilizing short-chain acyl-CoA monomers. When producing polyketides through heterologous hosts, one must not only functionally express the synthase itself, but activate the machinery used to generate the required substrate acyl-CoA's. As a result, metabolic engineering of these pathways is necessary for high-level production of heterologous polyketides. In this study, we over-express three different pathways for provision of the two substrates (propionyl-CoA and (2S)-methylmalonyl-CoA) utilized for the biosynthesis of 6-deoxyerythronolide B (6-dEB; the macrolactone precursor of erythromycin): (1) a propionate → propionyl-CoA → (2S)-methylmalonyl-CoA pathway, (2) a methylmalonate → methylmalonyl-CoA → propionyl-CoA pathway, and (3) a succinate → succinyl-CoA → (2R)-methylmalonyl-CoA → (2S)-methylmalonyl-CoA → propionyl-CoA pathway. The current study revealed that propionate is a necessary component for greater than 5 mg L(-1) titers. Deletion of the propionyl-CoA:succinate CoA transferase (ygfH) or over-expression of the transcriptional activator of short chain fatty acid uptake improved titer to over 100 mg L(-1), while the combination of the two improved titer to over 130 mg L(-1). The addition of exogenous methylmalonate could also improve titer to over 100 mg L(-1). Expression of a Streptomyces coelicolor A3(2) methylmalonyl-CoA epimerase, in conjunction with over-expression of Escherichia coli's native methylmalonyl-CoA mutase, allowed for the incorporation of exogenously fed succinate into the 6-dEB core.

Genome-based characterization of two prenylation steps in the assembly of the stephacidin and notoamide anticancer agents in a marine-derived Aspergillus sp

Stephacidin and notoamide natural products belong to a group of prenylated indole alkaloids containing a core bicyclo[2.2.2]diazaoctane ring system. These bioactive fungal secondary metabolites have a range of unusual structural and stereochemical features but their biosynthesis has remained uncharacterized. Herein, we report the first biosynthetic gene cluster for this class of fungal alkaloids based on whole genome sequencing of a marine-derived Aspergillus sp. Two central pathway enzymes catalyzing both normal and reverse prenyltransfer reactions were characterized in detail. Our results establish the early steps for creation of the prenylated indole alkaloid structure and suggest a scheme for the biosynthesis of stephacidin and notoamide metabolites. The work provides the first genetic and biochemical insights for understanding the structural diversity of this important family of fungal alkaloids.

Titer improvement of iso-migrastatin in selected heterologous Streptomyces hosts and related analysis of mRNA expression by quantitative RT-PCR

iso-Migrastatin (iso-MGS) has been actively pursued recently as an outstanding candidate of antimetastasis agents. Having characterized the iso-MGS biosynthetic gene cluster from its native producer Streptomyces platensis NRRL 18993, we have recently succeeded in producing iso-MGS in five selected heterologous Streptomyces hosts, albeit the low titers failed to meet expectations and cast doubt on the utility of this novel technique for large-scale production. To further explore and capitalize on the production capacity of these hosts, a thorough investigation of these five engineered strains with three fermentation media for iso-MGS production was undertaken. Streptomyces albus J1074 and Streptomyces lividans K4-114 were found to be preferred heterologous hosts, and subsequent analysis of carbon and nitrogen sources revealed that sucrose and yeast extract were ideal for iso-MGS production. After the initial optimization, the titers of iso-MGS in all five hosts were considerably improved by 3-18-fold in the optimized R2YE medium. Furthermore, the iso-MGS titer of S. albus J1074 (pBS11001) was significantly improved to 186.7 mg/L by a hybrid medium strategy. Addition of NaHCO(3) to the latter finally afforded an optimized iso-MGS titer of 213.8 mg/L, about 5-fold higher than the originally reported system. With S. albus J1074 (pBS11001) as a model host, the expression of iso-MGS gene cluster in four different media was systematically studied via the quantitative RT-PCR technology. The resultant comparison revealed the correlation of gene expression and iso-MGS production for the first time; synchronous expression of the whole gene cluster was crucial for optimal iso-MGS production. These results reveal new insights into the iso-MGS biosynthetic machinery in heterologous hosts and provide the primary data to realize large-scale production of iso-MGS for further preclinical studies.