Engineering Salinispora tropica for heterologous expression of natural product biosynthetic gene clusters

Glucose-1-phosphate thymidylyltransferase promotes the production of 3-O-α-mycarosylerythronolide B in Streptomyces coelicolor

AIMS

The main objective of this study was to produce erythronolide B (EB) and 3-O-α-mycarosylerythronolide B (MEB) in Streptomyces coelicolor and enhance the MEB production by expressing the glucose-1-phosphate thymidylyltransferase (RfbA).

METHODS AND RESULTS

We expressed eryF and eryB genes (eryBII, eryBIII, eryBIV, eryBV, eryBVI, and eryBVII) to produce EB and MEB. The expression was confirmed by quantitative real-time polymerase chain reaction. Furthermore, the MEB's production was improved by more than 100-fold by expressing an enzyme, RfbA, which is absent from the erythromycin gene cluster, to promote the biosynthesis of TDP-L-mycarose. We discuss the feasibility of alternative Streptomyces species for erythromycin production based on the presence or absence of RfbA.

CONCLUSIONS

The RbfA enzyme from Saccharopolyspora erythraea was expressed in S. coelicolor M1152 along with the MEB biosynthesis pathway, resulting in a large increase in MEB production (>100-fold).

Heterologous expression facilitates the discovery and characterization of marine microbial natural products

Microbial natural products and their derivatives have been developed as a considerable part of clinical drugs and agricultural chemicals. Marine microbial natural products exhibit diverse chemical structures and bioactivities with substantial potential for the development of novel pharmaceuticals. However, discovering compounds with new skeletons from marine microbes remains challenging. In recent decades, multiple approaches have been developed to discover novel marine microbial natural products, among which heterologous expression has proven to be an effective method. Facilitated by large DNA cloning and comparative metabolomic technologies, a few novel bioactive natural products from marine microorganisms have been identified by the expression of their biosynthetic gene clusters (BGCs) in heterologous hosts. Heterologous expression is advantageous for characterizing gene functions and elucidating the biosynthetic mechanisms of natural products. This review provides an overview of recent progress in heterologous expression-guided discovery, biosynthetic mechanism elucidation, and yield optimization of natural products from marine microorganisms and discusses the future directions of the heterologous expression strategy in facilitating novel natural product exploitation.

A Semisynthesis Platform for the Efficient Production and Exploration of Didemnin-Based Drugs

Plitidepsin (or dehydrodidemnin B), an approved anticancer drug, belongs to the didemnin family of cyclic depsipeptides, which are found in limited quantities in marine tunicate extracts. Herein, we introduce a new approach that integrates microbial and chemical synthesis to generate plitidepsin and its analogues. We screened a Tistrella strain library to identify a potent didemnin B producer, and then introduced a second copy of the didemnin biosynthetic gene cluster into its genome, resulting in a didemnin B titer of approximately 75 mg/L. Next, we developed two straightforward chemical strategies to convert didemnin B into plitidepsin, one of which involved a one-step synthetic route giving over 90 % overall yield. Furthermore, we synthesized 13 new didemnin derivatives and three didemnin probes, enabling research into structure-activity relationships and interactions between didemnin and proteins. Our study highlights the synergistic potential of biosynthesis and chemical synthesis in overcoming the challenge of producing complex natural products sustainably and at scale.

Antibiotics from rare actinomycetes, beyond the genus Streptomyces

Throughout the golden age of antibiotic discovery, Streptomyces have been unsurpassed for their ability to produce bioactive metabolites. Yet, this success has been hampered by rediscovery. As we enter a new stage of biodiscovery, omics data and existing scientific repositories can enable informed choices on the biodiversity that may yield novel antibiotics. Here, we focus on the chemical potential of rare actinomycetes, defined as bacteria within the order Actinomycetales, but not belonging to the genus Streptomyces. They are named as such due to their less-frequent isolation under standard laboratory practices, yet there is increasing evidence to suggest these biologically diverse genera harbour considerable biosynthetic and chemical diversity. In this review, we focus on examples of successful isolation and genera that have been the focus of more concentrated biodiscovery efforts, we survey the representation of rare actinomycete taxa, compared with Streptomyces, across natural product data repositories in addition to its biosynthetic potential. This is followed by an overview of clinically useful drugs produced by rare actinomycetes and considerations for future biodiscovery efforts. There is much to learn about these underexplored taxa, and mounting evidence suggests that they are a fruitful avenue for the discovery of novel antimicrobials.

Yeast-based heterologous production of the Colletochlorin family of fungal secondary metabolites

Transcriptomic studies have revealed that fungal pathogens of plants activate the expression of numerous biosynthetic gene clusters (BGC) exclusively when in presence of a living host plant. The identification and structural elucidation of the corresponding secondary metabolites remain challenging. The aim was to develop a polycistronic system for heterologous expression of fungal BGCs in Saccharomyces cerevisiae. Here we adapted a polycistronic vector for efficient, seamless and cost-effective cloning of biosynthetic genes using in vivo assembly (also called transformation-assisted recombination) directly in Escherichia coli followed by heterologous expression in S. cerevisiae. Two vectors were generated with different auto-inducible yeast promoters and selection markers. The effectiveness of these vectors was validated with fluorescent proteins. As a proof-of-principle, we applied our approach to the Colletochlorin family of molecules. These polyketide secondary metabolites were known from the phytopathogenic fungus Colletotrichum higginsianum but had never been linked to their biosynthetic genes. Considering the requirement for a halogenase, and by applying comparative genomics, we identified a BGC putatively involved in the biosynthesis of Colletochlorins in C. higginsianum. Following the expression of those genes in S. cerevisiae, we could identify the presence of the precursor Orsellinic acid, Colletochlorins and their non-chlorinated counterparts, the Colletorins. In conclusion, the polycistronic vectors described herein were adapted for the host S. cerevisiae and allowed to link the Colletochlorin compound family to their corresponding biosynthetic genes. This system will now enable the production and purification of infection-specific secondary metabolites of fungal phytopathogens. More widely, this system could be applied to any fungal BGC of interest.

Recent Advances in the Heterologous Expression of Biosynthetic Gene Clusters for Marine Natural Products

Marine natural products (MNPs) are an important source of biologically active metabolites, particularly for therapeutic agent development after terrestrial plants and nonmarine microorganisms. Sequencing technologies have revealed that the number of biosynthetic gene clusters (BGCs) in marine microorganisms and the marine environment is much higher than expected. Unfortunately, the majority of them are silent or only weakly expressed under traditional laboratory culture conditions. Furthermore, the large proportion of marine microorganisms are either uncultivable or cannot be genetically manipulated. Efficient heterologous expression systems can activate cryptic BGCs and increase target compound yield, allowing researchers to explore more unknown MNPs. When developing heterologous expression of MNPs, it is critical to consider heterologous host selection as well as genetic manipulations for BGCs. In this review, we summarize current progress on the heterologous expression of MNPs as a reference for future research.

Microbial chassis engineering drives heterologous production of complex secondary metabolites

The cryptic secondary metabolite biosynthetic gene clusters (BGCs) far outnumber currently known secondary metabolites. Heterologous production of secondary metabolite BGCs in suitable chassis facilitates yield improvement and discovery of new-to-nature compounds. The two juxtaposed conventional model microorganisms, Escherichia coli, Saccharomyces cerevisiae, have been harnessed as microbial chassis to produce a bounty of secondary metabolites with the help of certain host engineering. In last decade, engineering non-model microbes to efficiently biosynthesize secondary metabolites has received increasing attention due to their peculiar advantages in metabolic networks and/or biosynthesis. The state-of-the-art synthetic biology tools lead the way in operating genetic manipulation in non-model microorganisms for phenotypic optimization or yields improvement of desired secondary metabolites. In this review, we firstly discuss the pros and cons of several model and non-model microbial chassis, as well as the importance of developing broader non-model microorganisms as alternative programmable heterologous hosts to satisfy the desperate needs of biosynthesis study and industrial production. Then we highlight the lately advances in the synthetic biology tools and engineering strategies for optimization of non-model microbial chassis, in particular, the successful applications for efficient heterologous production of multifarious complex secondary metabolites, e.g., polyketides, nonribosomal peptides, as well as ribosomally synthesized and post-translationally modified peptides. Lastly, emphasis is on the perspectives of chassis cells development to access the ideal cell factory in the artificial intelligence-driven genome era.

Gut Microbiota Feature of Senile Osteoporosis by Shallow Shotgun Sequencing Using Aged Rats Model

Senile osteoporosis is defined as an age-related bone metabolic disorder, which is characterized by bone loss and decreased bone fragility. Gut microbiota (GM) could regulate the bone metabolic process and be closely related to senile osteoporosis. Several genus-level GM were found to increase in osteoporotic animals and patients. However, to reveal the pathogenic bacteria in senile osteoporosis, further studies are still needed to investigate the complete characteristics of bacteria species. In the present study, the rats were equally divided into two groups: the control group (Con, 6-month-old) and the osteoporosis group (OP, 22-month-old). Fecal samples were freshly collected to conduct the shallow shotgun sequencing. Then, we compared the species numbers, microbial diversity, GM composition at genus and species-level, and functional metabolic pathways in the two groups. The results showed that the species number was lower in the OP group (1272) than in the control group (1413), and 1002 GM species were shared between the two groups. The OP group had the decreased α diversity compared with the control group. As for β diversity, The PCA revealed that samples in the two groups had distinguishable ecological distance in each coordinate. At the species level, Bacteroide coprocola (B. coprocola), Acinetobacter baumannii (A. baumannii), Parabacteroides distasonis (P. distasonis), and Prevotella copri (P. copri) were higher in the OP group, while Corynebacterium stationis (C. stationis), Akkermansia muciniphila (A. muciniphila), and Alistipes indistinctus (A. indistinctus) were decreased. Moreover, functional metabolic analysis revealed that metabolic pathways of fatty acid biosynthesis, valine/isoleucine biosynthesis, GABA biosynthesis, and ubiquinone biosynthesis were enriched in the senile osteoporotic rats. In conclusion, GM at the species level in senile osteoporotic rats was significantly altered in structure, composition, and function. The altered GM structure, increased GM species such as P. copri, and decreased GM species such as A. muciniphila might be linked with the development of senile osteoporosis.

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

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

Laboratory scale cultivation of Salinispora tropica in shake flasks and mechanically stirred bioreactors

OBJECTIVE

Marine actinomycetes from the genus Salinispora have an unexploited biotechnological potential. To accurately estimate their application potential however, data on their cultivation, including biomass growth kinetics, are needed but only incomplete information is currently available.

RESULTS

This work provides some insight into the effect of temperature, salinity, nitrogen source, glucose concentration and oxygen supply on growth rate, biomass productivity and yield of Salinispora tropica CBN-440^T. The experiments were carried out in unbaffled shake flasks and agitated laboratory-scale bioreactors. The results show that the optimum growth temperature lies within the range 28-30 °C, salinity is close to sea water and the initial glucose concentration is around 10 g/L. Among tested nitrogen sources, yeast extract and soy peptone proved to be the most suitable. The change from unbaffled to baffled flasks increased the volumetric oxygen transfer coefficient (k_La) as did the use of agitated bioreactors. The highest specific growth rate (0.0986 h^-1) and biomass productivity (1.11 g/L/day) were obtained at k_La = 28.3 h^-1. A further increase in k_La was achieved by increasing stirrer speed, but this led to a deterioration in kinetic parameters.

CONCLUSIONS

Improvement of S. tropica biomass growth kinetics of was achieved mainly by identifying the most suitable nitrogen sources and optimizing k_La in baffled flasks and agitated bioreactors.

Approaches to genetic tool development for rapid domestication of non-model microorganisms

Non-model microorganisms often possess complex phenotypes that could be important for the future of biofuel and chemical production. They have received significant interest the last several years, but advancement is still slow due to the lack of a robust genetic toolbox in most organisms. Typically, "domestication" of a new non-model microorganism has been done on an ad hoc basis, and historically, it can take years to develop transformation and basic genetic tools. Here, we review the barriers and solutions to rapid development of genetic transformation tools in new hosts, with a major focus on Restriction-Modification systems, which are a well-known and significant barrier to efficient transformation. We further explore the tools and approaches used for efficient gene deletion, DNA insertion, and heterologous gene expression. Finally, more advanced and high-throughput tools are now being developed in diverse non-model microbes, paving the way for rapid and multiplexed genome engineering for biotechnology.

Actinobacteria in natural products research: Progress and prospects

Actinobacteria are well-recognised biosynthetic factories that produce an extensive spectrum of secondary metabolites. Recent genomic insights seem to impact the exploitation of these metabolically versatile bacteria in several aspects. Notably, from the isolation of novel taxa to the discovery of new compounds, different approaches evolve at a steady pace. Here, we systematically discuss the enduring importance of Actinobacteria in the field of drug discovery, the current focus of isolation efforts targeting bioactive Actinobacteria from diverse sources, recent discoveries of novel compounds with different bioactivities, and the relative employment of different strategies in the search for novel compounds. Ultimately, we highlight notable progress that will have profound impacts on future quests for secondary metabolites of Actinobacteria.

Bacteria as genetically programmable producers of bioactive natural products

Next to plants, bacteria account for most of the biomass on Earth. They are found everywhere, although certain species thrive only in specific ecological niches. These microorganisms biosynthesize a plethora of both primary and secondary metabolites, defined, respectively, as those required for the growth and maintenance of cellular functions and those not required for survival but offering a selective advantage for the producer under certain conditions. As a result, bacterial fermentation has long been used to manufacture valuable natural products of nutritional, agrochemical and pharmaceutical interest. The interactions of secondary metabolites with their biological targets have been optimized by millions of years of evolution and they are, thus, considered to be privileged chemical structures, not only for drug discovery. During the last two decades, functional genomics has allowed for an in-depth understanding of the underlying biosynthetic logic of secondary metabolites. This has, in turn, paved the way for the unprecedented use of bacteria as programmable biochemical workhorses. In this Review, we discuss the multifaceted use of bacteria as biological factories in diverse applications and highlight recent advances in targeted genetic engineering of bacteria for the production of valuable bioactive compounds. Emphasis is on current advances to access nature's abundance of natural products.

Challenges and advances in genetic manipulation of filamentous actinomycetes - the remarkable producers of specialized metabolites

Covering: up to February 2019Actinomycetes are Gram positive bacteria of the phylum Actinobacteria. These organisms are one of the most important sources of structurally diverse, clinically used antibiotics and other valuable bioactive products, as well as biotechnologically relevant enzymes. Most strains were discovered by their ability to produce a given molecule and were often poorly characterized, physiologically and genetically. The development of genetic methods for Streptomyces and related filamentous actinomycetes has led to the successful manipulation of antibiotic biosynthesis to attain structural modification of microbial metabolites that would have been inaccessible by chemical means and improved production yields. Moreover, genome mining reveals that actinomycete genomes contain multiple biosynthetic gene clusters (BGCs), however only a few of them are expressed under standard laboratory conditions, leading to the production of the respective compound(s). Thus, to access and activate the so-called "silent" BGCs, to improve their biosynthetic potential and to discover novel natural products methodologies for genetic manipulation are required. Although different methods have been applied for many actinomycete strains, genetic engineering is still remaining very challenging for some "underexplored" and poorly characterized actinomycetes. This review summarizes the strategies developed to overcome the obstacles to genetic manipulation of actinomycetes and allowing thereby rational genetic engineering of this industrially relevant group of microorganisms. At the end of this review we give some tips to researchers with limited or no previous experience in genetic manipulation of actinomycetes. The article covers the most relevant literature published until February 2019.

Genetic platforms for heterologous expression of microbial natural products

Covering: 2005 up to 2019Natural products are of paramount importance in human medicine. Not only are most antibacterial and anticancer drugs derived directly from or inspired by natural products, many other branches of medicine, such as immunology, neurology, and cardiology, have similarly benefited from natural product-based drugs. Typically, the genetic material required to synthesize a microbial specialized product is arranged in a multigene biosynthetic gene cluster (BGC), which codes for proteins associated with molecule construction, regulation, and transport. The ability to connect natural product compounds to BGCs and vice versa, along with ever-increasing knowledge of biosynthetic machineries, has spawned the field of genomics-guided natural product genome mining for the rational discovery of new chemical entities. One significant challenge in the field of natural product genome mining is how to rapidly link orphan biosynthetic genes to their associated chemical products. This review highlights state-of-the-art genetic platforms to identify, interrogate, and engineer BGCs from diverse microbial sources, which can be broken into three stages: (1) cloning and isolation of genomic loci, (2) heterologous expression in a host organism, and (3) genetic manipulation of cloned pathways. In the future, we envision natural product genome mining will be rapidly accelerated by de novo DNA synthesis and refactoring of whole biosynthetic pathways in combination with systematic heterologous expression methodologies.

Heterologous production of small molecules in the optimized Streptomyces hosts

Time span of literature covered: 2010-2018The genome mining of streptomycetes has revealed their great biosynthetic potential to produce novel natural products. One of the most promising exploitation routes of this biosynthetic potential is the refactoring and heterologous expression of corresponding biosynthetic gene clusters in a panel of specifically selected and optimized chassis strains. This article will review selected recent reports on heterologous production of natural products in streptomycetes. In the first part, the importance of heterologous production for drug discovery will be discussed. In the second part, the review will discuss recently developed genetic control elements (such as promoters, ribosome binding sites, terminators) and their application to achieve successful heterologous expression of biosynthetic gene clusters. Finally, the most widely used Streptomyces hosts for heterologous expression of biosynthetic gene clusters will be compared in detail. The article will be of interest to natural product chemists, molecular biologists, pharmacists and all individuals working in the natural products drug discovery field.

Direct cloning and heterologous expression of natural product biosynthetic gene clusters by transformation-associated recombination

Heterologous expression of natural product biosynthetic gene clusters (BGCs) is a robust approach not only to decipher biosynthetic logic behind natural product (NP) biosynthesis, but also to discover new chemicals from uncharacterized BGCs. This approach largely relies on techniques used for cloning large BGCs into suitable expression vectors. Recently, several whole-pathway direct cloning approaches, including full-length RecE-mediated recombination in Escherichia coli, Cas9-assisted in vitro assembly, and transformation-associated recombination (TAR) in Saccharomyces cerevisiae, have been developed to accelerate BGC isolation. In this chapter, we summarize a protocol for TAR cloning large NP BGCs, detailing the process of choosing TAR plasmids, designing pathway-specific TAR vectors, generating yeast spheroplasts, performing yeast transformation, and heterologously expressing BGCs in various host strains. We believe that the established platforms can accelerate the process of discovering new NPs, understanding NP biosynthetic logic, and engineering biosynthetic pathways.