Phage p1-derived artificial chromosomes facilitate heterologous expression of the FK506 gene cluster

Promoter engineering of natural product biosynthetic gene clusters in actinomycetes: concepts and applications

Covering 2011 to 2022Low titers of natural products in laboratory culture or fermentation conditions have been one of the challenging issues in natural products research. Many natural product biosynthetic gene clusters (BGCs) are also transcriptionally silent in laboratory culture conditions, making it challenging to characterize the structures and activities of their metabolites. Promoter engineering offers a potential solution to this problem by providing tools for transcriptional activation or optimization of biosynthetic genes. In this review, we summarize the 10 years of progress in promoter engineering approaches in natural products research focusing on the most metabolically talented group of bacteria actinomycetes.

Employing synthetic biology to expand antibiotic discovery

Antimicrobial-resistant (AMR) bacterial pathogens are a continually growing threat as our methods for combating these infections continue to be overcome by the evolution of resistance mechanisms. Recent therapeutic methods have not staved off the concern of AMR infections, so continued research focuses on new ways of identifying small molecules to treat AMR pathogens. While chemical modification of existing antibiotics is possible, there has been rapid development of resistance by pathogens that were initially susceptible to these compounds. Synthetic biology is becoming a key strategy in trying to predict and induce novel, natural antibiotics. Advances in cloning and mutagenesis techniques applied through a synthetic biology lens can help characterize the native regulation of antibiotic biosynthetic gene clusters (BGCs) to identify potential modifications leading to more potent antibiotic activity. Additionally, many cryptic antibiotic BGCs are derived from non-ribosomal peptide synthase (NRPS) and polyketide synthase (PKS) biosynthetic pathways; complex, clustered genetic sequences that give rise to amino acid-derived natural products. Synthetic biology can be applied to modify and metabolically engineer these enzyme-based systems to promote rapid and sustainable production of natural products and their variants. This review will focus on recent advances related to synthetic biology as applied to genetic pathway characterization and identification of antibiotics from naturally occurring BGCs. Specifically, we will summarize recent efforts to characterize BGCs via general genomic mutagenesis, endogenous gene expression, and heterologous gene expression.

Heterologous Expression of the Formicamycin Biosynthetic Gene Cluster Unveils Glycosylated Fasamycin Congeners

Formicamycins and their biosynthetic intermediates the fasamycins are polyketide antibiotics produced by Streptomyces formicae KY5 from a pathway encoded by the for biosynthetic gene cluster. In this work the ability of Streptomyces coelicolor M1146 and the ability of Saccharopolyspora erythraea Δery to heterologously express the for biosynthetic gene cluster were assessed. This led to the identification of eight new glycosylated fasamycins modified at different phenolic groups with either a monosaccharide (glucose, galactose, or glucuronic acid) or a disaccharide comprised of a proximal hexose (either glucose or galactose), with a terminal pentose (arabinose) moiety. In contrast to the respective aglycones, minimal inhibitory screening assays showed these glycosylated congeners lacked antibacterial activity.

Optimization of FK-506 production in Streptomyces tsukubaensis by modulation of Crp-mediated regulation

FK-506 is a potent immunosuppressive macrocyclic polyketide with growing pharmaceutical interest, produced by Streptomyces tsukubaensis. However, due to low levels synthesized by the wild-type strain, biotechnological production of FK-506 is rather limited. Optimization strategies to enhance the productivity of S. tsukubaensis by means of genetic engineering have been established. In this work primarily global regulatory aspects with respect to the FK-506 biosynthesis have been investigated with the focus on the global Crp (cAMP receptor protein) regulator. In expression analyses and protein-DNA interaction studies, the role of Crp during FK-506 biosynthesis was elucidated. Overexpression of Crp resulted in two-fold enhancement of FK-506 production in S. tsukubaensis under laboratory conditions. Further optimizations using fermentors proved that the strategy described in this study can be transferred to industrial scale, presenting a new approach for biotechnological FK-506 production. KEY POINTS: • The role of the global Crp (cAMP receptor protein) regulator for FK-506 biosynthesis in S. tsukubaensis was demonstrated • Crp overexpression in S. tsukubaensis was applied as an optimization strategy to enhance FK-506 and FK-520 production resulting in two-fold yield increase.

Cloning and Expression of Metagenomic DNA in Streptomyces lividans and Its Subsequent Fermentation for Optimized Production

The choice of an expression system for the metagenomic DNA of interest is of vital importance for the detection of any particular gene or gene cluster. Most of the screens to date have used the Gram-negative bacterium Escherichia coli as a host for metagenomic gene libraries. However, the use of E. coli introduces a potential host bias since only 40% of the enzymatic activities may be readily recovered by random cloning in E. coli. To recover some of the remaining 60%, alternative cloning hosts such as Streptomyces spp. have been used. Streptomycetes are high-GC Gram-positive bacteria belonging to the Actinomycetales and they have been studied extensively for more than 25 years as an alternative expression system. They are extremely well suited for the expression of DNA from other actinomycetes and genomes of high GC content. Furthermore, due to its high innate, extracellular secretion capacity, Streptomyces can be a better system than E. coli for the production of many extracellular proteins. In this article, an overview is given about the materials and methods for growth and successful expression and secretion of heterologous proteins from diverse origin using Streptomyces lividans as a host. More in detail, an overview is given about the protocols of transformation, type of plasmids used and of vectors useful for integration of DNA into the host chromosome, and accompanying cloning strategies. In addition, various control elements for gene expression including synthetic promoters are discussed, and methods to compare their strength are described. Stable and efficient marker-less integration of the gene of interest under the control of the promoter of choice into S. lividans chromosome via homologous recombination using pAMR23A-based system will be explained. Finally, a basic protocol for bench-top bioreactor experiments which can form the start in the production process optimization and up-scaling will be provided.

BAC cloning and heterologous expression of a giant biosynthetic gene cluster encoding antifungal neotetrafibricin in streptomyces rubrisoli

Polyene natural products including nystatin A1, amphotericin B, ECO-02301, and mediomycin belong to a large family of valuable antifungal polyketide compounds typically produced by soil actinomycetes. A previous study (Park et al., Front. Bioeng. Biotechnol., 2021, 9, 692340) isolated Streptomyces rubrisoli Inha501 with strong antifungal activity and analyzed a large-sized biosynthetic gene cluster (BGC) of a linear polyene compound named Inha-neotetrafibricin (I-NTF) using whole genome sequencing and bioinformatics. In the present study, an entire I-NTF BGC (∼167 kb) was isolated through construction and screening of Streptomyces BAC library. Overexpression of the cloned I-NTF BGC in the wild-type S. rubrisoli Inha501 and its heterologous expression in S. lividans led to 2.6-fold and 2.8-fold increase in I-NTF yields, respectively. The qRT-PCR confirmed that the transcription levels of I-NTF BGC were significantly increased in both homologous and heterologous hosts containing the BAC integration of I-NTF BGC. In addition, the I-NTF aglycone-producing strains were constructed by a target-specific deletion of glycosyltransferase gene present in I-NTF BGC. A comparison of the in vitro biological activities of I-NTF and I-NTF aglycone confirmed that the rhamnose sugar motif of I-NTF plays a critical role in both antifungal and antibacterial activities. These results suggest that the Streptomyces BAC cloning of a large-sized natural product BGC is a valuable approach for natural product titer improvement and biological activity screening of natural product in actinomycetes.

Streptomyces as Microbial Chassis for Heterologous Protein Expression

Heterologous production of recombinant proteins is gaining increasing interest in biotechnology with respect to productivity, scalability, and wide applicability. The members of genus Streptomyces have been proposed as remarkable hosts for heterologous production due to their versatile nature of expressing various secondary metabolite biosynthetic gene clusters and secretory enzymes. However, there are several issues that limit their use, including low yield, difficulty in genetic manipulation, and their complex cellular features. In this review, we summarize rational engineering approaches to optimizing the heterologous production of secondary metabolites and recombinant proteins in Streptomyces species in terms of genetic tool development and chassis construction. Further perspectives on the development of optimal Streptomyces chassis by the design-build-test-learn cycle in systems are suggested, which may increase the availability of secondary metabolites and recombinant proteins.

Multiplying the heterologous production of spinosad through tandem amplification of its biosynthetic gene cluster in Streptomyces coelicolor

Heterologous expression of the biosynthetic gene cluster (BGC) is important for studying the microbial natural products (NPs), especially for those kept in silent or poorly expressed in their original strains. Here, we cloned the spinosad BGC through the Cas9-Assisted Targeting of Chromosome segments and amplified it to five copies through a ZouA-dependent DNA amplification system in Streptomyces coelicolor M1146. The resulting strain produced 1253.9 ± 78.2 μg l^-1 of spinosad, which was about 224-fold compared with that of the parent strain carrying only one copy of the spinosad BGC. Moreover, we further increased spinosad to 1958.9 ± 73.5 μg l^-1 by the dynamic regulation of intracellular triacylglycerol degradation. Our study indicates that tandem amplification of the targeted gene cluster is particularly suitable to enhance the heterologous production of valuable NPs with efficiency and simplicity.

Activation and Identification of a Griseusin Cluster in Streptomyces sp. CA-256286 by Employing Transcriptional Regulators and Multi-Omics Methods

Streptomyces are well-known producers of a range of different secondary metabolites, including antibiotics and other bioactive compounds. Recently, it has been demonstrated that "silent" biosynthetic gene clusters (BGCs) can be activated by heterologously expressing transcriptional regulators from other BGCs. Here, we have activated a silent BGC in Streptomyces sp. CA-256286 by overexpression of a set of SARP family transcriptional regulators. The structure of the produced compound was elucidated by NMR and found to be an N-acetyl cysteine adduct of the pyranonaphtoquinone polyketide 3'-O-α-d-forosaminyl-(+)-griseusin A. Employing a combination of multi-omics and metabolic engineering techniques, we identified the responsible BGC. These methods include genome mining, proteomics and transcriptomics analyses, in combination with CRISPR induced gene inactivations and expression of the BGC in a heterologous host strain. This work demonstrates an easy-to-implement workflow of how silent BGCs can be activated, followed by the identification and characterization of the produced compound, the responsible BGC, and hints of its biosynthetic pathway.

Discovery and Characterization of Epemicins A and B, New 30-Membered Macrolides from Kutzneria sp. CA-103260

Actinobacteria have been a rich source of novel, structurally complex natural products for many decades. Although the largest genus is Streptomyces, from which the majority of antibiotics in current and past clinical use were originally isolated, other less common genera also have the potential to produce a wealth of novel secondary metabolites. One example is the Kutzneria genus, which currently contains only five reported species. One of these species is Kutzneria albida DSM 43870T, which has 46 predicted biosynthetic gene clusters and is known to produce the macrolide antibiotic aculeximycin. Here, we report the isolation and structural characterization of two novel 30-membered glycosylated macrolides, epemicins A and B, that are structurally related to aculeximycin, from a rare Kutzneria sp. The absolute configuration for all chiral centers in the two compounds is proposed based on extensive 1D and 2D NMR studies and bioinformatics analysis of the gene cluster. Through heterologous expression and genetic inactivation, we have confirmed the link between the biosynthetic gene cluster and the new molecules. These findings show the potential of rare Actinobacteria to produce new, structurally diverse metabolites. Furthermore, the gene inactivation represents the first published report to genetically manipulate a representative of the Kutzneria genus.

Recent Advances in Strategies for the Cloning of Natural Product Biosynthetic Gene Clusters

Microbial natural products (NPs) are a major source of pharmacological agents. Most NPs are synthesized from specific biosynthetic gene clusters (BGCs). With the rapid increase of sequenced microbial genomes, large numbers of NP BGCs have been discovered, regarded as a treasure trove of novel bioactive compounds. However, many NP BGCs are silent in native hosts under laboratory conditions. In order to explore their therapeutic potential, a main route is to activate these silent NP BGCs in heterologous hosts. To this end, the first step is to accurately and efficiently capture these BGCs. In the past decades, a large number of effective technologies for cloning NP BGCs have been established, which has greatly promoted drug discovery research. Herein, we describe recent advances in strategies for BGC cloning, with a focus on the preparation of high-molecular-weight DNA fragment, selection and optimization of vectors used for carrying large-size DNA, and methods for assembling targeted DNA fragment and appropriate vector. The future direction into novel, universal, and high-efficiency methods for cloning NP BGCs is also prospected.

Genetic Engineering in Combination with Semi-Synthesis Leads to a New Route for Gram-Scale Production of the Immunosuppressive Natural Product Brasilicardin A

Brasilicardin A (1) consists of an unusual anti/syn/anti-perhydrophenanthrene skeleton with a carbohydrate side chain and an amino acid moiety. It exhibits potent immunosuppressive activity, yet its mode of action differs from standard drugs that are currently in use. Further pre-clinical evaluation of this promising, biologically active natural product is hampered by restricted access to the ready material, as its synthesis requires both a low-yielding fermentation process using a pathogenic organism and an elaborate, multi-step total synthesis. Our semi-synthetic approach included a) the heterologous expression of the brasilicardin A gene cluster in different non-pathogenic bacterial strains producing brasilicardin A aglycone (5) in excellent yield and b) the chemical transformation of the aglycone 5 into the trifluoroacetic acid salt of brasilicardin A (1 a) via a short and straightforward five-steps synthetic route. Additionally, we report the first preclinical data for brasilicardin A.

Recent Advances in Silent Gene Cluster Activation in Streptomyces

Natural products (NPs) are critical sources of drug molecules for decades. About two-thirds of natural antibiotics are produced by Streptomyces. Streptomyces have a large number of secondary metabolite biosynthetic gene clusters (SM-BGCs) that may encode NPs. However, most of these BGCs are silent under standard laboratory conditions. Hence, activation of these silent BGCs is essential to current natural products discovery research. In this review, we described the commonly used strategies for silent BGC activation in Streptomyces from two aspects. One focused on the strategies applied in heterologous host, including methods to clone and reconstruct BGCs along with advances in chassis engineering; the other focused on methods applied in native host which includes engineering of promoters, regulatory factors, and ribosomes. With the metabolic network being elucidated more comprehensively and methods optimized more high-thoroughly, the discovery of NPs will be greatly accelerated.

Bioengineering of Anti-Inflammatory Natural Products

Inflammatory processes occur as a generic response of the immune system and can be triggered by various factors, such as infection with pathogenic microorganisms or damaged tissue. Due to the complexity of the inflammation process and its role in common diseases like asthma, cancer, skin disorders or Alzheimer's disease, anti-inflammatory drugs are of high pharmaceutical interest. Nature is a rich source for compounds with anti-inflammatory properties. Several studies have focused on the structural optimization of natural products to improve their pharmacological properties. As derivatization through total synthesis is often laborious with low yields and limited stereoselectivity, the use of biosynthetic, enzyme-driven reactions is an attractive alternative for synthesizing and modifying complex bioactive molecules. In this minireview, we present an outline of the biotechnological methods used to derivatize anti-inflammatory natural products, including precursor-directed biosynthesis, mutasynthesis, combinatorial biosynthesis, as well as whole-cell and in vitro biotransformation.

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.

GPAHex-A synthetic biology platform for Type IV-V glycopeptide antibiotic production and discovery

Glycopeptide antibiotics (GPAs) are essential for the treatment of severe infectious diseases caused by Gram-positive bacteria. The emergence and spread of GPA resistance have propelled the search for more effective GPAs. Given their structural complexity, genetic intractability, and low titer, expansion of GPA chemical diversity using synthetic or medicinal chemistry remains challenging. Here we describe a synthetic biology platform, GPAHex (GPA Heterologous expression), which exploits the genes required for the specialized GPA building blocks, regulation, antibiotic transport, and resistance for the heterologous production of GPAs. Application of the GPAHex platform results in: (1) a 19-fold increase of corbomycin titer compared to the parental strain, (2) the discovery of a teicoplanin-class GPA from an Amycolatopsis isolate, and (3) the overproduction and characterization of a cryptic nonapeptide GPA. GPAHex provides a platform for GPA production and mining of uncharacterized GPAs and provides a blueprint for chassis design for other natural product classes.

Expansion of the chemical diversity of glycopeptide antibiotics (GPAs) to deal with the emergence and spread of GPA resistance is challenging. Here, the authors report a GPA synthetic biology platform in Streptomyces coelicolor for Type IV–V glycopeptide antibiotic production and discovery.

The Onset of Tacrolimus Biosynthesis in Streptomyces tsukubaensis Is Dependent on the Intracellular Redox Status

The oxidative stress response is a key mechanism that microorganisms have to adapt to changeling environmental conditions. Adaptation is achieved by a fine-tuned molecular response that extends its influence to primary and secondary metabolism. In the past, the role of the intracellular redox status in the biosynthesis of tacrolimus in Streptomyces tsukubaensis has been briefly acknowledged. Here, we investigate the impact of the oxidative stress response on tacrolimus biosynthesis in S. tsukubaensis. Physiological characterization of S. tsukubaensis showed that the onset of tacrolimus biosynthesis coincided with the induction of catalase activity. In addition, tacrolimus displays antioxidant properties and thus a controlled redox environment would be beneficial for its biosynthesis. In addition, S. tsukubaensis ∆ahpC strain, a strain defective in the H₂O₂-scavenging enzyme AhpC, showed increased production of tacrolimus. Proteomic and transcriptomic studies revealed that the tacrolimus over-production phenotype was correlated with a metabolic rewiring leading to increased availability of tacrolimus biosynthetic precursors. Altogether, our results suggest that the carbon source, mainly used for cell growth, can trigger the production of tacrolimus by modulating the oxidative metabolism to favour a low oxidizing intracellular environment and redirecting the metabolic flux towards the increase availability of biosynthetic precursors.

A chromatogram-simplified Streptomyces albus host for heterologous production of natural products

Cloning natural product biosynthetic gene clusters from cultured or uncultured sources and their subsequent expression by genetically tractable heterologous hosts is an essential strategy for the elucidation and characterisation of novel microbial natural products. The availability of suitable expression hosts is a critical aspect of this workflow. In this work, we mutagenised five endogenous biosynthetic gene clusters from Streptomyces albus S4, which reduced the complexity of chemical extracts generated from the strain and eliminated antifungal and antibacterial bioactivity. We showed that the resulting quintuple mutant can express foreign biosynthetic gene clusters by heterologously producing actinorhodin, cinnamycin and prunustatin. We envisage that our strain will be a useful addition to the growing suite of heterologous expression hosts available for exploring microbial secondary metabolism.

Model-Driven Engineering of N-Linked Glycosylation in Chinese Hamster Ovary Cells

Chinese hamster ovary (CHO) cells are used for industrial production of protein-based therapeutics (i.e., "biologics"). Here we describe a method for combining systems-level kinetic models with a synthetic biology platform for multigene overexpression to rationally perturb N-linked glycosylation. Specifically, we sought to increase galactose incorporation on a secreted Immunoglobulin G (IgG) protein. We rationally design, build, and test a total of 23 transgenic cell pools that express single or three-gene glycoengineering cassettes comprising a total of 100 kilobases of engineered DNA sequence. Through iterative engineering and model refinement, we rationally increase the fraction of bigalactosylated glycans five-fold from 11.9% to 61.9% and simultaneously decrease the glycan heterogeneity on the secreted IgG. Our approach allows for rapid hypothesis testing and identification of synergistic behavior from genetic perturbations by bridging systems and synthetic biology.

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.

Engineering actinomycetes for biosynthesis of macrolactone polyketides

Actinobacteria are characterized as the most prominent producer of natural products (NPs) with pharmaceutical importance. The production of NPs from these actinobacteria is associated with particular biosynthetic gene clusters (BGCs) in these microorganisms. The majority of these BGCs include polyketide synthase (PKS) or non-ribosomal peptide synthase (NRPS) or a combination of both PKS and NRPS. Macrolides compounds contain a core macro-lactone ring (aglycone) decorated with diverse functional groups in their chemical structures. The aglycon is generated by megaenzyme polyketide synthases (PKSs) from diverse acyl-CoA as precursor substrates. Further, post-PKS enzymes are responsible for allocating the structural diversity and functional characteristics for their biological activities. Macrolides are biologically important for their uses in therapeutics as antibiotics, anti-tumor agents, immunosuppressants, anti-parasites and many more. Thus, precise genetic/metabolic engineering of actinobacteria along with the application of various chemical/biological approaches have made it plausible for production of macrolides in industrial scale or generation of their novel derivatives with more effective biological properties. In this review, we have discussed versatile approaches for generating a wide range of macrolide structures by engineering the PKS and post-PKS cascades at either enzyme or cellular level in actinobacteria species, either the native or heterologous producer strains.

Refactoring the Cryptic Streptophenazine Biosynthetic Gene Cluster Unites Phenazine, Polyketide, and Nonribosomal Peptide Biochemistry

The disconnect between the genomic prediction of secondary metabolite biosynthetic potential and the observed laboratory production profile of microorganisms is well documented. While heterologous expression of biosynthetic gene clusters (BGCs) is often seen as a potential solution to bridge this gap, it is not immune to many challenges including impaired regulation, the inability to recruit essential building blocks, and transcriptional and/or translational silence of the biosynthetic genes. Here we report the discovery, cloning, refactoring, and heterologous expression of a cryptic hybrid phenazine-type BGC (spz) from the marine actinomycete Streptomyces sp. CNB-091. Overexpression of the engineered spz pathway resulted in increased production and chemical diversity of phenazine natural products belonging to the streptophenazine family, including bioactive members containing an unprecedented N-formylglycine attachment. An atypical discrete adenylation enzyme in the spz cluster is required to introduce the formylglycine moiety and represents a phylogenetically distinct class of adenylation proteins.

Synthetic biology and metabolic engineering of actinomycetes for natural product discovery

Actinomycetes are one of the most valuable sources of natural products with industrial and medicinal importance. After more than half a century of exploitation, it has become increasingly challenging to find novel natural products with useful properties as the same known compounds are often repeatedly re-discovered when using traditional approaches. Modern genome mining approaches have led to the discovery of new biosynthetic gene clusters, thus indicating that actinomycetes still harbor a huge unexploited potential to produce novel natural products. In recent years, innovative synthetic biology and metabolic engineering tools have greatly accelerated the discovery of new natural products and the engineering of actinomycetes. In the first part of this review, we outline the successful application of metabolic engineering to optimize natural product production, focusing on the use of multi-omics data, genome-scale metabolic models, rational approaches to balance precursor pools, and the engineering of regulatory genes and regulatory elements. In the second part, we summarize the recent advances of synthetic biology for actinomycetal metabolic engineering including cluster assembly, cloning and expression, CRISPR/Cas9 technologies, and chassis strain development for natural product overproduction and discovery. Finally, we describe new advances in reprogramming biosynthetic pathways through polyketide synthase and non-ribosomal peptide synthetase engineering. These new developments are expected to revitalize discovery and development of new natural products with medicinal and other industrial applications.

Heterologous expression of bacterial natural product biosynthetic pathways

The review highlights the 2013–2018 literature on the heterologous expression of bacterial natural product biosynthetic pathways and emphasises new techniques, heterologous hosts, and novel chemistry.

Heterologous expression of bacterial natural product biosynthetic pathways

The review highlights the 2013–2018 literature on the heterologous expression of bacterial natural product biosynthetic pathways and emphasises new techniques, heterologous hosts, and novel chemistry.

Streptomycetes: Surrogate hosts for the genetic manipulation of biosynthetic gene clusters and production of natural products

Due to the worldwide prevalence of multidrug-resistant pathogens and high incidence of diseases such as cancer, there is an urgent need for the discovery and development of new drugs. Nearly half of the FDA-approved drugs are derived from natural products that are produced by living organisms, mainly bacteria, fungi, and plants. Commercial development is often limited by the low yield of the desired compounds expressed by the native producers. In addition, recent advances in whole genome sequencing and bioinformatics have revealed an abundance of cryptic biosynthetic gene clusters within microbial genomes. Genetic manipulation of clusters in the native host is commonly used to awaken poorly expressed or silent gene clusters, however, the lack of feasible genetic manipulation systems in many strains often hinders our ability to engineer the native producers. The transfer of gene clusters into heterologous hosts for expression of partial or entire biosynthetic pathways is an approach that can be used to overcome this limitation. Heterologous expression also facilitates the chimeric fusion of different biosynthetic pathways, leading to the generation of "unnatural" natural products. The genus Streptomyces is especially known to be a prolific source of drugs/antibiotics, its members are often used as heterologous expression hosts. In this review, we summarize recent applications of Streptomyces species, S. coelicolor, S. lividans, S. albus, S. venezuelae and S. avermitilis, as heterologous expression systems.

Heterologous expression-facilitated natural products' discovery in actinomycetes

Actinomycetes produce many of the drugs essential for human and animal health as well as crop protection. Genome sequencing projects launched over the past two decades reveal dozens of cryptic natural product biosynthetic gene clusters in each actinomycete genome that are not expressed under regular laboratory conditions. This so-called 'chemical dark matter' represents a potentially rich untapped resource for drug discovery in the genomic era. Through improved understanding of natural product biosynthetic logic coupled with the development of bioinformatic and genetic tools, we are increasingly able to access this 'dark matter' using a wide variety of strategies with downstream potential application in drug development. In this review, we discuss recent research progress in the field of cloning of natural product biosynthetic gene clusters and their heterologous expression in validating the potential of this methodology to drive next-generation drug discovery.

Trichlorination of a Teicoplanin-Type Glycopeptide Antibiotic by the Halogenase StaI Evades Resistance

Glycopeptide antibiotics (GPAs) include clinically important drugs used for the treatment of infections caused by Gram-positive pathogens. These antibiotics are specialized metabolites produced by several genera of actinomycete bacteria. While many GPAs are highly chemically modified, A47934 is a relatively unadorned GPA lacking sugar or acyl modifications, common to other members of the class, but which is chlorinated at three distinct sites. The biosynthesis of A47934 is encoded by a 68-kb gene cluster in Streptomyces toyocaensis NRRL 15009. The cluster includes all necessary genes for the synthesis of A47934, including two predicted halogenase genes, staI and staK In this study, we report that only one of the halogenase genes, staI, is necessary and essential for A47934 biosynthesis. Chlorination of the A47934 scaffold is important for antibiotic activity, as assessed by binding affinity for the target N-acyl-d-Ala-d-Ala. Surprisingly, chlorination is also vital to avoid activation of enterococcal and Streptomyces VanB-type GPA resistance through induction of resistance genes. Phenotypic assays showed stronger induction of GPA resistance by the dechlorinated compared to the chlorinated GPA. Correspondingly, the relative expression of the enterococcal vanA resistance gene was shown to be increased by the dechlorinated compared to the chlorinated compound. These results provide insight into the biosynthesis of GPAs and the biological function of GPA chlorination for this medically important class of antibiotic.

Substrate Specificity of Acyltransferase Domains for Efficient Transfer of Acyl Groups

Acyltransferase domains (ATs) of polyketide synthases (PKSs) are critical for loading of acyl groups on acyl carrier protein domains (A) via self- and trans-acylation reactions, to produce structurally diverse polyketides. However, the interaction specificity between ATs and unusual acyl units is rarely documented. In Streptomycestsukubaensis YN06, we found that AT4_FkbB [an AT in the fourth module of tacrolimus (FK506) PKS] transferred both allylmalonyl (allmal) and emthylmalonyl (ethmal) units to ACPs, which was supposed responsible for the production of both FK506 and its analog FK520, respectively. Mutations of five residues in AT4_FkbB (Q119A, L185I-V186D-V187T, and F203L) caused decreased efficiency of allmal transfer, but a higher ratio of ethmal transfer, supposedly due to less nucleophilic attacks between Ser599 in the active site of AT4_FkbB and the carbonyl carbon in the allmal unit, as observed from molecular dynamics simulations. Furthermore, reverse mutations of these five residues in ethmal-specific ATs to the corresponding residues of AT4_FkbB increased its binding affinity to allmal-CoA. Among these residues, Val187 of AT4_FkbB mainly contributed to allmal recognition, and V187K mutant produced less FK520 than wild type. Our findings thus suggested that five critical residues within AT4_FkbB were important for AT functionality in polyketide extension and potentially for targeting biosynthesis by generating desirable products and eliminating undesirable analogs.

Boosting Secondary Metabolite Production and Discovery through the Engineering of Novel Microbial Biosensors

Bacteria are a source of a large number of secondary metabolites with several biomedical and biotechnological applications. In recent years, there has been tremendous progress in the development of novel synthetic biology approaches both to increase the production rate of secondary metabolites of interest in native producers and to mine and reconstruct novel biosynthetic gene clusters in heterologous hosts. Here, we present the recent advances toward the engineering of novel microbial biosensors to detect the synthesis of secondary metabolites in bacteria and in the development of synthetic promoters and expression systems aiming at the construction of microbial cell factories for the production of these compounds. We place special focus on the potential of Gram-negative bacteria as a source of biosynthetic gene clusters and hosts for pathway assembly, on the construction and characterization of novel promoters for native hosts, and on the use of computer-aided design of novel pathways and expression systems for secondary metabolite production. Finally, we discuss some of the potentials and limitations of the approaches that are currently being developed and we highlight new directions that could be addressed in the field.

Unraveling Nutritional Regulation of Tacrolimus Biosynthesis in Streptomyces tsukubaensis through omic Approaches

Streptomyces tsukubaensis stands out among actinomycetes by its ability to produce the immunosuppressant tacrolimus. Discovered about 30 years ago, this macrolide is widely used as immunosuppressant in current clinics. Other potential applications for the treatment of cancer and as neuroprotective agent have been proposed in the last years. In this review we introduce the discovery of S. tsukubaensis and tacrolimus, its biosynthetic pathway and gene cluster (fkb) regulation. We have focused this work on the omic studies performed in this species in order to understand tacrolimus production. Transcriptomics, proteomics and metabolomics have improved our knowledge about the fkb transcriptional regulation and have given important clues about nutritional regulation of tacrolimus production that can be applied to improve production yields. Finally, we address some points of S. tsukubaensis biology that deserve more attention.

Mode of Action and Heterologous Expression of the Natural Product Antibiotic Vancoresmycin

Antibiotics that interfere with the bacterial cytoplasmic membrane have long-term potential for the treatment of infectious diseases as this mode of action is anticipated to result in low resistance frequency. Vancoresmycin is an understudied natural product antibiotic consisting of a terminal tetramic acid moiety fused to a linear, highly oxygenated, stereochemically complex polyketide chain. Vancoresmycin shows minimum inhibitory concentrations (MICs) from 0.125 to 2 μg/mL against a range of clinically relevant, antibiotic-resistant Gram-positive bacteria. Through a comprehensive mode-of-action study, utilizing Bacillus subtilis reporter strains, DiSC₃(5) depolarization assays, and fluorescence microscopy, we have shown that vancoresmycin selectively targets the cytoplasmic membrane of Gram-positive bacteria via a non-pore-forming, concentration-dependent depolarization mechanism. Whole genome sequencing of the producing strain allowed identification of the 141 kbp gene cluster encoding for vancoresmycin biosynthesis and a preliminary model for its biosynthesis. The size and complex structure of vancoresmycin could confound attempts to generate synthetic analogues. To overcome this problem and facilitate future studies, we identified, cloned, and expressed the 141 kbp biosynthetic gene cluster in Streptomyces coelicolor M1152. Elucidation of the mode-of-action of vancoresmycin, together with the heterologous expression system, will greatly facilitate further studies of this and related molecules.

dRNA-seq transcriptional profiling of the FK506 biosynthetic gene cluster in Streptomyces tsukubaensis NRRL18488 and general analysis of the transcriptome

FK506 (tacrolimus) is a valuable immunosuppressant produced by several Streptomyces strains. In the genome of the wild type producer Streptomyces tsukubaensis NRRL18488, FK506 biosynthesis is encoded by a gene cluster that spans 83.5 (kb). A whole transcriptome differential shotgun sequencing (dRNA-seq) of S. tsukubaensis was performed to analyze transcription at 2 different time points; before and during active FK506 production. In total, 8,914 transcription start sites were identified in either condition, which enabled precise determination of the 5'-UTR length of the corresponding transcripts as well as the identification of 2 consensus sequence motifs in the promoter regions. The transcription start sites of all gene operons within the FK506 cluster were identified, including 3 examples of leaderless RNA transcripts. These data provide detailed insight into the transcription of the FK506 biosynthetic gene cluster to support future regulatory studies, genetic manipulation, and industrial production.

Formicamycins, antibacterial polyketides produced by Streptomyces formicae isolated from African Tetraponera plant-ants

We report a new Streptomyces species named S. formicae that was isolated from the African fungus-growing plant-ant Tetraponera penzigi and show that it produces novel pentacyclic polyketides that are active against MRSA and VRE. The chemical scaffold of these compounds, which we have called the formicamycins, is similar to the fasamycins identified from the heterologous expression of clones isolated from environmental DNA, but has significant differences that allow the scaffold to be decorated with up to four halogen atoms. We report the structures and bioactivities of 16 new molecules and show, using CRISPR/Cas9 genome editing, that biosynthesis of these compounds is encoded by a single type 2 polyketide synthase biosynthetic gene cluster in the S. formicae genome. Our work has identified the first antibiotic from the Tetraponera system and highlights the benefits of exploring unusual ecological niches for new actinomycete strains and novel natural products.

Cloning and Heterologous Expression of a Large-sized Natural Product Biosynthetic Gene Cluster in Streptomyces Species

Actinomycetes family including Streptomyces species have been a major source for the discovery of novel natural products (NPs) in the last several decades thanks to their structural novelty, diversity and complexity. Moreover, recent genome mining approach has provided an attractive tool to screen potentially valuable NP biosynthetic gene clusters (BGCs) present in the actinomycetes genomes. Since many of these NP BGCs are silent or cryptic in the original actinomycetes, various techniques have been employed to activate these NP BGCs. Heterologous expression of BGCs has become a useful strategy to produce, reactivate, improve, and modify the pathways of NPs present at minute quantities in the original actinomycetes isolates. However, cloning and efficient overexpression of an entire NP BGC, often as large as over 100 kb, remain challenging due to the ineffectiveness of current genetic systems in manipulating large NP BGCs. This mini review describes examples of actinomycetes NP production through BGC heterologous expression systems as well as recent strategies specialized for the large-sized NP BGCs in Streptomyces heterologous hosts.

Cloning and Expression of Metagenomic DNA in Streptomyces lividans and Subsequent Fermentation for Optimized Production

The choice of an expression system for the metagenomic DNA of interest is of vital importance for the detection of any particular gene or gene cluster. Most of the screens to date have used the gram-negative bacterium Escherichia coli as a host for metagenomic gene libraries. However, the use of E. coli introduces a potential host bias since only 40 % of the enzymatic activities may be readily recovered by random cloning in E. coli. To recover some of the remaining 60 %, alternative cloning hosts such as Streptomyces spp. have been used. Streptomycetes are high-GC gram-positive bacteria belonging to the Actinomycetales and they have been studied extensively for more than 15 years as an alternative expression system. They are extremely well suited for the expression of DNA from other actinomycetes and genomes of high GC content. Furthermore, due to its high innate, extracellular secretion capacity, Streptomyces can be a better system than E. coli for the production of many extracellular proteins. In this article an overview is given about the materials and methods for growth and successful expression and secretion of heterologous proteins from diverse origin using Streptomyces lividans has a host. More in detail, an overview is given about the protocols of transformation, type of plasmids used and of vectors useful for integration of DNA into the host chromosome, and accompanying cloning strategies. In addition, various control elements for gene expression including synthetic promoters are discussed, and methods to compare their strength are described. Integration of the gene of interest under the control of the promoter of choice into S. lividans chromosome via homologous recombination using pAMR4-based system is explained. Finally a basic protocol for benchtop bioreactor experiments which can form the start in the production process optimization and upscaling is provided.

The contribution of genome mining strategies to the understanding of active principles of PGPR strains

Pathogenic microorganisms and insects affecting plant health are a major and chronic threat to food production and the ecosystem worldwide. As agricultural production has intensified over the years, the use of agrochemicals has in turn increased. However, this extensive usage has had several detrimental effects, with a pervasive environmental impact and the emergence of pathogen resistance. In addition, there is an increasing tendency among consumers to give preference to pesticide-free food products. Biological control, through the employment of plant growth-promoting rhizobacteria (PGPR), is therefore considered a possible route to the reduction, even the elimination, of the use of agrochemicals. PGPR exert their beneficial influence by a multitude of mechanisms, often involving antibiotics and proteins, to defend the host plant against pathogens. To date, these key metabolites have been uncovered only by systematic investigation or by serendipity; their discovery has nevertheless been propelled by the genomic revolution of recent years, as increasing numbers of genomic studies have been integrated into this field, facilitating a holistic view of this topic and the rapid identification of ecologically important metabolites. This review surveys the highlights and advances of genome-driven compound and protein discovery in the field of bacterial PGPR strains, and aims to advocate for the benefits of this strategy.

How To Make a Glycopeptide: A Synthetic Biology Approach To Expand Antibiotic Chemical Diversity

Modification of natural product backbones is a proven strategy for the development of clinically useful antibiotics. Such modifications have traditionally been achieved through medicinal chemistry strategies or via in vitro enzymatic activities. In an orthogonal approach, engineering of biosynthetic pathways using synthetic biology techniques can generate chemical diversity. Here we report the use of a minimal teicoplanin class glycopeptide antibiotic (GPA) scaffold expressed in a production-optimized Streptomyces coelicolor strain to expand GPA chemical diversity. Thirteen scaffold-modifying enzymes from 7 GPA biosynthetic gene clusters in different combinations were introduced into S. coelicolor, enabling us to explore the criteria for in-cell GPA modification. These include identifying specific isozymes that tolerate the unnatural GPA scaffold and modifications that prevent or allow further elaboration by other enzymes. Overall, 15 molecules were detected, 9 of which have not been reported previously. Some of these compounds showed activity against GPA-resistant bacteria. This system allows us to observe the complex interplay between substrates and both non-native and native tailoring enzymes in a cell-based system and establishes rules for GPA synthetic biology and subsequent expansion of GPA chemical diversity.

Target genes of the Streptomyces tsukubaensis FkbN regulator include most of the tacrolimus biosynthesis genes, a phosphopantetheinyl transferase and other PKS genes

Tacrolimus (FK506) is a 23-membered macrolide immunosuppressant used in current clinics. Understanding how the tacrolimus biosynthetic gene cluster is regulated is important to increase its industrial production. Here, we analysed the effect of the disruption of fkbN (encoding a LAL-type positive transcriptional regulator) on the whole transcriptome of the tacrolimus producer Streptomyces tsukubaensis using microarray technology. Transcription of fkbN in the wild type strain increases from 70 h of cultivation reaching a maximum at 89 h, prior to the onset of tacrolimus biosynthesis. Disruption of fkbN in S. tsukubaensis does not affect growth but prevents tacrolimus biosynthesis. Inactivation of fkbN reduces the transcription of most of the fkb cluster genes, including some all (for allylmalonyl-CoA biosynthesis) genes but does not affect expression of allMNPOS or fkbR (encoding a LysR-type regulator). Disruption of fkbN does not suppress transcription of the cistron tcs6-fkbQ-fkbN; thus, FkbN self-regulates only weakly its own expression. Interestingly, inactivation of FkbN downregulates the transcription of a 4'-phosphopantetheinyl transferase coding gene, which product is involved in tacrolimus biosynthesis, and upregulates the transcription of a gene cluster containing a cpkA orthologous gene, which encodes a PKS involved in coelimycin P1 biosynthesis in Streptomyces coelicolor. We propose an information theory-based model for FkbN binding sequences. The consensus FkbN binding sequence consists of 14 nucleotides with dyad symmetry containing two conserved inverted repeats of 7 nt each. This FkbN target sequence is present in the promoters of FkbN-regulated genes.

Genetic engineering of modular PKSs: from combinatorial biosynthesis to synthetic biology

This reviews covers on-going efforts at engineering the gigantic modular polyketide synthases (PKSs), highlighting both notable successes and failures.

Biosynthesis of the Novel Macrolide Antibiotic Anthracimycin

We report the identification of the biosynthetic gene cluster for the unusual antibiotic anthracimycin (atc) from the marine derived producer strain Streptomyces sp. T676 isolated off St. John's Island, Singapore. The 53 253 bps atc locus includes a trans-acyltransferase (trans-AT) polyketide synthase (PKS), and heterologous expression in Streptomyces coelicolor resulted in anthracimycin production. Analysis of the atc cluster revealed that anthracimycin is likely generated by four PKS gene products AtcC-AtcF without involvement of post-PKS tailoring enzymes, and a biosynthetic pathway is proposed. The availability of the atc cluster provides a basis for investigating the biosynthesis of anthracimycin and its subsequent bioengineering to provide novel analogues with improved pharmacological properties.