New tools for reconstruction and heterologous expression of natural product biosynthetic gene clusters

CRISETR: an efficient technology for multiplexed refactoring of biosynthetic gene clusters

The efficient refactoring of natural product biosynthetic gene clusters (BGCs) for activating silent BGCs is a central challenge for the discovery of new bioactive natural products. Herein, we have developed a simple and robust CRISETR (CRISPR/Cas9 and RecET-mediated Refactoring) technique, combining clustered regulatory interspaced short palindromic repeats (CRISPR)/Cas9 and RecET, for the multiplexed refactoring of natural product BGCs. By this approach, natural product BGCs can be refactored through the synergistic interaction between RecET-mediated efficient homologous recombination and the CRISPR/Cas9 system. We first performed a proof-of-concept validation of the ability of CRISETR, and CRISETR can achieve simultaneous replacement of four promoter sites and marker-free replacement of single promoter site in natural product BGCs. Subsequently, we applied CRISETR to the promoter engineering of the 74-kb daptomycin BGC containing a large number of direct repeat sequences for enhancing the heterologous production of daptomycin. We used combinatorial design to build multiple refactored daptomycin BGCs with diverse combinations of promoters different in transcriptional strengths, and the yield of daptomycin was improved 20.4-fold in heterologous host Streptomyces coelicolor A3(2). In general, CRISETR exhibits enhanced tolerance to repetitive sequences within gene clusters, enabling efficient refactoring of diverse and complex BGCs, which would greatly accelerate discovery of novel bioactive metabolites present in microorganism.

Genus-specific secondary metabolome in Allokutzneria and Kibdelosporangium

Rare actinomycete genera are highly recognized as a promising source of structurally diverse and bioactive natural products. Among these genera, Allokutzneria and Kibdelosporangium are two phylogenetically closely related and have been reported to encode some valuable biosynthetic enzymes and secondary metabolites. However, there is currently no relevant systematic research available to outline the linkage of genomic and metabolomics for specific secondary metabolites in these two promising genera. In this study, we first investigated the genus-specific secondary metabolic potential in Allokutzneria and Kibdelosporangium by comparing the diversity and novelty of their secondary metabolite biosynthetic gene clusters (BGCs). The specific secondary metabolites produced by two representative strains of these genera were comprehensively investigated using untargeted metabolomics techniques. The findings unveiled that the majority (95.4%) of the gene cluster families (GCFs) encoded by Allokutzneria and Kibdelosporangium were genus-specific, including NRPS GCFs encoding siderophores. The untargeted metabolomics analysis revealed that the metabolic profiles of two representative strains exhibit extensive specificity, with the culture medium having a big impact on the metabolic profiles. Besides, an MS-cluster featuring a series of hydroxamate-type siderophores was identified from Allokutzneria albata JCM 9917, with two of them, including a novel one (N-deoxy arthrobactin A), being experimentally validated. The present study offers valuable insights for the targeted discovery of genus-specific natural products from microorganisms.

Streptomyces spp. as biocatalyst sources in pulp and paper and textile industries: Biodegradation, bioconversion and valorization of waste

Complex polymers represent a challenge for remediating environmental pollution and an opportunity for microbial-catalysed conversion to generate valorized chemicals. Members of the genus Streptomyces are of interest because of their potential use in biotechnological applications. Their versatility makes them excellent sources of biocatalysts for environmentally responsible bioconversion, as they have a broad substrate range and are active over a wide range of pH and temperature. Most Streptomyces studies have focused on the isolation of strains, recombinant work and enzyme characterization for evaluating their potential for biotechnological application. This review discusses reports of Streptomyces-based technologies for use in the textile and pulp-milling industry and describes the challenges and recent advances aimed at achieving better biodegradation methods featuring these microbial catalysts. The principal points to be discussed are (1) Streptomyces' enzymes for use in dye decolorization and lignocellulosic biodegradation, (2) biotechnological processes for textile and pulp and paper waste treatment and (3) challenges and advances for textile and pulp and paper effluent treatment.

Charting the Evolution of Chemoenzymatic Strategies in the Syntheses of Complex Natural Products

Bolstered by recent advances in bioinformatics, genetics, and enzyme engineering, the field of chemoenzymatic synthesis has enjoyed a rapid increase in popularity and utility. This Perspective explores the integration of enzymes into multistep chemical syntheses, highlighting the unique potential of biocatalytic transformations to streamline the synthesis of complex natural products. In particular, we identify four primary conceptual approaches to chemoenzymatic synthesis and illustrate each with a number of landmark case studies. Future opportunities and challenges are also discussed.

Recent Advances and Future Prospects of Mycosporine-like Amino Acids

Mycosporine-like amino acids (MAAs) are a class of water-soluble active substances produced by various aquatic organisms. However, due to the limitations of low accumulation of MAAs in organisms, the cumbersome extraction process, difficult identification, and high cost, MAAs have not yet been widely used in human life. Recently, there has been an emergence of heterologous synthesis for MAAs, making increasing yield the key to the quantification and application of MAAs. This review summarizes the latest research progress of MAAs, including: (1) introducing the biodistribution of MAAs and the content differences among different species to provide a reference for the selection of research subjects; (2) elaborating the species and molecular information of MAAs; (3) dissecting the synthesis mechanism and sorting out the synthesis pathways of various MAAs; (4) summarizing the methods of extraction and identification, summarizing the advantages and disadvantages, and providing a reference for the optimization of extraction protocols; (5) examining the heterologous synthesis method; and (6) summarizing the physiological functions of MAAs. This paper comprehensively updates the latest research status of MAAs and the various problems that need to be addressed, especially emphasizing the potential advantages of heterologous synthesis in the future production of MAAs.

Genomics-Guided Efficient Identification of 2,5-Diketopiperazine Derivatives from Actinobacteria

Secondary metabolites derived from microorganism constitute an important part of natural products. Mining of the microbial genomes revealed a large number of uncharacterized biosynthetic gene clusters, indicating their greater potential to synthetize specialized or secondary metabolites (SMs) than identified by classic fermentation and isolation approaches. Various bioinformatics tools have been developed to analyze and identify such gene clusters, thus accelerating significantly the mining process. Heterologous expression of an individual biosynthetic gene cluster has been proven as an efficient way to activate the genes and identify the encoded metabolites that cannot be detected under normal laboratory cultivation conditions. Herein, we describe a concept of genomics-guided approach by performing genome mining and heterologous expression to uncover novel CDPS-derived DKPs and functionally characterize novel tailoring enzymes embedded in the biosynthetic pathways. Recent works focused on the identification of the nucleobase-related and dimeric DKPs are also presented.

Breaking the Bottleneck in Anticancer Drug Development: Efficient Utilization of Synthetic Biology

Natural products have multifarious bioactivities against bacteria, fungi, viruses, cancers and other diseases due to their diverse structures. Nearly 65% of anticancer drugs are natural products or their derivatives. Thus, natural products play significant roles in clinical cancer therapy. With the development of biosynthetic technologies, an increasing number of natural products have been discovered and developed as candidates for clinical cancer therapy. Here, we aim to summarize the anticancer natural products approved from 1950 to 2021 and discuss their molecular mechanisms. We also describe the available synthetic biology tools and highlight their applications in the development of natural products.

Genetically engineered bacterium: Principles, practices, and prospects

Advances in synthetic biology and the clinical application of bacteriotherapy enable the use of genetically engineered bacteria (GEB) to combat various diseases. GEB act as a small 'machine factory' in the intestine or other tissues to continuously produce heterologous proteins or molecular compounds and, thus, diagnose or cure disease or work as an adjuvant reagent for disease treatment by regulating the immune system. Although the achievements of GEBs in the treatment or adjuvant therapy of diseases are promising, the practical implementation of this new therapeutic modality remains a grand challenge, especially at the initial stage. In this review, we introduce the development of GEBs and their advantages in disease management, summarize the latest research advances in microbial genetic techniques, and discuss their administration routes, performance indicators and the limitations of GEBs used as platforms for disease management. We also present several examples of GEB applications in the treatment of cancers and metabolic diseases and further highlight their great potential for clinical application in the near future.

The role of NAD and NAD precursors on longevity and lifespan modulation in the budding yeast, Saccharomyces cerevisiae

Molecular causes of aging and longevity interventions have witnessed an upsurge in the last decade. The resurgent interests in the application of small molecules as potential geroprotectors and/or pharmacogenomics point to nicotinamide adenine dinucleotide (NAD) and its precursors, nicotinamide riboside, nicotinamide mononucleotide, nicotinamide, and nicotinic acid as potentially intriguing molecules. Upon supplementation, these compounds have shown to ameliorate aging related conditions and possibly prevent death in model organisms. Besides being a molecule essential in all living cells, our understanding of the mechanism of NAD metabolism and its regulation remain incomplete owing to its omnipresent nature. Here we discuss recent advances and techniques in the study of chronological lifespan (CLS) and replicative lifespan (RLS) in the model unicellular organism Saccharomyces cerevisiae. We then follow with the mechanism and biology of NAD precursors and their roles in aging and longevity. Finally, we review potential biotechnological applications through engineering of microbial lifespan, and laid perspective on the promising candidature of alternative redox compounds for extending lifespan.

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.

Towards the sustainable discovery and development of new antibiotics

An ever-increasing demand for novel antimicrobials to treat life-threatening infections caused by the global spread of multidrug-resistant bacterial pathogens stands in stark contrast to the current level of investment in their development, particularly in the fields of natural-product-derived and synthetic small molecules. New agents displaying innovative chemistry and modes of action are desperately needed worldwide to tackle the public health menace posed by antimicrobial resistance. Here, our consortium presents a strategic blueprint to substantially improve our ability to discover and develop new antibiotics. We propose both short-term and long-term solutions to overcome the most urgent limitations in the various sectors of research and funding, aiming to bridge the gap between academic, industrial and political stakeholders, and to unite interdisciplinary expertise in order to efficiently fuel the translational pipeline for the benefit of future generations.

Rational construction of genome-reduced Burkholderiales chassis facilitates efficient heterologous production of natural products from proteobacteria

Heterologous expression of biosynthetic gene clusters (BGCs) avails yield improvements and mining of natural products, but it is limited by lacking of more efficient Gram-negative chassis. The proteobacterium Schlegelella brevitalea DSM 7029 exhibits potential for heterologous BGC expression, but its cells undergo early autolysis, hindering further applications. Herein, we rationally construct DC and DT series genome-reduced S. brevitalea mutants by sequential deletions of endogenous BGCs and the nonessential genomic regions, respectively. The DC5 to DC7 mutants affect growth, while the DT series mutants show improved growth characteristics with alleviated cell autolysis. The yield improvements of six proteobacterial natural products and successful identification of chitinimides from Chitinimonas koreensis via heterologous expression in DT mutants demonstrate their superiority to wild-type DSM 7029 and two commonly used Gram-negative chassis Escherichia coli and Pseudomonas putida. Our study expands the panel of Gram-negative chassis and facilitates the discovery of natural products by heterologous expression.

The Role of Metabolic Engineering Technologies for the Production of Fatty Acids in Yeast

Simple Summary

Metabolic engineering involves the sustainable production of high-value products. E. coli and yeast, in particular, are used for such processes. Using metabolic engineering, the biosynthetic pathways of these cells are altered to obtain a high production of desired products. Fatty acids (FAs) and their derivatives are products produced using metabolic engineering. However, classical methods used for engineering yeast metabolic pathways for the production of fatty acids and their derivatives face problems such as the low supply of key precursors and product tolerance. This review introduces the different ways FAs are being produced in E. coli and yeast and the genetic manipulations for enhanced production of FAs. The review also summarizes the latest techniques (i.e., CRISPR–Cas and synthetic biology) for developing FA-producing yeast cell factories.

Abstract

Metabolic engineering is a cutting-edge field that aims to produce simple, readily available, and inexpensive biomolecules by applying different genetic engineering and molecular biology techniques. Fatty acids (FAs) play an important role in determining the physicochemical properties of membrane lipids and are precursors of biofuels. Microbial production of FAs and FA-derived biofuels has several advantages in terms of sustainability and cost. Conventional yeast Saccharomyces cerevisiae is one of the models used for FA synthesis. Several genetic manipulations have been performed to enhance the citrate accumulation and its conversation into acetyl-CoA, a precursor for FA synthesis. Success has been achieved in producing different chemicals, including FAs and their derivatives, through metabolic engineering. However, several hurdles such as slow growth rate, low oleaginicity, and cytotoxicity are still need to be resolved. More robust research needs to be conducted on developing microbes capable of resisting diverse environments, chemicals, and cost-effective feed requirements. Redesigning microbes to produce FAs with cutting-edge synthetic biology and CRISPR techniques can solve these problems. Here, we reviewed the technological progression of metabolic engineering techniques and genetic studies conducted on S. cerevisiae, making it suitable as a model organism and a great candidate for the production of biomolecules, especially FAs.

Natural Products from Nocardia and Their Role in Pathogenicity

Nocardia spp. are filamentous Actinobacteria of the order Corynebacteriales and mostly known for their ability to cause localized and systemic infections in humans. However, the onset and progression of nocardiosis is only poorly understood, in particular the mechanisms of strain-specific presentations. Recent genome sequencing has revealed an extraordinary capacity for the production of specialized small molecules. Such secondary metabolites are often crucial for the producing microbe to survive the challenges of different environmental conditions. An interesting question thus concerns the role of these natural products in Nocardia-associated pathogenicity and immune evasion in a human host. In this review, a summary and discussion of Nocardia metabolites is presented, which may play a part in nocardiosis because of their cytotoxic, immunosuppressive and metal-chelating properties or otherwise vitally important functions. This review also contains so far unpublished data concerning the biosynthesis of these molecules that were obtained by detailed bioinformatic analyses.

The Design-Build-Test-Learn cycle for metabolic engineering of Streptomycetes

Streptomycetes are producers of a wide range of specialized metabolites of great medicinal and industrial importance, such as antibiotics, antifungals, or pesticides. Having been the drivers of the golden age of antibiotics in the 1950s and 1960s, technological advancements over the last two decades have revealed that very little of their biosynthetic potential has been exploited so far. Given the great need for new antibiotics due to the emerging antimicrobial resistance crisis, as well as the urgent need for sustainable biobased production of complex molecules, there is a great renewed interest in exploring and engineering the biosynthetic potential of streptomycetes. Here, we describe the Design-Build-Test-Learn (DBTL) cycle for metabolic engineering experiments in streptomycetes and how it can be used for the discovery and production of novel specialized metabolites.

Synthetic biology-inspired strategies and tools for engineering of microbial natural product biosynthetic pathways

Microbial-derived natural products (NPs) and their derivative products are of great importance and used widely in many fields, especially in pharmaceutical industries. However, there is an immediate need to establish innovative approaches, strategies, and techniques to discover new NPs with novel or enhanced biological properties, due to the less productivity and higher cost on traditional drug discovery pipelines from natural bioresources. Revealing of untapped microbial cryptic biosynthetic gene clusters (BGCs) using DNA sequencing technology and bioinformatics tools makes genome mining possible for NP discovery from microorganisms. Meanwhile, new approaches and strategies in the area of synthetic biology offer great potentials for generation of new NPs by engineering or creating synthetic systems with improved and desired functions. Development of approaches, strategies and tools in synthetic biology can facilitate not only exploration and enhancement in supply, and also in the structural diversification of NPs. Here, we discussed recent advances in synthetic biology-inspired strategies, including bioinformatics and genetic engineering tools and approaches for identification, cloning, editing/refactoring of candidate biosynthetic pathways, construction of heterologous expression hosts, fitness optimization between target pathways and hosts and detection of NP production.

Synergizing the potential of bacterial genomics and metabolomics to find novel antibiotics

Antibiotic development based on natural products has faced a long lasting decline since the 1970s, while both the speed and the extent of antimicrobial resistance (AMR) development have been severely underestimated. The discovery of antimicrobial natural products of bacterial and fungal origin featuring new chemistry and previously unknown mode of actions is increasingly challenged by rediscovery issues. Natural products that are abundantly produced by the corresponding wild type organisms often featuring strong UV signals have been extensively characterized, especially the ones produced by extensively screened microbial genera such as streptomycetes. Purely synthetic chemistry approaches aiming to replace the declining supply from natural products as starting materials to develop novel antibiotics largely failed to provide significant numbers of antibiotic drug leads. To cope with this fundamental issue, microbial natural products science is being transformed from a 'grind-and-find' study to an integrated approach based on bacterial genomics and metabolomics. Novel technologies in instrumental analytics are increasingly employed to lower detection limits and expand the space of detectable substance classes, while broadening the scope of accessible and potentially bioactive natural products. Furthermore, the almost exponential increase in publicly available bacterial genome data has shown that the biosynthetic potential of the investigated strains by far exceeds the amount of detected metabolites. This can be judged by the discrepancy between the number of biosynthetic gene clusters (BGC) encoded in the genome of each microbial strain and the number of secondary metabolites actually detected, even when considering the increased sensitivity provided by novel analytical instrumentation. In silico annotation tools for biosynthetic gene cluster classification and analysis allow fast prioritization in BGC-to-compound workflows, which is highly important to be able to process the enormous underlying data volumes. BGC prioritization is currently accompanied by novel molecular biology-based approaches to access the so-called orphan BGCs not yet correlated with a secondary metabolite. Integration of metabolomics, in silico genomics and molecular biology approaches into the mainstream of natural product research will critically influence future success and impact the natural product field in pharmaceutical, nutritional and agrochemical applications and especially in anti-infective research.

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.

Discovery, Bioactivity Evaluation, Biosynthetic Gene Cluster Identification, and Heterologous Expression of Novel Albofungin Derivatives

The crude extract of Streptomyces chrestomyceticus exhibited strong and broad activities against most “ESKAPE pathogens.” We conducted a comprehensive chemical investigation for secondary metabolites from the S. chrestomyceticus strain and identified two novel albofungin (alb) derivatives, i.e., albofungins A (1) and B (2), along with two known compounds, i.e., albofungin (3) and chloroalbofungin (4). The chemical structures of the novel compounds were elucidated using HRMS, 1D and 2D NMR, and electronic circular dichroism spectroscopy. The draft genome of S. chrestomyceticus was sequenced, and a 72 kb albofungin (alb) gene cluster with 72 open reading frames encoding type II polyketide synthases (PKSs), regulators, and transporters, and tailoring enzymes were identified using bioinformatics analysis. The alb gene cluster was confirmed using the heterologous expression in Streptomyces coelicolor, which successfully produced the compounds 3 and 4. Furthermore, compounds 1–4 displayed remarkable activities against Gram-positive bacteria and antitumor activities toward various cancer cells. Notably, compounds 1 and 3 showed potent activities against Gram-negative pathogenic bacteria. The terminal deoxynucleotidyl transferase (dUTP) nick-end labeling and flow cytometry analysis verified that compound 1 inhibited cancer cell proliferation by inducing cellular apoptosis. These results indicated that albofungins might be potential candidates for the development of antibiotics and antitumor drugs.

Chemical Toolbox to Decode the Microbiota Lexicon

The human microbiota deploys a diverse range of molecules and metabolites to engage in chemical communications with the host, mediating fundamental aspects of host health. Studies of the structures and activities of bioactive molecules produced by the microbiota are imperative to address their implications in microbiota associated diseases in human. By drawing experiences from different research fields, chemists and chemical biologists, who are experts in dealing with chemical molecules, are uniquely positioned to contribute to the emerging knowledge of human microbiota. In this minireview, we discuss the current chemical tools and methods that are pertinent to the discovery of microbiota molecules and metabolites, characterizations of their protein targets, as well as evaluations of their biodistributions in hosts. These are key aspects in understanding the chemical underpinnings of the microbiota-host interactions that would enable future development of diagnostics and therapeutics targeting the human microbiota.

Efficient Multiplex Genome Editing in Streptomyces via Engineered CRISPR-Cas12a Systems

Streptomyces strains produce a great number of valuable natural products. With the development of genome sequencing, a vast number of biosynthetic gene clusters with high potential for use in the discovery of valuable clinical drugs have been revealed. Therefore, emerging needs for tools to manipulate these biosynthetic pathways are presented. Although the clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas 9) system has exhibited great capabilities for gene editing in multiple Streptomyces strains, it has failed to work in some newly discovered strains and some important industrial strains. Additionally, the protospacer adjacent motif (PAM) recognition scope of this system sometimes limits its applications for generating precise site mutations and insertions. Here, we developed three efficient CRISPR-FnCas12a systems for multiplex genome editing in several Streptomyces strains. Each system exhibited advantages for different applications. The CRISPR-FnCas12a1 system was efficiently applied in the industrial strain Streptomyces hygroscopicus, in which SpCas9 does not work well. The CRISPR-FnCas12a2 system was used to delete large fragments ranging from 21.4 to 128 kb. Additionally, the CRISPR-FnCas12a3 system employing the engineered FnCas12a mutant EP16, which recognizes a broad spectrum of PAM sequences, was used to precisely perform site mutations and insertions. The CRISPR-FnCas12a3 system addressed the limitation of TTN PAM recognition in Streptomyces strains with high GC contents. In summary, all the CRISPR-FnCas12a systems developed in this study are powerful tools for precise and multiplex genome editing in Streptomyces strains.

Promoter-Library-Based Pathway Optimization for Efficient (2S)-Naringenin Production from p-Coumaric Acid in Saccharomyces cerevisiae

Pathway optimization plays an important role in fine-tuning metabolic pathways. In most conditions, more than three genes are involved in the biosynthesis pathway of a specific target product. To improve the titer of products, rational regulation of a group of genes by a series of promoters with different strengths is essential. On the basis of a series of RNA-Seq data, a set of 66 native promoters was chosen to fine-tune gene expression in Saccharomyces cerevisiae. Promoter strength was characterized by measuring the fluorescence strength of the enhanced green fluorescent protein through fluorescence-activated cell sorting. The expressions of P_TDH1, P_PGK1, P_INO1, P_SED1, and P_CCW12 were stronger than that of P_TDH3, whereas those of another 15 promoters were stronger than that of P_TEF1. Then, 30 promoters were chosen to optimize the biosynthesis pathway of (2S)-naringenin from p-coumaric acid. With a high-throughput screening method, the highest titer of (2S)-naringenin in a 5 L bioreactor reached 1.21 g/L from p-coumaric acid, which is the highest titer according to the currently available reports.

Bioprospecting Through Cloning of Whole Natural Product Biosynthetic Gene Clusters

Since the discovery of penicillin, natural products and their derivatives have been a valuable resource for drug discovery. With recent development of genome mining approaches in the post-genome era, a great number of natural product biosynthetic gene clusters (BGCs) have been identified and these can potentially be exploited for the discovery of novel natural products that can find application as pharmaceuticals. Since many BGCs are silent or do not express in native hosts under laboratory conditions, heterologous expression of BGCs in genetically tractable hosts becomes an attractive route to activate these BGCs to discover the corresponding products. Here, we highlight recent achievements in cloning and discovery of natural product biosynthetic pathways via intact BGC capturing, and discuss the prospects of high-throughput and multiplexed cloning of rational-designed gene clusters in the future.

Antibiotic drug discovery: Challenges and perspectives in the light of emerging antibiotic resistance

Amid a rising threat of antimicrobial resistance in a global scenario, our huge investments and high-throughput technologies injected for rejuvenating the key therapeutic scaffolds to suppress these rising superbugs has been diminishing severely. This has grasped world-wide attention, with increased consideration being given to the discovery of new chemical entities. Research has now proven that the relatively tiny and simpler microbes possess enhanced capability of generating novel and diverse chemical constituents with huge therapeutic leads. The usage of these beneficial organisms could help in producing new chemical scaffolds that govern the power to suppress the spread of obnoxious superbugs. Here in this review, we have explicitly focused on several appealing strategies employed for the generation of new chemical scaffolds. Also, efforts on providing novel insights on some of the unresolved questions in the production of metabolites, metabolic profiling and also the serendipity of getting "hit molecules" have been rigorously discussed. However, we are highly aware that biosynthetic pathway of different classes of secondary metabolites and their biosynthetic route is a vast topic, thus we have avoided discussion on this topic.

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.

Heterologous Metabolic Pathways: Strategies for Optimal Expression in Eukaryotic Hosts

Heterologous pathways are linked series of biochemical reactions occurring in a host organism after the introduction of foreign genes. Incorporation of metabolic pathways into host organisms is a major strategy used to increase the production of valuable secondary metabolites. Unfortunately, simple introduction of the pathway genes into the heterologous host in most cases does not result in successful heterologous expression. Extensive modification of heterologous genes and the corresponding enzymes on many different levels is required to achieve high target metabolite production rates. This review summarizes the essential techniques used to create heterologous biochemical pathways, with a focus on the key challenges arising in the process and the major strategies for overcoming them.

Linking Genes to Molecules in Eukaryotic Sources: An Endeavor to Expand Our Biosynthetic Repertoire

The discovery of natural products continues to interest chemists and biologists for their utility in medicine as well as facilitating our understanding of signaling, pathogenesis, and evolution. Despite an attenuation in the discovery rate of new molecules, the current genomics and transcriptomics revolution has illuminated the untapped biosynthetic potential of many diverse organisms. Today, natural product discovery can be driven by biosynthetic gene cluster (BGC) analysis, which is capable of predicting enzymes that catalyze novel reactions and organisms that synthesize new chemical structures. This approach has been particularly effective in mining bacterial and fungal genomes where it has facilitated the discovery of new molecules, increased the understanding of metabolite assembly, and in some instances uncovered enzymes with intriguing synthetic utility. While relatively less is known about the biosynthetic potential of non-fungal eukaryotes, there is compelling evidence to suggest many encode biosynthetic enzymes that produce molecules with unique bioactivities. In this review, we highlight how the advances in genomics and transcriptomics have aided natural product discovery in sources from eukaryotic lineages. We summarize work that has successfully connected genes to previously identified molecules and how advancing these techniques can lead to genetics-guided discovery of novel chemical structures and reactions distributed throughout the tree of life. Ultimately, we discuss the advantage of increasing the known biosynthetic space to ease access to complex natural and non-natural small molecules.

DeepRiPP integrates multiomics data to automate discovery of novel ribosomally synthesized natural products

Microbial natural products represent a rich resource of evolved chemistry that forms the basis for the majority of pharmacotherapeutics. Ribosomally synthesized and posttranslationally modified peptides (RiPPs) are a particularly interesting class of natural products noted for their unique mode of biosynthesis and biological activities. Analyses of sequenced microbial genomes have revealed an enormous number of biosynthetic loci encoding RiPPs but whose products remain cryptic. In parallel, analyses of bacterial metabolomes typically assign chemical structures to only a minority of detected metabolites. Aligning these 2 disparate sources of data could provide a comprehensive strategy for natural product discovery. Here we present DeepRiPP, an integrated genomic and metabolomic platform that employs machine learning to automate the selective discovery and isolation of novel RiPPs. DeepRiPP includes 3 modules. The first, NLPPrecursor, identifies RiPPs independent of genomic context and neighboring biosynthetic genes. The second module, BARLEY, prioritizes loci that encode novel compounds, while the third, CLAMS, automates the isolation of their corresponding products from complex bacterial extracts. DeepRiPP pinpoints target metabolites using large-scale comparative metabolomics analysis across a database of 10,498 extracts generated from 463 strains. We apply the DeepRiPP platform to expand the landscape of novel RiPPs encoded within sequenced genomes and to discover 3 novel RiPPs, whose structures are exactly as predicted by our platform. By building on advances in machine learning technologies, DeepRiPP integrates genomic and metabolomic data to guide the isolation of novel RiPPs in an automated manner.

Nonribosomal peptides in fungal cell factories: from genome mining to optimized heterologous production

Fungi are notoriously prolific producers of secondary metabolites including nonribosomal peptides (NRPs). The structural complexity of NRPs grants them interesting activities such as antibiotic, anti-cancer, and anti-inflammatory properties. The discovery of these compounds with attractive activities can be achieved by using two approaches: either by screening samples originating from various environments for their biological activities, or by identifying the related clusters in genomic sequences thanks to bioinformatics tools. This genome mining approach has grown tremendously due to recent advances in genome sequencing, which have provided an incredible amount of genomic data from hundreds of microbial species. Regarding fungal organisms, the genomic data have revealed the presence of an unexpected number of putative NRP-related gene clusters. This highlights fungi as a goldmine for the discovery of putative novel bioactive compounds. Recent development of NRP dedicated bioinformatics tools have increased the capacity to identify these gene clusters and to deduce NRPs structures, speeding-up the screening process for novel metabolites discovery. Unfortunately, the newly identified compound is frequently not or poorly produced by native producers due to a lack of expression of the related genes cluster. A frequently employed strategy to increase production rates consists in transferring the related biosynthetic pathway in heterologous hosts. This review aims to provide a comprehensive overview about the topic of NRPs discovery, from gene cluster identification by genome mining to the heterologous production in fungal hosts. The main computational tools and methods for genome mining are herein presented with an emphasis on the particularities of the fungal systems. The different steps of the reconstitution of NRP biosynthetic pathway in heterologous fungal cell factories will be discussed, as well as the key factors to consider for maximizing productivity. Several examples will be developed to illustrate the potential of heterologous production to both discover uncharacterized novel compounds predicted in silico by genome mining, and to enhance the productivity of interesting bio-active natural products.

Novel bioactive natural products from bacteria via bioprospecting, genome mining and metabolic engineering

For over seven decades, bacteria served as a valuable source of bioactive natural products some of which were eventually developed into drugs to treat infections, cancer and immune system-related diseases. Traditionally, novel compounds produced by bacteria were discovered via conventional bioprospecting based on isolation of potential producers and screening their extracts in a variety of bioassays. Over time, most of the natural products identifiable by this approach were discovered, and the pipeline for new drugs based on bacterially produced metabolites started to run dry. This mini-review highlights recent developments in bacterial bioprospecting for novel compounds that are based on several out-of-the-box approaches, including the following: (i) targeting bacterial species previously unknown to produce any bioactive natural products, (ii) exploring non-traditional environmental niches and methods for isolation of bacteria and (iii) various types of 'genome mining' aimed at unravelling genetic potential of bacteria to produce secondary metabolites. All these approaches have already yielded a number of novel bioactive compounds and, if used wisely, will soon revitalize drug discovery pipeline based on bacterial natural products.

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.

Synthetic biology strategies for microbial biosynthesis of plant natural products

Metabolic engineers endeavor to create a bio-based manufacturing industry using microbes to produce fuels, chemicals, and medicines. Plant natural products (PNPs) are historically challenging to produce and are ubiquitous in medicines, flavors, and fragrances. Engineering PNP pathways into new hosts requires finding or modifying a suitable host to accommodate the pathway, planning and implementing a biosynthetic route to the compound, and discovering or engineering enzymes for missing steps. In this review, we describe recent developments in metabolic engineering at the level of host, pathway, and enzyme, and discuss how the field is approaching ever more complex biosynthetic opportunities.

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.

Recent advances in natural products exploitation in Streptomyces via synthetic biology

Natural products of microbial origin have proven to be the wellspring of clinically useful compounds for human therapeutics. Streptomyces species are predominant sources of bioactive compounds, most of which serve as potential drug candidates. While the exploitation of natural products has been severely reduced over the past two decades, the growing crisis of evolution and dissemination of drug resistant pathogens have again attracted great interest in this field. The emerging synthetic biology has been heralded as a new bioengineering platform to discover novel bioactive compounds and expand bioactive natural products diversity and production. Herein, we review recent advances in the natural products exploitation of Streptomyces with the applications of synthetic biology from three major aspects, including recently developed synthetic biology tools, natural products biosynthetic pathway engineering strategies as well as chassis host modifications.

Bioactive molecules from Nocardia: diversity, bioactivities and biosynthesis

Nocardia spp. are catalase positive, aerobic, and non-motile Gram-positive filamentous bacteria. Many Nocarida spp. have been reported as unusual causes of diverse clinical diseases in both humans and animals. Therefore, they have been studied for a long time, primarily focusing on strain characterization, taxonomic classification of new isolates, and host pathophysiology. Currently, there are emerging interests in isolating bioactive molecules from diverse actinobacteria including Nocardia spp. and studying their biosynthetic mechanisms. In addition, these species possess significant metabolic capacity, which has been utilized for generating diverse functionalized bioactive molecules by whole cell biotransformation. This review summarizes the structural diversity and biological activities of compounds biosynthesized or biotransformed by Nocardia spp. Furthermore, the recent advances on biosynthetic mechanisms and genetic engineering approaches for enhanced production or structural/functional modification are presented.

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.

CRISPR-Cpf1-Assisted Multiplex Genome Editing and Transcriptional Repression in Streptomyces

Streptomyces has a strong capability for producing a large number of bioactive natural products and remains invaluable as a source for the discovery of novel drug leads. Although the Streptococcus pyogenes CRISPR-Cas9-assisted genome editing tool has been developed for rapid genetic engineering in Streptomyces, it has a number of limitations, including the toxicity of SpCas9 expression in some important industrial Streptomyces strains and the need for complex expression constructs when targeting multiple genomic loci. To address these problems, in this study, we developed a high-efficiency CRISPR-Cpf1 system (from Francisella novicida) for multiplex genome editing and transcriptional repression in Streptomyces Using an all-in-one editing plasmid with homology-directed repair (HDR), our CRISPR-Cpf1 system precisely deletes single or double genes at efficiencies of 75 to 95% in Streptomyces coelicolor When no templates for HDR are present, random-sized DNA deletions are achieved by FnCpf1-induced double-strand break (DSB) repair by a reconstituted nonhomologous end joining (NHEJ) pathway. Furthermore, a DNase-deactivated Cpf1 (ddCpf1)-based integrative CRISPRi system is developed for robust, multiplex gene repression using a single customized crRNA array. Finally, we demonstrate that FnCpf1 and SpCas9 exhibit different suitability in tested industrial Streptomyces species and show that FnCpf1 can efficiently promote HDR-mediated gene deletion in the 5-oxomilbemycin-producing strain Streptomyces hygroscopicus SIPI-KF, in which SpCas9 does not work well. Collectively, FnCpf1 is a powerful and indispensable addition to the Streptomyces CRISPR toolbox.IMPORTANCE Rapid, efficient genetic engineering of Streptomyces strains is critical for genome mining of novel natural products (NPs) as well as strain improvement. Here, a novel and high-efficiency Streptomyces genome editing tool is established based on the FnCRISPR-Cpf1 system, which is an attractive and powerful alternative to the S. pyogenes CRISPR-Cas9 system due to its unique features. When combined with HDR or NHEJ, FnCpf1 enables the creation of gene(s) deletion with high efficiency. Furthermore, a ddCpf1-based integrative CRISPRi platform is established for simple, multiplex transcriptional repression. Of importance, FnCpf1-based genome editing proves to be a highly efficient tool for genetic modification of some important industrial Streptomyces strains (e.g., S. hygroscopicus SIPI-KF) that cannot utilize the SpCRISPR-Cas9 system. We expect the CRISPR-Cpf1-assisted genome editing tool to accelerate discovery and development of pharmaceutically active NPs in Streptomyces as well as other actinomycetes.

CRISPR-Cpf1-Assisted Multiplex Genome Editing and Transcriptional Repression in Streptomyces

Streptomyces has a strong capability for producing a large number of bioactive natural products and remains invaluable as a source for the discovery of novel drug leads. Although the Streptococcus pyogenes CRISPR-Cas9-assisted genome editing tool has been developed for rapid genetic engineering in Streptomyces, it has a number of limitations, including the toxicity of SpCas9 expression in some important industrial Streptomyces strains and the need for complex expression constructs when targeting multiple genomic loci. To address these problems, in this study, we developed a high-efficiency CRISPR-Cpf1 system (from Francisella novicida) for multiplex genome editing and transcriptional repression in Streptomyces Using an all-in-one editing plasmid with homology-directed repair (HDR), our CRISPR-Cpf1 system precisely deletes single or double genes at efficiencies of 75 to 95% in Streptomyces coelicolor When no templates for HDR are present, random-sized DNA deletions are achieved by FnCpf1-induced double-strand break (DSB) repair by a reconstituted nonhomologous end joining (NHEJ) pathway. Furthermore, a DNase-deactivated Cpf1 (ddCpf1)-based integrative CRISPRi system is developed for robust, multiplex gene repression using a single customized crRNA array. Finally, we demonstrate that FnCpf1 and SpCas9 exhibit different suitability in tested industrial Streptomyces species and show that FnCpf1 can efficiently promote HDR-mediated gene deletion in the 5-oxomilbemycin-producing strain Streptomyces hygroscopicus SIPI-KF, in which SpCas9 does not work well. Collectively, FnCpf1 is a powerful and indispensable addition to the Streptomyces CRISPR toolbox.IMPORTANCE Rapid, efficient genetic engineering of Streptomyces strains is critical for genome mining of novel natural products (NPs) as well as strain improvement. Here, a novel and high-efficiency Streptomyces genome editing tool is established based on the FnCRISPR-Cpf1 system, which is an attractive and powerful alternative to the S. pyogenes CRISPR-Cas9 system due to its unique features. When combined with HDR or NHEJ, FnCpf1 enables the creation of gene(s) deletion with high efficiency. Furthermore, a ddCpf1-based integrative CRISPRi platform is established for simple, multiplex transcriptional repression. Of importance, FnCpf1-based genome editing proves to be a highly efficient tool for genetic modification of some important industrial Streptomyces strains (e.g., S. hygroscopicus SIPI-KF) that cannot utilize the SpCRISPR-Cas9 system. We expect the CRISPR-Cpf1-assisted genome editing tool to accelerate discovery and development of pharmaceutically active NPs in Streptomyces as well as other actinomycetes.

Need for Laboratory Ecosystems To Unravel the Structures and Functions of Soil Microbial Communities Mediated by Chemistry

The chemistry underpinning microbial interactions provides an integrative framework for linking the activities of individual microbes, microbial communities, plants, and their environments. Currently, we know very little about the functions of genes and metabolites within these communities because genome annotations and functions are derived from the minority of microbes that have been propagated in the laboratory. Yet the diversity, complexity, inaccessibility, and irreproducibility of native microbial consortia limit our ability to interpret chemical signaling and map metabolic networks. In this perspective, we contend that standardized laboratory ecosystems are needed to dissect the chemistry of soil microbiomes. We argue that dissemination and application of standardized laboratory ecosystems will be transformative for the field, much like how model organisms have played critical roles in advancing biochemistry and molecular and cellular biology. Community consensus on fabricated ecosystems ("EcoFABs") along with protocols and data standards will integrate efforts and enable rapid improvements in our understanding of the biochemical ecology of microbial communities.

Raistrickiones A-E from a Highly Productive Strain of Penicillium raistrickii Generated through Thermo Change

Three new diastereomers of polyketides (PKs), raistrickiones A−C (1⁻3), together with two new analogues, raistrickiones D and E (4 and 5), were isolated from a highly productive strain of Penicillium raistrickii, which was subjected to an experimental thermo-change strategy to tap its potential of producing new secondary metabolites. Metabolites 1 and 2 existed in a diastereomeric mixture in the crystal packing according to the X-ray data, and were laboriously separated by semi-preparative HPLC on a chiral column. The structures of 1⁻5 were determined on the basis of the detailed analyses of the spectroscopic data (UV, IR, HRESIMS, 1D, and 2D NMR), single-crystal X-ray diffractions, and comparison of the experimental and calculated electronic circular dichroism spectra. Compounds 1⁻5 represented the first case of 3,5-dihydroxy-4-methylbenzoyl derivatives of natural products. Compounds 1⁻5 exhibited moderate radical scavenging activities against 1,1-diphenyl-2-picrylhydrazyl radical 2,2-diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl (DPPH).