High-efficiency multiplex genome editing of Streptomyces species using an engineered CRISPR/Cas system

Machine-Learning Analysis of Streptomyces coelicolor Transcriptomes Reveals a Transcription Regulatory Network Encompassing Biosynthetic Gene Clusters

Streptomyces produces diverse secondary metabolites of biopharmaceutical importance, yet the rate of biosynthesis of these metabolites is often hampered by complex transcriptional regulation. Therefore, a fundamental understanding of transcriptional regulation in Streptomyces is key to fully harness its genetic potential. Here, independent component analysis (ICA) of 454 high-quality gene expression profiles of the model species Streptomyces coelicolor is performed, of which 249 profiles are newly generated for S. coelicolor cultivated on 20 different carbon sources and 64 engineered strains with overexpressed sigma factors. ICA of the transcriptome dataset reveals 117 independently modulated groups of genes (iModulons), which account for 81.6% of the variance in the dataset. The genes in each iModulon are involved in specific cellular responses, which are often transcriptionally controlled by specific regulators. Also, iModulons accurately predict 25 secondary metabolite biosynthetic gene clusters encoded in the genome. This systemic analysis leads to reveal the functions of previously uncharacterized genes, putative regulons for 40 transcriptional regulators, including 30 sigma factors, and regulation of secondary metabolism via phosphate- and iron-dependent mechanisms in S. coelicolor. ICA of large transcriptomic datasets thus enlightens a new and fundamental understanding of transcriptional regulation of secondary metabolite synthesis along with interconnected metabolic processes in Streptomyces.

Rational construction of genome-minimized Streptomyces host for the expression of secondary metabolite gene clusters

Streptomyces offer a wealth of naturally occurring compounds with diverse structures, many of which possess significant pharmaceutical values. However, new product exploration and increased yield of specific compounds in Streptomyces have been technically challenging due to their slow growth rate, complex culture conditions and intricate genetic backgrounds. In this study, we screened dozens of Streptomyces strains inhabiting in a plant rhizosphere for fast-growing candidates, and further employed CRISPR/Cas-based engineering techniques for stepwise refinement of a particular strain, Streptomyces sp. A-14 that harbors a 7.47 Mb genome. After strategic removal of nonessential genomic regions and most gene clusters, we reduced its genome size to 6.13 Mb, while preserving its growth rate to the greatest extent. We further demonstrated that cleaner metabolic background of this engineered strain was well suited for the expression and characterization of heterologous gene clusters, including the biosynthetic pathways of actinorhodin and polycyclic tetramate macrolactams. Moreover, this streamlined genome is anticipated to facilitate directing the metabolic flux towards the production of desired compounds and increasing their yields.

Advances in CRISPR-Cas systems for human bacterial disease

Prokaryotic adaptive immune systems called CRISPR-Cas systems have transformed genome editing by allowing for precise genetic alterations through targeted DNA cleavage. This system comprises CRISPR-associated genes and repeat-spacer arrays, which generate RNA molecules that guide the cleavage of invading genetic material. CRISPR-Cas is classified into Class 1 (multi-subunit effectors) and Class 2 (single multi-domain effectors). Its applications span combating antimicrobial resistance (AMR), targeting antibiotic resistance genes (ARGs), resensitizing bacteria to antibiotics, and preventing horizontal gene transfer (HGT). CRISPR-Cas3, for example, effectively degrades plasmids carrying resistance genes, providing a precise method to disarm bacteria. In the context of ESKAPE pathogens, CRISPR technology can resensitize bacteria to antibiotics by targeting specific resistance genes. Furthermore, in tuberculosis (TB) research, CRISPR-based tools enhance diagnostic accuracy and facilitate precise genetic modifications for studying Mycobacterium tuberculosis. CRISPR-based diagnostics, leveraging Cas endonucleases' collateral cleavage activity, offer highly sensitive pathogen detection. These advancements underscore CRISPR's transformative potential in addressing AMR and enhancing infectious disease management.

Genome Mining and Genetic Manipulation Reveal New Isofuranonaphthoquinones in Nocardia Species

The identification of specialized metabolites isolated from microorganisms is urgently needed to determine their roles in treating cancer and controlling multidrug-resistant pathogens. Naphthoquinones act as anticancer agents in various types of cancers, but some toxicity indicators have been limited in their appropriate application. In this context, new isofuranonaphthoquinones (ifnq) that are less toxic to humans could be promising lead compounds for developing anticancer drugs. The aim of this study is to identify and characterize novel furanonaphthoquinones (fnqs) from Nocardia sp. CS682 and to evaluate their potential therapeutic applications. Analysis of the genome of Nocardia sp. CS682 revealed the presence of a furanonaphthoquinone (fnq) gene cluster, which displays a similar genetic organization and high nucleotide sequence identity to the ifnq gene cluster from Streptomyces sp. RI-77, a producer of the naphthoquinones JBIR-76 and JBIR-77. In this study, the overexpression of the Streptomyces antibiotic regulatory protein (SARP) in Nocardia sp. CS682DR (nargenicin gene-deleted mutant) explicitly produced new fnqs, namely, NOC-IBR1 and NOC-IBR2. Subsequently, the role of the SARP regulator was confirmed by gene inactivation using CRISPR-Cas9 and complementation studies. Furthermore, antioxidant, antimicrobial, and cytotoxicity assays were performed for the isolated compounds, and it was found that NOC-IBR2 exhibited superior activities to NOC-IBR1. In addition, a flexible methyltransferase substrate, ThnM3, was found to be involved in terminal methylation of NOC-IBR1, which was confirmed by in vitro enzyme assays. Thus, this study supports the importance of genome mining and genome editing approaches for exploring new specialized metabolites in a rare actinomycete called Nocardia.

Prokaryotic RNA N1-Methyladenosine Erasers Maintain tRNA m1A Modification Levels in Streptomyces venezuelae

tRNA modifications help maintain tRNA structure and facilitate translation and stress response. Found in all three kingdoms of life, m1A tRNA modification occurs in the T loop of many tRNAs, stabilizes tertiary tRNA structure, and impacts translation. M1A in the T loop is reversible by three mammalian demethylase enzymes, which bypasses the need of turning over the tRNA molecule to adjust its m1A levels in cells. However, no prokaryotic tRNA demethylase enzyme has been identified that acts on endogenous RNA modifications. Using Streptomyces venezuelae as a model organism, we confirmed the presence and quantitative m1A tRNA signatures using mass spectrometry and high-throughput tRNA sequencing. We identified two RNA demethylases that can remove m1A in tRNA and validated the activity of a previously annotated tRNA m1A writer. Using single-gene knockouts of these erasers and the m1A writer, we found dynamic changes of m1A levels in many tRNAs under stress conditions. Phenotypic characterization highlighted changes in their growth and altered antibiotic production. Our identification of the first prokaryotic tRNA demethylase enzyme paves the way for investigating new mechanisms of translational regulation in bacteria.

Bacillus subtilis as a host for natural product discovery and engineering of biosynthetic gene clusters

Covering: up to October 2023Many bioactive natural products are synthesized by microorganisms that are either difficult or impossible to cultivate under laboratory conditions, or that produce only small amounts of the desired compound. By transferring biosynthetic gene clusters (BGCs) into alternative host organisms that are more easily cultured and engineered, larger quantities can be obtained and new analogues with potentially improved biological activity or other desirable properties can be generated. Moreover, expression of cryptic BGCs in a suitable host can facilitate the identification and characterization of novel natural products. Heterologous expression therefore represents a valuable tool for natural product discovery and engineering as it allows the study and manipulation of their biosynthetic pathways in a controlled setting, enabling innovative applications. Bacillus is a genus of Gram-positive bacteria that is widely used in industrial biotechnology as a host for the production of proteins from diverse origins, including enzymes and vaccines. However, despite numerous successful examples, Bacillus species remain underexploited as heterologous hosts for the expression of natural product BGCs. Here, we review important advantages that Bacillus species offer as expression hosts, such as high secretion capacity, natural competence for DNA uptake, and the increasing availability of a wide range of genetic tools for gene expression and strain engineering. We evaluate different strain optimization strategies and other critical factors that have improved the success and efficiency of heterologous natural product biosynthesis in B. subtilis. Finally, future perspectives for using B. subtilis as a heterologous host are discussed, identifying research gaps and promising areas that require further exploration.

Engineered cytosine base editor enabling broad-scope and high-fidelity gene editing in Streptomyces

Base editing (BE) faces protospacer adjacent motif (PAM) constraints and off-target effects in both eukaryotes and prokaryotes. For Streptomyces, renowned as one of the most prolific bacterial producers of antibiotics, the challenges are more pronounced due to its diverse genomic content and high GC content. Here, we develop a base editor named eSCBE3-NG-Hypa, tailored with both high efficiency and -fidelity for Streptomyces. Of note, eSCBE3-NG-Hypa recognizes NG PAM and exhibits high activity at challenging sites with high GC content or GC motifs, while displaying minimal off-target effects. To illustrate its practicability, we employ eSCBE3-NG-Hypa to achieve precise key amino acid conversion of the dehydratase (DH) domains within the modular polyketide synthase (PKS) responsible for the insecticide avermectins biosynthesis, achieving domains inactivation. The resulting DH-inactivated mutants, while ceasing avermectins production, produce a high yield of oligomycin, indicating competitive relationships among multiple biosynthetic gene clusters (BGCs) in Streptomyces avermitilis. Leveraging this insight, we use eSCBE3-NG-Hypa to introduce premature stop codons into competitor gene cluster of ave in an industrial S. avermitilis, with the mutant Δolm exhibiting the highest 4.45-fold increase in avermectin B1a compared to the control. This work provides a potent tool for modifying biosynthetic pathways and advancing metabolic engineering in Streptomyces.

Sigma Factor Engineering in Actinoplanes sp. SE50/110: Expression of the Alternative Sigma Factor Gene ACSP50_0507 (σHAs) Enhances Acarbose Yield and Alters Cell Morphology

Sigma factors are transcriptional regulators that are part of complex regulatory networks for major cellular processes, as well as for growth phase-dependent regulation and stress response. Actinoplanes sp. SE50/110 is the natural producer of acarbose, an α-glucosidase inhibitor that is used in diabetes type 2 treatment. Acarbose biosynthesis is dependent on growth, making sigma factor engineering a promising tool for metabolic engineering. ACSP50_0507 is a homolog of the developmental and osmotic-stress-regulating Streptomyces coelicolor σHSc. Therefore, the protein encoded by ACSP50_0507 was named σHAs. Here, an Actinoplanes sp. SE50/110 expression strain for the alternative sigma factor gene ACSP50_0507 (sigHAs) achieved a two-fold increased acarbose yield with acarbose production extending into the stationary growth phase. Transcriptome sequencing revealed upregulation of acarbose biosynthesis genes during growth and at the late stationary growth phase. Genes that are transcriptionally activated by σHAs frequently code for secreted or membrane-associated proteins. This is also mirrored by the severely affected cell morphology, with hyperbranching, deformed and compartmentalized hyphae. The dehydrated cell morphology and upregulation of further genes point to a putative involvement in osmotic stress response, similar to its S. coelicolor homolog. The DNA-binding motif of σHAs was determined based on transcriptome sequencing data and shows high motif similarity to that of its homolog. The motif was confirmed by in vitro binding of recombinantly expressed σHAs to the upstream sequence of a strongly upregulated gene. Autoregulation of σHAs was observed, and binding to its own gene promoter region was also confirmed.

Unleashing the potential: type I CRISPR-Cas systems in actinomycetes for genome editing

Covering: up to the end of 2023Type I CRISPR-Cas systems are widely distributed, found in over 40% of bacteria and 80% of archaea. Among genome-sequenced actinomycetes (particularly Streptomyces spp.), 45.54% possess type I CRISPR-Cas systems. In comparison to widely used CRISPR systems like Cas9 or Cas12a, these endogenous CRISPR-Cas systems have significant advantages, including better compatibility, wide distribution, and ease of operation (since no exogenous Cas gene delivery is needed). Furthermore, type I CRISPR-Cas systems can simultaneously edit and regulate genes by adjusting the crRNA spacer length. Meanwhile, most actinomycetes are recalcitrant to genetic manipulation, hindering the discovery and engineering of natural products (NPs). The endogenous type I CRISPR-Cas systems in actinomycetes may offer a promising alternative to overcome these barriers. This review summarizes the challenges and recent advances in CRISPR-based genome engineering technologies for actinomycetes. It also presents and discusses how to establish and develop genome editing tools based on type I CRISPR-Cas systems in actinomycetes, with the aim of their future application in gene editing and the discovery of NPs in actinomycetes.

Polyester degradation by soil bacteria: identification of conserved BHETase enzymes in Streptomyces

The rising use of plastic results in an appalling amount of waste which is scattered into the environment. One of these plastics is PET which is mainly used for bottles. We have identified and characterized an esterase from Streptomyces, annotated as LipA, which can efficiently degrade the PET-derived oligomer BHET. The Streptomyces coelicolor ScLipA enzyme exhibits varying sequence similarity to several BHETase/PETase enzymes, including IsPETase, TfCut2, LCC, PET40 and PET46. Of 96 Streptomyces strains, 18% were able to degrade BHET via one of three variants of LipA, named ScLipA, S2LipA and S92LipA. SclipA was deleted from S. coelicolor resulting in reduced BHET degradation. Overexpression of all LipA variants significantly enhanced BHET degradation. All variants were expressed in E. coli for purification and biochemical analysis. The optimum conditions were determined as pH 7 and 25 °C for all variants. The activity on BHET and amorphous PET film was investigated. S2LipA efficiently degraded BHET and caused roughening and indents on the surface of PET films, comparable to the activity of previously described TfCut2 under the same conditions. The abundance of the S2LipA variant in Streptomyces suggests an environmental advantage towards the degradation of more polar substrates including these polluting plastics.

A Guanidinobenzol-Rich Polymer Overcoming Cascade Delivery Barriers for CRISPR-Cas9 Genome Editing

The efficient cytosolic delivery of the CRISPR-Cas9 machinery remains a challenge for genome editing. Herein, we performed ligand screening and identified a guanidinobenzol-rich polymer to overcome the cascade delivery barriers of CRISPR-Cas9 ribonucleoproteins (RNPs) for genome editing. RNPs were stably loaded into the polymeric nanoparticles (PGBA NPs) by their inherent affinity. The polymer facilitated rapid endosomal escape of RNPs via a dynamic multiple-step cascade process. Importantly, the incorporation of fluorescence in the polymer helps to identify the correlation between cellular uptake and editing efficiency, increasing the efficiency up to 70% from the initial 30% for the enrichment of edited cells. The PGBA NPs efficiently deliver RNPs for in vivo gene editing via both local and systemic injections and dramatically reduce PCSK9 level. These results indicate that PGBA NPs enable the cascade delivery of RNPs for genome editing, showing great promise in broadening the therapeutic potential of the CRISPR-Cas9 technique.

Antifungal Properties of Streptomyces bacillaris S8 for Biological Control Applications

Soybean (Glycine max), a crucial global crop, experiences yearly yield reduction due to diseases such as anthracnose (Colletotrichum truncatum) and root rot (Fusarium spp.). The use of fungicides, which have traditionally been employed to control these phytopathogens, is now facing challenges due to the emergence of fungicide-resistant strains. Streptomyces bacillaris S8 strain S8 is previously known to produce valinomycin t through a nonribosomal peptide synthetase (NRPS) pathway. The objective of this study was to evaluate the antifungal activity of S. bacillaris S8 against C. truncatum and Fusarium sp., assessing its efficacy against soybean pathogens. The results indicate that strain S8 effectively controlled both above-ground and underground soybean diseases, using the NRPS and NRPS-related compound, suggesting its potential as a biological control in plant-microbe interactions. These findings underscore the pivotal role of the stain S8 in fostering healthy soybean microbial communities and emphasize the significance of microbiota structure studies in unveiling potent biocontrol agents.

Multiplex CRISPR-Cas Genome Editing: Next-Generation Microbial Strain Engineering

Genome editing is a crucial technology for obtaining desired phenotypes in a variety of species, ranging from microbes to plants, animals, and humans. With the advent of CRISPR-Cas technology, it has become possible to edit the intended sequence by modifying the target recognition sequence in guide RNA (gRNA). By expressing multiple gRNAs simultaneously, it is possible to edit multiple targets at the same time, allowing for the simultaneous introduction of various functions into the cell. This can significantly reduce the time and cost of obtaining engineered microbial strains for specific traits. In this review, we investigate the resolution of multiplex genome editing and its application in engineering microorganisms, including bacteria and yeast. Furthermore, we examine how recent advancements in artificial intelligence technology could assist in microbial genome editing and engineering. Based on these insights, we present our perspectives on the future evolution and potential impact of multiplex genome editing technologies in the agriculture and food industry.

Engineered Biosynthesis and Anticancer Studies of Ring-Expanded Antimycin-Type Depsipeptides

Respirantins are 18-membered antimycin-type depsipeptides produced by Streptomyces sp. and Kitasatospora sp. These compounds have shown extraordinary anticancer activities against a panel of cancer cell lines with nanomolar levels of IC50 values. However, further investigation has been impeded by the low titers of the natural producers and the challenging chemical synthesis due to their structural complexity. The biosynthetic gene cluster (BGC) of respirantin was previously proposed based on a bioinformatic comparison of the four members of antimycin-type depsipeptides. In this study, we report the first successful reconstitution of respirantin in Streptomyces albus using a synthetic BGC. This heterologous system serves as an accessible platform for the production and diversification of respirantins. Through polyketide synthase pathway engineering, biocatalysis, and chemical derivatization, we generated nine respirantin compounds, including six new derivatives. Cytotoxicity screening against human MCF-7 and Hela cancer cell lines revealed a unique biphasic dose-response profile of respirantin. Furthermore, a structure-activity relationship study has elucidated the essential functional groups that contribute to its remarkable cytotoxicity. This work paves the way for respirantin-based anticancer drug discovery and development.

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.

Application of Cas12j for Streptomyces Editing

In recent years, CRISPR-Cas toolboxes for Streptomyces editing have rapidly accelerated natural product discovery and engineering. However, Cas efficiencies are oftentimes strain-dependent, and the commonly used Streptococcus pyogenes Cas9 (SpCas9) is notorious for having high levels of off-target toxicity effects. Thus, a variety of Cas proteins is required for greater flexibility of genetic manipulation within a wider range of Streptomyces strains. This study explored the first use of Acidaminococcus sp. Cas12j, a hypercompact Cas12 subfamily, for genome editing in Streptomyces and its potential in activating silent biosynthetic gene clusters (BGCs) to enhance natural product synthesis. While the editing efficiencies of Cas12j were not as high as previously reported efficiencies of Cas12a and Cas9, Cas12j exhibited higher transformation efficiencies compared to SpCas9. Furthermore, Cas12j demonstrated significantly improved editing efficiencies compared to Cas12a in activating BGCs in Streptomyces sp. A34053, a strain wherein both SpCas9 and Cas12a faced limitations in accessing the genome. Overall, this study expanded the repertoire of Cas proteins for genome editing in actinomycetes and highlighted not only the potential of recently characterized Cas12j in Streptomyces but also the importance of having an extensive genetic toolbox for improving the editing success of these beneficial microbes.

Genetic toolbox for Photorhabdus and Xenorhabdus: pSEVA based heterologous expression systems and CRISPR/Cpf1 based genome editing for rapid natural product profiling

BACKGROUND

Bacteria of the genus Photorhabdus and Xenorhabdus are motile, Gram-negative bacteria that live in symbiosis with entomopathogenic nematodes. Due to their complex life cycle, they produce a large number of specialized metabolites (natural products) encoded in biosynthetic gene clusters (BGC). Genetic tools for Photorhabdus and Xenorhabdus have been rare and applicable to only a few strains. In the past, several tools have been developed for the activation of BGCs and the deletion of individual genes. However, these often have limited efficiency or are time consuming. Among the limitations, it is essential to have versatile expression systems and genome editing tools that could facilitate the practical work.

RESULTS

In the present study, we developed several expression vectors and a CRISPR-Cpf1 genome editing vector for genetic manipulations in Photorhabdus and Xenorhabdus using SEVA plasmids. The SEVA collection is based on modular vectors that allow exchangeability of different elements (e.g. origin of replication and antibiotic selection markers with the ability to insert desired sequences for different end applications). Initially, we tested different SEVA vectors containing the broad host range origins and three different resistance genes for kanamycin, gentamycin and chloramphenicol, respectively. We demonstrated that these vectors are replicative not only in well-known representatives, e.g. Photorhabdus laumondii TTO1, but also in other rarely described strains like Xenorhabdus sp. TS4. For our CRISPR/Cpf1-based system, we used the pSEVA231 backbone to delete not only small genes but also large parts of BGCs. Furthermore, we were able to activate and refactor BGCs to obtain high production titers of high value compounds such as safracin B, a semisynthetic precursor for the anti-cancer drug ET-743.

CONCLUSIONS

The results of this study provide new inducible expression vectors and a CRISPR/CPf1 encoding vector all based on the SEVA (Standard European Vector Architecture) collection, which can improve genetic manipulation and genome editing processes in Photorhabdus and Xenorhabdus.

Low-Toxicity and High-Efficiency Streptomyces Genome Editing Tool Based on the Miniature Type V-F CRISPR/Cas Nuclease AsCas12f1

Genome editing tools based on SpCas9 and FnCpf1 have facilitated strain improvements for natural product production and novel drug discovery in Streptomyces. However, due to high toxicity, their editing requires high DNA transformation efficiency, which is unavailable in most streptomycetes. The transformation efficiency of an all-in-one editing tool based on miniature Cas nuclease AsCas12f1 was significantly higher than those of SpCas9 and FnCpf1 in tested streptomycetes, which is due to its small size and weak DNA cleavage activity. Using this tool, in Streptomyces coelicolor, we achieved 100% efficiency for single gene or gene cluster deletion and 46.7 and 40% efficiency for simultaneous deletion of two genes and two gene clusters, respectively. AsCas12f1 was successfully extended to Streptomyces hygroscopicus SIPI-054 for efficient genome editing, in which SpCas9/FnCpf1 does not work well. Collectively, this work offers a low-toxicity, high-efficiency genome editing tool for streptomycetes, particularly those with low DNA transformation efficiency.

Amycolatopsis mediterranei: A Sixty-Year Journey from Strain Isolation to Unlocking Its Potential of Rifamycin Analogue Production by Combinatorial Biosynthesis

Ever since the isolation of Amycolatopsis mediterranei in 1957, this strain has been the focus of research worldwide. In the last 60 years or more, our understanding of the taxonomy, development of cloning vectors and conjugation system, physiology, genetics, genomics, and biosynthetic pathway of rifamycin B production in A. mediterranei has substantially increased. In particular, the development of cloning vectors, transformation system, characterization of the rifamycin biosynthetic gene cluster, and the regulation of rifamycin B production by the pioneering work of Heinz Floss have made the rifamycin polyketide biosynthetic gene cluster (PKS) an attractive target for extensive genetic manipulations to produce rifamycin B analogues which could be effective against multi-drug-resistant tuberculosis. Additionally, a better understanding of the regulation of rifamycin B production and the application of newer genomics tools, including CRISPR-assisted genome editing systems, might prove useful to overcome the limitations associated with low production of rifamycin analogues.

Discovery and engineering of ribosomally synthesized and post-translationally modified peptide (RiPP) natural products

Ribosomally synthesized and post-translationally modified peptides (RiPPs) represent a diverse superfamily of natural products with immense potential for drug development. This review provides a concise overview of the recent advances in the discovery of RiPP natural products, focusing on rational strategies such as bioactivity guided screening, enzyme or precursor-based genome mining, and biosynthetic engineering. The challenges associated with activating silent biosynthetic gene clusters and the development of elaborate catalytic systems are also discussed. The logical frameworks emerging from these research studies offer valuable insights into RiPP biosynthesis and engineering, paving the way for broader pharmaceutic applications of these peptide natural products.

Bacillus and Streptomyces spp. as hosts for production of industrially relevant enzymes

The application of enzymes is expanding across diverse industries due to their nontoxic and biodegradable characteristics. Another advantage is their cost-effectiveness, reflected in reduced processing time, water, and energy consumption. Although Gram-positive bacteria, Bacillus, and Streptomyces spp. are successfully used for production of industrially relevant enzymes, they still lag far behind Escherichia coli as hosts for recombinant protein production. Generally, proteins secreted by Bacillus and Streptomyces hosts are released into the culture medium; their native conformation is preserved and easier recovery process enabled. Given the resilience of both hosts in harsh environmental conditions and their spore-forming capability, a deeper understanding and broader use of Bacillus and Streptomyces as expression hosts could significantly enhance the robustness of industrial bioprocesses. This mini-review aims to compare two expression hosts, emphasizing their specific advantages in industrial surroundings such are chemical, detergent, textile, food, animal feed, leather, and paper industries. The homologous sources, heterologous hosts, and molecular tools used for the production of recombinant proteins in these hosts are discussed. The potential to use both hosts as biocatalysts is also evaluated. Undoubtedly, Bacillus and Streptomyces spp. as production hosts possess the potential to take on a more substantial role, providing superior (bio-based) process robustness and flexibility. KEY POINTS: • Bacillus and Streptomyces spp. as robust hosts for enzyme production. • Industrially relevant enzyme groups for production in alternative hosts highlighted. • Molecular biology techniques are enabling easier utilization of both hosts.

CRISPR-aided genome engineering for secondary metabolite biosynthesis in Streptomyces

The demand for discovering novel microbial secondary metabolites is growing to address the limitations in bioactivities such as antibacterial, antifungal, anticancer, anthelmintic, and immunosuppressive functions. Among microbes, the genus Streptomyces holds particular significance for secondary metabolite discovery. Each Streptomyces species typically encodes approximately 30 secondary metabolite biosynthetic gene clusters (smBGCs) within its genome, which are mostly uncharacterized in terms of their products and bioactivities. The development of next-generation sequencing has enabled the identification of a large number of potent smBGCs for novel secondary metabolites that are imbalanced in number compared with discovered secondary metabolites. The clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) system has revolutionized the translation of enormous genomic potential into the discovery of secondary metabolites as the most efficient genetic engineering tool for Streptomyces. In this review, the current status of CRISPR/Cas applications in Streptomyces is summarized, with particular focus on the identification of secondary metabolite biosynthesis gene clusters and their potential applications.This review summarizes the broad range of CRISPR/Cas applications in Streptomyces for natural product discovery and production.

ONE-SENTENCE SUMMARY

This review summarizes the broad range of CRISPR/Cas applications in Streptomyces for natural product discovery and production.

Manipulation and epigenetic control of silent biosynthetic pathways in actinobacteria

Most biosynthetic gene clusters (BGCs) of Actinobacteria are either silent or expressed less than the detectable level. The non-genetic approaches including biological interactions, chemical agents, and physical stresses that can be used to awaken silenced pathways are compared in this paper. These non-genetic induction strategies often need screening approaches, including one strain many compounds (OSMAC), reporter-guided mutant selection, and high throughput elicitor screening (HiTES) have been developed. Different types of genetic manipulations applied in the induction of cryptic BGCs of Actinobacteria can be categorized as genome-wide pleiotropic and targeted approaches like manipulation of global regulatory systems, modulation of regulatory genes, ribosome and engineering of RNA polymerase or phosphopantheteine transferases. Targeted approaches including genome editing by CRISPR, mutation in transcription factors and modification of BGCs promoters, inactivation of the highly expressed biosynthetic pathways, deleting the suppressors or awakening the activators, heterologous expression, or refactoring of gene clusters can be applied for activation of pathways which are predicted to synthesize new bioactive structures in genome mining studies of Acinobacteria. In this review, the challenges and advantages of employing these approaches in induction of Actinobacteria BGCs are discussed. Further, novel natural products needed as drug for pharmaceutical industry or as biofertilizers in agricultural industry can be discovered even from known species of Actinobactera by the innovative approaches of metabolite biosynthesis elicitation.

Development of platensimycin, platencin, and platensilin overproducers by biosynthetic pathway engineering and fermentation medium optimization

The platensimycin (PTM), platencin (PTN), and platensilin (PTL) family of natural products continues to inspire the discovery of new chemistry, enzymology, and medicine. Engineered production of this emerging family of natural products, however, remains laborious due to the lack of practical systems to manipulate their biosynthesis in the native-producing Streptomyces platensis species. Here we report solving this technology gap by implementing a CRISPR-Cas9 system in S. platensis CB00739 to develop an expedient method to manipulate the PTM, PTN, and PTL biosynthetic machinery in vivo. We showcase the utility of this technology by constructing designer recombinant strains S. platensis SB12051, SB12052, and SB12053, which, upon fermentation in the optimized PTM-MS medium, produced PTM, PTN, and PTL with the highest titers at 836 mg L-1, 791 mg L-1, and 40 mg L-1, respectively. Comparative analysis of these resultant recombinant strains also revealed distinct chemistries, catalyzed by PtmT1 and PtmT3, two diterpene synthases that nature has evolved for PTM, PTN, and PTL biosynthesis. The ΔptmR1/ΔptmT1/ΔptmT3 triple mutant strain S. platensis SB12054 could be envisaged as a platform strain to engineer diterpenoid biosynthesis by introducing varying ent-copalyl diphosphate-acting diterpene synthases, taking advantage of its clean metabolite background, ability to support diterpene biosynthesis in high titers, and the promiscuous tailoring biosynthetic machinery.

ONE-SENTENCE SUMMARY

Implementation of a CRISPR-Cas9 system in Streptomyces platensis CB00739 enabled the construction of a suite of designer recombinant strains for the overproduction of platensimycin, platencin, and platensilin, discovery of new diterpene synthase chemistries, and development of platform strains for future diterpenoid biosynthesis engineering.

CRISPR/Cas9-Mediated Promoter Engineering in Saccharopolyspora erythraea

In situ promoter engineering is an effective way to alter target gene expression without introducing excess DNA sequences. Recently, the CRISPR/Cas9 technologies have been proved to be efficient tools for genome editing in actinomycetes, making it easier and more efficient to perform gene insertion and substitution in actinomycetes in a scarless manner. In this chapter, we describe a routine protocol for CRISPR/Cas9-mediated promoter engineering in Saccharopolyspora erythraea NRRL 23338, which is the wild-type producer of erythromycin. This protocol can be adapted to CRISPR/Cas9-mediated gene editing, not limited to promoter engineering, in other actinomycetes, with modifications.

CRISPR-Cas9-Mediated Genome Editing in Paenibacillus polymyxa

In recent years, the clustered regularly interspaced palindromic repeats-Cas (CRISPR-Cas) technology has become the method of choice for precision genome editing in many organisms due to its simplicity and efficacy. Multiplex genome editing, point mutations, and large genomic modifications are attractive features of the CRISPR-Cas9 system. These applications facilitate both the ease and velocity of genetic manipulations and the discovery of novel functions. In this protocol chapter, we describe the use of a CRISPR-Cas9 system for multiplex integration and deletion modifications, and deletions of large genomic regions by the use of a single guide RNA (sgRNA), and, finally, targeted point mutation modifications in Paenibacillus polymyxa.

Systems-wide analysis of the ROK-family regulatory gene rokL6 and its role in the control of glucosamine toxicity in Streptomyces coelicolor

IMPORTANCE

Central metabolism plays a key role in the control of growth and antibiotic production in streptomycetes. Specifically, aminosugars act as signaling molecules that affect development and antibiotic production, via metabolic interference with the global repressor DasR. While aminosugar metabolism directly connects to other major metabolic routes such as glycolysis and cell wall synthesis, several important aspects of their metabolism are yet unresolved. Accumulation of N-acetylglucosamine 6-phosphate or glucosamine 6-phosphate is lethal to many bacteria, a yet unresolved phenomenon referred to as "aminosugar sensitivity." We made use of this concept by selecting for suppressors in genes related to glucosamine toxicity in nagB mutants, which showed that the gene pair of rok-family regulatory gene rokL6 and major facilitator superfamily transporter gene sco1448 forms a cryptic rescue mechanism. Inactivation of rokL6 resulted in the expression of sco1448, which then prevents the toxicity of amino sugar-derived metabolites in Streptomyces. The systems biology of RokL6 and its transcriptional control of sco1448 shed new light on aminosugar metabolism in streptomycetes and on the response of bacteria to aminosugar toxicity.

Effects of static magnetic field on the sulfate metabolic pathway involved in Magnetospirillum magneticum AMB-1 cell growth and magnetosome formation

AIMS

Magnetotactic bacteria (MTB) can use their unique intracellular magnetosome organelles to swim along the Earth's magnetic field. They play important roles in the biogeochemical cycles of iron and sulfur. Previous studies have shown that the applied magnetic fields could affect the magnetosome formation and antioxidant defense systems in MTB. However, the molecular mechanisms by which magnetic fields affect MTB cells remain unclear. We aim to better understand the dark at 28°C-29°C for 20 h, as shownthe interactions between magnetic fields and cells, and the mechanism of MTB adaptation to magnetic field at molecular levels.

METHODS AND RESULTS

We performed microbiological, transcriptomic, and genetic experiments to analyze the effects of a weak static magnetic field (SMF) exposure on the cell growth and magnetosome formation in the MTB strain Magnetospirillum magneticum AMB-1. The results showed that a 1.5 mT SMF significantly promoted the cell growth but reduced magnetosome formation in AMB-1, compared to the geomagnetic field. Transcriptomic analysis revealed decreased expression of genes primarily involved in the sulfate reduction pathway. Consistently, knockout mutant lacking adenylyl-sulfate kinase CysC did no more react to the SMF and the differences in growth and Cmag disappeared. Together with experimental findings of increased reactive oxidative species in the SMF-treated wild-type strain, we proposed that cysC, as a key gene, can participate in the cell growth and mineralization in AMB-1 by SMF regulation.

CONCLUSIONS

This study suggests that the magnetic field exposure can trigger a bacterial oxidative stress response involved in AMB-1 growth and magnetosome mineralization by regulating the sulfur metabolism pathway. CysC may serve as a pivotal enzyme in mediating sulfur metabolism to synchronize the impact of SMF on both growth and magnetization of AMB-1.

Endogenous CRISPR-Cas mediated in situ genome editing: State-of-the-art and the road ahead for engineering prokaryotes

The CRISPR-Cas systems have shown tremendous promise as heterologous tools for genome editing in various prokaryotes. However, the perturbation of DNA homeostasis and the inherent toxicity of Cas9/12a proteins could easily lead to cell death, which led to the development of endogenous CRISPR-Cas systems. Programming the widespread endogenous CRISPR-Cas systems for in situ genome editing represents a promising tool in prokaryotes, especially in genetically intractable species. Here, this review briefly summarizes the advances of endogenous CRISPR-Cas-mediated genome editing, covering aspects of establishing and optimizing the genetic tools. In particular, this review presents the application of different types of endogenous CRISPR-Cas tools for strain engineering, including genome editing and genetic regulation. Notably, this review also provides a detailed discussion of the transposon-associated CRISPR-Cas systems, and the programmable RNA-guided transposition using endogenous CRISPR-Cas systems to enable editing of microbial communities for understanding and control. Therefore, they will be a powerful tool for targeted genetic manipulation. Overall, this review will not only facilitate the development of standard genetic manipulation tools for non-model prokaryotes but will also enable more non-model prokaryotes to be genetically tractable.

Transposon-based identification of genes involved in the rimocidin biosynthesis in Streptomyces rimosus M527

The transposon mutagenesis strategy has been employed to generate random insertion mutants and analyze the correlation between genes and secondary metabolites in the genus Streptomyces. In this study, our primary objective was to identify an unknown gene involved in rimocidin biosynthesis and elucidate its role in rimocidin production in Streptomyces rimosus M527. To achieve this, we established a random mutant library of S. rimosus M527 using a Tn5 transposon-mediated random mutagenesis strategy. Among the 137 isolated mutants, M527-G10 and M527-W5 exhibited the most significant variations in antagonistic activity against the plant pathogenic fungus Fusarium oxysporum f. sp. cucumerinum. Specifically, M527-G10 displayed a 72.93% reduction, while M527-W5 showed a 49.8% increase in rimocidin production compared to the wild-type (WT) strain S. rimosus M527. Subsequently, we employed a plasmid rescue strategy to identify the insertion loci of the transposon in the genomes of mutants M527-G10 and M527-W5, revealing a response regulator transcription factor (rrt) and a hypothetical protein (hyp), respectively. The roles of rrt and hyp in rimocidin biosynthesis were determined through gene deletion, overexpression in the WT strain, and complemented expression in the transposon mutants. Notably, the gene-deletion mutants M527-ΔRRT and M527-ΔHYP exhibited similar behavior in rimocidin production compared to the corresponding transposon mutants M527-G10 and M527-W5, suggesting that transposon insertions in genes rrt and hyp led to alterations in rimocidin production. Furthermore, both gene deletion and overexpression of rrt and hyp had no discernible effects on cell growth. These results reveal that genes rrt and hyp have positive and negative impacts on rimocidin production in S. rimosus M527, respectively.

A unique dual acyltransferase system shared in the polyketide chain initiation of kidamycinone and rubiflavinone biosynthesis

The pluramycin family of natural products has diverse substituents at the C2 position, which are closely related to their biological activity. Therefore, it is important to understand the biosynthesis of C2 substituents. In this study, we describe the biosynthesis of C2 moieties in Streptomyces sp. W2061, which produces kidamycin and rubiflavinone C-1, containing anthrapyran aglycones. Sequence analysis of the loading module (Kid13) of the PKS responsible for the synthesis of these anthrapyran aglycones is useful for confirming the incorporation of atypical primer units into the corresponding products. Kid13 is a ketosynthase-like decarboxylase (KSQ)-type loading module with unusual dual acyltransferase (AT) domains (AT1-1 and AT1-2). The AT1-2 domain primarily loads ethylmalonyl-CoA and malonyl-CoA for rubiflavinone and kidamycinone and rubiflavinone, respectively; however, the AT1-1 domain contributed to the functioning of the AT1-2 domain to efficiently load ethylmalonyl-CoA for rubiflavinone. We found that the dual AT system was involved in the production of kidamycinone, an aglycone of kidamycin, and rubiflavinone C-1 by other shared biosynthetic genes in Streptomyces sp. W2061. This study broadens our understanding of the incorporation of atypical primer units into polyketide products.

Development of a CRISPR/Cas9D10A Nickase (nCas9)-Mediated Genome Editing Tool in Streptomyces

Streptomycetes have a strong ability to produce a vast array of bioactive natural products (NPs) widely used in agriculture and veterinary/human medicine. The recently developed CRISPR/Cas9-based genome editing tools have greatly facilitated strain improvement for target NP overproduction as well as novel NP discovery in Streptomyces. However, CRISPR/Cas9 shows high toxicity to the host, limiting its application in many Streptomyces strains with a low DNA transformation efficiency. In this study, we developed a low-toxicity CRISPR/Cas9D10A nickase (nCas9)-based genome editing tool in the model strain Streptomyces coelicolor M145. We showed that in the presence of both targeting sgRNA and Cas proteins, utilization of nCas9 instead of Cas9 significantly reduced the toxicity to the host and greatly enhanced cell survival. Using this tool, we achieved deletion of single genes and gene clusters with efficiencies of 87-100 and 63-87%, and simultaneous deletion of two genes or gene clusters with efficiencies of 47 and 43%, respectively. The editing efficiency of nCas9 is comparable to that of the Cas9-mediated editing tool. Finally, the nCas9-based editing tool was successfully applied for genome editing in the industrial rapamycin-producing strain Streptomyces rapamycinicus, in which CRISPR/Cas9 cannot work well. We achieved the deletion of three tested genes with an efficiency of 27.2-30%. Collectively, the CRISPR/nCas9-based editing tool offers a convenient and efficient genetic modification system for the engineering of streptomycetes, particularly those with low DNA transformation efficiency.

CUBIC: A Versatile Cumate-Based Inducible CRISPRi System in Streptomyces

Streptomyces, a genus of Gram-positive bacteria, is known as nature's medicine maker, producing a plethora of natural products that have huge benefits for human health, agriculture, and biotechnology. To take full advantage of this treasure trove of bioactive molecules, better genetic tools are required for the genetic engineering and synthetic biology of Streptomyces. We therefore developed CUBIC, a novel CUmate-Based Inducible CRISPR interference (CRISPRi) system that allows highly efficient and inducible gene knockdown in Streptomyces. Its broad application is shown by the specific and nondisruptive knockdown of genes involved in growth, development and antibiotic production in various Streptomyces species. To facilitate hyper-efficient plasmid construction, we adapted the Golden Gate assembly to achieve 100% cloning efficiency of the protospacers. We expect that the versatile plug-and-play CUBIC system will create new opportunities for research and innovation in the field of Streptomyces.

Multicopy Chromosome Integration and Deletion of Negative Global Regulators Significantly Increased the Heterologous Production of Aborycin in Streptomyces coelicolor

Aborycin is a type I lasso peptide with a stable interlocked structure, offering a favorable framework for drug development. The aborycin biosynthetic gene cluster gul from marine sponge-associated Streptomyces sp. HNS054 was cloned and integrated into the chromosome of S. coelicolor hosts with different copies. The three-copy gul-integration strain S. coelicolor M1346::3gul showed superior production compared to the one-copy or two-copy gul-integration strains, and the total titer reached approximately 10.4 mg/L, i.e., 2.1 times that of the native strain. Then, five regulatory genes, phoU (SCO4228), wblA (SCO3579), SCO1712, orrA (SCO3008) and gntR (SCO1678), which reportedly have negative effects on secondary metabolism, were further knocked out from the M1346::3gul genome by CRISPR/Cas9 technology. While the ΔSCO1712 mutant showed a significant decrease (4.6 mg/L) and the ΔphoU mutant showed no significant improvement (12.1 mg/L) in aborycin production, the ΔwblA, ΔorrA and ΔgntR mutations significantly improved the aborycin titers to approximately 23.6 mg/L, 56.3 mg/L and 48.2 mg/L, respectively, which were among the highest heterologous yields for lasso peptides in both Escherichia coli systems and Streptomyces systems. Thus, this study provides important clues for future studies on enhancing antibiotic production in Streptomyces systems.

Methyl halide transferase-based gas reporters for quantification of filamentous bacteria in microdroplet emulsions

The application of microfluidic techniques in experimental and environmental studies is a rapidly emerging field. Water-in-oil microdroplets can serve readily as controllable micro-vessels for studies that require spatial structure. In many applications, it is useful to monitor cell growth without breaking or disrupting the microdroplets. To this end, optical reporters based on color, fluorescence, or luminescence have been developed. However, optical reporters suffer from limitations when used in microdroplets such as inaccurate readings due to strong background interference or limited sensitivity during early growth stages. In addition, optical detection is typically not amenable to filamentous or biofilm-producing organisms that have significant nonlinear changes in opacity and light scattering during growth. To overcome such limitations, we show that volatile methyl halide gases produced by reporter cells expressing a methyl halide transferase (MHT) can serve as an alternative nonoptical detection approach suitable for microdroplets. In this study, an MHT-labeled Streptomyces venezuelae reporter strain was constructed and characterized. Protocols were established for the encapsulation and incubation of S. venezuelae in microdroplets. We observed the complete life cycle for S. venezuelae including the vegetative expansion of mycelia, mycelial fragmentation, and late-stage sporulation. Methyl bromide (MeBr) production was detected by gas chromatography-mass spectrometry (GC-MS) from S. venezuelae gas reporters incubated in either liquid suspension or microdroplets and used to quantitatively estimate bacterial density. Overall, using MeBr production as a means of quantifying bacterial growth provided a 100- to 1,000-fold increase in sensitivity over optical or fluorescence measurements of a comparable reporter strain expressing fluorescent proteins. IMPORTANCE Quantitative measurement of bacterial growth in microdroplets in situ is desirable but challenging. Current optical reporter systems suffer from limitations when applied to filamentous or biofilm-producing organisms. In this study, we demonstrate that volatile methyl halide gas production can serve as a quantitative nonoptical growth assay for filamentous bacteria encapsulated in microdroplets. We constructed an S. venezuelae gas reporter strain and observed a complete life cycle for encapsulated S. venezuelae in microdroplets, establishing microdroplets as an alternative growth environment for Streptomyces spp. that can provide spatial structure. We detected MeBr production from both liquid suspension and microdroplets with a 100- to 1,000-fold increase in signal-to-noise ratio compared to optical assays. Importantly, we could reliably detect bacteria with densities down to 106 CFU/mL. The combination of quantitative gas reporting and microdroplet systems provides a valuable approach to studying fastidious organisms that require spatial structure such as those found typically in soils.

Development of Single Molecule Techniques for Sensing and Manipulation of CRISPR and Polymerase Enzymes

Clustered regularly interspaced short palindromic repeats (CRISPR) and polymerases are powerful enzymes and their diverse applications in genomics, proteomics, and transcriptomics have revolutionized the biotechnology industry today. CRISPR has been widely adopted for genomic editing applications and Polymerases can efficiently amplify genomic transcripts via polymerase chain reaction (PCR). Further investigations into these enzymes can reveal specific details about their mechanisms that greatly expand their use. Single-molecule techniques are an effective way to probe enzymatic mechanisms because they may resolve intermediary conformations and states with greater detail than ensemble or bulk biosensing techniques. This review discusses various techniques for sensing and manipulation of single biomolecules that can help facilitate and expedite these discoveries. Each platform is categorized as optical, mechanical, or electronic. The methods, operating principles, outputs, and utility of each technique are briefly introduced, followed by a discussion of their applications to monitor and control CRISPR and Polymerases at the single molecule level, and closing with a brief overview of their limitations and future prospects.

Construction and application of a new CRISPR/Cas12a system in Stenotrophomonas AGS-1 from aerobic granular sludge

Aerobic granular sludge (AGS) is a microbial aggregate with a biofilm structure. Thus, investigating AGS in the aspect of biofilm and microbial attachment at the genetic level would help to reveal the mechanism of granule biofilm formation. In this work, a two-plasmid clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated proteins (Cas)12a genome editing system was constructed to identify attachment genes for the first time in Stenotrophomonas AGS-1 from AGS. One plasmid contained a Cas12a cassette driven by an arabinose-inducible promoter, and another contained the specific crRNA and homologous arms (HAs). Acidaminococcus sp. Cas12a (AsCas12a) was adopted and proven to have mild toxicity (compared to Cas9) and strong cleavage activity for AGS-1. CRISPR/Cas12a-mediated rmlA knockout decreased attachment ability by 38.26%. Overexpression of rmlA in AGS-1 resulted in an increase of 30.33% in attachment ability. These results showed that the modulation of rmlA was an important factor for the biofilm formation of AGS-1. Moreover, two other genes (xanB and rpfF) were knocked out by CRISPR/Cas12a and identified as attachment-related genes in AGS-1. Also, this system could achieve point mutations. These data indicated that the CRISPR/Cas12a system could be an effective molecular platform for attachment gene function identification, which would be useful for the development of AGS in wastewater treatment.

Systems Analysis of Highly Multiplexed CRISPR-Base Editing in Streptomycetes

CRISPR tools, especially Cas9n-sgRNA guided cytidine deaminase base editors such as CRISPR-BEST, have dramatically simplified genetic manipulation of streptomycetes. One major advantage of CRISPR base editing technology is the possibility to multiplex experiments in genomically instable species. Here, we demonstrate scaled up Csy4 based multiplexed genome editing using CRISPR-mcBEST in Streptomyces coelicolor. We evaluated the system by simultaneously targeting 9, 18, and finally all 28 predicted specialized metabolite biosynthetic gene clusters in a single experiment. We present important insights into the performance of Csy4 based multiplexed genome editing at different scales. Using multiomics analysis, we investigated the systems wide effects of such extensive editing experiments and revealed great potentials and important bottlenecks of CRISPR-mcBEST. The presented analysis provides crucial data and insights toward the development of multiplexed base editing as a novel paradigm for high throughput engineering of Streptomyces chassis and beyond.

Identification and Analysis of the Biosynthetic Gene Cluster for the Hydrazide-Containing Aryl Polyene Spinamycin

Natural products containing nitrogen-nitrogen (N-N) bonds have attracted much attention because of their bioactivities and chemical features. Several recent studies have revealed the nitrous acid-dependent N-N bond-forming machinery. However, the catalytic mechanisms of hydrazide synthesis using nitrous acid remain unknown. Herein, we focused on spinamycin, a hydrazide-containing aryl polyene produced by Streptomyces albospinus JCM3399. In the S. albospinus genome, we discovered a putative spinamycin biosynthetic gene (spi) cluster containing genes that encode a type II polyketide synthase and genes for the secondary metabolism-specific nitrous acid biosynthesis pathway. A gene inactivation experiment showed that this cluster was responsible for spinamycin biosynthesis. A feeding experiment using stable isotope-labeled sodium nitrite and analysis of nitrous acid-synthesizing enzymes in vitro strongly indicated that one of the nitrogen atoms of the hydrazide group was derived from nitrous acid. In vitro substrate specificity analysis of SpiA3, which is responsible for loading a starter substrate onto polyketide synthase, indicated that N-N bond formation occurs after starter substrate loading. In vitro analysis showed that the AMP-dependent ligase SpiA7 catalyzes the diazotization of an amino group on a benzene ring without a hydroxy group, resulting in a highly reactive diazo intermediate, which may be the key step in hydrazide group formation. Therefore, we propose the overall biosynthetic pathway of spinamycin. This study expands our knowledge of N-N bond formation in microbial secondary metabolism.

Natural products of pentacyclic triterpenoids: from discovery to heterologous biosynthesis

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

Improving the Efficiency of Precise Genome Editing with CRISPR/Cas9 to Generate Goats Overexpressing Human Butyrylcholinesterase

The CRISPR/Cas9 system is widely used for genome editing in livestock production, although off-target effects can occur. It is the main method to produce genome-edited goats by somatic cell nuclear transfer (SCNT) of CRISPR/Cas9-mediated genome-edited primary goat fetal fibroblast cells (GFFs). Improving the double-strand break (DSB) efficiency of Cas9 in primary cells would improve the homologous repair (HR) efficiency. The low efficiency of HR remains a major hurdle in CRISPR/Cas9-mediated precise genome editing, increasing the work required to screen the genome-edited primary cell clones. In this study, we modified several essential parameters that affect the efficiency of the CRISPR/Cas9-mediated knock-in GFF cloning system, including establishing a high-efficiency transfection system for primary cells via nucleofection and optimizing homology arm (HA) length during HR. Here, we specifically inserted a recombinant human butyrylcholinesterase gene (rhBChE) into the goat fibroblast growth factor (FGF)-5 locus through the CRISPR/Cas9 system, thereby achieving simultaneous rhBChE insertion and FGF5 knock-out. First, this study introduced the Cas9, FGF5 knock-out small guide RNA, and rhBChE knock-in donors into GFFs by electroporation and obtained positive cell clones without off-target effects. Then, we demonstrated the expression of rhBChE in GFF clones and verified its function. Finally, we obtained a CRISPR/Cas9-mediated rhBChE-overexpression goat.

Evidence of a role for CutRS and actinorhodin in the secretion stress response in Streptomyces coelicolor M145

CutRS was the first two-component system to be identified in Streptomyces species and is highly conserved in this genus. It was reported >25 years ago that deletion of cutRS increases the production of the antibiotic actinorhodin in Streptomyces coelicolor. However, despite this early work, the function of CutRS has remained enigmatic until now. Here we show that deletion of cutRS upregulates the production of the actinorhodin biosynthetic enzymes up to 300-fold, explaining the increase in actinorhodin production. However, while ChIP-seq identified 85 CutR binding sites in S. coelicolor none of these are in the actinorhodin biosynthetic gene cluster, meaning the effect is indirect. The directly regulated CutR targets identified in this study are implicated in extracellular protein folding, including two of the four highly conserved HtrA-family foldases: HtrA3 and HtrB, and a putative VKOR enzyme, which is predicted to recycle DsbA following its catalysis of disulphide bond formation in secreted proteins. Thus, we tentatively propose a role for CutRS in sensing and responding to protein misfolding outside the cell. Since actinorhodin can oxidise cysteine residues and induce disulphide bond formation in proteins, its over production in the ∆cutRS mutant may be a response to protein misfolding on the extracellular face of the membrane.

Bacterial synthetic biology: tools for novel drug discovery

INTRODUCTION

Bacterial synthetic biology has provided powerful tools to revolutionize the drug discovery process. These tools can be harnessed to generate bacterial novel pharmaceutical compounds with enhanced bioactivity and selectivity or to create genetically modified microorganisms as living drugs.

AREAS COVERED

This review provides a current overview of the state-of-the-art in bacterial synthetic biology tools for novel drug discovery. The authors discuss the application of these tools including bioinformatic tools, CRISPR tools, engineered bacterial transcriptional regulators, and synthetic biosensors for novel drug discovery. Additionally, the authors present the recent progress on reprogramming bacteriophages as living drugs to fight against antibiotic-resistant pathogens.

EXPERT OPINION

The field of using bacterial synthetic biology tools for drug discovery is rapidly advancing. However, challenges remain in developing reliable and robust methods to engineer bacteria. Further advancements in synthetic biology hold promise to speed up drug discovery, facilitating the development of novel therapeutics against various diseases.

Modern Approaches to the Genome Editing of Antibiotic Biosynthetic Clusters in Actinomycetes

Representatives of the phylum Actinomycetota are one of the main sources of secondary metabolites, including antibiotics of various classes. Modern studies using high-throughput sequencing techniques enable the detection of dozens of potential antibiotic biosynthetic genome clusters in many actinomycetes; however, under laboratory conditions, production of secondary metabolites amounts to less than 5% of the total coding potential of producer strains. However, many of these antibiotics have already been described. There is a continuous "rediscovery" of known antibiotics, and new molecules become almost invisible against the general background. The established approaches aimed at increasing the production of novel antibiotics include: selection of optimal cultivation conditions by modifying the composition of nutrient media; co-cultivation methods; microfluidics, and the use of various transcription factors to activate silent genes. Unfortunately, these tools are non-universal for various actinomycete strains, stochastic in nature, and therefore do not always lead to success. The use of genetic engineering technologies is much more efficient, because they allow for a directed and controlled change in the production of target metabolites. One example of such technologies is mutagenesis-based genome editing of antibiotic biosynthetic clusters. This targeted approach allows one to alter gene expression, suppressing the production of previously characterized molecules, and thereby promoting the synthesis of other unknown antibiotic variants. In addition, mutagenesis techniques can be successfully applied both to new producer strains and to the genes of known isolates to identify new compounds.

Global Effects of the Developmental Regulator BldB in Streptomyces venezuelae

In Streptomyces, the Bld (Bald) regulators control formation of the reproductive aerial hyphae. The functions of some of these regulators have been well characterized, but BldB has remained enigmatic. In addition to the bldB gene itself, Streptomyces venezuelae has 10 paralogs of bldB that sit next to paralogs of whiJ and abaA. Transcriptome sequencing (RNA-seq) revealed that loss of BldB function causes the dramatic transcriptional upregulation of the abaA paralogs and a novel inhibitor of sporulation, iosA, and that cooverexpression of just two of these genes, iosA and abaA6, was sufficient to recapitulate the bldB mutant phenotype. Further RNA-seq analysis showed that the transcription factor WhiJ9 is required for the activation of iosA seen in the bldB mutant, and biochemical studies showed that WhiJ9 mediates the activation of iosA expression by binding to direct repeats in the iosA-whiJ9 intergenic region. BldB and BldB9 hetero-oligomerize, providing a potential link between BldB and the iosA-whiJ9-bldB9 locus. This work greatly expands our overall understanding of the global effects of the BldB developmental regulator. IMPORTANCE To reproduce and disperse, the filamentous bacterium Streptomyces develops specialized reproductive structures called aerial hyphae. The formation of these structures is controlled by the bld (bald) genes, many of which encode transcription factors whose functions have been characterized. An exception is BldB, a protein whose biochemical function is unknown. In this study, we gain insight into the global effects of BldB function by examining the genome-wide transcriptional effects of deleting bldB. We identify a small set of genes that are dramatically upregulated in the absence of BldB. We show that their overexpression causes the bldB phenotype and characterize a transcription factor that mediates the upregulation of one of these target genes. Our results provide new insight into how BldB influences Streptomyces development.

CRISPR-Based Gene Editing in Acinetobacter baumannii to Combat Antimicrobial Resistance

Antimicrobial resistance (AMR) poses a significant threat to the health, social, environment, and economic sectors on a global scale and requires serious attention to addressing this issue. Acinetobacter baumannii was given top priority among infectious bacteria because of its extensive resistance to nearly all antibiotic classes and treatment options. Carbapenem-resistant A. baumannii is classified as one of the critical-priority pathogens on the World Health Organization (WHO) priority list of antibiotic-resistant bacteria for effective drug development. Although available genetic manipulation approaches are successful in A. baumannii laboratory strains, they are limited when employed on newly acquired clinical strains since such strains have higher levels of AMR than those used to select them for genetic manipulation. Recently, the CRISPR-Cas (Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein) system has emerged as one of the most effective, efficient, and precise methods of genome editing and offers target-specific gene editing of AMR genes in a specific bacterial strain. CRISPR-based genome editing has been successfully applied in various bacterial strains to combat AMR; however, this strategy has not yet been extensively explored in A. baumannii. This review provides detailed insight into the progress, current scenario, and future potential of CRISPR-Cas usage for AMR-related gene manipulation in A. baumannii.

Establishment of CRISPR-Cas9 system in Bifidobacteria animalis AR668

Bifidobacteria are representative intestinal probiotics that have extremely high application value in the food and medical fields. However, the lack of molecular biology tools limits the research on functional genes and mechanisms of bifidobacteria. The application of an accurate and efficient CRISPR system to genome engineering can fill the gap in efficient genetic tools for bifidobacteria. In this study, CRISPR system of B. animalis AR668 was established, which successfully knocked out gene 0348 and gene 0208. The influence of different homology arms and fragments on the knockout effect of the system was explored. In addition, the inducible plasmid curing system of bifidobacteria was innovatively established. This study contributes to the genetic modification and functional mechanism analysis of bifidobacteria.

Optimized De Novo Eriodictyol Biosynthesis in Streptomyces albidoflavus Using an Expansion of the Golden Standard Toolkit for Its Use in Actinomycetes

Eriodictyol is a hydroxylated flavonoid displaying multiple pharmaceutical activities, such as antitumoral, antiviral or neuroprotective. However, its industrial production is limited to extraction from plants due to its inherent limitations. Here, we present the generation of a Streptomyces albidoflavus bacterial factory edited at the genome level for an optimized de novo heterologous production of eriodictyol. For this purpose, an expansion of the Golden Standard toolkit (a Type IIS assembly method based on the Standard European Vector Architecture (SEVA)) has been created, encompassing a collection of synthetic biology modular vectors (adapted for their use in actinomycetes). These vectors have been designed for the assembly of transcriptional units and gene circuits in a plug-and-play manner, as well as for genome editing using CRISPR-Cas9-mediated genetic engineering. These vectors have been used for the optimization of the eriodictyol heterologous production levels in S. albidoflavus by enhancing the flavonoid-3'-hydroxylase (F3'H) activity (by means of a chimera design) and by replacing three native biosynthetic gene clusters in the bacterial chromosome with the plant genes matBC (involved in extracellular malonate uptake and its intracellular activation into malonyl-CoA), therefore allowing more malonyl-CoA to be devoted to the heterologous production of plant flavonoids in this bacterial factory. These experiments have allowed an increase in production of 1.8 times in the edited strain (where the three native biosynthetic gene clusters have been deleted) in comparison with the wild-type strain and a 13 times increase in eriodictyol overproduction in comparison with the non-chimaera version of the F3'H enzyme.

Efficient Knocking Out of the Organophosphorus Insecticides Degradation Gene opdB in Cupriavidus nantongensis X1T via CRISPR/Cas9 with Red System

Cupriavidus nantongensis X1^T is a type strain of the genus Cupriavidus, that can degrade eight kinds of organophosphorus insecticides (OPs). Conventional genetic manipulations in Cupriavidus species are time-consuming, difficult, and hard to control. The clustered regularly interspaced short palindromic repeat (CRISPR)/associated protein 9 (Cas9) system has emerged as a powerful tool for genome editing applied in prokaryotes and eukaryotes due to its simplicity, efficiency, and accuracy. Here, we combined CRISPR/Cas9 with the Red system to perform seamless genetic manipulation in the X1^T strain. Two plasmids, pACasN and pDCRH were constructed. The pACasN plasmid contained Cas9 nuclease and Red recombinase, and the pDCRH plasmid contained the dual single-guide RNA (sgRNA) of organophosphorus hydrolase (OpdB) in the X1^T strain. For gene editing, two plasmids were transferred to the X1^T strain and a mutant strain in which genetic recombination had taken place, resulting in the targeted deletion of opdB. The incidence of homologous recombination was over 30%. Biodegradation experiments suggested that the opdB gene was responsible for the catabolism of organophosphorus insecticides. This study was the first to use the CRISPR/Cas9 system for gene targeting in the genus Cupriavidus, and it furthered our understanding of the process of degradation of organophosphorus insecticides in the X1^T strain.