CtcS, a MarR family regulator, regulates chlortetracycline biosynthesis

MarR family regulator LcbR2 activates lincomycin biosynthesis in multiple ways

Lincomycin, produced by the actinomycete Streptomyces lincolnensis, is highly effective against Gram-positive bacteria and protozoans, making it widely used in clinical settings. This study identified LcbR2, a MarR family transcriptional regulator, as an activator of lincomycin biosynthesis. Knocking out the lcbR2 gene reduced lincomycin production by 63.0 % without affecting growth or morphology. Quantitative real-time PCR, electrophoretic mobility shift assays, and XylE reporter assays demonstrated that LcbR2 binds to a 13-bp imperfect palindromic sequence -TTGCCnnnnnCAA-, repressing the expression of lcbR2 Further analysis revealed that LcbR2 directly activates the expression of lincomycin biosynthesis genes (lmbD, lmbJ, lmbK, lmbV, and lmbW), enhancing lincomycin production. It also regulates lincomycin resistance genes (lmrA and lmrB), increasing the self-tolerance of S. lincolnensis to lincomycin. Additionally, LcbR2 modulates other regulatory genes (lmbU, adpA, aflQ1, bldD, and lcbR1), affecting lincomycin production in a cascade manner. LcbR2 also influences the expression of genes related to carbon, nitrogen, phosphorus, and sulfur metabolism, indirectly impacting lincomycin production. Moreover, the binding of LcbR2 to DNA can be attenuated by apramycin. This study thus characterized LcbR2 as a novel transcriptional regulator with a broad regulatory scope.

Biosynthesis of the benzylpyrrolidine precursor in anisomycin by a unique ThDP-dependent enzyme

Anisomycin (compound 1), a multifunctional pyrrolidine antibiotic, primarily inhibits protein biosynthesis by binding to the ribosome. Upon binding to the ribosome, the para-phenol moiety of anisomycin inserts completely into the hydrophobic crevice of the A-site and blocks the access of the incoming aminoacyl-tRNAs, disrupting peptide bond formation. Hence, the para-methoxyphenyl group serves as a starting point for developing novel anisomycin analogs with potent antifungal and insecticidal properties. However, the activation and condensation mechanism of phenylpyruvic acid has not yet been elucidated. In this study, genetic manipulations of aniP and its homologue siAniP confirmed their indispensable role in 1 biosynthesis. Bioinformatics analysis suggested that AniP and siAniP function as transketolase. siAniP was found to catalyzed condensation between 4-hydroxyphenylpyruvic acid (3) and glyceraldehyde (GA), initiating pyrrolidine synthesis. siAniP was specific for aromatic keto acids and tolerant of aliphatic and aromatic aldehydes, and was able to catalyze the asymmetric intermolecular condensation of two keto acids, leading to the formation of 24 α-hydroxy ketone. To the best of our knowledge, siAniP is the first TK that catalyzes the transfer of a C2 ketol and symmetrical intermolecular coupling using aromatic keto acids as donor substrates. Structural analysis, docking model construction, and site-directed mutagenesis identified that I220, H275, R322 and W391 were crucial for substrate binding. Moreover, sequence similarity network (SSN)-based genome neighborhood network (GNN) analyses of AniP suggested the widespread occurrence of the AniP-like-mediated reaction in the biosynthesis of 1 and its analogs, particularly in the assembly of benzylpyrrolidine. These findings not only expand the repertoire of TKs but also provide a potent biocatalyst that could be used for the structural innovation of 1 and its derivatives.

Transcriptional regulators of secondary metabolite biosynthesis in Streptomyces

In the post-genome era, great progress has been made in metabolic engineering using recombinant DNA technology to enhance the production of high-value products by Streptomyces. With the development of microbial genome sequencing techniques and bioinformatic tools, a growing number of secondary metabolite (SM) biosynthetic gene clusters in Streptomyces and their biosynthetic logics have been uncovered and elucidated. In order to increase our knowledge about transcriptional regulators in SM of Streptomyces, this review firstly makes a comprehensive summary of the characterized factors involved in enhancing SM production and awakening SM biosynthesis. Future perspectives on transcriptional regulator engineering for new SM biosynthesis by Streptomyces are also provided.

MarR Family Transcriptional Regulators and Their Roles in Plant-Interacting Bacteria

The relationship between plants and associated soil microorganisms plays a major role in ecosystem functioning. Plant-bacteria interactions involve complex signaling pathways regulating various processes required by bacteria to adapt to their fluctuating environment. The establishment and maintenance of these interactions rely on the ability of the bacteria to sense and respond to biotic and abiotic environmental signals. In this context, MarR family transcriptional regulators can use these signals for transcriptional regulation, which is required to establish adapted responses. MarR-like transcriptional regulators are essential for the regulation of the specialized functions involved in plant-bacteria interactions in response to a wide range of molecules associated with the plant host. The conversion of environmental signals into changes in bacterial physiology and behavior allows the bacteria to colonize the plant and ensure a successful interaction. This review focuses on the mechanisms of plant-signal perception by MarR-like regulators, namely how they (i) allow bacteria to cope with the rhizosphere and plant endosphere, (ii) regulate the beneficial functions of Plant-Growth-Promoting Bacteria and (iii) regulate the virulence of phytopathogenic bacteria.

Metabolic engineering of Streptomyces to enhance the synthesis of valuable natural products

The mycelial bacterium Streptomyces is a workhorse for producing natural products, serving as a key source of drugs and other valuable chemicals. However, its complicated life cycle, silent biosynthetic gene clusters (BGCs), and poorly characterized metabolic mechanisms limit efficient production of natural products. Therefore, a metabolic engineering strategy, including traditional and emerging tools from different disciplines, was developed to further enhance natural product synthesis by Streptomyces. Here, current trends in systems metabolic engineering, including tools and strategies, are reviewed. Particularly, this review focuses on recent developments in the selection of methods for regulating the Streptomyces life cycle, strategies for the activation of silent gene clusters, and the exploration of regulatory mechanisms governing antibiotic production. Finally, future challenges and prospects are discussed.

Positive regulation of MarR-type regulator slnO and improving salinomycin production of Streptomyces albus by multiple transcriptional regulations

The purpose of this study is to explore the function of MarR-family regulator slnO. In addition, the high-yield strain of salinomycin was constructed by using combined regulation strategies. Firstly the slnO gene over-expression strain (GO) was constructed in Streptomyces albus. Compared to wild type (WT) strain，salinomycin production in GO strain was increased about 28%. Electrophoretic mobility gel shift assays (EMSAs) confirmed that SlnO protein can bind specifically to the intergenic region of slnN-slnO, slnQ-slnA1 and slnF-slnT. qRT-PCR experiments also showed that slnA1, slnF, and slnT1 were significantly up-regulated, while the expression level of the slnN gene was down-regulated in GO strain. Secondly, slnN gene deletion strain (slnNDM) was used as the starting strain, and the pathway specific gene slnR in salinomycin gene cluster was over expressed in slnNDM. The new strain was named ZJUS01. The yield of salinomycin in ZJUS01 strain was 25% and 56% higher than that in slnNDM strain and WT strain. Above results indicate that the slnO gene has a positive regulation effect on the biosynthesis of salinomycin. Meanwhile, the yield of salinomycin could be greatly increased by manipulating multiple transcriptional regulations.

Uncovering and Engineering a Mini-Regulatory Network of the TetR-Family Regulator SACE_0303 for Yield Improvement of Erythromycin in Saccharopolyspora erythraea

Erythromycins produced by Saccharopolyspora erythraea have broad-spectrum antibacterial activities. Recently, several TetR-family transcriptional regulators (TFRs) were identified to control erythromycin production by multiplex control modes; however, their regulatory network remains poorly understood. In this study, we report a novel TFR, SACE_0303, positively correlated with erythromycin production in Sac. erythraea. It directly represses its adjacent gene SACE_0304 encoding a MarR-family regulator and indirectly stimulates the erythromycin biosynthetic gene eryAI and resistance gene ermE. SACE_0304 negatively regulates erythromycin biosynthesis by directly inhibiting SACE_0303 as well as eryAI and indirectly repressing ermE. Then, the SACE_0303 binding site within the SACE_0303-SACE_0304 intergenic region was defined. Through genome scanning combined with in vivo and in vitro experiments, three additional SACE_0303 target genes (SACE_2467 encoding cation-transporting ATPase, SACE_3156 encoding a large transcriptional regulator, SACE_5222 encoding α-ketoglutarate permease) were identified and proved to negatively affect erythromycin production. Finally, by coupling CRISPRi-based repression of those three targets with SACE_0304 deletion and SACE_0303 overexpression, we performed stepwise engineering of the SACE_0303-mediated mini-regulatory network in a high-yield strain, resulting in enhanced erythromycin production by 67%. In conclusion, the present study uncovered the regulatory network of a novel TFR for control of erythromycin production and provides a multiplex tactic to facilitate the engineering of industrial actinomycetes for yield improvement of antibiotics.

Acyltransferase AniI, a Tailoring Enzyme with Broad Substrate Tolerance for High-Level Production of Anisomycin

Anisomycin (compound 1), a pyrrolidine antibiotic, exhibits diverse biological and pharmacologic activities. The biosynthetic gene cluster of compound 1 has been identified previously, and the multistep assembly of the core benzylpyrrolidine scaffold was characterized. However, enzymatic modifications, such as acylation, involved in compound 1 biosynthesis are unknown. In this study, the genetic manipulation of aniI proved that it encoded an indispensable acetyltransferase for compound 1 biosynthesis. Bioinformatics analysis suggested AniI as a member of maltose (MAT) and galactoside O-acetyltransferases (GAT) with C-terminal left-handed parallel beta-helix (LbH) subdomain, which were referred to as LbH-MAT-GAT sugar O-acetyltransferases. However, the biochemical assay identified that its target site was the hydroxyl group of the pyrrolidine ring. AniI was found to be tolerant of acyl donors with different chain lengths for the biosynthesis of compound 1 and derivatives 12 and 13 with butyryl and isovaleryl groups, respectively. Meanwhile, it showed comparable activity toward biosynthetic intermediates and synthesized analogues, suggesting promiscuity to the pyrrolidine ring structure of compound 1. These data may inspire new viable synthetic routes for the construction of more complex pyrrolidine ring scaffolds in compound 1. Finally, the overexpression of aniI under the control of strong promoters contributed to the higher productivities of compound 1 and its analogues. These findings reported here not only improve the understanding of anisomycin biosynthesis but also expand the substrate scope of O-acetyltransferase working on the pyrrolidine ring and pave the way for future metabolic engineering construction of high-yield strains. IMPORTANCE Acylation is an important tailoring reaction during natural product biosynthesis. Acylation could increase the structural diversity and affect the chemical stability, volatility, biological activity, and even the cellular localization of specialized compounds. Many acetyltransferases have been reported in natural product biosynthesis. The typical example of the LbH-MAT-GAT sugar O-acetyltransferase subfamily was reported to catalyze the coenzyme A (CoA)-dependent acetylation of the 6-hydroxyl group of sugars. However, no protein of this family has been characterized to acetylate a nonsugar secondary metabolic product. Here, AniI was found to catalyze the acylation of the hydroxyl group of the pyrrolidine ring and be tolerant of diverse acyl donors and acceptors, which made the biosynthesis more efficient and exclusive for biosynthesis of compound 1 and its derivatives. Moreover, the overexpression of aniI serves as a successful example of genetic manipulation of a modification gene for the high production of final products and might set the stage for future metabolic engineering.

Joint engineering of SACE_Lrp and its target MarR enhances the biosynthesis and export of erythromycin in Saccharopolyspora erythraea

The Lrp and MarR families are two groups of transcriptional regulators widely distributed among prokaryotes. However, the hierarchical-regulatory relationship between the Lrp family and the MarR family remains unknown. Our previous study found that an Lrp (SACE_Lrp) from Saccharopolyspora erythraea indirectly repressed the biosynthesis of erythromycin. In this study, we characterized a novel MarR family protein (SACE_6745) from S. erythraea, which is controlled by SACE_Lrp and plays a direct regulatory role in erythromycin biosynthesis and export. SACE_Lrp directly regulated the expression of marR by specifically binding a precise site OM (5'-CTCCGGGAACCATT-3'). Gene disruption of marR increased the production of erythromycin by 45% in S. erythraea A226. We found that MarR has direct DNA-binding activity for the promoter regions of the erythromycin biosynthetic genes, as well as an ABC exporter SACE_2701-2702 which was genetically proved to be responsible for erythromycin efflux. Disruption of SACE_Lrp in industrial S. erythraea WB was an efficient strategy to enhance erythromycin production. Herein, we jointly engineered SACE_Lrp and its target MarR by deleting marR in WBΔSACE_Lrp, resulting in 20% increase in erythromycin yield in mutant WBΔLrpΔmarR compared to WBΔSACE_Lrp, and 39% to WB. Overall, our findings provide new insights into the hierarchical-regulatory relationship of Lrp and MarR proteins and new avenues for coordinating antibiotic biosynthesis and export by joint engineering regulators in actinomycetes. KEY POINTS: • The hierarchical-regulatory relationship between SACE_Lrp and MarR was identified. • MarR directly controlled the expression of erythromycin biosynthesis and export genes. • Joint engineering of SACE_Lrp-MarR regulatory element enhanced erythromycin production.

Extracts of endophytic fungi from leaves of selected Nigerian ethnomedicinal plants exhibited antioxidant activity

Background

Plants with an ethnobotanical history are known to harbor diverse group of endophytic fungi, which constitute major natural sources of bioactive compounds. In the present study, we evaluated the antioxidant activity of endophytic fungi from eight Nigerian ethnomedicinal plants. Endophytic fungi were isolated from the leaves of Acalypha ornata, Albizia zygia, Alchornea cordifolia, Chrysophyllum albidum, Ficus exasperata, Gomphrena celosioides, Millettia thonningii, and Newbouldia laevis.

Methods

Endophytic fungi were isolated from the leaves of selected plants via surface sterilization. Isolated fungi were identified by internal transcribed spacer (ITS-rDNA) sequence analysis. Pure fungal strains were subjected to fermentation process on solid rice medium and metabolites extracted using ethyl-acetate. Fungal crude extracts were screened for antioxidant activity using 2, 2- diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and reduction of ferric ion assays. Gas chromatography/mass spectrometry (GC/MS) analysis was used to identify the major chemical constituents in active fungal extracts.

Results

A total of eighteen fungal endophytes with fungal codes CU (061 and 062); ZA (161, 162, 163, and 164); LO (261); CA (041, 042, and 043); FE (081, 082, and 084); GE (091); MO (211 and 212); and NA (021 and 022) were isolated from the eight ethnomedicinal plants A. ornata, A. zygia, A. cordifolia, C. albidum, F. exasperata, G. celosioides, M. thonningii, and N. laevis respectively. ZA 163 and MO 211 fungal extracts showed significant (p < 0.05) radical scavenging activity with IC₅₀ values of 50.53 ± 0.01 and 86.69 ± 0.02 μg/ml respectively. Fungal extract CA 041 demonstrated significantly (p < 0.01) higher iron chelating activity than standard gallic acid with absorbance values of 0.803 and 1.107 at 250 and 500 μg/ml concentrations respectively. Pyrogallol, phenol, 2,6-dimethoxy-, phytol, dl-alpha-tocopherol, alpha-tocospiro, oleamide, methyl stearate, oleic acid, palmitic acid, campesterol, stigmasterol, β-sitosterol, urs-12-en-24-oic acid, 3-oxo-, methyl ester, lup-20(29)-en-3-one, and lupeol were detected in the selected active extracts.

Conclusion

These results showed that leaves of the selected Nigerian plants harbor diverse group of endophytic fungi, which can be potential antioxidant resource.

Graphical abstract

Flavin Adenine Dinucleotide-Dependent Halogenase XanH and Engineering of Multifunctional Fusion Halogenases

Xantholipin (compound 1), a polycyclic xanthone antibiotic, exhibited strong antibacterial activities and showed potent cytotoxicity. The biosynthetic gene cluster of compound 1 has been identified in our previous work, and the construction of xanthone nucleus has been well demonstrated. However, limited information of the halogenation involved in compound 1 biosynthesis is available. In this study, based on the genetic manipulation and biochemical assay, we characterized XanH as an indispensable flavin adenine dinucleotide (FAD)-dependent halogenase (FDH) for the biosynthesis of compound 1. XanH was found to be a bifunctional protein capable of flavin reduction and chlorination and exclusively used the NADH. However, the reduced flavin could not be fully and effectively utilized, and the presence of an extra flavin reductase (FDR) and chemical-reducing agent could promote the halogenation. XanH accepted its natural free-standing substrate with angular fused polycyclic aromatic systems. Meanwhile, it exhibited moderate halogenation activity and possessed high substrate specificity. The requirement of extra FDR for higher halogenation activity is tedious for future engineering. To facilitate efforts in engineering XanH derivative proteins, we constructed the self-sufficient FDR-XanH fusion proteins. The fusion protein E1 with comparable activities to that of XanH could be used as a good alternative for future protein engineering. Taken together, these findings reported here not only improve the understanding of polycyclic xanthones biosynthesis but also expand the substrate scope of FDH and pave the way for future engineering of biocatalysts for new active substance synthesis.IMPORTANCE Halogenation is important in medicinal chemistry and plays an essential role in the biosynthesis of active secondary metabolites. Halogenases have evolved to catalyze reactions with high efficiency and selectivity, and engineering efforts have been made to engage the selective reactivity in natural product biosynthesis. The enzymatic halogenations are an environmentally friendly approach with high regio- and stereoselectivity, which make it a potential complement to organic synthesis. FDHs constitute one of the most extensively elucidated class of halogenases; however, the inventory awaits to be expanded for biotechnology applications and for the generation of halogenated natural product analogues. In this study, XanH was found to reduce flavin and halogenated the freely diffusing natural substrate with an angular fused hexacyclic scaffold, findings which were different from those for the exclusively studied FDHs. Moreover, the FDR-XanH fusion protein E1 with comparable reactivity to that of XanH serves as a successful example of genetic fusions and sets an important stage for future protein engineering.

Metabolic engineering of a methyltransferase for production of drug precursors demecycline and demeclocycline in Streptomyces aureofaciens

Demecycline (DMTC) and demeclocycline (DMCTC) are C6-demethylated derivatives of tetracycline (TC) and chlortetracycline (CTC), respectively. They are precursors of minocycline and tigecycline, which showed remarkable bioactivity against TC-resistant bacteria and have been used clinically for decades. In order to biosynthesize drug precursors DMTC and DMCTC, the function of a possible C-methyltransferase encoding gene ctcK was studied systematically in the CTC high-yielding industrial strain Streptomyces aureofaciens F3. The ΔctcK mutant accumulated two new products, which were turned out to be DMTC and DMCTC. Meanwhile, time-course analysis of the fermentation products detected the epimers of DMTC and DMCTC transformed spontaneously. Finally, an engineering strain with higher productivity of DMCTC was constructed by deleting ctcK and overexpressing ctcP of three extra copies simultaneously. Construction of these two engineering strains not only served as a successful example of synthesizing required products through metabolic engineering, but also provided original strains for following elaborate engineering to synthesize more effective tetracycline derivatives.

Coordinated regulation for nature products discovery and overproduction in Streptomyces

Streptomyces is an important treasure trove for natural products discovery. In recent years, many scientists focused on the genetic modification and metabolic regulation of Streptomyces to obtain diverse bioactive compounds with high yields. This review summarized the commonly used regulatory strategies for natural products discovery and overproduction in Streptomyces from three main aspects, including regulator-related strategies, promoter engineering, as well as other strategies employing transposons, signal factors, or feedback regulations. It is expected that the metabolic regulation network of Streptomyces will be elucidated more comprehensively to shed light on natural products research in the future.