Activation of Secondary Metabolite Gene Clusters in Streptomyces clavuligerus by the PimM Regulator of Streptomyces natalensis

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.

Development of Integrated Vectors with Strong Constitutive Promoters for High-Yield Antibiotic Production in Mangrove-Derived Streptomyces

It is important to improve the production of bioactive secondary products for drug development. The Escherichia coli-Streptomyces shuttle vector pSET152 and its derived vector pIB139 containing a strong constitutive promoter ermEp* are commonly used as integrative vectors in actinomycetes. Four new integrative vectors carrying the strong constitutive promoter kasOp*, hrdBp, SCO5768p, and SP44, respectively, were constructed and proven to be functional in different mangrove-derived Streptomyces host strains by using kanamycin resistance gene neo as a reporter. Some biosynthetic genes of elaiophylins, azalomycin Fs, and armeniaspirols were selected and inserted into these vectors to overexpress in their producers including Streptomyces sp. 219807, Streptomyces sp. 211726, and S. armeniacus DSM 43125, resulting in an approximately 1.1-1.4-fold enhancement of the antibiotic yields.

Metabolic perturbation of Streptomyces albulus by introducing NADP-dependent glyceraldehyde 3-phosphate dehydrogenase

The available resources of Streptomyces represent a valuable repository of bioactive natural products that warrant exploration. Streptomyces albulus is primarily utilized in the industrial synthesis of ε-poly-L-lysine (ε-PL). In this study, the NADP-dependent glyceraldehyde 3-phosphate dehydrogenase (GapN) from Streptococcus mutans was heterologously expressed in S. albulus CICC11022, leading to elevated intracellular NADPH levels and reduced NADH and ATP concentrations. The resulting perturbation of S. albulus metabolism was comprehensively analyzed using transcriptomic and metabolomic methodologies. A decrease in production of ε-PL was observed. The expression of gapN significantly impacted on 23 gene clusters responsible for the biosynthesis of secondary metabolites. A comprehensive analysis revealed a total of 21 metabolites exhibiting elevated levels both intracellularly and extracellularly in the gapN expressing strain compared to those in the control strain. These findings underscore the potential of S. albulus to generate diverse bioactive natural products, thus offering valuable insights for the utilization of known Streptomyces resources through genetic manipulation.

Phylogenetic classification of natural product biosynthetic gene clusters based on regulatory mechanisms

The natural products (NPs) biosynthetic gene clusters (BGCs) represent the adapting biochemical toolkit for microorganisms to thrive different microenvironments. Despite their high diversity, particularly at the genomic level, detecting them in a shake-flask is challenging and remains the primary obstacle limiting our access to valuable chemicals. Studying the molecular mechanisms that regulate BGC expression is crucial to design of artificial conditions that derive on their expression. Here, we propose a phylogenetic analysis of regulatory elements linked to biosynthesis gene clusters, to classify BGCs to regulatory mechanisms based on protein domain information. We utilized Hidden Markov Models from the Pfam database to retrieve regulatory elements, such as histidine kinases and transcription factors, from BGCs in the MIBiG database, focusing on actinobacterial strains from three distinct environments: oligotrophic basins, rainforests, and marine environments. Despite the environmental variations, our isolated microorganisms share similar regulatory mechanisms, suggesting the potential to activate new BGCs using activators known to affect previously characterized BGCs.

Targeted Discovery of Cryptic Metabolites with Antiproliferative Activity

Microorganisms have provided a rich source of therapeutically valuable natural products. Recent advances in whole genome sequencing and bioinformatics have revealed immense untapped potential for new natural products in the form of silent or "cryptic" biosynthetic genes. We herein conducted high-throughput elicitor screening (HiTES) in conjunction with cytotoxicity assays against selected cancer cell lines with the goal of uncovering otherwise undetectable cryptic metabolites with antiproliferative activity. Application to Streptomyces clavuligerus facilitated identification of clavamates A and B, two bioactive metabolites with unusual structural features, as well as facile activation of a gene cluster coding for tunicamycin, which exhibited strong growth-inhibitory activity. The elicitor we identified was pleiotropic, additionally leading to the discovery of a modified, bicyclic pentapeptide natural product. Our results highlight the utility of this approach in identifying new molecules with antiproliferative activity from even overexploited microbial strains.

An integrative-omics analysis of an industrial clavulanic acid-overproducing Streptomyces clavuligerus

Clavulanic acid (CA) is a clinically important secondary metabolite used to treat infectious diseases. We aimed to decipher complex regulatory mechanisms acting in CA biosynthesis by analyzing transcriptome- and proteome-wide alterations in an industrial CA overproducer Streptomyces clavuligerus strain, namely DEPA and its wild-type counterpart NRRL3585. A total of 924 differentially expressed genes (DEGs) and 271 differentially produced proteins (DPPs) were obtained by RNA-seq and nanoLC-MS/MS analyses, respectively. In particular, CA biosynthetic genes, namely, car (cad), cas2, oat2, pah, bls, ceas2, orf12, and claR, a cluster situated regulatory (CSR) gene, were significantly upregulated as shown by RNA-seq. Enzymes of clavam biosynthesis were downregulated considerably in the DEPA strain, while the genes involved in the arginine biosynthesis, one of the precursors of CA pathway, were overexpressed. However, the biosynthesis of the other CA precursor, glyceraldehyde-3-phosphate (G3P), was not affected. CA overproduction in the DEPA strain was correlated with BldD, BldG, BldM, and BldN (AdsA) overrepresentation. In addition, TetR, WhiB, and Xre family transcriptional regulators were shown to be significantly overrepresented. Several uncharacterized/unknown proteins differentially expressed in the DEPA strain await further studies for functional characterization. Correlation analysis indicated an acceptable degree of consistency between the transcriptome and proteome data. The study represents the first integrative-omics analysis in a CA overproducer S. clavuligerus strain, providing insights into the critical control points and potential rational engineering targets for a purposeful increase of CA yields in strain improvement. KEY POINTS: ∙ Transcriptome and proteome-wide alterations in industrial CA overproducer strain DEPA ∙ An acceptable degree of consistency between the transcriptome and proteome data ∙ New targets to be exploited for rational engineering.

Modulation of Multiple Gene Clusters’ Expression by the PAS-LuxR Transcriptional Regulator PteF

PAS-LuxR transcriptional regulators are conserved proteins governing polyene antifungal biosynthesis. PteF is the regulator of filipin biosynthesis from Streptomyces avermitilis. Its mutation drastically abates filipin, but also oligomycin production, a macrolide ATP-synthase inhibitor, and delays sporulation; thus, it has been considered a transcriptional activator. Transcriptomic analyses were performed in S. avermitilis DpteF and its parental strain. Both strains were grown in a YEME medium without sucrose, and the samples were taken at exponential and stationary growth phases. A total of 257 genes showed an altered expression in the mutant, most of them at the exponential growth phase. Surprisingly, despite PteF being considered an activator, most of the genes affected showed overexpression, thereby suggesting a negative modulation. The affected genes were related to various metabolic processes, including genetic information processing; DNA, energy, carbohydrate, and lipid metabolism; morphological differentiation; and transcriptional regulation, among others, but were particularly related to secondary metabolite biosynthesis. Notably, 10 secondary metabolite gene clusters out of the 38 encoded by the genome showed altered expression profiles in the mutant, suggesting a regulatory role for PteF that is wider than expected. The transcriptomic results were validated by quantitative reverse-transcription polymerase chain reaction. These findings provide important clues to understanding the intertwined regulatory machinery that modulates antibiotic biosynthesis in Streptomyces.

Characterization of Microbial Diversity in Decayed Wood from a Spanish Forest: An Environmental Source of Industrially Relevant Microorganisms

Rotting wood is inhabited by a large diversity of bacteria, fungi, and insects with complex environmental relationships. The aim of this work was to study the composition of the microbiota (bacteria and fungi) in decaying wood from a northwest Spanish forest as a source of industrially relevant microorganisms. The analyzed forest is situated in a well-defined biogeographic area combining Mediterranean and temperate macrobioclimates. Bacterial diversity, determined by metagenome analyses, was higher than fungal heterogeneity. However, a total of 194 different cultivable bacterial isolates (mainly Bacillaceae, Streptomycetaceae, Paenibacillaceae, and Microbacteriaceae) were obtained, in contrast to 343 fungal strains (mainly Aspergillaceae, Hypocreaceae, and Coniochaetaceae). Isolates traditionally known as secondary metabolite producers, such as Actinobacteria and members of the Penicillium genus, were screened for their antimicrobial activity by the detection of antibiotic biosynthetic clusters and competitive bioassays against fungi involved in wood decay. In addition, the ability of Penicillium isolates to degrade cellulose and release ferulic acid from wood was also examined. These results present decaying wood as an ecologically rich niche and a promising source of biotechnologically interesting microorganisms.

Genome editing reveals that pSCL4 is required for chromosome linearity in Streptomyces clavuligerus

Streptomyces clavuligerus is an industrially important actinomycete whose genetic manipulation is limited by low transformation and conjugation efficiencies, low levels of recombination of introduced DNA, and difficulty in obtaining consistent sporulation. We describe the construction and application of versatile vectors for Cas9-mediated genome editing of this strain. To design spacer sequences with confidence, we derived a highly accurate genome assembly for an isolate of the type strain (ATCC 27064). This yielded a chromosome assembly (6.75 Mb) plus assemblies for pSCL4 (1795 kb) and pSCL2 (149 kb). The strain also carries pSCL1 (12 kb), but its small size resulted in only partial sequence coverage. The previously described pSCL3 (444 kb) is not present in this isolate. Using our Cas9 vectors, we cured pSCL4 with high efficiency by targeting the plasmid's parB gene. Five of the resulting pSCL4-cured isolates were characterized and all showed impaired sporulation. Shotgun genome sequencing of each of these derivatives revealed large deletions at the ends of the chromosomes in all of them, and for two clones sufficient sequence data was obtained to show that the chromosome had circularized. Taken together, these data indicate that pSCL4 is essential for the structural stability of the linear chromosome.

Streptomyces clavuligerus: The Omics Era

The Streptomyces clavuligerus genome consists in a linear chromosome of about 6.7 Mb and four plasmids (pSCL1 to pSCL4), the latter one of 1.8 Mb. Deletion of pSCL4, results in viable mutants with high instability in the chromosome arms, which may lead to chromosome circularisation. Transcriptomic and proteomic studies comparing different mutants with the wild-type strain improved our knowledge on the biosynthesis and regulation of clavulanic acid, cephamycin C and holomycin. Additional knowledge has been obtained on the SARP-type CcaR activator and the network of connections with other regulators (Brp, AreB, AdpA, BldG, RelA) controlling ccaR expression. The transcriptional pattern of the cephamycin and clavulanic acid clusters is supported by the binding of CcaR to different promoters and confirmed that ClaR is a CcaR-dependent activator that controls the late steps of clavulanic biosynthesis. Metabolomic studies allowed the detection of new metabolites produced by S. clavuligerus such as naringenin, desferroxamines, several N-acyl tunicamycins, the terpenes carveol and cuminyl alcohol or bafilomycin J. Heterologous expression of S. clavuligerus terpene synthases resulted in the formation of no less than 15 different terpenes, although none of them was detected in S. clavuligerus culture broth. In summary, application of the Omic tools results in a better understanding of the molecular biology of S. clavuligerus, that allows the use of this strain as an industrial actinobacterial platform and helps to improve CA production.

Extended Plant Metarhizobiome: Understanding Volatile Organic Compound Signaling in Plant-Microbe Metapopulation Networks

Plant rhizobiomes consist of microbes that are influenced by the physical, chemical, and biological properties of the plant root system. While plant-microbe interactions are generally thought to be local, accumulating evidence suggests that topologically disconnected bulk soil microbiomes could be linked with plants and their associated rhizospheric microbes through volatile organic compounds (VOCs). While several studies have focused on the effect of soil physicochemical properties for VOC movement, it is less clear how VOC signaling is affected by microbial communities themselves when VOCs travel across soils. To gain a better understanding of this, we propose that soil microbe-plant communities could be viewed as “metarhizobiomes,” where VOC-mediated interactions extend the plant rhizobiome further out through interconnected microbial metapopulation networks. In this minireview, we mainly focus on soil microbial communities and first discuss how microbial interactions within a local population affect VOC signaling, leading to changes in the amount, type, and ecological roles of produced VOCs. We then consider how VOCs could connect spatially separated microbial populations into a larger metapopulation network and synthesize how (i) VOC effects cascade in soil matrix when moving away from the source of origin and (ii) how microbial metapopulation composition and diversity shape VOC-signaling between plants and microbes at the landscape level. Finally, we propose new avenues for experimentally testing VOC movement in plant-microbe metapopulation networks and suggest how VOCs could potentially be used for managing plant health in natural and agricultural soils.

TCA Cycle and Its Relationship with Clavulanic Acid Production: A Further Interpretation by Using a Reduced Genome-Scale Metabolic Model of Streptomyces clavuligerus

Streptomyces clavuligerus (S. clavuligerus) has been widely studied for its ability to produce clavulanic acid (CA), a potent inhibitor of β-lactamase enzymes. In this study, S. clavuligerus cultivated in 2D rocking bioreactor in fed-batch operation produced CA at comparable rates to those observed in stirred tank bioreactors. A reduced model of S. clavuligerus metabolism was constructed by using a bottom-up approach and validated using experimental data. The reduced model was implemented for in silico studies of the metabolic scenarios arisen during the cultivations. Constraint-based analysis confirmed the interrelations between succinate, oxaloacetate, malate, pyruvate, and acetate accumulations at high CA synthesis rates in submerged cultures of S. clavuligerus. Further analysis using shadow prices provided a first view of the metabolites positive and negatively associated with the scenarios of low and high CA production.

Specialized Metabolites from Ribosome Engineered Strains of Streptomyces clavuligerus

Bacterial specialized metabolites are of immense importance because of their medicinal, industrial, and agricultural applications. Streptomyces clavuligerus is a known producer of such compounds; however, much of its metabolic potential remains unknown, as many associated biosynthetic gene clusters are silent or expressed at low levels. The overexpression of ribosome recycling factor (frr) and ribosome engineering (induced rpsL mutations) in other Streptomyces spp. has been reported to increase the production of known specialized metabolites. Therefore, we used an overexpression strategy in combination with untargeted metabolomics, molecular networking, and in silico analysis to annotate 28 metabolites in the current study, which have not been reported previously in S. clavuligerus. Many of the newly described metabolites are commonly found in plants, further alluding to the ability of S. clavuligerus to produce such compounds under specific conditions. In addition, the manipulation of frr and rpsL led to different metabolite production profiles in most cases. Known and putative gene clusters associated with the production of the observed compounds are also discussed. This work suggests that the combination of traditional strain engineering and recently developed metabolomics technologies together can provide rapid and cost-effective strategies to further speed up the discovery of novel natural products.

Genomic scanning enabling discovery of a new antibacterial bicyclic carbamate-containing alkaloid

Non-ribosomal peptides are a group of structurally diverse natural products with various important therapeutic and agrochemical applications. Bacterial pyrrolizidine alkaloids (PAs), containing a scaffold of two fused five-membered ring system with a nitrogen atom at the bridgehead, have been found to originate from a multidomain non-ribosomal peptide synthetase to generate indolizidine intermediates, followed by multistep oxidation, catalysed by single Bayer-Villiger (BV) enzymes, to yield PA scaffolds. Although bacterial PAs are rare in natural product inventory, bioinformatics analysis suggested that the biosynthetic gene clusters (BGCs) that are likely to be responsible for the production of PA-like metabolites are widely distributed in bacterial genomes. However, most of the strains containing PA-like BGCs are not deposited in the public domain, therefore preventing further assessment of the chemical spaces of this group of bioactive metabolites. Here, we report a genomic scanning strategy to assess the potential of PA metabolites production in our culture collection without prior knowledge of genome information. Among the strains tested, we found fifteen contain the key BV enzymes that are likely to be involved in the last step of PA ring formation. Subsequently one-strain-many-compound (OSMAC) method, supported by a combination of HR-MS, NMR, SMART 2.0 technology, and GNPS analysis, allowed identification and characterization of a new [5 + 7] heterobicyclic carbamate, legoncarbamate, together with five known PAs, bohemamine derivatives, from Streptomyces sp. CT37, a Ghanaian soil isolate. The absolute stereochemistry of legoncarbamate was determined by comparison of measured and calculated ECD spectra. Legoncarbamate displays antibacterial activity against E. coli ATCC 25922 with an MIC value of 3.1 μg/mL. Finally, a biosynthetic model of legoncarbamate and other bohemamines was proposed based on the knowledge we have gained so far.

Clavulanic Acid Production by Streptomyces clavuligerus: Insights from Systems Biology, Strain Engineering, and Downstream Processing

Clavulanic acid (CA) is an irreversible β-lactamase enzyme inhibitor with a weak antibacterial activity produced by Streptomyces clavuligerus (S. clavuligerus). CA is typically co-formulated with broad-spectrum β‑lactam antibiotics such as amoxicillin, conferring them high potential to treat diseases caused by bacteria that possess β‑lactam resistance. The clinical importance of CA and the complexity of the production process motivate improvements from an interdisciplinary standpoint by integrating metabolic engineering strategies and knowledge on metabolic and regulatory events through systems biology and multi-omics approaches. In the large-scale bioprocessing, optimization of culture conditions, bioreactor design, agitation regime, as well as advances in CA separation and purification are required to improve the cost structure associated to CA production. This review presents the recent insights in CA production by S. clavuligerus, emphasizing on systems biology approaches, strain engineering, and downstream processing.

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.

A Hierarchical Network of Four Regulatory Genes Controlling Production of the Polyene Antibiotic Candicidin in Streptomyces sp. Strain FR-008

The four regulatory genes fscR1 to fscR4 in Streptomyces sp. strain FR-008 form a genetic arrangement that is widely distributed in macrolide-producing bacteria. Our previous work has demonstrated that fscR1 and fscR4 are critical for production of the polyene antibiotic candicidin. In this study, we further characterized the roles of the other two regulatory genes, fscR2 and fscR3, focusing on the relationship between these four regulatory genes. Disruption of a single or multiple regulatory genes did not affect bacterial growth, but transcription of genes in the candicidin biosynthetic gene cluster decreased, and candicidin production was abolished, indicating a critical role for each of the four regulatory genes, including fscR2 and fscR3, in candicidin biosynthesis. We found that fscR1 to fscR4, although differentially expressed throughout the growth phase, displayed similar temporal expression patterns, with an abrupt increase in the early exponential phase, coincident with initial detection of antibiotic production in the same phase. Our data suggest that the four regulatory genes fscR1 to fscR4 have various degrees of control over structural genes in the biosynthetic cluster under the conditions examined. Extensive transcriptional analysis indicated that complex regulation exists between these four regulatory genes, forming a regulatory network, with fscR1 and fscR4 functioning at a lower level. Comprehensive cross-complementation analysis indicates that functional complementation is restricted among the four regulators and unidirectional, with fscR1 complementing the loss of fscR3 or -4 and fscR4 complementing loss of fscR2 Our study provides more insights into the roles of, and the regulatory network formed by, these four regulatory genes controlling production of an important pharmaceutical compound.IMPORTANCE The regulation of antibiotic biosynthesis by Streptomyces species is complex, especially for biosynthetic gene clusters with multiple regulatory genes. The biosynthetic gene cluster for the polyene antibiotic candicidin contains four consecutive regulatory genes, which encode regulatory proteins from different families and which form a subcluster within the larger biosynthetic gene cluster in Streptomyces sp. FR-008. Syntenic arrangements of these regulatory genes are widely distributed in polyene gene clusters, such as the amphotericin and nystatin gene clusters, suggesting a conserved regulatory mechanism controlling production of these clinically important medicines. However, the relationships between these multiple regulatory genes are unknown. In this study, we determined that each of these four regulatory genes is critical for candicidin production. Additionally, using transcriptional analyses, bioassays, high-performance liquid chromatography (HPLC) analysis, and genetic cross-complementation, we showed that FscR1 to FscR4 comprise a hierarchical regulatory network that controls candicidin production and is likely representative of how expression of other polyene biosynthetic gene clusters is controlled.

Disclosing the Potential of the SARP-Type Regulator PapR2 for the Activation of Antibiotic Gene Clusters in Streptomycetes

Streptomyces antibiotic regulatory protein (SARP) family regulators are well-known activators of antibiotic biosynthesis in streptomycetes. The respective genes occur in various types of antibiotic gene clusters encoding, e.g., for polyketides, ribosomally and non-ribosomally synthesized peptides, or β-lactam antibiotics. We found that overexpression of the SARP-type regulator gene papR2 from Streptomyces pristinaespiralis in Streptomyces lividans leads to the activation of the silent undecylprodigiosin (Red) gene cluster. The activation happens upon the inducing function of PapR2, which takes over the regulatory role of RedD, the latter of which is the intrinsic SARP regulator of Red biosynthesis in S. lividans. Due to the broad abundance of SARP genes in different antibiotic gene clusters of various actinomycetes and the uniform activating principle of the encoded regulators, we suggest that this type of regulator is especially well suited to be used as an initiator of antibiotic biosynthesis in actinomycetes. Here, we report on a SARP-guided strategy to activate antibiotic gene clusters. As a proof of principle, we present the PapR2-driven activation of the amicetin/plicacetin gene cluster in the novel Indonesian strain isolate Streptomyces sp. SHP22-7.

Homologous expression of lysA encoding diaminopimelic acid (DAP) decarboxylase reveals increased antibiotic production in Streptomyces clavuligerus

lysA gene encoding meso-diaminopimelic acid (DAP) decarboxylase enzyme that catalyzes L-lysine biosynthesis in the aspartate pathway in Streptomyces clavuligerus was overexpressed, and its effects on cephamycin C (CephC), clavulanic acid (CA), and tunicamycin productions were investigated. Multicopy expression of lysA gene under the control of glpF promoter (glpFp) in S. clavuligerus pCOlysA led to higher expression levels ranging from 2- to 6-fold increase at both lysA gene and CephC biosynthetic gene cluster at T₃₆ and T₄₈ of TSBG fermentation. These results accorded well with CephC production. Thus, 1.86- and 3.14-fold higher volumetric as well as 1.26- and 1.71-fold increased specific CephC yields were recorded in S. clavuligerus pCOlysA in comparison with the wild-type and its control strain, respectively, at 48th h. Increasing the expression of lysA provided 4.3 times more tunicamycin yields in the recombinant strain. These findings suggested that lysA overexpression in S. clavuligerus made the strain more productive for CephC and tunicamycin. The results also supported the presence of complex interactions among antibiotic biosynthesis pathways in S. clavuligerus.

Comparative Genomics and Metabolomics Analyses of Clavulanic Acid-Producing Streptomyces Species Provides Insight Into Specialized Metabolism

Clavulanic acid is a bacterial specialized metabolite, which inhibits certain serine β-lactamases, enzymes that inactivate β-lactam antibiotics to confer resistance. Due to this activity, clavulanic acid is widely used in combination with penicillin and cephalosporin (β-lactam) antibiotics to treat infections caused by β-lactamase-producing bacteria. Clavulanic acid is industrially produced by fermenting Streptomyces clavuligerus, as large-scale chemical synthesis is not commercially feasible. Other than S. clavuligerus, Streptomyces jumonjinensis and Streptomyces katsurahamanus also produce clavulanic acid along with cephamycin C, but information regarding their genome sequences is not available. In addition, the Streptomyces contain many biosynthetic gene clusters thought to be "cryptic," as the specialized metabolites produced by them are not known. Therefore, we sequenced the genomes of S. jumonjinensis and S. katsurahamanus, and examined their metabolomes using untargeted mass spectrometry along with S. clavuligerus for comparison. We analyzed the biosynthetic gene cluster content of the three species to correlate their biosynthetic capacities, by matching them with the specialized metabolites detected in the current study. It was recently reported that S. clavuligerus can produce the plant-associated metabolite naringenin, and we describe more examples of such specialized metabolites in extracts from the three Streptomyces species. Detailed comparisons of the biosynthetic gene clusters involved in clavulanic acid (and cephamycin C) production were also performed, and based on our analyses, we propose the core set of genes responsible for producing this medicinally important metabolite.

The two-component system CepRS regulates the cephamycin C biosynthesis in Streptomyces clavuligerus F613-1

During industrial fermentation, Streptomyces clavuligerus F613-1 simultaneously produces primary product clavulanic acid (CA) and cephamycin C. The cephamycin C biosynthetic gene cluster and pathway have been basically elucidated and the CcaR positive regulator was found to control the cephamycin genes expression. However, additional mechanisms of regulation cannot be excluded. The BB341_RS13780/13785 gene pair in S. clavuligerus F613-1 (annotated as SCLAV_2960/2959 in S. clavuligerus ATCC27064) encodes a bacterial two-component system (TCS) and were designated as CepRS (for cephamycin regulator/sensor). CepRS significantly affects cephamycin C production but only slightly affects CA production. To further understand the regulation of cephamycin C biosynthesis, the cepRS genes were deleted from S. clavuligerus F613-1. The deletion mutant resulted in decreased cephamycin C production but had no phenotypic effects. Real-time quantitative polymerase chain reaction analysis revealed that CepRS regulates the expression of most genes involved in cephamycin C biosynthesis, with electrophoretic mobility shift assays showing that CepR interacts with the cefD-cmcI intergenic region. These results demonstrate that the CepR response regulator serves as a transcriptional activator of cephamycin C biosynthesis, which may provide an approach for metabolic engineering methods for CA production by S. clavuligerus F613-1 in future.