Identification of PimR as a positive regulator of pimaricin biosynthesis in Streptomyces natalensis

Novel mechanism of hydrogen peroxide for promoting efficient natamycin synthesis in Streptomyces

The mechanism of regulation of natamycin biosynthesis by Streptomyces in response to oxidative stress is unclear. Here, we first show cholesterol oxidase SgnE, which catalyzes the formation of H2O2 from sterols, triggered a series of redox-dependent interactions to stimulate natamycin production in S. gilvosporeus. In response to reactive oxygen species, residues Cys212 and Cys221 of the H2O2-sensing consensus sequence of OxyR were oxidized, resulting in conformational changes in the protein: OxyR extended its DNA-binding domain to interact with four motifs of promoter p sgnM . This acted as a redox-dependent switch to turn on/off gene transcription of sgnM, which encodes a cluster-situated regulator, by controlling the affinity between OxyR and p sgnM , thus regulating the expression of 12 genes in the natamycin biosynthesis gene cluster. OxyR cooperates with SgnR, another cluster-situated regulator and an upstream regulatory factor of SgnM, synergistically modulated natamycin biosynthesis by masking/unmasking the -35 region of p sgnM depending on the redox state of OxyR in response to the intracellular H2O2 concentration. IMPORTANCE Cholesterol oxidase SgnE is an indispensable factor, with an unclear mechanism, for natamycin biosynthesis in Streptomyces. Oxidative stress has been attributed to the natamycin biosynthesis. Here, we show that SgnE catalyzes the formation of H2O2 from sterols and triggers a series of redox-dependent interactions to stimulate natamycin production in S. gilvosporeus. OxyR, which cooperates with SgnR, acted as a redox-dependent switch to turn on/off gene transcription of sgnM, which encodes a cluster-situated regulator, by masking/unmasking its -35 region, to control the natamycin biosynthesis gene cluster. This work provides a novel perspective on the crosstalk between intracellular ROS homeostasis and natamycin biosynthesis. Application of these findings will improve antibiotic yields via control of the intracellular redox pressure in Streptomyces.

Identification of RimR2 as a positive pathway-specific regulator of rimocidin biosynthesis in Streptomyces rimosus M527

BACKGROUND

Streoptomyces rimosus M527 is a producer of the polyene macrolide rimocidin which shows activity against various plant pathogenic fungi. Notably, the regulatory mechanisms underlying rimocidin biosynthesis are yet to be elucidated.

RESULTS

In this study, using domain structure and amino acid alignment and phylogenetic tree construction, rimR2, which located in the rimocidin biosynthetic gene cluster, was first found and identified as a larger ATP-binding regulators of the LuxR family (LAL) subfamily regulator. The rimR2 deletion and complementation assays were conducted to explore its role. Mutant M527-ΔrimR2 lost its ability to produce rimocidin. Complementation of M527-ΔrimR2 restored rimocidin production. The five recombinant strains, M527-ER, M527-KR, M527-21R, M527-57R, and M527-NR, were constructed by overexpressing rimR2 gene using the promoters permE^*, kasOp^*, SPL21, SPL57, and its native promoter, respectively, to improve rimocidin production. M527-KR, M527-NR, and M527-ER exhibited 81.8%, 68.1%, and 54.5% more rimocidin production, respectively, than the wild-type (WT) strain, while recombinant strains M527-21R and M527-57R exhibited no obvious differences in rimocidin production compared with the WT strain. RT-PCR assays revealed that the transcriptional levels of the rim genes were consistent with the changes in rimocidin production in the recombinant strains. Using electrophoretic mobility shift assays, we confirmed that RimR2 can bind to the promoter regions of rimA and rimC.

CONCLUSION

A LAL regulator RimR2 was identified as a positive specific-pathway regulator of rimocidin biosynthesis in M527. RimR2 regulates the rimocidin biosynthesis by influencing the transcriptional levels of rim genes and binding to the promoter regions of rimA and rimC.

MilR3, a unique SARP family pleiotropic regulator in Streptomyces bingchenggensis

Streptomyces bingchenggensis is the main industrial producer of milbemycins, which are a group of 16-membered macrocylic lactones with excellent insecticidal activities. In the past several decades, scientists have made great efforts to solve its low productivity. However, a lack of understanding of the regulatory network of milbemycin biosynthesis limited the development of high-producing strains using a regulatory rewiring strategy. SARPs (Streptomyces Antibiotic Regulatory Proteins) family regulators are widely distributed and play key roles in regulating antibiotics production in actinobacteria. In this paper, MilR3 (encoded by sbi_06842) has been screened out for significantly affecting milbemycin production from all the 19 putative SARP family regulators in S. bingchenggensis with the DNase-deactivated Cpf1-based integrative CRISPRi system. Interestingly, milR3 is about 7 Mb away from milbemycin biosynthetic gene cluster and adjacent to a putative type II PKS (the core minimal PKS encoding genes are sbi_06843, sbi_06844, sbi_06845 and sbi_06846) gene cluster, which was proved to be responsible for producing a yellow pigment. The quantitative real-time PCR analysis proved that MilR3 positively affected the transcription of specific genes within milbemycin BGC and those from the type II PKS gene cluster. Unlike previous "small" SARP family regulators that played pathway-specific roles, MilR3 was probably a unique SARP family regulator and played a pleotropic role. MilR3 was an upper level regulator in the MilR3-MilR regulatory cascade. This study first illustrated the co-regulatory role of this unique SARP regulator. This greatly enriches our understanding of SARPs and lay a solid foundation for milbemycin yield enhancement in the near future.

Genome Sequence-Guided Finding of Lucensomycin Production by Streptomyces achromogenes Subsp. streptozoticus NBRC14001

Information on microbial genome sequences is a powerful resource for accessing natural products with significant activities. We herein report the unveiling of lucensomycin production by Streptomyces achromogenes subsp. streptozoticus NBRC14001 based on the genome sequence of the strain. The genome sequence of strain NBRC14001 revealed the presence of a type I polyketide synthase gene cluster with similarities to a biosynthetic gene cluster for natamycin, which is a polyene macrolide antibiotic that exhibits antifungal activity. Therefore, we investigated whether strain NBRC14001 produces antifungal compound(s) and revealed that an extract from the strain inhibited the growth of Candida albicans. A HPLC analysis of a purified compound exhibiting antifungal activity against C. albicans showed that the compound differed from natamycin. Based on HR-ESI-MS spectrometry and a PubChem database search, the compound was predicted to be lucensomycin, which is a tetraene macrolide antibiotic, and this prediction was supported by the results of a MS/MS analysis. Furthermore, the type I polyketide synthase gene cluster in strain NBRC14001 corresponded well to lucesomycin biosynthetic gene cluster (lcm) in S. cyanogenus, which was very recently reported. Therefore, we concluded that the antifungal compound produced by strain NBRC14001 is lucensomycin.

Eliciting the silent lucensomycin biosynthetic pathway in Streptomyces cyanogenus S136 via manipulation of the global regulatory gene adpA

Actinobacteria are among the most prolific sources of medically and agriculturally important compounds, derived from their biosynthetic gene clusters (BGCs) for specialized (secondary) pathways of metabolism. Genomics witnesses that the majority of actinobacterial BGCs are silent, most likely due to their low or zero transcription. Much effort is put into the search for approaches towards activation of silent BGCs, as this is believed to revitalize the discovery of novel natural products. We hypothesized that the global transcriptional factor AdpA, due to its highly degenerate operator sequence, could be used to upregulate the expression of silent BGCs. Using Streptomyces cyanogenus S136 as a test case, we showed that plasmids expressing either full-length adpA or its DNA-binding domain led to significant changes in the metabolome. These were evident as changes in the accumulation of colored compounds, bioactivity, as well as the emergence of a new pattern of secondary metabolites as revealed by HPLC-ESI-mass spectrometry. We further focused on the most abundant secondary metabolite and identified it as the polyene antibiotic lucensomycin. Finally, we uncovered the entire gene cluster for lucensomycin biosynthesis (lcm), that remained elusive for five decades until now, and outlined an evidence-based scenario for its adpA-mediated activation.

The Application of Regulatory Cascades in Streptomyces: Yield Enhancement and Metabolite Mining

Streptomyces is taken as an important resource for producing the most abundant antibiotics and other bio-active natural products, which have been widely used in pharmaceutical and agricultural areas. Usually they are biosynthesized through secondary metabolic pathways encoded by cluster situated genes. And these gene clusters are stringently regulated by interweaved transcriptional regulatory cascades. In the past decades, great advances have been made to elucidate the regulatory mechanisms involved in antibiotic production in Streptomyces. In this review, we summarized the recent advances on the regulatory cascades of antibiotic production in Streptomyces from the following four levels: the signals triggering the biosynthesis, the global regulators, the pathway-specific regulators and the feedback regulation. The production of antibiotic can be largely enhanced by rewiring the regulatory networks, such as overexpression of positive regulators, inactivation of repressors, fine-tuning of the feedback and ribosomal engineering in Streptomyces. The enormous amount of genomic sequencing data implies that the Streptomyces has potential to produce much more antibiotics for the great diversities and wide distributions of biosynthetic gene clusters in Streptomyces genomes. Most of these gene clusters are defined cryptic for unknown or undetectable natural products. In the synthetic biology era, activation of the cryptic gene clusters has been successfully achieved by manipulation of the regulatory genes. Chemical elicitors, rewiring regulatory gene and ribosomal engineering have been employed to crack the potential of cryptic gene clusters. These have been proposed as the most promising strategy to discover new antibiotics. For the complex of regulatory network in Streptomyces, we proposed that the discovery of new antibiotics and the optimization of industrial strains would be greatly promoted by further understanding the regulatory mechanism of antibiotic production.

Actinomycete-Derived Polyketides as a Source of Antibiotics and Lead Structures for the Development of New Antimicrobial Drugs

Actinomycetes are remarkable producers of compounds essential for human and veterinary medicine as well as for agriculture. The genomes of those microorganisms possess several sets of genes (biosynthetic gene cluster (BGC)) encoding pathways for the production of the valuable secondary metabolites. A significant proportion of the identified BGCs in actinomycetes encode pathways for the biosynthesis of polyketide compounds, nonribosomal peptides, or hybrid products resulting from the combination of both polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs). The potency of these molecules, in terms of bioactivity, was recognized in the 1940s, and started the "Golden Age" of antimicrobial drug discovery. Since then, several valuable polyketide drugs, such as erythromycin A, tylosin, monensin A, rifamycin, tetracyclines, amphotericin B, and many others were isolated from actinomycetes. This review covers the most relevant actinomycetes-derived polyketide drugs with antimicrobial activity, including anti-fungal agents. We provide an overview of the source of the compounds, structure of the molecules, the biosynthetic principle, bioactivity and mechanisms of action, and the current stage of development. This review emphasizes the importance of actinomycetes-derived antimicrobial polyketides and should serve as a "lexicon", not only to scientists from the Natural Products field, but also to clinicians and others interested in this topic.

Characterization and overproduction of cell-associated cholesterol oxidase ChoD from Streptomyces lavendulae YAKB-15

Cholesterol oxidases are important enzymes with a wide range of applications from basic research to industry. In this study, we have discovered and described the first cell-associated cholesterol oxidase, ChoD, from Streptomyces lavendulae YAKB-15. This strain is a naturally high producer of ChoD, but only produces ChoD in a complex medium containing whole yeast cells. For characterization of ChoD, we acquired a draft genome sequence of S. lavendulae YAKB-15 and identified a gene product containing a flavin adenine dinucleotide binding motif, which could be responsible for the ChoD activity. The enzymatic activity was confirmed in vitro with histidine tagged ChoD produced in Escherichia coli TOP10, which lead to the determination of basic kinetic parameters with K_m 15.9 µM and k_cat 10.4/s. The optimum temperature and pH was 65 °C and 5, respectively. In order to increase the efficiency of production, we then expressed the cholesterol oxidase, choD, gene heterologously in Streptomyces lividans TK24 and Streptomyces albus J1074 using two different expression systems. In S. albus J1074, the ChoD activity was comparable to the wild type S. lavendulae YAKB-15, but importantly allowed production of ChoD without the presence of yeast cells.

Challenges and opportunities for natural product discovery, production, and engineering in native producers versus heterologous hosts

Recent advances and emerging technologies for metabolic pathway engineering and synthetic biology have transformed the field of natural product discovery, production, and engineering. Despite these advancements, there remain many challenges in understanding how biosynthetic gene clusters are silenced or activated, including changes in the transcription of key biosynthetic and regulatory genes. This knowledge gap is highlighted by the success and failed attempts of manipulating regulatory genes within biosynthetic gene clusters in both native producers and heterologous hosts. These complexities make the choice of native producers versus heterologous hosts, fermentation medium, and supply of precursors crucial factors in achieving the production of the target natural products and engineering designer analogs. Nature continues to serve as inspiration for filling the knowledge gaps and developing new research strategies. By exploiting the evolutionary power of nature, alternative producers, with the desired genetic amenability and higher titers of the target natural products, and new strains, harboring gene clusters that encode evolutionary optimized congeners of the targeted natural product scaffolds, can be discovered. These newly identified strains can serve as an outstanding biotechnology platform for the engineered production of sufficient quantities of the target natural products and their analogs, enabling biosynthetic studies and potential therapeutic applications. These challenges and opportunities are showcased herein using fredericamycin, iso-migrastatin, platencin and platensimycin, the enediynes of C-1027, tiancimycin, and yangpumicin, and the leinamycin family of natural products.

Characterization of the positive SARP family regulator PieR for improving piericidin A1 production in Streptomyces piomogeues var. Hangzhouwanensis

Piericidin A1, a member of ɑ-pyridone antibiotic, exhibits various biological activities such as antimicrobial, antifungal, and antitumor properties and possesses potent respiration-inhibitory activity against insects due to its competitive binding capacity to mitochondrial complex I. The biosynthetic pathway of piericidin A1 has been reported in Streptomyces piomogeues var. Hangzhouwanensis, while the regulatory mechanism remains poorly understood. In this study, a Streptomyces antibiotic regulatory protein (SARP) family transcriptional regulator PieR was characterized. Genetic disruption and complementation manipulations revealed that PieR positively regulated the production of piericidin A1. Moreover, the overexpression of pieR contributed to the improvement of piericidin A1 productivity. The real-time quantitative PCR (RT-qPCR) was carried out and the data showed that pieR stimulated the transcription of all the biosynthesis-related genes for piericidin A1. In order to explore the regulatory mechanism, electrophoresis mobility shift assays (EMSA) and DNase I footprinting experiments have been conducted. A protected region covering 50 nucleotides within the upstream region of pieR was identified and two 5-nt direct repeat sequences (5′-CCGGA-3′) in the protected region were found. These findings, taken together, set stage for transcriptional control engineering in the view of optimizing piericidin A1 production and thus provide a viable potent route for the construction of strains with high productivity.

Identification of wysPII as an Activator of Morphological Development in Streptomyces albulus CK-15

Wuyiencin is produced by Streptomyces albulus var. wuyiensis and used widely in agriculture to control a variety of fungal diseases, such as cucumber downy mildew, strawberry powdery mildew, and tomato gray mold. As an industrially-produced biopesticide, reducing production costs is very important for popularization of this approach. To obtain a rapidly growing strain that effectively shortens the fermentation time, we investigated the effects of knockout and overexpression of the wysPII gene, a member of the LuxR regulatory gene family, in S. albulus strain CK-15. The ΔwysPII mutant exhibited a reduced rate of growth and sporulation. The time taken to reach the greatest mycelial biomass was approximately 18 h shorter in the ooPII (wysPII overexpressing) strain compared with that of the wild-type (WT) strain. In addition, the time to reach the greatest wuyiencin production was 56 h in the ooPII strain compared with 62 h in the WT strain. Furthermore, wysPII was shown to act as an activator of morphological development without affecting wuyiencin production. Thus, the ooPII strain can be used to reduce costs and increase efficiency in industrial fermentation processes for wuyiencin production.

Regulation of antibiotic biosynthesis in actinomycetes: Perspectives and challenges

Actinomycetes are the main sources of antibiotics. The onset and level of production of each antibiotic is subject to complex control by multi-level regulators. These regulators exert their functions at hierarchical levels. At the lower level, cluster-situated regulators (CSRs) directly control the transcription of neighboring genes within the gene cluster. Higher-level pleiotropic and global regulators exert their functions mainly through modulating the transcription of CSRs. Advances in understanding of the regulation of antibiotic biosynthesis in actinomycetes have inspired us to engineer these regulators for strain improvement and antibiotic discovery.

Global evolution of glycosylated polyene macrolide antibiotic biosynthesis

Antibiotics are the most marvelous evolutionary products of microbes to obtain competitive advantage and maintain ecological balance. However, the origination and development of antibiotics has yet to be explicitly investigated. Due to diverse structures and similar biosynthesis, glycosylated polyene macrolides (gPEMs) were chosen to explore antibiotic evolution. A total of 130 candidate and 38 transitional gPEM clusters were collected from actinomycetes genomes, providing abundant references for phenotypic gaps in gPEM evolution. The most conserved parts of gPEM biosynthesis were found and used for phylogeny construction. On this basis, we proposed ancestral gPEM clusters at different evolutionary stages and interpreted the possible evolutionary histories in detail. The results revealed that gPEMs evolved from small rings to large rings and continuously increased structural diversity through acquiring, discarding and exchanging genes from different evolutionary origins, as well as co-evolution of functionally related proteins. The combination of horizontal gene transfers, environmental effects and host preference resulted in the diversity and worldwide distribution of gPEMs. This study is not only a useful exploration on antibiotic evolution but also an inspiration for diversity and biogeographic investigations on antibiotics in the era of Big Data.

Promoter Engineering Reveals the Importance of Heptameric Direct Repeats for DNA Binding by Streptomyces Antibiotic Regulatory Protein-Large ATP-Binding Regulator of the LuxR Family (SARP-LAL) Regulators in Streptomyces natalensis

The biosynthesis of small-size polyene macrolides is ultimately controlled by a couple of transcriptional regulators that act in a hierarchical way. A Streptomyces antibiotic regulatory protein–large ATP-binding regulator of the LuxR family (SARP-LAL) regulator binds the promoter of a PAS-LuxR regulator-encoding gene and activates its transcription, and in turn, the gene product of the latter activates transcription from various promoters of the polyene gene cluster directly. The primary operator of PimR, the archetype of SARP-LAL regulators, contains three heptameric direct repeats separated by four-nucleotide spacers, but the regulator can also bind a secondary operator with only two direct repeats separated by a 3-nucleotide spacer, both located in the promoter region of its unique target gene, pimM. A similar arrangement of operators has been identified for PimR counterparts encoded by gene clusters for different antifungal secondary metabolites, including not only polyene macrolides but peptidyl nucleosides, phoslactomycins, or cycloheximide. Here, we used promoter engineering and quantitative transcriptional analyses to determine the contributions of the different heptameric repeats to transcriptional activation and final polyene production. Optimized promoters have thus been developed. Deletion studies and electrophoretic mobility assays were used for the definition of DNA-binding boxes formed by 22-nucleotide sequences comprising two conserved heptameric direct repeats separated by four-nucleotide less conserved spacers. The cooperative binding of PimR^SARP appears to be the mechanism involved in the binding of regulator monomers to operators, and at least two protein monomers are required for efficient binding.

IMPORTANCE Here, we have shown that a modulation of the production of the antifungal pimaricin in Streptomyces natalensis can be accomplished via promoter engineering of the PAS-LuxR transcriptional activator pimM. The expression of this gene is controlled by the Streptomyces antibiotic regulatory protein–large ATP-binding regulator of the LuxR family (SARP-LAL) regulator PimR, which binds a series of heptameric direct repeats in its promoter region. The structure and importance of such repeats in protein binding, transcriptional activation, and polyene production have been investigated. These findings should provide important clues to understand the regulatory machinery that modulates antibiotic biosynthesis in Streptomyces and open new possibilities for the manipulation of metabolite production. The presence of PimR orthologues encoded by gene clusters for different secondary metabolites and the conservation of their operators suggest that the improvements observed in the activation of pimaricin biosynthesis by Streptomyces natalensis could be extrapolated to the production of different compounds by other species.

Bidirectional Regulation of AdpAch in Controlling the Expression of scnRI and scnRII in the Natamycin Biosynthesis of Streptomyces chattanoogensis L10

AdpA, an AraC/XylS family protein, had been proved as a key regulator for secondary metabolism and morphological differentiation in Streptomyces griseus. Here, we identify AdpA_ch, an ortholog of AdpA, as a "higher level" pleiotropic regulator of natamycin biosynthesis with bidirectional regulatory ability in Streptomyces chattanoogensis L10. DNase I footprinting revealed six AdpA_ch-binding sites in the scnRI-scnRII intergenic region. Further analysis using the xylE reporter gene fused to the scnRI-scnRII intergenic region of mutated binding sites demonstrated that the expression of scnRI and scnRII was under the control of AdpA_ch. AdpA_ch showed a bi-stable regulatory ability where it firstly binds to the Site C and Site D to activate the transcription of the two pathway-specific genes, scnRI and scnRII, and then binds to other sites where it acts as an inhibitor. When Site A and Site F were mutated in vivo, the production of natamycin was increased by 21% and 25%, respectively. These findings indicated an autoregulatory mechanism where AdpA_ch serves as a master switch with bidirectional regulation for natamycin biosynthesis.

Comparative Genomic and Regulatory Analyses of Natamycin Production of Streptomyces lydicus A02

Streptomyces lydicus A02 is used by industry because it has a higher natamycin-producing capacity than the reference strain S. natalensis ATCC 27448. We sequenced the complete genome of A02 using next-generation sequencing platforms, and to achieve better sequence coverage and genome assembly, we utilized single-molecule real-time (SMRT) sequencing. The assembled genome comprises a 9,307,519-bp linear chromosome with a GC content of 70.67%, and contained 8,888 predicted genes. Comparative genomics and natamycin biosynthetic gene cluster (BGC) analysis showed that BGC are highly conserved among evolutionarily diverse strains, and they also shared closer genome evolution compared with other Streptomyces species. Forty gene clusters were predicted to involve in the secondary metabolism of A02, and it was richly displayed in two-component signal transduction systems (TCS) in the genome, indicating a complex regulatory systems and high diversity of metabolites. Disruption of the phoP gene of the phoR-phoP TCS and nsdA gene confirmed phosphate sensitivity and global negative regulation of natamycin production. The genome sequence and analyses presented in this study provide an important molecular basis for research on natamycin production in Streptomyces, which could facilitate rational genome modification to improve the industrial use of A02.

Regulatory genes and their roles for improvement of antibiotic biosynthesis in Streptomyces

The numerous secondary metabolites in Streptomyces spp. are crucial for various applications. For example, cephamycin C is used as an antibiotic, and avermectin is used as an insecticide. Specifically, antibiotic yield is closely related to many factors, such as the external environment, nutrition (including nitrogen and carbon sources), biosynthetic efficiency and the regulatory mechanisms in producing strains. There are various types of regulatory genes that work in different ways, such as pleiotropic (or global) regulatory genes, cluster-situated regulators, which are also called pathway-specific regulatory genes, and many other regulators. The study of regulatory genes that influence antibiotic biosynthesis in Streptomyces spp. not only provides a theoretical basis for antibiotic biosynthesis in Streptomyces but also helps to increase the yield of antibiotics via molecular manipulation of these regulatory genes. Currently, more and more emphasis is being placed on the regulatory genes of antibiotic biosynthetic gene clusters in Streptomyces spp., and many studies on these genes have been performed to improve the yield of antibiotics in Streptomyces. This paper lists many antibiotic biosynthesis regulatory genes in Streptomyces spp. and focuses on frequently investigated regulatory genes that are involved in pathway-specific regulation and pleiotropic regulation and their applications in genetic engineering.

Constitutive overexpression of asm18 increases the production and diversity of maytansinoids in Actinosynnema pretiosum

Ansamitocins isolated from Actinosynnema pretiosum, potent antitumor compounds, belong to the family of maytansinoids, and the antibody-maytansinoid conjugates are currently under different phases of clinical trials. The clinical applications of ansamitocins have stimulated extensive studies to improve their production yields. In this study, we investigated the function of a pathway-specific S treptomyces antibiotic regulatory protein (SARP) family regulator, Asm18, and observed that ectopic overexpression of the asm18 gene increased the production of N-demethyl-4,5-desepoxy-maytansinol (2) to 50 mg/L in the HGF052 + pJTU824-asm18 strain, an increase by 4.7-fold compared to that of the control strain HGF052 + pJTU824. Real-time PCR analysis showed that the overexpression of the asm18 gene selectively increased the transcription levels of the genes involved in the biosynthesis of the starter unit (asm43), polyketide assembly (asmA), post-PKS modification (asm21), as well as the transcription levels of the regulatory gene (asm8), which is a specific LAL-type activator in ansamitocin biosynthesis. With the increase of fermentation titre, seven ansamitocin analogs (1-7) including three new ones (1, 5, and 6) and maytansinol (7) were isolated from the HGF052 + pJTU824-asm18 strain. Our results not only pave the way for further improving the production of ansamitocin analogs but also indicate that the post-PKS modifications of ansamitocin biosynthesis are flexible, which brings a potential of producing maytansinol, the most fascinating intermediate for the synthesis of antibody-maytansinoid conjugates, by optimizing the HGF052 and/or HGF052 + pJTU824-asm18 strains.

Overproduction of lactimidomycin by cross-overexpression of genes encoding Streptomyces antibiotic regulatory proteins

The glutarimide-containing polyketides represent a fascinating class of natural products that exhibit a multitude of biological activities. We have recently cloned and sequenced the biosynthetic gene clusters for three members of the glutarimide-containing polyketides-iso-migrastatin (iso-MGS) from Streptomyces platensis NRRL 18993, lactimidomycin (LTM) from Streptomyces amphibiosporus ATCC 53964, and cycloheximide (CHX) from Streptomyces sp. YIM56141. Comparative analysis of the three clusters identified mgsA and chxA, from the mgs and chx gene clusters, respectively, that were predicted to encode the PimR-like Streptomyces antibiotic regulatory proteins (SARPs) but failed to reveal any regulatory gene from the ltm gene cluster. Overexpression of mgsA or chxA in S. platensis NRRL 18993, Streptomyces sp. YIM56141 or SB11024, and a recombinant strain of Streptomyces coelicolor M145 carrying the intact mgs gene cluster has no significant effect on iso-MGS or CHX production, suggesting that MgsA or ChxA regulation may not be rate-limiting for iso-MGS and CHX production in these producers. In contrast, overexpression of mgsA or chxA in S. amphibiosporus ATCC 53964 resulted in a significant increase in LTM production, with LTM titer reaching 106 mg/L, which is five-fold higher than that of the wild-type strain. These results support MgsA and ChxA as members of the SARP family of positive regulators for the iso-MGS and CHX biosynthetic machinery and demonstrate the feasibility to improve glutarimide-containing polyketide production in Streptomyces strains by exploiting common regulators.

Biotechnological production and application of the antibiotic pimaricin: biosynthesis and its regulation

Pimaricin (natamycin) is a small polyene macrolide antibiotic used worldwide. This efficient antimycotic and antiprotozoal agent, produced by several soil bacterial species of the genus Streptomyces, has found application in human therapy, in the food and beverage industries and as pesticide. It displays a broad spectrum of activity, targeting ergosterol but bearing a particular mode of action different to other polyene macrolides. The biosynthesis of this only antifungal agent with a GRAS status has been thoroughly studied, which has permitted the manipulation of producers to engineer the biosynthetic gene clusters in order to generate several analogues. Regulation of its production has been largely unveiled, constituting a model for other polyenes and setting the leads for optimizing the production of these valuable compounds. This review describes and discusses the molecular genetics, uses, mode of action, analogue generation, regulation and strategies for increasing pimaricin production yields.

Genetic manipulation of secondary metabolite biosynthesis for improved production in Streptomyces and other actinomycetes

Actinomycetes continue to be important sources for the discovery of secondary metabolites for applications in human medicine, animal health, and crop protection. With the maturation of actinomycete genome mining as a robust approach to identify new and novel cryptic secondary metabolite gene clusters, it is critical to continue developing methods to activate and enhance secondary metabolite biosynthesis for discovery, development, and large-scale manufacturing. This review covers recent reports on promising new approaches and further validations or technical improvements of existing approaches to strain improvement applicable to a wide range of Streptomyces species and other actinomycetes.

Functional analysis of filipin tailoring genes from Streptomyces filipinensis reveals alternative routes in filipin III biosynthesis and yields bioactive derivatives

Background

Streptomyces filipinensis is the industrial producer of filipin, a pentaene macrolide, archetype of non-glycosylated polyenes, and widely used for the detection and the quantitation of cholesterol in biological membranes and as a tool for the diagnosis of Niemann–Pick type C disease. Genetic manipulations of polyene biosynthetic pathways have proven useful for the discovery of products with improved properties. Here, we describe the late biosynthetic steps for filipin III biosynthesis and strategies for the generation of bioactive filipin III derivatives at high yield.

Results

A region of 13,778 base pairs of DNA from the S. filipinensis genome was isolated, sequenced, and characterized. Nine complete genes and two truncated ORFs were located. Disruption of genes proved that this genomic region is part of the biosynthetic cluster for the 28-membered ring of the polyene macrolide filipin. This set of genes includes two cytochrome P450 monooxygenase encoding genes, filC and filD, which are proposed to catalyse specific hydroxylations of the macrolide ring at C26 and C1′ respectively. Gene deletion and complementation experiments provided evidence for their role during filipin III biosynthesis. Filipin III derivatives were accumulated by the recombinant mutants at high yield. These have been characterized by mass spectrometry and nuclear magnetic resonance following high-performance liquid chromatography purification thus revealing the post-polyketide steps during polyene biosynthesis. Two alternative routes lead to the formation of filipin III from the initial product of polyketide synthase chain assembly and cyclization filipin I, one trough filipin II, and the other one trough 1′-hydroxyfilipin I, all filipin III intermediates being biologically active. Moreover, minimal inhibitory concentration values against Candida utilis and Saccharomyces cerevisiae were obtained for all filipin derivatives, finding that 1′-hydroxyfilipin and especially filipin II show remarkably enhanced antifungal bioactivity. Complete nuclear magnetic resonance assignments have been obtained for the first time for 1′-hydroxyfilipin I.

Conclusions

This report reveals the existence of two alternative routes for filipin III formation and opens new possibilities for the generation of biologically active filipin derivatives at high yield and with improved properties.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0307-4) contains supplementary material, which is available to authorized users.

Engineered biosynthesis of pimaricin derivatives with improved antifungal activity and reduced cytotoxicity

Pimaricin is an important antifungal antibiotic for antifungal therapy and prevention of mould contamination in the food industry. In this study, three new pimaricin derivatives, 12-decarboxy-12-methyl pimaricin (1), 4,5-desepoxy-12-decarboxy-12-methyl pimaricin (2), and 2-hydro-3-hydroxy-4,5-desepoxy-12-decarboxy-12-methyl pimaricin (3), were generated through the inactivation of P450 monooxygenase gene scnG in Streptomyces chattanoogensis L10. Compared with pimaricin, 1 displayed a twofold increase in antifungal activity against Candida albicans ATCC 14053 and a 4.5-fold decrease in cytotoxicity with erythrocytes, and 2 had comparable antifungal activity and reduced cytotoxicity, whereas 3 showed nearly no antifungal and hemolytic activities. Genetic and biochemical analyses proved that 1 is converted from 2 by P450 monooxygenase ScnD. Therefore, the overexpression of scnD in scnG-null strain eliminated the accumulation of 2 and improved the yield of 1 by 20 %. Conversely, scnG/scnD double mutation abolished the production of 1 and improved the yield of 2 to 2.3-fold. These results indicate that the pimaricin derivatives with improved pharmacological properties obtained by genetic engineering can be further developed into antifungal agents for potential clinical application.

Identification of a cluster-situated activator of oxytetracycline biosynthesis and manipulation of its expression for improved oxytetracycline production in Streptomyces rimosus

Background

Oxytetracycline (OTC) is a broad-spectrum antibiotic commercially produced by Streptomyces rimosus. Despite its importance, little is known about the regulation of OTC biosynthesis, which hampered any effort to improve OTC production via engineering regulatory genes.

Results

A gene encoding a Streptomyces antibiotic regulatory protein (SARP) was discovered immediately adjacent to the otrB gene of oxy cluster in S. rimosus and designated otcR. Deletion and complementation of otcR abolished or restored OTC production, respectively, indicating that otcR encodes an essential activator of OTC biosynthesis. Then, the predicted consensus SARP-binding sequences were extracted from the promoter regions of oxy cluster. Transcriptional analysis in a heterologous GFP reporter system demonstrated that OtcR directly activated the transcription of five oxy promoters in E. coli, further mutational analysis of a SARP-binding sequence of oxyI promoter proved that OtcR directly interacted with the consensus repeats. Therefore, otcR was chosen as an engineering target, OTC production was significantly increased by overexpression of otcR as tandem copies each under the control of strong SF14 promoter.

Conclusions

A SARP activator, OtcR, was identified in oxy cluster of S. rimosus; it was shown to directly activate five promoters from oxy cluster. Overexpression of otcR at an appropriate level dramatically increased OTC production by 6.49 times compared to the parental strain, thus demonstrating the great potential of manipulating OtcR to improve the yield of OTC production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0231-7) contains supplementary material, which is available to authorized users.

Pathway-specific regulation revisited: cross-regulation of multiple disparate gene clusters by PAS-LuxR transcriptional regulators

PAS-LuxR regulators are highly conserved proteins devoted to the control of antifungal production by binding to operators located in given promoters of polyene biosynthetic genes. The canonical operator of PimM, archetype of this class of regulators, has been used here to search for putative targets of orthologous protein PteF in the genome of Streptomyces avermitilis, finding 97 putative operators outside the pentaene filipin gene cluster (pte). The processes putatively affected included genetic information processing; energy, carbohydrate, and lipid metabolism; DNA replication and repair; morphological differentiation; secondary metabolite biosynthesis; and transcriptional regulation, among others. Seventeen of these operators were selected, and their binding to PimM DNA-binding domain was assessed by electrophoretic mobility shift assays. Strikingly, the protein bound all predicted operators suggesting a direct control over targeted processes. As a proof of concept, we studied the biosynthesis of the ATP-synthase inhibitor oligomycin whose gene cluster included two operators. Regulator mutants showed a severe loss of oligomycin production, whereas gene complementation of the mutant restored phenotype, and gene duplication in the wild-type strain boosted oligomycin production. Comparative gene expression analyses in parental and mutant strains by reverse transcription-quantitative polymerase chain reaction of selected olm genes corroborated production results. These results demonstrate that PteF is able to cross-regulate the biosynthesis of two related secondary metabolites, filipin and oligomycin, but might be extended to all the processes indicated above. This study highlights the complexity of the network of interactions in which PAS-LuxR regulators are involved and opens new possibilities for the manipulation of metabolite production in Streptomycetes.

Production of the antibiotic FR-008/candicidin in Streptomyces sp. FR-008 is co-regulated by two regulators, FscRI and FscRIV, from different transcription factor families

In Streptomyces sp. FR-008, the biosynthetic gene cluster of the polyene antibiotic FR-008, also known as candicidin, consists of 21 genes, including four regulatory genes, fscRI-fscRIV. Our bioinformatics analyses indicate that FscRI has an N-terminal PAS domain, whereas the other three regulators have N-terminal AAA domains and are members of the LAL (large ATP-binding regulators of the LuxR type) family. Deletion of fscRI abolished the production of FR-008, with production restored in the complemented strain, supporting a critical role for FscRI in FR-008 biosynthesis. Consistent with these findings, transcription of genes involved in the biosynthesis and efflux of FR-008 was greatly downregulated in a ΔfscRI mutant. Interestingly, the regulatory gene fscRIV was also downregulated in the ΔfscRI mutant. Production of FR-008 was reduced, but not abrogated, in an fscRIV deletion mutant, and although structural genes were downregulated in ΔfscRIV, the changes were much less dramatic than in ΔfscRI, suggesting a stronger regulatory role for FscRI. Remarkably, transcription of fscRI was also decreased in ΔfscRIV. Expression of fscRI restored antibiotic production in a ΔfscRIV mutant, but not vice versa. Putative binding sequences for FscRI were identified upstream of fscRIV and the three structural genes fscA, fscB and fscD, which encode large modular polyketide synthases. Our findings suggest that fscRI and fscRIV are interregulatory, whereas expression of fscRII and fscRIII appears to be independent of fscRI and fscRIV. This study demonstrates that the regulation of polyene antibiotic synthesis can involve mutually regulated transcriptional activators that belong to different families.

PAS-LuxR transcriptional control of filipin biosynthesis in S. avermitilis

The DNA region encoding the filipin gene cluster in Streptomyces avermitilis (pte) contains a PAS-LuxR regulatory gene, pteF, orthologue to pimM, the final pathway-specific positive regulatory protein of pimaricin biosynthesis in Streptomyces natalensis. Gene replacement of the gene from S. avermitilis chromosome resulted in a severe loss of filipin production and delayed spore formation in comparison to that of the wild-type strain, suggesting that it acts as a positive regulator of filipin biosynthesis and that it may also have a role in sporulation. Complementation of the mutant with a single copy of the gene integrated into the chromosome restored wild-type phenotypes. Heterologous complementation with the regulatory counterpart from S. natalensis also restored parental phenotypes. Gene expression analyses in S. avermitilis wild-type and the mutant by reverse transcription-quantitative polymerase chain reaction of the filipin gene cluster suggested the targets for the regulatory protein. Transcription start points of all the genes of the cluster were studied by 5'-rapid amplification of complementary DNA ends. Transcription start point analysis of the pteF gene revealed that the annotated sequence in the databases is incorrect. Confirmation of target promoters was performed by in silico search of binding sites among identified promoters and the binding of the orthologous regulator for pimaricin biosynthesis PimM to gene promoters by electrophoretic mobility shift assays. Precise binding regions were investigated by DNAse I protection studies. Our results indicate that PteF activates the transcription from two promoters of polyketide synthase genes directly, and indirectly of other genes of the cluster.

Transcriptomic analysis of Streptomyces clavuligerus ΔccaR::tsr: effects of the cephamycin C-clavulanic acid cluster regulator CcaR on global regulation

Streptomyces clavuligerus ATCC 27064 and S. clavuligerus ΔccaR::tsr cultures were grown in asparagine-starch medium, and samples were taken in the exponential and stationary growth phases. Transcriptomic analysis showed that the expression of 186 genes was altered in the ccaR-deleted mutant. These genes belong to the cephamycin C gene cluster, clavulanic acid gene cluster, clavams, holomycin, differentiation, carbon, nitrogen, amino acids or phosphate metabolism and energy production. All the clavulanic acid biosynthesis genes showed Mc values in the order of -4.23. The blip gene-encoding a β-lactamase inhibitory protein was also controlled by the cephamycin C-clavulanic acid cluster regulator (Mc -2.54). The expression of the cephamycin C biosynthesis genes was greatly reduced in the mutant (Mc values up to -7.1), while the genes involved in putative β-lactam resistance were less affected (Mc average -0.88). Genes for holomycin biosynthesis were upregulated. In addition, the lack of clavulanic acid and cephamycin production negatively affected the expression of genes for the clavulanic acid precursor arginine and of miscellaneous genes involved in nitrogen metabolism (amtB, glnB, glnA3, glnA2, glnA1). The transcriptomic results were validated by quantative reverse transcription polymerase chain reaction and luciferase assay of luxAB-coupled promoters. Transcriptomic analysis of the homologous genes of S. coelicolor validated the results obtained for S. clavuligerus primary metabolism genes.

Genome-wide analysis of the regulation of pimaricin production in Streptomyces natalensis by reactive oxygen species

To investigate the molecular mechanisms that interplay between oxygen metabolism and secondary metabolism in Streptomyces natalensis, we compared the transcriptomes of the strains CAM.02 (ΔsodF), pimaricin under-producer phenotype, and CAM.04 (ΔahpCD), pimaricin over-producer phenotype, with that of the wild type at late exponential and stationary growth phases. Microarray data interpretation was supported by characterization of the mutant strains regarding enzymatic activities, phosphate uptake, oxygen consumption and pimaricin production.Both mutant strains presented a delay in the transcription activation of the PhoRP system and pimaricin biosynthetic gene cluster that correlated with the delayed inorganic phosphate (Pi) depletion in the medium and late onset of pimaricin production, respectively. The carbon flux of both mutants was also altered: a re-direction from glycolysis to the pentose phosphate pathway (PPP) in early exponential phase followed by a transcriptional activation of both pathways in subsequent growth phases was observed. Mutant behavior diverged at the respiratory chain/tricarboxylic acid cycle (TCA) and the branched chain amino acid (BCAA) metabolism. CAM.02 (ΔsodF) presented an impaired TCA cycle and an inhibition of the BCAA biosynthesis and degradation pathways. Conversely, CAM.04 (ΔahpCD) presented a global activation of BCAA metabolism.The results highlight the cellular NADPH/NADH ratio and the availability of biosynthetic precursors via the BCAA metabolism as the main pimaricin biosynthetic bottlenecks under oxidative stress conditions. Furthermore, new evidences are provided regarding a crosstalk between phosphate metabolism and oxidative stress in Streptomyces.

Improvement of natamycin production by engineering of phosphopantetheinyl transferases in Streptomyces chattanoogensis L10

Phosphopantetheinyl transferases (PPTases) are essential to the activities of type I/II polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs) through converting acyl carrier proteins (ACPs) in PKSs and peptidyl carrier proteins (PCPs) in NRPSs from inactive apo-forms into active holo-forms, leading to biosynthesis of polyketides and nonribosomal peptides. The industrial natamycin (NTM) producer, Streptomyces chattanoogensis L10, contains two PPTases (SchPPT and SchACPS) and five PKSs. Biochemical characterization of these two PPTases shows that SchPPT catalyzes the phosphopantetheinylation of ACPs in both type I PKSs and type II PKSs, SchACPS catalyzes the phosphopantetheinylation of ACPs in type II PKSs and fatty acid synthases (FASs), and the specificity of SchPPT is possibly controlled by its C terminus. Inactivation of SchPPT in S. chattanoogensis L10 abolished production of NTM but not the spore pigment, while overexpression of the SchPPT gene not only increased NTM production by about 40% but also accelerated productions of both NTM and the spore pigment. Thus, we elucidated a comprehensive phosphopantetheinylation network of PKSs and improved polyketide production by engineering the cognate PPTase in bacteria.

Molecular regulation of antibiotic biosynthesis in streptomyces

Streptomycetes are the most abundant source of antibiotics. Typically, each species produces several antibiotics, with the profile being species specific. Streptomyces coelicolor, the model species, produces at least five different antibiotics. We review the regulation of antibiotic biosynthesis in S. coelicolor and other, nonmodel streptomycetes in the light of recent studies. The biosynthesis of each antibiotic is specified by a large gene cluster, usually including regulatory genes (cluster-situated regulators [CSRs]). These are the main point of connection with a plethora of generally conserved regulatory systems that monitor the organism's physiology, developmental state, population density, and environment to determine the onset and level of production of each antibiotic. Some CSRs may also be sensitive to the levels of different kinds of ligands, including products of the pathway itself, products of other antibiotic pathways in the same organism, and specialized regulatory small molecules such as gamma-butyrolactones. These interactions can result in self-reinforcing feed-forward circuitry and complex cross talk between pathways. The physiological signals and regulatory mechanisms may be of practical importance for the activation of the many cryptic secondary metabolic gene cluster pathways revealed by recent sequencing of numerous Streptomyces genomes.

Biosynthesis of clavam metabolites

Naturally occurring clavam metabolites include the valuable β-lactamase inhibitor, clavulanic acid, as well as stereochemical variants with side-chain modifications, called the 5S clavams. Because of the clinical importance of clavulanic acid, most studies of clavam biosynthesis are based on the industrial producer species Streptomyces clavuligerus. Well-characterized early steps in clavam biosynthesis are outlined, and less well understood late steps in 5S clavam biosynthesis are proposed. The complex genetic organization of the clavam biosynthetic genes in S. clavuligerus is described and, where possible, comparisons with other producer species are presented.

Genome mining of the biosynthetic gene cluster of the polyene macrolide antibiotic tetramycin and characterization of a P450 monooxygenase involved in the hydroxylation of the tetramycin B polyol segment

A polyene macrolide antibiotic tetramycin biosynthetic gene cluster was identified by genome mining and isolated from Streptomyces hygrospinosus var. beijingensis. Genetic and in silico analyses gave insights into the mechanism of biosynthesis of tetramycin, and a model of the tetramycin biosynthetic pathway is proposed. Inactivation of a cytochrome P450 monooxygenase gene, tetrK, resulted in the production of a tetramycin B precursor: tetramycin A, which lacks a hydroxy group in its polyol region. TetrK was subsequently overexpressed heterologously in E. coli with a His(6) tag, and purified TetrK efficiently hydroxylated tetramycin A to afford tetramycin B. Kinetic studies revealed no inhibition of TetrK by substrate or product. Surprisingly, sequence-alignment analysis showed that TetrK, as a hydroxylase, has much higher homology with epoxidase PimD than with hydroxylases NysL and AmphL. The 3D structure of TetrK was then constructed by homology modeling with PimD as reference. Although TetrK and PimD catalyzed different chemical reactions, homology modeling indicated that they might share the same catalytic sites, despite also possessing some different sites correlated with substrate binding and substrate specificity. These findings offer good prospects for the production of improved antifungal polyene analogues.

Identification of phoslactomycin biosynthetic gene clusters from Streptomyces platensis SAM-0654 and characterization of PnR1 and PnR2 as positive transcriptional regulators

Phoslactomycins (PLMs) are inhibitors of protein serine/threonine phosphatase 2A showing diverse and important antifungal, antibacterial and antitumor activity. PLMs are polyketide natural products and produced by several Streptomyces species. The PLMs biosynthetic gene clusters were identified from Streptomyces platensis SAM-0654 and localized in two separate genomic regions, consisting of 27 open reading frames that encode polyketide synthases (PKSs), enzymes for cyclohexanecarboxyl-CoA (CHC-CoA) and ethylmalonyl-CoA (Em-CoA) synthesis, enzymes for post-PKS modifications, proposed regulators, and putative resistance transporters. Bioinformatic analysis and inactivation experiment of regulatory genes suggest that PnR1 and PnR2 are two positive regulators of PLMs biosynthesis. Gene transcription analysis by reverse transcriptase PCR (RT-PCR) of the PLMs gene cluster demonstrated that PnR1 and PnR2 activate the transcription of the structural biosynthetic genes while PnR2 specially governs the transcription of pnR1 in a higher level.

5S clavam biosynthesis is controlled by an atypical two-component regulatory system in Streptomyces clavuligerus

Streptomyces clavuligerus produces a collection of five clavam metabolites, including the clinically important β-lactamase inhibitor clavulanic acid, as well as four structurally related metabolites called 5S clavams. The paralogue gene cluster of S. clavuligerus is one of three clusters of genes for the production of these clavam metabolites. A region downstream of the cluster was analyzed, and snk, res1, and res2, encoding elements of an atypical two-component regulatory system, were located. Mutation of any one of the three genes had no effect on clavulanic acid production, but snk and res2 mutants produced no 5S clavams, whereas res1 mutants overproduced 5S clavams. Reverse transcriptase PCR analyses showed that transcription of cvm7p (which encodes a transcriptional activator of 5S clavam biosynthesis) and 5S clavam biosynthetic genes was eliminated in snk and in res2 mutants but that snk and res2 transcription was unaffected in a cvm7p mutant. Both snk and res2 mutants could be complemented by introduction of cvm7p under the control of an independently regulated promoter. In vitro assays showed that Snk can autophosphorylate and transfer its phosphate group to both Res1 and Res2, and Snk-H365, Res1-D52, and Res2-D52 were identified as the phosphorylation sites for the system. Dephosphorylation assays indicated that Res1 stimulates dephosphorylation of Res2∼P. These results suggest a regulatory cascade in which Snk and Res2 form a two-component system controlling cvm7p transcription, with Res1 serving as a checkpoint to modulate phosphorylation levels. Cvm7P then activates transcription of 5S clavam biosynthetic genes.

Hierarchical control on polyene macrolide biosynthesis: PimR modulates pimaricin production via the PAS-LuxR transcriptional activator PimM

Control of polyene macrolide production in Streptomyces natalensis is mediated by the transcriptional activator PimR. This regulator combines an N-terminal domain corresponding to the Streptomyces antibiotic regulatory protein (SARP) family of transcriptional activators with a C-terminal half homologous to guanylate cyclases and large ATP-binding regulators of the LuxR family. The PimR SARP domain (PimR(SARP)) was expressed in Escherichia coli as a glutathione S-transferase (GST)-fused protein. Electrophoretic mobility shift assays showed that GST-PimR(SARP) binds a single target, the intergenic region between the regulatory genes pimR and pimMs in the pimaricin cluster. The PimR(SARP)-binding site was investigated by DNaseI protection studies, revealing that it contains three heptameric direct repeats adjusting to the consensus 5'-CGGCAAG-3'. Transcription start points of pimM and pimR promoters were identified by 5'-RACE, revealing that unlike other SARPs, PimR(SARP) does not interact with the -35 region of its target promoter. Quantitative transcriptional analysis of these regulatory genes on mutants on each of them has allowed the identification of the pimM promoter as the transcriptional target for PimR. Furthermore, the constitutive expression of pimM restored pimaricin production in a pimaricin-deficient strain carrying a deletion mutant of pimR. These results reveal that PimR exerts its positive effect on pimaricin production by controlling pimM expression level, a regulator whose gene product activates transcription from eight different promoters of pimaricin structural genes directly.

Cascades and networks of regulatory genes that control antibiotic biosynthesis

Onset of the biosynthesis of bioactive secondary metabolites in batch cultures of actinomycetes occurs after the rapid growth phase, following a transition phase which involves complex metabolic changes. This transition is triggered by nutrient starvation or by other environmental stress signals. Expression of genes encoding bioactive secondary metabolites is governed by cascades of pathway specific regulators and networks of cross-talking global regulators. Pathway specific regulators such as Streptomyces antibiotic regulatory proteins, LAL-type and LysR-type regulators respond to autoregulatory proteins that act in concert with their cognate ligands (e.g. γ-butyrolactone receptor proteins and their cognate γ-butyrolactone ligands). Global regulators such as PhoR-PhoP and other two component systems and orphan response regulators, such as GlnR, control set of genes affecting primary and secondary metabolism. GlnR and, therefore, nitrogen metabolism genes are under phosphate control exerted by binding of PhoP to PHO boxes located in the promoter region of GlnR. A few pleiotropic regulatory genes, such as areB (ndgR), dmdR1 or dasR connect primary metabolism (amino acid biosynthesis, N-acetylglucosamine or iron levels) with antibiotic biosynthesis. Some atypical response regulators that require specific small ligands appear to be involved in feedback control of antibiotic production. All these mechanisms together modulate, in a coordinated manner, different aspects of Streptomyces metabolism as a real "protection net" that prevents drastic changes in metabolism that may be deleterious for cell survival.

Waking up Streptomyces secondary metabolism by constitutive expression of activators or genetic disruption of repressors

Streptomycete bacteria are renowned as a prolific source of natural products with diverse biological activities. Production of these metabolites is often subject to transcriptional regulation: the biosynthetic genes remain silent until the required environmental and/or physiological signals occur. Consequently, in the laboratory environment, many gene clusters that direct the biosynthesis of natural products with clinical potential are not expressed or at very low level preventing the production/detection of the associated metabolite. Genetic engineering of streptomycetes can unleash the production of many new natural products. This chapter describes the overexpression of pathway-specific activators, the genetic disruption of pathway-specific repressors, and the main strategy used to identify and characterize new natural products from these engineered Streptomyces strains.

Crosstalk between ROS homeostasis and secondary metabolism in S. natalensis ATCC 27448: modulation of pimaricin production by intracellular ROS

Streptomyces secondary metabolism is strongly affected by oxygen availability. The increased culture aeration enhances pimaricin production in S. natalensis, however the excess of O₂ consumption can lead to an intracellular ROS imbalance that is harmful to the cell. The adaptive physiological response of S. natalensis upon the addition of exogenous H₂O₂ suggested that the modulation of the intracellular ROS levels, through the activation of the H₂O₂ inducible catalase during the late exponential growth phase, can alter the production of pimaricin. With the construction of defective mutants on the H₂O₂ related enzymes SodF, AhpCD and KatA1, an effective and enduring modulation of intracellular ROS was achieved. Characterization of the knock-out strains revealed different behaviours regarding pimaricin production: whilst the superoxide dismutase defective mutant presented low levels of pimaricin production compared to the wild-type, the mutants defective on the H₂O₂-detoxifying enzymes displayed a pimaricin overproducer phenotype. Using physiological and molecular approaches we report a crosstalk between oxidative stress and secondary metabolism regulatory networks. Our results reveal that the redox-based regulation network triggered by an imbalance of the intracellular ROS homeostasis is also able to modulate the biosynthesis of pimaricin in S. natalensis.

Functional conservation of PAS-LuxR transcriptional regulators in polyene macrolide biosynthesis

Control of polyene macrolide production in Streptomyces natalensis is mediated by the PAS-LuxR transcriptional activator PimM. Expression of target genes in this strain is positively regulated by binding of the regulator to 14-nucleotide sites showing dyad symmetry, and overlapping the -35 element of each promoter. These sequences have been found in the upstream regions of genes belonging to different polyene biosynthetic gene clusters. All the sequences in the amphotericin, nystatin, and filipin clusters were cloned and the binding of PimM to all of them has been shown by electrophoretic mobility shift assays. The precise binding regions were investigated by DNaseI protection studies. Results indicated that PAS-luxR regulators share the same regulatory pattern in different polyene-producing strains, these genes being responsible for polyketide chain construction, and when available, the genes for sugar dehydration and attachment, and the ABC transporters, the targets for regulation. Information content analysis of the 24 sequences protected in target promoters was used to refine the information-based model of the binding site. This site now spans 16 nucleotides and adjusts to the consensus CTVGGGAWWTCCCBAG. Gene complementation of S. natalensis ΔpimM with a single copy of heterologous regulators of the PAS/LuxR class integrated into the chromosome, such as amphRIV, nysRIV, or pteF, restored antifungal production, thus proving the functional conservation of these regulators. Introduction of a single copy of pimM into the amphotericin producing strain Streptomyces nodosus, or into the filipin producing strain S. avermitilis, boosted the production of both polyenes, thus indicating that the expression of the PAS-LuxR regulator constitutes a bottleneck in the biosynthesis of the antifungal, and also that these regulators are fully exchangeable. This work is the first report of a general mechanism regulating polyene production.

Identification and characterization of wblA-dependent tmcT regulation during tautomycetin biosynthesis in Streptomyces sp. CK4412

Tautomycetin (TMC) is an unusual linear polyketide compound esterified with a cyclic anhydride. It exhibits novel activated T cell-specific immunosuppressant as well as anti-cancer activities. Previously, we isolated and characterized the entire TMC biosynthetic gene cluster from Streptomyces sp. CK4412, including a TMC pathway-specific gene, tmcN, the over-expression of which led to a significant increase in TMC productivity. In addition, we also reported that WblA acts as a global down-regulator of antibiotic biosynthesis through pathway-specific regulation in Streptomyces species. Here, we confirm that TmcT acts as another TMC pathway-specific regulator within the TMC biosynthetic cluster. Specifically, tmcT deletion resulted in the complete loss of TMC production, whereas complementation with a tmcT-carrying integrative plasmid significantly restored TMC biosynthesis. We also identified a 0.39kb wblA ortholog (named wblA(tmc)) from Streptomyces sp. CK4412 via genomic DNA library screening that showed 96% amino acid identity compared to a previously-known S. coelicolor wblA. Targeted gene disruption of wblA(tmc) in Streptomyces sp. CK4412 exhibited approximately 3-fold higher TMC productivity than that in the wild-type strain. Moreover, transcription analyses of the TMC biosynthetic and regulatory genes revealed that the expression of tmcT was strongly down-regulated by wblA(tmc). These results imply that the TMC biosynthetic regulation network is controlled by two pathway-specific positive regulator, WblA(tmc)-dependent TmcT as well as WblA(tmc)-independent TmcN in Streptomyces sp. CK4412.

The pleitropic regulator AdpAch is required for natamycin biosynthesis and morphological differentiation in Streptomyces chattanoogensis

The complete natamycin (NTM) biosynthetic gene cluster of Streptomyces chattanoogensis was cloned and confirmed by the disruption of pathway-specific activator genes. Comparative cluster analysis with its counterpart in Streptomyces natalensis revealed different cluster architecture between these two clusters. Compared with the highly conserved coding sequences, sequence variations appear to occur frequently in the intergenic regions. The evolutionary change of nucleotide sequence in the intergenic regions has given rise to different transcriptional organizations in the two clusters and resulted in altered gene regulation. These results provide insight into the evolution of antibiotic biosynthetic gene clusters. In addition, we cloned a pleitropic regulator gene, adpAch , in S. chattanoogensis. Using the genetic system that we developed for this strain, adpAch was deleted from the genome of S. chattanoogensis. The ΔadpAch mutant showed a conditionally sparse aerial mycelium formation phenotype and defects in sporulation; it also lost the ability to produce NTM and a diffusible yellow pigment normally produced by S. chattanoogensis. RT-PCR analysis revealed that transcription of adpAch was constitutive in YEME liquid medium. By using rapid amplification of 5′ complementary DNA ends, two transcription start sites were identified upstream of the adpAch coding region. Quantitative transcriptional analysis showed that the expression level of the NTM regulatory gene scnRI decreased 20-fold in the ΔadpAch mutant strain, while the transcription of the other activator gene scnRII was not significantly affected. Electrophoretic mobility shift assay (EMSA) showed that AdpAch binds to its own promoter but fails to bind to the promoter region of scnRI, indicating that the control of scnRI by AdpAch is exerted in an indirect way. This work not only provides a platform and a new potential target for increasing the titre of NTM by genetic manipulation, but also advances the understanding of the regulation of NTM biosynthesis.

Enhanced production of aureofuscin by over-expression of AURJ3M, positive regulator of aureofuscin biosynthesis in Streptomyces aureofuscus

AIMS

 The production of aureofuscin is very low in the wild-type strain. We attempt to increase the production of aureofuscin by over-expression of a controlling gene in the wild-type strain.

METHODS AND RESULTS

The aurj3M gene was PCR-amplified from Streptomyces aureofuscus SYAU0709, ligated into vector pMD19 and sequenced. The predicted translation of the 579-bp cloned fragment was 97% similar to pimM from Streptomyces natalensis, which has an N-terminal PAS domain and a LuxR-type C-terminal helix-turn-helix. Recombinant bacterial strains were constructed by transforming SYAU0709 with an expression plasmid (pBJJ3M) that contained aurj3M, thereby increasing the number of aurj3M gene copies.

CONCLUSIONS

Bioassays for the antibiotic compound aureofuscin indicated that the recombinant bacteria had greater antifungal activity than the wild-type strain. Specifically, the recombinant strain produced approx. 600% more aureofuscin, as quantified by high-performance liquid chromatography analysis.

SIGNIFICANCE AND IMPACT OF THE STUDY

To our knowledge, this approach has not been attempted in S. aureofuscus before and few genes in the aureofuscin pathway have been cloned and characterized.