wblA, a pleiotropic regulatory gene modulating morphogenesis and daptomycin production in Streptomyces roseosporus

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.

Transcriptional regulation of the fidaxomicin gene cluster and cellular development in Actinoplanes deccanensis YP-1 by the pleiotropic regulator MtrA

IMPORTANCE

Cascade regulation networks are almost present in various kinds of microorganisms, but locating and systematically elucidating specific pleiotropic regulators related to a certain gene cluster can be a tricky problem. Here, based on the promoter of the fidaxomicin pathway-specific regulator FadR1, we utilized a "DNA to Proteins" affinity purification method and captured a global regulator MtrA, which positively regulates fidaxomicin biosynthesis. In the mtrA overexpressed strain, the production of fidaxomicin was improved by 37% compared to the native strain. Then, we combined the "Protein to DNAs" affinity purification method (DAP-seq) with the results of RNA-seq and systematically elucidated the primary and secondary metabolic processes in which MtrA directly or indirectly participates. Thus, our work brought up a new way to improve fidaxomicin production from the perspective of global regulation and analyzed the regulatory mechanism of MtrA. Meanwhile, we provided a novel methodology for the research of cascade regulation networks and vital secondary metabolites.

Transcriptional Regulator DasR Represses Daptomycin Production through Both Direct and Cascade Mechanisms in Streptomyces roseosporus

Daptomycin, produced by Streptomyces roseosporus, is a clinically important cyclic lipopeptide antibiotic used for the treatment of human infections caused by drug-resistant Gram-positive pathogens. In contrast to most Streptomyces antibiotic biosynthetic gene clusters (BGCs), daptomycin BGC has no cluster-situated regulator (CSR) genes. DasR, a GntR-family transcriptional regulator (TR) widely present in the genus, was shown to regulate antibiotic production in model species S. coelicolor by binding to promoter regions of CSR genes. New findings reported here reveal that DasR pleiotropically regulates production of daptomycin and reddish pigment, and morphological development in S. roseosporus. dasR deletion enhanced daptomycin production and morphological development, but reduced pigment production. DasR inhibited daptomycin production by directly repressing dpt structural genes and global regulatory gene adpA (whose product AdpA protein activates daptomycin production and morphological development). DasR-protected regions on dptEp and adpAp contained a 16 nt sequence similar to the consensus DasR-binding site dre in S. coelicolor. AdpA was shown to target dpt structural genes and dptR2 (which encodes a DeoR-family TR required for daptomycin production). A 10 nt sequence similar to the consensus AdpA-binding site was found on target promoter regions dptAp and dptR2p. This is the first demonstration that DasR regulates antibiotic production both directly and through a cascade mechanism. The findings expand our limited knowledge of the regulatory network underlying daptomycin production, and will facilitate methods for construction of daptomycin overproducers.

Protein X0P338, a GntR-type pleiotropic regulator for morphological differentiation and secondary metabolites production in Streptomyces diastatochromogenes 1628

The nucleoside antibiotic, toyocamycin (TM) exhibits excellent potent activity against several phytopathogenic fungi. Despite of its importance, little is known about key factors regulating TM biosynthesis and morphological differentiation in S. diastatochromogenes 1628. Based on proteomics data obtained from the analysis between wild-type (WT) S. diastatochromogenes 1628 strain and mutant strain 1628-T62 having a low-yield of TM, we observed that the differentially expressed protein, X0P338, which was proposed to be a regulator of the GntR-family, exhibited a higher expression level in S. diastatochromogenes 1628. Therefore, in this study, to explore whether protein X0P338 was involved in morphological differentiation and biosynthesis of secondary metabolites, especially TM, the gene called the gntR _sd -encoding protein X0P338 was cloned and over-expressed in WT strain 1628 and mutant strain 1628-T62, respectively. The results indicated that the over-expression of gntR _sd enhanced TM production in both strain 1628 (120.6 mg/L vs. 306.6 mg/L) and strain 1628-T62 (15.6 mg/L vs. 258.9 mg/L). Besides, the over-expression of gntR _sd had positive and negative effects on morphological differentiation in strain 1628 and strain 1628-T62, respectively. The results also showed opposite effects on tetraene macrolide production during the over-expression of gntR _sd in strain 1628 and strain 1628-T62. Moreover, transcription levels of genes involved in morphological differentiation and secondary metabolites production were affected by the over-expression of gntR _sd gene, both in strain 1628 and strain 1628-T62. These results confirm that X0P338 as a GntR-type pleiotropic regulator that regulates the morphological differentiation and biosynthesis of secondary metabolites, and especially has a positive effect on TM biosynthesis. This article is protected by copyright. All rights reserved.

The regulatory gene wblA is a target of the orphan response regulator OrrA in Streptomyces coelicolor

Our previous study using transposon mutagenesis indicated that disruption of the putative response regulator gene orrA impacted antibiotic production in Streptomyces coelicolor. In this study, the role of OrrA was further characterized by comparing the phenotypes and transcriptomic profiles of the wild-type S. coelicolor strain M145 and ΔorrA, a strain with an inactivated orrA gene. Chromatin immunoprecipitation using a strain expressing OrrA fused with FLAG showed that OrrA binds the promoter of wblA, whose expression was downregulated in ΔorrA. The interaction of OrrA with the wblA promoter was further validated by a pull-down assay. Similar to ΔorrA, the deletion mutant of wblA (ΔwblA) was defective in development, and developmental genes were expressed at similar levels in ΔorrA and ΔwblA. Although both OrrA and WblA downregulated actinorhodin and undecylprodigiosin, their roles in regulation of the calcium-dependent antibiotic and yellow-pigmented type I polyketide differed. sco1375, a gene of unknown function, was identified as another OrrA target, and overexpression of either sco1375 or wblA in ΔorrA partially restored the wild-type phenotype, indicating that these genes mediate some of the effects of OrrA. This study revealed targets of OrrA and provided more insights into the role of the orphan response regulator OrrA in Streptomyces. This article is protected by copyright. All rights reserved.

Top-down synthetic biology approach for titer improvement of clinically important antibiotic daptomycin in Streptomyces roseosporus

Secondary metabolites are produced at low titers by native producers due to tight regulations of their productions in response to environmental conditions. Synthetic biology provides a rational engineering principle for transcriptional optimization of secondary metabolite BGCs (biosynthetic gene clusters). Here, we demonstrate the use of synthetic biology principles for the development of a high-titer strain of the clinically important antibiotic daptomycin. Due to the presence of large NRPS (non-ribosomal peptide synthetase) genes with multiple direct repeats, we employed a top-down approach that allows transcriptional optimization of genes in daptomycin BGC with the minimum inputs of synthetic DNAs. The repeat-free daptomycin BGC was created through partial codon-reprogramming of a NRPS gene and cloned into a shuttle BAC vector, allowing BGC refactoring in a host with a powerful recombination system. Then, transcriptions of functionally divided operons were sequentially optimized through three rounds of DBTL (design-build-test-learn) cycles that resulted in up to ~2300% improvement in total lipopeptide titers compared to the wild-type strain. Upon decanoic acid feeding, daptomycin accounted for ∼ 40% of total lipopeptide production. To the best of our knowledge, this is the highest improvement of daptomycin titer ever achieved through genetic engineering of S. roseosporus. The top-down engineering approach we describe here could be used as a general strategy for the development of high-titer industrial strains of secondary metabolites produced by BGCs containing genes of large multi-modular NRPS and PKS enzymes.

Activation of Cryptic Antibiotic Biosynthetic Gene Clusters Guided by RNA-seq Data from Both Streptomyces ansochromogenes and ΔwblA

With the increase of drug resistance caused by the improper use and abuse of antibiotics, human beings are facing a global health crisis. Sequencing of Streptomyces genomes revealed the presence of an important reservoir of secondary metabolic gene clusters for previously unsuspected products with potentially valuable bioactivity. It has therefore become necessary to activate these cryptic pathways through various strategies. Here, we used RNA-seq data to perform a comparative transcriptome analysis of Streptomyces ansochromogenes (wild-type, WT) and its global regulatory gene disruption mutant ΔwblA, in which some differentially expressed genes are associated with the abolished nikkomycin biosynthesis and activated tylosin analogue compounds (TACs) production, and also with the oviedomycin production that is induced by the genetic manipulation of two differentially expressed genes (san7324 and san7324L) encoding RsbR. These results provide a significant clue for the discovery of new drug candidates and the activation of cryptic biosynthetic gene clusters.

Streptomyces: host for refactoring of diverse bioactive secondary metabolites

Microbial secondary metabolites are intensively explored due to their demands in pharmaceutical, agricultural and food industries. Streptomyces are one of the largest sources of secondary metabolites having diverse applications. In particular, the abundance of secondary metabolites encoding biosynthetic gene clusters and presence of wobble position in Streptomyces strains make it potential candidate as a native or heterologous host for secondary metabolite production including several cryptic gene clusters expression. Here, we have discussed the developments in Streptomyces strains genome mining, its exploration as a suitable host and application of synthetic biology for refactoring genetic systems for developing chassis for enhanced as well as novel secondary metabolites with reduced genome and cleaned background.

Comparative Transcriptome Analysis Demonstrates the Positive Effect of the Cyclic AMP Receptor Protein Crp on Daptomycin Biosynthesis in Streptomyces roseosporus

Daptomycin, which is produced by Streptomyces roseosporus, has been characterized as a novel cyclic lipopeptide antibiotic that is effective against Gram-positive bacteria. The biosynthesis of daptomycin is regulated by various factors. In the present study, we demonstrated that the cyclic AMP receptor protein (Crp) plays an important role in producing daptomycin in the S. roseosporus industrial strain. We found that daptomycin production from the crp deletion strain decreased drastically, whereas production from the crp overexpression strain increased by 22.1%. Transcriptome and qPCR analyses showed that some genes related to the daptomycin biosynthetic gene cluster (dpt) and the pleiotropic regulator (adpA) were significantly upregulated. RNA-seq also shows Crp to be a multifunctional regulator that modulates primary metabolism and enhances precursor flux to secondary metabolite biosynthesis. These results provide guidance for the development and improvement of potential natural products.

Molecular mechanism of mureidomycin biosynthesis activated by introduction of an exogenous regulatory gene ssaA into Streptomyces roseosporus

Mureidomycins (MRDs), a group of unique uridyl-peptide antibiotics, exhibit antibacterial activity against the highly refractory pathogen Pseudomonas aeruginosa. Our previous study showed that the cryptic MRD biosynthetic gene cluster (BGC) mrd in Streptomyces roseosporus NRRL 15998 could not be activated by its endogenous regulator 02995 but activated by an exogenous activator SsaA from sansanmycin’s BGC ssa of Streptomyces sp. strain SS. Here we report the molecular mechanism for this inexplicable regulation. EMSAs and footprinting experiments revealed that SsaA could directly bind to a 14-nt palindrome sequence of 5′-CTGRCNNNNGTCAG-3′ within six promoter regions of mrd. Disruption of three representative target genes (SSGG-02981, SSGG-02987 and SSGG-02994) showed that the target genes directly controlled by SsaA were essential for MRD production. The regulatory function was further investigated by replacing six regions of SSGG-02995 with those of ssaA. Surprisingly, only the replacement of 343–450 nt fragment encoding the 115–150 amino acids (AA) of SsaA could activate MRD biosynthesis. Further bioinformatics analysis showed that the 115–150 AA situated between two conserved domains of SsaA. Our findings significantly demonstrate that constitutive expression of a homologous exogenous regulatory gene is an effective strategy to awaken cryptic biosynthetic pathways in Streptomyces.

WblA, a global regulator of antibiotic biosynthesis in Streptomyces

Streptomyces species are soil-dwelling bacteria that produce vast numbers of pharmaceutically valuable secondary metabolites (SMs), such as antibiotics, immunosuppressants, antiviral, and anticancer drugs. On the other hand, the biosynthesis of most SMs remains very low due to tightly controlled regulatory networks. Both global and pathway-specific regulators are involved in the regulation of a specific SM biosynthesis in various Streptomyces species. Over the past few decades, many of these regulators have been identified and new ones are still being discovered. Among them, a global regulator of SM biosynthesis named WblA was identified in several Streptomyces species. The identification and understanding of the WblAs have greatly contributed to increasing the productivity of several Streptomyces SMs. This review summarizes the characteristics and applications on WblAs reported to date, which were found in various Streptomyces species and other actinobacteria.

Pleiotropic regulation of daptomycin synthesis by DptR1, a LuxR family transcriptional regulator

Daptomycin, produced by Streptomyces roseosporus is a novel cyclic lipopeptide antibiotic for treatment of Gram-positive bacteria caused infections. While, the regulatory mechanism of daptomycin synthesis has not been fully understood. Here we reported that DptR1, a LuxR family transcriptional regulator, played a pleiotropic regulatory role on daptomycin synthesis, for the first time. Deletion or over-expressing of dptR1 decreases the daptomycin's production, increases the transcriptional levels of the core dpt genes of day 3 and decreases the transcriptional levels of the core dpt genes of day 4, sharply, which indicates the transcriptional regulation of DptR1 on daptomycin synthesis is complex and time-ordered. The transcriptional levels of dptR2 increase in dptR1 deletion mutant (DR1), but decrease in dptR1 over-expression mutant (OR1), dramatically, compared to the starting strain of Streptomyces roseosporus N3 (WT), on the 3rd day, which indicates that DptR1 represses the transcription of dptR2. While, the transcriptional levels of dptR3 both in DR1 and OR1 decrease obviously, compared to WT, on the 3rd and 4th day. Comparative analysis of promoters' activities, using xylE gene as the reporter, shows that DptR1 activated the transcription of its own gene of dptR1 and represses the transcription of the dptR3 by affecting the promoter activities. While DptR1 may affect the expression of dptR2 indirectly, not by affecting the promoter activity of dptR2. DptR1, a LuxR family transcriptional regulator, played a pleiotropic regulation role on daptomycin synthesis.

Construction of Inducible Genetic Switch for the Global Regulator WblA To Sustain Both Overproduction of Tiancimycins and On-Demand Sporulation in Streptomyces sp. CB03234

The complex life cycle of streptomycetes is closely related to their secondary metabolisms, all controlled by cascade regulations. Tiancimycins (TNMs) are ten-membered enediynes possessing great potential for antitumor drug development. However, their low yields in Streptomyces sp. CB03234 have greatly limited subsequent studies. Through transcriptome analysis and genetic characterization, we proved that WblA is one pivotal global regulator to repress the biosynthesis of TNMs. The deletion of wblA could significantly enhance the production of TNMs, but also abolish the sporulation in CB03234. By constructing the NitR-ε-caprolactam inducible genetic switch, the expression of wblA was governed in CB03234-NRW, thereby sustaining the overproduction of TNMs and recovering the normal sporulation upon induction, which were practical for the scaled-up production of TNMs. Considering the prevalence and conserved regulatory roles of WblA in streptomycetes, our developed strategy shall provide an effective and practical approach to facilitate titer improvement and discovery of natural products.

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.

Secondary nucleotide messenger c-di-GMP exerts a global control on natural product biosynthesis in streptomycetes

Cyclic dimeric 3'-5' guanosine monophosphate, c-di-GMP, is a ubiquitous second messenger controlling diverse cellular processes in bacteria. In streptomycetes, c-di-GMP plays a crucial role in a complex morphological differentiation by modulating an activity of the pleiotropic regulator BldD. Here we report that c-di-GMP plays a key role in regulating secondary metabolite production in streptomycetes by altering the expression levels of bldD. Deletion of cdgB encoding a diguanylate cyclase in Streptomycesghanaensis reduced c-di-GMP levels and the production of the peptidoglycan glycosyltransferase inhibitor moenomycin A. In contrast to the cdgB mutant, inactivation of rmdB, encoding a phosphodiesterase for the c-di-GMP hydrolysis, positively correlated with the c-di-GMP and moenomycin A accumulation. Deletion of bldD adversely affected the synthesis of secondary metabolites in S. ghanaensis, including the production of moenomycin A. The bldD-deficient phenotype is partly mediated by an increase in expression of the pleiotropic regulatory gene wblA. Genetic and biochemical analyses demonstrate that a complex of c-di-GMP and BldD effectively represses transcription of wblA, thus preventing sporogenesis and sustaining antibiotic synthesis. These results show that manipulation of the expression of genes controlling c-di-GMP pool has the potential to improve antibiotic production as well as activate the expression of silent gene clusters.

The regulatory cascades of antibiotic production in Streptomyces

Streptomyces is famous for its capability to produce the most abundant antibiotics in all kingdoms. All Streptomyces antibiotics are natural products, whose biosynthesis from the so-called gene clusters are elaborately regulated by pyramidal transcriptional regulatory cascades. In the past decades, scientists have striven to unveil the regulatory mechanisms involved in antibiotic production in Streptomyces. Here we mainly focus on three aspects of the regulation on antibiotic production. 1. The onset of antibiotic production triggered by hormones and their coupled receptors as regulators; 2. The cascades of global and pathway-specific regulators governing antibiotic production; 3. The feedback regulation of antibiotics and/or intermediates on the gene cluster expression for their coordinated production. This review will summarize how the antibiotic production is stringently regulated in Streptomyces based on the signaling, and lay a theoretical foundation for improvement of antibiotic production and potentially drug discovery.

BldD, a master developmental repressor, activates antibiotic production in two Streptomyces species

BldD generally functions as a repressor controlling morphological development of Streptomyces. In this work, evidences that BldD also activates antibiotic production are provided. In Streptomyces roseosporus (which produces daptomycin widely used for treatment of human infections), deletion of bldD notably reduced daptomycin production, but enhanced sporulation. BldD stimulated daptomycin production by directly activating transcription of dpt structural genes and dptR3 (which encodes an indirect activator of daptomycin production), and repressed its own gene. BldD-binding sites on promoter regions of dptE, dptR3, and bldD were all found to contain BldD box-like sequences, facilitating prediction of new BldD targets. Two Streptomyces global regulatory genes, adpA and afsR, were confirmed to be directly activated by BldD. The protein AfsR was shown to act as an activator of daptomycin production, but a repressor of development. BldD directly represses nine key developmental genes. In Streptomyces avermitilis (which produces effective anthelmintic agents avermectins), BldD homolog (BldDsav) directly activates avermectin production through ave structural genes and cluster-situated activator gene aveR. This is the first report that BldD activates antibiotic biosynthesis both directly and via a cascade mechanism. BldD homologs are widely distributed among Streptomyces, our findings suggest that BldD may activate antibiotic production in other Streptomyces species.

Regulation of antibiotic production in Actinobacteria: new perspectives from the post-genomic era

Covering: 2000 to 2018 The antimicrobial activity of many of their natural products has brought prominence to the Streptomycetaceae, a family of Gram-positive bacteria that inhabit both soil and aquatic sediments. In the natural environment, antimicrobial compounds are likely to limit the growth of competitors, thereby offering a selective advantage to the producer, in particular when nutrients become limited and the developmental programme leading to spores commences. The study of the control of this secondary metabolism continues to offer insights into its integration with a complex lifecycle that takes multiple cues from the environment and primary metabolism. Such information can then be harnessed to devise laboratory screening conditions to discover compounds with new or improved clinical value. Here we provide an update of the review we published in NPR in 2011. Besides providing the essential background, we focus on recent developments in our understanding of the underlying regulatory networks, ecological triggers of natural product biosynthesis, contributions from comparative genomics and approaches to awaken the biosynthesis of otherwise silent or cryptic natural products. In addition, we highlight recent discoveries on the control of antibiotic production in other Actinobacteria, which have gained considerable attention since the start of the genomics revolution. New technologies that have the potential to produce a step change in our understanding of the regulation of secondary metabolism are also described.

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.

Nystatin-like Pseudonocardia polyene B1, a novel disaccharide-containing antifungal heptaene antibiotic

Polyene macrolides such as nystatin A1 and amphotericin B belong to a large family of very valuable antifungal polyketide compounds typically produced by soil actinomycetes. Recently, nystatin-like Pseudonocardia polyene (NPP) A1 has been identified as a unique disaccharide-containing tetraene antifungal macrolide produced by Pseudonocardia autotrophica. Despite its significantly increased water solubility and decreased hemolytic activity, its antifungal activity remains limited compared with that of nystatin A1. In this study, we developed NPP B1, a novel NPP A1 derivative harboring a heptaene core structure, by introducing two amino acid substitutions in the putative NADPH-binding motif of the enoyl reductase domain in module 5 of the NPP A1 polyketide synthase NppC. The low level NPP B1 production yield was successfully improved by eliminating the native plasmid encoding a polyketide biosynthetic gene cluster present in P. autotrophica. In vitro and in vivo antifungal activity and toxicity studies indicated that NPP B1 exhibited comparable antifungal activity against Candida albicans and was less toxic than the most potent heptaene antifungal, amphotericin B. Moreover, NPP B1 showed improved pharmacokinetic parameters compared to those of amphotericin B, suggesting that NPP B1 could be a promising candidate for development into a pharmacokinetically improved and less-toxic polyene antifungal antibiotic.

Rational engineering of Streptomyces albus J1074 for the overexpression of secondary metabolite gene clusters

BACKGROUND

Genome sequencing revealed that Streptomyces sp. can dedicate up to ~ 10% of their genomes for the biosynthesis of bioactive secondary metabolites. However, the majority of these biosynthetic gene clusters are only weakly expressed or not at all. Indeed, the biosynthesis of natural products is highly regulated through integrating multiple nutritional and environmental signals perceived by pleiotropic and pathway-specific transcriptional regulators. Although pathway-specific refactoring has been a proved, productive approach for the activation of individual gene clusters, the construction of a global super host strain by targeting pleiotropic-specific genes for the expression of multiple diverse gene clusters is an attractive approach.

RESULTS

Streptomyces albus J1074 is a gifted heterologous host. To further improve its secondary metabolite expression capability, we rationally engineered the host by targeting genes affecting NADPH availability, precursor flux, cell growth and biosynthetic gene transcriptional activation. These studies led to the activation of the native paulomycin pathway in engineered S. albus strains and importantly the upregulated expression of the heterologous actinorhodin gene cluster.

CONCLUSIONS

Rational engineering of Streptomyces albus J1074 yielded a series of mutants with improved capabilities for native and heterologous expression of secondary metabolite gene clusters.