Induction of antimicrobial activities in heterologous streptomycetes using alleles of the Streptomyces coelicolor gene absA1

Streptomyces extracellular vesicles are a broad and permissive antimicrobial packaging and delivery system

Streptomyces are the primary source of bioactive specialized metabolites used in research and medicine, including many antimicrobials. These are presumed to be secreted and function as freely soluble compounds. However, increasing evidence suggests that extracellular vesicles are an alternative secretion system. We assessed environmental and lab-adapted Streptomyces (sporulating filamentous actinomycetes) and found frequent production of antimicrobial vesicles. The molecular cargo included actinomycins, anthracyclines, candicidin, and actinorhodin, reflecting both diverse chemical properties and diverse antibacterial and antifungal activity. The levels of packaged antimicrobials correlated with the level of inhibitory activity of the vesicles, and a strain knocked out for the production of anthracyclines produced vesicles that lacked antimicrobial activity. We demonstrated that antimicrobial containing vesicles achieve direct delivery of the cargo to other microbes. Notably, this delivery via membrane fusion occurred to a broad range of microbes, including pathogenic bacteria and yeast. Vesicle encapsulation offers a broad and permissive packaging and delivery system for antimicrobial specialized metabolites, with important implications for ecology and translation.IMPORTANCEExtracellular vesicle encapsulation changes our picture of how antimicrobial metabolites function in the environment and provides an alternative translational approach for the delivery of antimicrobials. We find many Streptomyces strains are capable of releasing antimicrobial vesicles, and at least four distinct classes of compounds can be packaged, suggesting this is widespread in nature. This is a striking departure from the primary paradigm of the secretion and action of specialized metabolites as soluble compounds. Importantly, the vesicles deliver antimicrobial metabolites directly to other microbes via membrane fusion, including pathogenic bacteria and yeast. This suggests future applications in which lipid-encapsulated natural product antibiotics and antifungals could be used to solve some of the most pressing problems in drug resistance.

Bacterial synthetic biology: tools for novel drug discovery

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Two-Component Systems of Streptomyces coelicolor: An Intricate Network to Be Unraveled

Bacteria of the Streptomyces genus constitute an authentic biotech gold mine thanks to their ability to produce a myriad of compounds and enzymes of great interest at various clinical, agricultural, and industrial levels. Understanding the physiology of these organisms and revealing their regulatory mechanisms is essential for their manipulation and application. Two-component systems (TCSs) constitute the predominant signal transduction mechanism in prokaryotes, and can detect a multitude of external and internal stimuli and trigger the appropriate cellular responses for adapting to diverse environmental conditions. These global regulatory systems usually coordinate various biological processes for the maintenance of homeostasis and proper cell function. Here, we review the multiple TCSs described and characterized in Streptomyces coelicolor, one of the most studied and important model species within this bacterial group. TCSs are involved in all cellular processes; hence, unravelling the complex regulatory network they form is essential for their potential biotechnological application.

Identification of pulvomycin as an inhibitor of the futalosine pathway

Menaquinone is an essential cofactor in the electron-transfer pathway for bacteria. Menaquinone is biosynthesized from chorismate using either the well-known canonical pathway established by pioneering studies in model microorganisms or the futalosine pathway, which we discovered in Streptomyces. Because Helicobacter pylori, which causes stomach cancer, uses the futalosine pathway and most beneficial intestinal bacteria including lactobacilli use the canonical pathway, the futalosine pathway will be a great target to develop antibiotics specific for H. pylori. Here, we searched for such compounds from metabolites produced by actinomycetes and identified pulvomycin from culture broth of Streptomyces sp. K18-0194 as a specific inhibitor of the futalosine pathway.

Application of Streptomyces Antimicrobial Compounds for the Control of Phytopathogens

One of the relevant problems in today's agriculture is related to phytopathogenic microorganisms that cause between 30–40% of crop losses. Synthetic chemical pesticides and antibiotics have brought human and environmental health problems and microbial resistance to these treatments. So, the search for natural alternatives is necessary. The genus Streptomyces have broad biotechnological potential, being a promising candidate for the biocontrol of phytopathogenic microorganisms. The efficacy of some species of this genus in plant protection and their continued presence in the intensely competitive rhizosphere is due to its great potential to produce a wide variety of soluble bioactive secondary metabolites and volatile organic compounds. However, more attention is still needed to develop novel formulations that could increase the shelf life of streptomycetes, ensuring their efficacy as a microbial pesticide. In this sense, encapsulation offers an advantageous and environmentally friendly option. The present review aims to describe some phytopathogenic microorganisms with economic importance that require biological control. In addition, it focuses mainly on the Streptomyces genus as a great producer of secondary metabolites that act on other microorganisms and plants, exercising its role as biological control. The review also covers some strategies and products based on Streptomyces and the problems of its application in the field.

A Natural Product Chemist's Guide to Unlocking Silent Biosynthetic Gene Clusters

Microbial natural products have provided an important source of therapeutic leads and motivated research and innovation in diverse scientific disciplines. In recent years, it has become evident that bacteria harbor a large, hidden reservoir of potential natural products in the form of silent or cryptic biosynthetic gene clusters (BGCs). These can be readily identified in microbial genome sequences but do not give rise to detectable levels of a natural product. Herein, we provide a useful organizational framework for the various methods that have been implemented for interrogating silent BGCs. We divide all available approaches into four categories. The first three are endogenous strategies that utilize the native host in conjunction with classical genetics, chemical genetics, or different culture modalities. The last category comprises expression of the entire BGC in a heterologous host. For each category, we describe the rationale, recent applications, and associated advantages and limitations.

Research Tales from the Clardy Laboratory: Function-Driven Natural Product Discovery

Over the past 70 years, the search for small molecules from nature has transformed biomedical research: natural products are the basis for half of all pharmaceuticals; the quest for total synthesis of natural products fueled development of methodologies for organic synthesis; and their biosynthesis presented unprecedented biochemical transformations, expanding our chemo-enzymatic toolkit. Initially, the discovery of small molecules was driven by bioactivity-guided fractionation. However, this approach yielded the frequent rediscovery of already known metabolites. As a result, focus shifted to identifying novel scaffolds through either structure-first methods or genome mining, relegating function as a secondary concern. Over the past two decades, the laboratory of Jon Clardy has taken an alternative route and focused on an ecology-driven, function-first approach in pursuit of uncovering bacterial small molecules with biological activity. In this review, we highlight several examples that showcase this ecology-first approach. Though the highlighted systems are diverse, unifying themes are (1) to understand how microbes interact with their host or environment, (2) to gain insights into the environmental roles of microbial metabolites, and (3) to explore pharmaceutical potential from these ecologically relevant metabolites.

Benzanthric Acid, a Novel Metabolite From Streptomyces albus Del14 Expressing the Nybomycin Gene Cluster

Streptomycetes constitute a diverse bacterial group able to produce a wide variety of secondary metabolites with potential applications in the pharmacy industry. However, the genes responsible for the biosynthesis of these compounds are very frequently inactive or expressed at very low levels under standard laboratory cultivation conditions. Therefore, the activation or upregulation of secondary metabolite biosynthesis genes is a crucial step for the discovery of new bioactive natural products. We have recently reported the discovery of the biosynthetic genes for the antibiotic nybomycin (nyb genes) in Streptomyces albus subsp. chlorinus. The nyb genes were expressed in the heterologous host Streptomyces albus Del14, which produces not only nybomycin, but also a novel compound. In this study, we describe the isolation, purification, and structure elucidation of the new substance named benzanthric acid.

Engineering Streptomyces coelicolor for production of monomethyl branched chain fatty acids

Branched chain fatty acids (BCFA) are an appealing biorefinery-driven target of fatty acid (FA) production. BCFAs typically have lower melting points compared to straight chain FAs, making them useful in lubricants and biofuels. Actinobacteria, especially Streptomyces species, have unique secondary metabolism that are capable of producing not only antibiotics, but also high percentage of BCFAs in their membrane lipids. Since biosynthesis of polyketide (PK) and FA partially share common pathways to generate acyl-CoA precursors, in theory, Streptomyces sp. with high levels of PK antibiotics production can be easily manipulated into strains producing high levels of BCFAs. To increase the percentage of the BCFA moieties in lipids, we redirected acyl-CoA precursor fluxes from PK into BCFAs using S. coelicolor M1146 (M1146) as a host strain. In addition, 3-ketoacyl acyl carrier protein synthase III and branched chain α-keto acid dehydrogenase were overexpressed to push fluxes of branched chain acyl-CoA precursors towards FA synthesis. The maximum titer of 354.1 mg/L BCFAs, 90.3% of the total FA moieties, was achieved using M1146_dD-B, fadD deletion and bkdABC overexpression mutant of M1146 strain. Cell specific yield of 64.4 mg/L/g_cell was also achieved. The production titer and specific yield are the highest ever reported in bacterial cells, which provides useful insights to develop an efficient host strain for BCFAs.

Heterologous Expression of the Nybomycin Gene Cluster from the Marine Strain Streptomyces albus subsp. chlorinus NRRL B-24108

Streptomycetes represent an important reservoir of active secondary metabolites with potential applications in the pharmaceutical industry. The gene clusters responsible for their production are often cryptic under laboratory growth conditions. Characterization of these clusters is therefore essential for the discovery of new microbial pharmaceutical drugs. Here, we report the identification of the previously uncharacterized nybomycin gene cluster from the marine actinomycete Streptomyces albus subsp. chlorinus through its heterologous expression. Nybomycin has previously been reported to act against quinolone-resistant Staphylococcus aureus strains harboring a mutated gyrA gene but not against those with intact gyrA. The nybomycin-resistant mutants generated from quinolone-resistant mutants have been reported to be caused by a back-mutation in the gyrA gene that restores susceptibility to quinolones. On the basis of gene function assignment from bioinformatics analysis, we suggest a model for nybomycin biosynthesis.

The role of inter-species interactions in Salinispora specialized metabolism

Bacterial genome sequences consistently contain many more biosynthetic gene clusters encoding specialized metabolites than predicted by the compounds discovered from the respective strains. One hypothesis invoked to explain the cryptic nature of these gene clusters is that standard laboratory conditions do not provide the environmental cues needed to trigger gene expression. A potential source of such cues is other members of the bacterial community, which are logical targets for competitive interactions. In this study, we examined the effects of such interactions on specialized metabolism in the marine actinomycete Salinispora tropica. The results show that antibiotic activities and the concentration of some small molecules increase in the presence of co-occurring bacterial strains relative to monocultures. Some increases in antibiotic activity could be linked to nutrient depletion by the competitor as opposed to the production of a chemical cue. Other increases were correlated with the production of specific compounds by S. tropica. In particular, one interaction with a Vibrio sp. consistently induced antibiotic activity and was associated with parent ions that were unique to this interaction, although the associated compound could not be identified. This study provides insight into the metabolomic complexities of bacterial interactions and baseline information for future genome mining efforts.

Control of Specialized Metabolism by Signaling and Transcriptional Regulation: Opportunities for New Platforms for Drug Discovery?

Specialized metabolites are bacterially produced small molecules that have an extraordinary diversity of important biological activities. They are useful as biochemical probes of living systems, and they have been adapted for use as drugs for human afflictions ranging from infectious diseases to cancer. The biosynthetic genes for these molecules are controlled by a dense network of regulatory mechanisms: Cell-cell signaling and nutrient sensing are conspicuous features of this network. While many components of these mechanisms have been identified, important questions about their biological roles remain shrouded in mystery. In addition to identifying new molecules and solving their mechanisms of action (a central preoccupation in this field), we suggest that addressing questions of quorum sensing versus diffusion sensing and identifying the dominant nutritional and environmental cues for specialized metabolism are important directions for research.

Streptomyces Differentiation in Liquid Cultures as a Trigger of Secondary Metabolism

Streptomyces is a diverse group of gram-positive microorganisms characterised by a complex developmental cycle. Streptomycetes produce a number of antibiotics and other bioactive compounds used in the clinic. Most screening campaigns looking for new bioactive molecules from actinomycetes have been performed empirically, e.g., without considering whether the bacteria are growing under the best developmental conditions for secondary metabolite production. These screening campaigns were extremely productive and discovered a number of new bioactive compounds during the so-called “golden age of antibiotics” (until the 1980s). However, at present, there is a worrying bottleneck in drug discovery, and new experimental approaches are needed to improve the screening of natural actinomycetes. Streptomycetes are still the most important natural source of antibiotics and other bioactive compounds. They harbour many cryptic secondary metabolite pathways not expressed under classical laboratory cultures. Here, we review the new strategies that are being explored to overcome current challenges in drug discovery. In particular, we focus on those aimed at improving the differentiation of the antibiotic-producing mycelium stage in the laboratory.

Production of pikromycin using branched chain amino acid catabolism in Streptomyces venezuelae ATCC 15439

Branched chain amino acids (BCAA) are catabolized into various acyl-CoA compounds, which are key precursors used in polyketide productions. Because of that, BCAA catabolism needs fine tuning of flux balances for enhancing the production of polyketide antibiotics. To enhance BCAA catabolism for pikromycin production in Streptomyces venezuelae ATCC 15439, three key enzymes of BCAA catabolism, 3-ketoacyl acyl carrier protein synthase III, acyl-CoA dehydrogenase, and branched chain α-keto acid dehydrogenase (BCDH) were manipulated. BCDH overexpression in the wild type strain resulted in 1.3 fold increase in pikromycin production compared to that of WT, resulting in total 25 mg/L of pikromycin. To further increase pikromycin production, methylmalonyl-CoA mutase linked to succinyl-CoA production was overexpressed along with BCDH. Overexpression of the two enzymes resulted in the highest titer of total macrolide production of 43 mg/L, which was about 2.2 fold increase compared to that of the WT. However, it accumulated and produced dehydroxylated forms of pikromycin and methymycin, including their derivatives as well. It indicated that activities of pikC, P450 monooxygenase, newly became a bottleneck in pikromycin synthesis.

Identification of butenolide regulatory system controlling secondary metabolism in Streptomyces albus J1074

A large majority of genome-encrypted chemical diversity in actinobacteria remains to be discovered, which is related to the low level of secondary metabolism genes expression. Here, we report the application of a reporter-guided screening strategy to activate cryptic polycyclic tetramate macrolactam gene clusters in Streptomyces albus J1074. The analysis of the S. albus transcriptome revealed an overall low level of secondary metabolism genes transcription. Combined with transposon mutagenesis, reporter-guided screening resulted in the selection of two S. albus strains with altered secondary metabolites production. Transposon insertion in the most prominent strain, S. albus ATGSal2P2::TN14, was mapped to the XNR_3174 gene encoding an unclassified transcriptional regulator. The mutant strain was found to produce the avenolide-like compound butenolide 4. The deletion of the gene encoding a putative acyl-CoA oxidase, an orthologue of the Streptomyces avermitilis avenolide biosynthesis enzyme, in the S. albus XNR_3174 mutant caused silencing of secondary metabolism. The homologues of XNR_3174 and the butenolide biosynthesis genes were found in the genomes of multiple Streptomyces species. This result leads us to believe that the discovered regulatory elements comprise a new condition-dependent system that controls secondary metabolism in actinobacteria and can be manipulated to activate cryptic biosynthetic pathways.

Elfamycins: inhibitors of elongation factor-Tu

Elfamycins are a relatively understudied group of antibiotics that target the essential process of translation through impairment of EF-Tu function. For the most part, the utility of these compounds has been as laboratory tools for the study of EF-Tu and the ribosome, as their poor pharmacokinetic profile and solubility has prevented implementation as therapeutic agents. However, due to the slowing of the antibiotic pipeline and the rapid emergence of resistance to approved antibiotics, this group is being reconsidered. Some researchers are using screens for novel naturally produced variants, while others are making directed, systematic chemical improvements on publically disclosed compounds. As an example of the latter approach, a GE2270 A derivative, LFF571, has completed phase 2 clinical trials, thus demonstrating the potential for elfamycins to become more prominent antibiotics in the future.

Opportunities for natural products in 21st century antibiotic discovery

Natural products and their derivatives are mainstays of our antibiotic drugs, but they are increasingly in peril. The combination of widespread multidrug resistance in once susceptible bacterial pathogens, disenchantment with natural products as sources of new drugs, lack of success using synthetic compounds and target-based discovery methods, along with shifting economic and regulatory issues, conspire to move investment in research and development away from the antibiotics arena. The result is a growing crisis in antibiotic drug discovery that threatens modern medicine. 21^st century natural product research is perfectly positioned to fill the antibiotic discovery gap and bring new drug candidates to the clinic. Innovations in genomics and techniques to explore new sources of antimicrobial chemical matter are revealing new chemistry. Increasing appreciation of the value of narrow-spectrum drugs and re-examination of once discarded chemical scaffolds coupled with synthetic biology methods to generate new compounds and improve yields offer new strategies to revitalize once moribund natural product programs. The increasing awareness that the combination of antibiotics with adjuvants, non-antibiotic compounds that overcome resistance and enhance drug activity, can rescue older chemical scaffolds, and concepts such as blocking pathogen virulence present orthogonal strategies to traditional antibiotics. In all these areas, natural products offer chemical matter, shaped by natural selection, that is privileged in this therapeutic area. Natural product research is poised to regain prominence in delivering new drugs to solve the antibiotic crisis.

An Engineered Allele of afsQ1 Facilitates the Discovery and Investigation of Cryptic Natural Products

New approaches to antimicrobial discovery are needed to address the growing threat of antibiotic resistance. The Streptomyces genus, a proven source of antibiotics, is recognized as having a large reservoir of untapped secondary metabolic genes, many of which are likely to produce uncharacterized compounds. However, most of these compounds are currently inaccessible, as they are not expressed under standard laboratory conditions. Here, we present a novel methodology for activating these "cryptic" metabolites by heterologously expressing a constitutively active pleiotropic regulator. By screening wild Streptomyces isolates, we identified the antibiotic siamycin-I, a lasso peptide that we show is active against multidrug pathogens. We further revealed that siamycin-I interferes with cell wall integrity via lipid II. This new technology has the potential to be broadly applied for use in the discovery of additional "cryptic" metabolites.

Identification of a Novel Lincomycin Resistance Mutation Associated with Activation of Antibiotic Production in Streptomyces coelicolor A3(2)

Comparative genome sequencing analysis of a lincomycin-resistant strain of Streptomyces coelicolor A3(2) and the wild-type strain identified a novel mutation conferring a high level of lincomycin resistance. Surprisingly, the new mutation was an in-frame DNA deletion in the genes SCO4597 and SCO4598, resulting in formation of the hybrid gene linR. SCO4597 and SCO4598 encode two histidine kinases, which together with SCO4596, encoding a response regulator, constitute a unique two-component system. Sequence analysis indicated that these three genes and their arrangement patterns are ubiquitous among all Streptomyces genomes sequenced to date, suggesting these genes play important regulatory roles. Gene replacement showed that this mutation was responsible for the high level of lincomycin resistance, the overproduction of the antibiotic actinorhodin, and the enhanced morphological differentiation of this strain. Moreover, heterologous expression of the hybrid gene linR in Escherichia coli conferred resistance to lincomycin in this organism. Introduction of the hybrid gene linR in various Streptomyces strains by gene engineering technology may widely activate and/or enhance antibiotic production.

Activity-Independent Discovery of Secondary Metabolites Using Chemical Elicitation and Cheminformatic Inference

Most existing antibiotics were discovered through screens of environmental microbes, particularly the streptomycetes, for the capacity to prevent the growth of pathogenic bacteria. This "activity-guided screening" method has been largely abandoned because it repeatedly rediscovers those compounds that are highly expressed during laboratory culture. Most of these metabolites have already been biochemically characterized. However, the sequencing of streptomycete genomes has revealed a large number of "cryptic" secondary metabolic genes that are either poorly expressed in the laboratory or that have biological activities that cannot be discovered through standard activity-guided screens. Methods that reveal these uncharacterized compounds, particularly methods that are not biased in favor of the highly expressed metabolites, would provide direct access to a large number of potentially useful biologically active small molecules. To address this need, we have devised a discovery method in which a chemical elicitor called Cl-ARC is used to elevate the expression of cryptic biosynthetic genes. We show that the resulting change in product yield permits the direct discovery of secondary metabolites without requiring knowledge of their biological activity. We used this approach to identify three rare secondary metabolites and find that two of them target eukaryotic cells and not bacterial cells. In parallel, we report the first paired use of cheminformatic inference and chemical genetic epistasis in yeast to identify the target. In this way, we demonstrate that oxohygrolidin, one of the eukaryote-active compounds we identified through activity-independent screening, targets the V1 ATPase in yeast and human cells and secondarily HSP90.

David and Goliath: chemical perturbation of eukaryotes by bacteria

Environmental microbes produce biologically active small molecules that have been mined extensively as antibiotics and a smaller number of drugs that act on eukaryotic cells. It is known that there are additional bioactives to be discovered from this source. While the discovery of new antibiotics is challenged by the frequent discovery of known compounds, we contend that the eukaryote-active compounds may be less saturated. Indeed, despite there being far fewer eukaryotic-active natural products these molecules interact with a far richer diversity of molecular and cellular targets.

Discovery of microbial natural products by activation of silent biosynthetic gene clusters

Microorganisms produce a wealth of structurally diverse specialized metabolites with a remarkable range of biological activities and a wide variety of applications in medicine and agriculture, such as the treatment of infectious diseases and cancer, and the prevention of crop damage. Genomics has revealed that many microorganisms have far greater potential to produce specialized metabolites than was thought from classic bioactivity screens; however, realizing this potential has been hampered by the fact that many specialized metabolite biosynthetic gene clusters (BGCs) are not expressed in laboratory cultures. In this Review, we discuss the strategies that have been developed in bacteria and fungi to identify and induce the expression of such silent BGCs, and we briefly summarize methods for the isolation and structural characterization of their metabolic products.

The two kinases, AbrC1 and AbrC2, of the atypical two-component system AbrC are needed to regulate antibiotic production and differentiation in Streptomyces coelicolor

Two-component systems (TCSs) are the most important sensing mechanisms in bacteria. In Streptomyces, TCSs-mediated responses to environmental stimuli are involved in the regulation of antibiotic production. This study examines the individual role of two histidine kinases (HKs), AbrC1 and AbrC2, which form part of an atypical TCS in Streptomyces coelicolor. qRT-PCR analysis of the expression of both kinases demonstrated that both are expressed at similar levels in NB and NMMP media. Single deletion of abrC1 elicited a significant increase in antibiotic production, while deletion of abrC2 did not have any clear effect. The origin of this phenotype, probably related to the differential phosphorylation ability of the two kinases, was also explored indirectly, analyzing the toxic phenotypes associated with high levels of phosphorylated RR. The higher the AbrC3 regulator phosphorylation rate, the greater the cell toxicity. For the first time, the present work shows in Streptomyces the combined involvement of two different HKs in the response of a regulator to environmental signals. Regarding the possible applications of this research, the fact that an abrC1 deletion mutant overproduces three of the S. coelicolor antibiotics makes this strain an excellent candidate as a host for the heterologous production of secondary metabolites.

Identification of a novel unpaired histidine sensor kinase affecting secondary metabolism and morphological differentiation in Streptomyces acidiscabies ATCC 49003

Two-component systems (TCSs) are an important signaling transduction pathway that adapt to changing environments. Commonly, a TCS comprises a sensor kinase that is usually an integral membrane histidine sensor kinase and a response regulator that mediates the cellular responses. Presently, however, we cloned a novel sensor kinase gene (tcsK) that is not adjacent to its cognate response regulator from Streptomyces acidiscabies that produces two secondary metabolites, thaxtomin A and WS5995B, and identified its functional involvement in the production of secondary metabolites and morphological differentiation. The elevated expression and disruption of the tcsK gene enhanced 7.1-fold and almost abolished WS5995B production in S. acidiscabies, respectively, but did not affect the production of thaxtomin A. In addition, spore formation of S. acidiscabies was decreased 120-fold by the disruption of tcsK, and the actinorhodin production of Streptomyces lividans TK24 was increased 5.7-fold by the high expression of tcsK. These results indicate that the novel unpaired tcsK gene may be related to the control of secondary metabolite production and spore formation in actinomycetes.

The re-emerging role of microbial natural products in antibiotic discovery

New classes of antibacterial compounds are urgently needed to respond to the high frequency of occurrence of resistances to all major classes of known antibiotics. Microbial natural products have been for decades one of the most successful sources of drugs to treat infectious diseases but today, the emerging unmet clinical need poses completely new challenges to the discovery of novel candidates with the desired properties to be developed as antibiotics. While natural products discovery programs have been gradually abandoned by the big pharma, smaller biotechnology companies and research organizations are taking over the lead in the discovery of novel antibacterials. Recent years have seen new approaches and technologies being developed and integrated in a multidisciplinary effort to further exploit microbial resources and their biosynthetic potential as an untapped source of novel molecules. New strategies to isolate novel species thought to be uncultivable, and synthetic biology approaches ranging from genome mining of microbial strains for cryptic biosynthetic pathways to their heterologous expression have been emerging in combination with high throughput sequencing platforms, integrated bioinformatic analysis, and on-site analytical detection and dereplication tools for novel compounds. These different innovative approaches are defining a completely new framework that is setting the bases for the future discovery of novel chemical scaffolds that should foster a renewed interest in the identification of novel classes of natural product antibiotics from the microbial world.

Something old, something new: revisiting natural products in antibiotic drug discovery

Antibiotic discovery is in crisis. Despite a growing need for new drugs resulting from the increasing number of multi-antibiotic-resistant pathogens, there have been only a handful of new antibiotics approved for clinical use in the past 2 decades. Faced with scientific, economic, and regulatory challenges, the pharmaceutical sector seems unable to respond to what has been called an “apocalyptic” threat. Natural products produced by bacteria and fungi are genetically encoded products of natural selection that have been the mainstay sources of the antibiotics in current clinical use. The pharmaceutical industry has largely abandoned these compounds in favor of large libraries of synthetic molecules because of difficulties in identifying new natural product antibiotics scaffolds. Advances in next-generation genome sequencing, bioinformatics, and analytical chemistry are combining to overcome barriers to natural products. Coupled with new strategies in antibiotic discovery, including inhibition of resistance, novel drug combinations, and new targets, natural products are poised for a renaissance to address what is a pressing health care crisis.

Two-component systems in Streptomyces: key regulators of antibiotic complex pathways

Streptomyces, the main antibiotic-producing bacteria, responds to changing environmental conditions through a complex sensing mechanism and two-component systems (TCSs) play a crucial role in this extraordinary "sensing" device.Moreover, TCSs are involved in the biosynthetic control of a wide range of secondary metabolites, among them commercial antibiotics. Increased knowledge about TCSs can be a powerful asset in the manipulation of bacteria through genetic engineering with a view to obtaining higher efficiencies in secondary metabolite production. In this review we summarise the available information about Streptomyces TCSs, focusing specifically on their connections to antibiotic production.

Activating secondary metabolism with stress and chemicals

The available literature on the secondary or nonessential metabolites of the streptomycetes bacteria suggests that there may be poorly expressed or "cryptic" compounds that have yet to be identified and that may have significant medical utility. In addition, it is clear that there is a large and complex regulatory network that controls the production of these molecules in the laboratory and in nature. Two approaches that have been taken to manipulating the yields of secondary metabolites are the use of various stress responses and, more recently, the use of precision chemical probes. Here, we review the status of this work and outline the challenges and opportunities afforded by each of them.

A cryptic polyene biosynthetic gene cluster in Streptomyces calvus is expressed upon complementation with a functional bldA gene

Streptomyces calvus is best known as the producer of the fluorinated natural product nucleocidin. This strain of Streptomycetes is also unusual for displaying a "bald" phenotype that is deficient in the formation of aerial mycelium and spores. Genome sequencing of this organism revealed a point mutation in the bldA gene that is predicted to encode a misfolded Leu-tRNA(UUA) molecule. Complementation of S. calvus with a correct copy of bldA restored sporulation and additionally promoted production of a polyeneoic acid amide, 4-Z-annimycin, and a minor amount of the isomer, 4-E-annimycin. Bioassays reveal that these compounds inhibit morphological differentiation in other Actinobacteria. The annimycin gene cluster encoding a type 1 polyketide synthase was identified and verified through disruption studies. This study underscores the importance of the bldA gene in regulating the expression of cryptic biosynthetic genes.

Towards a new science of secondary metabolism

Secondary metabolites are a reliable and very important source of medicinal compounds. While these molecules have been mined extensively, genome sequencing has suggested that there is a great deal of chemical diversity and bioactivity that remains to be discovered and characterized. A central challenge to the field is that many of the novel or poorly understood molecules are expressed at low levels in the laboratory-such molecules are often described as the 'cryptic' secondary metabolites. In this review, we will discuss evidence that research in this field has provided us with sufficient knowledge and tools to express and purify any secondary metabolite of interest. We will describe 'unselective' strategies that bring about global changes in secondary metabolite output as well as 'selective' strategies where a specific biosynthetic gene cluster of interest is manipulated to enhance the yield of a single product.

Chemical perturbation of secondary metabolism demonstrates important links to primary metabolism

Bacterially produced secondary metabolites are used as antibiotics, anticancer drugs, and for many other medicinal applications. The mechanisms that limit the production of these molecules in the laboratory are not well understood, and this has impeded the discovery of many important compounds. We have identified small molecules that remodel the yields of secondary metabolites in many actinomycetes and show that one set of these molecules does so by inhibiting fatty acid biosynthesis. This demonstrates a particularly intimate relationship between this primary metabolic pathway and secondary metabolism and suggests an approach to enhance the yields of metabolites for discovery and biochemical characterization.

A magic bullet for antibiotic discovery

Activation/exploitation of biosynthetic pathways for useful metabolites is a major current interest. The metabolism remodeling approach developed by Craney and colleagues in this issue of (Chemistry & Biology), in which small molecule probes alter the secondary metabolites produced by streptomycetes, could lead to discovery of a multitude of novel antibiotics and other drugs.

Crp is a global regulator of antibiotic production in streptomyces

Cyclic AMP receptor protein (Crp) is a transcription regulator controlling diverse cellular processes in many bacteria. In Streptomyces coelicolor, it is well established that Crp plays a critical role in spore germination and colony development. Here, we demonstrate that Crp is a key regulator of secondary metabolism and antibiotic production in S. coelicolor and show that it may additionally coordinate precursor flux from primary to secondary metabolism. We found that crp deletion adversely affected the synthesis of three well-characterized antibiotics in S. coelicolor: actinorhodin (Act), undecylprodigiosin (Red), and calcium-dependent antibiotic (CDA). Using chromatin immunoprecipitation-microarray (ChIP-chip) assays, we determined that eight (out of 22) secondary metabolic clusters encoded by S. coelicolor contained Crp-associated sites. We followed the effect of Crp induction using transcription profiling analyses and found secondary metabolic genes to be significantly affected: included in this Crp-dependent group were genes from six of the clusters identified in the ChIP-chip experiments. Overexpressing Crp in a panel of Streptomyces species led to enhanced antibiotic synthesis and new metabolite production, suggesting that Crp control over secondary metabolism is broadly conserved in the streptomycetes and that Crp overexpression could serve as a powerful tool for unlocking the chemical potential of these organisms.

Streptomyces produces a remarkably diverse array of secondary metabolites, including many antibiotics. In recent years, genome sequencing has revealed that these products represent only a small proportion of the total secondary metabolite potential of Streptomyces. There is, therefore, considerable interest in discovering ways to stimulate the production of new metabolites. Here, we show that Crp (the classical regulator of carbon catabolite repression in Escherichia coli) is a master regulator of secondary metabolism in Streptomyces. It binds to eight of 22 secondary metabolic gene clusters in the Streptomyces coelicolor genome and directly affects the expression of six of these. Deletion of crp in S. coelicolor leads to dramatic reductions in antibiotic levels, while Crp overexpression enhances antibiotic production. We find that the antibiotic-stimulatory capacity of Crp extends to other streptomycetes, where its overexpression activates the production of “cryptic” metabolites that are not otherwise seen in the corresponding wild-type strain.

Better chemistry through regulation

Finding ways to increase the biosynthesis of medically important microbial secondary metabolites is a challenge of microbial chemical biology. Lechner et al. (in this issue of Chemistry & Biology) show that transcriptional regulation can be manipulated to selectively increase the production of a desired metabolite.

The regulation of the secondary metabolism of Streptomyces: new links and experimental advances

Streptomycetes and other actinobacteria are renowned as a rich source of natural products of clinical, agricultural and biotechnological value. They are being mined with renewed vigour, supported by genome sequencing efforts, which have revealed a coding capacity for secondary metabolites in vast excess of expectations that were based on the detection of antibiotic activities under standard laboratory conditions. Here we review what is known about the control of production of so-called secondary metabolites in streptomycetes, with an emphasis on examples where details of the underlying regulatory mechanisms are known. Intriguing links between nutritional regulators, primary and secondary metabolism and morphological development are discussed, and new data are included on the carbon control of development and antibiotic production, and on aspects of the regulation of the biosynthesis of microbial hormones. Given the tide of antibiotic resistance emerging in pathogens, this review is peppered with approaches that may expand the screening of streptomycetes for new antibiotics by awakening expression of cryptic antibiotic biosynthetic genes. New technologies are also described that have potential to greatly further our understanding of gene regulation in what is an area fertile for discovery and exploitation

Novel two-component systems implied in antibiotic production in Streptomyces coelicolor

The abundance of two-component systems (TCSs) in Streptomyces coelicolor A3(2) genome indicates their importance in the physiology of this soil bacteria. Currently, several TCSs have been related to antibiotic regulation, and the purpose in this study was the characterization of five TCSs, selected by sequence homology with the well-known absA1A2 system, that could also be associated with this important process. Null mutants of the five TCSs were obtained and two mutants (ΔSCO1744/1745 and ΔSCO4596/4597/4598) showed significant differences in both antibiotic production and morphological differentiation, and have been renamed as abr (antibiotic regulator). No detectable changes in antibiotic production were found in the mutants in the systems that include the ORFs SCO3638/3639, SCO3640/3641 and SCO2165/2166 in any of the culture conditions assayed. The system SCO1744/1745 (AbrA1/A2) was involved in negative regulation of antibiotic production, and acted also as a negative regulator of the morphological differentiation. By contrast, the system SCO4596/4597/4598 (AbrC1/C2/C3), composed of two histidine kinases and one response regulator, had positive effects on both morphological development and antibiotic production. Microarray analyses of the ΔabrC1/C2/C3 and wild-type transcriptomes revealed downregulation of actII-ORF4 and cdaR genes, the actinorhodin and calcium-dependent antibiotic pathway-specific regulators respectively. These results demonstrated the involvement of these new two-component systems in antibiotic production and morphological differentiation by different approaches. One is a pleiotropic negative regulator: abrA1/A2. The other one is a positive regulator composed of three elements, two histidine kinases and one response regulator: abrC1/C2/C3.

An ecological perspective of microbial secondary metabolism

Bacteria and fungi produce a remarkable array of bioactive small molecules. Many of these have found use in medicine as chemotherapies to treat diseases ranging from infection and cancer to hyperlipidemia and autoimmune disorders. The applications may or may not reflect the actual targets for these compounds. Through careful studies of microbes, their associated molecules and their targets, a growing understanding of the ecology of microbial secondary metabolism is emerging that exposes the central role of secondary metabolites in many complex biological systems.

How to discover new antibiotics: harvesting the parvome

There is a dire need for new antibiotics; commercial discovery programs have essentially dried up and there is talk of 'a return to the pre-antibiotic era'. Natural products are an inexhaustible source of bioactive compounds (antibiotics among them), and recent technical advances such as DNA sequencing and bioinformatics offer new approaches to small molecule discovery. Given that nucleotide sequence studies of actinomycetes genomes reveal the presence of 20 or more pathways for the synthesis of bioactive compounds, 'mining' these sequences offers the potential of expanding the repertoire of antibiotics and other drugs. Combined with advanced chemical separation and characterization techniques, the construction of large chemically diverse libraries of bioactive compounds for therapeutic applications is a realistic near-term goal.