The MtrAB two-component system controls antibiotic production in Streptomyces coelicolor A3(2)

Characterization of a pleiotropic regulator MtrA in Streptomyces avermitilis controlling avermectin production and morphological differentiation

BACKGROUND

The macrolide antibiotic avermectin, a natural product derived from Streptomyces avermitilis, finds extensive applications in agriculture, animal husbandry and medicine. The mtrA (sav_5063) gene functions as a transcriptional regulator belonging to the OmpR family. As a pleiotropic regulator, mtrA not only influences the growth, development, and morphological differentiation of strains but also modulates genes associated with primary metabolism. However, the regulatory role of MtrA in avermectin biosynthesis remains to be elucidated.

RESULTS

In this study, we demonstrated that MtrA, a novel OmpR-family transcriptional regulator in S. avermitilis, exerts global regulator effects by negatively regulating avermectin biosynthesis and cell growth while positively controlling morphological differentiation. The deletion of the mtrA gene resulted in an increase in avermectin production, accompanied by a reduction in biomass and a delay in the formation of aerial hyphae and spores. The Electrophoretic Mobility Shift Assay (EMSA) revealed that MtrA exhibited binding affinity towards the upstream region of aveR, the intergenic region between aveA1 and aveA2 genes, as well as the upstream region of aveBVIII in vitro. These findings suggest that MtrA exerts a negative regulatory effect on avermectin biosynthesis by modulating the expression of avermectin biosynthesis cluster genes. Transcriptome sequencing and fluorescence quantitative PCR analysis showed that mtrA deletion increased the transcript levels of the cluster genes aveR, aveA1, aveA2, aveC, aveE, aveA4 and orf-1, which explains the observed increase in avermectin production in the knockout strain. Furthermore, our findings demonstrate that MtrA positively regulates the cell division and differentiation genes bldM and ssgC, while exerting a negative regulatory effect on bldD, thereby modulating the primary metabolic processes associated with cell division, differentiation and growth in S. avermitilis, consequently impacting avermectin biosynthesis.

CONCLUSIONS

In this study, we investigated the negative regulatory effect of the global regulator MtrA on avermectin biosynthesis and its effects on morphological differentiation and cell growth, and elucidated its transcriptional regulatory mechanism. Our findings indicate that MtrA plays crucial roles not only in the biosynthesis of avermectin but also in coordinating intricate physiological processes in S. avermitilis. These findings provide insights into the synthesis of avermectin and shed light on the primary and secondary metabolism of S. avermitilis mediated by OmpR-family regulators.

AlphaFold2 captures the conformational landscape of the HAMP signaling domain

In this study, we present a conformational landscape of 5000 AlphaFold2 models of the Histidine kinases, Adenyl cyclases, Methyl-accepting proteins and Phosphatases (HAMP) domain, a short helical bundle that transduces signals from sensors to effectors in two-component signaling proteins such as sensory histidine kinases and chemoreceptors. The landscape reveals the conformational variability of the HAMP domain, including rotations, shifts, displacements, and tilts of helices, many combinations of which have not been observed in experimental structures. HAMP domains belonging to a single family tend to occupy a defined region of the landscape, even when their sequence similarity is low, suggesting that individual HAMP families have evolved to operate in a specific conformational range. The functional importance of this structural conservation is illustrated by poly-HAMP arrays, in which HAMP domains from families with opposite conformational preferences alternate, consistent with the rotational model of signal transduction. The only poly-HAMP arrays that violate this rule are predicted to be of recent evolutionary origin and structurally unstable. Finally, we identify a family of HAMP domains that are likely to be dynamic due to the presence of a conserved pi-helical bulge. All code associated with this work, including a tool for rapid sequence-based prediction of the rotational state in HAMP domains, is deposited at https://github.com/labstructbioinf/HAMPpred.

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.

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

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

Trends in the two-component system's role in the synthesis of antibiotics by Streptomyces

Despite the advances in understanding the regulatory networks for secondary metabolite production in Streptomyces, the participation of the two-component systems (TCS) in this process still requires better characterization. These sensing systems and their responses to environmental stimuli have been described by evaluating mutant strains with techniques that allow in-depth regulatory responses. However, defining the stimulus that triggers their activation is still a task. The transmembrane nature of the sensor kinases and the high content of GC in the streptomycetes represent significant challenges in their study. In some examples, adding elements to the assay medium has determined the respective ligand. However, a complete TCS description and characterization requires specific amounts of the involved proteins that are most difficult to obtain. The availability of enough sensor histidine kinase concentrations could facilitate the identification of the ligand-protein interaction, and besides would allow the establishment of its phosphorylation mechanisms and determine their tridimensional structure. Similarly, the advances in the development of bioinformatics tools and novel experimental techniques also promise to accelerate the TCSs description and provide knowledge on their participation in the regulation processes of secondary metabolite formation. This review aims to summarize the recent advances in the study of TCSs involved in antibiotic biosynthesis and to discuss alternatives to continue their characterization. KEY POINTS: • TCSs are the environmental signal transducers more abundant in nature. • The Streptomyces have some of the highest number of TCSs found in bacteria. • The study of signal transduction between SHKs and RRs domains is a big challenge.

Osmotic stress responses and the biology of the second messenger c-di-AMP in Streptomyces

Streptomyces are prolific antibiotic producers that thrive in soil, where they encounter diverse environmental cues, including osmotic challenges caused by rainfall and drought. Despite their enormous value in the biotechnology sector, which often relies on ideal growth conditions, how Streptomyces react and adapt to osmotic stress is heavily understudied. This is likely due to their complex developmental biology and an exceptionally broad number of signal transduction systems. With this review, we provide an overview of Streptomyces' responses to osmotic stress signals and draw attention to open questions in this research area. We discuss putative osmolyte transport systems that are likely involved in ion balance control and osmoadaptation and the role of alternative sigma factors and two-component systems (TCS) in osmoregulation. Finally, we highlight the current view on the role of the second messenger c-di-AMP in cell differentiation and the osmotic stress responses with specific emphasis on the two models, S. coelicolor and S. venezuelae.

An overview on the two-component systems of Streptomyces coelicolor

The two-component system (TCS) found in various organisms is a regulatory system, which is involved in the response by the organism to stimuli, thereby regulating the internal behavior of the cell. It is commonly found in prokaryotes and is an important signaling system in bacteria. TCSs are involved in the regulation of physiological and morphological differentiation of the industrially important microbes from the genus Streptomyces, which produce a vast array of bioactive secondary metabolites (SMs). Genetic engineering of TCSs can substantially increase the yield of target SMs, which is valuable for industrial-scale production. Research on TCS has mainly been completed in the model strain Streptomyces coelicolor. In this review, we summarize the recent advances in the functional identification and elucidation of the regulatory mechanisms of various TCSs in S. coelicolor, with a focus on their roles in the biosynthesis of important SMs.

Diversity, mechanisms and beneficial features of phosphate-solubilizing Streptomyces in sustainable agriculture: A review

Currently, the use of phosphate (P) biofertilizers among many bioformulations has attracted a large amount of interest for sustainable agriculture. By acting as growth promoters, members of the Streptomyces genus can positively interact with plants. Several studies have shown the great potential of this bacterial group in supplementing P in a soluble, plant-available form by several mechanisms. Furthermore, some P-solubilizing Streptomyces (PSS) species are known as plant growth-promoting rhizobacteria that are able to promote plant growth through other means, such as increasing the availability of soil nutrients and producing a wide range of antibiotics, phytohormones, bioactive compounds, and secondary metabolites other than antimicrobial compounds. Therefore, the use of PSS with multiple plant growth-promoting activities as an alternative strategy appears to limit the negative impacts of chemical fertilizers in agricultural practices on environmental and human health, and the potential effects of these PSS on enhancing plant fitness and crop yields have been explored. However, compared with studies on the use of other gram-positive bacteria, studies on the use of Streptomyces as P solubilizers are still lacking, and their results are unclear. Although PSS have been reported as potential bioinoculants in both greenhouse and field experiments, no PSS-based biofertilizers have been commercialized to date. In this regard, this review provides an overview mainly of the P solubilization activity of Streptomyces species, including their use as P biofertilizers in competitive agronomic practices and the mechanisms through which they release P by solubilization/mineralization, for both increasing P use efficiency in the soil and plant growth. This review further highlights and discusses the beneficial association of PSS with plants in detail with the latest developments and research to expand the knowledge concerning the use of PSS as P biofertilizers for field applications by exploiting their numerous advantages in improving crop production to meet global food demands.

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.

Comparative Proteomic Analysis of Transcriptional and Regulatory Proteins Abundances in S. lividans and S. coelicolor Suggests a Link between Various Stresses and Antibiotic Production

Streptomyces coelicolor and Streptomyces lividans constitute model strains to study the regulation of antibiotics biosynthesis in Streptomyces species since these closely related strains possess the same pathways directing the biosynthesis of various antibiotics but only S. coelicolor produces them. To get a better understanding of the origin of the contrasted abilities of these strains to produce bioactive specialized metabolites, these strains were grown in conditions of phosphate limitation or proficiency and a comparative analysis of their transcriptional/regulatory proteins was carried out. The abundance of the vast majority of the 355 proteins detected greatly differed between these two strains and responded differently to phosphate availability. This study confirmed, consistently with previous studies, that S. coelicolor suffers from nitrogen stress. This stress likely triggers the degradation of the nitrogen-rich peptidoglycan cell wall in order to recycle nitrogen present in its constituents, resulting in cell wall stress. When an altered cell wall is unable to fulfill its osmo-protective function, the bacteria also suffer from osmotic stress. This study thus revealed that these three stresses are intimately linked in S. coelicolor. The aggravation of these stresses leading to an increase of antibiotic biosynthesis, the connection between these stresses, and antibiotic production are discussed.

Biocontrol of Wilt disease of rice seedlings incited by Fusarium oxysporum through soil application of Streptomyces chilikensis RC1830

The genus Streptomyces includes many antifungal metabolite producing novel strains. Fusarium oxysporum a soil-inhabiting pathogenic fungi, that affects rice to cause wilt disease. This work demonstrates the efficacy of novel Streptomyces chilikensis strain RC1830, previously isolated from estuarine habitat Chilika Lake in preventing the F. oxysporum wilting/root rot disease and promoting the growth of rice (Var. Swarna) seedlings. A total of 25 different compounds were identified from crude extracts of S. chilikensis RC1830 by GC-MS. In pot trial experiments, Streptomyces treated rice seedlings showed significantly reduced Disease severity index (DSI) by 80.51%. The seedlings growth parameters (root length, root fresh weight and root dry weight )were also increased by 53.91%, 62.5%, 73.46% respectively in Streptomyces treated groups of seedlings compared to Fusarium infected seedlings. Similarly, the shoot length, shoot dry weight and shoot fresh weight were also increased by 26%, 58% and 34.4% respectively in Streptomyces treated groups of seedlings compared to Fusarium infected seedlings. Formulations of the strain were prepared using seven organic & inorganic wastes as the carrier material and the shelf lives of the propagules were also monitored. Vermiculite and activated charcoal formulations stored at 4°C exhibited a higher viable cell count after 3 months of storage.

ActinoBase: tools and protocols for researchers working on Streptomyces and other filamentous actinobacteria

Actinobacteria is an ancient phylum of Gram-positive bacteria with a characteristic high GC content to their DNA. The ActinoBase Wiki is focused on the filamentous actinobacteria, such as Streptomyces species, and the techniques and growth conditions used to study them. These organisms are studied because of their complex developmental life cycles and diverse specialised metabolism which produces many of the antibiotics currently used in the clinic. ActinoBase is a community effort that provides valuable and freely accessible resources, including protocols and practical information about filamentous actinobacteria. It is aimed at enabling knowledge exchange between members of the international research community working with these fascinating bacteria. ActinoBase is an anchor platform that underpins worldwide efforts to understand the ecology, biology and metabolic potential of these organisms. There are two key differences that set ActinoBase apart from other Wiki-based platforms: [1] ActinoBase is specifically aimed at researchers working on filamentous actinobacteria and is tailored to help users overcome challenges working with these bacteria and [2] it provides a freely accessible resource with global networking opportunities for researchers with a broad range of experience in this field.

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.

Mutation of MtrA at the Predicted Phosphorylation Site Abrogates Its Role as a Global Regulator in Streptomyces venezuelae

The global regulator MtrA controls development and primary and secondary metabolism in Streptomyces species. However, residues critical for its function have not yet been characterized. In this study, we identified residue D53 as the potential phosphorylation site of MtrA from Streptomyces venezuelae, a model Streptomyces strain. MtrA variants with amino acid substitutions at the D53 site were generated, and the effects of these substitutions were evaluated in vitro and in vivo. We showed that, although substitutions at D53 did not alter MtrA's secondary structure, the MtrA D53 protein variants lost the ability to bind known MtrA recognition sequences (MtrA sites) in electrophoretic mobility shift assays. Complementation of the ΔmtrA strain with MtrA D53 protein variants did not affect overall strain growth. However, in comparison to the wild-type strain, chloramphenicol and jadomycin production were aberrant in the D53 variant strains, with levels similar to the levels in the ΔmtrA strain. Transcriptional analysis showed that the expression patterns of genes were also similar in the ΔmtrA strain and the D53 variant strains. Although the D53 protein variants and wild-type MtrA were produced at similar levels in S. venezuelae, chromatin immunoprecipitation-quantitative PCR results indicated that replacing the D53 residue rendered the altered proteins unable to bind MtrA sites in vivo, including MtrA sites that regulate genes involved in nitrogen metabolism and in chloramphenicol and jadomycin biosynthesis. In conclusion, our study demonstrates that the predicted phosphorylation site D53 is critical for the role of MtrA in regulation and suggests that MtrA functions in a phosphorylated form in the genus Streptomyces. IMPORTANCE Although phosphorylation has been shown to be essential for the activation of many response regulator proteins of two-component systems, the role of the phosphorylation site in the function of the global regulator MtrA in the genus Streptomyces has not been reported. In this study, we generated Streptomyces mutants that had amino acid substitutions at the predicted phosphorylation site of MtrA, and the effects of the substitutions were investigated by comparing the phenotypes of the resulting strains and their gene expression patterns with those of the wild-type strain and an MtrA deletion mutant. The ability of the altered proteins to bind known promoter targets in vitro was also evaluated. Our analyses showed that the predicted phosphorylation site D53 is critical for MtrA binding in vitro and for the normal functioning of MtrA in vivo. These studies further demonstrate the importance of MtrA as a global regulator in the genus Streptomyces.

Effects of dual deletion of glnR and mtrA on expression of nitrogen metabolism genes in Streptomyces venezuelae

GlnR activates nitrogen metabolism genes under nitrogen‐limited conditions, whereas MtrA represses these genes under nutrient‐rich conditions in Streptomyces. In this study, we compared the transcription patterns of nitrogen metabolism genes in a double deletion mutant (ΔmtrA‐glnR) lacking both mtrA and glnR and in mutants lacking either mtrA (ΔmtrA) or glnR (ΔglnR). The nitrogen metabolism genes were expressed similarly in ΔmtrA‐glnR and ΔglnR under both nitrogen‐limited and nutrient‐rich conditions, with patterns distinctly different from that of ΔmtrA, suggesting a decisive role for GlnR in the control of nitrogen metabolism genes and further suggesting that regulation of these genes by MtrA is GlnR‐dependent. MtrA and GlnR utilize the same binding sites upstream of nitrogen metabolism genes, and we showed stronger in vivo binding of MtrA to these sites under nutrient‐rich conditions and of GlnR under nitrogen‐limited conditions, consistent with the higher levels of MtrA or GlnR under those respective conditions. In addition, we showed that both mtrA and glnR are self‐regulated. Our study provides new insights into the regulation of nitrogen metabolism genes in Streptomyces.

The Response Regulator MacR and its Potential in Improvement of Antibiotic Production in Streptomyces coelicolor

We previously reported that the two-component system MacRS regulates morphogenesis and production of the blue-pigmented antibiotic actinorhodin (ACT) in Streptomyces coelicolor. In this study, the role of MacRS was further extended to include control of the production of the red-pigmented antibiotic undecylprodigiosin (RED) and the calcium-dependent antibiotic (CDA), and control of other important cellular activities. Our data indicated that disruption of the MacRS TCS reduced production not only of ACT but also of RED and CDA. RNA-Seq analysis revealed that genes involved in both secondary metabolism and primary metabolism are differentially expressed in the MacRS deletion mutant ΔmacRS. Moreover, we found that genes of the Zur regulon are also markedly downregulated in ΔmacRS, suggesting a role for macRS in zinc homeostasis. In addition to previously identified MacR sites with strong matches to the MacR consensus recognition sequence, a genome-wide search revealed over one hundred less-stringent matches, including potential sites upstream of absR1, crgA, and smeA. Electrophoretic mobility shift assays demonstrated that MacR binds some of these sites in vitro. Although there is no strong MacR site upstream of the ACT regulatory gene actII-orf4 (sco5085), we showed that an engineered MacR site enhanced ACT production, providing an approach for modulating production of useful compounds. Altogether, our work suggests an important role for MacRS in a range of cellular activities in Streptomyces and its potential application in strain engineering.

Impact of MtrA on phosphate metabolism genes and the response to altered phosphate conditions in Streptomyces

Phosphate metabolism is known to be regulated by the PhoPR regulatory system in Streptomyces and some other bacteria. In this study, we report that MtrA also regulates phosphate metabolism in Streptomyces. Our data showed that, in Streptomyces coelicolor, MtrA regulates not only phosphate metabolism genes such as phoA but also phoP under different phosphate conditions, including growth on rich complex media without added inorganic phosphate and on defined media with low or high concentrations of inorganic phosphate. Cross-regulation was also observed among mtrA, phoP and glnR under these conditions. We demonstrated both in vitro and in vivo binding of MtrA to the promoter regions of genes associated with phosphate metabolism and to the intergenic region between phoR and phoU, indicating that these phosphate metabolism genes are targets of MtrA. We further showed that MtrA in S. lividans and S. venezuelae has detectable regulatory effects on expression of phosphate metabolism genes. Additionally, the MtrA homologue from Corynebacterium glutamicum bound predicted MtrA sites of multiple phosphate metabolism genes, implying its potential for regulating phosphate metabolism in this species. Overall, our findings support MtrA as a major regulator for phosphate metabolism in Streptomyces and also potentially in other actinobacteria.

Identification of the cognate response regulator of the orphan histidine kinase OhkA involved in both secondary metabolism and morphological differentiation in Streptomyces coelicolor

In the model actinomycete strain, Streptomyces coelicolor, an orphan histidine kinase (HK) named OhkA (encoded by SCO1596), which belongs to bacterial two-component regulatory systems (TCSs), has been identified as being involved in the regulation of both antibiotic biosynthesis and morphological development. However, its cognate response regulator (RR) remains unknown due to its isolated genetic location on the genome, which impedes the elucidation of the mechanism underlying OhkA-mediated regulation. Here, we identified the orphan RR OrrA (encoded by SCO3008) as the cognate RR of OhkA according to mutant phenotypic changes, transcriptomics analysis, and bacterial two-hybrid experiment. Considering that the partner RR of the orphan HK is also orphan, a library of mutants with in-frame individual deletion of these functionally unknown orphan RR-encoding genes were generated. Through phenotypic analysis, it was found that the ∆orrA mutant exhibited similar phenotypic changes as that of the ∆ohkA mutant, showing increased production of actinorhodin (ACT) and undecylprodigiosin (RED), and pink colony surface. Further transcriptomics analysis showed these two mutants exhibited highly similar transcriptomics profiles. Finally, the direct interaction between OhkA and OrrA was revealed by bacterial two-hybrid system. The identification of the partner RR of OhkA lays a good foundation for an in-depth elucidation of the molecular mechanism underlying OhkA-mediated regulation of development and antibiotic biosynthesis in Streptomyces. KEY POINTS: • OrrA was identified as the partner RR of the orphan histidine kinase OhkA. • The ∆orrA and ∆ohkA mutants showed similar phenotype and transcriptomic profiling. • Specific interaction of OrrA and OhkA was revealed by bacterial two-hybrid system.

Genetic Network Architecture and Environmental Cues Drive Spatial Organization of Phenotypic Division of Labor in Streptomyces coelicolor

A number of bacteria are known to differentiate into cells with distinct phenotypic traits during processes such as biofilm formation or the development of reproductive structures. These cell types, by virtue of their specialized functions, embody a division of labor. However, how bacteria build spatial patterns of differentiated cells is not well understood. Here, we examine the factors that drive spatial patterns in divisions of labor in colonies of Streptomyces coelicolor, a multicellular bacterium capable of synthesizing an array of antibiotics and forming complex reproductive structures (e.g., aerial hyphae and spores). Using fluorescent reporters, we demonstrate that the pathways for antibiotic biosynthesis and aerial hypha formation are activated in distinct waves of gene expression that radiate outwards in S. coelicolor colonies. We also show that the spatiotemporal separation of these cell types depends on a key activator in the developmental pathway, AdpA. Importantly, when we manipulated local gradients by growing competing microbes nearby, or through physical disruption, expression in these pathways could be decoupled and/or disordered, respectively. Finally, the normal spatial organization of these cell types was partially restored with the addition of a siderophore, a public good made by these organisms, to the growth medium. Together, these results indicate that spatial divisions of labor in S. coelicolor colonies are determined by a combination of physiological gradients and regulatory network architecture, key factors that also drive patterns of cellular differentiation in multicellular eukaryotic organisms.

Differential regulation of undecylprodigiosin biosynthesis in the yeast-scavenging Streptomyces strain MBK6

Streptomyces are efficient chemists with a capacity to generate diverse and potent chemical scaffolds. The secondary metabolism of these soil-dwelling prokaryotes is stimulated upon interaction with other microbes in their complex ecosystem. We observed such an interaction when a Streptomyces isolate was cultivated in a media supplemented with dead yeast cells. Whole-genome analysis revealed that Streptomyces sp. MBK6 harbors the red cluster that is cryptic under normal environmental conditions. An interactive culture of MBK6 with dead yeast triggered the production of the red pigments metacycloprodigiosin and undecylprodigiosin. Streptomyces sp. MBK6 scavenges dead-yeast cells and preferentially grows in aggregates of sequestered yeasts within its mycelial network. We identified that the activation depends on the cluster-situated regulator, mbkZ, which may act as a cross-regulator. Cloning of this master regulator mbkZ in S. coelicolor with a constitutive promoter and promoter-deprived conditions generated different production levels of the red pigments. These surprising results were further validated by DNA–protein binding assays. The presence of the red cluster in Streptomyces sp. MBK6 provides a vivid example of horizontal gene transfer of an entire metabolic pathway followed by differential adaptation to a new environment through mutations in the receiver domain of the key regulatory protein MbkZ.

Systems and synthetic biology to elucidate secondary metabolite biosynthetic gene clusters encoded in Streptomyces genomes

Covering: 2010 to 2020 Over the last few decades, Streptomyces have been extensively investigated for their ability to produce diverse bioactive secondary metabolites. Recent advances in Streptomyces research have been largely supported by improvements in high-throughput technology 'omics'. From genomics, numerous secondary metabolite biosynthetic gene clusters were predicted, increasing their genomic potential for novel bioactive compound discovery. Additional omics, including transcriptomics, translatomics, interactomics, proteomics and metabolomics, have been applied to obtain a system-level understanding spanning entire bioprocesses of Streptomyces, revealing highly interconnected and multi-layered regulatory networks for secondary metabolism. The comprehensive understanding derived from this systematic information accelerates the rational engineering of Streptomyces to enhance secondary metabolite production, integrated with the exploitation of the highly efficient 'Design-Build-Test-Learn' cycle in synthetic biology. In this review, we describe the current status of omics applications in Streptomyces research to better understand the organism and exploit its genetic potential for higher production of valuable secondary metabolites and novel secondary metabolite discovery.

Antibiotic Production and Antibiotic Resistance: The Two Sides of AbrB1/B2, a Two-Component System of Streptomyces coelicolor

Antibiotic resistance currently presents one of the biggest threats to humans. The development and implementation of strategies against the spread of superbugs is a priority for public health. In addition to raising social awareness, approaches such as the discovery of new antibiotic molecules and the elucidation of resistance mechanisms are common measures. Accordingly, the two-component system (TCS) of Streptomyces coelicolor AbrB1/B2, offer amenable ways to study both antibiotic production and resistance. Global transcriptomic comparisons between the wild-type strain S. coelicolor M145 and the mutant ΔabrB, using RNA-Seq, showed that the AbrB1/B2 TCS is implicated in the regulation of different biological processes associated with stress responses, primary and secondary metabolism, and development and differentiation. The ΔabrB mutant showed the up-regulation of antibiotic biosynthetic gene clusters and the down-regulation of the vancomycin resistance gene cluster, according to the phenotypic observations of increased antibiotic production of actinorhodin and undecylprodigiosin, and greater susceptibility to vancomycin. The role of AbrB1/B2 in vancomycin resistance has also been shown by an in silico analysis, which strongly indicates that AbrB1/B2 is a homolog of VraR/S from Staphylococcus aureus and LiaR/S from Enterococcus faecium/Enterococcus faecalis, both of which are implied in vancomycin resistance in these pathogenic organisms that present a serious threat to public health. The results obtained are interesting from a biotechnological perspective since, on one hand, this TCS is a negative regulator of antibiotic production and its high degree of conservation throughout Streptomyces spp. makes it a valuable tool for improving antibiotic production and the discovery of cryptic metabolites with antibiotic action. On the other hand, AbrB1/B2 contributes to vancomycin resistance and is a homolog of VraR/S and LiaR/S, important regulators in clinically relevant antibiotic-resistant bacteria. Therefore, the study of AbrB1/B2 could provide new insight into the mechanism of this type of resistance.

Impact on Multiple Antibiotic Pathways Reveals MtrA as a Master Regulator of Antibiotic Production in Streptomyces spp. and Potentially in Other Actinobacteria

Regulation of antibiotic production by Streptomyces is complex. We report that the response regulator MtrA is a master regulator for antibiotic production in Streptomyces Deletion of MtrA altered production of actinorhodin, undecylprodigiosin, calcium-dependent antibiotic, and the yellow-pigmented type I polyketide and resulted in altered expression of the corresponding gene clusters in S. coelicolor Integrated in vitro and in vivo analyses identified MtrA binding sites upstream of cdaR, actII-orf4, and redZ and between cpkA and cpkD MtrA disruption also led to marked changes in chloramphenicol and jadomycin production and in transcription of their biosynthetic gene clusters (cml and jad, respectively) in S. venezuelae, and MtrA sites were identified within cml and jad MtrA also recognized predicted sites within the avermectin and oligomycin pathways in S. avermitilis and in the validamycin gene cluster of S. hygroscopicus The regulator GlnR competed for several MtrA sites and impacted production of some antibiotics, but its effects were generally less dramatic than those of MtrA. Additional potential MtrA sites were identified in a range of other antibiotic biosynthetic gene clusters in Streptomyces species and other actinobacteria. Overall, our study suggests a universal role for MtrA in antibiotic production in Streptomyces and potentially other actinobacteria.IMPORTANCE In natural environments, the ability to produce antibiotics helps the producing host to compete with surrounding microbes. In Streptomyces, increasing evidence suggests that the regulation of antibiotic production is complex, involving multiple regulatory factors. The regulatory factor MtrA is known to have additional roles beyond controlling development, and using bioassays, transcriptional studies, and DNA-binding assays, our study identified MtrA recognition sequences within multiple antibiotic pathways and indicated that MtrA directly controls the production of multiple antibiotics. Our analyses further suggest that this role of MtrA is evolutionarily conserved in Streptomyces species, as well as in other actinobacterial species, and also suggest that MtrA is a major regulatory factor in antibiotic production and in the survival of actinobacteria in nature.

System-level understanding of gene expression and regulation for engineering secondary metabolite production in Streptomyces

The gram-positive bacterium, Streptomyces, is noticed for its ability to produce a wide array of pharmaceutically active compounds through secondary metabolism. To discover novel bioactive secondary metabolites and increase the production, Streptomyces species have been extensively studied for the past decades. Among the cellular components, RNA molecules play important roles as the messengers for gene expression and diverse regulations taking place at the RNA level. Thus, the analysis of RNA-level regulation is critical to understanding the regulation of Streptomyces’ metabolism and secondary metabolite production. A dramatic advance in Streptomyces research was made recently, by exploiting high-throughput technology to systematically understand RNA levels. In this review, we describe the current status of the system-wide investigation of Streptomyces in terms of RNA, toward expansion of its genetic potential for secondary metabolite synthesis.

A MarR-family transcriptional factor MapR positively regulates actinorhodin production in Streptomyces coelicolor

Production of secondary metabolites is tightly regulated by transcriptional regulators in Streptomyces. Many regulators have been identified by gene disruption in Streptomyces; however, there are still unknown regulatory genes yet to be revealed due to gene silence under laboratory culture conditions. In present study, a T7 expression system was used to identify novel regulators by gene deletion and constitutive-expression of nine candidates. We reported that the constitutive-expression of a MarR family transcriptional regulator MapR (SCO2398) resulted in increase of actinorhodin (ACT) production. The expression profiling indicated that MapR regulates ACT production through a transcriptional cascade. Collectively, our study suggests that MapR serves as a novel positive regulator of ACT production.

DNA-Binding and Transcription Activation by Unphosphorylated Response Regulator AgrR From Cupriavidus metallidurans Involved in Silver Resistance

Even though silver and silver nanoparticles at low concentrations are considered safe for human health, their steadily increasing use and associated release in nature is not without risk since it may result in the selection of silver-resistant microorganisms, thus impeding the utilization of silver as antimicrobial agent. Furthermore, increased resistance to metals may be accompanied by increased antibiotic resistance. Inactivation of the histidine kinase and concomitant upregulation of the cognate response regulator (RR) of the AgrRS two-component system was previously shown to play an important role in the increased silver resistance of laboratory adapted mutants of Cupriavidus metallidurans. However, binding of AgrR, a member of the OmpR/PhoP family of RRs with a conserved phosphoreceiver aspartate residue, to potential target promoters has never been demonstrated. Here we identify differentially expressed genes in the silver-resistant mutant NA4S in non-selective conditions by RNA-seq and demonstrate sequence-specific binding of AgrR to six selected promoter regions of upregulated genes and divergent operons. We delimit binding sites by DNase I and in gel copper-phenanthroline footprinting of AgrR-DNA complexes, and establish a high resolution base-specific contact map of AgrR-DNA interactions using premodification binding interference techniques. We identified a 16-bp core AgrR binding site (AgrR box) arranged as an imperfect inverted repeat of 6 bp (ATTACA) separated by 4 bp variable in sequence (6-4-6). AgrR interacts with two major groove segments and the intervening minor groove, all aligned on one face of the helix. Furthermore, an additional in phase imperfect direct repeat of the half-site may be observed slightly up and/or downstream of the inverted repeat at some operators. Mutant studies indicated that both inverted and direct repeats contribute to AgrR binding in vitro and AgrR-mediated activation in vivo. From the position of the AgrR box it appears that AgrR may act as a Type II activator for most investigated promoters, including positive autoregulation. Furthermore, we show in vitro binding and in vivo activation with dephosphomimetic AgrR mutant D51A, indicating that unphosphorylated AgrR is the active form of the RR in mutant NA4S.

The Balance Metabolism Safety Net: Integration of Stress Signals by Interacting Transcriptional Factors in Streptomyces and Related Actinobacteria

Soil dwelling Streptomyces species are faced with large variations in carbon or nitrogen sources, phosphate, oxygen, iron, sulfur, and other nutrients. These drastic changes in key nutrients result in an unbalanced metabolism that have undesirable consequences for growth, cell differentiation, reproduction, and secondary metabolites biosynthesis. In the last decades evidence has accumulated indicating that mechanisms to correct metabolic unbalances in Streptomyces species take place at the transcriptional level, mediated by different transcriptional factors. For example, the master regulator PhoP and the large SARP-type regulator AfsR bind to overlapping sequences in the afsS promoter and, therefore, compete in the integration of signals of phosphate starvation and S-adenosylmethionine (SAM) concentrations. The cross-talk between phosphate control of metabolism, mediated by the PhoR-PhoP system, and the pleiotropic orphan nitrogen regulator GlnR, is very interesting; PhoP represses GlnR and other nitrogen metabolism genes. The mechanisms of control by GlnR of several promoters of ATP binding cassettes (ABC) sugar transporters and carbon metabolism are highly elaborated. Another important cross-talk that governs nitrogen metabolism involves the competition between GlnR and the transcriptional factor MtrA. GlnR and MtrA exert opposite effects on expression of nitrogen metabolism genes. MtrA, under nitrogen rich conditions, represses expression of nitrogen assimilation and regulatory genes, including GlnR, and competes with GlnR for the GlnR binding sites. Strikingly, these sites also bind to PhoP. Novel examples of interacting transcriptional factors, discovered recently, are discussed to provide a broad view of this interactions. Altogether, these findings indicate that cross-talks between the major transcriptional factors protect the cell metabolic balance. A detailed analysis of the transcriptional factors binding sequences suggests that the transcriptional factors interact with specific regions, either by overlapping the recognition sequence of other factors or by binding to adjacent sites in those regions. Additional interactions on the regulatory backbone are provided by sigma factors, highly phosphorylated nucleotides, cyclic dinucleotides, and small ligands that interact with cognate receptor proteins and with TetR-type transcriptional regulators. We propose to define the signal integration DNA regions (so called integrator sites) that assemble responses to different stress, nutritional or environmental signals. These integrator sites constitute nodes recognized by two, three, or more transcriptional factors to compensate the unbalances produced by metabolic stresses. This interplay mechanism acts as a safety net to prevent major damage to the metabolism under extreme nutritional and environmental conditions.

Dissolution of the Disparate: Co-ordinate Regulation in Antibiotic Biosynthesis

Discovering new antibiotics is vital to combat the growing threat of antimicrobial resistance. Most currently used antibiotics originate from the natural products of actinomycete bacteria, particularly Streptomyces species, that were discovered over 60 years ago. However, genome sequencing has revealed that most antibiotic-producing microorganisms encode many more natural products than previously thought. Biosynthesis of these natural products is tightly regulated by global and cluster situated regulators (CSRs), most of which respond to unknown environmental stimuli, and this likely explains why many biosynthetic gene clusters (BGCs) are not expressed under laboratory conditions. One approach towards novel natural product discovery is to awaken these cryptic BGCs by re-wiring the regulatory control mechanism(s). Most CSRs bind intergenic regions of DNA in their own BGC to control compound biosynthesis, but some CSRs can control the biosynthesis of multiple natural products by binding to several different BGCs. These cross-cluster regulators present an opportunity for natural product discovery, as the expression of multiple BGCs can be affected through the manipulation of a single regulator. This review describes examples of these different mechanisms, including specific examples of cross-cluster regulation, and assesses the impact that this knowledge may have on the discovery of novel natural products.

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.

Two amino acids missing of MtrA resulted in increased erythromycin level and altered phenotypes in Saccharopolyspora erythraea

The MtrA-MtrB two-component regulatory system is highly conserved in Actinobacteria and plays crucial roles in cell cycle progression, cell morphology, antibiotic resistance, and osmoprotection. Previously, we revealed that the MtrA protein of Saccharopolyspora erythraea E3 strain (a high erythromycin-producing strain) had a two amino acid (H197 and V198) deletion in the DNA recognition helices of the C-terminal domain compared to the wild type S. erythraea strain NRRL2338. Here, we identified mepA (encoding a membrane protein related to metalloendopeptidases) as an MtrA target gene, and found that deleting the two amino acids in MtrA (MtrA^del) resulted in the loss of its DNA-binding activity for the mepA gene. The mutant MtrA^del lost its regulatory activity and affected various physiological functions consistent with mtrA deletion, including increased erythromycin biosynthesis, enhanced antibiotic resistance, deregulated osmoprotection, and improved transport of substances. The introduction of the wild type mtrA gene into the S. erythraea E3 strain with the mtrA^del gene decreased the erythromycin yield by approximately 50%, confirming that MtrA repressed erythromycin production. These findings demonstrate that MtrA is an important pleiotropic regulator of erythromycin biosynthesis, antibiotic resistance, osmoprotection, and substance transport in S. erythraea and provide new insights for improving erythromycin production. Future studies linking the molecular effects of MtrA to these phenotypes will improve our understanding of the MtrA-MtrB two-component regulatory system in Actinobacteria.

The developmental regulator MtrA binds GlnR boxes and represses nitrogen metabolism genes in Streptomyces coelicolor

In Streptomyces, GlnR is an activator protein that activates nitrogen-assimilation genes under nitrogen-limiting conditions. However, less is known regarding the regulation of these genes under nitrogen-rich conditions. We determined that the developmental regulator MtrA represses nitrogen-assimilation genes in nitrogen-rich media and that it competes with GlnR for binding to GlnR boxes. The GlnR boxes upstream of multiple nitrogen genes, such as amtB, were confirmed as MtrA binding sites in vitro by electrophoretic mobility shift assays and in vivo by ChIP-qPCR analysis. Transcriptional analysis indicated that, on nutrient-rich medium, MtrA profoundly repressed expression of nitrogen-associated genes, indicating opposing roles for MtrA and GlnR in the control of nitrogen metabolism. Using in vitro and in vivo analysis, we also showed that glnR is itself a direct target of MtrA and that MtrA represses glnR transcription. We further demonstrated functional conservation of MtrA homologues in the recognition of GlnR boxes upstream of nitrogen genes from different actinobacterial species. As mtrA and glnR are widespread among actinomycetes, this mechanism of potential competitive control over nitrogen metabolism genes may be common in this group, adding a major new layer of complexity to the known regulatory network for nitrogen metabolism in Streptomyces and related species.

Novel Two-Component System MacRS Is a Pleiotropic Regulator That Controls Multiple Morphogenic Membrane Protein Genes in Streptomyces coelicolor

As with most annotated two-component systems (TCSs) of Streptomyces coelicolor, the function of TCS SCO2120/2121 was unknown. Based on our findings, we have designated this TCS MacRS, for morphogenesis and actinorhodin regulator/sensor. Our study indicated that either single or double mutation of MacRS largely blocked production of actinorhodin but enhanced formation of aerial mycelium. Chromatin immunoprecipitation (ChIP) sequencing, using an S. coelicolor strain expressing MacR-Flag fusion protein, identified in vivo targets of MacR, and DNase I footprinting of these targets revealed a consensus sequence for MacR binding, TGAGTACnnGTACTCA, containing two 7-bp inverted repeats. A genome-wide search revealed sites identical or highly similar to this consensus sequence upstream of six genes encoding putative membrane proteins or lipoproteins. These predicted sites were confirmed as MacR binding sites by DNase I footprinting and electrophoretic mobility shift assays in vitro and by ChIP-quantitative PCR in vivo, and transcriptional analyses demonstrated that MacR significantly impacts expression of these target genes. Disruption of three of these genes, sco6728, sco4924, and sco4011, markedly accelerated aerial mycelium formation, indicating that their gene products are novel morphogenic factors. Two-hybrid assays indicated that these three proteins, which we have named morphogenic membrane protein A (MmpA; SCO6728), MmpB (SCO4924), and MmpC (SCO4011), interact with one another and with the putative membrane protein and MacR target SCO4225. Notably, SAV6081/82 and SVEN1780/81, homologs of MacRS TCS from S. avermitilis and S. venezuelae, respectively, can substitute for MacRS, indicating functional conservation. Our findings reveal a role for MacRS in cellular morphogenesis and secondary metabolism in Streptomyces IMPORTANCE TCSs help bacteria adapt to environmental stresses by altering gene expression. However, the roles and corresponding regulatory mechanisms of most TCSs in the Streptomyces model strain S. coelicolor are unknown. We investigated the previously uncharacterized MacRS TCS and identified the core DNA recognition sequence, two seven-nucleotide inverted repeats, for the DNA-binding protein MacR. We further found that MacR directly controls a group of membrane proteins, including MmpA-C, which are novel morphogenic factors that delay formation of aerial mycelium. We also discovered that these membrane proteins interact with one another and that other Streptomyces species have conserved MacRS homologs. Our findings suggest a conserved role for MacRS in morphogenesis and/or other membrane-associated activities. Additionally, our study showed that MacRS impacts, albeit indirectly, the production of the signature metabolite actinorhodin, further suggesting that MacRS and its homologs function as novel pleiotropic regulatory systems in Streptomyces.

Streptomyces: implications and interactions in plant growth promotion

With the impending increase of the world population by 2050, more activities have been directed toward the improvement of crop yield and a safe environment. The need for chemical-free agricultural practices is becoming eminent due to the effects of these chemicals on the environment and human health. Actinomycetes constitute a significant percentage of the soil microbial community. The Streptomyces genus, which is the most abundant and arguably the most important actinomycetes, is a good source of bioactive compounds, antibiotics, and extracellular enzymes. These genera have shown over time great potential in improving the future of agriculture. This review highlights and buttresses the agricultural importance of Streptomyces through its biocontrol and plant growth-promoting activities. These activities are highlighted and discussed in this review. Some biocontrol products from this genus are already being marketed while work is still ongoing on this productive genus. Compared to more focus on its biocontrol ability, less work has been done on it as a biofertilizer until recently. This genus is as efficient as a biofertilizer as it is as a biocontrol.

Regulation of specialised metabolites in Actinobacteria - expanding the paradigms

The increase in availability of actinobacterial whole genome sequences has revealed huge numbers of specialised metabolite biosynthetic gene clusters, encoding a range of bioactive molecules such as antibiotics, antifungals, immunosuppressives and anticancer agents. Yet the majority of these clusters are not expressed under standard laboratory conditions in rich media. Emerging data from studies of specialised metabolite biosynthesis suggest that the diversity of regulatory mechanisms is greater than previously thought and these act at multiple levels, through a range of signals such as nutrient limitation, intercellular signalling and competition with other organisms. Understanding the regulation and environmental cues that lead to the production of these compounds allows us to identify the role that these compounds play in their natural habitat as well as provide tools to exploit this untapped source of specialised metabolites for therapeutic uses. Here, we provide an overview of novel regulatory mechanisms that act in physiological, global and cluster-specific regulatory manners on biosynthetic pathways in Actinobacteria and consider these alongside their ecological and evolutionary implications.

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.

Essentiality and function of WalK/WalR two-component system: the past, present, and future of research

The WalK/WalR two-component system (TCS), originally identified in Bacillus subtilis, is very highly conserved in gram-positive bacteria, including several important pathogens. The WalK/WalR TCS appears to be involved in the growth of most bacterial species encoding it. Previous studies have indicated conserved functions of this system, defining this signal transduction pathway as a crucial regulatory system for cell wall metabolism. Because of such effects on essential functions, this system is considered a potential target for anti-infective therapeutics. In this review, we discuss the role of WalK/WalR TCS in different bacterial cells, focusing on the function of the genes in its regulon as well as the variations in walRK operon structure, its auxiliary proteins, and the composition of its regulon. We also discuss recent experimental data addressing its essential function and the potential type of signal being sensed by B. subtilis. This review also focuses on the potential future research.