A response-regulator-like activator of antibiotic synthesis from Streptomyces coelicolor A3(2) with an amino-terminal domain that lacks a phosphorylation pocket

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.

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.

Harnessing intercellular signals to engineer the soil microbiome

Covering: Focus on 2015 to 2020Plant and soil microbiomes consist of diverse communities of organisms from across kingdoms and can profoundly affect plant growth and health. Natural product-based intercellular signals govern important interactions between microbiome members that ultimately regulate their beneficial or harmful impacts on the plant. Exploiting these evolved signalling circuits to engineer microbiomes towards beneficial interactions with crops is an attractive goal. There are few reports thus far of engineering the intercellular signalling of microbiomes, but this article argues that it represents a tremendous opportunity for advancing the field of microbiome engineering. This could be achieved through the selection of synergistic consortia in combination with genetic engineering of signal pathways to realise an optimised microbiome.

Fnr Negatively Regulates Prodigiosin Synthesis in Serratia sp. ATCC 39006 During Aerobic Fermentation

The well-known Crp/Fnr family regulator Fnr has long been recognized as an oxygen sensor to regulate multiple biological processes, including the switch between aerobic/anaerobic metabolism, nitrogen fixation, bioluminescence, infection, and virulence. In most cases, Fnr was found to be active under anaerobic conditions. However, its role in aerobic antibiotic metabolism has not yet been revealed. In this research, we report that in the model organism, Serratia sp. ATCC 39006, Fnr (Ser39006_013370) negatively regulates prodigiosin production by binding to the spacer between the −10 and −35 region in the promoter of prodigiosin biosynthetic gene cluster under aerobic conditions. Fnr was also shown to modulate the anti-bacterial activity and motility by regulating pathway-specific regulatory genes, indicating that Fnr acts as a global regulator in Serratia sp. ATCC 39006. For the first time, we describe that Fnr regulates antibiotic synthesis in the presence of oxygen, which expands the known physiological functions of Fnr and benefits the further investigation of this important transcriptional regulator.

Transcriptional regulation of congocidine (netropsin) biosynthesis and resistance

The production of specialized metabolites by Streptomyces bacteria is usually temporally regulated. This regulation is complex and frequently involves both global and pathway-specific mechanisms. Streptomyces ambofaciens ATCC23877 produces several specialized metabolites, including spiramycins, stambomycins, kinamycins and congocidine. The production of the first three molecules has been shown to be controlled by one or several cluster-situated transcriptional regulators. However, nothing is known regarding the regulation of congocidine biosynthesis. Congocidine (netropsin) belongs to the family of pyrrolamide metabolites, which also includes distamycin and anthelvencins. Most pyrrolamides bind into the minor groove of DNA, specifically in A/T-rich regions, which gives them numerous biological activities, such as antimicrobial and antitumoral activities. We previously reported the characterization of the pyrrolamide biosynthetic gene clusters of congocidine (cgc) in S. ambofaciens ATCC23877, distamycin (dst) in Streptomyces netropsis DSM40846 and anthelvencins (ant) in Streptomyces venezuelae ATCC14583. The three gene clusters contain a gene encoding a putative transcriptional regulator, cgc1, dst1 and ant1 respectively. Cgc1, Dst1 and Ant1 present a high percentage of amino acid sequence similarity. We demonstrate here that Cgc1, an atypical orphan response regulator, activates the transcription of all cgc genes in the stationary phase of S. ambofaciens growth. We also show that the cgc cluster is constituted of eight main transcriptional units. Finally, we show that congocidine induces the expression of the transcriptional regulator Cgc1 and of the operon containing the resistance genes (cgc20 and cgc21, coding for an ABC transporter), and propose a model for the transcriptional regulation of the cgc gene cluster. Importance Understanding the mechanisms of regulation of specialized metabolite production can have important implications both at the level of specialized metabolism study (expression of silent gene clusters) and the biotechnological level (increase of the production of a metabolite of interest). We report here a study on the regulation of the biosynthesis of a metabolite from the pyrrolamide family, congocidine. We show that congocidine biosynthesis and resistance is controlled by Cgc1, a cluster-situated regulator. As the gene clusters directing the biosynthesis of the pyrrolamides distamycin and anthelvencin encode a homolog of Cgc1, our findings may be relevant for the biosynthesis of other pyrrolamides. In addition, our results reveal a new type of feed-forward induction mechanism, in which congocidine induces its own biosynthesis through the induction of the transcription of cgc1.

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.

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.

Stringent Starvation Protein Regulates Prodiginine Biosynthesis via Affecting Siderophore Production in Pseudoalteromonas sp. Strain R3

Prodiginines are a family of red-pigmented secondary metabolites with multiple biological activities. The biosynthesis of prodiginines is affected by various physiological and environmental factors. Thus, prodiginine biosynthesis regulation is highly complex and multifaceted. Although the regulatory mechanism for prodiginine biosynthesis has been extensively studied in Serratia and Streptomyces species, little is known about that in the marine betaproteobacterium Pseudoalteromonas In this study, we report that stringent starvation protein A (SspA), an RNA polymerase-associated regulatory protein, is required for the biosynthesis of prodiginine in Pseudoalteromonas sp. strain R3. The strain lacking sspA (ΔsspA) fails to produce prodiginine, which resulted from the downregulation of the prodiginine biosynthetic gene (pig) cluster. The effect of SspA on prodiginine biosynthesis is independent of histone-like nucleoid structuring protein (H-NS) and RpoS (σ^S). Further analysis demonstrates that the ΔsspA strain has a significant decrease in the transcription of the siderophore biosynthesis gene (pvd) cluster, leading to the inhibition of siderophore production and iron uptake. The ΔsspA strain regains the ability to synthesize prodiginine by cocultivation with siderophore producers or the addition of iron. Therefore, we conclude that SspA-regulated prodiginine biosynthesis is due to decreased siderophore levels and iron deficiency. We further show that the iron homeostasis master regulator Fur is also essential for pig transcription and prodiginine biosynthesis. Overall, our results suggest that SspA indirectly regulates the biosynthesis of prodiginine, which is mediated by the siderophore-dependent iron uptake pathway.IMPORTANCE The red-pigmented prodiginines are attracting increasing interest due to their broad biological activities. As with many secondary metabolites, the biosynthesis of prodiginines is regulated by both environmental and physiological factors. At present, studies on the regulation of prodiginine biosynthesis are mainly restricted to Serratia and Streptomyces species. This work focused on the regulatory mechanism of prodiginine biosynthesis in Pseudoalteromonas sp. R3. We found that stringent starvation protein A (SspA) positively regulates prodiginine biosynthesis via affecting the siderophore-dependent iron uptake pathway. The connections among SspA, iron homeostasis, and prodiginine biosynthesis were investigated. These findings uncover a novel regulatory mechanism for prodigiosin biosynthesis.

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.

Genome-wide analysis of the role of the antibiotic biosynthesis regulator AbsA2 in Streptomyces coelicolor A3(2)

The AbsA1-AbsA2 two component signalling system of Streptomyces coelicolor has long been known to exert a powerful negative influence on the production of the antibiotics actinorhodin, undecylprodiginine and the Calcium-Dependent Antibiotic (CDA). Here we report the analysis of a ΔabsA2 deletion strain, which exhibits the classic precocious antibiotic hyper-production phenotype, and its complementation by an N-terminal triple-FLAG-tagged version of AbsA2. The complemented and non-complemented ΔabsA2 mutant strains were used in large-scale microarray-based time-course experiments to investigate the effect of deleting absA2 on gene expression and to identify the in vivo AbsA2 DNA-binding target sites using ChIP-on chip. We show that in addition to binding to the promoter regions of redZ and actII-orfIV AbsA2 binds to several previously unidentified sites within the cda biosynthetic gene cluster within and/or upstream of SCO3215—SCO3216, SCO3217, SCO3229—SCO3230, and SCO3226, and we relate the pattern of AbsA2 binding to the results of the transcriptomic study and antibiotic phenotypic assays. Interestingly, dual ‘biphasic’ ChIP peaks were observed with AbsA2 binding across the regulatory genes actII-orfIV and redZ and the absA2 gene itself, while more conventional single promoter-proximal peaks were seen at the CDA biosynthetic genes suggesting a different mechanism of regulation of the former loci. Taken together the results shed light on the complex mechanism of regulation of antibiotic biosynthesis in Streptomyces coelicolor and the important role of AbsA2 in controlling the expression of three antibiotic biosynthetic gene clusters.

A LuxR family transcriptional regulator AniF promotes the production of anisomycin and its derivatives in Streptomyces hygrospinosus var. beijingensis

The protein synthesis inhibitor anisomycin features a unique benzylpyrrolidine system and exhibits potent selective activity against pathogenic protozoa and fungi. It is one of the important effective components in Agricultural Antibiotic120, which has been widely used as naturally-originated agents for treatment of crop decay in China. The chemical synthesis of anisomycin has recently been reported, but the complex process with low productivity made the biosynthesis still to be a vital mainstay in efforts. The biosynthetic gene cluster (BGC) of anisomycin in Streptomyces hygrospinosus var. beijingensis has been identified in our previous work, while poor understanding of the regulatory mechanism limited the yield enhancement via regulation engineering of S. hygrospinosus var. beijingensis. In this study here, we characterized AniF as an indispensable LuxR family transcriptional regulator for the activation of anisomycin biosynthesis. The genetic manipulations of aniF and the real-time quantitative PCR (RT-qPCR) revealed that it positively regulated the transcription of the anisomycin BGC. Moreover, the overexpression of aniF contributed to the improvement of the production of anisomycin and its derivatives. Dissection of the mechanism underlying the function of AniF revealed that it directly activated the transcription of the genes aniR-G involved in anisomycin biosynthesis. Especially, one AniF-binding site in the promoter region of aniR was identified by DNase I footprinting assay and an inverted repeat sequence (5′-GGGC-3′) composed of two 4-nt half sites in the protected region was found. Taken together, our systematic study confirmed the positive regulatory role of AniF and might facilitate the future construction of engineering strains with high productivity of anisomycin and its derivatives.

Global regulator BldA regulates morphological differentiation and lincomycin production in Streptomyces lincolnensis

Global regulator BldA, the only tRNA for a rare leucine codon UUA, is best known for its ability to affect morphological differentiation and secondary metabolism in the genus Streptomyces. In this study, we confirmed the regulatory function of the bldA gene (Genbank accession no. EU124663.1) in Streptomyces lincolnensis. Disruption of bldA hinders the sporulation and lincomycin production, that can recur when complemented with a functional bldA gene. Western blotting assays demonstrate that translation of the lmbB2 gene which encodes a L-tyrosine hydroxylase is absolutely dependent on BldA; however, mistranslation of the lmbU gene which encodes a cluster-situated regulator (CSR) is observed in a bldA mutant. Intriguingly, when the preferential cognate codon CTG was used, the expression level of LmbU was not the highest compared to the usage of rare codon TTA or CTA, indicating the rare codon in this position is significant for the regulation of lmbU expression. Moreover, replacement of TTA codons in both genes with another leucin codon in the bldA mutant did not restore lincomycin production. Thus, we believe that the bldA gene regulates lincomycin production via controlling the translation of not only lmbB2 and lmbU, but also the other TTA-containing genes. In conclusion, the present study demonstrated the importance of the bldA gene in morphological differentiation and lincomycin production in S. lincolnensis.

New Kid on the Block: LmbU Expands the Repertoire of Specialized Metabolic Regulators in Streptomyces

Streptomyces has an extensive natural product repertoire, including most of the naturally derived antibiotics. Understanding the control of natural product biosynthesis is central to antibiotic discovery and production optimization. Here, Hou et al. (J. Bacteriol. 200:00447-17, 2018, https://doi.org/10.1128/JB.00447-17) report the identification and characterization of a novel regulator-LmbU-that functions primarily as an activator of lincomycin production in Streptomyces lincolnensis Importantly, members of this new regulator family are associated with natural product biosynthetic clusters throughout the streptomycetes and their actinomycete relatives.

Identification of truncated form of NosP as a transcription factor to regulate the biosynthesis of nosiheptide

Nosiheptide (NOS), a typical member of the thiopeptides, possesses strong activities against multidrug-resistant, gram-positive bacterial pathogens. Similar to other thiopeptides, the biosynthetic pathway of NOS belongs to a ribosomally synthesized and posttranslationally modified peptide system. Bioinformatics analysis of the NOS gene cluster suggests that nosP gene encodes a homologous protein of the Streptomyces antibiotic regulatory protein (SARP) family. In the present study, the actual initiation codon of nosP was identified by comparison of potential initiation codons GUG and AUG. In contrast to previous predictions of starting with GUG, AUG, corresponding to methionine residue as the 53rd residue in the original sequence, is actually the initiation codon of nosP, indicating that a truncated form of NosP (NosP_53-323) is a functional protein. For better understanding of the transcriptional regulation for NOS biosynthesis, the binding region was subsequently investigated with NosP_53-323, demonstrating that NosP_53-323 specifically binds the bidirectional nosL-nosM promoter region. Additionally, NosP_53-323 was confirmed to serve as a transcription factor to activate the transcription of all 15 structural genes in the gene cluster. The present study provides new insights into pathway-specific regulation of the biosynthesis of NOS, which would be beneficial to the investigation of the regulatory function of similar SARP proteins in the gene clusters of other thiopeptides.-Wu, X., Jin, L., Zhang, H., Tong, R., Ma, M., Chen, Y. Identification of truncated form of NosP as a transcription factor to regulate the biosynthesis of nosiheptide.

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

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

To ∼P or Not to ∼P? Non-canonical activation by two-component response regulators

Bacteria sense and respond to their environment through the use of two-component regulatory systems. The ability to adapt to a wide range of environmental stresses is directly related to the number of two-component systems an organism possesses. Recent advances in this area have identified numerous variations on the archetype systems that employ a sensor kinase and a response regulator. It is now evident that many orphan regulators that lack cognate kinases do not rely on phosphorylation for activation and new roles for unphosphorylated response regulators have been identified. The significance of recent findings and suggestions for further research are discussed.

Prodigiosin and its potential applications

Since a decade, there has been a strong consumer demand for more natural products. This has augmented inclination towards substitution of synthetic colorants with natural pigments. Natural pigments not only have the capacity to increase the marketability of products, they also demonstrate valuable biological activities as antioxidants and anticancer agents. There is a long history of exploitation of natural products produced by bacteria as sources of pharmaceutically important, bioactive compounds. Among natural pigments, pigments from microbial sources are potentially suitable alternatives to synthetic pigments. The red pigment prodigiosin (PG) has unusual properties, which have long been documented. The red-pigmented prodiginines are bioactive secondary metabolites produced by both Gram-negative and Gram-positive bacteria. Prodigiosins are characterized by a common pyrrolyl pyrromethene skeleton, and the biological role of these pigments in the producer organisms remains unclear. Bacterial prodigiosins and their synthetic derivatives are effective proapoptotic agents against various cancer cell lines, with multiple cellular targets including multi-drug resistant cells with little or no toxicity towards normal cell lines. However, research into the biology of pigment production will stimulate interest in the bioengineering of strains to synthesize useful prodiginine derivatives. This review article highlights the characteristics and potential applications of prodigiosin pigment from Serratia as prodigiosins are real potential therapeutic drugs.

PapR6, a putative atypical response regulator, functions as a pathway-specific activator of pristinamycin II biosynthesis in Streptomyces pristinaespiralis

There are up to seven regulatory genes in the pristinamycin biosynthetic gene cluster of Streptomyces pristinaespiralis, which infers a complicated regulation mechanism for pristinamycin production. In this study, we revealed that PapR6, a putative atypical response regulator, acts as a pathway-specific activator of pristinamycin II (PII) biosynthesis. Deletion of the papR6 gene resulted in significantly reduced PII production, and its overexpression led to increased PII formation, compared to that of the parental strain HCCB 10218. However, either papR6 deletion or overexpression had very little effect on pristinamycin I (PI) biosynthesis. Electrophoretic mobility shift assays (EMSAs) demonstrated that PapR6 bound specifically to the upstream region of snaF, the first gene of the snaFE1E2GHIJK operon, which is likely responsible for providing the precursor isobutyryl-coenzyme A (isobutyryl-CoA) and the intermediate C11 αβ-unsaturated thioester for PII biosynthesis. A signature PapR6-binding motif comprising two 4-nucleotide (nt) inverted repeat sequences (5'-GAGG-4 nt-CCTC-3') was identified. Transcriptional analysis showed that inactivation of the papR6 gene led to markedly decreased expression of snaFE1E2GHIJK. Furthermore, we found that a mutant (snaFmu) with base substitutions in the identified PapR6-binding sequence in the genome exhibited the same phenotype as that of the ΔpapR6 strain. Therefore, it may be concluded that pathway-specific regulation of PapR6 in PII biosynthesis is possibly exerted via controlling the provision of isobutyryl-CoA as well as the intermediate C11 αβ-unsaturated thioester.

A comparison of key aspects of gene regulation in Streptomyces coelicolor and Escherichia coli using nucleotide-resolution transcription maps produced in parallel by global and differential RNA sequencing

Streptomyces coelicolor is a model for studying bacteria renowned as the foremost source of natural products used clinically. Post-genomic studies have revealed complex patterns of gene expression and links to growth, morphological development and individual genes. However, the underlying regulation remains largely obscure, but undoubtedly involves steps after transcription initiation. Here we identify sites involved in RNA processing and degradation as well as transcription within a nucleotide-resolution map of the transcriptional landscape. This was achieved by combining RNA-sequencing approaches suited to the analysis of GC-rich organisms. Escherichia coli was analysed in parallel to validate the methodology and allow comparison. Previously, sites of RNA processing and degradation had not been mapped on a transcriptome-wide scale for E. coli. Through examples, we show the value of our approach and data sets. This includes the identification of new layers of transcriptional complexity associated with several key regulators of secondary metabolism and morphological development in S. coelicolor and the identification of host-encoded leaderless mRNA and rRNA processing associated with the generation of specialized ribosomes in E. coli. New regulatory small RNAs were identified for both organisms. Overall the results illustrate the diversity in mechanisms used by different bacterial groups to facilitate and regulate gene expression.

Pleiotropic regulatory genes bldA, adpA and absB are implicated in production of phosphoglycolipid antibiotic moenomycin

Unlike the majority of actinomycete secondary metabolic pathways, the biosynthesis of peptidoglycan glycosyltransferase inhibitor moenomycin in Streptomyces ghanaensis does not involve any cluster-situated regulators (CSRs). This raises questions about the regulatory signals that initiate and sustain moenomycin production. We now show that three pleiotropic regulatory genes for Streptomyces morphogenesis and antibiotic production—bldA, adpA and absB—exert multi-layered control over moenomycin biosynthesis in native and heterologous producers. The bldA gene for tRNA^Leu_UAA is required for the translation of rare UUA codons within two key moenomycin biosynthetic genes (moe), moeO5 and moeE5. It also indirectly influences moenomycin production by controlling the translation of the UUA-containing adpA and, probably, other as-yet-unknown repressor gene(s). AdpA binds key moe promoters and activates them. Furthermore, AdpA interacts with the bldA promoter, thus impacting translation of bldA-dependent mRNAs—that of adpA and several moe genes. Both adpA expression and moenomycin production are increased in an absB-deficient background, most probably because AbsB normally limits adpA mRNA abundance through ribonucleolytic cleavage. Our work highlights an underappreciated strategy for secondary metabolism regulation, in which the interaction between structural genes and pleiotropic regulators is not mediated by CSRs. This strategy might be relevant for a growing number of CSR-free gene clusters unearthed during actinomycete genome mining.

SsaA, a member of a novel class of transcriptional regulators, controls sansanmycin production in Streptomyces sp. strain SS through a feedback mechanism

Sansanmycins, produced by Streptomyces sp. strain SS, are uridyl peptide antibiotics with activities against Pseudomonas aeruginosa and multidrug-resistant Mycobacterium tuberculosis. In this work, the biosynthetic gene cluster of sansanmycins, comprised of 25 open reading frames (ORFs) showing considerable amino acid sequence identity to those of the pacidamycin and napsamycin gene cluster, was identified. SsaA, the archetype of a novel class of transcriptional regulators, was characterized in the sansanmycin gene cluster, with an N-terminal fork head-associated (FHA) domain and a C-terminal LuxR-type helix-turn-helix (HTH) motif. The disruption of ssaA abolished sansanmycin production, as well as the expression of the structural genes for sansanmycin biosynthesis, indicating that SsaA is a pivotal activator for sansanmycin biosynthesis. SsaA was proved to directly bind several putative promoter regions of biosynthetic genes, and comparison of sequences of the binding sites allowed the identification of a consensus SsaA binding sequence, GTMCTGACAN₂TGTCAGKAC. The DNA binding activity of SsaA was inhibited by sansanmycins A and H in a concentration-dependent manner. Furthermore, sansanmycins A and H were found to directly interact with SsaA. These results indicated that SsaA strictly controls the production of sansanmycins at the transcriptional level in a feedback regulatory mechanism by sensing the accumulation of the end products. As the first characterized regulator of uridyl peptide antibiotic biosynthesis, the understanding of this autoregulatory process involved in sansanmycin biosynthesis will likely provide an effective strategy for rational improvements in the yields of these uridyl peptide antibiotics.

Molecular regulation of antibiotic biosynthesis in streptomyces

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

Cascades and networks of regulatory genes that control antibiotic biosynthesis

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

Strict regulation of morphological differentiation and secondary metabolism by a positive feedback loop between two global regulators AdpA and BldA in Streptomyces griseus

AdpA is a global transcriptional regulator that is induced by the microbial hormone A-factor and activates many genes required for morphological differentiation and secondary metabolism in Streptomyces griseus. We confirmed that the regulatory tRNA gene bldA was required for translation of TTA-containing adpA. We also demonstrated that AdpA bound two sites upstream of the bldA promoter and activated transcription of bldA. Thus, we revealed a unique positive feedback loop between AdpA and BldA in S. griseus. Forced expression of bldA in an A-factor-deficient mutant resulted in the partial restoration of aerial mycelium formation and streptomycin production, suggesting that the positive feedback loop could prevent premature transcriptional activation of the AdpA-target genes in the wild-type strain. We revealed that the morphological defect of the bldA mutant could be attributed mainly to the TTA codons of only two genes: adpA and amfR. amfR encodes a transcriptional activator essential for aerial mycelium formation and is a member of the AdpA regulon. Thus, amfR is regulated by a feedforward mechanism involving AdpA and BldA. We concluded that the central regulatory unit composed of AdpA and BldA plays important roles in the initiation of morphological differentiation and secondary metabolism triggered by A-factor.

An orphan histidine kinase, OhkA, regulates both secondary metabolism and morphological differentiation in Streptomyces coelicolor

We report here the physiological and genetic characterization of an orphan histidine kinase (HK) (OhkA, SCO1596) in Streptomyces coelicolor and its homolog (OhkAsav, SAV_6741) in Streptomyces avermitilis. The physiological analysis showed that the ohkA mutant of S. coelicolor exhibits impaired aerial mycelium formation and sporulation and overproduction of multiple antibiotics on mannitol-soy flour (MS) medium, especially actinorhodin (ACT) and calcium-dependent antibiotic (CDA), and disruption of ohkAsav in S. avermitilis also led to the similar phenotypes of impaired morphological differentiation and significantly increased oligomycin A production. DNA microarray analysis combined with real-time reverse transcription-PCR (RT-PCR) and RNA dot blot assay in the S. coelicolor ohkA deletion mutant confirmed the physiological results by showing the upregulation of genes involved in the biosynthesis of ACT, CDA, undecylprodigiosin (RED), a yellow type I polyketide (CPK, SCO6273-6289), and a sesquiterpene antibiotic, albaflavenone (SCO5222-5223). The results also suggested that the increased production of ACT and RED in the mutant could be partly ascribed to the enhanced precursor malonyl coenzyme A (malonyl-CoA) supply through increased transcription of genes encoding acetyl-CoA carboxylase (ACCase). Interestingly, DNA microarray analysis also showed that deletion of ohkA greatly downregulated the transcription of chpABCDEFGH genes essential for aerial mycelium formation by S. coelicolor on MS medium but significantly increased transcription of ramS/C/R, which is responsible for SapB formation and regulation and is normally absent on MS medium. Moreover, many other genes involved in development, such as bldM/N, whiG/H/I, ssgA/B/E/G/R, and whiE, were also significantly downregulated upon ohkA deletion. The results clearly demonstrated that OhkA is an important global regulator for both morphological differentiation and secondary metabolism in S. coelicolor and S. avermitilis.

Repression of antibiotic production and sporulation in Streptomyces coelicolor by overexpression of a TetR family transcriptional regulator

The overexpression of a regulatory gene of the TetR family (SCO3201) originating either from Streptomyces lividans or from Streptomyces coelicolor was shown to strongly repress antibiotic production (calcium-dependent antibiotic [CDA], undecylprodigiosin [RED], and actinorhodin [ACT]) of S. coelicolor and of the ppk mutant strain of S. lividans. Curiously, the overexpression of this gene also had a strong inhibitory effect on the sporulation process of S. coelicolor but not on that of S. lividans. SCO3201 was shown to negatively regulate its own transcription, and its DNA binding motif was found to overlap its -35 promoter sequence. The interruption of this gene in S. lividans or S. coelicolor did not lead to any obvious phenotypes, indicating that when overexpressed SCO3201 likely controls the expression of target genes of other TetR regulators involved in the regulation of the metabolic and morphological differentiation process in S. coelicolor. The direct and functional interaction of SCO3201 with the promoter region of scbA, a gene under the positive control of the TetR-like regulator, ScbR, was indeed demonstrated by in vitro as well as in vivo approaches.

Regulation of the biosynthesis of the macrolide antibiotic spiramycin in Streptomyces ambofaciens

Streptomyces ambofaciens synthesizes the macrolide antibiotic spiramycin. The biosynthetic gene cluster for spiramycin has been characterized for S. ambofaciens. In addition to the regulatory gene srmR (srm22), previously identified (M. Geistlich et al., Mol. Microbiol. 6:2019-2029, 1992), three putative regulatory genes had been identified by sequence analysis. Gene expression analysis and gene inactivation experiments showed that only one of these three genes, srm40, plays a major role in the regulation of spiramycin biosynthesis. The disruption of srm22 or srm40 eliminated spiramycin production while their overexpression increased spiramycin production. Expression analysis was performed by reverse transcription-PCR (RT-PCR) for all the genes of the cluster in the wild-type strain and in the srm22 (srmR) and srm40 deletion mutants. The results from the expression analysis, together with the ones from the complementation experiments, indicated that Srm22 is required for srm40 expression, Srm40 being a pathway-specific activator that controls most, if not all, of the spiramycin biosynthetic genes.

Unphosphorylated CsgD controls biofilm formation in Salmonella enterica serovar Typhimurium

The transcriptional regulator CsgD of Salmonella enterica serovar Typhimurium (S. Typhimurium) is a major regulator of biofilm formation required for the expression of csgBA, which encodes curli fimbriae, and adrA, coding for a diguanylate cyclase. CsgD is a response regulator with an N-terminal receiver domain with a conserved aspartate (D59) as a putative target site for phosphorylation and a C-terminal LuxR-like helix-turn-helix DNA binding motif, but the mechanisms of target gene activation remained unclear. To study the DNA-binding properties of CsgD we used electrophoretic mobility shift assays and DNase I footprint analysis to show that unphosphorylated CsgD-His(6) binds specifically to the csgBA and adrA promoter regions. In vitro transcription analysis revealed that CsgD-His(6) is crucial for the expression of csgBA and adrA. CsgD-His(6) is phosphorylated by acetyl phosphate in vitro, which decreases its DNA-binding properties. The functional impact of D59 in vivo was demonstrated as S. Typhimurium strains expressing modified CsgD protein (D59E and D59N) were dramatically reduced in biofilm formation due to decreased protein stability and DNA-binding properties in the case of D59E. In summary, our findings suggest that the response regulator CsgD functions in its unphosphorylated form under the conditions of biofilm formation investigated in this study.

Quorum sensing-controlled Evr regulates a conserved cryptic pigment biosynthetic cluster and a novel phenomycin-like locus in the plant pathogen, Pectobacterium carotovorum

Pectobacterium carotovorum SCRI193 is a phytopathogenic Gram-negative bacterium. In this study, we have identified a novel cryptic pigment biosynthetic locus in P. carotovorum SCRI193 which we have called the Pectobacterium orange pigment (pop) cluster. The pop cluster is flanked by two tRNA genes and contains genes that encode non-ribosomal peptide synthases and polyketide synthase and produces a negatively charged polar orange pigment. Orange pigment production is activated when an adjacent transcriptional activator sharing sequence similarity with the Erwinia virulence regulator (Evr) is overexpressed. Evr was shown to positively activate its own transcription and that of the pigment biosynthetic genes and an unlinked locus encoding a phenomycin homologue. In addition, the expression of Evr and orange pigment production was shown to be regulated by N-(3-oxohexanoyl)-HSL (OHHL) quorum sensing and have a virulence phenotype in potato. Finally, by comparative genomics and Southern blotting we demonstrate that this pigment biosynthetic cluster is present in multiple P. carotovorum spp., Pectobacterium brasiliensis 1692 and a truncated version of the cluster is present in Pectobacterium atrosepticum. The conserved nature of this cluster in P. carotovorum and P. brasiliensis suggests that the pop cluster has an important function in these broad-host-range soft rotting bacteria, which is no longer required in the narrow-host-range P. atrosepticum SCRI1043.

Autoregulation of antibiotic biosynthesis by binding of the end product to an atypical response regulator

In bacteria, many "atypical" response regulators (ARRs) lack the conserved residues important for phosphorylation by which typical response regulators switch their output response, suggesting the existence of alternative regulatory mechanisms. However, such mechanisms have not been established. JadR1, an OmpR-type ARR of Streptomyces venezuelae, appears to activate the transcription of jadomycin B (JdB) biosynthetic genes while repressing its own gene. JadR1 activities were inhibited in cells induced to produce JdB, which was found to bind directly to the N-terminal receiver domain of JadR1, causing JadR1 to dissociate from target promoters. The activity of a NarL-type ARR, RedZ, that regulates production of another antibiotic was likewise modulated by the end product (undecylprodigisines), implying that end-product-mediated control of antibiotic pathway-specific ARRs may be widespread. These results could prove relevant to knowledge-based improvements in yield of commercially important antibiotics.

Hierarchical control of virginiamycin production in Streptomyces virginiae by three pathway-specific regulators: VmsS, VmsT and VmsR

Two regulatory genes encoding a Streptomyces antibiotic regulatory protein (vmsS) and a response regulator (vmsT) of a bacterial two-component signal transduction system are present in the left-hand region of the biosynthetic gene cluster of the antibiotic virginiamycin, which is composed of virginiamycin M (VM) and virginiamycin S (VS), in Streptomyces virginiae. Disruption of vmsS abolished both VM and VS biosynthesis, with drastic alteration of the transcriptional profile for virginiamycin biosynthetic genes, whereas disruption of vmsT resulted in only a loss of VM biosynthesis, suggesting that vmsS is a pathway-specific regulator for both VM and VS biosynthesis, and that vmsT is a pathway-specific regulator for VM biosynthesis alone. Gene expression profiles determined by semiquantitative RT-PCR on the virginiamycin biosynthetic gene cluster demonstrated that vmsS controls the biosynthetic genes for VM and VS, and vmsT controls unidentified gene(s) of VM biosynthesis located outside the biosynthetic gene cluster. In addition, transcriptional analysis of a deletion mutant of vmsR located in the clustered regulatory region in the virginiamycin cluster (and which also acts as a SARP-family activator for both VM and VS biosynthesis) indicated that the expression of vmsS and vmsT is under the control of vmsR, and vmsR also contributes to the expression of VM and VS biosynthetic genes, independent of vmsS and vmsT. Therefore, coordinated virginiamycin biosynthesis is controlled by three pathway-specific regulators which hierarchically control the expression of the biosynthetic gene cluster.

The SmpB-tmRNA tagging system plays important roles in Streptomyces coelicolor growth and development

The ssrA gene encodes tmRNA that, together with a specialized tmRNA-binding protein, SmpB, forms part of a ribonucleoprotein complex, provides a template for the resumption of translation elongation, subsequent termination and recycling of stalled ribosomes. In addition, the mRNA-like domain of tmRNA encodes a peptide that tags polypeptides derived from stalled ribosomes for degradation. Streptomyces are unique bacteria that undergo a developmental cycle culminating at sporulation that is at least partly controlled at the level of translation elongation by the abundance of a rare tRNA that decodes UUA codons found in a relatively small number of open reading frames prompting us to examine the role of tmRNA in S. coelicolor. Using a temperature sensitive replicon, we found that the ssrA gene could be disrupted only in cells with an extra-copy wild type gene but not in wild type cells or cells with an extra-copy mutant tmRNA (tmRNA(DD)) encoding a degradation-resistant tag. A cosmid-based gene replacement method that does not include a high temperature step enabled us to disrupt both the ssrA and smpB genes separately and at the same time suggesting that the tmRNA tagging system may be required for cell survival under high temperature. Indeed, mutant cells show growth and sporulation defects at high temperature and under optimal culture conditions. Interestingly, even though these defects can be completely restored by wild type genes, the DeltassrA strain was only partially corrected by tmRNA(DD). In addition, wildtype tmRNA can restore the hygromycin-resistance to DeltassrA cells while tmRNA(DD) failed to do so suggesting that degradation of aberrant peptides is important for antibiotic resistance. Overall, these results suggest that the tmRNA tagging system plays important roles during Streptomyces growth and sporulation under both normal and stress conditions.

Combinatorial regulation by a novel arrangement of FruA and MrpC2 transcription factors during Myxococcus xanthus development

Myxococcus xanthus is a gram-negative soil bacterium that undergoes multicellular development upon nutrient limitation. Intercellular signals control cell movements and regulate gene expression during the developmental process. C-signal is a short-range signal essential for aggregation and sporulation. C-signaling regulates the fmgA gene by a novel mechanism involving cooperative binding of the response regulator FruA and the transcription factor/antitoxin MrpC2. Here, we demonstrate that regulation of the C-signal-dependent fmgBC operon is under similar combinatorial control by FruA and MrpC2, but the arrangement of binding sites is different than in the fmgA promoter region. MrpC2 was shown to bind to a crucial cis-regulatory sequence in the fmgBC promoter region. FruA was required for MrpC and/or MrpC2 to associate with the fmgBC promoter region in vivo, and expression of an fmgB-lacZ fusion was abolished in a fruA mutant. Recombinant FruA was shown to bind to an essential regulatory sequence located slightly downstream of the MrpC2-binding site in the fmgBC promoter region. Full-length FruA, but not its C-terminal DNA-binding domain, enhanced the formation of complexes with fmgBC promoter region DNA, when combined with MrpC2. This effect was nearly abolished with fmgBC DNA fragments having a mutation in either the MrpC2- or FruA-binding site, indicating that binding of both proteins to DNA is important for enhancement of complex formation. These results are similar to those observed for fmgA, where FruA and MrpC2 bind cooperatively upstream of the promoter, except that in the fmgA promoter region the FruA-binding site is located slightly upstream of the MrpC2-binding site. Cooperative binding of FruA and MrpC2 appears to be a conserved mechanism of gene regulation that allows a flexible arrangement of binding sites and coordinates multiple signaling pathways.

Characterization of rrdA, a TetR family protein gene involved in the regulation of secondary metabolism in Streptomyces coelicolor

Streptomyces not only exhibits complex morphological differentiation but also produces a plethora of secondary metabolites, particularly antibiotics. To improve our general understanding of the complex network of undecylprodigiosin (Red) biosynthesis regulation, we used an in vivo transposition system to identify novel regulators that influence Red production in Streptomyces coelicolor M145. Using this screening system, we obtained 25 Red-deficient mutants. Twenty-four of these mutants had a transposon inserted in the previously described Red biosynthetic gene cluster and produced different amounts of another secondary metabolite, actinorhodin (Act). One mutant was shown to have an insertion in a different region of the chromosome upstream of the previously uncharacterized gene rrdA (regulator of redD, sco1104), which encodes a putative TetR family transcription factor. Compared with wild-type strain M145, the rrdA null mutant exhibited increased Red production and decreased Act production. A high level of rrdA expression resulted in a severe reduction in Red production and Act overproduction. Reverse transcription-PCR analysis showed that RrdA negatively regulated Red production by controlling redD mRNA abundance, while no change was observed at the transcript level of the Act-specific activator gene, actII-orf4. The effects on Act biosynthesis might arise from competition for precursors that are common to both pathways.

A combination of unusual transcription factors binds cooperatively to control Myxococcus xanthus developmental gene expression

Myxococcus xanthus is a bacterium that undergoes multicellular development requiring coordinate regulation of multiple signaling pathways. One pathway governs aggregation and sporulation of some cells in a starving population and requires C-signaling, whereas another pathway causes programmed cell death and requires the MazF toxin. In response to starvation, the levels of the bifunctional transcription factor/antitoxin MrpC and its related proteolytic fragment MrpC2 are increased, inhibiting the cell death pathway via direct interaction of MrpC with MazF. Herein, we demonstrate that MrpC2 plays a direct role in the transcriptional response to C-signaling. We show that MrpC2 binds to sequences upstream of the C-signal-dependent fmgA promoter. These sequences are present in other C-signal-dependent promoter regions, indicating a general role for MrpC2 in developmental gene regulation. Association of MrpC and/or MrpC2 with the fmgA promoter region in vivo requires FruA, a protein that is similar to response regulators of 2-component signal transduction systems, but may not be phosphorylated. DNA binding studies showed that this association likely involves an unusual mechanism for a response regulator in which FruA and MrpC2 bind cooperatively to adjacent sites upstream of the fmgA promoter. We propose that this unusual mechanism of combinatorial control allows coordination of morphogenetic C-signaling with starvation signaling and cell death, determining spatiotemporal gene expression and cell fate.

Chapter 5. Applying the genetics of secondary metabolism in model actinomycetes to the discovery of new antibiotics

The actinomycetes, including in particular members of the filamentous genus Streptomyces, are the industrial source of a large number of bioactive small molecules employed as antibiotics and other drugs. They produce these molecules as part of their "secondary" or nonessential metabolism. The number and diversity of secondary metabolic pathways is enormous, with some estimates suggesting that this one genus can produce more than 100,000 distinct molecules. However, the discovery of new antimicrobials is hampered by the fact that many wild isolates fail to express all or sometimes any of their secondary metabolites under laboratory conditions. Furthermore, the use of previously successful screening strategies frequently results in the rediscovery of known molecules: the all-important novel structures have proven to be elusive. Mounting evidence suggests that streptomycetes possess many regulatory pathways that control the biosynthetic gene clusters for these secondary metabolic pathways and that cell metabolism plays a significant role in limiting or potentiating expression as well. In this article we explore the idea that manipulating metabolic conditions and regulatory pathways can "awaken" silent gene clusters and lead to the discovery of novel antimicrobial activities.

Extracellular signalling, translational control, two repressors and an activator all contribute to the regulation of methylenomycin production in Streptomyces coelicolor

Bioinformatic analysis of the plasmid-linked gene cluster associated with biosynthesis of methylenomycin (Mm) suggested that part of the cluster directs synthesis of a gamma-butyrolactone-like autoregulator. Autoregulator activity could be extracted from culture fluids, but differed from gamma-butyrolactones in being alkali resistant. The activity has recently been shown to comprise a series of novel autoregulator molecules, the methylenomycin furans (termed MMF). MMF autoregulator activity is shown to account for the ability of certain Mm non-producing mutants to act as 'secretors' in cosynthesis with other 'convertor' mutants. Three genes implicated in MMF biosynthesis are flanked by two regulatory genes, which are related to genes for gamma-butyrolactone-binding proteins. Genetic evidence suggests that these two genes encode components of a hetero-oligomeric repressor of MMF and Mm biosynthesis. The Mm biosynthetic genes themselves depend on the activator gene mmyB, which appears to be repressed by the putative MmyR/MmfR complex until enough MMF accumulates to release repression. The presence of TTA codons in mmyB and the main MMF biosynthetic gene causes Mm production to be dependent on the pleiotropically acting bldA gene, which encodes the tRNA for the rarely used UUA codon.

Feast or famine: the global regulator DasR links nutrient stress to antibiotic production by Streptomyces

Members of the soil-dwelling prokaryotic genus Streptomyces produce many secondary metabolites, including antibiotics and anti-tumour agents. Their formation is coupled with the onset of development, which is triggered by the nutrient status of the habitat. We propose the first complete signalling cascade from nutrient sensing to development and antibiotic biosynthesis. We show that a high concentration of N-acetylglucosamine-perhaps mimicking the accumulation of N-acetylglucosamine after autolytic degradation of the vegetative mycelium-is a major checkpoint for the onset of secondary metabolism. The response is transmitted to antibiotic pathway-specific activators through the pleiotropic transcriptional repressor DasR, the regulon of which also includes all N-acetylglucosamine-related catabolic genes. The results allowed us to devise a new strategy for activating pathways for secondary metabolite biosynthesis. Such 'cryptic' pathways are abundant in actinomycete genomes, thereby offering new prospects in the fight against multiple drug-resistant pathogens and cancers.

New pleiotropic effects of eliminating a rare tRNA from Streptomyces coelicolor, revealed by combined proteomic and transcriptomic analysis of liquid cultures

BACKGROUND

In Streptomyces coelicolor, bldA encodes the only tRNA for a rare leucine codon, UUA. This tRNA is unnecessary for growth, but is required for some aspects of secondary metabolism and morphological development. We describe a transcriptomic and proteomic analysis of the effects of deleting bldA on cellular processes during submerged culture: conditions relevant to the industrial production of antibiotics.

RESULTS

At the end of rapid growth, a co-ordinated transient up-regulation of about 100 genes, including many for ribosomal proteins, was seen in the parent strain but not the DeltabldA mutant. Increased basal levels of the signal molecule ppGpp in the mutant strain may be responsible for this difference. Transcripts or proteins from a further 147 genes classified as bldA-influenced were mostly expressed late in culture in the wild-type, though others were significantly transcribed during exponential growth. Some were involved in the biosynthesis of seven secondary metabolites; and some have probable roles in reorganising metabolism after rapid growth. Many of the 147 genes were "function unknown", and may represent unknown aspects of Streptomyces biology. Only two of the 147 genes contain a TTA codon, but some effects of bldA could be traced to TTA codons in regulatory genes or polycistronic operons. Several proteins were affected post-translationally by the bldA deletion. There was a statistically significant but weak positive global correlation between transcript and corresponding protein levels. Different technical limitations of the two approaches were a major cause of discrepancies in the results obtained with them.

CONCLUSION

Although deletion of bldA has very conspicuous effects on the gross phenotype, the bldA molecular phenotype revealed by the "dualomic" approach has shown that only about 2% of the genome is affected; but this includes many previously unknown effects at a variety of different levels, including post-translational changes in proteins and global cellular physiology.

Phosphorylated AbsA2 negatively regulates antibiotic production in Streptomyces coelicolor through interactions with pathway-specific regulatory gene promoters

The AbsA two-component signal transduction system, comprised of the sensor kinase AbsA1 and the response regulator AbsA2, acts as a negative regulator of antibiotic production in Streptomyces coelicolor, for which the phosphorylated form of AbsA2 (AbsA2 approximately P) is the agent of repression. In this study, we used chromatin immunoprecipitation to show that AbsA2 binds the promoter regions of actII-ORF4, cdaR, and redZ, which encode pathway-specific activators for actinorhodin, calcium-dependent antibiotic, and undecylprodigiosin, respectively. We confirm that these interactions also occur in vitro and that the binding of AbsA2 to each gene is enhanced by phosphorylation. Induced expression of actII-ORF4 and redZ in the hyperrepressive absA1 mutant (C542) brought about pathway-specific restoration of actinorhodin and undecylprodigiosin production, respectively. Our results suggest that AbsA2 approximately P interacts with as many as four sites in the region that includes the actII-ORF4 promoter. These data suggest that AbsA2 approximately P inhibits antibiotic production by directly interfering with the expression of pathway-specific regulators of antibiotic biosynthetic gene clusters.

An unusual response regulator influences sporulation at early and late stages in Streptomyces coelicolor

WhiI, a regulator required for efficient sporulation septation in the aerial mycelium of Streptomyces coelicolor, resembles response regulators of bacterial two-component systems but lacks some conserved features of typical phosphorylation pockets. Four amino acids of the abnormal "phosphorylation pocket" were changed by site-directed mutagenesis. Unlike whiI null mutations, these point mutations did not interfere with sporulation septation but had various effects on spore maturation. Transcriptome analysis was used to compare gene expression in the wild-type strain, a D27A mutant (pale gray spores), a D69E mutant (wild-type spores), and a null mutant (white aerial mycelium, no spores) (a new variant of PCR targeting was used to introduce the point mutations into the chromosomal copy of whiI). The results revealed 45 genes that were affected by the deletion of whiI. Many of these showed increased expression in the wild type at the time when aerial growth and development were taking place. About half of them showed reduced expression in the null mutant, and about half showed increased expression. Some, but not all, of these 45 genes were also affected by the D27A mutation, and a few were affected by the D69E mutation. The results were consistent with a model in which WhiI acts differently at sequential stages of development. Consideration of the functions of whiI-influenced genes provides some insights into the physiology of aerial hyphae. Mutation of seven whiI-influenced genes revealed that three of them play roles in spore maturation.

The biosynthesis and regulation of bacterial prodiginines

The red-pigmented prodiginines are bioactive secondary metabolites produced by both Gram-negative and Gram-positive bacteria. Recently, these tripyrrole molecules have received renewed attention owing to reported immunosuppressive and anticancer properties. The enzymes involved in the biosynthetic pathways for the production of two of these molecules, prodigiosin and undecylprodigiosin, are now known. However, the biochemistry of some of the reactions is still poorly understood. The physiology and regulation of prodiginine production in Serratia and Streptomyces are now well understood, although the biological role of these pigments in the producer organisms remains unclear. However, research into the biology of pigment production will stimulate interest in the bioengineering of strains to synthesize useful prodiginine derivatives.

Transcriptional regulation of the macs1-fadD1 operon encoding two acyl-CoA synthases involved in the physiological differentiation of Streptomyces coelicolor

The long-chain acyl-CoA synthase (ACS) FadD1 plays an important role in timing the levels of antibiotic production in Streptomyces coelicolor. fadD1 and macs1, encoding a putative medium-chain ACS, are part of a two-gene operon, whose expression is induced during the stationary phase of growth. Here it is reported that transcription of the macs1-fadD1 operon is positively regulated by AcsR, a LuxR-type transcriptional regulator. In an acsR mutant, expression of the macs1-fadD1 genes loses its normal up-regulation and the mutant becomes deficient in antibiotic production, in a clear correlation with the phenotype shown by a fadD1 null mutant. The absence of macs1-fadD1 induction in the acsR mutant was restored by complementation with a wild-type copy of the acsR gene, showing a strict link between AcsR and induction of the macs1-fadD1 operon. Gel mobility shift assays and DNase I footprinting indicated that AcsR binds to specific sequences about +162 nucleotides downstream of the macs1 transcriptional start site. In the putative operator sequence three almost identical direct tandem repeats of seven nucleotides were identified where the central sequence is essential for AcsR recognition and binding. Transcriptional fusions of the divergent pacsR and pmacs1 promoters indicated that AcsR does not regulate its own transcription, and that it binds to the operator region to control exclusively the growth-phase induction of the macs1-fadD1 operon.

Streptomyces inside-out: a new perspective on the bacteria that provide us with antibiotics

Many of the antibiotics used today are made by a group of bacteria called Streptomyces. Streptomycetes evolved about 450 million years ago as branched filamentous organisms adapted to the utilization of plant remains. They reproduce by sending up specialized aerial branches, which form spores. Aerial growth is parasitic on the primary colony, which is digested and reused for aerial growth. The reproductive phase is coordinated with the secretion of antibiotics, which may protect the colony against invading bacteria during aerial growth. A clue to the integration of antibiotic production and aerial growth is provided by bldA mutants, which are defective in both processes. These mutants lack the ability to translate a particularly rare codon, UUA, in the genetic code. The UUA codon (TTA in DNA) is present in several regulatory genes that control sets of antibiotic production genes, and in one, bldH that controls aerial mycelium formation. The regulatory genes for antibiotic production are all involved in self-reinforcing regulatory systems that potentially amplify the regulatory significance of small changes in the efficiency of translation of UUA codons. One of the regulatory targets of bldH is an extracellular protease inhibitor protein that is likely to delay the digestion of the primary biomass until the colony is ready for aerial growth. The use of the UUA codon to orchestrate different aspects of extracellular biology appeared very early in Streptomyces evolution.