Regulation of tylosin production: role of a TylP-interactive ligand

Regulatory Control of Rishirilide(s) Biosynthesis in Streptomyces bottropensis

Streptomycetes are well-known producers of numerous bioactive secondary metabolites widely used in medicine, agriculture, and veterinary. Usually, their genomes encode 20-30 clusters for the biosynthesis of natural products. Generally, the onset and production of these compounds are tightly coordinated at multiple regulatory levels, including cluster-situated transcriptional factors. Rishirilides are biologically active type II polyketides produced by Streptomyces bottropensis. The complex regulation of rishirilides biosynthesis includes the interplay of four regulatory proteins encoded by the rsl-gene cluster: three SARP family regulators (RslR1-R3) and one MarR-type transcriptional factor (RslR4). In this work, employing gene deletion and overexpression experiments we revealed RslR1-R3 to be positive regulators of the biosynthetic pathway. Additionally, transcriptional analysis indicated that rslR2 is regulated by RslR1 and RslR3. Furthermore, RslR3 directly activates the transcription of rslR2, which stems from binding of RslR3 to the rslR2 promoter. Genetic and biochemical analyses demonstrated that RslR4 represses the transcription of the MFS transporter rslT4 and of its own gene. Moreover, DNA-binding affinity of RslR4 is strictly controlled by specific interaction with rishirilides and some of their biosynthetic precursors. Altogether, our findings revealed the intricate regulatory network of teamworking cluster-situated regulators governing the biosynthesis of rishirilides and strain self-immunity.

Investigation of the Autoregulator-Receptor System in the Pristinamycin Producer Streptomyces pristinaespiralis

Pristinamycin biosynthesis in Streptomyces pristinaespiralis is governed by a complex hierarchical signaling cascade involving seven different transcriptional regulators (SpbR, PapR1, PapR2, PapR3, PapR4, PapR5, and PapR6). The signaling cascade is triggered by γ-butyrolactone (GBL)-like effector molecules, whereby the chemical structure of the effector, as well as its biosynthetic origin is unknown so far. Three of the pristinamycin transcriptional regulators (SpbR, PapR3, and PapR5) belong to the type of γ-butyrolactone receptor (GBLR). GBLRs are known to either act as “real” GBLRs, which bind GBLs as ligands or as “pseudo” GBLRs binding antibiotics or intermediates thereof as effector molecules. In this study, we performed electromobility shift assays (EMSAs) with SpbR, PapR3, and PapR5, respectively, in the presence of potential ligand samples. Thereby we could show that all three GBLRs bind synthetic 1,4-butyrolactone but not pristinamycin as ligand, suggesting that SpbR, PapR3, and PapR5 act as “real” GBLRs in S. pristinaespiralis. Furthermore, we identified a cytochrome P450 monooxygenase encoding gene snbU as potential biosynthesis gene for the GBLR-interacting ligand. Inactivation of snbU resulted in an increased pristinamycin production, which indicated that SnbU has a regulatory influence on pristinamycin production. EMSAs with culture extract samples from the snbU mutant did not influence the target binding ability of SpbR, PapR3, and PapR5 anymore, in contrast to culture supernatant samples from the S. pristinaespiralis wild-type or the pristinamycin deficient mutant papR2::apra, which demonstrates that SnbU is involved in the synthesis of the GBLR-interacting ligand.

The Streptomyces filipinensis Gamma-Butyrolactone System Reveals Novel Clues for Understanding the Control of Secondary Metabolism

Streptomyces GBLs are important signaling molecules that trigger antibiotic production in a quorum sensing-dependent manner. We have characterized the GBL system from S. filipinensis, finding that two key players of this system, the GBL receptor and the pseudo-receptor, each counteracts the transcription of the other for the modulation of filipin production and that such control over antifungal production involves an indirect effect on the transcription of filipin biosynthetic genes. Additionally, the two regulators bind the same sites, are self-regulated, and repress the transcription of three other genes of the GBL cluster, including that encoding the GBL synthase. In contrast to all the GBL receptors known, SfbR activates its own synthesis. Moreover, the pseudo-receptor was identified as the receptor of antimycin A, thus extending the range of examples supporting the idea of signaling effects of antibiotics in Streptomyces. The intricate regulatory network depicted here should provide important clues for understanding the regulatory mechanism governing secondary metabolism.

Streptomyces γ-butyrolactones (GBLs) are quorum sensing communication signals triggering antibiotic production. The GBL system of Streptomyces filipinensis, the producer of the antifungal agent filipin, has been investigated. Inactivation of sfbR (for S. filipinensis γ-butyrolactone receptor), a GBL receptor, resulted in a strong decrease in production of filipin, and deletion of sfbR2, a pseudo-receptor, boosted it, in agreement with lower and higher levels of transcription of filipin biosynthetic genes, respectively. It is noteworthy that none of the mutations affected growth or morphological development. While no ARE (autoregulatory element)-like sequences were found in the promoters of filipin genes, suggesting indirect control of production, five ARE sequences were found in five genes of the GBL cluster, whose transcription has been shown to be controlled by both S. filipinensis SfbR and SfbR2. In vitro binding of recombinant SfbR and SfbR2 to such sequences indicated that such control is direct. Transcription start points were identified by 5′ rapid amplification of cDNA ends, and precise binding regions were investigated by the use of DNase I protection studies. Binding of both regulators took place in the promoter of target genes and at the same sites. Information content analysis of protected sequences in target promoters yielded an 18-nucleotide consensus ARE sequence. Quantitative transcriptional analyses revealed that both regulators are self-regulated and that each represses the transcription of the other as well as that of the remaining target genes. Unlike other GBL receptor homologues, SfbR activates its own transcription whereas SfbR2 has a canonical autorepression profile. Additionally, SfbR2 was found here to bind the antifungal antimycin A as a way to modulate its DNA-binding activity.

IMPORTANCEStreptomyces GBLs are important signaling molecules that trigger antibiotic production in a quorum sensing-dependent manner. We have characterized the GBL system from S. filipinensis, finding that two key players of this system, the GBL receptor and the pseudo-receptor, each counteracts the transcription of the other for the modulation of filipin production and that such control over antifungal production involves an indirect effect on the transcription of filipin biosynthetic genes. Additionally, the two regulators bind the same sites, are self-regulated, and repress the transcription of three other genes of the GBL cluster, including that encoding the GBL synthase. In contrast to all the GBL receptors known, SfbR activates its own synthesis. Moreover, the pseudo-receptor was identified as the receptor of antimycin A, thus extending the range of examples supporting the idea of signaling effects of antibiotics in Streptomyces. The intricate regulatory network depicted here should provide important clues for understanding the regulatory mechanism governing secondary metabolism.

Exploring the molecular basis for substrate specificity in homologous macrolide biosynthetic cytochromes P450

Cytochromes P450 (P450s) are nature's catalysts of choice for performing demanding and physiologically vital oxidation reactions. Biochemical characterization of these enzymes over the past decades has provided detailed mechanistic insight and highlighted the diversity of substrates P450s accommodate and the spectrum of oxidative transformations they catalyze. Previously, we discovered that the bacterial P450 MycCI from the mycinamicin biosynthetic pathway in Micromonospora griseorubida possesses an unusually broad substrate scope, whereas the homologous P450 from tylosin-producing Streptomyces fradiae (TylHI) exhibits a high degree of specificity for its native substrate. Here, using biochemical, structural, and computational approaches, we aimed to understand the molecular basis for the disparate reactivity profiles of these two P450s. Turnover and equilibrium binding experiments with substrate analogs revealed that TylHI strictly prefers 16-membered ring macrolides bearing the deoxyamino sugar mycaminose. To help rationalize these results, we solved the X-ray crystal structure of TylHI in complex with its native substrate at 1.99-Å resolution and assayed several site-directed mutants. We also conducted molecular dynamics simulations of TylHI and MycCI and biochemically characterized a third P450 homolog from the chalcomycin biosynthetic pathway in Streptomyces bikiniensis These studies provided a basis for constructing P450 chimeras to gain further insight into the features dictating the differences in reaction profile among these structurally and functionally related enzymes, ultimately unveiling the central roles of key loop regions in influencing substrate binding and turnover. Our work highlights the complex nature of P450/substrate interactions and raises interesting questions regarding the evolution of functional diversity among biosynthetic enzymes.

A Novel TetR Family Transcriptional Regulator, CalR3, Negatively Controls Calcimycin Biosynthesis in Streptomyces chartreusis NRRL 3882

Calcimycin is a unique ionophoric antibiotic that is widely used in biochemical and pharmaceutical applications, but the genetic basis underlying the regulatory mechanisms of calcimycin biosynthesis are unclear. Here, we identified the calR3 gene, which encodes a novel TetR family transcriptional regulator and exerts a negative effect on calcimycin biosynthesis. Disruption of calR3 in Streptomyces chartreusis NRRL 3882 led to significantly increased calcimycin and its intermediate cezomycin. Gene expression analysis showed that the transcription of calR3 and its adjacent calT gene were dramatically enhanced (30- and 171-fold, respectively) in GLX26 (ΔcalR3) mutants compared with the wild-type strains. Two CalR3-binding sites within the bidirectional calR3-calT promoter region were identified using a DNase I footprinting assay, indicating that CalR3 directly repressed the transcription of its own gene and the calT gene. In vitro electrophoretic mobility shift assays suggested that both calcimycin and cezomycin can act as CalR3 ligands to induce CalR3 to dissociate from its binding sites. These findings indicate negative feedback for the regulation of CalR3 in calcimycin biosynthesis and suggest that calcimycin production can be improved by manipulating its biosynthetic machinery.

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.

Functional insights into the mode of DNA and ligand binding of the TetR family regulator TylP from Streptomyces fradiae

Tetracycline repressors (TetRs) modulate multidrug efflux pathways in several pathogenic bacteria. In Streptomyces, they additionally regulate secondary metabolic pathways like antibiotic production. For instance, in the antibiotic producer Streptomyces fradiae, a layered network of TetRs regulates the levels of the commercially important antibiotic tylosin, with TylP occupying the top of this cascading network. TetRs exist in two functional states, the DNA-bound and the ligand-bound form, which are allosterically regulated. Here, to develop deeper insights into the factors that govern allostery, the crystal structure of TylP was solved to a resolution of 2.3 Å. The structure revealed that TylP possesses several unique features; notably, it harbors a unique C-terminal helix-loop extension that spans the entire length of the structure. This anchor connects the DNA-binding domain (DBD) with the ligand-binding domain (LBD) via a mix of positively charged and hydrogen-bonding interactions. Supporting EMSA studies with a series of ΔC truncated versions show that a systematic deletion of this region results in complete loss of DNA binding. The structure additionally revealed that TylP is markedly different in the orientation of its DBD and LBD architecture and the dimeric geometry from its hypothesized Streptomyces homologue CprB, which is a γ-butyrolactone regulator. Rather, TylP is closer in structural design to macrolide-binding TetRs found in pathogens. Supporting molecular dynamic studies suggested that TylP binds a macrolide intermediate in the tylosin pathway. Collectively, the structure along with corroborating biochemical studies provided insights into the novel mode of regulation of TetRs in antibiotic-producing organisms.

Crystallization and X-ray crystallographic analysis of recombinant TylP, a putative γ-butyrolactone receptor protein from Streptomyces fradiae

TylP is one of five regulatory proteins involved in the regulation of antibiotic (tylosin) production, morphological and physiological differentiation in Streptomyces fradiae. Its function is similar to those of various γ-butyrolactone receptor proteins. In this report, N-terminally His-tagged recombinant TylP protein (rTylP) was overproduced in Escherichia coli and purified to homogeneity. The rTylP protein was crystallized from a reservoir solution comprising 34%(v/v) ethylene glycol and 5%(v/v) glycerol. The protein crystals diffracted X-rays to 3.05 Å resolution and belonged to the trigonal space group P3₁21, with unit-cell parameters a = b = 126.62, c = 95.63 Å.

A Complex Signaling Cascade Governs Pristinamycin Biosynthesis in Streptomyces pristinaespiralis

Pristinamycin production in Streptomyces pristinaespiralis Pr11 is tightly regulated by an interplay between different repressors and activators. A γ-butyrolactone receptor gene (spbR), two TetR repressor genes (papR3 and papR5), three SARP (Streptomyces antibiotic regulatory protein) genes (papR1, papR2, and papR4), and a response regulator gene (papR6) are carried on the large 210-kb pristinamycin biosynthetic gene region of Streptomyces pristinaespiralis Pr11. A detailed investigation of all pristinamycin regulators revealed insight into a complex signaling cascade, which is responsible for the fine-tuned regulation of pristinamycin production in S. pristinaespiralis.

A γ-butyrolactone-sensing activator/repressor, JadR3, controls a regulatory mini-network for jadomycin biosynthesis

Two regulatory genes, jadR2 and jadR3, in the jadomycin (jad) biosynthetic gene cluster of Streptomyces venezuelae encode homologues of γ-butyrolactone receptor. JadR2 was previously shown to be a pseudo γ-butyrolactone receptor. jadR3 is situated at the upstream of jadW123 encoding putative enzymes for γ-butyrolactone biosynthesis. Disruption of jadR3 resulted in markedly decreased production of jadomycin. Transcriptional analysis revealed that JadR3 represses jadW1, jadR2 and jadR3 but activates jadR1, the key activator gene for jadomycin biosynthesis. DNase I footprinting showed that JadR3 has four binding sites in the intergenic regions of jadR2-jadR1 and jadR3-jadW1. A JadR3 interactive molecule, SVB1, was purified from a large-scale fermentation and its structure found to be the same as SCB3, a γ-butyrolactone from Streptomyces coelicolor, and was absent from a jadW123 mutant lacking jadomycin production. Addition of SVB1 or extract from S. coelicolor to the mutant restored jadomycin production. Overall, our results revealed that the association of JadR3 and SVB1 plays an important role in controlling a regulatory mini-network governing jadomycin biosynthesis, providing new insights into the ways in which γ-butyrolactone/receptor systems modulate antibiotic biosynthesis in Streptomyces.

Structural and functional basis of transcriptional regulation by TetR family protein CprB from S. coelicolor A3(2)

Antibiotic production and resistance pathways in Streptomyces are dictated by the interplay of transcriptional regulatory proteins that trigger downstream responses via binding to small diffusible molecules. To decipher the mode of DNA binding and the associated allosteric mechanism in the sub-class of transcription factors that are induced by γ-butyrolactones, we present the crystal structure of CprB in complex with the consensus DNA element to a resolution of 3.25 Å. Binding of the DNA results in the restructuring of the dimeric interface of CprB, inducing a pendulum-like motion of the helix-turn-helix motif that inserts into the major groove. The crystal structure revealed that, CprB is bound to DNA as a dimer of dimers with the mode of binding being analogous to the broad spectrum multidrug transporter protein QacR from the antibiotic resistant strain Staphylococcus aureus. It was demonstrated that the CprB displays a cooperative mode of DNA binding, following a clamp and click model. Experiments performed on a subset of DNA sequences from Streptomyces coelicolor A3(2) suggest that CprB is most likely a pleiotropic regulator. Apart from serving as an autoregulator, it is potentially a part of a network of proteins that modulates the γ-butyrolactone synthesis and antibiotic regulation pathways in S. coelicolor A3(2).

The TetR family of regulators

The most common prokaryotic signal transduction mechanisms are the one-component systems in which a single polypeptide contains both a sensory domain and a DNA-binding domain. Among the >20 classes of one-component systems, the TetR family of regulators (TFRs) are widely associated with antibiotic resistance and the regulation of genes encoding small-molecule exporters. However, TFRs play a much broader role, controlling genes involved in metabolism, antibiotic production, quorum sensing, and many other aspects of prokaryotic physiology. There are several well-established model systems for understanding these important proteins, and structural studies have begun to unveil the mechanisms by which they bind DNA and recognize small-molecule ligands. The sequences for more than 200,000 TFRs are available in the public databases, and genomics studies are identifying their target genes. Three-dimensional structures have been solved for close to 200 TFRs. Comparison of these structures reveals a common overall architecture of nine conserved α helices. The most important open question concerning TFR biology is the nature and diversity of their ligands and how these relate to the biochemical processes under their control.

Avenolide, a Streptomyces hormone controlling antibiotic production in Streptomyces avermitilis

Gram-positive bacteria of the genus Streptomyces are industrially important microorganisms, producing >70% of commercially important antibiotics. The production of these compounds is often regulated by low-molecular-weight bacterial hormones called autoregulators. Although 60% of Streptomyces strains may use γ-butyrolactone-type molecules as autoregulators and some use furan-type molecules, little is known about the signaling molecules used to regulate antibiotic production in many other members of this genus. Here, we purified a signaling molecule (avenolide) from Streptomyces avermitilis--the producer of the important anthelmintic agent avermectin with annual world sales of $850 million--and determined its structure, including stereochemistry, by spectroscopic analysis and chemical synthesis as (4S,10R)-10-hydroxy-10-methyl-9-oxo-dodec-2-en-1,4-olide, a class of Streptomyces autoregulator. Avenolide is essential for eliciting avermectin production and is effective at nanomolar concentrations with a minimum effective concentration of 4 nM. The aco gene of S. avermitilis, which encodes an acyl-CoA oxidase, is required for avenolide biosynthesis, and homologs are also present in Streptomyces fradiae, Streptomyces ghanaensis, and Streptomyces griseoauranticus, suggesting that butenolide-type autoregulators may represent a widespread and another class of Streptomyces autoregulator involved in regulating antibiotic production.

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

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

Strain improvement in actinomycetes in the postgenomic era

With the recent advances in DNA sequencing technologies, it is now feasible to sequence multiple actinomycete genomes rapidly and inexpensively. An important observation that emerged from early Streptomyces genome sequencing projects was that each strain contains genes that encode 20 or more potential secondary metabolites, only a fraction of which are expressed during fermentation. More recently, this observation has been extended to many other actinomycetes with large genomes. The discovery of a wealth of orphan or cryptic secondary metabolite biosynthetic gene clusters has suggested that sequencing large numbers of actinomycete genomes may provide the starting materials for a productive new approach to discover novel secondary metabolites. The key issue for this approach to be successful is to find ways to turn on or turn up the expression of cryptic or poorly expressed pathways to provide material for structure elucidation and biological testing. In this review, I discuss several genetic approaches that are potentially applicable to many actinomycetes for this application.

Characterization and manipulation of the pathway-specific late regulator AlpW reveals Streptomyces ambofaciens as a new producer of Kinamycins

The genome sequence of Streptomyces ambofaciens, a species known to produce the congocidine and spiramycin antibiotics, has revealed the presence of numerous gene clusters predicted to be involved in the biosynthesis of secondary metabolites. Among them, the type II polyketide synthase-encoding alp cluster was shown to be responsible for the biosynthesis of a compound with antibacterial activity. Here, by means of a deregulation approach, we gained access to workable amounts of the antibiotics for structure elucidation. These compounds, previously designated as alpomycin, were shown to be known members of kinamycin family of antibiotics. Indeed, a mutant lacking AlpW, a member of the TetR regulator family, was shown to constitutively produce kinamycins. Comparative transcriptional analyses showed that expression of alpV, the essential regulator gene required for activation of the biosynthetic genes, is strongly maintained during the stationary growth phase in the alpW mutant, a stage at which alpV transcripts and thereby transcripts of the biosynthetic genes normally drop off. Recombinant AlpW displayed DNA binding activity toward specific motifs in the promoter region of its own gene and that of alpV and alpZ. These recognition sequences are also targets for AlpZ, the γ-butyrolactone-like receptor involved in the regulation of the alp cluster. However, unlike that of AlpZ, the AlpW DNA-binding ability seemed to be insensitive to the signaling molecules controlling antibiotic biosynthesis. Together, the results presented in this study reveal S. ambofaciens to be a new producer of kinamycins and AlpW to be a key late repressor of the cellular control of kinamycin biosynthesis.

Role of sgcR3 in positive regulation of enediyne antibiotic C-1027 production of Streptomyces globisporus C-1027

Background

C-1027, produced by Streptomyces globisporus C-1027, is one of the most potent antitumoral agents. The biosynthetic gene cluster of C-1027, previously cloned and sequenced, contains at least three putative regulatory genes, i.e. sgcR1, sgcR2 and sgcR3. The predicted gene products of these genes share sequence similarities to StrR, regulators of AraC/XylS family and TylR. The purpose of this study was to investigate the role of sgcR3 in C-1027 biosynthesis.

Results

Overexpression of sgcR3 in S. globisporus C-1027 resulted in a 30–40% increase in C-1027 production. Consistent with this, disruption of sgcR3 abolished C-1027 production. Complementation of the sgcR3-disrupted strain R3KO with intact sgcR3 gene could restore C-1027 production. The results from real time RT-PCR analysis in R3KO mutant and wild type strain indicated that not only transcripts of biosynthetic structural genes such as sgcA1 and sgcC4, but also putative regulatory genes, sgcR1 and sgcR2, were significantly decreased in R3KO mutant. The cross-complementation studies showed that sgcR1R2 could functionally complement sgcR3 disruption in trans. Purified N-terminal His₁₀-tagged SgcR3 showed specific DNA-binding activity to the promoter region of sgcR1R2.

Conclusion

The role of SgcR3 has been proved to be a positive regulator of C-1027 biosynthesis in S. globisporus C-1027. SgcR3 occupies a higher level than SgcR1 and SgcR2 in the regulatory hierarchy that controls C-1027 production and activates the transcription of sgcR1 and sgcR2 by binding directly to the promoter region of sgcR1R2.

Analysis of RegA, a pathway-specific regulator of the friulimicin biosynthesis in Actinoplanes friuliensis

The rare actinomycete Actinoplanes friuliensis is the producer of the lipopeptide antibiotic friulimicin, which is active against a broad range of Gram-positive bacteria such as methicillin-resistant Enterococcus spec. and Staphylococcus aureus (MRE, MRSA) strains. Friulimicin consists of a decapeptide core and an acyl residue linked to an exocyclic amino acid. The complete biosynthetic gene cluster consisting of 24 open reading frames was characterized by sequence analysis and the transcription units were subsequently determined by RT-PCR experiments. In addition to several genes for biosynthesis, self-resistance and transport four different regulatory genes (regA, regB, regC and regD) were identified within the cluster. To analyse the role of the pathway-specific regulatory protein RegA in the friulimicin biosynthesis, the corresponding gene was inactivated resulting in friulimicin non-producing mutants. Furthermore, several protein-binding sites within the friulimicin gene cluster were identified by gel retardation assays. By real-time RT-PCR experiments, it was shown that the majority of the friulimicin biosynthetic genes is positively regulated by RegA.

Identification of genes involved in the butyrolactone autoregulator cascade that modulates secondary metabolism in Streptomyces lavendulae FRI-5

The gamma-butyrolactone-autoregulator signalling system is widely distributed across many Streptomyces species and it controls secondary metabolism and/or morphological differentiation. IM-2 [(2R,3R,1'R)-2-1'-hydroxybutyl-3-hydroxymethyl-gamma-butanolide] is a gamma-butyrolactone autoregulator which, in Streptomyces lavendulae FRI-5, switches off the production of D-cycloserine, but switches on the production of several nucleoside antibiotics and blue pigment. In the IM-2 system, an IM-2 specific receptor (FarA) plays a critical role in the biosynthetic regulation of these metabolites, including IM-2 itself. Here, we identified five additional regulatory genes in the farA-flanking region and demonstrated that, in addition to farA, at least two more genes (farR1 and farR2) are involved in the IM-2/FarA system as the direct transcriptional target of FarA. The gel-shift assay revealed that FarA was bound to the upstream region of the four genes (including farR1 and farR2) in an IM-2-dependent manner. The FarA-binding sites were localized by DNase I footprinting to 27- to 33-bp palindromic structures, suggesting that FarA-binding sequences consist of two conserved hexamers separated by six nucleotides. Both farR1 and farR2 were transcribed in a growth-dependent manner, and marked expression was induced in the presence of IM-2, whereas transcripts of other two genes were not detected under the cultivation conditions used. The FarA-binding sites of farR1 and far2 overlap the promoter regions, suggesting that FarA represses the transcription of these two genes in the absence of IM-2 by inhibiting RNA polymerase access.

A bistable gene switch for antibiotic biosynthesis: the butyrolactone regulon in Streptomyces coelicolor

Many microorganisms, including bacteria of the class Streptomycetes, produce various secondary metabolites including antibiotics to gain a competitive advantage in their natural habitat. The production of these compounds is highly coordinated in a population to expedite accumulation to an effective concentration. Furthermore, as antibiotics are often toxic even to their producers, a coordinated production allows microbes to first arm themselves with a defense mechanism to resist their own antibiotics before production commences. One possible mechanism of coordination among individuals is through the production of signaling molecules. The gamma-butyrolactone system in Streptomyces coelicolor is a model of such a signaling system for secondary metabolite production. The accumulation of these signaling molecules triggers antibiotic production in the population. A pair of repressor-amplifier proteins encoded by scbA and scbR mediates the production and action of one particular gamma-butyrolactone, SCB1. Based on the proposed interactions of scbA and scbR, a mathematical model was constructed and used to explore the ability of this system to act as a robust genetic switch. Stability analysis shows that the butyrolactone system exhibits bistability and, in response to a threshold SCB1 concentration, can switch from an OFF state to an ON state corresponding to the activation of genes in the cryptic type I polyketide synthase gene cluster, which are responsible for production of the hypothetical polyketide. The switching time is inversely related to the inducer concentration above the threshold, such that short pulses of low inducer concentration cannot switch on the system, suggesting its possible role in noise filtering. In contrast, secondary metabolite production can be triggered rapidly in a population of cells producing the butyrolactone signal due to the presence of an amplification loop in the system. S. coelicolor was perturbed experimentally by varying concentrations of SCB1, and the model simulations match the experimental data well. Deciphering the complexity of this butyrolactone switch will provide valuable insights into how robust and efficient systems can be designed using "simple" two-protein networks.

Regulation of the synthesis of the angucyclinone antibiotic alpomycin in Streptomyces ambofaciens by the autoregulator receptor AlpZ and its specific ligand

Streptomyces ambofaciens produces an orange pigment and the antibiotic alpomycin, both of which are products of a type II polyketide synthase gene cluster identified in each of the terminal inverted repeats of the linear chromosome. Five regulatory genes encoding Streptomyces antibiotic regulatory proteins (alpV, previously shown to be an essential activator gene; alpT; and alpU) and TetR family receptors (alpZ and alpW) were detected in this cluster. Here, we demonstrate that AlpZ, which shows high similarity to gamma-butyrolactone receptors, is at the top of a pathway-specific regulatory hierarchy that prevents synthesis of the alp polyketide products. Deletion of the two copies of alpZ resulted in the precocious production of both alpomycin and the orange pigment, suggesting a repressor role for AlpZ. Consistent with this, expression of the five alp-located regulatory genes and of two representative biosynthetic structural genes (alpA and alpR) was induced earlier in the alpZ deletion strain. Furthermore, recombinant AlpZ was shown to bind to specific DNA sequences within the promoter regions of alpZ, alpV, and alpXW, suggesting direct transcriptional control of these genes by AlpZ. Analysis of solvent extracts of S. ambofaciens cultures identified the existence of a factor which induces precocious production of alpomycin and pigment in the wild-type strain and which can disrupt the binding of AlpZ to its DNA targets. This activity is reminiscent of gamma-butyrolactone-type molecules. However, the AlpZ-interacting molecule(s) was shown to be resistant to an alkali treatment capable of inactivating gamma-butyrolactones, suggesting that the AlpZ ligand(s) does not possess a lactone functional group.

Organization of the biosynthetic gene cluster for the macrolide antibiotic spiramycin in Streptomyces ambofaciens

Spiramycin, a 16-membered macrolide antibiotic used in human medicine, is produced by Streptomyces ambofaciens; it comprises a polyketide lactone, platenolide, to which three deoxyhexose sugars are attached. In order to characterize the gene cluster governing the biosynthesis of spiramycin, several overlapping cosmids were isolated from an S. ambofaciens gene library, by hybridization with various probes (spiramycin resistance or biosynthetic genes, tylosin biosynthetic genes), and the sequences of their inserts were determined. Sequence analysis showed that the spiramycin biosynthetic gene cluster spanned a region of over 85 kb of contiguous DNA. In addition to the five previously described genes that encode the type I polyketide synthase involved in platenolide biosynthesis, 45 other genes have been identified. It was possible to propose a function for most of the inferred proteins in spiramycin biosynthesis, in its regulation, in resistance to the produced antibiotic or in the provision of extender units for the polyketide synthase. Two of these genes, predicted to be involved in deoxysugar biosynthesis, were inactivated by gene replacement, and the resulting mutants were unable to produce spiramycin, thus confirming their involvement in spiramycin biosynthesis. This work reveals the main features of spiramycin biosynthesis and constitutes a first step towards a detailed molecular analysis of the production of this medically important antibiotic.

Gamma-butyrolactone-dependent expression of the Streptomyces antibiotic regulatory protein gene srrY plays a central role in the regulatory cascade leading to lankacidin and lankamycin production in Streptomyces rochei

Our previous studies revealed that the srrX and srrA genes carried on the large linear plasmid pSLA2-L constitute a gamma-butyrolactone-receptor system in Streptomyces rochei. Extensive transcriptional analysis has now showed that the Streptomyces antibiotic regulatory protein gene srrY, which is also carried on pSLA2-L, is a target of the receptor/repressor SrrA and plays a central role in lankacidin and lankamycin production. The srrY gene was expressed in a growth-dependent manner, slightly preceding antibiotic production. The expression of srrY was undetectable in the srrX mutant but was restored in the srrX srrA double mutant. In addition, SrrA was bound specifically to the promoter region of srrY, and this binding was prevented by the addition of the S. rochei gamma-butyrolactone fraction, while the W119A mutant receptor SrrA was kept bound even in the presence of S. rochei gamma-butyrolactone. Furthermore, the introduction of an intact srrY gene under the control of a foreign promoter into the srrX or srrA(W119A) mutant restored antibiotic production. All of these results confirmed the signaling pathway from srrX through srrA to srrY, leading to lankacidin and lankamycin production.

Engineering a regulatory region of jadomycin gene cluster to improve jadomycin B production in Streptomyces venezuelae

Streptomyces venezuelae ISP5230 produces a group of jadomycin congeners with cytotoxic activities. To improve jadomycin fermentation process, a genetic engineering strategy was designed to replace a 3.4-kb regulatory region of jad gene cluster that contains four regulatory genes (3' end 272 bp of jadW2, jadW3, jadR2, and jadR1) and the native promoter upstream of jadJ (P(J)) with the ermEp* promoter sequence so that ermEp* drives the expression of the jadomycin biosynthetic genes from jadJ in the engineered strain. As expected, the mutant strain produced jadomycin B without ethanol treatment, and the yield increased to about twofold that of the stressed wild-type. These results indicated that manipulation of the regulation of a biosynthetic gene cluster is an effective strategy to increase product yield.