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

Promoter Engineering Reveals the Importance of Heptameric Direct Repeats for DNA Binding by Streptomyces Antibiotic Regulatory Protein-Large ATP-Binding Regulator of the LuxR Family (SARP-LAL) Regulators in Streptomyces natalensis.

The biosynthesis of small-size polyene macrolides is ultimately controlled by a couple of transcriptional regulators that act in a hierarchical way. A Streptomyces antibiotic regulatory protein-large ATP-binding regulator of the LuxR family (SARP-LAL) regulator binds the promoter of a PAS-LuxR regulator-encoding gene and activates its transcription, and in turn, the gene product of the latter activates transcription from various promoters of the polyene gene cluster directly. The primary operator of PimR, the archetype of SARP-LAL regulators, contains three heptameric direct repeats separated by four-nucleotide spacers, but the regulator can also bind a secondary operator with only two direct repeats separated by a 3-nucleotide spacer, both located in the promoter region of its unique target gene, pimM A similar arrangement of operators has been identified for PimR counterparts encoded by gene clusters for different antifungal secondary metabolites, including not only polyene macrolides but peptidyl nucleosides, phoslactomycins, or cycloheximide. Here, we used promoter engineering and quantitative transcriptional analyses to determine the contributions of the different heptameric repeats to transcriptional activation and final polyene production. Optimized promoters have thus been developed. Deletion studies and electrophoretic mobility assays were used for the definition of DNA-binding boxes formed by 22-nucleotide sequences comprising two conserved heptameric direct repeats separated by four-nucleotide less conserved spacers. The cooperative binding of PimRSARP appears to be the mechanism involved in the binding of regulator monomers to operators, and at least two protein monomers are required for efficient binding.IMPORTANCE Here, we have shown that a modulation of the production of the antifungal pimaricin in Streptomyces natalensis can be accomplished via promoter engineering of the PAS-LuxR transcriptional activator pimM The expression of this gene is controlled by the Streptomyces antibiotic regulatory protein-large ATP-binding regulator of the LuxR family (SARP-LAL) regulator PimR, which binds a series of heptameric direct repeats in its promoter region. The structure and importance of such repeats in protein binding, transcriptional activation, and polyene production have been investigated. These findings should provide important clues to understand the regulatory machinery that modulates antibiotic biosynthesis in Streptomyces and open new possibilities for the manipulation of metabolite production. The presence of PimR orthologues encoded by gene clusters for different secondary metabolites and the conservation of their operators suggest that the improvements observed in the activation of pimaricin biosynthesis by Streptomyces natalensis could be extrapolated to the production of different compounds by other species.

Biotechnological production and application of the antibiotic pimaricin: biosynthesis and its regulation.

Pimaricin (natamycin) is a small polyene macrolide antibiotic used worldwide. This efficient antimycotic and antiprotozoal agent, produced by several soil bacterial species of the genus Streptomyces, has found application in human therapy, in the food and beverage industries and as pesticide. It displays a broad spectrum of activity, targeting ergosterol but bearing a particular mode of action different to other polyene macrolides. The biosynthesis of this only antifungal agent with a GRAS status has been thoroughly studied, which has permitted the manipulation of producers to engineer the biosynthetic gene clusters in order to generate several analogues. Regulation of its production has been largely unveiled, constituting a model for other polyenes and setting the leads for optimizing the production of these valuable compounds. This review describes and discusses the molecular genetics, uses, mode of action, analogue generation, regulation and strategies for increasing pimaricin production yields.

Functional analysis of filipin tailoring genes from Streptomyces filipinensis reveals alternative routes in filipin III biosynthesis and yields bioactive derivatives.

BACKGROUND: Streptomyces filipinensis is the industrial producer of filipin, a pentaene macrolide, archetype of non-glycosylated polyenes, and widely used for the detection and the quantitation of cholesterol in biological membranes and as a tool for the diagnosis of Niemann-Pick type C disease. Genetic manipulations of polyene biosynthetic pathways have proven useful for the discovery of products with improved properties. Here, we describe the late biosynthetic steps for filipin III biosynthesis and strategies for the generation of bioactive filipin III derivatives at high yield. RESULTS: A region of 13,778 base pairs of DNA from the S. filipinensis genome was isolated, sequenced, and characterized. Nine complete genes and two truncated ORFs were located. Disruption of genes proved that this genomic region is part of the biosynthetic cluster for the 28-membered ring of the polyene macrolide filipin. This set of genes includes two cytochrome P450 monooxygenase encoding genes, filC and filD, which are proposed to catalyse specific hydroxylations of the macrolide ring at C26 and C1' respectively. Gene deletion and complementation experiments provided evidence for their role during filipin III biosynthesis. Filipin III derivatives were accumulated by the recombinant mutants at high yield. These have been characterized by mass spectrometry and nuclear magnetic resonance following high-performance liquid chromatography purification thus revealing the post-polyketide steps during polyene biosynthesis. Two alternative routes lead to the formation of filipin III from the initial product of polyketide synthase chain assembly and cyclization filipin I, one trough filipin II, and the other one trough 1'-hydroxyfilipin I, all filipin III intermediates being biologically active. Moreover, minimal inhibitory concentration values against Candida utilis and Saccharomyces cerevisiae were obtained for all filipin derivatives, finding that 1'-hydroxyfilipin and especially filipin II show remarkably enhanced antifungal bioactivity. Complete nuclear magnetic resonance assignments have been obtained for the first time for 1'-hydroxyfilipin I. CONCLUSIONS: This report reveals the existence of two alternative routes for filipin III formation and opens new possibilities for the generation of biologically active filipin derivatives at high yield and with improved properties.

Pathway-specific regulation revisited: cross-regulation of multiple disparate gene clusters by PAS-LuxR transcriptional regulators.

PAS-LuxR regulators are highly conserved proteins devoted to the control of antifungal production by binding to operators located in given promoters of polyene biosynthetic genes. The canonical operator of PimM, archetype of this class of regulators, has been used here to search for putative targets of orthologous protein PteF in the genome of Streptomyces avermitilis, finding 97 putative operators outside the pentaene filipin gene cluster (pte). The processes putatively affected included genetic information processing; energy, carbohydrate, and lipid metabolism; DNA replication and repair; morphological differentiation; secondary metabolite biosynthesis; and transcriptional regulation, among others. Seventeen of these operators were selected, and their binding to PimM DNA-binding domain was assessed by electrophoretic mobility shift assays. Strikingly, the protein bound all predicted operators suggesting a direct control over targeted processes. As a proof of concept, we studied the biosynthesis of the ATP-synthase inhibitor oligomycin whose gene cluster included two operators. Regulator mutants showed a severe loss of oligomycin production, whereas gene complementation of the mutant restored phenotype, and gene duplication in the wild-type strain boosted oligomycin production. Comparative gene expression analyses in parental and mutant strains by reverse transcription-quantitative polymerase chain reaction of selected olm genes corroborated production results. These results demonstrate that PteF is able to cross-regulate the biosynthesis of two related secondary metabolites, filipin and oligomycin, but might be extended to all the processes indicated above. This study highlights the complexity of the network of interactions in which PAS-LuxR regulators are involved and opens new possibilities for the manipulation of metabolite production in Streptomycetes.

Genome-wide analysis of the regulation of pimaricin production in Streptomyces natalensis by reactive oxygen species.

To investigate the molecular mechanisms that interplay between oxygen metabolism and secondary metabolism in Streptomyces natalensis, we compared the transcriptomes of the strains CAM.02 (ΔsodF), pimaricin under-producer phenotype, and CAM.04 (ΔahpCD), pimaricin over-producer phenotype, with that of the wild type at late exponential and stationary growth phases. Microarray data interpretation was supported by characterization of the mutant strains regarding enzymatic activities, phosphate uptake, oxygen consumption and pimaricin production.Both mutant strains presented a delay in the transcription activation of the PhoRP system and pimaricin biosynthetic gene cluster that correlated with the delayed inorganic phosphate (Pi) depletion in the medium and late onset of pimaricin production, respectively. The carbon flux of both mutants was also altered: a re-direction from glycolysis to the pentose phosphate pathway (PPP) in early exponential phase followed by a transcriptional activation of both pathways in subsequent growth phases was observed. Mutant behavior diverged at the respiratory chain/tricarboxylic acid cycle (TCA) and the branched chain amino acid (BCAA) metabolism. CAM.02 (ΔsodF) presented an impaired TCA cycle and an inhibition of the BCAA biosynthesis and degradation pathways. Conversely, CAM.04 (ΔahpCD) presented a global activation of BCAA metabolism.The results highlight the cellular NADPH/NADH ratio and the availability of biosynthetic precursors via the BCAA metabolism as the main pimaricin biosynthetic bottlenecks under oxidative stress conditions. Furthermore, new evidences are provided regarding a crosstalk between phosphate metabolism and oxidative stress in Streptomyces.

PAS-LuxR transcriptional control of filipin biosynthesis in S. avermitilis.

The DNA region encoding the filipin gene cluster in Streptomyces avermitilis (pte) contains a PAS-LuxR regulatory gene, pteF, orthologue to pimM, the final pathway-specific positive regulatory protein of pimaricin biosynthesis in Streptomyces natalensis. Gene replacement of the gene from S. avermitilis chromosome resulted in a severe loss of filipin production and delayed spore formation in comparison to that of the wild-type strain, suggesting that it acts as a positive regulator of filipin biosynthesis and that it may also have a role in sporulation. Complementation of the mutant with a single copy of the gene integrated into the chromosome restored wild-type phenotypes. Heterologous complementation with the regulatory counterpart from S. natalensis also restored parental phenotypes. Gene expression analyses in S. avermitilis wild-type and the mutant by reverse transcription-quantitative polymerase chain reaction of the filipin gene cluster suggested the targets for the regulatory protein. Transcription start points of all the genes of the cluster were studied by 5'-rapid amplification of complementary DNA ends. Transcription start point analysis of the pteF gene revealed that the annotated sequence in the databases is incorrect. Confirmation of target promoters was performed by in silico search of binding sites among identified promoters and the binding of the orthologous regulator for pimaricin biosynthesis PimM to gene promoters by electrophoretic mobility shift assays. Precise binding regions were investigated by DNAse I protection studies. Our results indicate that PteF activates the transcription from two promoters of polyketide synthase genes directly, and indirectly of other genes of the cluster.

Draft genome of Streptomyces tsukubaensis NRRL 18488, the producer of the clinically important immunosuppressant tacrolimus (FK506).

The macrocyclic polyketide tacrolimus (FK506) is a potent immunosuppressant that prevents T-cell proliferation produced solely by Streptomyces species. We report here the first draft genome sequence of a true FK506 producer, Streptomyces tsukubaensis NRRL 18488, the first tacrolimus-producing strain that was isolated and that contains the full tacrolimus biosynthesis gene cluster.

Molecular control of polyene macrolide biosynthesis: direct binding of the regulator PimM to eight promoters of pimaricin genes and identification of binding boxes.

Control of polyene macrolide production in Streptomyces natalensis is mediated by the transcriptional activator PimM. This regulator, which combines an N-terminal PAS domain with a C-terminal helix-turn-helix motif, is highly conserved among polyene biosynthetic gene clusters. PimM, truncated forms of the protein without the PAS domain (PimM(ΔPAS)), and forms containing just the DNA-binding domain (DBD) (PimM(DBD)) were overexpressed in Escherichia coli as GST-fused proteins. GST-PimM binds directly to eight promoters of the pimaricin cluster, as demonstrated by electrophoretic mobility shift assays. Assays with truncated forms of the protein revealed that the PAS domain does not mediate specificity or the distinct recognition of target genes, which rely on the DBD domain, but significantly reduces binding affinity up to 500-fold. Transcription start points were identified by 5'-rapid amplification of cDNA ends, and the binding regions of PimM(DBD) were investigated by DNase I protection studies. In all cases, binding took place covering the -35 hexamer box of each promoter, suggesting an interaction of PimM and RNA polymerase to cause transcription activation. Information content analysis of the 16 sequences protected in target promoters was used to deduce the structure of the PimM-binding site. This site displays dyad symmetry, spans 14 nucleotides, and adjusts to the consensus TVGGGAWWTCCCBA. Experimental validation of this binding site was performed by using synthetic DNA duplexes. Binding of PimM to the promoter region of one of the polyketide synthase genes from the Streptomyces nodosus amphotericin cluster containing the consensus binding site was also observed, thus proving the applicability of the findings reported here to other antifungal polyketides.

Modulation of Multiple Gene Clusters' Expression by the PAS-LuxR Transcriptional Regulator PteF.

PAS-LuxR transcriptional regulators are conserved proteins governing polyene antifungal biosynthesis. PteF is the regulator of filipin biosynthesis from Streptomyces avermitilis. Its mutation drastically abates filipin, but also oligomycin production, a macrolide ATP-synthase inhibitor, and delays sporulation; thus, it has been considered a transcriptional activator. Transcriptomic analyses were performed in S. avermitilis DpteF and its parental strain. Both strains were grown in a YEME medium without sucrose, and the samples were taken at exponential and stationary growth phases. A total of 257 genes showed an altered expression in the mutant, most of them at the exponential growth phase. Surprisingly, despite PteF being considered an activator, most of the genes affected showed overexpression, thereby suggesting a negative modulation. The affected genes were related to various metabolic processes, including genetic information processing; DNA, energy, carbohydrate, and lipid metabolism; morphological differentiation; and transcriptional regulation, among others, but were particularly related to secondary metabolite biosynthesis. Notably, 10 secondary metabolite gene clusters out of the 38 encoded by the genome showed altered expression profiles in the mutant, suggesting a regulatory role for PteF that is wider than expected. The transcriptomic results were validated by quantitative reverse-transcription polymerase chain reaction. These findings provide important clues to understanding the intertwined regulatory machinery that modulates antibiotic biosynthesis in Streptomyces.

Understanding microRNAs in the Context of Infection to Find New Treatments against Human Bacterial Pathogens.

The development of RNA-based anti-infectives has gained interest with the successful application of mRNA-based vaccines. Small RNAs are molecules of RNA of <200 nucleotides in length that may control the expression of specific genes. Small RNAs include small interference RNAs (siRNAs), Piwi-interacting RNAs (piRNAs), or microRNAs (miRNAs). Notably, the role of miRNAs on the post-transcriptional regulation of gene expression has been studied in detail in the context of cancer and many other genetic diseases. However, it is also becoming apparent that some human miRNAs possess important antimicrobial roles by silencing host genes essential for the progress of bacterial or viral infections. Therefore, their potential use as novel antimicrobial therapies has gained interest during the last decade. The challenges of the transport and delivery of miRNAs to target cells are important, but recent research with exosomes is overcoming the limitations in RNA-cellular uptake, avoiding their degradation. Therefore, in this review, we have summarised the latest developments in the exosomal delivery of miRNA-based therapies, which may soon be another complementary treatment to pathogen-targeted antibiotics that could help solve the problem caused by multidrug-resistant bacteria.

Functional conservation of PAS-LuxR transcriptional regulators in polyene macrolide biosynthesis.

Control of polyene macrolide production in Streptomyces natalensis is mediated by the PAS-LuxR transcriptional activator PimM. Expression of target genes in this strain is positively regulated by binding of the regulator to 14-nucleotide sites showing dyad symmetry, and overlapping the -35 element of each promoter. These sequences have been found in the upstream regions of genes belonging to different polyene biosynthetic gene clusters. All the sequences in the amphotericin, nystatin, and filipin clusters were cloned and the binding of PimM to all of them has been shown by electrophoretic mobility shift assays. The precise binding regions were investigated by DNaseI protection studies. Results indicated that PAS-luxR regulators share the same regulatory pattern in different polyene-producing strains, these genes being responsible for polyketide chain construction, and when available, the genes for sugar dehydration and attachment, and the ABC transporters, the targets for regulation. Information content analysis of the 24 sequences protected in target promoters was used to refine the information-based model of the binding site. This site now spans 16 nucleotides and adjusts to the consensus CTVGGGAWWTCCCBAG. Gene complementation of S. natalensis ΔpimM with a single copy of heterologous regulators of the PAS/LuxR class integrated into the chromosome, such as amphRIV, nysRIV, or pteF, restored antifungal production, thus proving the functional conservation of these regulators. Introduction of a single copy of pimM into the amphotericin producing strain Streptomyces nodosus, or into the filipin producing strain S. avermitilis, boosted the production of both polyenes, thus indicating that the expression of the PAS-LuxR regulator constitutes a bottleneck in the biosynthesis of the antifungal, and also that these regulators are fully exchangeable. This work is the first report of a general mechanism regulating polyene production.

Production, Detection, Extraction, and Quantification of Polyene Antibiotics.

Polyene antibiotics are macrolide antifungal compounds obtained by fermentation of producer Streptomyces strains. Here we describe commonly used methods for polyene production, detection, and their subsequent extraction and purification. While bioassays are used to detect these compounds based on their biological activity, quantification by spectrophotometry or high-performance liquid chromatography (HPLC ) relies on their physiochemical properties and is more reliable.

Structural and Bioactivity Characterization of Filipin Derivatives from Engineered Streptomyces filipinensis Strains Reveals Clues for Reduced Haemolytic Action.

The rise in the number of immunocompromised patients has led to an increased incidence of fungal infections, with high rates of morbidity and mortality. Furthermore, misuse of antifungals has boosted the number of resistant strains to these agents; thus, there is urgent need for new drugs against these infections. Here, the in vitro antifungal activity of filipin III metabolic intermediates has been characterized against a battery of opportunistic pathogenic fungi-Candida albicans, Candida glabrata, Candida krusei, Cryptococcus neoformans, Trichosporon cutaneum, Trichosporon asahii, Aspergillus nidulans, Aspergillus niger, and Aspergillus fumigatus-using the Clinical and Laboratory Standards Institute broth microdilution method. Structural characterization of these compounds was undertaken by mass spectrometry (MS) and nuclear magnetic resonance (NMR) following HPLC purification. Complete NMR assignments were obtained for the first time for filipins I and II. In vitro haemolytic assays revealed that the haemolytic action of these compounds relies largely on the presence of a hydroxyl function at C26, since derivatives lacking such moiety show remarkably reduced activity. Two of these derivatives, 1'-hydroxyfilipin I and filipin I, show decreased toxicity towards cholesterol-containing membranes while retaining potent antifungal activity, and could constitute excellent leads for the development of efficient pharmaceuticals, particularly against Cryptococcosis.

PimT, an amino acid exporter controls polyene production via secretion of the quorum sensing pimaricin-inducer PI-factor in Streptomyces natalensis.

BACKGROUND: Polyenes represent a major class of antifungal agents characterised by the presence of a series of conjugated double bonds in their planar hydroxylated macrolide ring structure. Despite their general interest, very little is known about the factors that modulate their biosynthesis. Among these factors, we have recently discovered a new inducing compound (PI-factor) in the pimaricin producer Streptomyces natalensis, which elicits polyene production in a manner characteristic of quorum sensing. Here, we describe the involvement of an amino-acid exporter from S. natalensis in modulating the expression of pimaricin biosynthetic genes via secretion of the quorum-sensing pimaricin-inducer PI-factor. RESULTS: Adjacent to the pimaricin gene cluster lies a member of the RhtB family of amino-acid exporters. Gene deletion and complementation experiments provided evidence for a role for PimT in the export of L-homoserine, L-serine, and L-homoserine lactone. Expression of the gene was shown to be induced by homoserine and by the quorum-sensing pimaricin-inducer PI-factor. Interestingly, the mutant displayed 65% loss of pimaricin production, and also 50% decrease in the production of PI, indicating that PimT is used as PI-factor exporter, and suggesting that the effect in antifungal production might be due to limited secretion of the inducer. CONCLUSION: This report describes the involvement of an amino acid exporter (encoded by pimT in the vicinity of the pimaricin cluster) in modulating the expression of antibiotic biosynthetic genes via secretion of the quorum-sensing pimaricin-inducer PI-factor. The discovery of the participation of amino acid exporters in a signal transduction cascade for the production of polyene macrolides is unexpected, and represents an important step forward towards understanding the regulatory network for polyene regulation. Additionally, this finding constitutes the first detailed characterization of an amino-acid exporter in an Actinomycete, and to our knowledge, the first evidence for the implication of this type of exporters in quorum sensing.

Activation of Secondary Metabolite Gene Clusters in Streptomyces clavuligerus by the PimM Regulator of Streptomyces natalensis.

Expression of non-native transcriptional activators may be a powerful general method to activate secondary metabolites biosynthetic pathways. PAS-LuxR regulators, whose archetype is PimM, activate the biosynthesis of polyene macrolide antifungals and other antibiotics, and have been shown to be functionally preserved across multiple Streptomyces strains. In this work we show that constitutive expression of pimM in Streptomyces clavuligerus ATCC 27064 significantly affected its transcriptome and modifies secondary metabolism. Almost all genes in three secondary metabolite clusters were overexpressed, including the clusters responsible for the biosynthesis of the clinically important clavulanic acid and cephamycin C. In comparison to a control strain, this resulted in 10- and 7-fold higher production levels of these metabolites, respectively. Metabolomic and bioactivity studies of S. clavuligerus::pimM also revealed deep metabolic changes. Antifungal activity absent in the control strain was detected in S. clavuligerus::pimM, and determined to be the result of a fivefold increase in the production of the tunicamycin complex.

Cholesterol oxidases act as signaling proteins for the biosynthesis of the polyene macrolide pimaricin.

The gene cluster responsible for pimaricin biosynthesis in Streptomyces natalensis contains a cholesterol oxidase-encoding gene (pimE) surrounded by genes involved in pimaricin production. Gene-inactivation and -complementation experiments revealed that pimE encodes a functional cholesterol oxidase and, surprisingly, that it is also involved in pimaricin biosynthesis. This extracellular enzyme was purified from S. natalensis culture broths to homogeneity, and it was shown to restore pimaricin production when added to the mutant culture broths. Other cholesterol oxidases also triggered pimaricin production, suggesting that these enzymes could act as signaling proteins for polyene biosynthesis. This finding constitutes the description of a cholesterol oxidase gene with an involvement in antibiotic biosynthesis, and it broadens the scope of the biological functions for this type of oxidase.

Phosphate effect on filipin production and morphological differentiation in Streptomyces filipinensis and the role of the PhoP transcription factor.

The biosynthesis of the antifungal filipin in Streptomyces filipinensis is very sensitive to phosphate regulation. Concentrations as low as 2.5 mM block filipin production. This effect is, at least in part, produced by repression of the transcription of most filipin biosynthetic genes. The role of the two-component PhoRP system in this process was investigated. The phoRP system of S. filipinensis was cloned and transcriptionally characterised. PhoP binds to two PHO boxes present in one of its two promoters. Filipin production was greatly increased in ΔphoP and ΔphoRP mutants, in agreement with a higher transcription of the fil genes, and the effect of phosphate repression on the antibiotic production of these strains was significantly reduced. No PhoP binding was observed by electrophoretic mobility gel shift assays (EMSAs) with the promoter regions of the fil gene cluster thus suggesting an indirect effect of mutations. Binding assays with cell-free extracts from the wild-type and mutant strains on fil genes promoters revealed retardation bands in the parental strain that were absent in the mutants, thus suggesting that binding of the putative transcriptional regulator or regulators controlled by PhoP was PhoP dependent. Noteworthy, PhoP or PhoRP deletion also produced a dramatic decrease in sporulation ability, thus indicating a clear relationship between the phosphate starvation response mediated by PhoP and the sporulation process in S. filipinensis. This effect was overcome upon gene complementation, but also by phosphate addition, thus suggesting that alternative pathways take control in the absence of PhoRP.

An efficient gene transfer system for the pimaricin producer Streptomyces natalensis.

Streptomyces natalensis produces the antifungal polyene macrolide pimaricin. Genetic manipulation of its biosynthetic genes has been hampered by the lack of efficient gene transfer systems. We have developed a gene transfer system based on intergeneric conjugation from Escherichia coli. Using this approach, we managed to attain transformation efficiencies of 1 x 10(-4) exconjugants per recipient when using self-replicating vectors such as pHZ1358. The use of integrative vectors such as pSET152 or pSOK804 resulted in significantly lower efficiencies. Site-specific integration or the use of self-replicating plasmids did not affect pimaricin production or the essential functions of S. natalensis. Use of DNA methylation proficient E. coli donor strains resulted in no transformants, indicating the presence of methyl-specific restriction systems in S. natalensis. This methodology will enable easier manipulation of the genes responsible for pimaricin biosynthesis, and could prove valuable for the generation of new designer polyene macrolides with better antifungal activity and pharmacological properties. As an example of the validity of the method, we describe the introduction of Supercos-1-derived cosmid vectors into S. natalensis in order to promote gene replacements by double crossover recombination.

Generating novel polyene antifungal drugs.

In this issue of Chemistry & Biology, Francisco Malpartida and colleagues [1] report the formation of novel polyene amide derivatives upon transformation of the producer strain with SCP2*-derived vectors carrying the erythromycin resistance gene ermE. This unexpected finding provides a new tool for generating antifungal drugs by biotransformation.