The large linear plasmid pSLA2-L of Streptomyces rochei has an unusually condensed gene organization for secondary metabolism

Isolation of Hydrazide-alkenes with Different Amino Acid Origins from an Azoxy-alkene-Producing Mutant of Streptomyces rochei 7434AN4

A triple mutant (strain KA57) of Streptomyces rochei 7434AN4 produces an azoxy-alkene compound, KA57A, which was not detected in a parent strain or other single and double mutants. This strain accumulated several additional minor components, whose structures were elucidated. HPLC analysis of strain KA57 indicated the presence of two UV active components (KA57D1 and KA57D2) as minor components. They exhibited a maximum UV absorbance at 218 nm, whereas a UV absorbance of azoxy-alkene KA57A was detected at 236 nm, suggesting that both KA57D1 and KA57D2 contain a different chromophore from KA57A. KA57D1 has a molecular formula of C12H22N2O2, and NMR analysis revealed KA57D1 is a novel hydrazide-alkene compound, (Z)-N-acetyl-N'-(hex-1-en-1-yl)isobutylhydrazide. Labeling studies indicated that nitrogen Nβ of KA57D1 is derived from l-glutamic acid, and the isobutylamide unit (C-1 to C-3, 2-Me, and Nα) originates from valine. KA57D2 has a molecular formula of C13H24N2O2, and its structure was determined to be (Z)-N-acetyl-N'-(hex-1-en-1-yl)-2-methylbutanehydrazide, in which a 2-methylbutanamide unit was shown to originate from isoleucine. Different biogenesis of the Nα atom (l-serine for KA57A, l-valine for KA57D1, and l-isoleucine for KA57D2) indicates the relaxed substrate recognition for nitrogen-nitrogen bond formation in the biosyntheses of KA57A, KA57D1, and KA57D2.

Global Map of Specialized Metabolites Encoded in Prokaryotic Plasmids

Plasmids are the main mobile elements responsible for horizontal gene transfer (HGT) in microorganisms. These replicons extend the metabolic spectrum of their host cells by carrying functional genes. However, it is still unknown to what extent plasmids carry biosynthetic gene clusters (BGCs) related to the production of secondary or specialized metabolites (SMs). Here, we analyzed 9,183 microbial plasmids to unveil their potential to produce SMs, finding a large diversity of cryptic BGCs in a few varieties of prokaryotic host taxa. Some of these plasmids harbored 15 or more BGCs, and many others were exclusively dedicated to mobilizing BGCs. We found an occurrence pattern of BGCs within groups of homologous plasmids shared by a common taxon, mainly in host-associated microbes (e.g., Rhizobiales, Enterobacteriaceae members). Our results add to the knowledge of the ecological functions and potential industrial uses of plasmids and shed light on the dynamics and evolution of SMs in prokaryotes. IMPORTANCE Plasmids are mobile DNA elements that can be shared among microbial cells, and they are useful for bringing to fruition some microbial ecological traits. However, it is not known to what extent plasmids harbor genes related to the production of specialized/secondary metabolites (SMs). In microbes, these metabolites are frequently useful for defense purposes, signaling, etc. In addition, these molecules usually have biotechnological and clinical applications. Here, we analyzed the content, dynamics, and evolution of genes related to the production of SMs in >9,000 microbial plasmids. Our results confirm that some plasmids act as a reservoir of SMs. We also found that some families of biosynthetic gene clusters are exclusively present in some groups of plasmids shared among closely related microbes. Host-associated bacteria (e.g., plant and human microbes) harbor the majority of specialized metabolites encoded in plasmids. These results provide new knowledge about microbial ecological traits and might enable the discovery of novel metabolites.

Accumulation of lankamycin derivative with a branched-chain sugar from a blocked mutant of chalcose biosynthesis in Streptomyces rochei 7434AN4

Lankamycin, a macrolide antibiotic produced by Streptomyces rochei 7434AN4, exhibits a moderate antimicrobial activity and acts as a synergistic pair with carbocyclic antibiotic lankacidin C by binding to the ribosome exit tunnel. Its biosynthetic gene (lkm) cluster (orf24-orf53) is located on the largest plasmid pSLA2-L (210,614 bp). Our group possesses a variety of lankamycin derivatives and macrolide-modification enzymes including P450 enzymes and glycosyltransferases, which may lead to expand the chemical library of bioactive macrolides. Here we constructed a mutant of a 3-ketoreductase gene lkmCVI (orf42) involved in d-chalcose biosynthesis, and its metabolite was isolated and structure-elucidated. Accumulation of novel lankamycin derivative harboring a branched-chain deoxysugar, 5-O-(4',6'-dideoxy-3'-C-acetyl-d-ribo-hexopyranosyl)-3-O-(4″-O-acetyl-l-arcanosyl)-lankanolide, indicated that LkmCVI acts as a gate keeper enzyme for d-chalcose synthesis in lankamycin biosynthesis.

Pseudomonas fitomaticsae sp. nov., isolated at Marimurtra Botanical Garden in Blanes, Catalonia, Spain

In the framework of the research project called fitomatics, we have isolated and characterized a bacterial plant-endophyte from the rhizomes of Iris germanica, hereafter referred to as strain FIT81T. The bacterium is Gram negative, rod-shaped with lophotrichous flagella, and catalase- and oxidase-positive. The optimal growth temperature of strain FIT81T is 28 °C, although it can grow within a temperature range of 4–32 °C. The pH growth tolerance ranges between pH 5 and 10, and it tolerates 4% (w/v) NaCl. A 16S rRNA phylogenetic analysis positioned strain FIT81T within the genus Pseudomonas , and multilocus sequence analysis revealed that Pseudomonas gozinkensis IzPS32dT, Pseudomonas glycinae MS586T, Pseudomonas allokribbensis IzPS23T, 'Pseudomonas kribbensis' 46–2 and Pseudomonas koreensis PS9-14T are the top five most closely related species, which were selected for further genome-to-genome comparisons, as well as for physiological and chemotaxonomic characterization. The genome size of strain FIT81T is 6 492 796 base-pairs long, with 60.6 mol% of G+C content. Average nucleotide identity and digital DNA–DNA hybridization analyses yielded values of 93.6 and 56.1%, respectively, when the FIT81T genome was compared to that of the closest type strain P. gozinkensis IzPS32dT. Taken together, the obtained genomic, physiologic and chemotaxonomic data indicate that strain FIT81T is different from its closest relative species, which lead us to suggest that it is a novel species to be included in the list of type strains with the name Pseudomonas fitomaticsae sp. nov. (FIT81T=CECT 30374T=DSM 112699T).

Bioinspired computational design of lankacidin derivatives for improvement in antitumor activity

Background: The 17-membered polyketide, lankacidin C, exhibits considerable antitumor activity as a microtubule stabilizer by binding to the paclitaxel binding site. Method: Esterification of the C-7/C-13 hydroxyl in lankacidin C was performed with acetyl, cinnamoyl and hydrocinnamoyl groups and their antitumor activity was assessed to improve the cytotoxicity of lankacidins through bioinspired computational design. Results: Compared with the cytotoxicity of parent lankacidin C against the HeLa cell line, 13-O-cinnamoyl-lankacidin C demonstrated sevenfold higher cytotoxicity. Furthermore, 7,13-di-O-cinnamoyl-lankacidin C exhibited considerable antitumor activity against three tested cell lines. Conclusion: C13-esterification by a cinnamoyl group dramatically improved antitumor activity, in agreement with computational predictions. This finding provides a potential substrate for next-generation lankacidin derivatives with significant antitumor activity.

Overexpression of SRO_3163, a homolog of Streptomyces antibiotic regulatory protein, induces the production of novel cyclohexene-containing enamide in Streptomyces rochei

Streptomyces antibiotic regulatory proteins (SARPs) are well characterized as transcriptional activators for secondary metabolites in Streptomyces species. Streptomyces rochei 7434AN4 harbors 15 SARP genes, among which 3 were located on a giant linear plasmid pSLA2-L and others were on the chromosome. Some SARP genes were cloned into an integrative thiostrepton-inducible vector pIJ8600, and their recombinants were cultivated. The recombinant of SARP gene, SRO_3163, accumulated a UV-active compound YM3163-A, which was not detected in the parent strain and other SARP recombinants. Its molecular formula was established to be C8H11NO. Extensive NMR analysis revealed that YM3163-A is a novel enamide, 2-(cyclohex-2-en-1-ylidene)acetamide, and its structure was confirmed by chemical synthesis including Horner-Wadsworth-Emmons reaction and ammonolysis.

Chemoenzymatic synthesis, computational investigation, and antitumor activity of monocyclic lankacidin derivatives

We investigated the importance of the δ-lactone ring (C1-C5) in lankacidin C using chemoenzymatic synthesis and computational prediction and assessing biological activity, including antitumor activity. Pyrroloquinoline quinone-dependent dehydrogenase (Orf23) in Streptomyces rochei was used in the chemoenzymatic synthesis of lankacyclinone C, a novel lankacidin C congener lacking the δ-lactone moiety. Orf23 could convert the monocyclic lankacidinol derivatives, lankacyclinol and 2-epi-lankacyclinol, to the C-24 keto compounds, lankacyclinone C and 2-epi-lankacyclinone C, respectively, elucidating the relaxed substrate specificity of Orf23. Computational prediction using molecular dynamics simulations and the molecular mechanics/generalized Born-surface area protocol indicated that binding energy values of all the monocyclic derivatives are very close to those of lankacidin C, which may reflect a comparable affinity to tubulin. Monocyclic lankacidin derivatives showed moderate antitumor activity when compared with bicyclic lankacidins, suggesting that the δ-lactone moiety is less important for antitumor activity in lankacidin-group antibiotics.

Beyond Self-Resistance: ABCF ATPase LmrC Is a Signal-Transducing Component of an Antibiotic-Driven Signaling Cascade Accelerating the Onset of Lincomycin Biosynthesis

In natural environments, antibiotics are important means of interspecies competition. At subinhibitory concentrations, they act as cues or signals inducing antibiotic production; however, our knowledge of well-documented antibiotic-based sensing systems is limited. Here, for the soil actinobacterium Streptomyces lincolnensis, we describe a fundamentally new ribosome-mediated signaling cascade that accelerates the onset of lincomycin production in response to an external ribosome-targeting antibiotic to synchronize antibiotic production within the population. The entire cascade is encoded in the lincomycin biosynthetic gene cluster (BGC) and consists of three lincomycin resistance proteins in addition to the transcriptional regulator LmbU: a lincomycin transporter (LmrA), a 23S rRNA methyltransferase (LmrB), both of which confer high resistance, and an ATP-binding cassette family F (ABCF) ATPase, LmrC, which confers only moderate resistance but is essential for antibiotic-induced signal transduction. Specifically, antibiotic sensing occurs via ribosome-mediated attenuation, which activates LmrC production in response to lincosamide, streptogramin A, or pleuromutilin antibiotics. Then, ATPase activity of the ribosome-associated LmrC triggers the transcription of lmbU and consequently the expression of lincomycin BGC. Finally, the production of LmrC is downregulated by LmrA and LmrB, which reduces the amount of ribosome-bound antibiotic and thus fine-tunes the cascade. We propose that analogous ABCF-mediated signaling systems are relatively common because many ribosome-targeting antibiotic BGCs encode an ABCF protein accompanied by additional resistance protein(s) and transcriptional regulators. Moreover, we revealed that three of the eight coproduced ABCF proteins of S. lincolnensis are clindamycin responsive, suggesting that the ABCF-mediated antibiotic signaling may be a widely utilized tool for chemical communication. IMPORTANCE Resistance proteins are perceived as mechanisms protecting bacteria from the inhibitory effect of their produced antibiotics or antibiotics from competitors. Here, we report that antibiotic resistance proteins regulate lincomycin biosynthesis in response to subinhibitory concentrations of antibiotics. In particular, we show the dual character of the ABCF ATPase LmrC, which confers antibiotic resistance and simultaneously transduces a signal from ribosome-bound antibiotics to gene expression, where the 5' untranslated sequence upstream of its encoding gene functions as a primary antibiotic sensor. ABCF-mediated antibiotic signaling can in principle function not only in the induction of antibiotic biosynthesis but also in selective gene expression in response to any small molecules targeting the 50S ribosomal subunit, including clinically important antibiotics, to mediate intercellular antibiotic signaling and stress response induction. Moreover, the resistance-regulatory function of LmrC presented here for the first time unifies functionally inconsistent ABCF family members involving antibiotic resistance proteins and translational regulators.

Identification, Isolation, and Characterization of Medipeptins, Antimicrobial Peptides From Pseudomonas mediterranea EDOX

The plant microbiome is a vastly underutilized resource for identifying new genes and bioactive compounds. Here, we used Pseudomonas sp. EDOX, isolated from the leaf endosphere of a tomato plant grown on a small farm in the Netherlands. To get more insight into its biosynthetic potential, the genome of Pseudomonas sp. EDOX was sequenced and subjected to bioinformatic analyses. The genome sequencing analysis identified strain EDOX as a member of the Pseudomonas mediterranea. In silico analysis for secondary metabolites identified a total of five non-ribosomally synthesized peptides synthetase (NRPS) gene clusters, related to the biosynthesis of syringomycin, syringopeptin, anikasin, crochelin A, and fragin. Subsequently, we purified and characterized several cyclic lipopeptides (CLPs) produced by NRPS, including some of the already known ones, which have biological activity against several plant and human pathogens. Most notably, mass spectrometric analysis led to the discovery of two yet unknown CLPs, designated medipeptins, consisting of a 22 amino acid peptide moiety with varying degrees of activity against Gram-positive and Gram-negative pathogens. Furthermore, we investigated the mode of action of medipeptin A. The results show that medipeptin A acts as a bactericidal antibiotic against Gram-positive pathogens, but as a bacteriostatic antibiotic against Gram-negative pathogens. Medipeptin A exerts its potent antimicrobial activity against Gram-positive bacteria via binding to both lipoteichoic acid (LTA) and lipid II as well as by forming pores in membranes. Collectively, our study provides important insights into the biosynthesis and mode of action of these novel medipeptins from P. mediterranea EDOX.

Biology and applications of co-produced, synergistic antimicrobials from environmental bacteria

Environmental bacteria, such as Streptomyces spp., produce specialized metabolites that are potent antibiotics and therapeutics. Selected specialized antimicrobials are co-produced and function together synergistically. Co-produced antimicrobials comprise multiple chemical classes and are produced by a wide variety of bacteria in different environmental niches, suggesting that their combined functions are ecologically important. Here, we highlight the exquisite mechanisms that underlie the simultaneous production and functional synergy of 16 sets of co-produced antimicrobials. To date, antibiotic and antifungal discovery has focused mainly on single molecules, but we propose that methods to target co-produced antimicrobials could widen the scope and applications of discovery programs.

Properties of a "Split-and-Stuttering" Module of an Assembly Line Polyketide Synthase

Notwithstanding the "one-module-one-elongation-cycle" paradigm of assembly line polyketide synthases (PKSs), some PKSs harbor modules that iteratively elongate their substrates through a defined number of cycles. While some insights into module iteration, also referred to as "stuttering", have been derived through in vivo and in vitro analysis of a few PKS modules, a general understanding of the mechanistic principles underlying module iteration remains elusive. This report serves as the first interrogation of a stuttering module from a trans-AT subfamily PKS that is also naturally split across two polypeptides. Previous work has shown that Module 5 of the NOCAP (nocardiosis associated polyketide) synthase iterates precisely three times in the biosynthesis of its polyketide product, resulting in an all-trans-configured triene moiety in the polyketide product. Here, we describe the intrinsic catalytic properties of this NOCAP synthase module. Through complementary experiments in vitro and in E. coli, the "split-and-stuttering" module was shown to catalyze up to five elongation cycles, although its dehydratase domain ceased to function after three cycles. Unexpectedly, the central olefinic group of this truncated product had a cis configuration. Our findings set the stage for further in-depth analysis of a structurally and functionally unusual PKS module with contextual biosynthetic plasticity.

Chlorinated metabolites from Streptomyces sp. highlight the role of biosynthetic mosaics and superclusters in the evolution of chemical diversity

LCMS-guided screening of a library of biosynthetically talented bacteria and fungi identified Streptomyces sp. MST- as a prolific producer of chlorinated metabolites. We isolated and characterised six new and nine reported compounds from MST-, belonging to three discrete classes - the depsipeptide svetamycins, the indolocarbazole borregomycins and the aromatic polyketide anthrabenzoxocinones. Following genome sequencing of MST-, we describe, for the first time, the svetamycin biosynthetic gene cluster (sve), its mosaic structure and its relationship to several distantly related gene clusters. Our analysis of the sve cluster suggested that the reported stereostructures of the svetamycins may be incorrect. This was confirmed by single-crystal X-ray diffraction analysis, allowing us to formally revise the absolute configurations of svetamycins A-G. We also show that the borregomycins and anthrabenzoxocinones are encoded by a single supercluster (bab) implicating superclusters as potential nucleation points for the evolution of biosynthetic gene clusters. These clusters highlight how individual enzymes and functional subclusters can be co-opted during the formation of biosynthetic gene clusters, providing a rare insight into the poorly understood mechanisms underpinning the evolution of chemical diversity.

antiSMASH 6.0: improving cluster detection and comparison capabilities

Many microorganisms produce natural products that form the basis of antimicrobials, antivirals, and other drugs. Genome mining is routinely used to complement screening-based workflows to discover novel natural products. Since 2011, the "antibiotics and secondary metabolite analysis shell—antiSMASH" (https://antismash.secondarymetabolites.org/) has supported researchers in their microbial genome mining tasks, both as a free-to-use web server and as a standalone tool under an OSI-approved open-source license. It is currently the most widely used tool for detecting and characterising biosynthetic gene clusters (BGCs) in bacteria and fungi. Here, we present the updated version 6 of antiSMASH. antiSMASH 6 increases the number of supported cluster types from 58 to 71, displays the modular structure of multi-modular BGCs, adds a new BGC comparison algorithm, allows for the integration of results from other prediction tools, and more effectively detects tailoring enzymes in RiPP clusters.

Substrate specificity of two cytochrome P450 monooxygenases involved in lankamycin biosynthesis

To elucidate the gross lankamycin biosynthetic pathway including two cytochrome P450 monooxygenases, LkmK and LkmF, we constructed two double mutants of P450 genes in combination with glycosyltransferase genes, lkmL and lkmI. An aglycon 8,15-dideoxylankanolide, a possible substrate for LkmK, was prepared from an lkmK-lkmL double mutant, while a monoglycoside 3-O-l-arcanosyl-8-deoxylankanolide, a substrate for LkmF, was from an lkmF-lkmI double mutant. Bioconversion of lankamycin derivatives was performed in the Escherichia coli recombinant for LkmK and the Streptomyces lividans recombinant for LkmF, respectively. LkmK catalyzes the C-15 hydroxylation on all 15-deoxy derivatives, including 8,15-dideoxylankanolide (a possible substrate), 8,15-dideoxylankamycin, and 15-deoxylankamycin, suggesting the relaxed substrate specificity of LkmK. On the other hand, LkmF hydroxylates the C-8 methine of 3-O-l-anosyl-8-deoxylankanolide. Other 8-deoxy lankamycin/lankanolide derivatives were not oxidized, suggesting the importance of a C-3 l-arcanosyl moiety for substrate recognition by LkmF in lankamycin biosynthesis. Thus, LkmF has a strict substrate specificity in lankamycin biosynthesis.

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.

Functional Analysis of P450 Monooxygenase SrrO in the Biosynthesis of Butenolide-Type Signaling Molecules in Streptomyces rochei

Streptomyces rochei 7434AN4 produces two structurally unrelated polyketide antibiotics lankacidin and lankamycin, and their biosynthesis is tightly controlled by butenolide-type signaling molecules SRB1 and SRB2. SRBs are synthesized by SRB synthase SrrX, and induce lankacidin and lankamycin production at 40 nM concentration. We here investigated the role of a P450 monooxygenase gene srrO (orf84), which is located adjacent to srrX (orf85), in SRB biosynthesis. An srrO mutant KA54 accumulated lankacidin and lankamycin at a normal level when compared with the parent strain. To elucidate the chemical structures of the signaling molecules accumulated in KA54 (termed as KA54-SRBs), this mutant was cultured (30 L) and the active components were purified. Two active components (KA54-SRB1 and KA54-SRB2) were detected in ESI-MS and chiral HPLC analysis. The molecular formulae for KA54-SRB1 and KA54-SRB2 are C₁₅H₂₆O₄ and C₁₆H₂₈O₄, whose values are one oxygen smaller and two hydrogen larger when compared with those for SRB1 and SRB2, respectively. Based on extensive NMR analysis, the signaling molecules in KA54 were determined to be 6'-deoxo-SRB1 and 6'-deoxo-SRB2. Gel shift analysis indicated that a ligand affinity of 6'-deoxo-SRB1 to the specific receptor SrrA was 100-fold less than that of SRB1. We performed bioconversion of the synthetic 6'-deoxo-SRB1 in the Streptomyces lividans recombinant carrying SrrO-expression plasmid. Substrate 6'-deoxo-SRB1 was converted through 6'-deoxo-6'-hydroxy-SRB1 to SRB1 in a time-dependent manner. Thus, these results clearly indicated that SrrO catalyzes the C-6' oxidation at a final step in SRB biosynthesis.

SrrB, a Pseudo-Receptor Protein, Acts as a Negative Regulator for Lankacidin and Lankamycin Production in Streptomyces rochei

Streptomyces rochei 7434AN4, a producer of lankacidin (LC) and lankamycin (LM), carries many regulatory genes including a biosynthesis gene for signaling molecules SRBs (srrX), an SRB receptor gene (srrA), and a SARP (Streptomyces antibiotic regulatory protein) family activator gene (srrY). Our previous study revealed that the main regulatory cascade goes from srrX through srrA to srrY, leading to LC production, whereas srrY further regulates a second SARP gene srrZ to synthesize LM. In this study we extensively investigated the function of srrB, a pseudo-receptor gene, by analyzing antibiotic production and transcription. Metabolite analysis showed that the srrB mutation increased both LC and LM production over four-folds. Transcription, gel shift, and DNase I footprinting experiments revealed that srrB and srrY are expressed under the SRB/SrrA regulatory system, and at the later stage, SrrB represses srrY expression by binding to the promoter region of srrY. These findings confirmed that SrrB acts as a negative regulator of the activator gene srrY to control LC and LM production at the later stage of fermentation in S. rochei.

Genome rearrangements and megaplasmid loss in the filamentous bacterium Kitasatospora viridifaciens are associated with protoplast formation and regeneration

Filamentous Actinobacteria are multicellular bacteria with linear replicons. Kitasatospora viridifaciens DSM 40239 contains a linear 7.8 Mb chromosome and an autonomously replicating plasmid KVP1 of 1.7 Mb. Here we show that lysozyme-induced protoplast formation of the multinucleated mycelium of K. viridifaciens drives morphological diversity. Characterisation and sequencing of an individual revertant colony that had lost the ability to differentiate revealed that the strain had not only lost most of KVP1 but also carried deletions in the right arm of the chromosome. Strikingly, the deletion sites were preceded by insertion sequence elements, suggesting that the rearrangements may have been caused by replicative transposition and homologous recombination between both replicons. These data indicate that protoplast formation is a stressful process that can lead to profound genetic changes.

Complete Genome Sequence of Acidithiobacillus Ferrooxidans YNTRS-40, a Strain of the Ferrous Iron- and Sulfur-Oxidizing Acidophile

Acidithiobacillus ferrooxidans YNTRS-40 (A. ferrooxidans) is a chemolithoautotrophic aerobic bacterium isolated from Tengchong hot springs, Yunnan Province, China, with a broad growth pH range of 1.0–4.5. This study reports the genome sequence of this strain and the information of genes related to the adaptation of diverse stresses and the oxidation of ferrous iron and sulfur. Results showed that YNTRS-40 possesses chromosomal DNA (3,209,933-bp) and plasmid DNA (47,104-bp). The complete genome of 3,257,037-bp consists of 3,349 CDS genes comprising 6 rRNAs, 52 tRNAs, and 6 ncRNAs. There are many encoded genes associated with diverse stresses adaptation and ferrous iron and sulfur oxidation such as rus operon, res operon, petI, petII, sqr, doxDA, cydAB, and cyoABCD. This work will provide essential information for further application of A. ferrooxidans YNTRS-40 in industry.

Comparison of Antibiotic Resistance Mechanisms in Antibiotic-Producing and Pathogenic Bacteria

Antibiotic resistance poses a tremendous threat to human health. To overcome this problem, it is essential to know the mechanism of antibiotic resistance in antibiotic-producing and pathogenic bacteria. This paper deals with this problem from four points of view. First, the antibiotic resistance genes in producers are discussed related to their biosynthesis. Most resistance genes are present within the biosynthetic gene clusters, but some genes such as paromomycin acetyltransferases are located far outside the gene cluster. Second, when the antibiotic resistance genes in pathogens are compared with those in the producers, resistance mechanisms have dependency on antibiotic classes, and, in addition, new types of resistance mechanisms such as Eis aminoglycoside acetyltransferase and self-sacrifice proteins in enediyne antibiotics emerge in pathogens. Third, the relationships of the resistance genes between producers and pathogens are reevaluated at their amino acid sequence as well as nucleotide sequence levels. Pathogenic bacteria possess other resistance mechanisms than those in antibiotic producers. In addition, resistance mechanisms are little different between early stage of antibiotic use and the present time, e.g., β-lactam resistance in Staphylococcus aureus. Lastly, guanine + cytosine (GC) barrier in gene transfer to pathogenic bacteria is considered. Now, the resistance genes constitute resistome composed of complicated mixture from divergent environments.

The genome sequence of Streptomyces rochei 7434AN4, which carries a linear chromosome and three characteristic linear plasmids

Streptomyces rochei 7434AN4 produces two structurally unrelated polyketide antibiotics, lankacidin and lankamycin, and carries three linear plasmids, pSLA2-L (211 kb), -M (113 kb), and -S (18 kb), whose nucleotide sequences were previously reported. The complete nucleotide sequence of the S. rochei chromosome has now been determined using the long-read PacBio RS-II sequencing together with short-read Illumina Genome Analyzer IIx sequencing and Roche 454 pyrosequencing techniques. The assembled sequence revealed an 8,364,802-bp linear chromosome with a high G + C content of 71.7% and 7,568 protein-coding ORFs. Thus, the gross genome size of S. rochei 7434AN4 was confirmed to be 8,706,406 bp including the three linear plasmids. Consistent with our previous study, a tap-tpg gene pair, which is essential for the maintenance of a linear topology of Streptomyces genomes, was not found on the chromosome. Remarkably, the S. rochei chromosome contains seven ribosomal RNA (rrn) operons (16S-23S-5S), although Streptomyces species generally contain six rrn operons. Based on 2ndFind and antiSMASH platforms, the S. rochei chromosome harbors at least 35 secondary metabolite biosynthetic gene clusters, including those for the 28-membered polyene macrolide pentamycin and the azoxyalkene compound KA57-A.

Stereoconfining macrocyclizations in the total synthesis of natural products

This review covers selected examples of point chirality-forming macrocyclizations in natural product total synthesis in the past three decades.

Probing the Flexibility of an Iterative Modular Polyketide Synthase with Non-Native Substrates in Vitro

In the search for molecular machinery for custom biosynthesis of valuable compounds, the modular type I polyketide synthases (PKSs) offer great potential. In this study, we investigate the flexibility of BorM5, the iterative fifth module of the borrelidin synthase, with a panel of non-native priming substrates in vitro. BorM5 differentially extends various aliphatic and substituted substrates. Depending on substrate size and substitution BorM5 can exceed the three iterations it natively performs. To probe the effect of methyl branching on chain length regulation, we engineered a BorM5 variant capable of incorporating methylmalonyl- and malonyl-CoA into its intermediates. Intermediate methylation did not affect overall chain length, indicating that the enzyme does not to count methyl branches to specify the number of iterations. In addition to providing regulatory insight about BorM5, we produced dozens of novel methylated intermediates that might be used for production of various hydrocarbons or pharmaceuticals. These findings enable rational engineering and recombination of BorM5 and inform the study of other iterative modules.

Unusual features of the large linear plasmid pSA3239 from Streptomyces aureofaciens CCM 3239

We previously identified the aur1 gene cluster, responsible for the production of the angucycline antibiotic auricin in Streptomyces aureofaciens CCM 3239. Pulse-field gel electrophoresis showed a single, 241kb linear plasmid, pSA3239, in this strain, and several approaches confirmed the presence of the aur1 cluster in this plasmid. We report here the nucleotide sequence of this 241,076-bp plasmid. pSA3239 contains an unprecedentedly small (13bp) telomeric sequence CCCGCGGAGCGGG, which is identical to the conserved Palindrome I sequence involved in the priming of end-patching replication. A bioinformatics analysis revealed 234 open reading frames with high number (28) of regulatory genes from various families. In contrast to most other linear plasmids, pSA3239 contains a pair of replication initiation genes (sa76 and sa75) located at its extreme left end, adjacent to the telomere. Together with similar proteins from several other linear plasmids (pFRL2, pSLA2-M, pSV2, pSDA1, and SAP1), they constitute a new family of replication initiation proteins. This left end also contains two genes, tpgSa and tapSa, encoding the terminal protein and the telomere associated-protein involved in telomere end-patching replication. pSA3239 also contains two genes homologous to the parAB partitioning system, and deletion of the parA homologue (sa43) affects structural stability of the plasmid. pSA3239 carries five potential secondary metabolite gene clusters. In addition to aur1 and a non-ribosomal peptide synthase (NRPS) gene cluster for the blue pigment indigoidine, it also contains a partial type II polyketide synthase (PKS) gene cluster, a partial type I PKS gene cluster, and a NRPS/PKSI gene cluster for unknown secondary metabolites. The last gene cluster contains a subcluster of seven genes (sa91-sa97), highly similar to part of the valanimycin biosynthetic cluster vlm. A S. aureofaciens strain lacking pSA3239 was prepared. This deletion did not substantially affect growth and differentiation. A comparative analysis of secondary metabolites between both strains did not identify any product, except auricin and indigoidine, which is dependent upon pSA3239. Thus, the other three identified gene clusters are likely silent under these conditions.

Cytochromes P450 for natural product biosynthesis in Streptomyces: sequence, structure, and function

Covering: up to January 2017Cytochrome P450 enzymes (P450s) are some of the most exquisite and versatile biocatalysts found in nature. In addition to their well-known roles in steroid biosynthesis and drug metabolism in humans, P450s are key players in natural product biosynthetic pathways. Natural products, the most chemically and structurally diverse small molecules known, require an extensive collection of P450s to accept and functionalize their unique scaffolds. In this review, we survey the current catalytic landscape of P450s within the Streptomyces genus, one of the most prolific producers of natural products, and comprehensively summarize the functionally characterized P450s from Streptomyces. A sequence similarity network of >8500 P450s revealed insights into the sequence-function relationships of these oxygen-dependent metalloenzymes. Although only ∼2.4% and <0.4% of streptomycete P450s have been functionally and structurally characterized, respectively, the study of streptomycete P450s involved in the biosynthesis of natural products has revealed their diverse roles in nature, expanded their catalytic repertoire, created structural and mechanistic paradigms, and exposed their potential for biomedical and biotechnological applications. Continued study of these remarkable enzymes will undoubtedly expose their true complement of chemical and biological capabilities.

Genome Sequence of the Filamentous Actinomycete Kitasatospora viridifaciens

The vast majority of antibiotics are produced by filamentous soil bacteria called actinomycetes. We report here the genome sequence of the tetracycline producer “Streptomyces viridifaciens” DSM 40239. Given that this species has the hallmark signatures characteristic of the Kitasatospora genus, we previously proposed to rename this organism Kitasatospora viridifaciens.

Iterative polyketide biosynthesis by modular polyketide synthases in bacteria

Modular polyketide synthases (type I PKSs) in bacteria are responsible for synthesizing a significant percentage of bioactive natural products. This group of synthases has a characteristic modular organization, and each module within a PKS carries out one cycle of polyketide chain elongation; thus each module is non-iterative in function. It was possible to predict the basic structure of a polyketide product from the module organization of the PKSs, since there generally existed a co-linearity between the number of modules and the number of chain elongations. However, more and more bacterial modular PKSs fail to conform to the canonical rules, and a particularly noteworthy group of non-canonical PKSs is the bacterial iterative type I PKSs. This review covers recent examples of iteratively used modular PKSs in bacteria. These non-canonical PKSs give rise to a large array of natural products with impressive structural diversity. The molecular mechanism behind the iterations is often unclear, presenting a new challenge to the rational engineering of these PKSs with the goal of generating new natural products. Structural elucidation of these synthase complexes and better understanding of potential PKS-PKS interactions as well as PKS-substrate recognition may provide new prospects and inspirations for the discovery and engineering of new bioactive polyketides.

Endophytes as in vitro production platforms of high value plant secondary metabolites

Many reports have been published on bioprospecting of endophytic fungi capable of producing high value bioactive molecules like, paclitaxel, vincristine, vinblastine, camptothecin and podophyllotoxin. However, commercial exploitation of endophytes for high value-low volume plant secondary metabolites remains elusive due to widely reported genomic instability of endophytes in the axenic culture. While most of the endophyte research focuses on screening endophytes for novel or existing high value biomolecules, very few reports seek to explore the possible mechanisms of production of host-plant associated or novel secondary metabolites in these organisms. With an overview of host-endophyte relationship and its possible impact on the secondary metabolite production potential of endophytes, the review highlights the evidence reported for and against the presence of host-independent biosynthetic machinery in endophytes. The review aims to address the question, why should and how can endophytes be exploited for large scale in vitro production of high value phytochemicals? In this regard, various bioprocess optimization strategies that have been applied to sustain and enhance the product yield from the endophytes have also been described in detail. Further, techniques like mixed fermentation/co-cultivation and use of epigenetic modifiers have also been discussed as potential strategies to activate cryptic gene clusters in endophytes, thereby aiding in novel metabolite discovery and overcoming the limitations associated with axenic culture of endophytes.

Butyrolactone signalling circuits for synthetic biology

Signalling circuits based on quorum sensing mechanisms have been popular tools for synthetic biology. Recent advances in our understanding of the analogous systems regulating antibiotics production in soil bacteria suggest that these might provide useful complementary tools to increase the complexity of possible circuit designs. Here we discuss the diversity of these natural circuits, which use γ-butyrolactones (GBLs) as their main inter-cellular signal, highlighting the range of new building blocks they could provide, as well as a number of exciting recent applications of GBL-based circuits in heterologous systems. We conclude by presenting examples of the novel circuit complexity that could become accessible through the use of GBL-based designs.

The tap-tpg gene pair on the linear plasmid functions to maintain a linear topology of the chromosome in Streptomyces rochei

Streptomyces rochei 7434AN4 carries three linear plasmids, pSLA2-L (211 kb), pSLA2-M (113 kb) and pSLA2-S (18 kb), their complete nucleotide sequences having been determined. Restriction and sequencing analysis revealed that the telomere sequences at both ends of the linear chromosome are identical to each other, are 98.5% identical to the right end sequences of pSLA2-L and pSLA2-M up to 3.1 kb from the ends and have homology to those of typical Streptomyces species. Mutant 2-39, which lost all the three linear plasmids, was found to carry a circularized chromosome. Sequence comparison of the fusion junction and both deletion ends revealed that chromosomal circularization occurred by terminal deletions followed by nonhomologous recombination. Curing of pSLA2-L from strain 51252, which carries only pSLA2-L, also resulted in terminal deletions in newly obtained mutants. The tap-tpg gene pair, which encodes a telomere-associated protein and a terminal protein for end patching, is located on pSLA2-L and pSLA2-M but has not hitherto been found on the chromosome. These results led us to the idea that the tap-tpg of pSLA2-L or pSLA2-M functions to maintain a linear chromosome in strain 7434AN4. This hypothesis was finally confirmed by complementation and curing experiments of the tap-tpg of pSLA2-M.

Blockage of the early step of lankacidin biosynthesis caused a large production of pentamycin, citreodiol and epi-citreodiol in Streptomyces rochei

In our effort to find the key intermediates of lankacidin biosynthesis in Streptomyces rochei, three UV-active compounds were isolated from mutant FS18, a gene disruptant of lkcA encoding a non-ribosomal peptide synthetase (NRPS)-polyketide synthase (PKS) hybrid enzyme. Their structures were elucidated on the basis of spectroscopic data of NMR and MS. Two compounds of a higher mobile spot on silica gel TLC (Rf=0.45 in CHCl3-MeOH=20:1) were determined to be an epimeric mixture of citreodiol and epi-citreodiol at the C-6 position in the ratio of 2:1. In contrast, the compound of a lower mobile spot (Rf=~0 in CHCl3-MeOH=20:1) was identical to a 28-membered polyene macrolide pentamycin. The yields of citreodiols and pentamycin in FS18 were 5- and 250-fold higher compared with the parent strain. Introduction of a second mutation of srrX, coding a biosynthetic gene of the signaling molecules SRBs, into mutant FS18 did not affect the production of three metabolites. Thus, their production was not regulated by the SRB signaling molecules in contrast to lankacidin or lankamycin.

An unusual dehydratase acting on glycerate and a ketoreducatse stereoselectively reducing α-ketone in polyketide starter unit biosynthesis

Polyketide synthases (PKSs) usually employ a ketoreductase (KR) to catalyze the reduction of a β-keto group, followed by a dehydratase (DH) that drives the dehydration to form a double bond between the α- and β-carbon atoms. Herein, a DH*-KR* involved in FR901464 biosynthesis was characterized: DH* acts on glyceryl-S-acyl carrier protein (ACP) to yield ACP-linked pyruvate; subsequently KR* reduces α-ketone that yields L-lactyl-S-ACP as starter unit for polyketide biosynthesis. Genetic and biochemical evidence was found to support a similar pathway that is involved in the biosynthesis of lankacidins. These results not only identified new PKS domains acting on different substrates, but also provided additional options for engineering the PKS starter pathway or biocatalysis.

Genetic and biochemical analysis of the antibiotic biosynthetic gene clusters on the Streptomyces linear plasmid

We extensively analyzed the giant linear plasmid pSLA2-L in Streptomyces rochei 7434AN4, a producer of two structurally unrelated polyketide antibiotics, lankacidin and lankamycin. It was found that amine oxidase LkcE oxidizes an acyclic amine to an imine, which is in turn converted to the 17-membered carbocyclic lankacidin. Heterologous expression and translational fusion experiments indicated the modular-iterative mixed polyketide biosynthesis of lankacidin. Concerning to lankamycin biosynthesis, starter unit biosynthesis and the post-PKS modification pathway were elucidated by feeding and gene inactivation experiments. It was shown that pSLA2-L contains many regulatory genes, which constitute the signaling molecule/receptor system for antibiotic production and morphological differentiation in this strain. Two signaling molecules, SRB1 and SRB2, that induce production of lankacidin and lankamycin were further isolated and their structures were elucidated. Each contains a 2,3-disubstituted butenolide skeleton, and the stereochemistry at C-1' position is crucial for inducing activity.

A single Sfp-type phosphopantetheinyl transferase plays a major role in the biosynthesis of PKS and NRPS derived metabolites in Streptomyces ambofaciens ATCC23877

The phosphopantetheinyl transferases (PPTases) are responsible for the activation of the carrier protein domains of the polyketide synthases (PKS), non ribosomal peptide synthases (NRPS) and fatty acid synthases (FAS). The analysis of the Streptomyces ambofaciens ATCC23877 genome has revealed the presence of four putative PPTase encoding genes. One of these genes appears to be essential and is likely involved in fatty acid biosynthesis. Two other PPTase genes, samT0172 (alpN) and samL0372, are located within a type II PKS gene cluster responsible for the kinamycin production and an hybrid NRPS-PKS cluster involved in antimycin production, respectively, and their products were shown to be specifically involved in the biosynthesis of these secondary metabolites. Surprisingly, the fourth PPTase gene, which is not located within a secondary metabolite gene cluster, appears to play a pleiotropic role. Its product is likely involved in the activation of the acyl- and peptidyl-carrier protein domains within all the other PKS and NRPS complexes encoded by S. ambofaciens. Indeed, the deletion of this gene affects the production of the spiramycin and stambomycin macrolide antibiotics and of the grey spore pigment, all three being PKS-derived metabolites, as well as the production of the nonribosomally produced compounds, the hydroxamate siderophore coelichelin and the pyrrolamide antibiotic congocidine. In addition, this PPTase seems to act in concert with the product of samL0372 to activate the ACP and/or PCP domains of the antimycin biosynthesis cluster which is also responsible for the production of volatile lactones.

Genome mining of Streptomyces ambofaciens

Since the discovery of the streptomycin produced by Streptomyces griseus in the middle of the last century, members of this bacterial genus have been largely exploited for the production of secondary metabolites with wide uses in medicine and in agriculture. They have even been recognized as one of the most prolific producers of natural products among microorganisms. With the onset of the genomic era, it became evident that these microorganisms still represent a major source for the discovery of novel secondary metabolites. This was highlighted with the complete genome sequencing of Streptomyces coelicolor A3(2) which revealed an unexpected potential of this organism to synthesize natural products undetected until then by classical screening methods. Since then, analysis of sequenced genomes from numerous Streptomyces species has shown that a single species can carry more than 30 secondary metabolite gene clusters, reinforcing the idea that the biosynthetic potential of this bacterial genus is far from being fully exploited. This review highlights our knowledge on the potential of Streptomyces ambofaciens ATCC 23877 to synthesize natural products. This industrial strain was known for decades to only produce the drug spiramycin and another antibacterial compound, congocidine. Mining of its genome allowed the identification of 23 clusters potentially involved in the production of other secondary metabolites. Studies of some of these clusters resulted in the characterization of novel compounds and of previously known compounds but never characterized in this Streptomyces species. In addition, genome mining revealed that secondary metabolite gene clusters of phylogenetically closely related Streptomyces are mainly species-specific.

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.

Draft genome sequence of Streptomyces globisporus C-1027, which produces an antitumor antibiotic consisting of a nine-membered enediyne with a chromoprotein

Streptomyces globisporus C-1027 is the producer of antitumor antibiotic C-1027, a nine-membered enediyne-containing compound. Here we present a draft genome sequence of S. globisporus C-1027 containing the intact biosynthetic gene cluster for this antibiotic. The genome also carries numerous sets of genes for the biosynthesis of diverse secondary metabolites.

The structural role of the carrier protein--active controller or passive carrier

Common to all FASs, PKSs and NRPSs is a remarkable component, the acyl or peptidyl carrier protein (A/PCP). These take the form of small individual proteins in type II systems or discrete folded domains in the multi-domain type I systems and are characterized by a fold consisting of three major α-helices and between 60-100 amino acids. This protein is central to these biosynthetic systems and it must bind and transport a wide variety of functionalized ligands as well as mediate numerous protein-protein interactions, all of which contribute to efficient enzyme turnover. This review covers the structural and biochemical characterization of carrier proteins, as well as assessing their interactions with different ligands, and other synthase components. Finally, their role as an emerging tool in biotechnology is discussed.

PKMiner: a database for exploring type II polyketide synthases

Background

Bacterial aromatic polyketides are a pharmacologically important group of natural products synthesized by type II polyketide synthases (type II PKSs) in actinobacteria. Isolation of novel aromatic polyketides from microbial sources is currently impeded because of the lack of knowledge about prolific taxa for polyketide synthesis and the difficulties in finding and optimizing target microorganisms. Comprehensive analysis of type II PKSs and the prediction of possible polyketide chemotypes in various actinobacterial genomes will thus enable the discovery or synthesis of novel polyketides in the most plausible microorganisms.

Description

We performed a comprehensive computational analysis of type II PKSs and their gene clusters in actinobacterial genomes. By identifying type II PKS subclasses from the sequence analysis of 280 known type II PKSs, we developed highly accurate domain classifiers for these subclasses and derived prediction rules for aromatic polyketide chemotypes generated by different combinations of type II PKS domains. Using 319 available actinobacterial genomes, we predicted 231 type II PKSs from 40 PKS gene clusters in 25 actinobacterial genomes, and polyketide chemotypes corresponding to 22 novel PKS gene clusters in 16 genomes. These results showed that the microorganisms capable of producing aromatic polyketides are specifically distributed within a certain suborder of Actinomycetales such as Catenulisporineae, Frankineae, Micrococcineae, Micromonosporineae, Pseudonocardineae, Streptomycineae, and Streptosporangineae.

Conclusions

We could identify the novel candidates of type II PKS gene clusters and their polyketide chemotypes in actinobacterial genomes by comprehensive analysis of type II PKSs and prediction of aromatic polyketides. The genome analysis results indicated that the specific suborders in actinomycetes could be used as prolific taxa for polyketide synthesis. The chemotype-prediction rules with the suggested type II PKS modules derived using this resource can be used further for microbial engineering to produce various aromatic polyketides. All these resources, together with the results of the analysis, are organized into an easy-to-use database PKMiner, which is accessible at the following URL: http://pks.kaist.ac.kr/pkminer. We believe that this web-based tool would be useful for research in the discovery of novel bacterial aromatic polyketides.