Streptomyces sp. ADR1, Strain Producing β- and γ-Rubromycin Antibiotics, Isolated from Algerian Sahara Desert

A Gram-positive strain, ADR1, was isolated from soil collected from the Algerian Sahara Desert. The ethyl acetate extract of the fermentation broth showed cytotoxic activity against the PANC-1 cell line (37.1 ± 1.3% viability when applied at a concentration of 100 µg/mL). Fractionation and NMR analysis of two peaks absorbing at 490 nm revealed that they represented β- and γ-rubromycin, anticancer antibiotic compounds. The ADR1 strain contained LL-diaminopimelic acid in the whole-cell hydrolysate, and the partial 16S ribosomal RNA gene sequence (1392 bp, Accession No. KF947515) showed 99% sequence similarity to Streptomyces species. Therefore, the name Streptomyces sp. ADR1 was proposed and deposited in the Wellness Industries Culture Collection (WICC) of the Institute of Bioproduct Development, UTM, Malaysia, under the number (WICC- B86). In a 16 L stirred-tank bioreactor, the stain was adapted to submerged culture conditions and produced rubromycins at a relatively high concentration, with maximums of 24.58 mg/L and 356 mg/L for β- and γ-rubromycins, respectively.

Streptomyces derivatives as an insecticide: Current perspectives, challenges and future research needs for mosquito control

The pervasiveness of arboviruses in wreaking havoc on public health has lingered on international health agendas. A scarcity of mosquito-borne disease vaccines and therapies demands prompt attention, as billions of people worldwide are at risk of infections. It is widely known that vector control continues, and in some diseases, remains the only resort in suppressing disease transmissions we presently possess at its disposal. But the use of commercial insecticides is being crippled by the widespread insecticide resistance, which greatly menaces their efficacies, toxicological repercussions such as environmental pollution and human health risk. Rather, an environmentally benign technique of employing Streptomyces isolates from settings such as terrestrial soils, marine sediments, and mangrove soils for Culicidae management has recently received a lot of positive attention. Streptomyces' capacities to produce a wide range of bioactive secondary metabolites that contribute to pharmaceutical, agricultural and veterinarian, Streptomyces-derived bioactive compounds are increasingly being considered for use in vector control. Herein, we compiled all of the available datasets on the effectiveness of Streptomyces-derived compounds against major mosquito vectors of medical importance. Aedes, Anopheles, and Culex are used to assess the toxicity of crude extracts or fractions. This paper reviewed the promising ovicidal, larvicidal, and pupacidal effects of different Streptomyces strains. Notably, no research into the adulticidal effect of Streptomyces-derived compounds has yet been done. Aside from the genetic makeup, the production of secondary metabolites from Streptomyces depends on the growing conditions. And that, to optimise the maximum yield of highly potent bioactive compounds being extracted, solvents' choice is of paramount importance. Thus, both cultivation parameters and the choice of organic solvents for secondary metabolites extraction will be discussed. Furthermore, biases derived from different studies have implied the need for standardizing experimental procedures. While entomological data should be collected consistently across all studies to expedite evidence-based policymaking of bioinsecticides, the quality of data from vector control interventions - particularly the experimental design, execution, analysis, and presentation of results of vector control studies - will be thoroughly reviewed. Lastly, to promote consistency and reliability, these knowledge gaps are identified, along with a discussion of current perspectives on vector control, global bioinsecticide trends, challenges on commercializing bioinsecticides and future research needs.

Chiral Squaramide Catalyzed Asymmetric Spiroketalization toward Aromatic [6,5] Spiroketals

Herein is disclosed an efficient enantio- and diastereoselective spiroketalization of aromatic ketone tethered to ortho-homoformyl and enone moiety via in situ enol formation using quinine derived squaramide organocatalyst to access aromatic [6,5] spiroketals with complete atom economy. Furthermore, aromatic spiroketals undergo Brønsted acid catalyzed Piancatelli type rearrangement to provide dihydronaphtho[1,2-b]furans with retention of the enantioselectivities.

Genome mining of Streptomyces sp. CB00271 as a natural high-producer of β-rubromycin and the resulting discovery of β-rubromycin acid

β-rubromycin (β-RUB) (1) is an efficient inhibitor of human telomerase possessing a unique spiroketal moiety as a potential pharmacophore and regarded as a promising anticancer drug lead. But the development of (β-RUB) (1) has long been hampered by its low titer and very poor water solubility. By adopting a genome mining strategy, an FAD-dependent monooxygenase RubN involving with the formation of the spiro system was applied as the probe and Streptomyces sp. CB00271 was screened out from our strain collection as an alternative natural high producer of β-RUB (1). After a series of fermentation optimizations, CB00271 could produce 124.8 ± 3.4 mg/L β-RUB (1), which was the highest titer up to now. Moreover, the enhanced production of β-RUB (1) in fermentation broth also led to the discovery of a new congener β-RUB acid (7), which was structurally elucidated as the acid form of β-RUB (1). Comparing to β-RUB (1), the substituted carboxyl group endowed β-RUB acid (7) much better solubility in serum and resulted in its higher activity towards tumor cells. Our work set up a solid base for the pilot-scale production of β-RUB (1) and its congeners to facilitate their future development as promising anticancer drug leads, and also provide an alternative and practical strategy for the exploitation of other important microbial natural products.

Characterization of Streptomyces piniterrae sp. nov. and Identification of the Putative Gene Cluster Encoding the Biosynthesis of Heliquinomycins

A novel actinomycete producing heliquinomycin and 9’-methoxy-heliquinomycin, designated strain jys28^T, was isolated from rhizosphere soil of Pinus yunnanensis and characterized using a polyphasic approach. The strain had morphological characteristics and chemotaxonomic properties identical to those of members of the genus Streptomyces. It formed spiral chains of spores with spiny surfaces. The menaquinones detected were MK-9(H₆), MK-9(H₈) and MK-9(H₄). The major fatty acids were iso-C_16:0, C_15:0, C_16:1ω7с and anteiso-C_15:0. The phospholipids were diphosphatidylglycerol, phosphatidylmethylethanolamine, phosphatidylethanolamine and phosphatidylinositol mannoside. The DNA G + C content of the draft genome sequence, consisting of 8.5 Mbp, was 70.6%. Analysis of the 16S rRNA gene sequence showed that strain jys28^T belongs to the genus Streptomyces with the highest sequence similarities to Streptomyces chattanoogensis NBRC 13058^T (99.2%) and Streptomyces lydicus DSM 40002^T (99.2%) and phylogenetically clustered with them. Multilocus sequence analysis based on five other house-keeping genes (atpD, gyrB, rpoB, recA and trpB) and the low level of DNA–DNA relatedness and phenotypic differences allowed the novel isolate to be differentiated from its most closely related strains. Therefore, the strain is concluded to represent a novel species of the genus Streptomyces, for which the name Streptomycespiniterrae sp. nov. is proposed. Furthermore, the putative biosynthetic gene cluster of heliquinomycins was identified and the biosynthetic pathway was discussed. The type strain is jys28^T (=CCTCC AA 2018051^T =DSM 109823^T).

Diastereoselective construction of the benzannulated spiroketal core of chaetoquadrins enabled by a regiodivergent cascade

A remarkable acid-mediated methodology for the regiodivergent construction of a biologically interesting tricyclic benzannulated-spiroketal skeleton with diastereomeric specificity was uncovered to efficiently access analogs of chaetoquadrins .

Structure Determination, Functional Characterization, and Biosynthetic Implications of Nybomycin Metabolites from a Mining Reclamation Site-Associated Streptomyces

We report the isolation and characterization of three new nybomycins (nybomycins B-D, 1-3) and six known compounds (nybomycin, 4; deoxynyboquinone, 5; α-rubromycin, 6; β-rubromycin, 7; γ-rubromycin, 8; and [2α(1E,3E),4β]-2-(1,3-pentadienyl)-4-piperidinol, 9) from the Rock Creek (McCreary County, KY) underground coal mine acid reclamation site isolate Streptomyces sp. AD-3-6. Nybomycin D (3) and deoxynyboquinone (5) displayed moderate (3) to potent (5) cancer cell line cytotoxicity and displayed weak to moderate anti-Gram-(+) bacterial activity, whereas rubromycins 6-8 displayed little to no cancer cell line cytotoxicity but moderate to potent anti-Gram-(+) bacterial and antifungal activity. Assessment of the impact of 3 or 5 cancer cell line treatment on 4E-BP1 phosphorylation, a predictive marker of ROS-mediated control of cap-dependent translation, also revealed deoxynyboquinone (5)-mediated downstream inhibition of 4E-BP1p. Evaluation of 1-9 in a recently established axolotl embryo tail regeneration assay also highlighted the prototypical telomerase inhibitor γ-rubromycin (8) as a new inhibitor of tail regeneration. Cumulatively, this work highlights an alternative nybomycin production strain, a small set of new nybomycin metabolites, and previously unknown functions of rubromycins (antifungal activity and inhibition of tail regeneration) and also provides a basis for revision of the previously proposed nybomycin biosynthetic pathway.

Stereochemical Control of Enzymatic Carbon-Carbon Bond-Forming Michael-Type Additions by "Substrate Engineering"

The enzyme 4-oxalocrotonate tautomerase (4-OT) promiscuously catalyzes the Michael-type addition of acetaldehyde to β-nitrostyrene derivatives to yield chiral γ-nitroaldehydes, which are important precursors for pharmaceutically active γ-aminobutyric acids. In this study, we investigated the effect of different substituents at the aromatic ring of the Michael acceptor on the catalytic efficiency and stereoselectivity of the 4-OT-catalyzed acetaldehyde addition reactions. Highly enantioenriched (R)- and (S)-γ-nitroaldehydes and 4-substituted chroman-2-ol could be obtained in good to excellent yields by applying different substituents at appropriate positions of the aromatic substrate. Stereochemical control of these enzymatic Michael-type additions by "substrate engineering" allowed the enantioselective synthesis of valuable γ-aminobutyric acid precursors. In addition, the results suggest a novel enzymatic synthesis route towards precursors for chromans and derivatives, which are valuable scaffolds for preparing biologically active natural products.

Isolation, biological activity, biosynthesis and synthetic studies towards the rubromycin family of natural products

The rubromycins are a unique family of natural products. This review covers their isolation, biological activity, biosynthesis and a detailed discussion of the diverse chemistry employed for total synthesis.

An ortho-quinone methide based strategy towards the rubromycin spiroketal family

A method for the generation/in situhetero-Diels–Alder cycloaddition of a trisubstitutedortho-quinone methide (o-QM) is described.

Caryophyllene sesquiterpenes from the marine-derived fungus Ascotricha sp. ZJ-M-5 by the one strain-many compounds strategy

The marine-derived fungus Ascotricha sp. ZJ-M-5, which can produce cyclonerodiol analogues, a 3,4-seco lanostane triterpenoid, and diketopiperazines in an eutrophic medium, was subjected to a one strain-many compounds (OSMAC) analysis. It was found to produce three new caryophyllene derivatives (1-3) and the known 1,3,6-trihydroxy-8-methylxanthone (4) in an oligotrophic medium, Czapek Dox broth with or without Mg(2+). (+)-6-O-Demethylpestalotiopsin A (1) and (+)-6-O-demethylpestalotiopsin C (2), which have a five-membered hemiacetal structural moiety, showed growth inhibitory effects against HL-60 and K562 leukemia cell lines with the lowest GI50 value of 6.9 ± 0.4 μM. It can be concluded that modification of the culture media is still effective in the discovery of novel bioactive fungal secondary metabolites.

Hyaluromycin, a new hyaluronidase inhibitor of polyketide origin from marine Streptomyces sp

Hyaluromycin (1), a new member of the rubromycin family of antibiotics, was isolated from the culture extract of a marine-derived Streptomyces sp. as a HAase inhibitor on the basis of HAase activity screening. The structure of 1 was elucidated through the interpretation of NMR data for the compound and its 3″-O-methyl derivative in combination with an incorporation experiment with [1,2-¹³C₂]acetate. The compound’s absolute configuration was determined by the comparison of its circular dichroism (CD) spectrum with those of other rubromycins. Hyaluromycin (1) consists of a γ-rubromycin core structure possessing a 2-amino-3-hydroxycyclopent-2-enone (C₅N) unit as an amide substituent of the carboxyl function; both structural units have been reported only from actinomycetes. Hyaluromycin (1) displayed approximately 25-fold more potent hyaluronidase inhibitory activity against hyaluronidase than did glycyrrhizin, a known inhibitor of plant origin.

Total synthesis of (±)-δ-rubromycin

Transition-metal-catalyzed spiroketalization cyclization has been performed successfully and has led to the first total synthesis of (±)-δ-rubromycin with a longest linear sequence of 18 steps from commercially available guaiacol in a 2.7% overall yield.

New strategies for natural products containing chroman spiroketals

Two cycloaddition strategies are described that lead to various chroman spiroketals from assorted exocyclic enol ethers. Unlike conventional thermodynamic ketalization strategies, the stereochemical outcome for this approach is determined by a kinetic cycloaddition reaction. Thus, the stereochemical outcome reflects the olefin geometry of the starting materials along with the orientation of the associated transition state. However, the initial kinetic product can also be equilibrated by acid catalysis and reconstituted into a thermodynamic stereochemical arrangement. Thus, these strategies uniquely enable synthetic access to either the thermodynamic or kinetic conformation of the spiroketal stereocenter itself. Applications of these strategies in the syntheses of berkelic acid, β-rubromycin, and paecilospirone are presented along with the use of a chroman spiroketal for the construction of heliespirones A and C.

Alternative spiroketalization methods toward purpuromycin: a hemiketal conjugate addition strategy and use of an electron-rich isocoumarin precursor

Two methods are presented that were designed to circumvent the persistent problem of benzofuran formation and instead yield a spiroketal of the rubromycin family type. First, using an alternative disconnection, a hemiketal conjugate addition to a naphthaquinone electrophile was investigated. Synthesis of the requisite electrophile provided insight into the selective oxidation and functionalization of the naphthalene portion. Second, the electronic features of the isocoumarin ring system were adjusted, and the corresponding reactivity further supports the hypothesis that electron-rich isocoumarins are capable of spiroketalization. Robust, flexible syntheses from simple precursors were developed that allowed multiple reduced isocoumarins to be generated. Combined, the data presented herein give insight into the sensitivities of this family and illuminate other potential methods of spiroketalization. In addition, the convergent assembly of substrates containing different naphthaquinone and isocoumarin subunits highlights the utility of our 1,3-dipolar cycloaddition approach to generate analogs of these structures for SAR, as well as chemical reactivity studies.

An Imidazopyridinone and Further Metabolites from Streptomycetes [1]

Screening and chromatographic analysis of the extracts from the terrestrial streptomycete GW5127 and from the marine Streptomyces sp. isolate B7967 yielded four new metabolites, i.e. 8-ethyl-6,11-dihydroxy-1-methoxy-naphthacene-5,12-dione (1a), 4-acetyl-1,3-dihydro-imidazo[4,5-c]pyridin-2-one (4a), 5-hydroxmethyl-4-hydroxy-2,4-dimethyl-2-cyclopentenone (6), and N-[2-(3′-hydroxy-4′-methoxyphenyl)-ethyl]-acetamide (5a), along with five known compounds, β-rubromycin (2), γ-rubromycin (3), 2-(3′-hydroxy-4′-methoxyphenyl)ethanol (5b), uracil, and N-acetyltyramine. The structures of the new compounds were deduced from high resolution mass, 1D and 2D NMR spectra and by comparison with related compounds from the literature. The antibiotic activity of the crude extracts was due to 2 and 3.

Cleavage of four carbon-carbon bonds during biosynthesis of the griseorhodin a spiroketal pharmacophore

The rubromycins, such as gamma-rubromycin, heliquinomycin, and griseorhodin A, are a family of extensively modified aromatic polyketides that inhibit HIV reverse transcriptase and human telomerase. Telomerase inhibition crucially depends on the presence of a spiroketal moiety that is unique among aromatic polyketides. Biosynthetic incorporation of this pharmacophore into the rubromycins results in a dramatic distortion of the overall polyketide structure, but how this process is achieved by the cell has been obscure. To identify the enzymes involved in spiroketal construction, we generated 14 gene-deletion variants of the griseorhodin A biosynthetic gene cluster isolated from the tunicate-associated bacterium Streptomyces sp. JP95. Heterologous expression and metabolic analysis allowed for an assignment of most genes to various stages of griseorhodin tailoring and pharmacophore generation. The isolation of the novel advanced intermediate lenticulone, which exhibits cytotoxic, antibacterial, and elastase-inhibiting activity, provided direct evidence that the spiroketal is formed by cleavage of four carbon-carbon bonds in a pentangular polyketide precursor. This remarkable transformation is followed by an epoxidation catalyzed by an unusual cytochrome P450/NADPH:ubiquinone oxidoreductase pair that utilizes a saturated substrate. In addition, the absolute configuration of griseorhodin A was determined by quantum-chemical circular dichroism (CD) calculations in combination with experimental CD measurements.

Diastereoselective syntheses of chroman spiroketals via [4 + 2] cycloaddition of enol ethers and o-quinone methides

A variety of chroman spiroketals are synthesized via inverse-demand [4 + 2] cycloaddition of enol ethers and ortho-quinone methides (o-QMs). Low temperature o-QM generation in situ allows for the kinetic, diastereoselective construction of these motifs, providing entry to a number of unusual chroman spiroketal natural products.

Facile synthesis of naphthoquinone spiroketals by diastereoselective oxidative [3 + 2] cycloaddition

A highly selective oxidative [3 + 2] cycloaddition of chiral enol ethers and hydroxynaphthoquinone is described. This convergent strategy is amenable to an enantioselective synthesis of beta-rubromycin and related naphthoquinone spiroketals. Several compounds were found to inhibit DNA-polymerase and telomerase in a manner resembling alpha-rubromycin and beta-rubromycin.

Type II polyketide synthases: gaining a deeper insight into enzymatic teamwork

This review covers advances in understanding of the biosynthesis of polyketides produced by type II PKS systems at the genetic, biochemical and structural levels.

A gene cluster from a marine Streptomyces encoding the biosynthesis of the aromatic spiroketal polyketide griseorhodin A

The telomerase inhibitor griseorhodin A is probably the most heavily oxidized bacterial polyketide known and features a unique epoxyspiroketal moiety crucial for its activity. To gain insight into which tailoring enzymes generate this pharmacophore, we have cloned and fully sequenced the griseorhodin biosynthesis gene cluster. Among other unusual features, this aromatic polyketide synthase (PKS) system encodes an unprecedented number of functionally diverse oxidoreductases, which are involved in the oxidative modification of a polyaromatic tridecaketide precursor by cleavage of three carbon-carbon bonds. The cluster was highly unstable on a variety of shuttle plasmids but could finally be functionally expressed in its entirety in Streptomyces lividans using a novel integrative cosmid vector. The availability of the tailoring system now opens up the possibility of engineering nonnatural biosynthetic pathways yielding novel pharmacologically active analogs with a similar pharmacophore.