Retrograde sulfur flow from glucosinolates to cysteine in Arabidopsis thaliana.

Specialized (secondary) metabolic pathways in plants have long been considered one-way routes of leading primary metabolite precursors to bioactive end products. Conversely, endogenous degradation of such "end" products in plant tissues has been observed following environmental stimuli, including nutrition stress. Therefore, it is of general interest whether specialized metabolites can be reintegrated into primary metabolism to recover the invested resources, especially in the case of nitrogen- or sulfur-rich compounds. Here, we demonstrate that endogenous glucosinolates (GLs), a class of sulfur-rich plant metabolites, are exploited as a sulfur source by the reallocation of sulfur atoms to primary metabolites such as cysteine in Arabidopsis thaliana Tracer experiments using 34S- or deuterium-labeled GLs depicted the catabolic processing of GL breakdown products in which sulfur is mobilized from the thioglucoside group in GL molecules, potentially accompanied by the release of the sulfate group. Moreover, we reveal that beta-glucosidases BGLU28 and BGLU30 are the major myrosinases that initiate sulfur reallocation by hydrolyzing particular GL species, conferring sulfur deficiency tolerance in A. thaliana, especially during early development. The results delineate the physiological function of GL as a sulfur reservoir, in addition to their well-known functions as defense chemicals. Overall, our findings demonstrate the bidirectional interaction between primary and specialized metabolism, which enhances our understanding of the underlying metabolic mechanisms via which plants adapt to their environments.

Glutathione degradation activity of γ-glutamyl peptidase 1 manifests its dual roles in primary and secondary sulfur metabolism in Arabidopsis.

Glutathione (GSH) functions as a major sulfur repository and hence occupies an important position in primary sulfur metabolism. GSH degradation results in sulfur reallocation and is believed to be carried out mainly by γ-glutamyl cyclotransferases (GGCT2;1, GGCT2;2, and GGCT2;3), which, however, do not fully explain the rapid GSH turnover. Here, we discovered that γ-glutamyl peptidase 1 (GGP1) contributes to GSH degradation through a yeast complementation assay. Recombinant proteins of GGP1, as well as GGP3, showed high degradation activity of GSH, but not of oxidized glutathione (GSSG), in vitro. Notably, the GGP1 transcripts were highly abundant in rosette leaves, in agreement with the ggp1 mutants constantly accumulating more GSH regardless of nutritional conditions. Given the lower energy requirements of the GGP- than the GGCT-mediated pathway, the GGP-mediated pathway could be a more efficient route for GSH degradation than the GGCT-mediated pathway. Therefore, we propose a model wherein cytosolic GSH is degraded chiefly by GGP1 and likely also by GGP3. Another noteworthy fact is that GGPs are known to process GSH conjugates in glucosinolate and camalexin synthesis; indeed, we confirmed that the ggp1 mutant contained higher levels of O-acetyl-l-Ser, a signaling molecule for sulfur starvation, and lower levels of glucosinolates and their degradation products. The predicted structure of GGP1 further provided a rationale for this hypothesis. In conclusion, we suggest that GGP1 and possibly GGP3 play vital roles in both primary and secondary sulfur metabolism.

The Biosynthetic Landscape of Triceptides Reveals Radical SAM Enzymes That Catalyze Cyclophane Formation on Tyr- and His-Containing Motifs.

Peptide-derived cyclophanes inhabit a unique niche in the chemical space of macrocyclic peptides with several examples of pharmaceutical importance. Although both synthetic and biocatalytic methods are available for constructing these macrocycles, versatile (bio)catalysts able to incorporate a variety of amino acids that compose the macrocycle would be useful for the creation of diverse peptide cyclophanes. In this report, we synergized the use of bioinformatic tools to map the biosynthetic landscape of radical SAM enzymes (3-CyFEs) that catalyze three-residue cyclophane formation in the biosynthesis of a new family of RiPP natural products, the triceptides. This analysis revealed 3940 (3113 unique) putative precursor sequences predicted to be modified by 3-CyFEs. Several uncharacterized maturase systems were identified that encode unique precursor types. Functional studies were carried out in vivo in Escherichia coli to identify modified precursors containing His and Tyr residues. NMR analysis of the products revealed that Tyr and His can also be incorporated into cyclophane macrocycles by 3-CyFEs. Collectively, all aromatic amino acids can be incorporated by 3-CyFEs, and the cyclophane formation strictly occurs via a C(sp2)-C(sp3) cross-link between the (hetero)aromatic ring to Cß. In addition to 3-CyFEs, we functionally validated an Fe(II)/α-ketoglutarate-dependent hydroxylase, resulting in ß-hydroxylated residues within the cyclophane rings. This study reveals the potential breadth of triceptide precursors and a systematic approach for studying these enzymes to broaden the diversity of peptide macrocycles.

Publisher Correction: A cheminformatics approach to characterize metabolomes in stable-isotope-labeled organisms.

In the originally published Supplementary Information for this paper, the files presented as Supplementary Tables 3, 4, and 7 were duplicates of Supplementary Tables 5, 6, and 9, respectively. All Supplementary Table files are now correct online.

A cheminformatics approach to characterize metabolomes in stable-isotope-labeled organisms.

We report a computational approach (implemented in MS-DIAL 3.0; http://prime.psc.riken.jp/) for metabolite structure characterization using fully 13C-labeled and non-labeled plants and LC-MS/MS. Our approach facilitates carbon number determination and metabolite classification for unknown molecules. Applying our method to 31 tissues from 12 plant species, we assigned 1,092 structures and 344 formulae to 3,604 carbon-determined metabolite ions, 69 of which were found to represent structures currently not listed in metabolome databases.

Amphiol, an Antifungal Fungal Pigment from Pseudogymnoascus sp. PF1464.

An antifungal metabolite, designated amphiol (1), was isolated from the culture broth of the fungus Pseudogymnoascus sp. PF1464. It exists as a mixture of inseparable tautomers, an acetal form and a keto form. The chemical structure was determined by spectroscopic analyses and chemical derivatization. Amphiol (1) showed antifungal but not antibacterial activities, while yeast mutant cells lacking ergosterol biosynthetic genes were less sensitive, implying a fungal specific, membrane-related mechanism of action.

Leaf Endoplasmic Reticulum Bodies Identified in Arabidopsis Rosette Leaves Are Involved in Defense against Herbivory.

ER bodies are endoplasmic reticulum (ER)-derived organelles specific to the order Brassicales and are thought to function in plant defense against insects and pathogens. ER bodies are generally classified into two types: constitutive ER bodies in the epidermal cells of seedlings, and wound-inducible ER bodies in rosette leaves. Herein, we reveal a third type of ER body found in Arabidopsis (Arabidopsis thaliana) rosette leaves and designate them "leaf ERbodies" (L-ER bodies). L-ER bodies constitutively occurred in specific cells of the rosette leaves: marginal cells, epidermal cells covering the midrib, and giant pavement cells. The distribution of L-ER bodies was closely associated with the expression profile of the basic helix-loop-helix transcription factor NAI1, which is responsible for constitutive ER-body formation. L-ER bodies were seldom observed in nai1 mutant leaves, indicating that NAI1 is involved in L-ER body formation. Confocal imaging analysis revealed that L-ER bodies accumulated two types of ß-glucosidases: PYK10, the constitutive ER-body ß-glucosidase; and BETA-GLUCOSIDASE18 (BGLU18), the wound-inducible ER-body ß-glucosidase. Combined with the absence of L-ER bodies in the bglu18 pyk10 mutant, these results indicate that BGLU18 and PYK10 are the major components of L-ER bodies. A subsequent feeding assay with the terrestrial isopod Armadillidium vulgare revealed that bglu18 pyk10 leaves were severely damaged as a result of herbivory. In addition, the bglu18 pyk10 mutant was defective in the hydrolysis of 4-methoxyindol-3-ylmethyl glucosinolate These results suggest that L-ER bodies are involved in the production of defensive compound(s) from 4-methoxyindol-3-ylmethyl glucosinolate that protect Arabidopsis leaves against herbivory attack.

Identification of the common biosynthetic gene cluster for both antimicrobial streptoaminals and antifungal 5-alkyl-1,2,3,4-tetrahydroquinolines.

5-Alkyl-1,2,3,4-tetrahydroquinolines (5aTHQs) and streptoaminals (STAMs) are natural products isolated from the combined-culture of Streptomyces nigrescens HEK616 and Tsukamurella pulmonis TP-B0596. Despite their unique structures, their biosynthetic pathway has yet to be elucidated. In the present study, we conducted a feeding experiment using 13C-labeled acetates and demonstrated that 5aTHQs are likely synthesized by the action of polyketide synthase (PKS). Based on this observation, we identified the biosynthetic gene cluster for 5aTHQs. Interestingly, the same gene cluster was also responsible for the structurally-distinct STAMs. The gene cluster contains nine genes encoding one acyl carrier protein, two sets of ketosynthases (KSs) and chain length factors (CLFs), one aminotransferase/reductase bifunctional protein, two ketoreductases, and one thioesterase. KSs and CLFs are classified into the phylogenetically distinct clades from those of known type II PKSs. Heterologous expression of the biosynthetic genes and subsequent gene inactivation clearly indicated that all of the nine genes were required for the biosynthesis of both compounds. In the proposed biosynthetic pathway, chain elongation by PKS, reductive cleavage of a thioester bond, and subsequent transamination generate the core skeleton of both compounds. Differences in the oxidation states of the products result in a distinct cyclization mode to yield 5aTHQs and STAMs.

Chemical Interactions of Cryptic Actinomycete Metabolite 5-Alkyl-1,2,3,4-tetrahydroquinolines through Aggregate Formation.

Organisms often produce secondary metabolites as a mixture of biosynthetically related congeners. However, why are metabolites with minor chemical variations produced simultaneously? 5-Alkyl-1,2,3,4-tetrahydroquinolines (5aTHQs) are small, lipophilic metabolites produced by Streptomyces nigrescens HEK616 when cultured with Tsukamurella pulmonis TP-B0596. A mixture of 5aTHQs forms aggregates that show enhanced membrane affinity and biological activity. The ability to form aggregates and membrane-binding activity is regulated by the length of the alkyl chains. Aggregates with long alkyl chains were too stable to fuse with lipid membranes. However, if inactive 5aTHQ congener was mixed with active congener, the mixture showed increased membrane affinity, enabling cellular entry and biological activity. Therefore, it is shown that sloppiness in a biosynthetic pathway, by which minor structural variations can be produced, is functionally rational, as the metabolites show synergistic action.

Asymmetric Total Synthesis of Heronamides†A-C: Stereochemical Confirmation and Impact of Long-Range Stereochemical Communication on the Biological Activity.

Heronamides are biosynthetically related metabolites isolated from marine-derived actinomycetes. Heronamide C shows potent antifungal activity by targeting membrane phospholipids possessing saturated hydrocarbon chains with as-yet-unrevealed modes of action. In spite of their curious hypothesized biosynthesis and fascinating biological activities, there have been conflicts in regard to the reported stereochemistries of heronamides. Here, we describe the asymmetric total synthesis of the originally proposed and revised structures of heronamide C, which unambiguously confirmed the chemical structure of this molecule. We also demonstrated nonenzymatic synthesis of heronamides A and B from heronamide C, which not only proved the postulated biosynthesis, but also confirmed the correct structures of heronamides A and B. Investigation of the structure-activity relationship of synthetic and natural heronamides revealed the importance of both long-range stereochemical communication and the 20-membered macrolactam ring for the biological activity of these compounds.

Stereochemical Assignment and Biological Evaluation of BE-14106 Unveils the Importance of One Acetate Unit for the Antifungal Activity of Polyene Macrolactams.

Heronamides are a class of potent antifungal metabolites produced by marine-derived actinomycetes. The number of hydroxy groups and the stereochemistry of the two hydroxylated methine carbons are important for the activity of heronamide C, whereas the effect of the hydrocarbon chains is not known. In this study, the stereochemistry and the biological activity of BE-14106, another member of the heronamide class of antibiotics, isolated from an actinomycete Actinoalloteichus cyanogriseus IFM 11549 was investigated. Spectroscopic analysis coupled with photo- and O2-induced conversion revealed that BE-14106 and the heronamides had the same stereochemistry. BE-14106 showed potent growth inhibition against fission yeast cells with an MIC value of 0.50 µM (0.21 µg/mL), being 4 times less potent than heronamide C, which revealed the importance of the structure of the hydrocarbon tail for the activity.

Discovery and Total Synthesis of Streptoaminals: Antimicrobial [5,5]-Spirohemiaminals from the Combined-Culture of Streptomyces nigrescens and Tsukamurella pulmonis.

A series of lipidic spirohemiaminals, designated streptoaminals, is reported. These were discovered by surveying the unique molecular signatures identified in the mass spectrometry data of the combined-culture broth of Streptomyces nigrescens HEK616 and Tsukamurella pulmonis TP-B0596. Mass spectrometry analysis showed that streptoaminals appeared as a cluster of ion peaks, which were separated by 14 mass unit intervals, implying the presence of alkyl chains of different lengths. The chemical structures of these compounds were elucidated by spectroscopic analysis and total synthesis. Streptoaminals with globular structures showed broad antimicrobial activities, whereas the planar structures of the 5-alkyl-1,2,3,4-tetrahydroquinolines found in the same combined-culture did not. This work shows the application of microbes as reservoirs for a range of chemical scaffolds.

New insights into the catalytic active-site structure of multicopper oxidases.

Structural models determined by X-ray crystallography play a central role in understanding the catalytic mechanism of enzymes. However, X-ray radiation generates hydrated electrons that can cause significant damage to the active sites of metalloenzymes. In the present study, crystal structures of the multicopper oxidases (MCOs) CueO from Escherichia coli and laccase from a metagenome were determined. Diffraction data were obtained from a single crystal under low to high X-ray dose conditions. At low levels of X-ray exposure, unambiguous electron density for an O atom was observed inside the trinuclear copper centre (TNC) in both MCOs. The gradual reduction of copper by hydrated electrons monitored by measurement of the Cu K-edge X-ray absorption spectra led to the disappearance of the electron density for the O atom. In addition, the size of the copper triangle was enlarged by a two-step shift in the location of the type III coppers owing to reduction. Further, binding of O2 to the TNC after its full reduction was observed in the case of the laccase. Based on these novel structural findings, the diverse resting structures of the MCOs and their four-electron O2-reduction process are discussed.

Structure and biological activity of 8-deoxyheronamide C from a marine-derived Streptomyces sp.: heronamides target saturated hydrocarbon chains in lipid membranes.

Polyene macrolactams are a class of microbial metabolites, many of which show potent biological activities with unidentified modes of action. Here we report that 8-deoxyheronamide C, a new 20-membered polyene macrolactam from a marine-derived actinomycete Streptomyces sp., is a unique membrane binder. 8-Deoxyheronamide C showed a characteristic sensitivity profile against fission yeast sterol mutant cells, indicating that the metabolite targets cell membranes. We detected tight physical interaction between heronamides including 8-deoxyheronamide C and heronamide C and saturated hydrocarbon chains in lipid membranes using surface plasmon resonance experiments. We further show that heronamides induced abnormal cell wall morphology in fission yeast probably by perturbing the structure of membrane microdomains. This work will accelerate the biological and medical investigation of polyene macrolactams.

Post-translational formation of strained cyclophanes in bacteria.

Cyclic peptide natural products have served as important drug molecules, with several examples used clinically. Enzymatic or chemical macrocyclization is the key transformation for constructing these chemotypes. Methods to generate new and diverse cyclic peptide scaffolds enabling the modular and predictable synthesis of peptide libraries are desirable in drug discovery platforms. Here we identify a suite of post-translational modifying enzymes from bacteria that install single or multiple strained cyclophane macrocycles. The crosslinking occurs on three-residue motifs that include tryptophan or phenylalanine to form indole- or phenyl-bridged cyclophanes. The macrocycles display restricted rotation of the aromatic ring and induce planar chirality in the asymmetric indole bridge. The biosynthetic gene clusters originate from a broad range of bacteria derived from marine, terrestrial and human microbiomes. Three-residue cyclophane-forming enzymes define a new and significant natural product family and occupy a distinct region in sequence-function space.

Atypical Myrosinase as a Mediator of Glucosinolate Functions in Plants.

Glucosinolates (GLSs) are a well-known class of specialized plant metabolites, distributed mostly in the order Brassicales. A vast research field in basic and applied sciences has grown up around GLSs owing to their presence in important agricultural crops and the model plant Arabidopsis thaliana, and their broad range of bioactivities beneficial to human health. The major purpose of GLSs in plants has been considered their function as a chemical defense against predators. GLSs are physically separated from a specialized class of beta-thioglucosidases called myrosinases, at the tissue level or at the single-cell level. They are brought together as a consequence of tissue damage, primarily triggered by herbivores, and their interaction results in the release of toxic volatile chemicals including isothiocyanates. In addition, recent studies have suggested that plants may adopt other strategies independent of tissue disruption for initiating GLS breakdown to cope with certain biotic/abiotic stresses. This hypothesis has been further supported by the discovery of an atypical class of GLS-hydrolyzing enzymes possessing features that are distinct from those of the classical myrosinases. Nevertheless, there is only little information on the physiological importance of atypical myrosinases. In this review, we focus on the broad diversity of the beta-glucosidase subclasses containing known atypical myrosinases in A. thaliana to discuss the hypothesis that numerous members of these subclasses can hydrolyze GLSs to regulate their diverse functions in plants. Also, the increasingly broadening functional repertoires of known atypical/classical myrosinases are described with reference to recent findings. Assessment of independent insights gained from A. thaliana with respect to (1) the phenotype of mutants lacking genes in the GLS metabolic/breakdown pathways, (2) fluctuation in GLS contents/metabolism under specific conditions, and (3) the response of plants to exogenous GLSs or their hydrolytic products, will enable us to reconsider the physiological importance of GLS breakdown in particular situations, which is likely to be regulated by specific beta-glucosidases.

A novel resting form of the trinuclear copper center in the double mutant of a multicopper oxidase, CueO, Cys500Ser/Glu506Ala.

A multicopper oxidase, CueO was doubly mutated at its type I copper ligand, Cys500 and an acidic amino acid residue located in the proton transfer pathway, Glu506, to Ser and Ala, respectively. Cys500Ser/Glu506Ala was mainly in a novel resting form to afford the absorption band at ca. 400 nm and an EPR signal with a highly anisotropic character derived from type III copper. However, Cys500Ser/Glu506Ala gave the same reaction intermediate (peroxide intermediate) as that from Cys500Ser and Cys500Ser/Glu506Gln.

5-Alkyl-1,2,3,4-tetrahydroquinolines, new membrane-interacting lipophilic metabolites produced by combined culture of Streptomyces nigrescens and Tsukamurella pulmonis.

Eight novel 5-alkyl-1,2,3,4-tetrahydroquinolines (5aTHQs) bearing different side chains have been isolated from a combined culture of Streptomyces nigrescens HEK616 and Tsukamurella pulmonis TP-B0596. The chemical structures including the absolute configuration were elucidated by spectroscopic analysis and total synthesis. 5aTHQs inhibited the growth of wild-type fission yeast while only weakly inhibiting the growth of several mutant strains synthesizing premature ergosterol. These results demonstrate that 5aTHQs are novel antifungals that may target cell membranes.

ATR-FTIR study of the protonation states of the Glu residue in the multicopper oxidases, CueO and bilirubin oxidase.

Redox-induced protonation state changes of the Glu residue in the multicopper oxidases, CueO and bilirubin oxidase (BO), were studied by attenuated total reflectance-Fourier transform infrared spectroscopy. By monitoring IR bands of the carboxylic acid C=O stretch in the wild-type and Glu-to-Gln mutant enzymes the Glu506 of CueO (Glu463 of BO) was found to be unprotonated in the oxidised and protonated in the reduced forms. The results provided direct evidence for proton uptake by the Glu, suggesting it plays a key role in the proton donation to the activated oxygen species in the catalytic cycle.