Plant Host Traits Mediated by Foliar Fungal Symbionts and Secondary Metabolites.

Fungal symbionts living inside plant leaves ("endophytes") can vary from beneficial to parasitic, but the mechanisms by which the fungi affect the plant host phenotype remain poorly understood. Chemical interactions are likely the proximal mechanism of interaction between foliar endophytes and the plant, as individual fungal strains are often exploited for their diverse secondary metabolite production. Here, we go beyond single strains to examine commonalities in how 16 fungal endophytes shift plant phenotypic traits such as growth and physiology, and how those relate to plant metabolomics profiles. We inoculated individual fungi on switchgrass, Panicum virgatum L. This created a limited range of plant growth and physiology (2-370% of fungus-free controls on average), but effects of most fungi overlapped, indicating functional similarities in unstressed conditions. Overall plant metabolomics profiles included almost 2000 metabolites, which were broadly correlated with plant traits across all the fungal treatments. Terpenoid-rich samples were associated with larger, more physiologically active plants and phenolic-rich samples were associated with smaller, less active plants. Only 47 metabolites were enriched in plants inoculated with fungi relative to fungus-free controls, and of these, Lasso regression identified 12 metabolites that explained from 14 to 43% of plant trait variation. Fungal long-chain fatty acids and sterol precursors were positively associated with plant photosynthesis, conductance, and shoot biomass, but negatively associated with survival. The phytohormone gibberellin, in contrast, was negatively associated with plant physiology and biomass. These results can inform ongoing efforts to develop metabolites as crop management tools, either by direct application or via breeding, by identifying how associations with more beneficial components of the microbiome may be affected.

Mechanistic considerations of halogenating enzymes.

In nature, halogenation is a strategy used to increase the biological activity of secondary metabolites, compounds that are often effective as drugs. However, halides are not particularly reactive unless they are activated, typically by oxidation. The pace of discovery of new enzymes for halogenation is increasing, revealing new metalloenzymes, flavoenzymes, S-adenosyl-L-methionine (SAM)-dependent enzymes and others that catalyse halide oxidation using dioxygen, hydrogen peroxide and hydroperoxides, or that promote nucleophilic halide addition reactions.

Biosynthesis of amphi-enterobactin siderophores by Vibrio harveyi BAA-1116: identification of a bifunctional nonribosomal peptide synthetase condensation domain.

The genome of Vibrio harveyi BAA-1116 contains a nonribosomal peptide synthetase (NRPS) gene cluster (aebA-F) resembling that for enterobactin, yet enterobactin is not produced. A gene predicted to encode a long-chain fatty acid CoA ligase (FACL), similar to enzymes involved in the biosynthesis of acyl peptides, resides 15 kb away from the putative enterobactin-like biosynthetic gene cluster (aebG). The proximity of this FACL gene to the enterobactin-like synthetase suggested that V. harveyi may produce amphiphilic enterobactin-like siderophores. Extraction of the bacterial cell pellet of V. harveyi led to the isolation and structure determination of a suite of eight amphi-enterobactin siderophores composed of the cyclic lactone of tris-2,3-dihydroxybenzoyl-L-serine and acyl-L-serine. The FACL knockout mutant, ΔaebG V. harveyi, and the NRPS knockout mutant, ΔaebF V. harveyi, do not produce amphi-enterobactins. The amphi-enterobactin biosynthetic machinery was heterologously expressed in Escherichia coli and reconstituted in vitro, demonstrating the condensation domain of AebF has unique activity, catalyzing two distinct condensation reactions.

Human gut Actinobacteria boost drug absorption by secreting P-glycoprotein ATPase inhibitors.

Drug efflux transporters are a major determinant of drug efficacy and toxicity. A canonical example is P-glycoprotein (P-gp), an efflux transporter that controls the intestinal absorption of diverse compounds. Despite a rich literature on the dietary and pharmaceutical compounds that impact P-gp activity, its sensitivity to gut microbial metabolites remains an open question. Surprisingly, we found that the cardiac drug-metabolizing gut Actinobacterium Eggerthella lenta increases drug absorption in mice. Experiments in cell culture revealed that E. lenta produces a soluble factor that post-translationally inhibits P-gp ATPase efflux activity. P-gp inhibition is conserved in the Eggerthellaceae family but absent in other Actinobacteria. Comparative genomics identified genes associated with P-gp inhibition. Finally, activity-guided biochemical fractionation coupled to metabolomics implicated a group of small polar metabolites with P-gp inhibitory activity. These results highlight the importance of considering the broader relevance of the gut microbiome for drug disposition beyond first-pass metabolism.

Human gut bacterial metabolism drives Th17 activation and colitis.

Bacterial activation of T helper 17 (Th17) cells exacerbates mouse models of autoimmunity, but how human-associated bacteria impact Th17-driven disease remains elusive. We show that human gut Actinobacterium Eggerthella lenta induces intestinal Th17 activation by lifting inhibition of the Th17 transcription factor Rorγt through cell- and antigen-independent mechanisms. E. lenta is enriched in inflammatory bowel disease (IBD) patients and worsens colitis in a Rorc-dependent manner in mice. Th17 activation varies across E. lenta strains, which is attributable to the cardiac glycoside reductase 2 (Cgr2) enzyme. Cgr2 is sufficient to induce interleukin (IL)-17a, a major Th17 cytokine. cgr2+ E. lenta deplete putative steroidal glycosides in pure culture; related compounds are negatively associated with human IBD severity. Finally, leveraging the sensitivity of Cgr2 to dietary arginine, we prevented E. lenta-induced intestinal inflammation in mice. Together, these results support a role for human gut bacterial metabolism in driving Th17-dependent autoimmunity.

P2RY8 variants in lupus patients uncover a role for the receptor in immunological tolerance.

B cell self-tolerance is maintained through multiple checkpoints, including restraints on intracellular signaling and cell trafficking. P2RY8 is a receptor with established roles in germinal center (GC) B cell migration inhibition and growth regulation. Somatic P2RY8 variants are common in GC-derived B cell lymphomas. Here, we identify germline novel or rare P2RY8 missense variants in lupus kindreds or the related antiphospholipid syndrome, including a "de novo" variant in a child with severe nephritis. All variants decreased protein expression, F-actin abundance, and GPCR-RhoA signaling, and those with stronger effects increased AKT and ERK activity and cell migration. Remarkably, P2RY8 was reduced in B cell subsets from some SLE patients lacking P2RY8 gene variants. Low P2RY8 correlated with lupus nephritis and increased age-associated B cells and plasma cells. By contrast, P2RY8 overexpression in cells and mice restrained plasma cell development and reinforced negative selection of DNA-reactive developing B cells. These findings uncover a role of P2RY8 in immunological tolerance and lupus pathogenesis.

Host and gut bacteria share metabolic pathways for anti-cancer drug metabolism.

Pharmaceuticals have extensive reciprocal interactions with the microbiome, but whether bacterial drug sensitivity and metabolism is driven by pathways conserved in host cells remains unclear. Here we show that anti-cancer fluoropyrimidine drugs inhibit the growth of gut bacterial strains from 6 phyla. In both Escherichia coli and mammalian cells, fluoropyrimidines disrupt pyrimidine metabolism. Proteobacteria and Firmicutes metabolized 5-fluorouracil to its inactive metabolite dihydrofluorouracil, mimicking the major host mechanism for drug clearance. The preTA operon was necessary and sufficient for 5-fluorouracil inactivation by E. coli, exhibited high catalytic efficiency for the reductive reaction, decreased the bioavailability and efficacy of oral fluoropyrimidine treatment in mice and was prevalent in the gut microbiomes of colorectal cancer patients. The conservation of both the targets and enzymes for metabolism of therapeutics across domains highlights the need to distinguish the relative contributions of human and microbial cells to drug efficacy and side-effect profiles.

Chrysobactin siderophores produced by Dickeya chrysanthemi EC16.

The plant pathogen Dickeya chrysanthemi EC16 (formerly known as Petrobacterium chrysanthemi EC16 and Erwinia chrysanthemi EC16) was found to produce a new triscatecholamide siderophore, cyclic trichrysobactin, the related catecholamide compounds, linear trichrysobactin and dichrysobactin, and the previously reported monomeric siderophore unit, chrysobactin. Chrysobactin is comprised of L-serine, D-lysine, and 2,3-dihydroxybenzoic acid (DHBA). Trichrysobactin is a cyclic trimer of chrysobactin joined by a triserine lactone backbone. The chirality of the ferric complex of cyclic trichrysobactin is found to be in the Λ configuration, similar to Fe(III)-bacillibactin, which contains a glycine spacer between the DHBA and L-threonine components and is opposite that of Fe(III)-enterobactin, which contains DHBA ligated directly to L-serine.

Abcc1 and Ggt5 support lymphocyte guidance through export and catabolism of S-geranylgeranyl-l-glutathione.

P2RY8 promotes the confinement and growth regulation of germinal center (GC) B cells, and loss of human P2RY8 is associated with B cell lymphomagenesis. The metabolite S-geranylgeranyl-l-glutathione (GGG) is a P2RY8 ligand. The mechanisms controlling GGG distribution are poorly understood. Here, we show that gamma-glutamyltransferase-5 (Ggt5) expression in stromal cells was required for GGG catabolism and confinement of P2RY8-expressing cells to GCs. We identified the ATP-binding cassette subfamily C member 1 (Abcc1) as a GGG transporter and showed that Abcc1 expression by hematopoietic cells was necessary for P2RY8-mediated GC confinement. Furthermore, we discovered that P2RY8 and GGG negatively regulated trafficking of B and T cells to the bone marrow (BM). P2RY8 loss-of-function human T cells increased their BM homing. By defining how GGG distribution was determined and identifying sites of P2RY8 activity, this work helps establish how disruptions in P2RY8 function contribute to lymphomagenesis and other disease states.

Vanchrobactin and anguibactin siderophores produced by Vibrio sp. DS40M4.

The marine bacterium Vibrio sp. DS40M4 has been found to produce a new triscatechol amide siderophore, trivanchrobactin (1), a related new biscatecholamide compound, divanchrobactin (2), and the previously reported siderophores vanchrobactin (3) and anguibactin (4). Vanchrobactin is comprised of l-serine, d-arginine, and 2,3-dihydroxybenzoic acid, while trivanchrobactin is a linear trimer of vanchrobactin joined by two serine ester linkages. The cyclic trivanchrobactin product was not detected. In addition to siderophore production, extracts of Vibrio sp. DS40M4 were screened for biologically active molecules; anguibactin was found to be cytotoxic against the P388 murine leukemia cell line (IC(50) < 15 microM).

Loihichelins A-F, a suite of amphiphilic siderophores produced by the marine bacterium Halomonas LOB-5.

A suite of amphiphilic siderophores, loihichelins A-F, were isolated from cultures of the marine bacterium Halomonas sp. LOB-5. This heterotrophic Mn(II)-oxidizing bacterium was recently isolated from the partially weathered surfaces of submarine glassy pillow basalts and associated hydrothermal flocs of iron oxides collected from the southern rift zone of Loihi Seamount east of Hawai'i. The loihichelins contain a hydrophilic headgroup consisting of an octapeptide comprised of D-threo-beta-hydroxyaspartic acid, D-serine, L-glutamine, L-serine, L-N(delta)-acetyl-N(delta)-hydroxyornithine, dehydroamino-2-butyric acid, D-serine, and cyclic N(delta)-hydroxy-D-ornithine, appended by one of a series of fatty acids ranging from decanoic acid to tetradecanoic acid. The structure of loihichelin C was determined by a combination of amino acid and fatty acid analyses, tandem mass spectrometry, and NMR spectroscopy. The structures of the other loihichelins were inferred from the amino acid and fatty acid analyses and tandem mass spectrometry. The role of these siderophores in sequestering Fe(III) released during basaltic rock weathering, as well as their potential role in the promotion of Mn(II) and Fe(II) oxidation, is of considerable interest.

Effects of Discovery, Iteration, and Collaboration in Laboratory Courses on Undergraduates' Research Career Intentions Fully Mediated by Student Ownership.

Course-based undergraduate research experiences (CUREs) provide a promising avenue to attract a larger and more diverse group of students into research careers. CUREs are thought to be distinctive in offering students opportunities to make discoveries, collaborate, engage in iterative work, and develop a sense of ownership of their lab course work. Yet how these elements affect students' intentions to pursue research-related careers remain unexplored. To address this knowledge gap, we collected data on three design features thought to be distinctive of CUREs (discovery, iteration, collaboration) and on students' levels of ownership and career intentions from ∼800 undergraduates who had completed CURE or inquiry courses, including courses from the Freshman Research Initiative (FRI), which has a demonstrated positive effect on student retention in college and in science, technology, engineering, and mathematics. We used structural equation modeling to test relationships among the design features and student ownership and career intentions. We found that discovery, iteration, and collaboration had small but significant effects on students' intentions; these effects were fully mediated by student ownership. Students in FRI courses reported significantly higher levels of discovery, iteration, and ownership than students in other CUREs. FRI research courses alone had a significant effect on students' career intentions.

Biosynthetic considerations of triscatechol siderophores framed on serine and threonine macrolactone scaffolds.

Bacteria often produce siderophores to facilitate iron uptake. One of the most studied siderophores is enterobactin, the macrolactone trimer of 2,3-dihydroxybenzoyl-l-serine, produced by E. coli and many other enteric bacteria. Other siderophores are variants of enterobactin, with structural modifications including expansion of the tri-serine core to a tetra-serine macrolactone, substitution of l-serine with l-threonine, insertion of amino acids (i.e., Gly, l-Ala, d-Lys, d- and l-Arg, l-Orn), catechol glucosylation, and linearization of the tri-serine macrolactone core. In this review we summarize the current understanding of the biosyntheses of these enterobactin variants, placing them in contrast with the well-established biosynthesis of enterobactin.

Characterization of AntB, a promiscuous acyltransferase involved in antimycin biosynthesis.

The in vivo and in vitro characterization of AntB, a dedicated acyltransferase encoded in the antimycin biosynthetic gene cluster, which catalyzes the C-8 acyloxy formation is reported. It is demonstrated that AntB has broad substrate specificity toward both the acyl substrate and the acyl carrier and produces more antimycin analogues with varying C-8 acyloxy moieties.

Tandem enzymatic oxygenations in biosynthesis of epoxyquinone pharmacophore of manumycin-type metabolites.

Many natural products contain epoxyquinone pharmacophore with unknown biosynthetic mechanisms. Recent genetic analysis of the asukamycin biosynthetic gene cluster proposed enzyme candidates related to epoxyquinone formation for manumycin-type metabolites. Our biochemical studies reveal that 3-amino-4-hydroxyl benzoic acid (3,4-AHBA) precursor is activated and loaded on aryl carrier protein (AsuC12) by ATP-dependent adenylase (AsuA2). AsuE1 and AsuE3, both single-component flavin-dependent monooxygenases, catalyze the exquisite regio- and enantiospecific postpolyketide synthase (PKS) assembly oxygenations. AsuE1 installs a hydroxyl group on the 3,4-AHB ring to form a 4-hydroxyquinone moiety, which is epoxidized by AsuE3 to yield the epoxyquinone functionality. Despite being a single-component monooxygenase, AsuE1 activity is elicited by AsuE2, a pathway-specific flavin reductase. We further demonstrate that the epoxyquinone moiety is critical for anti-MRSA activity by analyzing the bioactivity of various manumycin-type metabolites produced through mutasynthesis.

Turnerbactin, a novel triscatecholate siderophore from the shipworm endosymbiont Teredinibacter turnerae T7901.

Shipworms are marine bivalve mollusks (Family Teredinidae) that use wood for shelter and food. They harbor a group of closely related, yet phylogenetically distinct, bacterial endosymbionts in bacteriocytes located in the gills. This endosymbiotic community is believed to support the host's nutrition in multiple ways, through the production of cellulolytic enzymes and the fixation of nitrogen. The genome of the shipworm endosymbiont Teredinibacter turnerae T7901 was recently sequenced and in addition to the potential for cellulolytic enzymes and diazotrophy, the genome also revealed a rich potential for secondary metabolites. With nine distinct biosynthetic gene clusters, nearly 7% of the genome is dedicated to secondary metabolites. Bioinformatic analyses predict that one of the gene clusters is responsible for the production of a catecholate siderophore. Here we describe this gene cluster in detail and present the siderophore product from this cluster. Genes similar to the entCEBA genes of enterobactin biosynthesis involved in the production and activation of dihydroxybenzoic acid (DHB) are present in this cluster, as well as a two-module non-ribosomal peptide synthetase (NRPS). A novel triscatecholate siderophore, turnerbactin, was isolated from the supernatant of iron-limited T. turnerae T7901 cultures. Turnerbactin is a trimer of N-(2,3-DHB)-L-Orn-L-Ser with the three monomeric units linked by Ser ester linkages. A monomer, dimer, dehydrated dimer, and dehydrated trimer of 2,3-DHB-L-Orn-L-Ser were also found in the supernatant. A link between the gene cluster and siderophore product was made by constructing a NRPS mutant, TtAH03. Siderophores could not be detected in cultures of TtAH03 by HPLC analysis and Fe-binding activity of culture supernatant was significantly reduced. Regulation of the pathway by iron is supported by identification of putative Fur box sequences and observation of increased Fe-binding activity under iron restriction. Evidence of a turnerbactin fragment was found in shipworm extracts, suggesting the production of turnerbactin in the symbiosis.

Enzymatic synthesis of dilactone scaffold of antimycins.

Antimycins are a family of natural products possessing outstanding biological activities and unique structures, which have intrigued chemists for over a half century. The antimycin structural skeleton is built on a nine-membered dilactone ring containing one alkyl, one acyloxy, two methyl moieties, and an amide linkage connecting to a 3-formamidosalicylic acid. Although a biosynthetic gene cluster for antimycins was recently identified, the enzymatic logic that governs the synthesis of antimycins has not yet been revealed. In this work, the biosynthetic pathway for antimycins was dissected by both genetic and enzymatic studies for the first time. A minimum set of enzymes needed for generation of the antimycin dilactone scaffold were identified, featuring a hybrid nonribosomal peptide synthetase (NRPS)-polyketide synthase (PKS) assembly line containing both cis- and trans-acting components. Several antimycin analogues were further produced using in vitro enzymatic total synthesis based on the substrate promiscuity of this NRPS-PKS machinery.

Vanadium bromoperoxidase from Delisea pulchra: enzyme-catalyzed formation of bromofuranone and attendant disruption of quorum sensing.

Vanadium bromoperoxidase was isolated and cloned from the marine red alga Delisea pulchra. This enzyme catalyzes the bromolactonization of 4-pentynoic acid forming 5E-bromo-methylidenetetrahydro-2-furanone, a compound which is shown herein to inhibit quorum sensing in the engineered reporter strain, Agrobacterium tumefaciens NTL4.