Enzymatic Synthesis of Diverse Heterocycles by a Noncanonical Nonribosomal Peptide Synthetase

Pseudomonas Virulence Factor Produces Autoinducer (S)-Valdiazen

Pseudomonas virulence factor (pvf) produces an autoinducing small-molecule signal that regulates bacterial cell-to-cell communication and virulence. While genes like pvf have been linked to the production of small molecules containing a diazeniumdiolate group, the specific chemical signal produced by pvf had not been identified. In this study, we reveal that (S)-valdiazen is the autoinducer produced by pvf in Pseudomonas entomophila, a model for pathogen-host interactions. The (S)-configuration is crucial for the signaling activity of valdiazen at physiological concentrations. We also define the (S)-stereochemistry of leudiazen, a similar signal produced by the plant pathogen Pseudomonas syringae. Using pvf genes needed for (S)-valdiazen signaling and production in P. entomophila, we bioinformatically identified 5383 bacterial organisms that may produce diazeniumdiolate signals. Signaling activity of valdiazen can be quenched by potassium permanganate, which oxidizes the diazeniumdiolate moiety. Identification of (S)-diazeniumdiolates from two bacterial species suggests stereospecific biosynthesis and transduction of these signals. Our findings set the stage for discovering diazeniumdiolate signals from other bacteria.

Antimicrobial Agent Trimethoprim Influences Chemical Interactions in Cystic Fibrosis Pathogens via the ham Gene Cluster

The fungus Aspergillus fumigatus and the bacterium Burkholderia cenocepacia cause fatal respiratory infections in immunocompromised humans and patients with lung disease, such as cystic fibrosis (CF). In dual infections, antagonistic interactions contribute to increased mortality. These interactions are further altered by the presence of antimicrobial and antifungal agents. However, studies performed to date on chemical interactions between clinical B. cenocepacia and A. fumigatus have focused on pathogens in isolation and do not include the most abundant chemical signal, i.e., clinically administered therapeutics, present in the lung. Here, we characterize small molecule-mediated interactions between B. cenocepacia and A. fumigatus and their shift in response to trimethoprim exposure by using metabolomics and mass spectrometry imaging. Using these methods, we report that the production of several small-molecule natural products of both the bacteria and the fungus is affected by cocultivation and exposure to trimethoprim. By systematic analysis of metabolomics data, we hypothesize that the B. cenocepacia-encoded ham gene cluster plays a role in the trimethoprim-mediated alteration of bacterial-fungal interactions. We support our findings by generating a genetically modified strain lacking the ham gene cluster and querying its interaction with A. fumigatus. Using comparative analyses of the extracts of wild-type and knockout strains, we report the inactivation of a bacterially produced antifungal compound, fragin, by A. fumigatus, which was verified by the addition of purified fragin to the A. fumigatus culture. Furthermore, we report that trimethoprim does not inhibit fungal growth, but affects the biochemical pathway for DHN-melanin biosynthesis, an important antifungal drug target, altering the pigmentation of the fungal conidia and is associated with modification of ergosterol to ergosteryl-3β-O-l-valine in coculture. This study demonstrates the impact of therapeutics on shaping microbial and fungal metabolomes, which influence interkingdom interactions and the expression of virulence factors. Our findings enhance the understanding of the complexity of chemical interactions between therapeutic compounds, bacteria, and fungi and may contribute to the development of selective treatments.

Antimicrobial and Anti-inflammatory Cyclic Tetrapeptides from the Co-cultures of Two Marine-Derived Fungi

Violaceotides B-E (1-4), four new cyclic tetrapeptides, along with seven known compounds, were identified from the sponge-associated Aspergillus insulicola IMB18-072 co-cultivated with the marine-derived Alternaria angustiovoidea IMB20-805. Their structures were elucidated by extensive analysis of spectroscopic data, including HRESIMS, 1D and 2D NMR, and MS/MS data. The absolute configurations were determined by the advanced Marfey's method. Compounds 2, 3, and violaceotide A (5) displayed selective antimicrobial activities against the aquatic pathogenic bacteria Edwardsiella tarda and E. ictaluri. In addition, compounds 1-5 showed inhibitory activities against the LPS-induced expression of the inflammatory mediator IL-6 in RAW264.7 cells at a concentration of 10 μM.

Function discovery of a non-ribosomal peptide synthetase-like encoding gene in the nematode-trapping fungus Arthrobotrys oligospora

In this study, the function of a non-ribosomal peptide synthetase-like (NRPS-like) encoding gene AOL_s00188g306 (g306) was investigated to reveal the association between NRPS and nematocidal activity in the nematode-trapping fungus Arthrobotrys oligospora. Sequence analysis indicated that the encoded product of g306 is an adenylation domain of non-ribosomal peptide synthetases and extended short-chain dehydrogenase/reductase domain-containing proteins, and displays a wide substrate spectrum. The Δg306 mutants were more sensitive to chemical stressors than the wild type. Disruption of g306 impeded the nematocidal efficiency of A. oligospora. Metabolomics analysis showed that secondary metabolite biosynthesis and lipid metabolism were altered in the mutants. The phenotypic changes in the mutants can be attributed to the down-regulation of various metabolites, including fatty acyls, prenol lipids, steroidsand steroid derivative, and amino acid derivatives, identified in the present study. This study investigated the association between the non-ribosomal polypeptide-encoding gene g306 and nematicidal activity in A. oligospora, providing a reference for resolving the predation mechanism of nematode-trapping fungus.

Pseudomonas virulence factor controls expression of virulence genes in Pseudomonas entomophila

Quorum sensing is a communication strategy that bacteria use to collectively alter gene expression in response to cell density. Pathogens use quorum sensing systems to control activities vital to infection, such as the production of virulence factors and biofilm formation. The Pseudomonas virulence factor (pvf) gene cluster encodes a signaling system (Pvf) that is present in over 500 strains of proteobacteria, including strains that infect a variety of plant and human hosts. We have shown that Pvf regulates the production of secreted proteins and small molecules in the insect pathogen Pseudomonas entomophila L48. Here, we identified genes that are likely regulated by Pvf using the model strain P. entomophila L48 which does not contain other known quorum sensing systems. Pvf regulated genes were identified through comparing the transcriptomes of wildtype P. entomophila and a pvf deletion mutant (ΔpvfA-D). We found that deletion of pvfA-D affected the expression of approximately 300 genes involved in virulence, the type VI secretion system, siderophore transport, and branched chain amino acid biosynthesis. Additionally, we identified seven putative biosynthetic gene clusters with reduced expression in ΔpvfA-D. Our results indicate that Pvf controls multiple virulence mechanisms in P. entomophila L48. Characterizing genes regulated by Pvf will aid understanding of host-pathogen interactions and development of anti-virulence strategies against P. entomophila and other pvf-containing strains.

Small NRPS-like enzymes in Aspergillus sections Flavi and Circumdati selectively form substituted pyrazinone metabolites

Aspergillus fungi produce mycotoxins that are detrimental to human and animal health. Two sections of aspergilli are of particular importance to cereal food crops such as corn and barley. Aspergillus section Flavi species like A. flavus and A. parasiticus produce aflatoxins, while section Circumdati species like A. ochraceus and A. sclerotiorum produce ochratoxin A. Mitigating these toxins in food and feed is a critical and ongoing worldwide effort. We have previously investigated biosynthetic gene clusters in Aspergillus flavus that are linked to fungal virulence in corn. We found that one such cluster, asa, is responsible for the production of aspergillic acid, an iron-binding, hydroxamic acid-containing pyrazinone metabolite. Furthermore, we found that the asa gene cluster is present in many other aflatoxin- and ochratoxin-producing aspergilli. The core gene in the asa cluster encodes the small nonribosomal peptide synthetase-like (NRPS-like) protein AsaC. We have swapped the asaC ortholog from A. sclerotiorum into A. flavus, replacing its native copy, and have also cloned both asaC orthologs into Saccharomyces cerevisiae. We show that AsaC orthologs in section Flavi and section Circumdati, while only containing adenylation-thiolation-reductase (ATR) domains, can selectively biosynthesize distinct pyrazinone natural products: deoxyaspergillic acid and flavacol, respectively. Because pyrazinone natural products and the gene clusters responsible for their production are implicated in a variety of important microbe-host interactions, uncovering the function and selectivity of the enzymes involved could lead to strategies that ultimately benefit human health.