Oxford Nanopore R10.4 long-read sequencing enables the generation of near-finished bacterial genomes from pure cultures and metagenomes without short-read or reference polishing.

Long-read Oxford Nanopore sequencing has democratized microbial genome sequencing and enables the recovery of highly contiguous microbial genomes from isolates or metagenomes. However, to obtain near-finished genomes it has been necessary to include short-read polishing to correct insertions and deletions derived from homopolymer regions. Here, we show that Oxford Nanopore R10.4 can be used to generate near-finished microbial genomes from isolates or metagenomes without short-read or reference polishing.

Heterologous expression of intact biosynthetic gene clusters in Fusarium graminearum.

Filamentous fungi such as species from the genus Fusarium are capable of producing a wide palette of interesting metabolites relevant to health, agriculture and biotechnology. Secondary metabolites are formed from large synthase/synthetase enzymes often encoded in gene clusters containing additional enzymes cooperating in the metabolite's biosynthesis. The true potential of fungal metabolomes remain untapped as the majority of secondary metabolite gene clusters are silent under standard laboratory growth conditions. One way to achieve expression of biosynthetic pathways is to clone the responsible genes and express them in a well-suited heterologous host, which poses a challenge since Fusarium polyketide synthase and non-ribosomal peptide synthetase gene clusters can be large (e.g. as large as 80 kb) and comprise several genes necessary for product formation. The major challenge associated with heterologous expression of fungal biosynthesis pathways is thus handling and cloning large DNA sequences. In this paper we present the successful workflow for cloning, reconstruction and heterologous production of two previously characterized Fusarium pseudograminearum natural product pathways in Fusarium graminearum. In vivo yeast recombination enabled rapid assembly of the W493 (NRPS32-PKS40) and the Fusarium Cytokinin gene clusters. F. graminearum transformants were obtained through protoplast-mediated and Agrobacterium tumefaciens-mediated transformation. Whole genome sequencing revealed isolation of transformants carrying intact copies the gene clusters was possible. Known Fusarium cytokinin metabolites; fusatin, 8-oxo-fusatin, 8-oxo-isopentenyladenine, fusatinic acid together with cis- and trans-zeatin were detected by liquid chromatography and mass spectrometry, which confirmed gene functionality in F. graminearum. In addition the non-ribosomal lipopeptide products W493 A and B was heterologously produced in similar amounts to that observed in the F. pseudograminearum doner. The Fusarium pan-genome comprises more than 60 uncharacterized putative secondary metabolite gene clusters. We nominate the well-characterized F. graminearum as a heterologous expression platform for Fusarium secondary metabolite gene clusters, and present our experience cloning and introducing gene clusters into this species. We expect the presented methods will inspire future endevours in heterologous production of Fusarium metabolites and potentially aid the production and characterization of novel natural products.

Chrysogine Biosynthesis Is Mediated by a Two-Module Nonribosomal Peptide Synthetase.

Production of chrysogine has been reported from several fungal genera including Penicillium, Aspergillus, and Fusarium. Anthranilic acid and pyruvic acid, which are expected precursors of chrysogine, enhance production of this compound. A possible route for the biosynthesis using these substrates is via a nonribosomal peptide synthetase (NRPS). Through comparative analysis of the NRPSs from genome-sequenced producers of chrysogine we identified a candidate NRPS cluster comprising five additional genes named chry2-6. Deletion of the two-module NRPS (NRPS14 = chry1) abolished chrysogine production in Fusarium graminearum, indicating that the gene cluster is responsible for chrysogine biosynthesis. Overexpression of NRPS14 enhanced chrysogine production, suggesting that the NRPS is the bottleneck in the biosynthetic pathway.

Identification of the non-ribosomal peptide synthetase responsible for biosynthesis of the potential anti-cancer drug sansalvamide in Fusarium solani.

Sansalvamide is a cyclic pentadepsipeptide produced by Fusarium solani and has shown promising results as potential anti-cancer drug. The biosynthetic pathway has until now remained unidentified, but here we used an Agrobacterium tumefaciens-mediated transformation (ATMT) approach to generate knockout mutants of two candidate non-ribosomal peptide synthetases (NRPS29 and NRPS30). Comparative studies of secondary metabolites in the two deletion mutants and wild type confirmed the absence of sansalvamide in the NRPS30 deletion mutant, implicating this synthetase in the biosynthetic pathway for sansalvamide. Sansalvamide is structurally related to the cyclic hexadepsipeptide destruxin, which both contain an α-hydroxyisocaproic acid (HICA) unit. A gene cluster responsible for destruxin production has previously been identified in Metarhizium robertsii together with a hypothetical biosynthetic pathway. Using comparative bioinformatic analyses of the catalytic domains in the destruxin and sansalvamide NRPSs, we were able to propose a model for sansalvamide biosynthesis. Orthologues of the gene clusters were also identified in species from several other genera including Acremonium chrysogenum and Trichoderma virens, which suggests that the ability to produce compounds related to destruxin and sansalvamide is widespread.

Functional Analysis of the Fusarielin Biosynthetic Gene Cluster.

Fusarielins are polyketides with a decalin core produced by various species of Aspergillus and Fusarium. Although the responsible gene cluster has been identified, the biosynthetic pathway remains to be elucidated. In the present study, members of the gene cluster were deleted individually in a Fusarium graminearum strain overexpressing the local transcription factor. The results suggest that a trans-acting enoyl reductase (FSL5) assists the polyketide synthase FSL1 in biosynthesis of a polyketide product, which is released by hydrolysis by a trans-acting thioesterase (FSL2). Deletion of the epimerase (FSL3) resulted in accumulation of an unstable compound, which could be the released product. A novel compound, named prefusarielin, accumulated in the deletion mutant of the cytochrome P450 monooxygenase FSL4. Unlike the known fusarielins from Fusarium, this compound does not contain oxygenized decalin rings, suggesting that FSL4 is responsible for the oxygenation.

Growth and single cell kinetics of the loricate choanoflagellate Diaphanoeca grandis.

Choanoflagellates are common members of planktonic communities. Some have complex life histories that involve transitions between multiple cell stages. We have grown the loricate choanoflagellate Diaphanoeca grandis on the bacterium Pantoea sp. and integrated kinetic observations at the culture level and at the single cell level. The life history of D. grandis includes a cell division cycle with a number of recognisable cell stages. Mature, loricate D. grandis were immobile and settled on the bottom substratum. Daughter cells were ejected from the lorica 30 min. after cell division, became motile and glided on the bottom substratum until they assembled a lorica. Single cell kinetics could explain overall growth kinetics in D. grandis cultures. The specific growth rate was 0.72 day-1 during exponential growth while mature D. grandis produced daughter cells at a rate of 0.9 day-1. Daughter cells took about 1.2 h to mature. D. grandis was able to abandon and replace its lorica, an event that delayed daughter cell formation by more than 2 days. The frequency of daughter cell formation varied considerably among individuals and single cell kinetics demonstrated an extensive degree of heterogeneity in D. grandis cultures, also when growth appeared to be balanced.

There it is! Fusarium pseudograminearum did not lose the fusaristatin gene cluster after all.

Fusarium pseudograminearum is a significant pathogen of cereals in arid regions worldwide and has the ability to produce numerous bioactive secondary metabolites. The genome sequences of seven F. pseudograminearum strains have been published and in one of these strains, C5834, we identified an intact gene cluster responsible for biosynthesis of the cyclic lipopeptide fusaristatin A. The high level of sequence identity of the fusaristatin cluster remnant in strains that do not produce fusaristatin suggests that the absence of the cluster evolved once, and subsequently the resulting locus with the cluster fragments became widely dispersed among strains of F. pseudograminearum in Australia. We examined a selection of 99 Australian F. pseudograminearum isolates to determine how widespread the ability to produce fusaristatin A is in F. pseudograminearum. We identified 15 fusaristatin producing strains, all originating from Western Australia. Phylogenetic analyses could not support a division of F. pseudograminearum into fusaristatin producing and nonproducing populations, which could indicate the loss has occurred relatively recent.

Fusaoctaxin A, an Example of a Two-Step Mechanism for Non-Ribosomal Peptide Assembly and Maturation in Fungi.

Fungal non-ribosomal peptide synthetase (NRPS) clusters are spread across the chromosomes, where several modifying enzyme-encoding genes typically flank one NRPS. However, a recent study showed that the octapeptide fusaoctaxin A is tandemly synthesized by two NRPSs in Fusarium graminearum. Here, we illuminate parts of the biosynthetic route of fusaoctaxin A, which is cleaved into the tripeptide fusatrixin A and the pentapeptide fusapentaxin A during transport by a cluster-specific ABC transporter with peptidase activity. Further, we deleted the histone H3K27 methyltransferase kmt6, which induced the production of fusaoctaxin A.

Feeding and growth of the marine heterotrophic nanoflagellates, Procryptobia sorokini and Paraphysomonas imperforata on a bacterium, Pseudoalteromonas sp. with an inducible defence against grazing.

Heterotrophic marine nanoflagellates are important grazers on bacteria in the water column. Some marine bacteria appear more resistant to grazing than do others. Marine nanoflagellates can be grown in the laboratory in batch cultures fed specific bacterial isolates. In some cultures, the flagellates appear unable to completely deplete the bacterial prey even when the bacterial strain otherwise is an excellent prey. This may indicate that some marine bacteria are able to induce defence mechanisms if they are grazed by nanoflagellates. Four morphologically distinct marine heterotrophic nanoflagellates, of which 3 were still identified as Procryptobia sorokini (Kinetoplastea) and one as Paraphysomonas imperforata (Chrysophyceae) were isolated from a coastal location along with 3 isolates of the marine bacterium Pseudoalteromonas sp. Flagellate growth and grazing on bacterial prey were analysed in batch cultures. Pseudoalteromonas was a suitable prey for all 4 flagellate isolates. They grazed and grew on Pseudoalteromonas as sole prey with maximal cell-specific growth rates of 0.1-0.25 h-1 and gross growth efficiencies of 38-61%. Exposure to dense flagellate cultures or their supernatants did, however, cause a fraction of the Pseudoalteromonas cells to aggregate and the bacterium became apparently resistant to grazing. Concentrations of suspended Pseudoalteromonas cells were therefore not decreased below 1,700-7,500 cells µL-1 by any of the flagellate isolates. These results indicate that Pseudoalteromonas sp. can be an excellent prey to marine nanoflagellates but also that is in possession of inducible mechanisms that protect against flagellate grazing.

Enhancing the Production of the Fungal Pigment Aurofusarin in Fusarium graminearum.

There is an increasing demand for products from natural sources, which includes a growing market for naturally-produced colorants. Filamentous fungi produce a vast number of chemically diverse pigments and are therefore explored as an easily accessible source. In this study we examine the positive regulatory effect of the transcription factor AurR1 on the aurofusarin gene cluster in Fusarium graminearum. Proteomic analyses showed that overexpression of AurR1 resulted in a significant increase of five of the eleven proteins belonging to the aurofusarin biosynthetic pathway. Further, the production of aurofusarin was increased more than threefold in the overexpression mutant compared to the wild type, reaching levels of 270 mg/L. In addition to biosynthesis of aurofusarin, several yet undescribed putative naphthoquinone/anthraquinone analogue compounds were observed in the overexpression mutant. Our results suggest that it is possible to enhance the aurofusarin production through genetic engineering.

The cereal pathogen Fusarium pseudograminearum produces a new class of active cytokinins during infection.

The fungal pathogen Fusarium pseudograminearum causes important diseases of wheat and barley. During a survey of secondary metabolites produced by this fungus, a novel class of cytokinins, herein termed Fusarium cytokinins, was discovered. Cytokinins are known for their growth-promoting and anti-senescence activities, and the production of a cytokinin mimic by what was once considered as a necrotrophic pathogen that promotes cell death and senescence challenges the simple view that this pathogen invades its hosts by employing a barrage of lytic enzymes and toxins. Through genome mining, a gene cluster in the F. pseudograminearum genome for the production of Fusarium cytokinins was identified and the biosynthetic pathway was established using gene knockouts. The Fusarium cytokinins could activate plant cytokinin signalling, demonstrating their genuine hormone mimicry. In planta analysis of the transcriptional response to one Fusarium cytokinin suggests extensive reprogramming of the host environment by these molecules, possibly through crosstalk with defence hormone signalling pathways.

The fungal community changes over time in developing wheat heads.

Under normal conditions, wheat is colonized by a multitude of fungi that can have beneficial or adverse effects on plant growth and yield. To study the effect of spraying wheat heads with fungicides on the fungal community from emergence to harvest we applied an amplicon sequencing approach on single wheat heads. The climatic data showed that the spring of 2014 was very dry and without precipitation in the two weeks around flowering. An initial quantitative PCR showed that the total amount of fungal DNA increased during the entire period, without significant difference between sprayed and control wheat heads. Amplicon sequencing of the internal transcribed spacer 2 (ITS2) region showed that operational taxonomic units (OTUs) identified as Sporobolomyces roseus dominated in the first weeks, whereas Alternaria infectoria OTUs dominated in the last weeks before harvest. The only observed significant difference was that the control wheat heads contained more of the powdery mildew causing Blumeria graminis f. sp. tritici OTUs compared with the sprayed wheat heads. The dry conditions around flowering most likely also had an effect on Fusarium head blight infection as Fusarium OTUs were only sporadically encountered. Analyses of secondary metabolites produced by Fusarium and Alternaria in the wheat heads confirmed the observations from the amplicon sequencing. Enniatin B was the most frequent contaminant present in four sprayed (49-538 ng/g) and three control (56-355 ng/g) wheat heads. The A. infectoria secondary metabolites infectopyrone and 4Z-infectopyrone were however consistently observed in all samples collected the last five weeks before harvest.