Biosynthesis of sphinganine-analog mycotoxins

Occurrence, transformation, and toxicity of fumonisins and their covert products during food processing

Fumonisins comprise structurally related metabolites mainly produced by Fusarium verticillioides and Fusarium proliferatum. Contamination with fumonisins causes incalculable damage to the economy and poses a great risk to animal and human health. Fumonisins and their covert products are found in cereals and cereal products. Food processing significantly affects the degradation of toxins and the formation of covert toxins. However, studies on fumonisins and their covert mycotoxins remain inadequate. This review aims to summarize changes in fumonisins and the generation of covert fumonisins during processing. It also investigates the toxicity and determination methods of fumonisins and covert fumonisins, and elucidates the factors affecting fumonisins and their covert forms during processing. In addition to the metabolic production by plants and fungi, covert fumonisins are mainly produced by covalent or noncovalent binding, complexation, or physical entrapment of fumonisins with other substances. The toxicity of covert fumonisins is similar to that of free fumonisins and is a non-negligible hazard. Covert fumonisins are commonly found in food matrices, and methods to analyze them have yet to be improved. Food processing significantly affects the conversion of fumonisins to their covert toxins.

FunOrder: A robust and semi-automated method for the identification of essential biosynthetic genes through computational molecular co-evolution

Secondary metabolites (SMs) are a vast group of compounds with different structures and properties that have been utilized as drugs, food additives, dyes, and as monomers for novel plastics. In many cases, the biosynthesis of SMs is catalysed by enzymes whose corresponding genes are co-localized in the genome in biosynthetic gene clusters (BGCs). Notably, BGCs may contain so-called gap genes, that are not involved in the biosynthesis of the SM. Current genome mining tools can identify BGCs, but they have problems with distinguishing essential genes from gap genes. This can and must be done by expensive, laborious, and time-consuming comparative genomic approaches or transcriptome analyses. In this study, we developed a method that allows semi-automated identification of essential genes in a BGC based on co-evolution analysis. To this end, the protein sequences of a BGC are blasted against a suitable proteome database. For each protein, a phylogenetic tree is created. The trees are compared by treeKO to detect co-evolution. The results of this comparison are visualized in different output formats, which are compared visually. Our results suggest that co-evolution is commonly occurring within BGCs, albeit not all, and that especially those genes that encode for enzymes of the biosynthetic pathway are co-evolutionary linked and can be identified with FunOrder. In light of the growing number of genomic data available, this will contribute to the studies of BGCs in native hosts and facilitate heterologous expression in other organisms with the aim of the discovery of novel SMs.

The discovery and description of novel fungal secondary metabolites promises novel antibiotics, pharmaceuticals, and other useful compounds. A way to identify novel secondary metabolites is to express the corresponding genes in a suitable expression host. Consequently, a detailed knowledge or an accurate prediction of these genes is necessary. In fungi, the genes are co-localized in so-called biosynthetic gene clusters. Notably, the clusters may also contain genes that are not necessary for the biosynthesis of the secondary metabolites, so-called gap genes. We developed a method to detect co-evolved genes within the clusters and demonstrated that essential genes are co-evolving and can thus be differentiated from the gap genes. This adds an additional layer of information, which can support researchers with their decisions on which genes to study and express for the discovery of novel secondary metabolites.

Biosynthesis, regulation, and biological significance of fumonisins in fungi: current status and prospects

Fumonisins are one of the most important mycotoxin classes due to their widespread occurrence and potential health threat to humans and animals. Currently, most of the research focuses on the control of fumonisin contamination in the food supply chain. In recent years, significant progress in biochemistry, enzymology, and genetic regulation of fumonisin biosynthesis has been achieved using molecular technology. Furthermore, new insights into the roles of fumonisins in the interaction between fungi and plant hosts have been reported. This review provides an overview of the current understanding of the biosynthesis and regulation of fumonisins. The ecological significance of fumonisins to Fusarium species that produce the toxins is discussed, and the complex regulatory networks of fumonisin synthesis is proposed.

Accessory Chromosome-Acquired Secondary Metabolism in Plant Pathogenic Fungi: The Evolution of Biotrophs Into Host-Specific Pathogens

Accessory chromosomes are strain- or pathotype-specific chromosomes that exist in addition to the core chromosomes of a species and are generally not considered essential to the survival of the organism. Among pathogenic fungal species, accessory chromosomes harbor pathogenicity or virulence factor genes, several of which are known to encode for secondary metabolites that are involved in plant tissue invasion. Accessory chromosomes are of particular interest due to their capacity for horizontal transfer between strains and their dynamic "crosstalk" with core chromosomes. This review focuses exclusively on secondary metabolism (including mycotoxin biosynthesis) associated with accessory chromosomes in filamentous fungi and the role accessory chromosomes play in the evolution of secondary metabolite gene clusters. Untargeted metabolomics profiling in conjunction with genome sequencing provides an effective means of linking secondary metabolite products with their respective biosynthetic gene clusters that reside on accessory chromosomes. While the majority of literature describing accessory chromosome-associated toxin biosynthesis comes from studies of Alternaria pathotypes, the recent discovery of accessory chromosome-associated biosynthetic genes in Fusarium species offer fresh insights into the evolution of biosynthetic enzymes such as non-ribosomal peptide synthetases (NRPSs), polyketide synthases (PKSs) and regulatory mechanisms governing their expression.

An Acetylenic Lipid from the New Zealand Ascidian Pseudodistoma cereum: Exemplification of an Improved Workflow for Determination of Absolute Configuration of Long-Chain 2-Amino-3-alkanols

An acetylenic 2-amino-3-alcohol, distaminolyne B (2), isolated from the New Zealand ascidian Pseudodistoma cereum, is reported. The isolation and structure elucidation of 2 and assignment of 2S,3S absolute configuration (AC) using the exciton coupled circular dichroism technique are described. Using a methodologically facile workflow, the same AC was also established by analysis of specific rotation, terminal methyl C-1 δ_C chemical shift, and NH δ_H and J values of the N,O-diacetate derivative.

Enantiomeric Variability of Distaminolyne A. Refinement of ECD and NMR Methods for Determining Optical Purity of 1-Amino-2-Alkanols

Sample configurations of distaminolyne A (1a); isolated from the ascidians Pseudodistoma opacum and P. cereum, and collected at different sites in New Zealand, were investigated by two methods: Exciton coupled electronic circular dichroism (EC ECD) of the corresponding N,O-dibenzoyl derivative 1b; and chiral reagent derivatization of 1a with (S)- and (R)-α-methoxyphenylacetic acid (MPA), followed by ¹H-NMR analysis. Configuration and optical purity of 1a (%ee) was found to vary depending on the geographic distribution of ascidian colonies. An improved method for preparing N,O-diarenoyl derivatives of 1a was optimized. The EC ECD method was found to be complementary to the MPA-NMR method at different ranges of %ee.

Occurrence, biochemistry and biological effects of host-selective plant mycotoxins

Host-selective mycotoxins (HSTs) are various secondary metabolites or proteinaceous compounds secreted by pathogenic necrotrophic fungi that feed off on dead tissues of certain plants. Research on the HSTs has not only fundamental but also practical importance. On one hand they are implicated in the onset of devastating crop diseases. On the other hand, they have been studied as a good model for revealing the intricate mechanisms of plant-pathogen interactions. At the cellular level, HSTs target different compartments and in most instances induce programmed cell death (PCD) by a wide range of mechanisms. Often the responses provoked by HSTs resemble the effector-triggered immunity used by plant cells to combat biotrophic pathogens, which suggests that HST-producing fungi exploit the plants' own defensive systems to derive benefits. Although by definition HSTs are active only in tissues of susceptible plant genotypes, it has been demonstrated that some of them are able to influence animal cells as well. The possible effects, like cytotoxicity or cytostasis, can be harmful or beneficial and thus HSTs may either pose a health risk for humans and livestock, or be of prospective use in the fields of pharmacology, medicine and agriculture.

Sphingosine kinase AtSPHK1 functions in fumonisin B1-triggered cell death in Arabidopsis

The fungal toxin Fumonisin B1 (FB1) is a strong inducer to trigger plant hypersensitive responses (HR) along with increased long chain bases (LCB) and long chain base phosphates (LCBP) contents, though the regulatory mechanism of FB1 action and how the LCB/LCBP signalling cassette functions during the process is still not fully understood. Here, we report sphingosine kinase 1 (SPHK1) as a key factor in FB1-induced HR by modulating the salicylic acid (SA) pathway and reactive oxygen species (ROS) accumulation in Arabidopsis thaliana. Overexpression of SPHK1 increases the FB1-induced accumulations of ROS and SA. The double mutant that simultaneously overexpresses SPHK1 and suppresses the SPPASE or DPL1, two enzymes are mainly responsible for Phyto-sphingosine-1-phosphate (Phyto-S1P) removal, showed enhanced susceptibility to FB1 killing and FB1-induced SA activation than the plants overexpress SPHK1 alone. Exogenous sphingosine-1-phosphate (S1P) can modulate the transcription of the SA-responsive marker gene PR1 in a concentration-dependent biphasic manner. Suppression of SPHK1 decreases SA production whereas promotes jasmonic acid (JA) biosynthesis in response to FB1 applications. Our findings indicate a role of SPHK1 in modulating FB1-triggered cell death via SA and JA pathway interactions.

Bioinformatics Prediction of Polyketide Synthase Gene Clusters from Mycosphaerella fijiensis

Mycosphaerella fijiensis, causal agent of black Sigatoka disease of banana, is a Dothideomycete fungus closely related to fungi that produce polyketides important for plant pathogenicity. We utilized the M. fijiensis genome sequence to predict PKS genes and their gene clusters and make bioinformatics predictions about the types of compounds produced by these clusters. Eight PKS gene clusters were identified in the M. fijiensis genome, placing M. fijiensis into the 23rd percentile for the number of PKS genes compared to other Dothideomycetes. Analysis of the PKS domains identified three of the PKS enzymes as non-reducing and two as highly reducing. Gene clusters contained types of genes frequently found in PKS clusters including genes encoding transporters, oxidoreductases, methyltransferases, and non-ribosomal peptide synthases. Phylogenetic analysis identified a putative PKS cluster encoding melanin biosynthesis. None of the other clusters were closely aligned with genes encoding known polyketides, however three of the PKS genes fell into clades with clusters encoding alternapyrone, fumonisin, and solanapyrone produced by Alternaria and Fusarium species. A search for homologs among available genomic sequences from 103 Dothideomycetes identified close homologs (>80% similarity) for six of the PKS sequences. One of the PKS sequences was not similar (< 60% similarity) to sequences in any of the 103 genomes, suggesting that it encodes a unique compound. Comparison of the M. fijiensis PKS sequences with those of two other banana pathogens, M. musicola and M. eumusae, showed that these two species have close homologs to five of the M. fijiensis PKS sequences, but three others were not found in either species. RT-PCR and RNA-Seq analysis showed that the melanin PKS cluster was down-regulated in infected banana as compared to growth in culture. Three other clusters, however were strongly upregulated during disease development in banana, suggesting that they may encode polyketides important in pathogenicity.

Critical Role of COI1-Dependent Jasmonate Pathway in AAL toxin induced PCD in Tomato Revealed by Comparative Proteomics

Alternaria alternata f.sp. Lycopersici (AAL) toxin induces programmed cell death (PCD) in susceptible tomato (Solanum lycopersicum) leaves. Jasmonate (JA) promotes AAL toxin induced PCD in a COI1 (coronatine insensitive 1, JA receptor)-dependent manner by enhancement of reactive oxygen species (ROS) production. To further elucidate the underlying mechanisms of this process, we performed a comparative proteomic analysis using tomato jasmonic acid insensitive1 ( jai1), the receptor mutant of JA, and its wild type (WT) after AAL toxin treatment with or without JA treatment. A total of 10367 proteins were identified in tomato leaves using isobaric tags for relative and absolute quantitation (iTRAQ) quantitative proteomics approach. 2670 proteins were determined to be differentially expressed in response to AAL toxin and JA. Comparison between AAL toxin treated jai1 and its WT revealed the COI1-dependent JA pathway regulated proteins, including pathways related to redox response, ceramide synthesis, JA, ethylene (ET), salicylic acid (SA) and abscisic acid (ABA) signaling. Autophagy, PCD and DNA damage related proteins were also identified. Our data suggest that COI1-dependent JA pathway enhances AAL toxin induced PCD through regulating the redox status of the leaves, other phytohormone pathways and/or important PCD components.

Vitroprocines, new antibiotics against Acinetobacter baumannii, discovered from marine Vibrio sp. QWI-06 using mass-spectrometry-based metabolomics approach

A robust and convenient research strategy integrating state-of-the-art analytical techniques is needed to efficiently discover novel compounds from marine microbial resources. In this study, we identified a series of amino-polyketide derivatives, vitroprocines A-J, from the marine bacterium Vibrio sp. QWI-06 by an integrated approach using imaging mass spectroscopy and molecular networking, as well as conventional bioactivity-guided fractionation and isolation. The structure-activity relationship of vitroprocines against Acinetobacter baumannii is proposed. In addition, feeding experiments with ¹³C-labeled precursors indicated that a pyridoxal 5′-phosphate-dependent mechanism is involved in the biosynthesis of vitroprocines. Elucidation of amino-polyketide derivatives from a species of marine bacteria for the first time demonstrates the potential of this integrated metabolomics approach to uncover marine bacterial biodiversity.

Classic myrosinase-dependent degradation of indole glucosinolate attenuates fumonisin B1-induced programmed cell death in Arabidopsis

The mycotoxin fumonisin B1 (FB1) causes the accumulation of reactive oxygen species (ROS) which then leads to programmed cell death (PCD) in Arabidopsis. In the process of studying FB1-induced biosynthesis of glucosinolates, we found that indole glucosinolate (IGS) is involved in attenuating FB1-induced PCD. Treatment with FB1 elevates the expression of genes related to the biosynthesis of camalexin and IGS. Mutants deficient in aliphatic glucosinolate (AGS) or camalexin biosynthesis display similar lesions to Col-0 upon FB1 infiltration; however, the cyp79B2 cyp79B3 double mutant, which lacks induction of both IGS and camalexin, displays more severe lesions. Based on the fact that the classic myrosinase β-thioglucoside glucohydrolase (TGG)-deficient double mutant tgg1 tgg2, rather than atypical myrosinase-deficient mutant pen2-2, is more sensitive to FB1 than Col-0, and the elevated expression of TGG1, but not of PEN2, correlates with the decrease in IGS, we conclude that TGG-dependent IGS hydrolysis is involved in FB1-induced PCD. Indole-3-acetonitrile (IAN) and indole-3-carbinol (I3C), the common derivatives of IGS, were used in feeding experiments, and this rescued the severe cell death phenotype, which is associated with reduced accumulation of ROS as well as increased activity of antioxidant enzymes and ROS-scavenging ability. Despite the involvement of indole-3-acetic acid (IAA) in restricting FB1-induced PCD, feeding of IAN and I3C attenuated FB1-induced PCD in the IAA receptor mutant tir1-1 just as in Col-0. Taken together, our results indicate that TGG-catalyzed breakdown products of IGS decrease the accumulation of ROS by their antioxidant behavior, and attenuate FB1 induced PCD in an IAA-independent way.

Tricarballylic ester formation during biosynthesis of fumonisin mycotoxins in Fusarium verticillioides

Fumonisins are agriculturally important mycotoxins produced by the maize pathogen Fusarium verticillioides. The chemical structure of fumonisins contains two tricarballylic esters, which are rare structural moieties and important for toxicity. The mechanism for the tricarballylic ester formation is not well understood. FUM7 gene of F. verticillioides was predicted to encode a dehydrogenase/reductase, and when it was deleted, the mutant produced tetradehydro fumonisins (DH₄–FB). MS and NMR analysis of DH₄–FB₁ indicated that the esters consist of aconitate with a 3′-alkene function, rather than a 2′-alkene function. Interestingly, the purified DH₄–FB₁ eventually yielded three chromatographic peaks in HPLC. However, MS revealed that the metabolites of the three peaks all had the same mass as the initial single-peak DH₄–FB₁. The results suggest that DH₄–FB₁ can undergo spontaneous isomerization, probably including both cis–trans stereoisomerization and 3′- to 2′-ene regioisomerization. In addition, when FUM7 was expressed in Escherichia coli and the resulting enzyme, Fum7p, was incubated with DH₄–FB, no fumonisin with typical tricarballylic esters was formed. Instead, new fumonisin analogs that probably contained isocitrate and/or oxalosuccinate esters were formed, which reveals new insight into fumonisin biosynthesis. Together, the data provided both genetic and biochemical evidence for the mechanism of tricarballylic ester formation in fumonisin biosynthesis.

Quantitative molecular networking to profile marine cyanobacterial metabolomes

Untargeted liquid chromatography-MS (LC-MS) is used to rapidly profile crude natural product (NP) extracts; however, the quantity of data produced can become difficult to manage. Molecular networking based on MS/MS data visualizes these complex data sets to aid their initial interpretation. Here, we developed an additional visualization step for the molecular networking workflow to provide relative and absolute quantitation of a specific compound in an extract. The new visualization also facilitates combination of several metabolomes into one network, and so was applied to an MS/MS data set from 20 crude extracts of cultured marine cyanobacteria. The resultant network illustrates the high chemical diversity present among marine cyanobacteria. It is also a powerful tool for locating producers of specific metabolites. In order to dereplicate and identify culture-based sources of known compounds, we added MS/MS data from 60 pure NPs and NP analogs to the 20-strain network. This dereplicated six metabolites directly and offered structural information on up to 30 more. Most notably, our visualization technique allowed us to identify and quantitatively compare several producers of the bioactive and biosynthetically intriguing lipopeptide malyngamide C. Our most prolific producer, a Panamanian strain of Okeania hirsuta (PAB10FEB10-01), was found to produce at least 0.024 mg of malyngamide C per mg biomass (2.4%, w/dw) and is now undergoing genome sequencing to access the corresponding biosynthetic machinery.

ESI-MS and MS/MS identification of the first ceramide analogues of fumonisin B₁ mycotoxin from a Fusarium verticillioides culture following RP-HPLC separation

Following the earlier detection of six new esterified fumonisin B₁ (EFB₁) isomers containing three acyl groups in a Fusarium verticillioides-inoculated rice culture, it was assumed that linoleic, palmitic or oleic acid esterifies one of the free OH groups on the fumonisin backbone. On the basis of the results of our recent investigations we now propose that these EFB₁ isomers are actually 3-O- and 5-O-acyl derivatives of FB₁ (3-O-linoleoyl-FB₁, 5-O-linoleoyl-FB₁, 3-O-palmitoyl-FB₁, 5-O-palmitoyl-FB₁, 3-O-oleoyl-FB₁ and 5-O-oleoyl-FB₁). A F. verticillioides strain was identified that produced not only O-acyl-FB₁ isomers, but also low amounts of three N-acyl derivatives (N-linoleoyl-FB₁, N-palmitoyl-FB₁ and N-oleoyl-FB₁), which eluted from the HPLC column after the six O-acyl compounds and in the same sequence as for the O-acyl compounds. The characteristic positive and negative ESI-MS/MS spectra obtained after solid-phase extraction of the culture extract facilitated identification of these N-acyl-FB₁ derivatives. The biosynthesis of N-palmitoyl-FB₁ by F. verticillioides was verified by spiking the culture extract with synthetic N-palmitoyl-FB₁. This is the first report of the separation and mass spectrometric identification of the six O-acyl- and three N-acyl-FB₁ derivatives extracted from a F. verticillioides culture.

Identification of early fumonisin biosynthetic intermediates by inactivation of the FUM6 gene in Fusarium verticillioides

Fumonisins are polyketide mycotoxins produced by the maize pathogen Fusarium verticillioides and are associated with multiple human and animal diseases. A fumonisin biosynthetic pathway has been proposed, but structures of early pathway intermediates have not been demonstrated. The F. verticillioides FUM6 gene is required for an early pathway step. Here, metabolites produced by strains of the fungus with an inactivated FUM6 gene were purified and shown by mass spectrometry and NMR spectroscopy to have fumonisin-like structures but without substitutions at C-14 and C-15. The major metabolite was 2-amino-12,16-dimethylicosane-3,10-diol. Lesser amounts of 3-keto and triol analogues of the metabolite were also identified. In precursor feeding experiments, 2-amino-12,16-dimethylicosane-3,10-diol was transformed to fumonisins by a F. verticillioides strain with an inactive fumonisin polyketide synthase gene. The results support the hypothesis that the FUM6-encoded enzyme catalyzes fumonisin C-14 and C-15 hydroxylation and provide direct spectroscopic and biochemical evidence for structures of early intermediates in fumonisin biosynthesis.

Recent advancements in the biosynthetic mechanisms for polyketide-derived mycotoxins

Polyketides (PKs) are a large group of natural products produced by microorganisms and plants. They are biopolymers of acetate and other short carboxylates and are biosynthesized by multifunctional enzymes called polyketide synthases (PKSs). This review discusses the biosynthesis of four toxic PK, aflatoxins, fumonisins, ochratoxins (OTs), and zearalenone. These metabolites are structurally diverse and differ in their mechanisms of toxicity. However, they are all of concern in food safety and agriculture because of their toxic properties and their frequent accumulation in crops used for food and feed. The focus is on the recent advancements in the understanding of the molecular mechanisms for the biosynthesis of these mycotoxins. Several of the mycotoxin PKSs have been genetically and biochemically studied while other PKSs remain to be investigated. Multiple post-PKS modifications are often required for the maturation of the mycotoxins. Many of these modification steps for aflatoxins and fumonisins are well established while the post-PKS modifications for zearalenone and OTs remain to be biochemically characterized. More efforts are needed to completely illustrate the biosynthetic mechanisms for this important group of PKs.

Fumonisin detection and analysis of potential fumonisin-producing Fusarium spp. in asparagus (Asparagus officinalis L.) in Zhejiang Province of China

BACKGROUND

Fumonisins are mycotoxins produced by a number of Fusarium species, including several pathogens of asparagus plants. China is one of the largest asparagus producers in the world. In this study, we analysed the contamination of fumonisins and fumonisin-producing fungi in asparagus spear samples from Zhejiang Province, the major asparagus production province in China.

RESULTS

The asparagus did not contain a detectable level of fumonisins. However, the recovery of Fusarium in asparagus was 72.7%, including F. proliferatum (40.9%), F. oxysporum (22.7%), F. acuminatum (4.55%) and F. equesti (4.55%). A multiplex PCR targeting the internal transcribed spacer sequence (ITS), translation elongation factor 1-alpha (TEF), and key biosynthetic genes FUM1 and FUM8, was used to simultaneously determine the identity and the biosynthetic ability of the fungal isolates. Fungal isolates containing the FUM genes also produced fumonisins in cultures, ranging from 28 to 4204 microg g(-1). F. proliferatum was the only fumonisin-producing Fusarium species in asparagus.

CONCLUSION

Although no fumonisin contamination was detected in asparagus in the current survey, we found that the majority of samples contained Fusarium spp. Because F. proliferatum is a high fumonisin-producing species, potential health risks for human consumption of asparagus exist, if the appropriate environmental conditions are present for this fungus.

Hereditary sensory neuropathy type 1 is caused by the accumulation of two neurotoxic sphingolipids

HSAN1 is an inherited neuropathy found to be associated with several missense mutations in the SPTLC1 subunit of serine palmitoyltransferase (SPT). SPT catalyzes the condensation of serine and palmitoyl-CoA, the initial step in the de novo synthesis of sphingolipids. Here we show that the HSAN1 mutations induce a shift in the substrate specificity of SPT, which leads to the formation of the two atypical deoxy-sphingoid bases (DSBs) 1-deoxy-sphinganine and 1-deoxymethyl-sphinganine. Both metabolites lack the C(1) hydroxyl group of sphinganine and can therefore neither be converted to complex sphingolipids nor degraded. Consequently, they accumulate in the cell, as demonstrated in HEK293 cells overexpressing mutant SPTLC1 and lymphoblasts of HSAN1 patients. Elevated DSB levels were also found in the plasma of HSAN1 patients and confirmed in three groups of HSAN1 patients with different SPTLC1 mutations. The DSBs show pronounced neurotoxic effects on neurite formation in cultured sensory neurons. The neurotoxicity co-occurs with a disturbed neurofilament structure in neurites when cultured in the presence of DSBs. Based on these observations, we conclude that HSAN1 is caused by a gain of function mutation, which results in the formation of two atypical and neurotoxic sphingolipid metabolites.

Proteome analysis of Aspergillus niger: lactate added in starch-containing medium can increase production of the mycotoxin fumonisin B2 by modifying acetyl-CoA metabolism

Background

Aspergillus niger is a filamentous fungus found in the environment, on foods and feeds and is used as host for production of organic acids, enzymes and proteins. The mycotoxin fumonisin B₂ was recently found to be produced by A. niger and hence very little is known about production and regulation of this metabolite. Proteome analysis was used with the purpose to reveal how fumonisin B₂ production by A. niger is influenced by starch and lactate in the medium.

Results

Fumonisin B₂ production by A. niger was significantly increased when lactate and starch were combined in the medium. Production of a few other A. niger secondary metabolites was affected similarly by lactate and starch (fumonisin B₄, orlandin, desmethylkotanin and pyranonigrin A), while production of others was not (ochratoxin A, ochratoxin alpha, malformin A, malformin C, kotanin, aurasperone B and tensidol B). The proteome of A. niger was clearly different during growth on media containing 3% starch, 3% starch + 3% lactate or 3% lactate. The identity of 59 spots was obtained, mainly those showing higher or lower expression levels on medium with starch and lactate. Many of them were enzymes in primary metabolism and other processes that affect the intracellular level of acetyl-CoA or NADPH. This included enzymes in the pentose phosphate pathway, pyruvate metabolism, the tricarboxylic acid cycle, ammonium assimilation, fatty acid biosynthesis and oxidative stress protection.

Conclusions

Lactate added in a medium containing nitrate and starch can increase fumonisin B₂ production by A. niger as well as production of some other secondary metabolites. Changes in the balance of intracellular metabolites towards a higher level of carbon passing through acetyl-CoA and a high capacity to regenerate NADPH during growth on medium with starch and lactate were found to be the likely cause of this effect. The results lead to the hypothesis that fumonisin production by A. niger is regulated by acetyl-CoA.

PKS and NRPS release mechanisms

This review covers the recent literature on the release mechanisms for polyketides and nonribosomal peptides produced by microorganisms. The emphasis is on the novel enzymology and mechanistic insights revealed by the biosynthetic studies of new natural products.

Introduction of the AAL-toxin polyketide synthase gene ALT1 into FUM1-disrupted Fusarium verticillioides produces metabolites with the fumonisin methylation pattern

Fumonisins and AAL toxins are polyketide-derived mycotoxins produced by several important fungal pathogens of crop plants. The toxins contain a linear, dimethylated polyketide chain, but differ in chain length and methylation positions. Here, we have isolated the major metabolites from a fumonisin null mutant, Fusarium verticillioides strain 5777, that has been transformed with the AAL-toxin polyketide synthase gene (ALT1). The results showed that the metabolites maintained the chain length and methylation pattern of fumonisins. This suggests that the timing and regioselectivity of the methyltransferase of the ALT1 PKS could be reprogrammed in this Fusarium transformant.

Metabolomics of Aspergillus fumigatus

Aspergillus fumigatus is the most important species in Aspergillus causing infective lung diseases. This species has been reported to produce a large number of extrolites, including secondary metabolites, acids, and proteins such as hydrophobins and extracellular enzymes. At least 226 potentially bioactive secondary metabolites have been reported from A. fumigatus that can be ordered into 24 biosynthetic families. Of these families we have detected representatives from the following families of secondary metabolites: fumigatins, fumigaclavines, fumiquinazolines, trypacidin and monomethylsulochrin, fumagillins, gliotoxins, pseurotins, chloroanthraquinones, fumitremorgins, verruculogen, helvolic acids, and pyripyropenes by HPLC with diode array detection and mass spectrometric detection. There is still doubt whether A. fumigatus can produce tryptoquivalins, but all isolates produce the related fumiquinazolines. We also tentatively detected sphingofungins in A. fumigatus Af293 and in an isolate of A. lentulus. The sphingofungins may have a similar role as the toxic fumonisins, found in A. niger. A further number of mycotoxins, including ochratoxin A, and other secondary metabolites have been reported from A. fumigatus, but in those cases either the fungus or its metabolite appear to be misidentified.

Biodiversity of sphingoid bases ("sphingosines") and related amino alcohols

"Sphingosin" was first described by J. L. W. Thudichum in 1884 and structurally characterized as 2S,3R,4E-2-aminooctadec-4-ene-1,3-diol in 1947 by Herb Carter, who also proposed the designation of "lipides derived from sphingosine as sphingolipides." This category of amino alcohols is now known to encompass hundreds of compounds that are referred to as sphingoid bases and sphingoid base-like compounds, which vary in chain length, number, position, and stereochemistry of double bonds, hydroxyl groups, and other functionalities. Some have especially intriguing features, such as the tail-to-tail combination of two sphingoid bases in the alpha,omega-sphingoids produced by sponges. Most of these compounds participate in cell structure and regulation, and some (such as the fumonisins) disrupt normal sphingolipid metabolism and cause plant and animal disease. Many of the naturally occurring and synthetic sphingoid bases are cytotoxic for cancer cells and pathogenic microorganisms or have other potentially useful bioactivities; hence, they offer promise as pharmaceutical leads. This thematic review gives an overview of the biodiversity of the backbones of sphingolipids and the broader field of naturally occurring and synthetic sphingoid base-like compounds.