Cytotoxicity of trichothecenes and fusarochromanone produced by Fusarium equiseti strains isolated from Norwegian cereals

The cytotoxicity and secondary metabolites of 28 Norwegian strains of Fusarium equiseti have been characterized. Trichothecenes and fusarochromanone (FUCH) in rice culture extracts of the strains were analysed by gas chromatography-mass spectrometry (GC-MS) and high performance liquid chromatography (HPLC). The following metabolites were found in all isolates: FUCH, nivalenol (NIV), scirpentriol (SCIRP), 4-acetylnivalenol (4-ac-NIV, also called fusarenon-X), 15-acetyl-nivalenol (15-ac-NIV), and diacetoxyscirpenol (DAS). 4,15-diacetyl-nivalenol (diacetyl-NIV) was found in 5 isolates. Porcine kidney epithelial cells (PK15. American Type Culture Collection) were exposed to rice culture extracts to study cytotoxicity. Descriptive statistics and factor analysis of the identified secondary metabolites show that their main metabolites were FUCH, NIV, SCIRP, DAS and 15-ac-NIV, consecutively. The individual trichothecenes were highly intercorrelated, whereas the production of acetylated NIV and DAS was slightly less. Stepwise multiple regression analysis of cytotoxicity and metabolite profiles of rice culture extracts ascribed the toxicity mainly to a combination of FUCH and 15-ac-NIV, though SCIRP or DAS are agents in the combined toxicity as well.

Inactivation of a cytochrome P-450 is a determinant of trichothecene diversity in Fusarium species

Species of the genus Fusarium produce a great diversity of agriculturally important trichothecene toxins that differ from each other in their pattern of oxygenation and esterification. T-2 toxin, produced by Fusarium sporotrichioides, and nivalenol (NIV), produced by some strains of F. graminearum, contain an oxygen at the C-4 position. Deoxynivalenol (DON), produced by other strains of F. graminearum, lacks a C-4 oxygen. NIV and DON are identical except for this difference, whereas T-2 differs from these trichothecenes at three other carbon positions. Sequence and Northern analyses of the F. sporotrichioides genomic region upstream of the previously described core trichothecene gene cluster have extended the cluster by two genes: TRI13 and TRI14. TRI13 shares significant similarity with the cytochrome P-450 class of enzymes, but TRI14 does not share similarity with any previously characterized proteins. Gene disruption and fermentation studies in F. sporotrichioides indicate that TRI13 is required for the addition of the C-4 oxygen of T-2 toxin, but that TRI14 is not required for trichothecene biosynthesis. PCR and sequence analyses indicate that the TRI13 homolog is functional in NIV-producing strains of F. graminearum but nonfunctional in DON-producing strains of the fungus. These genetic observations are consistent with chemical observations that biosynthesis of T-2 toxin and NIV requires a C-4 hydroxylase while biosynthesis of DON does not.

Production of culmorin compounds and other secondary metabolites by Fusarium culmorum and F. graminearum strains isolated from Norwegian cereals

Twenty-three Fusarium culmorum and 21 F. graminearum isolates were studied for their ability to produce mycotoxins and other secondary metabolites. The strains were cultivated on rice, and the extracts analysed by gas chromatography mass spectrometry (GC-MS) after derivatization with pentafluoropropionic (PFP) reagent. Two F. culmorum strains formed nivalenol and its acetylated derivatives (chemotype II), while all F. graminearum and the other F. culmorum isolates produced deoxynivalenol (DON) via 3-acetyldeoxynivalenol (3-acetyl-DON) (chemotype IA). 15-hydroxy-culmorin, followed by 5-hydroxy-culmorin were the main other metabolites produced F. culmorum, while 5-, 12- and an unidentified hydroxy-culmorin, suggested to be 14-hydroxy-culmorin, were the main metabolites of F. graminearum. The hydroxy-culmorin profile was found to be significantly different for the two Fusarium species. Minor amounts of about ten other hydroxy-culmorins, four hydroxy-culmorones and 3,13-dihydroxy- epiapotrichothecene were also detected in most cultures. Traces of sambucinol seemed to be present in some of the isolates, but were not detected in any significant amounts. The precursors in the biosynthetic sequence to 3-acetyldeoxynivalenol, 7,8-dihydroxycalonectrin and 15-deacetyl-7,8-dihydroxycalonectrin, were detected in most cultures. We also report the assignment of both the 1H and 13C NMR data of 15-deacetyl-7,8-dihydroxycalonectrin, which has only been reported incorrectly before.

Accumulation of type A trichothecenes in maize, wheat and rice by Fusarium sporotrichioides isolates under diverse culture conditions

Toxigenic isolates of Fusarium sporotrichioides were tested for the production of type A trichothecenes (T-2 toxin, HT-2 toxin, diacetoxyscirpenol and neosolaniol) when grown on three substrates (maize, rice and wheat) under various conditions of temperature and water activity in the laboratory for 3 weeks. Trichothecenes were determined by high-performance liquid chromatography with fluorescence detection, after derivatisation with coumarin-3-carbonyl chloride. This is the first time this analytical method has been applied to an extensive study of trichothecene accumulation. With minor exceptions, greater trichothecene production occurred when samples were incubated at 20 degrees C and moistened with 35% water (water activity 0.990) although incubation conditions affected the substrates studied in different ways. No correlation between the different pairs of trichothecenes was found except for neosolaniol and diacetoxyscirpenol (r=0.56). Principal component analysis results show that the data points can be grouped in three rough clusters related to cereal type, which points out that the composition of these cereals can influence the production of type A trichothecenes.

Real time detection of the tri5 gene in Fusarium species by lightcycler-PCR using SYBR Green I for continuous fluorescence monitoring

LightCycler technology combines rapid in vitro amplification of DNA with real time detection and quantification of the amount of target molecules present in a sample. The system enables a 35-cycle PCR with 32 samples do be completed in 45 min, including quantification and identification of the product. It is therefore well suited for routine analysis of large numbers of samples in quality control and for defining HACCP concepts. Based on PCR primers specific to the tri5 gene, a quantitative group specific assay was established for Fusarium species producing trichothecenes. In the assay, SYBR Green I was used as fluorescent dye enabling real time detection of PCR products. Characterisation of the amplicons was achieved by melting point analysis (85 +/- 0.1 degrees C). Nonspecific products such as primer dimers could readily be distinguished from the product by their lower melting points. Composition of the amplification buffer was optimised and various hot start methods were tested in order to achieve the highest sensitivity of the assay. Uracil DNA glycosylase was added to prevent amplification of nonspecific products due to DNA carryover. The spectrum of species detected was generally in accordance with the results found in conventional PCR using the Tox5 primer pair. Reproducibility in six parallel experiments of the assay was determined to be 98% in the range between 0.05 and 6 ng of purified Fusarium graminearum DNA. The assay was used to analyse 30 wheat samples contaminated with toxigenic Fusarium spp. Contamination ranged from 0% to 78% as revealed by mycological analysis, and this is compared with results from the LightCycler. This is the first report on the use of the LightCycler system in combination with SYBR Green I for the quantification and identification of fungal DNA in pure cultures and sample material.

Toxigenic potential of Fusarium culmorum strains isolated from French wheat

Sixty F. culmorum strains were isolated from wheat grains collected from different wheat-growing areas in France and from different cultivars. The isolates were grown on autoclaved wheat grain to assess their ability to produce trichothecenes and zearalenone. Fungal biomass was evaluated through the ergosterol grain content. All the isolates produced zearalenone (0.39-1660 mg kg(-1)). Thirty-five of the 60 F. culmorum produced nivalenol (0.11-11.7 mg kg(-1)), 12 of 60 produced fusarenone X (0.05-8.42 mg kg(-1)), five of 60 produced 15-acetyldeoxynivalenol (0.48-27.7 mg kg(-1)), 13 of 60 produced 3-acetyldeoxynivalenol (0.07-21.0 mg kg(-1) and 24 of 60 produced deoxynivalenol (0.92-51.9 mg kg(-1)). According to the results, the distribution of the different chemotypes as well as the high and the low mycotoxin-producing Fusarium strains could not be associated to geographical origin.

A novel regulatory gene, Tri10, controls trichothecene toxin production and gene expression

We report here the characterization of Tri10, a novel regulatory gene within the trichothecene gene cluster. Comparison of Tri10 genomic and mRNA sequences revealed that removal of a single 77-bp intron provided a 1,260-bp open reading frame, encoding a 420-amino-acid protein. Disruption of Tri10 in Fusarium sporotrichioides abolished T-2 toxin production and dramatically decreased the transcript accumulation for four trichothecene genes (Tri4, Tri5, Tri6, and Tri101) and an apparent farnesyl pyrophosphate synthetase (Fpps) gene. Conversely, homologous integration of a disruption vector by a single upstream crossover event significantly increased T-2 toxin production and elevated the transcript accumulation of the trichothecene genes and Fpps. Further analysis revealed that disruption of Tri10, and to a greater extent the disruption of Tri6, increased sensitivity to T-2 toxin under certain growth conditions. Although Tri10 is conserved in Fusarium graminearum and Fusarium sambucinum and clearly plays a central role in regulating trichothecene gene expression, it does not show any significant matches to proteins of known or predicted function or to motifs except a single transmembrane domain. We suggest a model in which Tri10 acts upstream of the cluster-encoded transcription factor TRI6 and is necessary for full expression of both the other trichothecene genes and the genes for the primary metabolic pathway that precedes the trichothecene biosynthetic pathway, as well as for wild-type levels of trichothecene self-protection. We further suggest the presence of a regulatory loop where Tri6 is not required for the transcription of Tri10 but is required to limit the expression of Tri10.

Trichothecene and moniliformin production by Fusarium species from western Canadian wheat

Fusarium graminearum, Fusarium culmorum, and Fusarium avenaceum, isolated from Fusarium-damaged wheat harvested in western Canada, were cultured and evaluated for mycotoxin production. Extracts of the culture media were assayed for trichothecenes by gas chromatography-mass spectrometry and for moniliformin by liquid chromatography. Deoxynivalenol (DON) was found in 28 of 42 isolates of F. graminearum and 42 of 42 isolates of F. culmorum at levels ranging from 0.5 to 25.0 microg/g. 15-AcetylDON was found in 28 of 42 isolates of F. graminearum at levels ranging from 1.0 to 7.1 microg/g. 3-AcetylDON was found in 41 of 42 isolates of F. culmorum at levels ranging from 0.8 to 13.0 microg/g. Several other trichothecenes were assayed but not detected in the culture medium. Moniliformin was present in 40 of 42 isolates of F. avenaceum at levels ranging from 1.3 to 138.1 microg/g, but was not present in any of the isolates of F. graminearum or F. culmorum.

Identification of deoxynivalenol- and nivalenol-producing chemotypes of Gibberella zeae by using PCR

Gibberella zeae, a major cause of cereal scab, may be divided into two chemotypes based on production of the trichothecenes deoxynivalenol (DON) and nivalenol (NIV). We cloned and sequenced the gene cluster for trichothecene biosynthesis from each chemotype. G. zeae H-11 is a DON producer isolated from corn, and G. zeae 88-1 is a NIV producer from barley. We sequenced a 23-kb gene cluster from H-11 and a 26-kb cluster from 88-1, along with the unlinked Tri101 genes. Each gene cluster contained 10 Tri gene homologues in the same order and transcriptional directions as those of Fusarium sporotrichioides. Between H-11 and 88-1 all of the Tri homologues except Tri7 were conserved, with identities ranging from 88 to 98% and 82 to 99% at the nucleotide and amino acid levels, respectively. The Tri7 sequences were only 80% identical at the nucleotide level. We aligned the Tri7 genes and found that the Tri7 open reading frame of H-11 carried several mutations and an insertion containing 10 copies of an 11-bp tandem repeat. The Tri7 gene from 88-1 carried neither the repeat nor the mutations. We assayed 100 G. zeae isolates of both chemotypes by PCR amplification with a primer pair derived from the Tri7 gene and could differentiate the chemotypes by polyacrylamide gel electrophoresis. The PCR-based method developed in this study should provide a simple and reliable diagnostic tool for differentiating the two chemotypes of G. zeae.

Effects of tebuconazole on morphology, structure, cell wall components and trichothecene production of Fusarium culmorum in vitro

The effects of tebuconazole, a systemic fungicide, on the morphology, structure, cell wall components and toxin production of Fusarium culmorum were investigated in vitro. Treatment was by application of four filter paper strips (0.75 cm x 5.0 cm) soaked in 20 micrograms ml-1 fungicide placed around a point inoculum in Petri dishes. Mycelial growth was strongly inhibited by fungicide treatment. Scanning electron microscopic observations showed that the fungicide caused irregular swelling and excessive branching of hyphae. The morphological changes induced by the fungicide at the ultrastructural level included considerable thickening of the hyphal cell walls, excessive septation, the formation of the incomplete septa, extensive vacuolisation, accumulation of lipid bodies and progressing necrosis or degeneration of the hyphal cytoplasm. Non-membrane inclusion bodies were often detected in the hyphal cytoplasm. Furthermore, the formation of new hyphae (daughter hyphae) inside collapsed hyphal cells was common following treatment. The daughter hyphae also displayed severe alterations such as irregular thickening of the cell walls and necrosis of the cytoplasm. Using cytochemical techniques, the labelling densities of chitin and beta-1,3-glucan in the cell walls of the fungicide-treated hyphae were more pronounced than in those of the control hyphae. Moreover, immunogold labelling with antiserum against deoxynivalenol (DON) revealed that Fusarium toxin DON was localized in the cell walls, cytoplasm, mitochondria and vacuoles of the hyphae from the control and the fungicide treatment, but the labelling density in the fungicide-treated hyphae decreased dramatically compared with the control hyphae, indicating that tebuconazole reduced Fusarium toxin production of the fungus.

Identification of deoxynivalenol, 3-acetyldeoxynivalenol and zearalenone in the galactose oxidase-producing fungus Dactylium dendroides

The galactose oxidase-producing fungus Dactylium dendroides was re-identified as a Fusarium species. Fungi of this genus are well known for the production of mycotoxins. Verification of growth of this fungus on rice, corn and liquid medium described for the production of galactose oxidase is provided to determine whether the fungus could produce Fusarium toxins, namely, moniliformin, fusaric acid, fumonisin, zearalenone and the trichothecenes, deoxynivalenol, 3-acetyldeoxynivalenol, fusarenone, nivalenol, diacetoxyscirpenol, neosolaniol, and toxin T-2. Under the culture conditions used, deoxynivalenol, 3-acetyldeoxynivalenol and zearalenone were detected in the fungal culture medium. The finding is consistent with the hypothesis that the fungus is in fact a Fusarium species.

Quantification of trichothecene-producing Fusarium species in harvested grain by competitive PCR to determine efficacies of fungicides against Fusarium head blight of winter wheat

We developed a PCR-based assay to quantify trichothecene-producing Fusarium based on primers derived from the trichodiene synthase gene (Tri5). The primers were tested against a range of fusarium head blight (FHB) (also known as scab) pathogens and found to amplify specifically a 260-bp product from 25 isolates belonging to six trichothecene-producing Fusarium species. Amounts of the trichothecene-producing Fusarium and the trichothecene mycotoxin deoxynivalenol (DON) in harvested grain from a field trial designed to test the efficacies of the fungicides metconazole, azoxystrobin, and tebuconazole to control FHB were quantified. No correlation was found between FHB severity and DON in harvested grain, but a good correlation existed between the amount of trichothecene-producing Fusarium and DON present within grain. Azoxystrobin did not affect levels of trichothecene-producing Fusarium compared with those of untreated controls. Metconazole and tebuconazole significantly reduced the amount of trichothecene-producing Fusarium in harvested grain. We hypothesize that the fungicides affected the relationship between FHB severity and the amount of DON in harvested grain by altering the proportion of trichothecene-producing Fusarium within the FHB disease complex and not by altering the rate of DON production. The Tri5 quantitative PCR assay will aid research directed towards reducing amounts of trichothecene mycotoxins in food and animal feed.

A Genetic and Biochemical Approach to Study Trichothecene Diversity in Fusarium sporotrichioides and Fusarium graminearum

The trichothecenes T-2 toxin and deoxynivalenol (DON) are natural fungal products that are toxic to both animals and plants. Their importance in the pathogenicity of Fusarium spp. on crop plants has inspired efforts to understand the genetic and biochemical mechanisms leading to trichothecene synthesis. In order to better understand T-2 toxin biosynthesis by Fusarium sporotrichioides and DON biosynthesis by F. graminearum, we compared the nucleotide sequence of the 23-kb core trichothecene gene cluster from each organism. This comparative genetic analysis allowed us to predict proteins encoded by two trichothecene genes, TRI9 and TRI10, that had not previously been described from either Fusarium species. Differences in gene structure also were correlated with differences in the types of trichothecenes that the two species produce. Gene disruption experiments showed that F. sporotrichioides TRI7 (FsTRI7) is required for acetylation of the oxygen on C-4 of T-2 toxin. Sequence analysis indicated that F. graminearum TRI7 (FgTRI7) is nonfunctional. This is consistent with the fact that the FgTRI7 product is not required for DON synthesis in F. graminearum because C-4 is not oxygenated.

Impact of competitive fungi on Trichothecene production by Fusarium graminearum

Bioassays were used to determine the production of the trichothecene mycotoxin, deoxynivalenol (DON), by two isolates of Fusarium graminearum when grown in association with potentially competitive fungi and an antifungal chemical, 6-pentyl-alpha-pyrone (6PAP). The presence of 6PAP in the culture medium reduced DON production by as much as 80%, but this effect was reduced for the F. graminearum isolate that most efficiently metabolized the added 6PAP. A 6PAP-producing Trichoderma isolate grown in a competition assay system with the F. graminearum isolates was also able to substantially reduce DON production. When Fusarium isolates (F. crookwellense, F. culmorum, F. subglutinans, F. poae, F. equiseti, F. avenaceum, and F. sambucinum), which co-occur with F. graminearum in New Zealand maize plants (Zea mays), were grown in competition assays, the effect on DON production was variable. However, all isolates of F. subglutinans tested were shown to cause reductions in DON production (by 13-76%, mean = 62%). F. subglutinans frequently co-occurs with F. graminearum, but its presence can vary with location and time of the season. When the competitive fungus tested was also a trichothecene producer (e.g., of nivalenol), both toxins were produced in the assay medium. The results indicate that mycotoxin production by F. graminearum can be affected by the presence of particular competitive fungi. These results have implications for an ecological understanding of pathogenicity and of mycotoxin accumulation in plants. Early establishment of F. subglutinans, for example, may act as a biological control mechanism providing a temporary protection against invasion by more commonly toxigenic fusaria such as F. graminearum.

Effects of the Fusarium spp. mycotoxins fusaric acid and deoxynivalenol on the growth of Ruminococcus albus and Methanobrevibacter ruminantium

The Fusarium spp. mycotoxins fusaric acid and deoxynivalenol (DON) were tested for antimicrobial activity against Ruminococcus albus and Methanobrevibacter ruminantium. The growth of both organisms was inhibited by fusaric acid as low as 15 micrograms/mL (84 microM) but not by DON, at levels as high as 100 micrograms/mL (338 microM). No synergistic inhibitory effect was observed with DON plus fusaric acid. Neither organism was able to adapt to the fusaric acid and responses of each organism to the compound were different. The optical density (OD) maximum for R. albus, but not for M. ruminantium, was diminished after 28 days incubation at concentrations of fusaric acid below 240 micrograms/mL. Inhibition of R. albus started before significant growth had occurred, while M. ruminantium doubled twice before the onset of inhibition. Responses to picolinic acid, an analog of fusaric acid, were also dramatically different between the two microorganisms with M. ruminantium exhibiting a severe lag followed by a complete recovery of growth, while R. albus was only slightly inhibited with no lag. These results suggest that the mechanism of fusaric acid inhibition is specific to each microorganism. This is the first demonstration of the common mycotoxin fusaric acid inhibiting the growth of rumen bacteria.

Altered regulation of 15-acetyldeoxynivalenol production in Fusarium graminearum

Most Fusarium graminearum isolates produce low or undetectable levels of trichothecenes in liquid shake cultures, making it difficult to perform biochemical studies of trichothecene biosynthesis. To develop strains with higher levels of trichothecene production under liquid shake conditions we transformed F. graminearum with both a reporter gene containing a homologous trichothecene pathway gene promoter (TRI5) and a gene encoding a heterologous trichothecene pathway transcription factor (TRI6). The TRI5 and TRI6 genes are part of the trichothecene pathway gene clusters of both Fusarium sporotrichioides and F. graminearum. These genes encode trichodiene synthase (encoded by TRI5), the first enzyme in the trichothecene pathway, and a transcription factor (encoded by TRI6) required for pathway gene expression. Transformation of F. graminearum with plasmids containing either an F. graminearum TRI5 promoter fragment (FGTRI5(P)) or FGTRI5(P) coupled with the beta-D-glucuronidase (GUS) reporter gene resulted in the identification of several transformants capable of producing 45 to 200 mg of 15-acetyldeoxynivalenol (15-ADON)/liter in liquid shake culture after 7 days. Increased 15-ADON production was only observed in transformants where plasmid integration occurred through the FGTRI5(P) sequence and was not accompanied by increased GUS expression. 15-ADON production was further increased in liquid culture up to 1,200 mg/liter following introduction of the F. sporotrichioides TRI6 gene (FSTRI16) into F. graminearum. The effects of FSTRI6 on 15-ADON production also depended on plasmid integration via homologous recombination of the FGTRI5(P) fragment and resulted in a 100-fold increase in GUS expression. High-level production of 15-ADON in liquid shake cultures provides a convenient method for large-scale trichothecene preparation. The results suggest that targeting transformation vector integration to FGTRI5(P) alters pathway gene expression and are consistent with the proposed conservation of TRI6 function between Fusarium species.

Effect of cycling temperatures on the production of deoxynivalenol and zearalenone by Fusarium graminearum NRRL 5883

The effects of three regimens of cycling incubation temperatures and incubation at constant 25 degrees C on the growth of Fusarium graminearum NRRL 5883 and production of deoxynivalenol (DON) and zearalenone (ZEN) on rice were compared. The effects of low-temperature stress were also studied by incubating rice cultures at a constant 15 degrees C for 4 weeks following incubation at constant 25 degrees C for 2 weeks. Both incubation temperature and time significantly (P < or = 0.05) affected growth of F. graminearum NRRL 5883 and production of DON and ZEN. The highest amount of free ergosterol (640 microg/g culture material) that was used as a measure of fungal growth was found in cultures incubated at temperatures cycling between 15 and 30 degrees C during a 6-week period. The highest amounts of DON (1,679 microg/g culture material) and ZEN (603 microg/g culture material) were produced in cultures incubated at a constant 25 degrees C for 2 weeks prior to incubation at a constant 15 degrees C for an additional 4 weeks. Under cycling incubation temperatures, maximum amounts of DON (850 microg/g culture material) and ZEN (98 microg/g culture material) were produced in cultures incubated at temperatures cycling between 15 and 30 degrees C for 6 weeks. Overall, there was no correlation between mold growth and production of either DON or ZEN. However, DON production and ZEN production were correlated.

The trichothecene biosynthesis regulatory gene from the type B producer Fusarium strains: sequence of Tri6 and its expression in Escherichia coli

A genomic DNA fragment containing Tri6, a transcription activator gene of trichothecene biosynthesis, was cloned by vectorette PCR from Fusarium graminearum F15, which produces type B trichothecene, deoxynivalenol. The nucleotide sequence of the gene showed 84% of identity to that of the type A trichothecene producer Fusarium sporotrichioides NRRL 3299, but the sequence around the initiation codon was not highly conserved between these producers. Based on the upstream and downstream sequences of the coding region of F. graminearum, Tri6 could be amplified by PCR from other type B trichothecene producers. Tri6 appeared to be expressed for only a limited period prior to the toxin production.

Biosynthesis of trichothecenes and apotrichothecenes

Fusarium culmorum produces two major trichothecenes, 3-acetyldeoxynivalenol and sambucinol, and some minor apotrichothecenes. It was desired to investigate if during their biosynthesis a C-11-keto intermediate was involved. To verify this postulate, trichodiene, a known precursor to trichothecenes, was synthesized with two deuteriums at C-11 and one at C-15. It was then fed to F. culmorum cultures, and the derived metabolites were purified and analyzed. The results ruled out the involvement of an 11-keto intermediate but revealed two novel apotrichothecenes. The characterization of their structures suggested that one of the 2-hydroxy-11alpha-apotrichothecene stereoisomers (2alpha or 2beta) could be converted to sambucinol. These apotrichothecenes were therefore synthesized labeled specifically with two deuteriums at C-4 and C-15 and fed to F. culmorum cultures. Indeed, the result established for the first time that 2alpha-hydroxy-11alpha-apotrichothecene was a precursor to sambucinol. A biosynthetic scheme for the production of trichothecenes and apotrichothecenes is described.

Biosynthesis of 3-acetyldeoxynivalenol and sambucinol. Identification of the two oxygenation steps after trichodiene

The first two oxygenation steps post-trichodiene in the biosyntheses of the trichothecenes 3-acetyldeoxynivalenol and sambucinol were investigated. The plausible intermediates 2-hydroxytrichodiene (2alpha- and 2beta-) and 12,13-epoxytrichodiene and the dioxygenated compounds 12,13-epoxy-9,10-trichoene-2-ol (2alpha- and 2beta-) were prepared specifically labeled with stable isotopes. They were then fed separately and/or together to Fusarium culmorum cultures, and the derived trichothecenes were isolated, purified, and analyzed. The stable isotopes enable easy localization of the labels in the products by 2H NMR, 13C NMR, and mass spectrometry. We found that 2alpha-hydroxytrichodiene is the first oxygenated step in the biosynthesis of both 3-acetyldeoxynivalenol and sambucinol. The stereoisomer 2beta-hydroxytrichodiene and 12,13-epoxytrichodiene are not biosynthetic intermediates and have not been isolated as metabolites. We also demonstrated that the dioxygenated 12, 13-epoxy-9,10-trichoene-2alpha-ol is a biosynthetic precursor to trichothecenes as had been suggested in a preliminary work. Its stereoisomer was not found in the pathway. A further confirmation of our results was the isolation of both oxygenated trichodiene derivatives 2alpha-hydroxytrichodiene and 12,13-epoxy-9, 10-trichoene-2alpha-ol as natural metabolites in F. culmorum cultures.

Characterization of a transcriptional activator controlling trichothecene toxin biosynthesis

Trichothecene biosynthetic pathway genes are localized within a gene cluster in Fusarium sporotrichioides and require the zinc-finger containing protein, TRI6, for expression. We show here that TRI6 is able to bind within the promoter regions of nine different pathway genes and that TRI6 binding is involved in pathway gene activation. TRI6 binding occurs at three distinct sites in the TRI5 promoter, all of which contain the sequence TNAGGCCT. DNA fragments from the promoter regions of six other pathway genes containing this sequence are also substrates for TRI6 binding. Specific nucleotide changes in the TNAGGCCT sequence dramatically reduced TRI6 binding. Analysis of TRI6 binding within the TRI3 and TRI11 promoters and the TRI4-TRI6 intergenic region which do not contain the TNAGGCCT motif suggests that the minimum sequence required for TRI6 binding is YNAGGCC. Two potential TRI6 binding sites, T4A and T4B, were identified within the intergenic region for the divergently transcribed TRI4 and TRI6 genes. Alteration or deletion of the T4A site resulted in the loss of nearly all in vitro TRI6 binding and was correlated with the loss of promoter activity in vivo as measured by the expression of mutant TRI4(p)/GUS fusions. This establishes a physiological role for TRI6 binding and demonstrates that TRI6 is directly involved in the regulation of pathway gene expression. To determine if a predicted Cys2His2 zinc-finger motif at the C-terminus of TRI6 is involved in DNA binding, a C187A mutant was constructed in TRI6 using site-directed mutagenesis. The C187A mutant did not bind promoter DNA fragments, supporting the role of C187 in DNA binding. In addition, a TRI6 homologue in the distantly related macrocyclic trichothecene pathway of Myrothecium roridum (MRTRI6) was also shown to bind to the same TRI5 and TRI4 promoter fragments bound by TRI6. Together, these data confirm our previous proposal that TRI6 is an activator of trichothecene pathway gene expression and that DNA binding employs the C-terminal region of TRI6 containing three predicted Cys2His2 zinc fingers.

Group specific PCR-detection of potential trichothecene-producing Fusarium-species in pure cultures and cereal samples

A PCR based assay (Tox5 PCR) which analyses Fusarium species potentially producing trichothecenes was developed using a pair of primers derived from the DNA-sequence of the trichodiene synthase gene (tri5). The primer pair was tested using DNA isolated from a variety of strains representing 64 species and varieties of Fusarium as well as from other fungi, bacteria and cereals. A 658 bp PCR fragment was specifically amplified with DNA isolated from strains of species belonging to the Fusarium sections Discolor, Sporotrichiella, Arthrosporiella, Gibbosum, and "Dlaminia". PCR products obtained were sequenced. Alignment to tri5 sequences given in the literature revealed a high degree of homology. Results of the PCR developed correlated well with literature data on the trichothecene producing capabilities of the respective species. Potential trichothecene producing fusaria were detected in contaminated cereals and malts using the Tox5 PCR assay. Intensity of the signals produced were well correlated with the concentration of deoxynivalenol (DON) in samples of wheat.

Study of toxin production by isolates of Stachybotrys chartarum and Memnoniella echinata isolated during a study of pulmonary hemosiderosis in infants

A cluster of cases of pulmonary hemosiderosis among infants was reported in Cleveland, Ohio, during 1993 and 1994. These unusual cases appeared only in infants ranging in age from 1 to 8 months and were characterized by pulmonary hemorrhage, which caused the babies to cough up blood. A case-control study identified major home water damage (from plumbing leaks, roof leaks, or flooding) as a risk factor for development of pulmonary hemorrhage in these infants. Because of an interest in the possibility that trichothecene mycotoxins might be involved in this illness, a number of isolates of Stachybotrys chartarum were grown in the laboratory on rice, and extracts were prepared and analyzed both for cytotoxicity and for specific toxins. Two isolates of Memnoniella echinata, a fungus closely related to S. chartarum, were also included in these studies. S. chartarum isolates collected from the homes were shown to produce a number of highly toxic compounds, and the profiles of toxic compounds from M. echinata were similar; the most notable difference was the fact that the principal metabolites produced by M. echinata were griseofulvins.