Aflastatin A, a novel inhibitor of aflatoxin production by aflatoxigenic fungi

Isolation of aflatoxin biosynthetic inhibitor from Chondrostereum purpureum mushroom culture filtrate

Aflatoxins (AFs) are highly toxic mycotoxins produced by the fungi, Aspergillus flavus and Aspergillus parasiticus. AFs pose severe health risks owing to their acute toxicity and carcinogenic properties. The control of AF contamination remains significantly challenging despite the extensive efforts toward controlling it. Here, we investigated the potential of mushroom extracts as a source of AF biosynthetic inhibitors. The A. parasiticus mutant strain, NFRI-95, that accumulates an AF biosynthesis intermediate, norsolorinic acid, was used in the bioassay to detect the inhibitory activity against AF biosynthesis. The screening of 195 mushroom extracts revealed that the culture filtrate extract of Chondrostereum purpureum exhibited strong inhibitory activity against AF biosynthesis. Next, large-scale culturing of C. purpureum was performed to isolate the compounds accounting for the inhibitory activity. The culture filtrate was extracted with ethyl acetate, after which the active compound was isolated by silica gel column chromatography and preparative high performance liquid chromatography (HPLC). The active compound was identified as cyclo(Val-Pro) by spectroscopic analyses. Further, four stereoisomers of cyclo(Val-Pro) were synthesized by the condensation of the N-Boc derivatives of d- and l-valine with the methyl esters of d- and l-proline. The naturally isolated compound was identified as cyclo(l-Val-l-Pro) by comparing its retention time with those of synthetic compounds by chiral HPLC analysis and CD spectra. The IC50 value of cyclo(L-Val-L-Pro) was 2.4 mM, whereas the LD, DL, and DD isomers exhibited weaker activities, with IC50 values of >5 mM.

Mycotoxin Contamination Status of Cereals in China and Potential Microbial Decontamination Methods

The presence of mycotoxins in cereals can pose a significant health risk to animals and humans. China is one of the countries that is facing cereal contamination by mycotoxins. Treating mycotoxin-contaminated cereals with established physical and chemical methods can lead to negative effects, such as the loss of nutrients, chemical residues, and high energy consumption. Therefore, microbial detoxification techniques are being considered for reducing and treating mycotoxins in cereals. This paper reviews the contamination of aflatoxins, zearalenone, deoxynivalenol, fumonisins, and ochratoxin A in major cereals (rice, wheat, and maize). Our discussion is based on 8700 samples from 30 provincial areas in China between 2005 and 2021. Previous research suggests that the temperature and humidity in the highly contaminated Chinese cereal-growing regions match the growth conditions of potential antagonists. Therefore, this review takes biological detoxification as the starting point and summarizes the methods of microbial detoxification, microbial active substance detoxification, and other microbial inhibition methods for treating contaminated cereals. Furthermore, their respective mechanisms are systematically analyzed, and a series of strategies for combining the above methods with the treatment of contaminated cereals in China are proposed. It is hoped that this review will provide a reference for subsequent solutions to cereal contamination problems and for the development of safer and more efficient methods of biological detoxification.

Natural inhibitors: A sustainable way to combat aflatoxins

Among a few hundred mycotoxins, aflatoxins had always posed a major threat to the world. Apart from A. flavus, A. parasiticus, and A. nomius of Aspergillus genus, which are most toxin-producing strains, several fungal bodies including Fusarium, Penicillium, and Alternaria that can biosynthesis aflatoxins. Basically, there are four different types of aflatoxins (Aflatoxin B1 (AFB1), Aflatoxin B2 (AFB2), Aflatoxin G1 (AFG1), Aflatoxin G2 (AFG2)) are produced as secondary metabolites. There are certainly other types of aflatoxins found but they are the by-products of these toxins. The fungal agents generally infect the food crops during harvesting, storing, and/or transporting; making a heavy post-harvest as well as economic loss in both developed and developing countries. And while ingesting the crop products, these toxins get into the dietary system causing aflatoxicosis, liver cirrhosis, etc. Therefore, it is imperative to search for certain ways to control the spread of infections and/or production of these toxins which may also not harm the crop harvest. In this review, we are going to discuss some sustainable methods that can effectively control the spread of infection and inhibit the biosynthesis of aflatoxins.

Total Synthesis of Aflastatin A

The total syntheses of aflastatin A and its C3-C48 degradation fragment (6a, R = H) have been accomplished. The syntheses feature several complex diastereoselective fragment couplings, including a Felkin-selective trityl-catalyzed Mukaiyama aldol reaction, a chelate-controlled aldol reaction involving soft enolization with magnesium, and an anti-Felkin-selective boron-mediated oxygenated aldol reaction. Careful comparison of the spectroscopic data for the synthetic C3-C48 degradation fragment to that reported by the isolation group revealed a structural misassignment in the lactol region of the naturally derived degradation product. Ultimately, the data reported for the naturally derived aflastatin A C3-C48 degradation lactol (6a, R = H) were attributed to its derivative lactol trideuteriomethyl ether (6c, R = CD₃). Additionally, the revised absolute configurations of six stereogenic centers (C8, C9, and C28-C31) were confirmed.

Current Practices for Reference Gene Selection in RT-qPCR of Aspergillus: Outlook and Recommendations for the Future

Aspergillus is a genus of filamentous fungi with vast geographic and ecological distributions. Species within this genus are clinically, agriculturally and biotechnologically relevant, leading to increasing interest in elucidating gene expression dynamics of key metabolic and physiological processes. Reverse-transcription quantitative Polymerase Chain Reaction (RT-qPCR) is a sensitive and specific method of quantifying gene expression. A crucial step for comparing RT-qPCR results between strains and experimental conditions is normalisation to experimentally validated reference gene(s). In this review, we provide a critical analysis of current reference gene selection and validation practices for RT-qPCR gene expression analyses of Aspergillus. Of 90 primary research articles obtained through our PubMed query, 17 experimentally validated the reference gene(s) used. Twenty reference genes were used across the 90 studies, with beta-tubulin being the most used reference gene, followed by actin, 18S rRNA and glyceraldehyde 3-phosphate dehydrogenase. Sixteen of the 90 studies used multiple reference genes for normalisation. Failing to experimentally validate the stability of reference genes can lead to conflicting results, as was the case for four studies. Overall, our review highlights the need to experimentally validate reference genes in RT-qPCR studies of Aspergillus.

Control of Aflatoxigenic Molds by Antagonistic Microorganisms: Inhibitory Behaviors, Bioactive Compounds, Related Mechanisms, and Influencing Factors

Aflatoxin contamination has been causing great concern worldwide due to the major economic impact on crop production and their toxicological effects to human and animals. Contamination can occur in the field, during transportation, and also in storage. Post-harvest contamination usually derives from the pre-harvest infection of aflatoxigenic molds, especially aflatoxin-producing Aspergilli such as Aspergillusflavus and A. parasiticus. Many strategies preventing aflatoxigenic molds from entering food and feed chains have been reported, among which biological control is becoming one of the most praised strategies. The objective of this article is to review the biocontrol strategy for inhibiting the growth of and aflatoxin production by aflatoxigenic fungi. This review focuses on comparing inhibitory behaviors of different antagonistic microorganisms including various bacteria, fungi and yeasts. We also reviewed the bioactive compounds produced by microorganisms and the mechanisms leading to inhibition. The key factors influencing antifungal activities of antagonists are also discussed in this review.

Synthesis of C3-C21 Segment of Aflastatin A Using Remote Asymmetric Induction Reactions

The C3-C21 segment of aflastatin A has been synthesized by converging three segments including the C3-C8 segment, the C9-C15 segment, and the C16-C21 segment. Each segment has been synthesized from a vinylketene silyl N,O-acetal possessing a chiral auxiliary by a wide-range stereocontrol strategy. The C3-C8 segment was constructed in seven steps including the stereoselective vinylogous Mukaiyama alkylation, while the C9-C15 segment and the C16-C21 segment were synthesized using the vinylogous Mukaiyama aldol reaction in seven and eight steps, respectively.

Aflatoxin-Exposure of Vibrio gazogenes as a Novel System for the Generation of Aflatoxin Synthesis Inhibitors

Aflatoxin is a mycotoxin and a secondary metabolite, and the most potent known liver carcinogen that contaminates several important crops, and represents a significant threat to public health and the economy. Available approaches reported thus far have been insufficient to eliminate this threat, and therefore provide the rational to explore novel methods for preventing aflatoxin accumulation in the environment. Many terrestrial plants and microbes that share ecological niches and encounter the aflatoxin producers have the ability to synthesize compounds that inhibit aflatoxin synthesis. However, reports of natural aflatoxin inhibitors from marine ecosystem components that do not share ecological niches with the aflatoxin producers are rare. Here, we show that a non-pathogenic marine bacterium, Vibrio gazogenes, when exposed to low non-toxic doses of aflatoxin B₁, demonstrates a shift in its metabolic output and synthesizes a metabolite fraction that inhibits aflatoxin synthesis without affecting hyphal growth in the model aflatoxin producer, Aspergillus parasiticus. The molecular mass of the predominant metabolite in this fraction was also different from the known prodigiosins, which are the known antifungal secondary metabolites synthesized by this Vibrio. Gene expression analyses using RT-PCR demonstrate that this metabolite fraction inhibits aflatoxin synthesis by down-regulating the expression of early-, middle-, and late- growth stage aflatoxin genes, the aflatoxin pathway regulator, aflR and one global regulator of secondary metabolism, laeA. Our study establishes a novel system for generation of aflatoxin synthesis inhibitors, and emphasizes the potential of the under-explored Vibrio’s silent genome for generating new modulators of fungal secondary metabolism.

Reduction of aflatoxin production by Aspergillus flavus and Aspergillus parasiticus in interaction with Streptomyces

The aim of this study is to investigate aflatoxin gene expression during Streptomyces-Aspergillus interaction. Aflatoxins are carcinogenic compounds produced mainly by Aspergillus flavus and Aspergillus parasiticus. A previous study has shown that Streptomyces-A. flavus interaction can reduce aflatoxin content in vitro. Here, we first validated this same effect in the interaction with A. parasiticus. Moreover, we showed that growth reduction and aflatoxin content were correlated in A. parasiticus but not in A. flavus. Secondly, we investigated the mechanisms of action by reverse-transcriptase quantitative PCR. As microbial interaction can lead to variations in expression of household genes, the most stable [act1, βtub (and cox5 for A. parasiticus)] were chosen using geNorm software. To shed light on the mechanisms involved, we studied during the interaction the expression of five genes (aflD, aflM, aflP, aflR and aflS). Overall, the results of aflatoxin gene expression showed that Streptomyces repressed gene expression to a greater level in A. parasiticus than in A. flavus. Expression of aflR and aflS was generally repressed in both Aspergillus species. Expression of aflM was repressed and was correlated with aflatoxin B1 content. The results suggest that aflM expression could be a potential aflatoxin indicator in Streptomyces species interactions. Therefore, we demonstrate that Streptomyces can reduce aflatoxin production by both Aspergillus species and that this effect can be correlated with the repression of aflM expression.

Streptomyces-Aspergillus flavus interactions: impact on aflatoxin B accumulation

The aim of this work was to investigate the potential of Streptomyces sp. as biocontrol agents against aflatoxins in maize. As such, we assumed that Streptomyces sp. could provide a complementary approach to current biocontrol systems such as Afla-guard(®) and we focused on biocontrol that was able to have an antagonistic contact with A. flavus. A previous study showed that 27 (out of 38) Streptomyces sp. had mutual antagonism in contact with A. flavus. Among these, 16 Streptomyces sp. were able to reduce aflatoxin content to below 17% of the residual concentration. We selected six strains to understand the mechanisms involved in the prevention of aflatoxin accumulation. Thus, in interaction with A. flavus, we monitored by RT-qPCR the gene expression of aflD, aflM, aflP, aflR and aflS. All the Streptomyces sp. were able to reduce aflatoxin concentration (24.0-0.2% residual aflatoxin B1). They all impacted on gene expression, but only S35 and S38 were able to repress expression significantly. Indeed, S35 significantly repressed aflM expression and S38 significantly repressed aflR, aflM and aflP. S6 reduced aflatoxin concentrations (2.3% residual aflatoxin B1) and repressed aflS, aflM and enhanced aflR expression. In addition, the S6 strain (previously identified as the most reducing pure aflatoxin B1) was further tested to determine a potential adsorption mechanism. We did not observe any adsorption phenomenon. In conclusion, this study showed that Streptomyces sp. prevent the production of (aflatoxin gene expression) and decontamination of (aflatoxin B1 reduction) aflatoxins in vitro.

In vitro interaction of actinomycetes isolates with Aspergillus flavus: impact on aflatoxins B1 and B2 production

This work aimed to study the interaction between Actinomycetal isolates and Aspergillus flavus to promote mutual antagonism in contact. Thirty-seven soilborn Streptomyces spp. isolates were chosen as potential candidates. After a 10-day in vitro co-incubation period, 27 isolates respond to the criteria, that is, mutual antagonism in contact. Further aflatoxins B1 and B2 analysis revealed that those 27 isolates reduced aflatoxin B1 residual concentration from 38·6 to 4·4%, depending on the isolate. We selected 12 isolates and tested their capacity to reduce AFB1 in pure culture to start identifying the mechanisms involved in its reduction. AFB1 was reduced by eight isolates. The remaining AFB1 concentration varied between 82·2 and 15·6%. These findings led us to suggest that these eight isolates could be used as biocontrol agents against AFB1 and B2 with low risk of impacting the natural microbial equilibrium.

SIGNIFICANCE AND IMPACT OF THE STUDY

Interaction between Aspergillus flavus and Actinomycetes isolates was conducted in vitro. Actinomycetes isolates having a mutual antagonism in contact with A. flavus were chosen for further aflatoxins production study. This is a new approach based to develop biocontrol against aflatoxins accumulation in maize while respecting natural microbial equilibrium.

Naturally occurring tetramic acid products: isolation, structure elucidation and biological activity

Natural products containing the tetramic acid core scaffold have been isolated from an assortment of terrestrial and marine species and often display wide ranging and potent biological activities including antibacterial, antiviral and antitumoral activities.

Conversion of 11-hydroxy-O-methylsterigmatocystin to aflatoxin G1 in Aspergillus parasiticus

In aflatoxin biosynthesis, aflatoxins G(1) (AFG(1)) and B(1) (AFB(1)) are independently produced from a common precursor, O-methylsterigmatocystin (OMST). Recently, 11-hydroxy-O-methylsterigmatocystin (HOMST) was suggested to be a later precursor involved in the conversion of OMST to AFB(1), and conversion of HOMST to AFB(1) was catalyzed by OrdA enzyme. However, the involvement of HOMST in AFG(1) formation has not been determined. In this work, HOMST was prepared by incubating OrdA-expressing yeast with OMST. Feeding Aspergillus parasiticus with HOMST allowed production of AFG(1) as well as AFB(1). In cell-free systems, HOMST was converted to AFG(1) when the microsomal fraction, the cytosolic fraction from A. parasiticus, and yeast expressing A. parasiticus OrdA were added. These results demonstrated (1) HOMST is produced from OMST by OrdA, (2) HOMST is a precursor of AFG(1) as well as AFB(1), and (3) three enzymes, OrdA, CypA, and NadA, and possibly other unknown enzymes are involved in conversion of HOMST to AFG(1).

Volatile profiling reveals intracellular metabolic changes in Aspergillus parasiticus: veA regulates branched chain amino acid and ethanol metabolism

Background

Filamentous fungi in the genus Aspergillus produce a variety of natural products, including aflatoxin, the most potent naturally occurring carcinogen known. Aflatoxin biosynthesis, one of the most highly characterized secondary metabolic pathways, offers a model system to study secondary metabolism in eukaryotes. To control or customize biosynthesis of natural products we must understand how secondary metabolism integrates into the overall cellular metabolic network. By applying a metabolomics approach we analyzed volatile compounds synthesized by Aspergillus parasiticus in an attempt to define the association of secondary metabolism with other metabolic and cellular processes.

Results

Volatile compounds were examined using solid phase microextraction - gas chromatography/mass spectrometry. In the wild type strain Aspergillus parasiticus SU-1, the largest group of volatiles included compounds derived from catabolism of branched chain amino acids (leucine, isoleucine, and valine); we also identified alcohols, esters, aldehydes, and lipid-derived volatiles. The number and quantity of the volatiles produced depended on media composition, time of incubation, and light-dark status. A block in aflatoxin biosynthesis or disruption of the global regulator veA affected the volatile profile. In addition to its multiple functions in secondary metabolism and development, VeA negatively regulated catabolism of branched chain amino acids and synthesis of ethanol at the transcriptional level thus playing a role in controlling carbon flow within the cell. Finally, we demonstrated that volatiles generated by a veA disruption mutant are part of the complex regulatory machinery that mediates the effects of VeA on asexual conidiation and sclerotia formation.

Conclusions

1) Volatile profiling provides a rapid, effective, and powerful approach to identify changes in intracellular metabolic networks in filamentous fungi. 2) VeA coordinates the biosynthesis of secondary metabolites with catabolism of branched chain amino acids, alcohol biosynthesis, and β-oxidation of fatty acids. 3) Intracellular chemical development in A. parasiticus is linked to morphological development. 4) Understanding carbon flow through secondary metabolic pathways and catabolism of branched chain amino acids is essential for controlling and customizing production of natural products.

Antineoplastic activities of MT81 and its structural analogue in Ehrlich ascites carcinoma-bearing Swiss Albino mice

Many fungal toxins exhibit in vitro and in vivo antineoplastic effects on various cancer cell types. Luteoskyrin,a hydroxyanthraquinone has been proved to be a potent inhibitor against Ehrlich ascites tumor cells. The comparative antitumor activity and antioxidant status of MT81 and its structural analogue [Acetic acid-MT81 (Aa-MT81)] having polyhydroxyanthraquinone structure were assessed against Ehrlich ascites carcinoma (EAC) tumor in mice. The in vitro cytotoxicity was measured by the viability of EAC cells after direct treatment of the said compounds. In in vivo study, MT81 and its structural analogue were administered (i.p.) at the two different doses (5, 7 mg MT81; 8.93, 11.48 mg Aa-MT81/kg body weight) for 7 days after 24 hrs. of tumor inoculation. The activities were assessed using mean survival time (MST), increased life span (ILS), tumor volume, viable tumor cell count, peritoneal cell count, protein percentage and hematological parameters. Antioxidant status was determined by malondialdehyde (MDA) and reduced glutathione (GSH) content, and by the activity of superoxide dismutase (SOD) and catalase (CAT). MT81 and its structural analogues increased the mean survival time, normal peritoneal cell count. They decreased the tumor volume, viable tumor cell count, hemoglobin percentage and packed cell volume. Differential counts of WBC, total counts of RBC & WBC that altered by EAC inoculation, were restored in a dose-dependent manner. Increased MDA and decreased GSH content and reduced activity of SOD, and catalase in EAC bearing mice were returned towards normal after the treatment of MT81 and its structural analogue. Being less toxic than parent toxin MT81, the structural analogue showed more prominent antineoplastic activities against EAC cells compared to MT81. At the same time, both compounds exhibit to some extent antioxidant potential for the EAC-bearing mice.

Isolation of microorganisms and substances inhibitory to aflatoxin production

Aflatoxins are toxic and carcinogenic secondary metabolites produced by Aspergillus flavus and A. parasiticus. The contamination of crops, feeds, and foods with aflatoxins can have serious effects on the health of humans and animals. Although many studies have been done to develop aflatoxin-control strategies, most are limited in their effectiveness. As part of an effort to develop control procedures, we have devised simple and safe methods that are useful for identifying microorganisms that effectively inhibit aflatoxin production by fungi. These include the microtitre agar plate assay using norsolorinic acid-accumulating mutant fungi, the ultraviolet light photography method using an instant film, the tip culture method, a convenient RNA extraction method for reverse transcription-polymerase chain reaction (RT-PCR) analysis, and other methods. Results of a recent trial have shown that Achromobacter xylosoxidans significantly inhibited aflatoxin production by A. parsiticus, and that the main inhibitory substance produced by the bacterium was cyclo(L-leucyl-L-prolyl). This result confirms that the methods described herein are useful for identifying microorganisms that inhibit aflatoxin production by fungi and could contribute to the development of methods to reduce aflatoxin contamination in commodities.

Dioctatin A is a strong inhibitor of aflatoxin production by Aspergillus parasiticus

Dioctatin A (DotA), a metabolite of Streptomyces, is known to be an inhibitor of human dipeptidyl aminopeptidase II. Here, it was found that DotA strongly inhibited aflatoxin production by Aspergillus parasiticus, with an IC50 value of 4.0 microM. The mycelial growth of the fungus was not affected by the addition of DotA at a concentration of 50 microM, but inhibition of conidiation was observed at the same concentration. DotA inhibited production of norsolorinic acid, an early biosynthetic intermediate of aflatoxin, and it strongly reduced the mRNA levels of genes encoding aflatoxin biosynthetic enzymes, and significantly decreased the mRNA level of aflR, which encodes a key regulatory protein for aflatoxin biosynthesis. In addition to these genes, the mRNA level of brlA, which encodes a conidiation-specific transcription factor, was also reduced by the addition of DotA. It was also found that DotA dramatically enhanced kojic acid production by the fungus. Furthermore, DotA inhibited production of sterigmatocystin, which is a toxic aflatoxin biosynthetic intermediate, and it also inhibited conidiation in Aspergillus nidulans. These results indicate that DotA has pleiotropic effects on regulatory mechanisms of fungal secondary metabolite production and differentiation, leading to inhibition of aflatoxin production.

Ultrastructural evidences of growth inhibitory effects of a novel biocide, Akacid(plus), on an aflatoxigenic Aspergillus parasiticus

The effects of Akacid(plus), a novel member of guanidine-based polymeric compounds recently introduced as a potent inhibitor of fungal growth and aflatoxin biosynthesis were studied on Aspergillus parasiticus by transmission electron microscopy (TEM). The toxigenic fungus was cultured on yeast extract-sucrose broth in presence of serial two-fold concentrations of Akacid(plus) (1.5-96 microL/50 mL medium) for 96 h at 28 degrees C with shaking. Mycelial samples exposed to fungistatic concentrations of compound (1.5-48 microL) were processed for TEM. Corresponding to the growth inhibition, TEM observations revealed morphological anomalies in fungal compartments. The results demonstrated that Akacid(plus) targets the plasma membrane of the hyphae by its breaking down at variable intervals with the formation of small membrane-bound vesicles inside the cytoplasm, while no obvious damage was observed on the cell wall. A marked depletion of cytoplasmic contents of hyphae accompanied with lysis and disruption of membranes of major organelles such as nuclei, mitochondria and endoplasmic reticulum indicates that in high fungistatic concentrations, Akacid(plus) passes not only through the cell wall but also through the plasma membrane and then interact with membranous structures of the cytoplasmic organelles. Ultrastructural changes of fungal compartments exposed to Akacid(plus) in relation to the fungal growth and aflatoxin biosynthesis are discussed.

Isolation of bacterial antagonists of Aspergillus flavus from almonds

Bacteria were isolated from California almond orchard samples to evaluate their potential antifungal activity against aflatoxin-producing Aspergillus flavus. Fungal populations from the same samples were examined to determine the incidence of aflatoxigenic Aspergillus species. Antagonistic activities of the isolated bacterial strains were screened against a nonaflatoxigenic nor mutant of A. flavus, which accumulates the pigmented aflatoxin precursor norsolorinic acid (NOR) under conditions conducive to aflatoxin production. Using solid and liquid media in coculture assays, 171 bacteria isolated from almond flowers, immature nut fruits, and mature nut fruits showed inhibition of A. flavus growth and/or inhibition of NOR accumulation. Bacterial isolates were further characterized for production of extracellular enzymes capable of hydrolyzing chitin or yeast cell walls. Molecular and physiological identification of the bacterial strains indicated that the predominant genera isolated were Bacillus, Pseudomonas, Ralstonia, and Burkholderia, as well as several plant-associated enteric and nonenteric bacteria. A set of 20 isolates was selected for further study based on their species identification, antifungal phenotypes, and extracellular enzyme production. Quantitative assays using these isolates in liquid coculture with a wild-type, aflatoxin-producing A. flavus strain showed that a number of strains completely inhibited fungal growth in three different media. These results indicate the potential for development of bacterial antagonists as biological control agents against aflatoxigenic aspergilli on almonds.

Cyclo(L-leucyl-L-prolyl) produced by Achromobacter xylosoxidans inhibits aflatoxin production by Aspergillus parasiticus

Aflatoxins are potent carcinogenic and toxic substances that are produced primarily by Aspergillus flavus and Aspergillus parasiticus. We found that a bacterium remarkably inhibited production of norsolorinic acid, a precursor of aflatoxin, by A. parasiticus. This bacterium was identified as Achromobacter xylosoxidans based on its 16S ribosomal DNA sequence and was designated A. xylosoxidans NFRI-A1. A. xylosoxidans strains commonly showed similar inhibition. The inhibitory substance(s) was excreted into the medium and was stable after heat, acid, or alkaline treatment. Although the bacterium appeared to produce several inhibitory substances, we finally succeeded in purifying a major inhibitory substance from the culture medium using Diaion HP20 column chromatography, thin-layer chromatography, and high-performance liquid chromatography. The purified inhibitory substance was identified as cyclo(L-leucyl-L-prolyl) based on physicochemical methods. The 50% inhibitory concentration for aflatoxin production by A. parasiticus SYS-4 (= NRRL2999) was 0.20 mg ml(-1), as determined by the tip culture method. High concentrations (more than 6.0 mg ml(-1)) of cyclo(L-leucyl-L-prolyl) further inhibited fungal growth. Similar inhibitory activities were observed with cyclo(D-leucyl-D-prolyl) and cyclo(L-valyl-L-prolyl), whereas cyclo(D-prolyl-L-leucyl) and cyclo(L-prolyl-D-leucyl) showed weaker activities. Reverse transcription-PCR analyses showed that cyclo(L-leucyl-L-prolyl) repressed transcription of the aflatoxin-related genes aflR, hexB, pksL1, and dmtA. This is the first report of a cyclodipeptide that affects aflatoxin production.

Involvement of two cytosolic enzymes and a novel intermediate, 5'-oxoaverantin, in the pathway from 5'-hydroxyaverantin to averufin in aflatoxin biosynthesis

During aflatoxin biosynthesis, 5'-hydroxyaverantin (HAVN) is converted to averufin (AVR). Although we had previously suggested that this occurs in one enzymatic step, we demonstrate here that this conversion is composed of two enzymatic steps by showing that the two enzyme activities in the cytosol fraction of Aspergillus parasiticus were clearly separated by Mono Q column chromatography. An enzyme, HAVN dehydrogenase, catalyzes the first reaction from HAVN to a novel intermediate, another new enzyme catalyzes the next reaction from the intermediate to AVR, and the intermediate is a novel substance, 5'-oxoaverantin (OAVN), which was determined by physicochemical methods. We also purified both of the enzymes, HAVN dehydrogenase and OAVN cyclase, from the cytosol fraction of A. parasiticus by using ammonium sulfate fractionation and successive chromatographic steps. The HAVN dehydrogenase is a homodimer composed of 28-kDa subunits, and it requires NAD, but not NADP, as a cofactor for its activity. Matrix-assisted laser desorption ionization-time of flight mass spectrometry analysis of tryptic peptides of the purified HAVN dehydrogenase revealed that this enzyme coincides with a protein deduced from the adhA gene in the aflatoxin gene cluster of A. parasiticus. Also, the OAVN cyclase enzyme is a homodimer composed of 79-kDa subunits which does not require any cofactor for its activity. Further characterizations of both enzymes were performed.

Inhibitory effect of aflastatin A on melanin biosynthesis by Colletotrichum lagenarium

The effect of aflastatin A (AsA), a novel inhibitor of aflatoxin production, on melanin biosynthesis of Colletotrichum lagenarium was examined. Addition of a low concentration of AsA (0.5 microg ml(-1)) to the culture medium almost completely inhibited the melanin production of this organism. AsA also inhibited the production of scytalone, an early intermediate of melanin biosynthesis. Melanin production was restored by addition of exogenous scytalone in the presence of AsA, suggesting that the late steps after the synthesis of scytalone were not significantly affected by AsA. This was confirmed by the results from RT-PCR analysis of the expression of genes encoding melanin biosynthetic enzymes (SCD1, THR1) and a regulatory gene (CMR1). By contrast, expression of PKS1 was severely impaired by AsA, although catalytic activity of a polyketide synthase (PKS1) was not inhibited by AsA. These results indicate that AsA inhibits an early step in melanin production, which suppresses the expression of PKS1.