Regulated expression of the nor-1 and ver-1 genes associated with aflatoxin biosynthesis

Activation of Aflatoxin Biosynthesis Alleviates Total ROS in Aspergillus parasiticus

An aspect of mycotoxin biosynthesis that remains unclear is its relationship with the cellular management of reactive oxygen species (ROS). Here we conduct a comparative study of the total ROS production in the wild-type strain (SU-1) of the plant pathogen and aflatoxin producer, Aspergillus parasiticus, and its mutant strain, AFS10, in which the aflatoxin biosynthesis pathway is blocked by disruption of its pathway regulator, aflR. We show that SU-1 demonstrates a significantly faster decrease in total ROS than AFS10 between 24 h to 48 h, a time window within which aflatoxin synthesis is activated and reaches peak levels in SU-1. The impact of aflatoxin synthesis in alleviation of ROS correlated well with the transcriptional activation of five superoxide dismutases (SOD), a group of enzymes that protect cells from elevated levels of a class of ROS, the superoxide radicals (O₂^-). Finally, we show that aflatoxin supplementation to AFS10 growth medium results in a significant reduction of total ROS only in 24 h cultures, without resulting in significant changes in SOD gene expression. Our findings show that the activation of aflatoxin biosynthesis in A. parasiticus alleviates ROS generation, which in turn, can be both aflR dependent and aflatoxin dependent.

The Fungal bZIP Transcription Factor AtfB Controls Virulence-Associated Processes in Aspergillus parasiticus

Fungal basic leucine zipper (bZIP) transcription factors mediate responses to oxidative stress. The ability to regulate stress response pathways in Aspergillus spp. was postulated to be an important virulence-associated cellular process, because it helps establish infection in humans, plants, and animals. Previous studies have demonstrated that the fungal transcription factor AtfB encodes a protein that is associated with resistance to oxidative stress in asexual conidiospores, and AtfB binds to the promoters of several stress response genes. Here, we conducted a gene silencing of AtfB in Aspergillus parasiticus, a well-characterized fungal pathogen of plants, animals, and humans that produces the secondary metabolite and carcinogen aflatoxin, in order to determine the mechanisms by which AtfB contributes to virulence. We show that AtfB silencing results in a decrease in aflatoxin enzyme levels, the down-regulation of aflatoxin accumulation, and impaired conidiospore development in AtfB-silenced strains. This observation is supported by a decrease of AtfB protein levels, and the down-regulation of many genes in the aflatoxin cluster, as well as genes involved in secondary metabolism and conidiospore development. Global expression analysis (RNA Seq) demonstrated that AtfB functionally links oxidative stress response pathways to a broader and novel subset of target genes involved in cellular defense, as well as in actin and cytoskeleton arrangement/transport. Thus, AtfB regulates the genes involved in development, stress response, and secondary metabolism in A. parasiticus. We propose that the bZIP regulatory circuit controlled by AtfB provides a large number of excellent cellular targets to reduce fungal virulence. More importantly, understanding key players that are crucial to initiate the cellular response to oxidative stress will enable better control over its detrimental impacts on humans.

Inhibitory Effect of Cinnamaldehyde, Citral, and Eugenol on Aflatoxin Biosynthetic Gene Expression and Aflatoxin B1 Biosynthesis in Aspergillus flavus

In order to reveal the inhibitory effects of cinnamaldehyde, citral, and eugenol on aflatoxin biosynthesis, the expression levels of 5 key aflatoxin biosynthetic genes were evaluated by real-time PCR. Aspergillus flavus growth and AFB1 production were completely inhibited by 0.80 mmol/L of cinnamaldehyde and 2.80 mmol/L of citral. However, at lower concentration, cinnamaldehyde (0.40 mmol/L), eugenol (0.80 mmol/L), and citral (0.56 mmol/L) significantly reduced AFB1 production with inhibition rate of 68.9%, 95.4%, and 41.8%, respectively, while no effect on fungal growth. Real-time PCR showed that the expressions of aflR, aflT, aflD, aflM, and aflP were down-regulated by cinnamaldehyde (0.40 mmol/L), eugenol (0.80 mmol/L), and citral (0.56 mmol/L). In the presence of cinnamaldehyde, AflM was highly down-regulated (average of 5963 folds), followed by aflP, aflR, aflD, and aflT with the average folds of 55, 18, 6.5, and 5.8, respectively. With 0.80 mmol/L of eugenol, aflP was highly down-regulated (average of 2061-folds), followed by aflM, aflR, aflD, and aflT with average of 138-, 15-, 5.2-, and 4.8-folds reduction, respectively. With 0.56 mmol/L of citral, aflT was completely inhibited, followed by aflM, aflP, aflR, and aflD with average of 257-, 29-, 3.5-, and 2.5-folds reduction, respectively. These results suggest that the reduction in AFB1 production by cinnamaldehyde, eugenol, and citral at low concentration may be due to the down-regulations of the transcription level of aflatoxin biosynthetic genes. Cinnamaldehyde and eugenol may be employed successfully as a good candidate in controlling of toxigenic fungi and subsequently contamination with aflatoxins in practice.

Evaluation of allium and its seasoning on toxigenic, nutritional, and sensorial profiles of groundnut oil

Mitigation of xerophilic storage fungi-associated aflatoxin threat in culinary oil will be a new technology advantage to food industries. Groundnut oil isolate Aspergillus flavus MTCC 10680 susceptibility to Allium species (A. sativum L., A. cepa L., and A. cepa var. aggregatum) extracts, composition, and in silico confirmation of extract's phytoconstituent aflatoxin synthesis inhibition were determined. The behavior of seasoning carrier medium groundnut oil in the presence of Allium was also determined. All the Allium species extracts exhibited concentration dependent in vitro inhibition on mycelial biomass, radial growth, and toxin elaboration. The gas chromatography-mass spectrometry revealed the presence of 28, 16, and 9 compounds in the extracts of A. sativum, A. cepa, A. cepa var. aggregatum, respectively. The Allium phytocostituents-like hexadecanoic acid, 5-Octanoyl-2,4,6(1H,3H,5H)-pyrimidinetrione, Guanosine, and so on, showed higher binding energy with aflatoxin synthesis key enzyme ver1. Allium seasoning increased the typical nutty odor of the groundnut oil with sweet aroma note as well as intensification of pale yellow color. Allium seasoning exhibited the highest aflatoxin detoxification and aroma development without any nutritional loss. Culinary oil Allium seasoning has anti-aflatoxin and food additive potential for use in food industries.

Analysis of volatile compounds emitted by filamentous fungi using solid-phase microextraction-gas chromatography/mass spectrometry

Here, we describe a solid-phase microextraction-gas chromatography/mass spectrometry (SPME-GC/MS) analytical approach that identifies and analyzes volatile compounds in the headspace above a live fungal culture. This approach is a sensitive, solvent-free, robust technique; most importantly from a practical standpoint, this approach is noninvasive and requires minimal sample handling. Aliquots of liquid fungal cultures are placed into vials equipped with inert septa and equilibrated at a constant temperature, and headspace gases are sampled using an SPME fiber inserted through the septum into the headspace above the fungal culture for a standardized period of time. The outer polymer coating of a fused silica fiber absorbs volatiles from the headspace; the volatiles are then desorbed in the hot GC inlet and chromatographed in the usual manner. The separated compounds are subsequently identified by mass spectrometry. All steps in volatile profiling of a single sample from volatile sorption on a fiber to obtaining a list of volatiles can take as little as 15 min or can be extended to several hours if longer sorption is required for compounds present at very low levels and/or have low rates of diffusion.

Expression of genes of the aflatoxin biosynthetic pathway in Aspergillus flavus isolates from keratitis

PURPOSE

To document transcriptional activation (expression) of key aflatoxin biosynthetic pathway genes in corneal isolates of Aspergillus flavus.

METHODS

The expression of certain regulatory (aflatoxin regulatory [aflR] and aflatoxin J [aflJ]) and structural (polyketide synthase acetate [pksA] and norsolonic acid-1 [nor-1]) genes in four corneal A. flavus isolates was evaluated by reverse transcription PCR. The aflatoxin-producing potential of each strain was determined by thin-layer chromatography and quantified by spectrophotometry. Four environmental isolates were used for comparison. The mean expression levels of these genes were compared in the corneal and environmental A. flavus isolates. In addition, the mean expression levels were also correlated with the aflatoxin production levels.

RESULTS

All isolates expressed aflJ, nor-1, and pksA, while all but one expressed aflR. Overall, significantly higher mean expression levels occurred in aflatoxigenic than in non-aflatoxigenic corneal isolates. A significant positive correlation was noted between the mean expression level of aflR and the quantum of aflatoxin production by the corneal isolates. Essentially similar patterns of expression of these genes were noted in four environmental A. flavus isolates used for comparison.

CONCLUSIONS

For the first time, isolates of A. flavus from human keratitis patients have been shown to express regulatory and structural aflatoxin biosynthetic pathway genes. Further studies are needed to clarify the precise influence of the corneal microenvironment on expression of these genes and aflatoxin production by A. flavus infecting the cornea.

Genetics of dothistromin biosynthesis in the peanut pathogen Passalora arachidicola

The peanut leaf spot pathogen Passalora arachidicola (Mycosphaerella arachidis) is known to produce dothistromin, a mycotoxin related to aflatoxin. This is a feature shared with the pine needle pathogen Dothistroma septosporum (Mycosphaerella pini). Dothistromin biosynthesis in D. septosporum commences at an unusually early stage of growth in culture compared to most other fungal secondary metabolites, and the biosynthetic genes are arranged in fragmented groups, in contrast to aflatoxin gene clusters. Dothistromin biosynthetic genes were identified and studied in P. arachidicola to determine if the attributes described in D. septosporum are shared by another dothistromin-producing species within the Class Dothideomycetes. It was shown that dothistromin biosynthesis is very similar in the two species with regard to gene sequence and gene synteny. Functional complementation of D. septosporum mutants with P. arachidicola dothistromin genes was also possible. These similarities support a vertical mode of dothistromin gene transmission. P. arachidicola also produced dothistromin at an early growth stage in culture, suggesting that this type of regulation pattern may be relevant to the biological role of dothistromin.

Stress-related transcription factor AtfB integrates secondary metabolism with oxidative stress response in aspergilli

In filamentous fungi, several lines of experimental evidence indicate that secondary metabolism is triggered by oxidative stress; however, the functional and molecular mechanisms that mediate this association are unclear. The basic leucine zipper (bZIP) transcription factor AtfB, a member of the bZIP/CREB family, helps regulate conidial tolerance to oxidative stress. In this work, we investigated the role of AtfB in the connection between oxidative stress response and secondary metabolism in the filamentous fungus Aspergillus parasiticus. This well characterized model organism synthesizes the secondary metabolite and carcinogen aflatoxin. Chromatin immunoprecipitation with specific anti-AtfB demonstrated AtfB binding at promoters of seven genes in the aflatoxin gene cluster that carry CREs. Promoters lacking CREs did not show AtfB binding. The binding of AtfB to the promoters occurred under aflatoxin-inducing but not under aflatoxin-noninducing conditions and correlated with activation of transcription of the aflatoxin genes. Deletion of veA, a global regulator of secondary metabolism and development, nearly eliminated this binding. Electrophoretic mobility shift analysis demonstrated that AtfB binds to the nor-1 (an early aflatoxin gene) promoter at a composite regulatory element that consists of highly similar, adjacent CRE1 and AP-1-like binding sites. The five nucleotides immediately upstream from CRE1, AGCC(G/C), are highly conserved in five aflatoxin promoters that demonstrate AtfB binding. We propose that AtfB is a key player in the regulatory circuit that integrates secondary metabolism and cellular response to oxidative stress.

Control of aflatoxin production of Aspergillus flavus and Aspergillus parasiticus using RNA silencing technology by targeting aflD (nor-1) gene

Aspergillus flavus and Aspergillus parasiticus are important pathogens of cotton, corn, peanuts and other oil-seed crops, producing toxins both in the field and during storage. We have designed three siRNA sequences (Nor-Ia, Nor-Ib, Nor-Ic) to target the mRNA sequence of the aflD gene to examine the potential for using RNA silencing technology to control aflatoxin production. Thus, the effect of siRNAs targeting of two key genes in the aflatoxin biosynthetic pathway, aflD (structural) and aflR (regulatory gene) and on aflatoxin B₁ (AFB₁), and aflatoxin G₁ (AFG₁) production was examined. The study showed that Nor-Ib gave a significant decrease in aflD mRNA, aflR mRNA abundance, and AFB₁ production (98, 97 and 97% when compared to the controls) in A. flavus NRRL3357, respectively. Reduction in aflD and aflR mRNA abundance and AFB₁ production increased with concentration of siRNA tested. There was a significant inhibition in aflD and AFB₁ production by A. flavus EGP9 and AFG₁ production by A. parasiticus NRRL 13005. However, there was no significant decrease in AFG₁ production by A. parasiticus SSWT 2999. Changes in AFB₁ production in relation to mRNA levels of aflD showed a good correlation (R = 0.88; P = 0.00001); changes in aflR mRNA level in relation to mRNA level of aflD also showed good correlation (R = 0.82; P = 0.0001). The correlations between changes in aflR and aflD gene expression suggests a strong relationship between these structural and regulatory genes, and that aflD could be used as a target gene to develop efficient means for aflatoxin control using RNA silencing technology.

Effects of sub-lethal food grade antioxidant doses and environmental stressors on growth, sclerotia, aflatoxins and aflD (nor-1) expression by Aspergillus parasiticus RCP08300

The aim of the work was to examine the effects of sub-lethal doses of butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) (1+1 mmol/l and 5+5 mmol/l), water activities (0.98, 0.95, 0.93, 0.89 aW) and temperatures (28, 20 °C) on growth, sclerotial characteristics, aflatoxin accumulation and aflD (=nor-1) transcript level by Aspergillus parasiticus RCP08300 on peanut based medium. Growth rate and aflatoxin production were inhibited by BHA-BHT mixture (1+1 mmol/l), regardless of environmental factor assayed. Although sclerotia number and aflD expression were stimulated by this treatment, sclerotia dry weight and volume were reduced by 62.3 and 31.2%, respectively. In contrast, when the fungus grew in presence of the higher dose of BHA-BHT mixture none or very low aflatoxin accumulation and aflD expression occurred. Similarly, A. parasiticus growth has been highly influenced by BHA-BHT (5+5 mmol/l) and interacting stress factors. Data show that sub-lethal antioxidant doses significantly reduced growth and aflatoxin accumulation by A. parasiticus but these treatments were not able to repress the expression of the early expression gene (aflD) involved in aflatoxin biosynthesis. Thus, this represents a high potential risk of stored peanuts contamination with aflatoxins.

Purification and Characterization of O-Methyltransferase I Involved in Conversion of Demethylsterigmatocystin to Sterigmatocystin and of Dihydrodemethylsterigmatocystin to Dihydrosterigmatocystin during Aflatoxin Biosynthesis

O-Methyltransferase I, which catalyzes conversions both of demethylsterigmatocystin (DMST) to sterigmatocystin (ST) and of dihydrodemethylsterigmatocystin (DHDMST) to dihydrosterigmatocystin (DHST) during aflatoxin biosynthesis, was purified to apparent homogeneity from the cytosol fraction of the mycelia of Aspergillus parasiticus NIAH-26 through the following chromatography series: phenyl-Sepharose, DEAE-Sepharose, phenyl-Sepharose, Sephacryl S-300, and Matrex gel Green A. The apparent molecular mass was estimated at 150 kDa based on Sephacryl S-300 gel filtration chromatography, and the denaturing molecular mass was 43 kDa based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The pI of the enzyme was 4.4, and the optimal pH for activity was broad, from 6.5 to 9.0. In competition experiments using the purified enzyme, the formation of ST from DMST was suppressed when DHDMST was added to the reaction mixture and DHST was newly formed. These results indicate that DMST and DHDMST commonly serve as substrates for the enzyme. The K(m) of the enzyme for DMST was 0.94 muM, and that for DHDMST was 2.5 muM. Interestingly, MT-I kinetics deviated substantially from standard Michaelis-Menten kinetics, demonstrating substrate inhibition at a higher substrate concentration.

Overexpression of aflR Leads to Upregulation of Pathway Gene Transcription and Increased Aflatoxin Production in Aspergillus flavus

The aflatoxin biosynthetic pathway regulatory gene, aflR, encodes a putative 47-kDa protein containing a zinc cluster DNA binding motif. It is required for the transcription of all of the characterized aflatoxin pathway genes in both Aspergillus flavus and Aspergillus parasiticus. The objective of this study was to examine the effects of aflR overexpression on temporal gene expression, aflatoxin production, and nitrate inhibition of aflatoxin biosynthesis in A. flavus. An inducible expression construct was made by fusing the coding region of aflR to the promoter region of the A. flavus adh1 gene. This construct was transformed into A. flavus 656-2 (FGSC A1010), a strain mutated at the aflR locus. Strain 656-2 containing the adh1(p)::aflR construct had induced transcription of two early aflatoxin pathway genes, nor-1 and pksA, and produced wild-type concentrations of aflatoxin in a temporal pattern similar to that of wild-type strains of A. flavus. Strains 656-2 and 86-10 (FGSC A1009) an aflatoxigenic strain, were transformed with a construct containing the constitutive promoter gpdA driving aflR. Transformants of these strains constitutively expressed aflR, fas-1A, pksA, nor-1, and omtA but did not constitutively produce aflatoxin. Strain 86-10 containing the gpdA(p)::aflR construct produced 50 times more aflatoxin than 86-10, but the temporal pattern of aflatoxin production was the same as for 86-10, and aflatoxin production was also induced by sucrose. The addition of 10 g of nitrate per liter to sucrose low salts medium inhibited aflatoxin production by both strain 86-10 and a transformant of 86-10 containing the gpdA(p)::aflR construct, indicating that nitrate inhibition of aflatoxin biosynthesis does not occur solely at the level of aflR transcription. These studies show that constitutive overexpression of the pathway transcriptional regulatory gene aflR leads to higher transcript accumulation of pathway genes and increased aflatoxin production but that the initiation of aflatoxin biosynthesis is not solely regulated by the transcriptional activities of the biosynthetic pathway.

Purification and characterization of two versiconal hemiacetal acetate reductases involved in aflatoxin biosynthesis

Two versiconal hemiacetal acetate (VHA) reductase activities (designated I and II), which catalyzed the reaction from VHA to versiconol acetate (VOAc) during aflatoxin biosynthesis, were purified to apparent homogeneity from the cytosol fraction of the mycelia of Aspergillus parasiticus mutant NIAH-26 through the following chromatography steps: first, fractionation with ammonium sulfate and then fractionation in succession with phenyl-Sepharose, DEAE-Sepharose, Sephacryl S-300, hydroxylapatite, and Matrex gel Green A chromatography. VHA reductase I and VHA reductase II were completely separated at the end of the DEAE-Sepharose step. The apparent molecular masses of reductase I and reductase II were estimated (by gel filtration) to be approximately 390 kDa; their denaturing molecular masses were 39- and 40-kDa, respectively (by sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Their pI values were 6.6 and 6.0, respectively (as determined by isoelectric focusing), and the optimal pH values were 8.0 and 9.0, respectively, although both enzymes exhibited a broad optimal pH range of between 7.5 and 9.0. The K(m) values of reductase I and reductase II for VHA were 35.4 and 25.4 muM, respectively. On the other hand, in the cell-free experiments involving either VHA reductase fraction and high-performance liquid chromatography, both (2'S)- and (2'R)-VOAc enantiomers were formed from racemic VHA and more of the 2'R isomer than the 2'S isomer was produced, indicating that the VHA reductase fractions have very similar stereospecificities to the substrate.

Functional expression and sub-cellular localization of the early aflatoxin pathway enzyme Nor-1 in Aspergillus parasiticus

Aflatoxin biosynthesis in Aspergillus parasiticus requires at least 17 enzyme activities (from acetate). Although the activities of most aflatoxin biosynthetic enzymes have been established, the mechanisms that govern transport and sub-cellular localization of these enzymes are not clear. We developed plasmid constructs that express Nor-1 fused to a green fluorescent protein reporter (EGFP) to monitor transport and localization of this early pathway enzyme in real time in Aspergillus parasiticus. Plasmids expressing EGFP fused to Nor-1 were introduced into A. parasiticus B62 (carries non-functional Nor-1). Transformants were screened for increased aflatoxin accumulation (restored Nor-1 activity) on coconut agar medium and for EGFP expression using fluorescence microscopy. Increased aflatoxin accumulation was confirmed by TLC and ELISA. Nor-1 fused to EGFP at either the N- or C- terminus functionally complemented non-functional Nor-1 in B62 and increased aflatoxin synthesis to wild-type (N-terminus) or lower levels (C-terminus). We detected full-length Nor-1 fusion proteins in transformants with increased aflatoxin accumulation (Western blot) and determined that the expression plasmid integrated at the nor-1 locus in these cells (Southern blot). Confocal laser scanning microscopy (CLSM) demonstrated that Nor-1 fusion proteins localized in the cytoplasm and vacuoles of fungal hyphae grown on aflatoxin-inducing solid media for 48h; control EGFP (no Nor-1) did not localize to vacuoles until 72h. The highest rate of aflatoxin synthesis coincided with the highest rate of transport of Nor-1 fusion proteins to the vacuole strongly suggesting that Nor-1 is synthesized in the cytoplasm and transported to the vacuole to carry out an early step in aflatoxin synthesis.

Strategies in Prevention of Preharvest Aflatoxin Contamination in Peanuts: Aflatoxin Biosynthesis, Genetics and Genomics

Peanut (Arachis hypogaea L.), or groundnut, is an important crop economically and nutritionally in many tropical and subtropical areas of the world. It is also one of the most susceptible host crops to Aspergillus flavus resulting in aflatoxin contamination. The prevention or elimination of aflatoxin contamination in preharvest and postharvest crops is a serious challenge facing scientists. The recent International Conference on Groundnut Aflatoxin Management and Genomics held in Guangzhou, China, provided an international forum for discussions on the latest accomplishments, the development of strategies, and the initiation of cooperative research for the prevention of aflatoxin contamination. This review summarizes the progress in genetic and genomic research of peanuts and the toxin-producing fungus A. flavus. In particular, the pathway for production and the genetic regulation of afaltoxin, and the peanut-Aspergillus interaction are discussed. The use of a peanut-Aspergillus microarray will help scientists to study the crop-pathogen interaction; aids in the identification of genes involved in both fungal invasion and crop resistance, and ultimately enhance research to find solutions that prevent aflatoxin contamination in agricultural commodities.

Function and regulation of aflJ in the accumulation of aflatoxin early pathway intermediate in Aspergillus flavus

aflJ resides within the aflatoxin biosynthetic gene cluster adjacent to the pathway regulatory gene aflR and is involved in aflatoxin production, but its function is unknown. Over-expression of aflJ in the aflatoxin-producing strain 86-10 resulted in increased aflatoxin. In an effort to study the function and regulation of aflJ, strain 649-1 lacking the entire biosynthetic cluster was transformed with either reporter constructs, expression constructs, or cosmid clones and analysed for gene expression or metabolite accumulation. Over-expression of aflJ did not result in elevated transcription of ver-1, omtA or aflR. To determine if over-expression of aflJ leads to an increase in early pathway intermediates, strain 649-1 was transformed with cosmid 5E6 and either gpdA::aflJ alone, gpdA::aflR alone, or aflJ and aflR together. Cosmid 5E6 contains the genes pksA, nor-1, fas-1, and fas-2, which are required for the biosynthesis of the early pathway intermediate averantin. 649-1 transformants containing 5E6 alone produced no detectable averantin. In contrast, 5E6 transformants with gpdA::aflR produced averantin, but only half as much as those transformants containing both aflR and aflJ. Northern blot analysis showed that 5E6 transformants containing both aflR and aflJ had five times more pksA transcripts and four times more nor-1 transcripts than 5E6 transformants containing gpdA::aflR alone. Further, aflJ transcription was regulated by aflR. Over-expression of aflR resulted in elevated aflJ transcription. aflJ appears to modulate the regulation of early genes in aflatoxin biosynthesis.

The initiation and pattern of spread of histone H4 acetylation parallel the order of transcriptional activation of genes in the aflatoxin cluster

The 27 genes involved in aflatoxin biosynthesis are clustered within a 70 kb region in the Aspergillus parasiticus genome. Using chromatin immunoprecipitation, we demonstrated a positive correlation between the initiation and spread of histone H4 acetylation in aflatoxin promoters and the onset of accumulation of aflatoxin proteins and aflatoxin. Histone H4 acetylation in the pksA (encodes an 'early' biosynthetic pathway enzyme) promoter peaked at 30 h, prior to the increased acetylation in the omtA and ordA (encode 'late' enzymes) promoters detected at 40 h. The specific order in which pksA, ver-1 (encodes a 'middle' enzyme) and omtA transcripts accumulated in cells paralleled the pattern of spread of histone H4 acetylation. Binding of AflR, a positive regulator of aflatoxin biosynthesis, to the ordA promoter showed a positive correlation with the spread of histone H4 acetylation. The data suggest that the order of genes within the aflatoxin cluster determines the timing and order of transcriptional activation, and that the site of initiation and spread of histone H4 acetylation mediate this process. Our data indicate that the aflatoxin and adjacent sugar utilization clusters are part of a larger 'regulatory unit'.

Ethylene inhibits aflatoxin biosynthesis in Aspergillus parasiticus grown on peanuts

The filamentous fungi Aspergillus parasiticus and Aspergillus flavus synthesize aflatoxins when they grow on a variety of susceptible food and feed crops. These mycotoxins are among the most carcinogenic naturally occurring compounds known and they pose significant health risks to humans and animals. We previously demonstrated that ethylene and CO2 act alone and together to reduce aflatoxin synthesis by A. parasiticus grown on laboratory media. To demonstrate the potential efficacy of treatment of stored seeds and grains with these gases, we tested ethylene and CO2 for ability to inhibit aflatoxin accumulation on Georgia Green peanuts stored for up to 5 days. We demonstrated an inverse relationship between A. parasiticus spore inoculum size and the level of toxin accumulation. We showed that ethylene inhibits aflatoxin synthesis in a dose-dependent manner on peanuts; CO2 also inhibits aflatoxin synthesis over a narrow dose range. Treatments had no discernable effect on mold growth. These observations support further exploration of this technology to reduce aflatoxin contamination of susceptible crops in the field and during storage.

Understanding nonaflatoxigenicity of Aspergillus sojae: a windfall of aflatoxin biosynthesis research

Aspergillus section Flavi includes aflatoxin-producing and nonproducing fungi. Aspergillus sojae is unable to produce aflatoxins and is generally recognized as safe for food fermentation. However, because of its taxonomical relatedness to aflatoxin-producing Aspergillus parasiticus and A. flavus, it is necessary to decipher the underlying mechanisms for its inability to produce aflatoxins. This review addresses the relationship between A. sojae and A. parasiticus and the advances that have been made in aflatoxin biosynthesis research, especially with regard to gene structure, genome organization, and gene regulation in A. parasiticus and A. flavus and how this has been used to assure the safety of A. sojae as an organism for food fermentation. The lack of aflatoxin-producing ability of A. sojae results primarily from an early termination point mutation in the pathway-specific aflR regulatory gene, which causes the truncation of the transcriptional activation domain of AflR and the abolishment of interaction between AflR and the AflJ co-activator. Both are required for gene expression. In addition, a defect in the polyketide synthase gene also contributes to its nonaflatoxigenicity.

Early expression of aflatoxin-like dothistromin genes in the forest pathogen Dothistroma septosporum

The forest pathogen Dothistroma septosporum produces the polyketide dothistromin, a mycotoxin very similar in structure to versicolorin B, a precursor of aflatoxin (AF). Dothistromin is a broad-range toxin and possibly involved in red-band needle blight disease. As the role of dothistromin in the disease is unknown the expression of dothistromin genes was studied to reveal clues to its function. Although the genes of AF and dothistromin biosynthesis are very similar, this study revealed remarkable differences in the timing of their expression. Secondary metabolites, like AF, are usually produced during late exponential phase. Previously identified dothistromin genes, as well as a newly reported versicolorin B synthase gene, vbsA, showed high levels of expression during the onset of exponential growth. This unusual early expression was also seen in transformants containing a green fluorescent protein (GFP) gene regulated by a dothistromin gene promoter, where the highest GFP expression occurred in young mycelium. Two hypotheses for the biological role of dothistromin are proposed based on these results. The study of dothistromin genes will improve current knowledge about secondary metabolite genes, their putative biological roles, and their regulation.

Biosynthesis of dothistromin

Dothistromin is a mycotoxin that is remarkably similar in structure to versicolorin B, a precursor of both aflatoxin and sterigmatocystin. Dothistromin-producing fungi also produce related compounds, including some aflatoxin precursors as well as alternative forms of dothistromin. Dothistromin is synthesized by pathogenic species of Dothistroma in the red bands of pine needles associated with needle blight, but is also made in culture where it is strongly secreted into the surrounding medium. Orthologs of aflatoxin and sterigmatocystin biosynthetic genes have been found that are required for the biosynthesis of dothistromin, along with others that are speculated to be involved in the same pathway on the basis of their sequence similarity to aflatoxin genes. An epoxide hydrolase gene that has no homolog in the aflatoxin or sterigmatocystin gene clusters is also clustered with the dothistromin genes, and all these genes appear to be located on a minichromosome in Dothistroma septosporum. The dothistromin genes are expressed at an early stage of growth, suggesting a role in the first stages of plant invasion by the fungus. Future studies are expected to reveal more about the role of dothistromin in needle blight and about the genomic organization and expression of dothistromin genes: these studies will provide for interesting comparisons with these aspects of aflatoxin and sterigmatocystin biosynthesis.

The aflatoxin pathway regulator AflR induces gene transcription inside and outside of the aflatoxin biosynthetic cluster

Aflatoxin contamination of food and feed is a major concern due to the carcinogenic properties of this mycotoxin. Previous studies using classical approaches have identified a cluster of genes responsible for aflatoxin production under the control of the pathway-specific transcriptional regulator aflR, but it is unknown whether aflR controls expression of other genes within the genome. Transcription profiling comparing wild type and DeltaaflR strains of Aspergillus parasiticus grown under conditions conducive for aflatoxin production identified only 23 upregulated genes in the wild type. These included 20 genes in the aflatoxin biosynthetic cluster, and three additional genes outside the aflatoxin biosynthetic cluster (nadA, hlyC, and niiA), all with AflR binding sites. This report is the first to demonstrate genes outside the biosynthetic cluster as being associated with aflR expression.

Role of cis-acting sites NorL, a TATA box, and AflR1 in nor-1 transcriptional activation in Aspergillus parasiticus

The transcription factor AflR is required for up-regulation of specific pathway genes involved in aflatoxin biosynthesis in the filamentous fungus Aspergillus. nor-1 encodes an early aflatoxin pathway enzyme; its promoter contains a consensus AflR binding site (AflR1). Proteins in Aspergillus parasiticus cell extracts and AflR expressed in Escherichia coli do not bind to A. parasiticus AflR1 in vitro, so it was not clear if this site was required for nor-1 expression or if other transcription factors contributed to gene regulation. In this study we defined the role of AflR1 in nor-1 expression in A. parasiticus and identified additional cis-acting sites required for maximum nor-1 transcriptional activation. Deletion and substitution of AflR1 in the nor-1 promoter in A. parasiticus nor-1::GUS reporter strains showed that this site is required for nor-1 transcriptional activation in vivo. Substitution of a putative TATA box in the nor-1 promoter resulted in nondetectable beta-glucuronidase (GUS) activity, demonstrating that this TATA box is functional in vivo. We also identified a novel cis-acting site, designated NorL, between residues -210 and -238 that was required for maximum nor-1 transcriptional activation in A. parasiticus grown in liquid medium and on solid medium. Using an electrophoretic mobility shift assay, we identified a specific NorL-dependent DNA-protein complex that relies on a functional AflR, either directly or indirectly, for maximum binding capacity. Because the NorL site appears only once in the aflatoxin gene cluster, its association with the nor-1 promoter may have important implications for the overall regulatory scheme for the aflatoxin pathway.

Completed sequence of aflatoxin pathway gene cluster in Aspergillus parasiticus

An 82-kb Aspergillus parasiticus genomic DNA region representing the completed sequence of the well-organized aflatoxin pathway gene cluster has been sequenced and annotated. In addition to the 19 reported and characterized aflatoxin pathway genes and the four sugar utilization genes in this cluster, we report here the identification of six newly identified genes which are putatively involved in aflatoxin formation. The function of these genes, the cluster organization and its significance in gene expression are discussed.

A novel cAMP-response element, CRE1, modulates expression of nor-1 in Aspergillus parasiticus

The level of aflatoxin accumulation in the filamentous fungus Aspergillus parasiticus is modulated by a variety of environmental cues. The presence of glucose (a preferred carbon source) in liquid and solid glucose minimal salts (GMS) growth media strongly stimulated aflatoxin accumulation. Peptone (a non-preferred carbon source) in peptone minimal salts (PMS) media stimulated only low levels of aflatoxin accumulation. Glucose stimulated transcription of the aflatoxin structural genes ver-1 and nor-1 to similar intermediate levels in liquid GMS, while on solid media, ver-1 transcription was stimulated to 20-fold higher levels than nor-1. PMS liquid and solid media stimulated very low or non-detectable levels of transcription of both genes. Electrophoretic mobility shift analysis using a nor-1 promoter fragment (norR) and A. parasiticus cell protein extracts revealed specific DNA-protein complexes of different mobility on GMS and PMS solid and liquid media. An imperfect cAMP-response element, CRE1, was identified in norR that mediated formation of the specific DNA-protein complexes. Mutation in CRE1 or AflR1 (AflR cis-acting site) caused up to a 3-fold decrease in cAMP-mediated stimulation of nor-1 promoter activity on GMS agar. South-Western blot analysis identified a 32-kDa protein that specifically bound to norR. p32 could be co-immunoprecipitated by anti-AflR antibody and co-purified with an AflR-maltose-binding protein fusion demonstrating a physical interaction between AflR and p32 in vitro. We hypothesize that p32 assists AflR in binding to the nor-1 promoter, thereby modulating nor-1 gene expression in response to environmental cues.

Dothistroma pini, a forest pathogen, contains homologs of aflatoxin biosynthetic pathway genes

Homologs of aflatoxin biosynthetic genes have been identified in the pine needle pathogen Dothistroma pini. D. pini produces dothistromin, a difuranoanthraquinone toxin with structural similarity to the aflatoxin precursor versicolorin B. Previous studies with purified dothistromin suggest a possible role for this toxin in pathogenicity. By using an aflatoxin gene as a hybridization probe, a genomic D. pini clone was identified that contained four dot genes with similarity to genes in aflatoxin and sterigmatocystin gene clusters with predicted activities of a ketoreductase (dotA), oxidase (dotB), major facilitator superfamily transporter (dotC), and thioesterase (dotD). A D. pini dotA mutant was made by targeted gene replacement and shown to be severely impaired in dothistromin production, confirming that dotA is involved in dothistromin biosynthesis. Accumulation of versicolorin A (a precursor of aflatoxin) by the dotA mutant confirms that the dotA gene product is involved in an aflatoxin-like biosynthetic pathway. Since toxin genes have been found to be clustered in fungi in every case analyzed so far, it is speculated that the four dot genes may comprise part of a dothistromin biosynthetic gene cluster. A fifth gene, ddhA, is not a homolog of aflatoxin genes and could be at one end of the dothistromin cluster. These genes will allow comparative biochemical and genetic studies of the aflatoxin and dothistromin biosynthetic pathways and may also lead to new ways to control Dothistroma needle blight.

Chromosomal location plays a role in regulation of aflatoxin gene expression in Aspergillus parasiticus

The nor-1 gene in the filamentous fungus Aspergillus parasiticus encodes a ketoreductase involved in aflatoxin biosynthesis. To study environmental influences on nor-1 expression, we generated plasmid pAPGUSNNB containing a nor-1 promoter-beta-glucuronidase (GUS) (encoded by uidA) reporter fusion with niaD (encodes nitrate reductase) as a selectable marker. niaD transformants of A. parasiticus strain NR-1 (niaD) carried pAPGUSNNB integrated predominantly at the nor-1 or niaD locus. Expression of the native nor-1 and nor-1::GUS reporter was compared in transformants grown under aflatoxin-inducing conditions by Northern and Western analyses and by qualitative and quantitative GUS activity assays. The timing and level of nor-1 promoter function with pAPGUSNNB integrated at nor-1 was similar to that observed for the native nor-1 gene. In contrast, nor-1 promoter activity in pAPGUSNNB and a second nor-1::GUS reporter construct, pBNG3.0, was not detectable when integration occurred at niaD. Because niaD-dependent regulation could account for the absence of expression at niaD, a third chromosomal location was analyzed using pAPGUSNP, which contained nor-1::GUS plus pyrG (encodes OMP decarboxylase) as a selectable marker. GUS expression was detectable only when pAPGUSNP integrated at nor-1 and was not detectable at pyrG, even under growth conditions that required pyrG expression. nor-1::GUS is regulated similarly to the native nor-1 gene when it is integrated at its homologous site within the aflatoxin gene cluster but is not expressed at native nor-1 levels at two locations outside of the aflatoxin gene cluster. We conclude that the GUS reporter system can be used effectively to measure nor-1 promoter activity and that nor-1 is subject to position-dependent regulation in the A. parasiticus chromosome.

In planta regulation of extension of an endophytic fungus and maintenance of high metabolic rates in its mycelium in the absence of apical extension

The fungus Neotyphodium lolii is an endophytic symbiont. It grows in the intercellular spaces of the perennial ryegrass Lolium perenne, producing secondary metabolites which enhance the fitness of the association over that of uninfected L. perenne. We report that the average number of hyphal strands in a given section of a leaf remains constant during the life of a leaf, indicating synchrony of leaf and hyphal extension, including cessation of hyphal extension when leaf extension ceases. We used a constitutively expressed reporter gene as an indicator of the mycelium's metabolic activity during and after hyphal extension. Reporter gene activity decreased when the mycelium stopped extending in liquid culture but not in planta. This indicates that in planta endophyte hyphae remain metabolically highly active when extension has ceased and throughout the life of the leaf they are colonizing. The behavior of the fungus in planta indicates the existence of signaling pathways which (i) synchronize the extension of leaf and hypha by regulating hyphal extension, (ii) suppress hyphal branching, and (iii) stop apical extension of fungal hyphae, without reducing the mycelium's metabolic activity. These signals may be crucial for the symbiosis, by allowing the endophyte to switch the focus of its metabolic activity from extension to the production of secondary metabolites.

Identification of Aspergillus nidulans genes essential for the accumulation of sterigmatocystin

The fungus Aspergillus nidulans (Emericella nidulans) was used as a genetic model for the identification of genes required for efficient accumulation of sterigmatocystin (ST). The required gene for sterigmatocystin expression was stc, which is an intermediate penultimate product in the aflatoxin biosynthetic pathway. Genetic analysis included studies of the sexual and parasexual cycles. The allelic segregation rates and recombination frequencies between linked and nonlinked genetic markers were determined by the crossing of the strains UT448 (stc) to UT196 (stc(+)) and UT448 (stc) to UT184 (stc). Low ST accumulation (4.0 ppm) in the UT196 strain and in 7.4% of the meiotic segregants allowed us to map the stc locus at chromosome I, 3.4% distant from riboA1. The diploid UT448 (stc)//UT184 (stc), prepared from nonproducing strains, was analyzed based on the parasexual cycle, and 28% of the haploid segregants accumulated the ST toxin. The results suggest that UT448 carries the stc mutant (or an inactivated) allele and that UT184, although carrying the stc(+) allele, is reactivated only by the R2(+) factor, which is located at chromosome VIII of UT448. In such a configuration, the diploid accumulates large amounts of sterigmatocystin (40 ppm). Another regulator factor (R1), located at the meth-w (II) chromosomic interval, was identified in the UT448 strain. At DNA level in chromosome I, the R1 product acts and blocks the stcZ(+) gene transcription. In a different genotypic configuration, the R1 product interacts with the R2 product (of chromosome VIII), allowing the stcZ(+) gene expression. Furthermore, the diploid UT448 (stc)//UT196 (stc(+)) accumulated the ST toxin at high level (40 ppm), indicating similar interaction between mentioned factors and the stc gene. Obtained data suggest that R1 (II) regulates the stcZ(+) transcription, by interacting with chromosome I (at the DNA level) and that R2 (VIII) controls R1 activity at the cytoplasm level. Based on these results, we propose a regulation model for the sterigmatocystin production.

Novel procedure for identification of compounds inhibitory to transcription of genes involved in mycotoxin biosynthesis

A novel assay is described for the identification and isolation of compounds that inhibit the transcription of genes involved in mycotoxin biosynthesis. The thin-layer chromatography-based assay was used to screen plant extracts for compounds that would inhibit the expression of the beta-glucuronidase reporter gene under the control of an aflatoxin biosynthesis gene promoter in Aspergillus parasiticus. The assay was used to track purification of an inhibitory compound, cp2, from extracts of black pepper (Piper nigrum). Cp2 did not inhibit mycelial growth or the expression of the beta-tubulin gene but did inhibit aflatoxin biosynthesis at the transcriptional level. Applications of cp2 to the control of mycotoxins are discussed.

Homologs of aflatoxin biosynthesis genes and sequence of aflR in Aspergillus oryzae and Aspergillus sojae

The presence, but not expression, of homologs of three structural genes and a regulatory gene necessary for aflatoxin biosynthesis in Aspergillus parasiticus and A. flavus was shown for A. oryzae and A. sojae. Homologs of the regulatory gene aflR were cloned and sequenced from A. oryzae and A. sojae.

Genetic organization and function of the aflatoxin B1 biosynthetic genes

Aflatoxins are secondary metabolites produced by Aspergillus flavus and Aspergillus parasiticus. Most of the genes involved in the biosynthesis of aflatoxin are contained within a single cluster in the genome of these filamentous fungi. Studies directed toward understanding the molecular biology of aflatoxin biosynthesis have led to a number of important discoveries. A pair of fatty acid synthase genes were identified that are involved uniquely in aflatoxin biosynthesis. Two genes were also characterized that represent new families of cytochrome P450 monooxygenases. Gene expression is coordinated during aflatoxin production and is under the control of a positive regulatory gene belonging to a family of fungal transcriptional activators associated with various metabolic pathways in fungi.

pH Regulation of Sterigmatocystin and Aflatoxin Biosynthesis in Aspergillus spp

ABSTRACT Aflatoxin (AF) and sterigmatocystin (ST) are toxic secondary metabolites produced by the same biochemical pathway found in several Aspergillus spp. The expression of the homologous ST/AF structural gene, stcU in A. nidulans and ver-1 in A. parasiticus, was affected by external pH of liquid growth media. Both stcU and ver-1 mRNAs appeared earlier and were expressed at higher levels in cultures grown in acidic media (pH 4 to 6) versus neutral (pH 7) and alkali (pH 8) media. Transcript levels correlated with ST/AF production. Visual and spectrophotometric analysis of production of the orange ST/AF intermediate, norsolorinic acid (NOR), also paralleled transcript patterns and indicated that the pH effects were operative in different nitrogen- and carbon-based solid growth media. Five- to 10-fold increases in ST, AF, and NOR were measured in cultures grown in pH 4 or 5 versus pH 8 media. An A. nidulans strain carrying a mutation resulting in constitutive activity of the pH regulatory factor, PacC, produced 10-fold less ST than did wild type. The stcU transcript was not noticeably affected by pH in this strain. The results suggest a general pattern of pH regulation of ST/AF biosynthesis that may override previously noted carbon and nitrogen effects.

Analysis of mechanisms regulating expression of the ver-1 gene, involved in aflatoxin biosynthesis

Previous studies have shown that ver-1A encodes an enzyme which is directly involved in the conversion of versicolorin A to demethylsterigmatocystin during aflatoxin B1 (AFB1) biosynthesis in the filamentous fungus Aspergillus parasiticus. In this study, two different tools were utilized to study the regulation of ver-1A expression at the level of transcription and protein accumulation. First, a ver-1A cDNA was expressed in Escherichia coli with the vector pMAL-c2. The resulting maltose-binding protein-Ver-1A fusion protein was purified and used to generate polyclonal antibodies. Western blot analyses showed that these antibodies specifically recognized the Ver-1 protein (approximately 28 kDa) in cell extracts of Aspergillus parasiticus SU1. Second, a GUS (uidA; encodes beta-glucuronidase) reporter system was developed by fusing the ver-1A promoter and transcription terminator to the GUS gene. Reporter constructs were transformed into A. parasiticus, resulting in a single copy of the ver-1A-GUS reporter integrated adjacent to the wild-type ver-1A gene (3' end) in the chromosome. Western blot analysis, Northern hybridization analysis, and a GUS activity assay were used to analyze transformants. The timing of appearance and pattern of accumulation of GUS transcript and GUS protein in transformants were consistent with the timing of appearance and pattern of accumulation of ver-1 transcript and Ver-1 protein. These data suggested that the GUS gene was under the same regulatory control as the wild-type ver-1 gene and confirmed that transcriptional regulation plays an important role in ver-1A expression. Integration of the ver-1A-GUS reporter construct at the niaD locus resulted in 500-fold-lower GUS activity, but the temporal pattern of accumulation of GUS activity was not affected. Therefore, chromosomal location can play a role in determining the level of gene expression in A. parasiticus and should be an important consideration when analyzing promoter function in this organism.

Isolation and characterization of the versicolorin B synthase gene from Aspergillus parasiticus. Expansion of the aflatoxin b1 biosynthetic gene cluster

Versicolorin B synthase catalyzes the side chain cyclization of racemic versiconal hemiacetal to the bisfuran ring system of(-)-versicolorin B, an essential transformation in the aflatoxin biosynthetic pathway of Aspergillus parasiticus. The dihydrobisfuran is key to the mutagenic nature of aflatoxin B1. The protein, which shows 58% similarity and 38% identity with glucose oxidase from Aspergillus niger, possesses an amino-terminal sequence homologous to the ADP-binding region of other flavoenzymes. However, this enzyme does not require flavin or nicotinamide cofactors for its cyclase activity. The 643-amino acid native enzyme contains three potential sites for N-linked glycosylation, Asn-Xaa-Thr or Asn-Xaa-Ser. The cDNA and genomic clones of versicolorin B synthase were isolated by screening the respective libraries with random-primed DNA probes generated from an exact copy of an internal vbs sequence. This probe was created through polymerase chain reaction by using nondegenerate polymerase chain reaction primers derived from the amino acid sequences of peptide fragments of the enzyme. The 1985-base genomic vbs DNA sequence is interrupted by one intron of 53 nucleotides. Southern blotting, nucleotide sequencing, and detailed restriction mapping of the vbs-containing genomic clones revealed the presence of omtA, a methyltransferase active in the biosynthesis, 3.3 kilobases upstream of vbs and oriented in the opposite direction from vbs. The presence of omtA in close proximity to vbs supports the theory that the genes encoding the aflatoxin biosynthetic enzymes in A. parasiticus are clustered.

Twenty-five coregulated transcripts define a sterigmatocystin gene cluster in Aspergillus nidulans

Sterigmatocystin (ST) and the aflatoxins (AFs), related fungal secondary metabolites, are among the most toxic, mutagenic, and carcinogenic natural products known. The ST biosynthetic pathway in Aspergillus nidulans is estimated to involve at least 15 enzymatic activities, while certain Aspergillus parasiticus, Aspergillus flavus, and Aspergillus nomius strains contain additional activities that convert ST to AF. We have characterized a 60-kb region in the A. nidulans genome and find it contains many, if not all, of the genes needed for ST biosynthesis. This region includes verA, a structural gene previously shown to be required for ST biosynthesis, and 24 additional closely spaced transcripts ranging in size from 0.6 to 7.2 kb that are coordinately induced only under ST-producing conditions. Each end of this gene cluster is demarcated by transcripts that are expressed under both ST-inducing and non-ST-inducing conditions. Deduced polypeptide sequences of regions within this cluster had a high percentage of identity with enzymes that have activities predicted for ST/AF biosynthesis, including a polyketide synthase, a fatty acid synthase (alpha and beta subunits), five monooxygenases, four dehydrogenases, an esterase, an 0-methyltransferase, a reductase, an oxidase, and a zinc cluster DNA binding protein. A revised system for naming the genes of the ST pathway is presented.

Structure and function of fas-1A, a gene encoding a putative fatty acid synthetase directly involved in aflatoxin biosynthesis in Aspergillus parasiticus

A novel gene, fas-1A, directly involved in aflatoxin B1 (AFB1) biosynthesis, was cloned by genetic complementation of an Aspergillus parasiticus mutant strain, UVM8, blocked at two unique sites in the AFB1 biosynthetic pathway. Metabolite conversion studies localized the two genetic blocks to early steps in the AFB1 pathway (nor-1 and fas-1A) and confirmed that fas-1A is blocked prior to nor-1. Transformation of UVM8 with cosmids NorA and NorB restored function in nor-1 and fas-1A, resulting in synthesis of AFB1. An 8-kb SacI subclone of cosmid NorA complemented fas-1A only, resulting in accumulation of norsolorinic acid. Gene disruption of the fas-1A locus blocked norsolorinic acid accumulation in A. parasiticus B62 (nor-1), which normally accumulates this intermediate. These data confirmed that fas-1A is directly involved in AFB1 synthesis. The predicted amino acid sequence of fas-1A showed a high level of identity with extensive regions in the enoyl reductase and malonyl/palmityl transferase functional domains in the beta subunit of yeast fatty acid synthetase. Together, these data suggest that fas-1A encodes a novel fatty acid synthetase which synthesizes part of the polyketide backbone of AFB1. Additional data support an interaction between AFB1 synthesis and sclerotium development.

Characterization of the polyketide synthase gene (pksL1) required for aflatoxin biosynthesis in Aspergillus parasiticus

Aflatoxins are potent toxic and carcinogenic compounds, produced by Aspergillus parasiticus and A. flavus as secondary metabolites. In this research, a polyketide synthase gene (pksL1), the key gene for aflatoxin biosynthesis initiation in A. parasiticus, has been functionally identified and molecularly characterized. PCR-derived DNA probes were used to find the pksL1 gene from subtracted, aflatoxin-related clones. Gene knockout experiments generated four pksL1 disruptants which lost both the ability to produce aflatoxins B1, B2, and G1 and the ability to accumulate norsolorinic acid and all other intermediates of the aflatoxin biosynthetic pathway. A pksL1 DNA probe detected a 6.6-kb poly(A)+ RNA transcript in Northern (RNA) hybridizations. This transcript, associated with aflatoxin production, exhibited a regulated expression that was influenced by growth phase, medium composition, and culture temperature. DNA sequencing of pksL1 revealed an open reading frame for a polypeptide (PKSL1) of 2,109 amino acids. Sequence analysis further recognized four functional domains in PKSL1, acyl carrier protein, beta-ketoacyl-acyl carrier protein synthase, acyltransferase, and thioesterase, all of which are usually present in polyketide synthases and fatty acid synthases. On the basis of these results, we propose that pksL1 encodes the polyketide synthase which synthesizes the backbone polyketide and initiates aflatoxin biosynthesis. In addition, the transcript of pksL1 exhibited heterogeneity at the polyadenylation site similar to that of plant genes.

Comparison of the omtA genes encoding O-methyltransferases involved in aflatoxin biosynthesis from Aspergillus parasiticus and A. flavus

O-methyltransferase (OMT) is one of the key enzymes in aflatoxin (AF) biosynthesis in the fungi, Aspergillus flavus (Af) and A. parasiticus (Ap). Genomic DNA clones containing the omtA genes from Ap strain SRRC 143 and Af strain CRA01-2B were sequenced. Comparison of the genomic DNA sequences with the cDNA of this Ap gene revealed the presence of four introns ranging from 52 to 60 bp in length in both species; the region encoding the putative S-adenosylmethionine-binding motif was located between the third and fourth introns. The coding sequence of omtA from Ap strain SRRC 143 demonstrated a greater than 97% sequence identity with that from Af strain CRA01-2B, within the coding region.

Sterigmatocystin biosynthesis in Aspergillus nidulans requires a novel type I polyketide synthase

A filamentous fungus, Aspergillus nidulans, produces the carcinogenic mycotoxin sterigmatocystin (ST), which is a polyketide-derived secondary metabolite. A gene (pksST) encoding the ST polyketide synthase (PKSst) in A. nidulans was cloned, sequenced, and characterized. Large induced deletion mutants, which did not make ST or any ST intermediates, were used to identify genes associated with ST biosynthesis. Among the transcripts detected within the deletion region, which showed developmental expression with ST production, was a 7.2-kb transcript. Functional inactivation of the gene encoding the 7.2-kb transcript blocked production of ST and all ST intermediate substrates but did not affect transcription of the pathway genes, indicating that this gene was involved in a very early step of ST biosynthesis. These results also indicate that PKSst was not associated with activation of other ST genes. Sequencing of the region spanning this gene revealed that it encoded a polypeptide with a deduced length of 2,181 amino acids that had high levels of similarity to many of the known polyketide synthases and FASs. This gene, pksST, encodes a multifunctional novel type I polyketide synthase which has as active sites a beta-ketoacyl acyl carrier protein synthase, an acyltransferase, duplicated acyl carrier proteins, and a thioesterase, all of these catalytic sites may be multiply used. In addition, a 1.9-kb transcript, which also showed developmental expression, was mapped adjacent to pksST, and the sequence of this gene revealed that it encoded a cytochrome P-450 monooxygenase-like peptide.

A beta-glucuronidase reporter gene construct for monitoring aflatoxin biosynthesis in Aspergillus flavus

Aflatoxins are toxic and carcinogenic secondary metabolites produced by the fungi Aspergillus flavus and A. parasiticus. Current research is directed at the elimination of these compounds in important food sources. The objective of this research was to develop a method to study the induction and regulation of aflatoxin biosynthesis by examining the expression of one aflatoxin pathway gene, ver1. The promoter region of ver1 was fused to the beta-glucuronidase (GUS) gene (uidA) from Escherichia coli to form the reporter construct, GAP13. A. flavus 656-2 was transformed with this construct. Aflatoxin production, GUS activity, and transcript accumulation were determined in transformants after shifting the cultures from a nonconducive medium to a medium conducive to aflatoxin biosynthesis. Transformants harboring GAP13 displayed GUS expression only when aflatoxin was detected in culture. Further, the transcription of the uidA gene driven by the ver1 promoter followed the same profile as for the ver1 genes. The results show that the GAP13 construct may be useful as a genetic tool to study the induction of aflatoxin in situ and to identify substances that affect the expression of genes involved in aflatoxin biosynthesis. The utility of this construct to detect inducers of aflatoxin biosynthesis in maize kernels was tested in a bioassay. A heat-stable inducer of aflatoxin with a molecular size of less than 10 kDa was detected in extracts from maize kernels colonized by A. flavus.

Physical and transcriptional map of an aflatoxin gene cluster in Aspergillus parasiticus and functional disruption of a gene involved early in the aflatoxin pathway

Two genes involved in aflatoxin B1 (AFB1) biosynthesis in Aspergillus parasiticus, nor-1 and ver-1, were localized to a 35-kb region on one A. parasiticus chromosome and to the genomic DNA fragment carried on a single cosmid, NorA. A physical and transcriptional map of the 35-kb genomic DNA insert in cosmid NorA was prepared to help determine whether other genes located in the nor-1-ver-1 region were involved in aflatoxin synthesis. Northern (RNA) analysis performed on RNA isolated from A. parasiticus SU1 grown in aflatoxin-inducing medium localized 14 RNA transcripts encoded by this region. Eight of these transcripts, previously unidentified, showed a pattern of accumulation similar to that of nor-1 and ver-1, suggesting possible involvement in AFB1 synthesis. To directly test this hypothesis, gene-1, encoding one of the eight transcripts, was disrupted in A. parasiticus CS10, which accumulates the aflatoxin precursor versicolorin A, by insertion of plasmid pAPNVES4. Thin-layer chromatography revealed that gene-1 disruptant clones no longer accumulated versicolorin A. Southern hybridization analysis of these clones indicated that gene-1 had been disrupted by insertion of the disruption vector. These data confirmed that gene-1 is directly involved in AFB1 synthesis. The predicted amino acid sequence of two regions of gene-1 showed a high degree of identity and similarity with the beta-ketoacyl-synthase and acyltransferase functional domains of polyketide synthases, consistent with a proposed role for gene-1 in polyketide backbone synthesis.

Comparative mapping of aflatoxin pathway gene clusters in Aspergillus parasiticus and Aspergillus flavus

Aflatoxins are toxic and carcinogenic secondary metabolites produced by the fungi Aspergillus flavus and A. parasiticus. Aflatoxins are synthesized by condensation of acetate units; their synthesis is estimated to involve at least 16 different enzymes. In this study we have shown that at least nine genes involved in the aflatoxin biosynthetic pathway are located within a 60-kb DNA fragment. Four of these genes, nor-1, aflR, ver-1, and omtA (previously named omt-1), have been cloned in A. flavus and A. parasiticus. In addition, five other genes, pksA, uvm8, aad, ord-1, and ord-2 have been recently cloned in A. parasiticus. The pksA, aad, and uvm8 genes exhibit sequence homologies to polyketide synthase, aryl-alcohol dehydrogenase, and fatty acid synthase genes, respectively. The cDNA sequences of ord-1 and ord-2 genes, which may be involved in later steps of aflatoxin biosynthesis, have been determined; the ord-1 gene product exhibits homology to cytochrome P-450-type enzymes. By characterizing the overlapping regions of the DNA inserts in different cosmid and lambda DNA clones, we have determined the order of these aflatoxin pathway genes within this 60-kb DNA region to be pksA, nor-1, uvm8, aflR, aad, ver-1, ord-1, ord-2, and omtA in A. parasiticus and nor-1, aflR, ver-1, ord-1, ord-2, and omtA in A. flavus. The order is related to the order in enzymatic steps required for aflatoxin biosynthesis. The physical distances (in kilobase pairs) and the directions of transcription of these genes have been determined for both aflatoxigenic species.

Structural and functional analysis of the nor-1 gene involved in the biosynthesis of aflatoxins by Aspergillus parasiticus

The nor-1 gene was cloned previously by complementation of a mutation (nor-1) in Aspergillus parasiticus SU-1 which blocked aflatoxin B1 biosynthesis, resulting in the accumulation of norsolorinic acid (NA). In this study, the nucleotide sequences of the cDNA and genomic DNA clones encompassing the coding region of the nor-1 gene were determined. The transcription initiation and polyadenylation sites of nor-1 were located by primer extension and RNase protection analyses and by comparison of the nucleotide sequences of the nor-1 genomic and cDNA clones. A plasmid, pNA51-82, was created for one-step disruption of the nor-1 gene by inserting a functional copy of the nitrate reductase (niaD) gene from A. parasiticus into the coding region of the nor-1 gene. Transformation of A. parasiticus NR-3 (niaD Afl+) with pNA51-82 resulted in niaD+ transformants that accumulated NA and produced reduced levels of aflatoxin as determined by thin-layer chromatography and enzyme-linked immunosorbent assay analyses of extracts from mycelia and the growth medium. Southern analysis of genomic DNA isolated from the NA-accumulating transformants indicated that the wild-type nor-1 gene in the chromosome had been replaced by the nonfunctional allele carried on pNA51-82. This recombinational inactivation event provides direct evidence that the nor-1 gene is functionally involved in aflatoxin biosynthesis. Comparison of the predicted nor-1 amino acid sequence with sequences in the GenBank and EMBL databases suggested that the protein is a member of the family of short-chain alcohol dehydrogenases, consistent with its proposed function as a keto reductase.

Aspergillus nidulans verA is required for production of the mycotoxin sterigmatocystin

Aspergillus nidulans produces the carcinogenic mycotoxin sterigmatocystin (ST), the next-to-last precursor in the aflatoxin (AF) biosynthetic pathway found in the closely related fungi Aspergillus flavus and Aspergillus parasiticus. We identified and characterized an A. nidulans gene, verA, that is required for converting the AF precursor versicolorin A to ST. verA is closely related to several polyketide biosynthetic genes involved in polyketide production in Streptomyces spp. and exhibits extended sequence similarity to A. parasiticus ver-1, a gene proposed to encode an enzyme involved in converting versicolorin A to ST. By performing a sequence analysis of the region 3' to verA, we identified two additional open reading frames, designated ORF1 and ORF2. ORF2 is closely related to a number of cytochrome P-450 monooxygenases, while ORF1 shares identity with the gamma subunit of translation elongation factor 1. Given that several steps in the ST-AF pathway may require monooxygenase activity and that AF biosynthetic genes are clustered in A. flavus and A. parasiticus, we suggest that verA may be part of a cluster of genes required for ST biosynthesis. We disrupted the verA coding region by inserting the A. nidulans argB gene into the center of the coding region and transformed an A. nidulans argB2 mutant to arginine prototrophy. Seven transformants that produced DNA patterns indicative of a verA disruption event were grown under ST-inducing conditions, and all of the transformants produced versicolorin A but negligible amounts of ST (200-fold to almost 1,000-fold less than the wild type), confirming the hypothesis that verA encodes an enzyme necessary for converting versicolorin A to ST.