Enzymes and aflatoxin biosynthesis

Transcriptomic and Proteomic Insights into the Effect of Sterigmatocystin on Aspergillus flavus

Aspergillus flavus is an important fungus that produces aflatoxins, among which aflatoxin B1 (AFB1) is the most toxic and contaminates food and poses a high risk to human health. AFB1 interacts with another mycotoxin sterigmatocystin (STC), which is also a precursor of AFB1. Herein, we determined the effect of STC on AFB1 by evaluating A. flavus transcriptomic and proteomic profiles in the presence or absence of STC by RNA-seq and isobaric tagging, respectively. Overall, 3377 differentially expressed genes were identified by RNA-seq. These genes were mainly associated with the cellular component organisation and biosynthesis, the synthesis of valine, leucine, and isoleucine, and the synthesis of aflatoxin. Clustered genes responsible for AFB1 biosynthesis exhibited varying degrees of downregulation, and norB expression was completely suppressed in the experimental group. During proteomic analysis, 331 genes were differentially expressed in response to STC. These differentially expressed proteins were associated with cell parts and catalytic and antioxidant activities. Differentially expressed proteins predominantly participated in metabolic pathways associated with aflatoxin biosynthesis, glycolysis/gluconeogenesis, glutathione metabolism, and carbon metabolism. Notably, the upregulated and downregulated enzymes in carbohydrate and glutathione metabolisms may serve as potential gateways for inhibiting aflatoxin biosynthesis. Moreover, twelve proteins including seven downregulated ones involved in aflatoxin biosynthesis were identified; among them, AflG was the most downregulated, suggesting that it may be the key enzyme responsible for inhibiting aflatoxin synthesis. These findings provide novel insights into A. flavus control and the mechanisms regulating mycotoxin production.

Discovery of long non-coding RNAs in Aspergillus flavus response to water activity, CO2 concentration, and temperature changes

Although the role of long non-coding RNAs (lncRNAs) in key biological processes in animals and plants has been confirmed for decades, their identification in fungi remains limited. In this study, we discovered and characterized lncRNAs in Aspergillus flavus in response to changes in water activity, CO₂ concentration, and temperature, and predicted their regulatory roles in cellular functions. A total of 472 lncRNAs were identified in the genome of A. flavus, consisting of 470 novel lncRNAs and 2 putative lncRNAs (EFT00053849670 and EFT00053849665). Our analysis of lncRNA expression revealed significant differential expression under stress conditions in A. flavus. Our findings indicate that lncRNAs in A. flavus, particularly down-regulated lncRNAs, may play pivotal regulatory roles in aflatoxin biosynthesis, respiratory activities, cellular survival, and metabolic maintenance under stress conditions. Additionally, we predicted that sense lncRNAs down-regulated by a temperature of 30 °C, osmotic stress, and CO₂ concentration might indirectly regulate proline metabolism. Furthermore, subcellular localization analysis revealed that up-and down-regulated lncRNAs are frequently localized in the nucleus under stress conditions, particularly at a water activity of 0.91, while most up-regulated lncRNAs may be located in the cytoplasm under high CO₂ concentration.

Advanced mycotoxin control and decontamination techniques in view of an increased aflatoxin risk in Europe due to climate change

Aflatoxins are toxic secondary metabolites produced by Aspergillus spp. found in staple food and feed commodities worldwide. Aflatoxins are carcinogenic, teratogenic, and mutagenic, and pose a serious threat to the health of both humans and animals. The global economy and trade are significantly affected as well. Various models and datasets related to aflatoxins in maize have been developed and used but have not yet been linked. The prevention of crop loss due to aflatoxin contamination is complex and challenging. Hence, the set-up of advanced decontamination is crucial to cope with the challenge of climate change, growing population, unstable political scenarios, and food security problems also in European countries. After harvest, decontamination methods can be applied during transport, storage, or processing, but their application for aflatoxin reduction is still limited. Therefore, this review aims to investigate the effects of environmental factors on aflatoxin production because of climate change and to critically discuss the present-day and novel decontamination techniques to unravel gaps and limitations to propose them as a tool to tackle an increased aflatoxin risk in Europe.

Quantitative analysis of metabolites in the aflatoxin biosynthesis pathway for early warning of aflatoxin contamination by UHPLC-HRMS combined with QAMS

Aflatoxins seriously threaten human health and food safety, and early warning benefits the reasonable use of control measures to reduce aflatoxin contamination. In this study, a novel method for quantifying aflatoxins and their precursors in the aflatoxin biosynthesis pathway was developed by combining ultra-high performance liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS) with quantitative analysis of multi-components by a single marker (QAMS). The stability of the relative correction factor (RCF) of QAMS was then systematically evaluated. The validation results showed that the relative deviation (RD) between QAMS and the external standard method (ESM) was less than 11.7%, indicating that the established QAMS method could replace ESM without the use of reference standards. This method was successfully employed to compare the time-course changes of metabolites in the aflatoxin biosynthesis pathway of Aspergillus flavus and Aspergillus parasitica. As a result, the precursors of (1'S,5'R)-5'-Hydroxyaverantin (HAVN) and Versicolorin B (VerB) could be used as potential markers for the early warning of aflatoxin contamination. This study provided a quantitative method of aflatoxins and their precursors in the biosynthesis pathway, and may serve as a reference for the extension of quantitative studies on other metabolic pathways.

Aflatoxin Biosynthesis and Genetic Regulation: A Review

The study of fungal species evolved radically with the development of molecular techniques and produced new evidence to understand specific fungal mechanisms such as the production of toxic secondary metabolites. Taking advantage of these technologies to improve food safety, the molecular study of toxinogenic species can help elucidate the mechanisms underlying toxin production and enable the development of new effective strategies to control fungal toxicity. Numerous studies have been made on genes involved in aflatoxin B1 (AFB1) production, one of the most hazardous carcinogenic toxins for humans and animals. The current review presents the roles of these different genes and their possible impact on AFB1 production. We focus on the toxinogenic strains Aspergillus flavus and A. parasiticus, primary contaminants and major producers of AFB1 in crops. However, genetic reports on A. nidulans are also included because of the capacity of this fungus to produce sterigmatocystin, the penultimate stable metabolite during AFB1 production. The aim of this review is to provide a general overview of the AFB1 enzymatic biosynthesis pathway and its link with the genes belonging to the AFB1 cluster. It also aims to illustrate the role of global environmental factors on aflatoxin production and the recent data that demonstrate an interconnection between genes regulated by these environmental signals and aflatoxin biosynthetic pathway.

Aflatoxins: Producing-Molds, Structure, Health Issues and Incidence in Southeast Asian and Sub-Saharan African Countries

This review aims to update the main aspects of aflatoxin production, occurrence and incidence in selected countries, and associated aflatoxicosis outbreaks. Means to reduce aflatoxin incidence in crops were also presented, with an emphasis on the environmentally-friendly technology using atoxigenic strains of Aspergillus flavus. Aflatoxins are unavoidable widespread natural contaminants of foods and feeds with serious impacts on health, agricultural and livestock productivity, and food safety. They are secondary metabolites produced by Aspergillus species distributed on three main sections of the genus (section Flavi, section Ochraceorosei, and section Nidulantes). Poor economic status of a country exacerbates the risk and the extent of crop contamination due to faulty storage conditions that are usually suitable for mold growth and mycotoxin production: temperature of 22 to 29 °C and water activity of 0.90 to 0.99. This situation paralleled the prevalence of high liver cancer and the occasional acute aflatoxicosis episodes that have been associated with these regions. Risk assessment studies revealed that Southeast Asian (SEA) and Sub-Saharan African (SSA) countries remain at high risk and that, apart from the regulatory standards revision to be more restrictive, other actions to prevent or decontaminate crops are to be taken for adequate public health protection. Indeed, a review of publications on the incidence of aflatoxins in selected foods and feeds from countries whose crops are classically known for their highest contamination with aflatoxins, reveals that despite the intensive efforts made to reduce such an incidence, there has been no clear tendency, with the possible exception of South Africa, towards sustained improvements. Nonetheless, a global risk assessment of the new situation regarding crop contamination with aflatoxins by international organizations with the required expertise is suggested to appraise where we stand presently.

Effect of Spanish smoked paprika "Pimentón de La Vera" on control of ochratoxin A and aflatoxins production on a dry-cured meat model system

Environmental conditions during ripening of dry-cured meat products favour growth of fungal population on their surface. Some of these moulds can produce mycotoxins. Paprika is one of the ingredients usually used in the formulation of raw-cured sausages, and its addition could influence the growth and production of mycotoxins of the moulds present in these products. In this work the effect of Spanish smoked paprika "Pimentón de la Vera" on growth of Aspergillus parasiticus and Penicillium nordicum and production of aflatoxins B₁ (AFB₁), G₁ (AFG₁) and ochratoxin A (OTA) respectively, was evaluated. Moulds were grown in a culture medium made from lyophilized fresh pork meat added with 4% salt and different concentrations of Spanish smoked paprika (1, 2 and 3%) at several water activity values (0.98, 0.94 and 0.87) and temperature (20-25 °C), to simulate conditions usually found during ripening of dry-cured meat products. Mould growth was evaluated by measuring the diameter of the colony every 24 h, and the production of mycotoxins by UHPLC-MS/MS every 2 days, during 10 days of incubation. Addition of paprika favours growth of the two mould species tested. However, the synthesis of mycotoxins was reduced at 0.94 and 0.98 a_w when at least a 2% of paprika was added. Therefore, the addition of Spanish smoked paprika at 2-3% in the formulations may help to minimize AFs and OTA production in dry-cured meat products such as loins or "chorizo" sausages.

Aflatoxin biosynthesis is a novel source of reactive oxygen species--a potential redox signal to initiate resistance to oxidative stress?

Aflatoxin biosynthesis in the filamentous fungus Aspergillus parasiticus involves a minimum of 21 enzymes, encoded by genes located in a 70 kb gene cluster. For aflatoxin biosynthesis to be completed, the required enzymes must be transported to specialized early and late endosomes called aflatoxisomes. Of particular significance, seven aflatoxin biosynthetic enzymes are P450/monooxygenases which catalyze reactions that can produce reactive oxygen species (ROS) as byproducts. Thus, oxidative reactions in the aflatoxin biosynthetic pathway could potentially be an additional source of intracellular ROS. The present work explores the hypothesis that the aflatoxin biosynthetic pathway generates ROS (designated as "secondary" ROS) in endosomes and that secondary ROS possess a signaling function. We used specific dyes that stain ROS in live cells and demonstrated that intracellular ROS levels correlate with the levels of aflatoxin synthesized. Moreover, feeding protoplasts with precursors of aflatoxin resulted in the increase in ROS generation. These data support the hypothesis. Our findings also suggest that secondary ROS may fulfill, at least in part, an important mechanistic role in increased tolerance to oxidative stress in germinating spores (seven-hour germlings) and in regulation of fungal development.

Real time PCR of Nor~1 (aflD) gene of aflatoxin producing fungi and its correlative quantization to aflatoxin levels in South African compound feeds

Aflatoxins (AFs) are naturally occurring secondary metabolites. This toxin is principally produced by Aspergillus flavus and Aspergillus parasiticus in compound feeds worldwide. Compound feeds are feeds blended from various raw materials and additives. Contaminations of these feeds by AFs and its possible transmission into edible materials like milk, egg and organs of the body, are a serious problem. Expression of the Nor~1 (aflD) gene is the main factor responsible for AFs production. For this reason, a study was carried out to establish a correlation between levels of AFs and determinant gene (Nor~1) in South African compound feeds. To achieve this, compound feeds (n=30) were analyzed for Nor~1 gene using real time polymerase chain reaction (RT-PCR), while AFs levels in similar samples were estimated using high-performance liquid chromatography (HPLC) after an immune-affinity clean-up extraction procedure. Results indicated that AFs levels in positive samples ranged from 0.7 to 33.0 ppb. These levels generally did not correlate (R(2)=0.093) with those of Nor~1 gene in similar samples. Consequently, Nor~1 gene levels established via RT-PCR cannot be used as a predicting model for AFs in compound feeds. Only four of the feeds analyzed, specifically poultry feeds, contained levels of AFs above the regulatory limits of 10 ppb established in South Africa (S.A.). This should be considered unsafe when consumed on a continuous basis and may pose some health related problems especially when AFs are found together with other significant mycotoxins such as ochratoxins (OTs) and/or fumonisins (FBs).

Current understanding on aflatoxin biosynthesis and future perspective in reducing aflatoxin contamination

Traditional molecular techniques have been used in research in discovering the genes and enzymes that are involved in aflatoxin formation and genetic regulation. We cloned most, if not all, of the aflatoxin pathway genes. A consensus gene cluster for aflatoxin biosynthesis was discovered in 2005. The factors that affect aflatoxin formation have been studied. In this report, the author summarized the current status of research progress and future possibilities that may be used for solving aflatoxin contamination.

Expression profiling of non-aflatoxigenic Aspergillus parasiticus mutants obtained by 5-azacytosine treatment or serial mycelial transfer

Aflatoxins are carcinogenic secondary metabolites produced by the fungi Aspergillus flavus and Aspergillus parasiticus. Previous studies found that repeated serial mycelial transfer or treatment of A. parasiticus with 5-azacytidine produced colonies with a fluffy phenotype and inability to produce aflatoxins. To understand how these treatments affect expression of genes involved in aflatoxin production and development, we carried out expressed sequence tag (EST)-based microarray assays to identify genes in treated clones that are differentially expressed compared to the wild-type. Expression of 183 genes was significantly dysregulated. Of these, 38 had at least two-fold or lower expression compared to the untreated control and only two had two-fold or higher expression. The most frequent change was downregulation of genes predicted to encode membrane-bound proteins. Based on this result we hypothesize that the treatments cause changes in the structure of cellular and organelle membranes that prevent normal development and aflatoxin biosynthesis.

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.

C(15)H(24) Volatile Compounds Unique to Aflatoxigenic Strains of Aspergillus flavus

Headspace volatiles from eight strains of Aspergillus flavus (four aflatoxigenic strains and four nonaflatoxigenic strains), grown for 1, 2, 3, 4, 8, and 10 days in submerged cultures, were collected in Tenax GC traps. The traps were desorbed onto a 50-m gas-liquid chromatography capillary column by heat and gas purge from an external direct injector device. The column was interfaced with a mass spectrometer data acquisition system. Peaks were identified by comparing retention times and mass spectra with those obtained from authentic compounds and by using a computer-assisted mass spectral data base. Aflatoxigenic strains of A. flavus produced several C(15)H(24) compounds (e.g., alpha-gurjunene, trans-caryophyllene, and cadinene) which peaked in 3-day cultures and were not present in earlier (1- and 2-day) or later (8- and 10-day) cultures. None of these volatiles were detected in nonaflatoxigenic strains of A. flavus. There was an apparent correlation between the release of C(15)H(24) volatile compounds and the initiation of aflatoxin biosynthesis, and a correlation between decline of aflatoxin synthesis and the disappearance of the C(15)H(24) compounds unique to aflatoxigenic A. flavus also existed.

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.

Study of the Effect of Methyl Jasmonate Concentration on Aflatoxin B(1) Biosynthesis by Aspergillus parasiticus in Yeast Extract Sucrose Medium

Aflatoxin B(1) (AFB(1)) is a carcinogenic metabolite produced by certain Aspergillus species on agricultural commodities. AFB(1) biosynthesis is affected by jasmonic acid and also by its methylester (MeJA), a plant growth regulator derived from linoleic acid. This study reports the effect of MeJA on the growth of A. parasiticus and AFB(1) output in yeast extract sucrose (YES) medium when added at three different concentrations; namely, 10(-2) M, 10(-4) M, and 10(-6) M. AFB(1) determination was performed by immunoaffinity and HPLC. MeJA at 10(-4) and 10(-6) M concentrations had no significant effect on mycelial growth but did affect AFB(1) production after the 7th day of incubation; on the 12th day, AFB(1) production was increased by 212.7% and 141.6% compared to the control samples (addition of 10(-6) M and 10(-4) M MeJA, resp.). Treatment of A. parasiticus cultures with 10(-2) M MeJA inhibited mycelial growth and AFB(1) production as well. These results suggest that the effect of MeJA on AFB(1) biosynthesis by A. parasiticus depends on the MeJA concentration used.

Predicted roles of the uncharacterized clustered genes in aflatoxin biosynthesis

Biosynthesis of the toxic and carcinogenic aflatoxins (AFs) requires the activity of more than 27 enzymes. The roles in biosynthesis of newly described enzymes are discussed in this review. We suggest that HypC catalyzes the oxidation of norsolorinic acid anthrone; AvfA (AflI), the ring-closure step in formation of hydroxyversicolorone; HypB, the second oxidation step in conversion of O-methylsterigmatocystin to AF; and HypE and NorA (AflE), the final two steps in AFB(1) formation. HypD, an integral membrane protein, affects fungal development and lowers AF production while AflJ (AflS), has a partial methyltransferase domain that may be important in its function as a transcriptional co-activator.

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.

Kinetics of the oxidoreductase involved in the conversion of O-methylsterigmatocystin to aflatoxin B1

Among the enzymatic steps in the aflatoxin biosynthetic pathway, the conversion of O-methylsterigmatocystin (OMST) to the potent environmental carcinogen aflatoxin B1 (AFB1), has been proposed to be catalysed by an oxidoreductase (OR) that requires a cytochrome P-450 type of oxidoreductase activity. This enzyme displays relative specificity towards OMST homologues in fungal whole cells. These studies were extended to the action of a cell-free enzyme system (CFES), on five OMST homologues, with a view to establish the kinetics. In the current study a CFES, containing an oxidoreductase, was derived from a blocked mutant of Aspergillus parasiticus (Wh1-11-105). The key experimental steps involved rapid concentration and efficient dialysis by membrane filtration to remove small biomolecules (MW<10,000), co-factors, primary and secondary metabolites. The kinetic parameters of the enzyme-substrate reactions indicated that the reaction follows a Michealis-Menten kinetics and OR activity decreased in the order: O-butylsterigmatocystin>O-propylsterigmatocystin>O-ethylsterigmatocystin>O-methylsterigmatocystin>O-acetylsterigmatocystin>O-benzoylsterigmatocystin. The 7-O-alkyl homologues were the best substrate for the CFES, thereby substantially supporting that the 7-O-methyl group of OMST is preferred for OR catalytic activity in the absence of any other alkylating groups in vitro. The Km was calculated as 5.65 microM for this CFES and varied marginally among the OMST homologues studied.

Molecular analysis of an inactive aflatoxin biosynthesis gene cluster in Aspergillus oryzae RIB strains

To help assess the potential for aflatoxin production by Aspergillus oryzae, the structure of an aflatoxin biosynthesis gene homolog cluster in A. oryzae RIB 40 was analyzed. Although most genes in the corresponding cluster exhibited from 97 to 99% similarity to those of Aspergillus flavus, three genes shared 93% similarity or less. A 257-bp deletion in the aflT region, a frameshift mutation in norA, and a base pair substitution in verA were found in A. oryzae RIB 40. In the aflR promoter, two substitutions were found in one of the three putative AreA binding sites and in the FacB binding site. PCR primers were designed to amplify homologs of aflT, nor-1, aflR, norA, avnA, verB, and vbs and were used to detect these genes in 210 A. oryzae strains. Based on the PCR results, the A. oryzae RIB strains were classified into three groups, although most of them fell into two of the groups. Group 1, in which amplification of all seven genes was confirmed, contained 122 RIB strains (58.1% of examined strains), including RIB 40. Seventy-seven strains (36.7%) belonged to group 2, characterized by having only vbs, verB, and avnA in half of the cluster. Although slight expression of aflR was detected by reverse transcription-PCR in some group 1 strains, including RIB 40, other genes (avnA, vbs, verB, and omtA) related to aflatoxin production were not detected. aflR was not detected in group 2 strains by Southern analysis.

Connection of propionyl-CoA metabolism to polyketide biosynthesis in Aspergillus nidulans

Propionyl-CoA is an intermediate metabolite produced through a variety of pathways including thioesterification of propionate and catabolism of odd chain fatty acids and select amino acids. Previously, we found that disruption of the methylcitrate synthase gene, mcsA, which blocks propionyl-CoA utilization, as well as growth on propionate impaired production of several polyketides-molecules typically derived from acetyl-CoA and malonyl-CoA-including sterigmatocystin (ST), a potent carcinogen, and the conidiospore pigment. Here we describe three lines of evidence that demonstrate that excessive propionyl-CoA levels in the cell can inhibit polyketide synthesis. First, inactivation of a putative propionyl-CoA synthase, PcsA, which converts propionate to propionyl-CoA, restored polyketide production and reduced cellular propionyl-CoA content in a DeltamcsA background. Second, inactivation of the acetyl-CoA synthase, FacA, which is also involved in propionate utilization, restored polyketide production in the DeltamcsA background. Third, fungal growth on several compounds (e.g., heptadecanoic acid, isoleucine, and methionine) whose catabolism includes the formation of propionyl-CoA, were found to inhibit ST and conidiospore pigment production. These results demonstrate that excessive propionyl-CoA levels in the cell can inhibit polyketide synthesis.

Aflatoxin biosynthesis cluster gene cypA is required for G aflatoxin formation

Aspergillus flavus isolates produce only aflatoxins B1 and B2, while Aspergillus parasiticus and Aspergillus nomius produce aflatoxins B1, B2, G1, and G2. Sequence comparison of the aflatoxin biosynthesis pathway gene cluster upstream from the polyketide synthase gene, pksA, revealed that A. flavus isolates are missing portions of genes (cypA and norB) predicted to encode, respectively, a cytochrome P450 monooxygenase and an aryl alcohol dehydrogenase. Insertional disruption of cypA in A. parasiticus yielded transformants that lack the ability to produce G aflatoxins but not B aflatoxins. The enzyme encoded by cypA has highest amino acid identity to Gibberella zeae Tri4 (38%), a P450 monooxygenase previously shown to be involved in trichodiene epoxidation. The substrate for CypA may be an intermediate formed by oxidative cleavage of the A ring of O-methylsterigmatocystin by OrdA, the P450 monooxygenase required for formation of aflatoxins B1 and B2.

Value-added food: single cell protein

The alarming rate of population growth has increased the demand for food production in third-world countries leading to a yawning gap in demand and supply. This has led to an increase in the number of hungry and chronically malnourished people. This situation has created a demand for the formulation of innovative and alternative proteinaceous food sources. Single cell protein (SCP) production is a major step in this direction. SCP is the protein extracted from cultivated microbial biomass. It can be used for protein supplementation of a staple diet by replacing costly conventional sources like soymeal and fishmeal to alleviate the problem of protein scarcity. Moreover, bioconversion of agricultural and industrial wastes to protein-rich food and fodder stocks has an additional benefit of making the final product cheaper. This would also offset the negative cost value of wastes used as substrate to yield SCP. Further, it would make food production less dependent upon land and relieve the pressure on agriculture. This article reviews diversified aspects of SCP as an alternative protein-supplementing source. Various potential strains and substrates that could be utilized for SCP production are described. Nutritive value and removal of nucleic acids and toxins from SCP as a protein-supplementing source are discussed. New processes need to be exploited to improve yield. In that direction the solid state fermentation (SSF) method and its advantages for SCP production are highlighted.

adhA in Aspergillus parasiticus is involved in conversion of 5'-hydroxyaverantin to averufin

Two routes for the conversion of 5'-hydroxyaverantin (HAVN) to averufin (AVF) in the synthesis of aflatoxin have been proposed. One involves the dehydration of HAVN to the lactone averufanin (AVNN), which is then oxidized to AVF. Another requires dehydrogenation of HAVN to 5'-ketoaverantin, the open-chain form of AVF, which then cyclizes spontaneously to AVF. We isolated a gene, adhA, from the aflatoxin gene cluster of Aspergillus parasiticus SU-1. The deduced ADHA amino acid sequence contained two conserved motifs found in short-chain alcohol dehydrogenases-a glycine-rich loop (GXXXGXG) that is necessary for interaction with NAD(+)-NADP(+), and the motif YXXXK, which is found at the active site. A. parasiticus SU-1, which produces aflatoxins, has two copies of adhA (adhA1), whereas A. parasiticus SRRC 2043, a strain that accumulates O-methylsterigmatocystin (OMST), has only one copy. Disruption of adhA in SRRC 2043 resulted in a strain that accumulates predominantly HAVN. This result suggests that ADHA is involved in the dehydrogenation of HAVN to AVF. Those adhA disruptants that still made small amounts of OMST also accumulated other metabolites, including AVNN, after prolonged culture.

Cloning and characterization of avfA and omtB genes involved in aflatoxin biosynthesis in three Aspergillus species

The biosynthesis of aflatoxins (B(1), G(1), B(2), and G(2)) is a multi-enzyme process controlled genetically by over 20 genes. In this study, we report the identification and characterization of the avfA gene, which was found to be involved in the conversion of averufin (AVF) to versiconal hemiacetal acetate (VHA), in Aspergillus parasiticus and A. flavus; a copy of avfA gene was also cloned from a non-aflatoxin producing strain A. sojae. Complementation of an averufin-accumulating, non-aflatoxigenic mutant strain of A. parasiticus, SRRC 165, with the avfA gene cloned from A. flavus, restored the ability of the mutant to convert AVF to VHA and to produce aflatoxins B(1), G(1), B(2), and G(2). Sequence analysis revealed that a single amino acid replacement from aspartic acid to asparagine disabled the function of the enzyme in the mutant strain SRRC 165. The A. parasiticus avfA was identified to be a homolog of previously sequenced, but functionally unassigned transcript, stcO, in A. nidulans based on sequence homology at both nucleotide (57%) and amino acid (55%) levels. In addition to avfA, another aflatoxin pathway gene, omtB, encoding for an O-methyltransferase involved in the conversion of demethylsterigmatocystin (DMST) to sterigmatocystin (ST) and dihydrodemethylsterigmatocystin (DHDMST) to dihydrosterigmatocystin (DHST), was cloned from A. parasiticus, A. flavus, and A. sojae. The omtB gene was found to be highly homologous to stcP from A. nidulans, which has been reported earlier to be involved in a similar enzymatic step for the sterigmatocystin formation in that species. RT-PCR data demonstrated that both the avfA and avfA1 as well as omtB genes in A. parasiticus were expressed only in the aflatoxin-conducive medium. An analysis of the degrees of homology for the two reported genes between the Aspergillus species A. parasiticus, A. flavus, A. nidulans and A. sojae was conducted.

Enzymatic function of the nor-1 protein in aflatoxin biosynthesis in Aspergillus parasiticus

The nor-1 gene is involved in aflatoxin biosynthesis in Aspergillus parasiticus and was predicted to encode a norsolorinic acid ketoreductase. Recombinant Nor-1 expressed in Escherichia coli converted the 1' keto group of norsolorinic acid to the 1' hydroxyl group of averantin in crude E. coli cell extracts in the presence of NADPH. The results confirm that Nor-1 functions as a ketoreductase in vitro.

Cloning and characterization of the O-methyltransferase I gene (dmtA) from Aspergillus parasiticus associated with the conversions of demethylsterigmatocystin to sterigmatocystin and dihydrodemethylsterigmatocystin to dihydrosterigmatocystin in aflatoxin biosynthesis

O-Methyltransferase I catalyzes both the conversion of demethylsterigmatocystin to sterigmatocystin and the conversion of dihydrodemethylsterigmatocystin to dihydrosterigmatocystin during aflatoxin biosynthesis. In this study, both genomic cloning and cDNA cloning of the gene encoding O-methyltransferase I were accomplished by using PCR strategies, such as conventional PCR based on the N-terminal amino acid sequence of the purified enzyme, 5' and 3' rapid amplification of cDNA ends PCR, and thermal asymmetric interlaced PCR (TAIL-PCR), and genes were sequenced by using Aspergillus parasiticus NIAH-26. A comparison of the genomic sequences with the cDNA of the dmtA region revealed that the coding region is interrupted by three short introns. The cDNA of the dmtA gene is 1,373 bp long and encodes a 386-amino-acid protein with a deduced molecular weight of 43,023, which is consistent with the molecular weight of the protein determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The C-terminal half of the deduced protein exhibits 76.3% identity with the coding region of the Aspergillus nidulans StcP protein, whereas the N-terminal half of dmtA exhibits 73.0% identity with the 5' flanking region of the stcP gene, suggesting that translation of the stcP gene may start at a site upstream from methionine that is different from the site that has been suggested previously. Also, an examination of the 5' and 3' flanking regions of the dmtA gene in which TAIL-PCR was used demonstrated that the dmtA gene is located in the aflatoxin biosynthesis cluster between (and in the same orientation as) the omtA and ord-2 genes. Northern blotting revealed that expression of the dmtA gene is influenced by both medium composition and culture temperature and that the pattern correlates with the patterns observed for other genes in the aflatoxin gene cluster. Furthermore, Southern blotting and PCR analyses of the dmtA gene showed that a dmtA homolog is present in Aspergillus oryzae SYS-2.

Enzymatic formation of G-group aflatoxins and biosynthetic relationship between G- and B-group aflatoxins

We detected biosynthetic activity for aflatoxins G(1) and G(2) in cell extracts of Aspergillus parasiticus NIAH-26. We found that in the presence of NADPH, aflatoxins G(1) and G(2) were produced from O-methylsterigmatocystin and dihydro-O-methylsterigmatocystin, respectively. No G-group aflatoxins were produced from aflatoxin B(1), aflatoxin B(2), 5-methoxysterigmatocystin, dimethoxysterigmatocystin, or sterigmatin, confirming that B-group aflatoxins are not the precursors of G-group aflatoxins and that G- and B-group aflatoxins are independently produced from the same substrates (O-methylsterigmatocystin and dihydro-O-methylsterigmatocystin). In competition experiments in which the cell-free system was used, formation of aflatoxin G(2) from dihydro-O-methylsterigmatocystin was suppressed when O-methylsterigmatocystin was added to the reaction mixture, whereas aflatoxin G(1) was newly formed. This result indicates that the same enzymes can catalyze the formation of aflatoxins G(1) and G(2). Inhibition of G-group aflatoxin formation by methyrapone, SKF-525A, or imidazole indicated that a cytochrome P-450 monooxygenase may be involved in the formation of G-group aflatoxins. Both the microsome fraction and a cytosol protein with a native mass of 220 kDa were necessary for the formation of G-group aflatoxins. Due to instability of the microsome fraction, G-group aflatoxin formation was less stable than B-group aflatoxin formation. The ordA gene product, which may catalyze the formation of B-group aflatoxins, also may be required for G-group aflatoxin biosynthesis. We concluded that at least three reactions, catalyzed by the ordA gene product, an unstable microsome enzyme, and a 220-kDa cytosol protein, are involved in the enzymatic formation of G-group aflatoxins from either O-methylsterigmatocystin or dihydro-O-methylsterigmatocystin.

Characterization of the critical amino acids of an Aspergillus parasiticus cytochrome P-450 monooxygenase encoded by ordA that is involved in the biosynthesis of aflatoxins B1, G1, B2, and G2

The conversion of O-methylsterigmatocystin (OMST) and dihydro-O-methylsterigmatocystin to aflatoxins B1, G1, B2, and G2 requires a cytochrome P-450 type of oxidoreductase activity. ordA, a gene adjacent to the omtA gene, was identified in the aflatoxin-biosynthetic pathway gene cluster by chromosomal walking in Aspergillus parasiticus. The ordA gene was a homolog of the Aspergillus flavus ord1 gene, which is involved in the conversion of OMST to aflatoxin B1. Complementation of A. parasiticus SRRC 2043, an OMST-accumulating strain, with the ordA gene restored the ability to produce aflatoxins B1, G1, B2, and G2. The ordA gene placed under the control of the GAL1 promoter converted exogenously supplied OMST to aflatoxin B1 in Saccharomyces cerevisiae. In contrast, the ordA gene homolog in A. parasiticus SRRC 2043, ordA1, was not able to carry out the same conversion in the yeast system. Sequence analysis revealed that the ordA1 gene had three point mutations which resulted in three amino acid changes (His-400-->Leu-400, Ala-143-->Ser-143, and Ile-528-->Tyr-528). Site-directed mutagenesis studies showed that the change of His-400 to Leu-400 resulted in a loss of the monooxygenase activity and that Ala-143 played a significant role in the catalytic conversion. In contrast, Ile-528 was not associated with the enzymatic activity. The involvement of the ordA gene in the synthesis of aflatoxins G1, and G2 in A. parasiticus suggests that enzymes required for the formation of aflatoxins G1 and G2 are not present in A. flavus. The results showed that in addition to the conserved heme-binding and redox reaction domains encoded by ordA, other seemingly domain-unrelated amino acid residues are critical for cytochrome P-450 catalytic activity. The ordA gene has been assigned to a new cytochrome P-450 gene family named CYP64 by The Cytochrome P450 Nomenclature Committee.

Characterization of aflJ, a gene required for conversion of pathway intermediates to aflatoxin

The genes encoding the aflatoxin biosynthetic pathway enzymes have been localized as a cluster to a 75-kb DNA fragment. The enzymatic functions of the products of most of the genes in the cluster are known, but there are a few genes that have not yet been characterized. We report here the characterization of one of these genes, a gene designated aflJ. This gene resides in the cluster adjacent to the pathway regulatory gene, aflR, and the two genes are divergently transcribed. Disruption of aflJ in Aspergillus flavus results in a failure to produce aflatoxins and a failure to convert exogenously added pathway intermediates norsolorinic acid, sterigmatocystin, and O-methylsterigmatocystin to aflatoxin. The disrupted strain does, however, accumulate pksA, nor-1, ver-1, and omtA transcripts under conditions conducive to aflatoxin biosynthesis. Therefore, disruption of aflJ does not affect transcription of these genes, and aflJ does not appear to have a regulatory function similar to that of aflR. Sequence analysis of aflJ and its putative peptide, AflJ, did not reveal any enzymatic domains or significant similarities to proteins of known function. The putative peptide does contain three regions predicted to be membrane-spanning domains and a microbodies C-terminal targeting signal.

Genetics and physiology of aflatoxin biosynthesis

Aflatoxins are the most thoroughly studied mycotoxins. Elegant early research on the biosynthetic scheme of the pathway has allowed a molecular characterization of aflatoxin biosynthesis and its regulation. Genetic studies on aflatoxin biosynthesis in Aspergillus flavus and A. parasiticus, and sterigmatocystin biosynthesis in A. nidulans, led to the cloning of 17 genes responsible for 12 enzymatic conversions in the AF/ST pathways. Pathway-specific regulation is by a Zn(II)2Cys6 DNA-binding protein that regulates the transcription of all pathway genes. Less is known about the global factors that regulate aflatoxin biosynthesis, but there is a clear link between development and aflatoxin biosynthesis. There is also a large body of information on physiological factors involved in aflatoxin biosynthesis, but it has been difficult to understand their role in the regulation of this pathway. This chapter discusses current knowledge on the molecular biology and genetics of the pathway, and provides a summary of the physiological factors known to influence aflatoxin formation.

Effect of Carotenoids on Aflatoxin B(1) Synthesis by Aspergillus flavus

ABSTRACT Carotenes and xanthophylls occurring in yellow corn and related terpenoids were tested for their effect on growth and aflatoxin B(1) production by Aspergillus flavus NRRL 3357, using the suspended disc culture method. Aflatoxin synthesis was inhibited at concentrations of beta-carotene, lutein, and zeaxanthin comparable to those found in the horny endosperm of mature corn. Usually growth was not significantly affected. Inhibition of aflatoxin biosynthesis was greater for compounds with an alpha-ionone-type ring (alpha-carotene, lutein, or alpha-ionone) compared with compounds with a beta-ionone ring. The presence of hydroxy groups on the rings tended to decrease inhibition, but did not override the effect of the ring type; lutein was similar to alpha-carotene and zeaxanthin was similar to beta-carotene in inhibition. A mutant accumulating norsolorinic acid (NA), A. parasiticus SRRC 162, incubated with alpha-carotene produced reduced levels of both NA and aflatoxin, indicating that inhibition occurred before NA. Additional A. flavus strains tested against 50 mug/ml of beta-carotene had 89 to 96% inhibition, which was significantly more sensitive than NRRL 3357. A. parasiticus strains were less sensitive and generally had similar or lower inhibition than NRRL 3357. The results indicate that the presence of carotenoids in endosperm may decrease the amount of aflatoxin produced by A. flavus.

avnA, a gene encoding a cytochrome P-450 monooxygenase, is involved in the conversion of averantin to averufin in aflatoxin biosynthesis in Aspergillus parasiticus

Recent studies have shown that at least 17 genes involved in the aflatoxin biosynthetic pathway are clustered within a 75-kb DNA fragment in the genome of Aspergillus parasiticus. Several additional transcripts have also been mapped to this gene cluster. A gene, avnA (previously named ord-1), corresponding to one of the two transcripts identified earlier between the ver-1 and omtA genes on the gene cluster was sequenced. The nucleotide sequence of the avnA gene contains a coding region for a protein of 495 amino acids with a calculated molecular mass of 56.3 kDa. The gene consists of three exons and two introns. Disruption of the avnA gene in the wild-type aflatoxigenic A. parasiticus strain (SU1-N3) resulted in a nonaflatoxigenic mutant which accumulated a bright yellow pigment. Thin-layer chromatographic studies with six different solvent systems showed that the migration patterns of the accumulated metabolite were identical to those of averantin, a known aflatoxin precursor. Precursor feeding studies with this mutant showed that norsolorinic acid and averantin were not converted to aflatoxin whereas 5'-hydroxyaverantin, averufanin, averufin, versicolorin A. sterigmatocystin, and O-methylsterigmatocystin were converted to aflatoxins. Southern blot analysis of the wild-type strain and avnA-disrupted mutant strain indicated that the avnA gene was disrupted in the mutant strain. A search of the GenBank database for similarity indicated that the avnA gene encodes a cytochrome P-450-type monooxygenase, and it has been assigned to a new P-450 gene family named CYP60A1. We have therefore concluded that the avnA gene encodes a fungal cytochrome P-450-type enzyme which is involved in the conversion of averantin to averufin in the aflatoxin biosynthetic pathway in A. parasiticus.

Relationship between aflatoxin biosynthesis and sporulation in Aspergillus parasiticus

Regulation of aflatoxin biosynthesis during differentiation of Aspergillus parasiticus was analyzed by using a drug that inhibits the development of the fungus and mutants affected in sporulation. Diaminobutanone, a competitive inhibitor of ornithine decarboxylase, repressed spore germination. If added after spore germination had occurred, it blocked sporulation completely and suppressed aflatoxin biosynthesis, but was only partially inhibitory of mycelial growth. Putrescine partially counteracted the inhibitory effect of the drug on both sporulation and aflatoxin biosynthesis. Analysis of mutants affected in sporulation confirmed the existence of a relationship between sporulation and aflatoxin formation. A nonsporulating mutant was unable to synthesize aflatoxins. In a sectorial mutant, the sporulating sector synthesized aflatoxins normally, whereas the asporogenous sector was unable to do so. It is suggested that regulation of aflatoxin biosynthesis is correlated with the sporulation process.

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.

Heterologous expression, isolation, and characterization of versicolorin B synthase from Aspergillus parasiticus. A key enzyme in the aflatoxin B1 biosynthetic pathway

Aflatoxin B1 is a potent environmental carcinogen produced by certain strains of Aspergillus. Central to the biosynthesis of this mycotoxin is the reaction catalyzed by versicolorin B synthase (VBS) in which a racemic substrate, versiconal hemiacetal, is cyclized to an optically active product whose absolute configuration is crucial to the interaction of aflatoxin B1 with DNA. Attempted over-production of VBS in Escherichia coli led principally to protein aggregated into inclusion bodies but also small amounts of soluble but catalytically inactive enzyme. Comparisons to wild-type VBS by SDS-polyacrylamide gel electrophoresis and after N-glycosidase F treatment revealed that extensive glycosylation accounted for the mass discrepancy (7,000+/-1,500 Da) between the native and bacterially expressed proteins. Several over-expression systems in Saccharomyces cerevisiae were surveyed in which one that incorporated a secretion signal was found most successful. VBS of indistinguishable mass on SDS-polyacrylamide gel electrophoresis and kinetic properties from the wild-type enzyme could be obtained in 50-100-fold greater amounts and whose catalytic behavior has been examined. The translated protein sequence of VBS showed three potential N-glycosylation sites (Asn-Xaa-Ser/Thr) consistent with the modifications observed above and unexpectedly revealed extensive homology to the ADP-binding region prominently conserved in the glucose-methanol-choline (GMC) family of flavoenzymes. Over-production of VBS in yeast marks the first aflatoxin biosynthetic enzyme to be so obtained and opens the way to direct study of the enzymology of this complex biosynthetic pathway.

Aspergillus has distinct fatty acid synthases for primary and secondary metabolism

Aspergillus nidulans contains two functionally distinct fatty acid synthases (FASs): one required for primary fatty acid metabolism (FAS) and the other required for secondary metabolism (sFAS). FAS mutants require long-chain fatty acids for growth, whereas sFAS mutants grow normally but cannot synthesize sterigmatocystin (ST), a carcinogenic secondary metabolite structurally and biosynthetically related to aflatoxin. sFAS mutants regain the ability to synthesize ST when provided with hexanoic acid, supporting the model that the ST polyketide synthase uses this short-chain fatty acid as a starter unit. The characterization of both the polyketide synthase and FAS may provide novel means for modifying secondary metabolites.

Characterization of the function of the ver-1A and ver-1B genes, involved in aflatoxin biosynthesis in Aspergillus parasiticus

The ver-1A gene was cloned and its nucleotide sequence was determined as part of a previous study on aflatoxin B1 (AFB1) biosynthesis in the filamentous fungus Aspergillus parasiticus SU-1. A second copy of this gene, ver-1B, was tentatively identified in this fungal strain. In this study, ver-1B was cloned by screening an A. parasiticus cosmid library with a ver-1A probe. The nucleotide sequence of ver-1B was determined. The predicted amino acid sequence of ver-1B had 95% identity with ver-1A. A translational stop codon, found in the ver-1B gene coding region, indicated that it encodes a truncated polypeptide. To confirm the function of the ver-1 genes in AFB1 synthesis, a plasmid (pDV-VA) was designed to disrupt ver-1A and/or ver-1B by transformation of the AFB1 producer A. parasiticus NR-1. One disruptant, VAD-102, which accumulated the pathway intermediate versicolorin A was obtained. Southern hybridization analysis of VAD-102 revealed that ver-1A but not ver-1B was disrupted. A functional ver-1A gene was transformed back into strain VAD-102. Transformants which received ver-1A produced AFB1, confirming that ver-1A is the only functional ver-1 gene in A. parasiticus SU-1 and that its gene product is involved in the conversion of versicolorin A to sterigmatocystin in AFB1 biosynthesis. A duplicated chromosomal region (approximately 12 kb) was identified upstream from ver-1A and ver-1B by Southern hybridization analysis. This duplicated region contained the aflR gene, which is proposed to be one regulator of AFB1, synthesis. A similar gene duplication was also identified in several other strains of A. parasiticus.

Conservation of structure and function of the aflatoxin regulatory gene aflR from Aspergillus nidulans and A. flavus

Under limiting growth conditions, Aspergillus nidulans produces a carcinogenic secondary metabolite related to aflatoxin and called sterigmatocystin (ST). The genes for ST biosynthesis are co-ordinately regulated and are all found within an approximately 60-kilobase segment of DNA. One of the genes within this region is predicted to encode a CX2CX6CX6CX2CX6CX2 zinc binuclear cluster DNA-binding protein that is related to the Aspergillus flavus and Aspergillus parasiticus aflatoxin regulatory gene aflR. Deletion of the A. nidulans aflR homolog resulted in an inability to induce expression of genes within the ST gene cluster and a loss of ST production. Because A. nidulans aflR mRNA accumulates specifically under conditions that favor ST production we expect that activation of ST biosynthetic genes is determined by A. nidulans aflR. In support of this hypothesis, we demonstrated that induced expression of the A. flavus aflR gene in A. nidulans, under conditions that normally suppress ST gene expression, resulted in activation of genes in the ST biosynthetic pathway. This result demonstrates that AflR function is conserved between Aspergillus spp. and that aflR expression is sufficient to activate genes in the ST pathway.

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.

stcS, a putative P-450 monooxygenase, is required for the conversion of versicolorin A to sterigmatocystin in Aspergillus nidulans

Sterigmatocystin (ST) and aflatoxin are carcinogenic end point metabolites derived from the same biochemical pathway, which is found in several Aspergillus spp. Recently, an ST gene cluster, containing approximately 25 distinct genes that are each proposed to function specifically in ST biosynthesis, has been identified in Aspergillus nidulans. Each of these structural genes is named stc (sterigmatocystin) followed by a consecutive letter of the alphabet. We have previously described stcU (formerly verA) as encoding a keto-reductase required for the conversion of versicolorin A to ST. We now describe a second A. nidulans gene, stcS (formerly verB), that is located within 2 kb of stcU in the ST gene cluster. An stcS-disrupted strain of A. nidulans, TSS17, was unable to produce ST and converted ST/aflatoxin precursors to versicolorin A rather than ST, indicating that stcS functions at the same point in the pathway as stcU. Genomic sequence analysis of stcS shows that it encodes a cytochrome P-450 monooxygenase and constitutes a novel P-450 family, CYP59. Assuming that StcU activity mimics that of similar P-450s, it is likely that StcU catalyzes one of the proposed oxidation steps necessary to convert versicolorin A to ST. These results constitute the first genetic proof that the conversion of versicolorin A to ST requires more than one enzymatic activity.

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.

The Aspergillus parasiticus polyketide synthase gene pksA, a homolog of Aspergillus nidulans wA, is required for aflatoxin B1 biosynthesis

Aflatoxins comprise a group of polyketide-derived carcinogenic mycotoxins produced by Aspergillus parasiticus and Aspergillus flavus. By transformation with a disruption construct, pXX, we disrupted the aflatoxin pathway in A. parasiticus SRRC 2043, resulting in the inability of this strain to produce aflatoxin intermediates as well as a major yellow pigment in the transformants. The disruption was attributed to a single-crossover, homologous integration event between pXX and the recipient A. parasiticus genome at a specific locus, designated pksA. Sequence analysis suggest that pksA is a homolog of the Aspergillus nidulans wA gene, a polyketide synthase gene involved in conidial wall pigment biosynthesis. The conserved beta-ketoacyl synthase, acyltransferase and acyl carrier-protein domains were present in the deduced amino acid sequence of the pksA product. No beta-ketoacyl reductase and enoyl reductase domains were found, suggesting that pksA does not encode catalytic activities for processing beta-carbon similar to those required for long chain fatty acid synthesis. The pksA gene is located in the aflatoxin pathway gene cluster and is linked to the nor-1 gene, an aflatoxin pathway gene required for converting norsolorinic acid to averantin. These two genes are divergently transcribed from a 1.5 kb intergenic region. We propose that pksA is a polyketide synthase gene required for the early steps of aflatoxin biosynthesis.

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.

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.