Isolation of averufin from a mutant of Aspergillus parasiticus impaired in aflatoxin biosynthesis

Palm Kernel: A Potential Substrate for Rapid Detection of Aflatoxigenic Fungi

Palm kernel is a cheap natural resource which is abundantly available in the tropics, parts of Asia and South and Central America. A culture medium was developed by incorporating fresh palm kernel extract for the detection of aflatoxigenic fungi. Aflatoxin positive isolates of Aspergilliexhibited a characteristic blue or blue green fluorescence of agar under long wave UV light against a pink background which was confirmed by thin layer chromatography. As compared to conventional desiccated coconut agar, the fluorescent nature of the medium, the intensity and diffusion of the hot water soluble fluorescent compounds of the fungus was unique on this medium. The optimal pH and temperature conditions of aflatoxin production were 7 and 30 ºC respectively. Additives (synthetic and natural) either had no effect or adversely affected the fluorescence of the medium. Aflatoxin detection was possible within 36h in palm kernel broth compared to 40 h in coconut broth. The optimal time of production of fluorescence was 44 h on palm kernel agar compared to 48 h on the conventional medium. Further tests with isolates from different sources showed that yellow pigmentation, fluorescence and aflatoxins were complementary thus obviating the need for UV light. It is thus possible to presumptively identify aflatoxin positive isolates.

Surveys of non-ribosomal peptide and polyketide assembly lines in fungi and prospects for their analysis in vitro and in vivo

With many bioactive non-ribosomal peptides and polyketides produced in fungi, studies of their biosyntheses are an active area of research. Practical limitations of working with mega-dalton synthetases including cell lysis and protein extraction to recombinant gene and pathway expression has slowed understanding of many secondary metabolic processes relative to bacterial counterparts. Recent advances in accessing fungal biosynthetic machinery are beginning to change this. Here we describe the successes of some studies of thiotemplate biosynthesis in fungal systems, along with very recent advances in chemical tagging and mass spectrometric strategies to selectively study biosynthetic conveyer belts in isolation, and within a few years, in endogenous fungal proteomes.

Selective inhibition of acyl-CoA:cholesterol acyltransferase 2 isozyme by flavasperone and sterigmatocystin from Aspergillus species

Five known fungal metabolites, aurasperone A, aurasperone D, averufanin, flavasperone and sterigmatocystin, were isolated from the culture broths of Aspergillus species as inhibitors of acyl-CoA:cholesterol acyltransferase (ACAT) in the cell-based assay using ACAT1- and ACAT2-expressing CHO cells. These compounds share a similar polycyclic skeleton. Among them, flavasperone and sterigmatocystin, having an angular skeleton, showed selective inhibition toward ACAT2 isozyme, while the others having a linear one had no selectivity in inhibition.

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.

Synthesis of sterigmatocystin derivatives and their biotransformation to aflatoxins by a blocked mutant of Aspergillus parasiticus

Seven alkyl and aryl homologues of O-methylsterigmatocystin (OMST) were synthesised and fed in separate experiments to a mutant of Aspergillus parasiticus capable of converting sterigmatocystin (ST) to aflatoxin B1 (AFB1). Their conversion to AFB1 was followed over a time period and it was found that O-propylsterigmatocystin (OPRST) was converted to AFB1 more rapidly than O-ethylsterigmatocystin (OEST) or OMST or ST itself. The aryl derivative O-benzoylsterigmatocystin (OBzST) was converted at the slowest rate. These results show that alkyl and aryl homologues of OMST may be converted to AFB1, suggesting that the methylation of ST is not an absolute requirement for its conversion to AFB1. It seems likely that whatever enzyme(s) are involved in this process exhibit relative specificity. As to whether alkylation of ST is an obligatory step in AFB1 biosynthesis is neither supported nor disproved as the fungal cells used are presumably capable of methylating ST. The fact that the propyl derivative showed fastest conversion is not necessarily significant as this may be due to faster diffusion of the least polar of the derivatives through the cell membrane.

Identification of aflatoxin biosynthesis genes by genetic complementation in an Aspergillus flavus mutant lacking the aflatoxin gene cluster

Aspergillus flavus mutant strain 649, which has a genomic DNA deletion of at least 120 kb covering the aflatoxin biosynthesis cluster, was transformed with a series of overlapping cosmids that contained DNA harboring the cluster of genes. The mutant phenotype of strain 649 was rescued by transformation with a combination of cosmid clones 5E6, 8B9, and 13B9, indicating that the cluster of genes involved in aflatoxin biosynthesis resides in the 90 kb of A. flavus genomic DNA carried by these clones. Transformants 5E6 and 20B11 and transformants 5E6 and 8B9 accumulated intermediate metabolites of the aflatoxin pathway, which were identified as averufanin and/or averufin, respectively. These data suggest that avf1, which is involved in the conversion of averufin to versiconal hemiacetal acetate, was present in the cosmid 13B9. Deletion analysis of 13B9 located the gene on a 7-kb DNA fragment of the cosmid. Transformants containing cosmid 8B9 converted exogenously supplied O-methylsterigmatocystin to aflatoxin, indicating that the oxidoreductase gene (ord1), which mediates the conversion of O-methylsterigmatocystin to aflatoxin, is carried by this cosmid. The analysis of transformants containing deletions of 8B9 led to the localization of ord1 on a 3.3-kb A. flavus genomic DNA fragment of the cosmid.

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.

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.

A red pigment synthesized by an Aspergillus parasiticus mutant as a possible new intermediate in the aflatoxin biosynthetic pathway

The isolation of a red pigment from an Aspergillus parasiticus mutant obtained by 366 nm u.v. light treatment of A. parasiticus NRRL 2999 is described. Studies of conversion in aflatoxin B1 and G1 suggest that the red pigment could be a possible new intermediate in the aflatoxin biosynthetic pathway not described to date, and this has been verified by studies in gas chromatography/mass spectrometry. The solubility and stability characteristics under refrigeration storage, and the influence of the temperature and the pH on its production by the A. parasiticus mutant were also studied. It grew best at 30 degrees C and pH 6. The red pigment was most soluble in ethyl acetate. The results obtained in water are emphasized where there was high stability.

Comparison of the use of UV light and nitrosoguanidine as mutagenic treatments in Aspergillus parasiticus

In the present work we study two different mutagenic methods in Aspergillus parasiticus: UV light and Nitrosoguanidine treatments. With UV light we obtained more suitable results, with survival percentage around 1% in many experiments. With Nitrosoguanidine our results were around 10% survival rate. We suggest the use of UV light to obtain mutants from A. parasiticus.

Cloning of a gene associated with aflatoxin B1 biosynthesis in Aspergillus parasiticus

A cosmid library was constructed by inserting genomic DNA isolated from a wild-type aflatoxin-producing strain of Aspergillus parasiticus (SU-1) into a cosmid vector containing an homologous nitrate reductase (niaD) gene as a selectable marker. One cosmid was isolated which complemented an aflatoxin-deficient, nitrate-nonutilizing mutant strain, A. parasiticus B62 (nor-1, niaD), to aflatoxin production. Deletion and complementation analyses showed that a 1.7 kb BglII-SphI DNA fragment isolated from this cosmid was responsible for renewed aflatoxin production. Northern hybridization analyses revealed that the major RNA transcribed from this DNA fragment was 1.4 kilonucleotides in size. Genetic complementation proved to be a useful strategy for cloning a gene associated with aflatoxin biosynthesis in A. parasiticus.

Enzymatic conversion of norsolorinic acid to averufin in aflatoxin biosynthesis

5'-Hydroxyaverantin (HAVN) was isolated from a mold, Emericella heterothallica IFO 30842. Aspergillus parasiticus NIAH-26, a UV-irradiated mutant of A. parasiticus SYS-4, produced neither aflatoxins nor precursors in yeast extract-sucrose (YES) medium. When the postmicrosome (cytosol) fraction of NIAH-26, which had been prepared from the culture in YES medium, was incubated with norsolorinic acid (NA) in the presence of NADH or NADPH, averantin (AVN) was produced. The reverse reaction from AVN to NA was promoted by the addition of NAD or NADP (dehydrogenase reaction). When the microsome fraction of NIAH-26 was incubated with AVN, HAVN was produced in the presence of NADPH (monooxygenase reaction). HAVN was, furthermore, oxidized to averufin (AVR) by the cytosol fraction of NIAH-26 in the presence of NAD or NADP (dehydrogenase reaction). In the feeding experiments with A. parasiticus NIAH-26, aflatoxins were produced from AVN, HAVN, NA, and AVR but not from averufanin or averythrin. These results indicate that the reaction sequence NA in equilibrium AVN----HAVN----AVR is involved in the biosynthetic pathway of aflatoxins. The enzyme activities described here were dependent on the culture medium, and no enzyme activities were observed in the nonaflatoxigenic strain A. oryzae SYS-2 (IFO 4251).

Transformation of Aspergillus parasiticus with a homologous gene (pyrG) involved in pyrimidine biosynthesis

The lack of efficient transformation methods for aflatoxigenic Aspergillus parasiticus has been a major constraint for the study of aflatoxin biosynthesis at the genetic level. A transformation system with efficiencies of 30 to 50 stable transformants per microgram of DNA was developed for A. parasiticus by using the homologous pyrG gene. The pyrG gene from A. parasiticus was isolated by in situ plaque hybridization of a lambda genomic DNA library. Uridine auxotrophs of A. parasiticus ATCC 36537, a mutant blocked in aflatoxin biosynthesis, were isolated by selection on 5-fluoroorotic acid following nitrosoguanidine mutagenesis. Isolates with mutations in the pyrG gene resulting in elimination of orotidine monophosphate (OMP) decarboxylase activity were detected by assaying cell extracts for their ability to convert [14C]OMP to [14C]UMP. Transformation of A. parasiticus pyrG protoplasts with the homologous pyrG gene restored the fungal cells to prototrophy. Enzymatic analysis of cell extracts of transformant clones demonstrated that these extracts had the ability to convert [14C]OMP to [14C]UMP. Southern analysis of DNA purified from transformant clones indicated that both pUC19 vector sequences and pyrG sequences were integrated into the genome. The development of this pyrG transformation system should allow cloning of the aflatoxin-biosynthetic genes, which will be useful in studying the regulation of aflatoxin biosynthesis and may ultimately provide a means for controlling aflatoxin production in the field.

Isolation and characterization of Aspergillus parasiticus mutants with impaired aflatoxin production by a novel tip culture method

A convenient procedure consisting of UV photography (K. Yabe, Y. Ando, M. Ito, and N. Terakado, Appl. Environ. Microbiol. 53:230-234, 1987) and a tip culture method has been devised for the isolation and characterization of Aspergillus parasiticus mutants relating to aflatoxin production. With the latter procedure, the production of aflatoxins excreted into the culture medium and precursors in the mycelium were easily measured quantitatively or semiquantitatively. A total of 38 mutants in which the aflatoxigenicity was decreased or lost were obtained by UV radiation; 3 were found to be blocked mutants, which accumulated the aflatoxin precursors versicolorin A or averantin.

Effect on aflatoxin production of competition between wild-type and mutant strains of Aspergillus parasiticus

Co-cultivation of a strain of Aspergillus parasiticus, capable of making aflatoxins, with blocked mutant strains, capable of producing none or only a low level of aflatoxins, reduced the net yield of aflatoxins more than that expected based on spore recovery. Yields of aflatoxins were 8-fold less for a norsolorinic acid-producing strain, 14-fold less for an averantin-producing strain, 6-fold less for an averufin-producing strain, and 21-fold less for a versicolorin A-producing strain when co-cultured in equal amounts with a wild-type strain of Aspergillus parasiticus. Even when the wild-type strain was initially present in 100-fold excess, with two of the mutant strains, reduced aflatoxin production was still observed.

Averufanin is an aflatoxin B1 precursor between averantin and averufin in the biosynthetic pathway

Wild-type Aspergillus parasiticus produces, in addition to the colorless aflatoxins, a number of pigmented secondary metabolites. Examination of these pigments demonstrated that a major component was an anthraquinone, averufanin. Radiolabeling studies with [14C]averufanin showed that 23% of the label was incorporated into aflatoxin B1 by the wild type and that 31% of the label was incorporated into O-methylsterigmatocystin by a non-aflatoxin-producing isolate. In similar studies with blocked mutants of A. parasiticus the 14C label from averufanin was accumulated in averufin (72%) and versicolorin A (54%) but not averantin. The results demonstrate that averufanin is a biosynthetic precursor of aflatoxin B1 between averantin and averufin.

Mutagenicity and toxicity of aflatoxin precursors

The Salmonella/microsome test and the chick embryo test were used to determine the mutagenicity and toxicity of five aflatoxin B1 precursors. A definite pattern emerges: the nearer the B1 an intermediate appears in the biosynthetic pathway, the more potent is its mutagenicity and toxicity.

Biogenesis of the C20 polyketide, aflatoxin. A review

Aflatoxin B2 is a secondary metabolite produced by the ubiquitous molds Aspergillus flavus and Aspergillus parasiticus. The toxin was first characterized in 1963 as the etiological agent responsible for the infamous "Turkey X' disease. Since that time, much information on its chemistry, toxicity and biological activity has accumulated. A significant amount of work has been done to elucidate its biosynthesis. Evidence indicates the polyketide route as its point of origin. The steps involved in the polyketide pathway, the six identified intermediated compounds, and the experimental techniques and analytical instrumentation used to procure information on aflatoxin biogenesis are included in this review.

Influence of tricarboxylic acid cycle intermediates and related metabolites on the biosynthesis of aflatoxin by resting cells of Aspergillus flavus

Resting cells of Aspergillus flavus synthesized aflatoxin from acetate as the sole carbon source after 36 h of incubation. Addition of pyruvate (5.5 mg/m) as cosubstrate to [1-14C]acetate and unlabeled acetate considerably reduced toxin production but increased the radioactivity on the tricarboxylic acid intermediates. This suggests that high tricarboxylic acid activity drastically affected toxin synthesis.

Precursor recognition by kinetic pulse-labeling in a toxigenic aflatoxin B1-producing strain of Aspergillus

Kinetic pulse-labeling of aflatoxin pathway compounds was carried out in Aspergillus parasiticus, beginning with radioactive acetate. Norsolorinic acid, averufin, versicolorin A, and sterigmatocystin (all known as compounds which can be incorporated into the aflatoxin molecule) were radiotraced to follow their order of appearance. Aflatoxin species B1, B2, G1, and G2 were included. Norsolorinic acid and averufin appeared as early transient intermediates followed in order by versicolorin A, aflatoxins, and sterigmatocystin. To date, a mutually confirming array of results has been obtained with established precursors in wild-type strains of A. parasiticus and A. versicolor (as well as with an aflatoxin pathway mutant of A. parasiticus), which together establish a practical methodology for recognition of new pathway intermediates. The kinetic of pulse-labeling for sterigmatocystin in relation to aflatoxins suggests that duel branchlets may exist to flatoxins; i.e., sterigmatocystin may not be an obligatory aflatoxin precursor.

Identification of averantin as an aflatoxin B1 precursor: placement in the biosynthetic pathway

A new blocked mutant of Aspergillus parasiticus produces no detectable aflatoxin B1, but accumulates several polyhydroxyanthraquinones. One of these pigments was identified as averantin. This is the first report of its formation by A. parasiticus. Radiotracer studies with [14C]averantin showed that 15.3% of label from averantin was incorporated into aflatoxin B1. This incorporation was blocked by dichlorvos. With radiotracers and other mutants, averantin was placed after norsolorinic acid and before averufin in the biosynthetic pathway in which the general steps are norsolorinic acid leads to averantin leads to averufin leads to versiconal hemiacetal acetate leads to versicolorin A leads to sterigmatocystin leads to aflatoxin B1.

Genetics of Aspergillus flavus: complementation and mapping of aflatoxin mutants

Mutants ofAspergillus flavusimpaired in aflatoxin production were induced withN-methyl-N′-nitrosoguanidine and analysed by means of the parasexual cycle. The gene symbolaflwas assigned to this type of mutation. Diploid complementation tests revealed that most of 14aflmutants belonged to different complementation groups. One mutant (afl-1) failed to complement or only partially complemented all other mutants. Haploidization of one diploid revealed the independent segregation of twoaflmutants. Heterozygous diploids were synthesized between twoaflmutants and tester strains genetically labelled on eight linkage groups. Haploidization of the diploids led to the assignment of two aft mutants to linkage groups. Linkage ofafl-4 tow+andafl-1 toleuon linkage groups II and VII, respectively, was demonstrated.

Mutagenicity of fungal metabolites related to aflatoxin biosynthesis

Fungal metabolites identified as the intermediates in aflatoxin biosynthetic pathway were screened for their mutagenic activity to Salmonella typhimurium TA98. Norsolorinic acid, averufin, and versiconal acetate were found to possess questionable mutagenic activity, but versicolorin A, and sterigmatocystin were significant mutagens relative to aflatoxin B1. The mutagenic activity appears to be related to the bisfuran and not the anthraquinone moiety of the molecule, even though the latter is a key structure of such potent carcinogenic mycotoxin as luteoskyrin.

Effect of temperature cycling on the production of aflatoxin by Asperfillus parasiticus

Aspergillus parasiticus (NRRL 2999) was grown under cycling temperature conditions on rice and nutmeat substrates. Under conditions of diurnal and nocturnal time-temperature sequencing, total heat input is an important factor of toxin production. When expressed in degree hours per day, thermal input becomes more definitive and provides a finite number, which can be related to observable changes in the culture such as sporulation and toxin biosynthesis. Three well-defined levels of response were observed in relation to heat input: no growth was detected at thermal inputs of less than 208 degree hours/day; mycelial growth as well as copious amounts of an orange pigment were observed at thermal inputs between 208 and 270 degree hours/day; sporulation and aflatoxin biosynthesis occurred above 270 degree hours/day. Between the optimum and minimum thermal input, cycling temperatures significantly reduced the period of the trophophase over cultures receiving equal heat input at a constant rate. Cycling temperatures at the low and high extremes of the temperature range had little or no effect upon the growth pattern of the culture. Regardless of how temperature was manipulated, these responses were consistent with the heat input received by the culture. A. parasiticus did not compete well when mixed with natural fungal isolates from nutmeats and was easily overgrown by the wild isolates even at relatively high thermal input and when present in superior numbers. This factor and heat input generally below that required for toxin biogenesis at harvest time appear to be two significant factors that limit occurrence of aflatoxin on nut crops of the Willamette Valley. These factors are likely to have significance for other crops grown and harvested under similar circumstances.

Step of dichlorvos inhibition in the pathway of aflatoxin biosynthesis

Dichlorvos (dimethyl 2,2-dichlorovinyl phosphate) inhibits the biosynthesis of aflatoxin by Aspergillus parasiticus. Cultures treated with dichlorvos excrete an orange pigment which can be converted into aflatoxin B(1) by the untreated mycelia. The orange pigment was partially identified as an acetyl derivative of versiconol-type compound. In the presence of dichlorvos, sterigmatocystin is converted into aflatoxin B(1) without being interfered, but averufin is converted into the orange pigment instead of aflatoxin B(1). Therefore, dichlorvos appears to block an enzymatic step in the aflatoxin biosynthetic pathway, which lies beyond averufin but before sterigmatocystin, at the formation of the orange pigment.