Purification and characterization of a methyltransferase from Aspergillus parasiticus SRRC 163 involved in aflatoxin biosynthetic pathway

Streptomyces-Aspergillus flavus interactions: impact on aflatoxin B accumulation

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

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.

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.

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.

Molecular genetic analysis and regulation of aflatoxin biosynthesis

Aflatoxins, produced by some Aspergillus species, are toxic and extremely carcinogenic furanocoumarins. Recent investigations of the molecular mechanism of AFB biosynthesis showed that the genes required for biosynthesis are in a 70 kb gene cluster. They encode a DNA-binding protein functioning in aflatoxin pathway gene regulation, and other enzymes such as cytochrome p450-type monooxygenases, dehydrogenases, methyltransferases, and polyketide and fatty acid synthases. Information gained from these studies has led to a better understanding of aflatoxin biosynthesis by these fungi. The characterization of genes involved in aflatoxin formation affords the opportunity to examine the mechanism of molecular regulation of the aflatoxin biosynthetic pathway, particularly during the interaction between aflatoxin-producing fungi and plants.

Function of native OmtA in vivo and expression and distribution of this protein in colonies of Aspergillus parasiticus

The activities of two enzymes, a 168-kDa protein and a 40-kDa protein, OmtA, purified from the filamentous fungus Aspergillus parasiticus were reported to convert the aflatoxin pathway intermediate sterigmatocystin to O-methylsterigmatocystin in vitro. Our initial goal was to determine if OmtA is necessary and sufficient to catalyze this reaction in vivo and if this reaction is necessary for aflatoxin synthesis. We generated A. parasiticus omtA-null mutant LW1432 and a maltose binding protein-OmtA fusion protein expressed in Escherichia coli. Enzyme activity analysis of OmtA fusion protein in vitro confirmed the reported catalytic function of OmtA. Feeding studies conducted with LW1432 demonstrated a critical role for OmtA, and the reaction catalyzed by this enzyme in aflatoxin synthesis in vivo. Because of a close regulatory link between aflatoxin synthesis and asexual sporulation (conidiation), we hypothesized a spatial and temporal association between OmtA expression and conidiospore development. We developed a novel time-dependent colony fractionation protocol to analyze the accumulation and distribution of OmtA in fungal colonies grown on a solid medium that supports both toxin synthesis and conidiation. OmtA-specific polyclonal antibodies were purified by affinity chromatography using an LW1432 protein extract. OmtA was not detected in 24-h-old colonies but was detected in 48-h-old colonies using Western blot analysis; the protein accumulated in all fractions of a 72-h-old colony, including cells (0 to 24 h) in which little conidiophore development was observed. OmtA in older fractions of the colony (24 to 72 h) was partly degraded. Fluorescence-based immunohistochemical analysis conducted on thin sections of paraffin-embedded fungal cells from time-fractionated fungal colonies demonstrated that OmtA is evenly distributed among different cell types and is not concentrated in conidiophores. These data suggest that OmtA is present in newly formed fungal tissue and then is proteolytically cleaved as cells in that section of the colony age.

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.

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.

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.

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.

Purification of a 40-kilodalton methyltransferase active in the aflatoxin biosynthetic pathway

The penultimate step in the aflatoxin biosynthetic pathway of the filamentous fungi Aspergillus flavus and A. parasiticus involves conversion of sterigmatocystin to O-methylsterigmatocystin. An S-adenosylmethionine-dependent methyltransferase that catalyzes this reaction was purified to homogeneity (> 90%) from 78-h-old mycelia of A. parasiticus SRRC 163. Purification of this soluble enzyme was carried out by five soft-gel chromatographic steps: cell debris remover treatment, QMA ACELL chromatography, hydroxylapatite-Ultrogel chromatography, DEAE-Spherodex chromatography, and Octyl Avidgel chromatography, followed by MA7Q high-performance liquid chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the protein peak from this step on silver staining identified a single band of approximately 40 kDa. This purified protein was distinct from the dimeric 168-kDa methyltransferase purified from the same fungal strain under identical growth conditions (D. Bhatnagar, A. H. J. Ullah, and T. E. Cleveland, Prep. Biochem. 18:321-349, 1988). The chromatographic behavior and N-terminal sequence of the 40-kDa enzyme were also distinct from those of the 168-kDa methyltransferase. The molar extinction coefficient of the 40-kDa enzyme at 278 nm was estimated to be 4.7 x 10(4) M-1 cm-1 in 50 mM potassium phosphate buffer (pH 7.5).

Cloning of the afl-2 gene involved in aflatoxin biosynthesis from Aspergillus flavus

Aflatoxins are extremely potent carcinogens produced by Aspergillus flavus and Aspergillus parasiticus. Cloning of genes in the aflatoxin pathway provides a specific approach to understanding the regulation of aflatoxin biosynthesis and, subsequently, to the control of aflatoxin contamination of food and feed. This paper reports the isolation of a gene involved in aflatoxin biosynthesis by complementation of an aflatoxin-nonproducing mutant with a wild-type genomic cosmid library of A. flavus. Strain 650-33, blocked in aflatoxin biosynthesis at the afl-2 allele, was complemented by a 32-kb cosmid clone (B9), resulting in the production of aflatoxin. The onset and profile of aflatoxin accumulation was similar for the transformed strain and the wild-type strain (NRRL 3357) of the fungus, indicating that the integrated gene is under the same control as in wild-type strains. Complementation analyses with DNA fragments from B9 indicated that the gene resides within a 2.2-kb fragment. Because this gene complements the mutated afl-2 allele, it was designated afl-2. Genetic evidence obtained from a double mutant showed that afl-2 is involved in aflatoxin biosynthesis before the formation of norsolorinic acid, the first stable intermediate identified in the pathway. Further, metabolite feeding studies with the mutant, transformed, and wild-type cultures and enzymatic activity measurements in cell extracts of these cultures suggest that afl-2 regulates gene expression or the activity of other aflatoxin pathway enzymes. This is the first reported isolation of a gene for aflatoxin biosynthesis in A. flavus.

Isolation and characterization of a gene from Aspergillus parasiticus associated with the conversion of versicolorin A to sterigmatocystin in aflatoxin biosynthesis

DNA isolated from the wild-type aflatoxin-producing (Afl+) fungus Aspergillus parasiticus NRRL 5862 was used to construct a cosmid genomic DNA library employing the homologous gene (pyrG) encoding orotidine monophosphate decarboxylase for selection of fungal transformants. The cosmid library was transformed into an Afl- mutant, A. parasiticus CS10 (ver-1 wh-1 pyrG), deficient in the conversion of the aflatoxin biosynthetic intermediate versicolorin A to sterigmatocystin. One pyrG+ Afl+ transformant was identified. DNA fragments from this transformant, recovered by marker rescue, contained part of the cosmid vector including the pyrG gene, the ampr gene, and a piece of the original genomic insert DNA. Transformation of these rescued DNA fragments into A. parasiticus CS10 resulted in production of wild-type levels of aflatoxin and abundant formation of sclerotia. The gene responsible for this complementation (ver-1) was identified by Northern RNA analysis and transformation with subcloned DNA fragments. The approximate locations of transcription initiation and polyadenylation sites of ver-1 were determined by an RNase protection assay and cDNA sequence analysis. The predicted amino acid sequence, deduced from the ver-1 genomic and cDNA nucleotide sequences, was compared with the EMBL and GenBank data bases. The search revealed striking similarity with Streptomyces ketoreductases involved in polyketide biosynthesis.

Aflatoxin production via cross-feeding of pathway intermediates during cofermentation of aflatoxin pathway-blocked Aspergillus parasiticus mutants

Cofermentation of Aspergillus parasiticus strains (SRRC 163 and SRRC 2043) blocked at different steps in the aflatoxin B1 (AFB1) biosynthetic pathway in a synthetic liquid medium or on seeds (cottonseed, corn kernels, and peanuts) resulted in production of AFB1. Strain SRRC 2043 accumulated O-methylsterigmatocystin (OMST), a late precursor in AFB1 biosynthesis, whereas SRRC 163 accumulated averantin, an early precursor in the pathway. Strain SRRC 2043 secreted large amounts of OMST in culture relative to the amounts of several other pathway intermediates secreted into media (by other AFB1 pathway-blocked strains). AFB1 production occurred even when colonies of SRRC 163 and SRRC 2043 strains (producing no detectable AFB1) were grown together on an agar medium while physically separated from each other by a filter membrane (0.22-micron pore size). In addition, when mycelia of strain SRRC 163 were added to culture filtrates (containing no mycelia but containing secreted OMST) of strain SRRC 2043, AFB1 production occurred. The results suggested a chemical (rather than genetic) mechanism of complementation for AFB1 production between AFB1 pathway-blocked strains, since no mycelial contact was required between these strains for AFB1 production. The mechanism for chemical complementation involves secretion of OMST by SRRC 2043 and subsequent absorption and conversion of OMST to AFB1 by mycelia of strain SRRC 163.

Two distinct O-methyltransferases in aflatoxin biosynthesis

The substances belonging to the sterigmatocystin group bear a close structural relationship to aflatoxins. When demethylsterigmatocystin (DMST) was fed to Aspergillus parasiticus NIAH-26, which endogenously produces neither aflatoxins nor precursors in YES medium, aflatoxins B1 and G1 were produced. When dihydrodemethylsterigmatocystin (DHDMST) was fed to this mutant, aflatoxins B2 and G2 were produced. Results of the cell-free experiment with S-adenosyl-[methyl-3H]methionine showed that first the C-6-OH groups of DMST and DHDMST are methylated to produce sterigmatocystin and dihydrosterigmatocystin (O-methyltransferase I) and then the C-7-OH groups are methylated to produce O-methylsterigmatocystin (OMST) and dihydro-O-methylsterigmatocystin (DHOMST) (O-methyltransferase II). However, no methyltransferase activity was observed when either OMST, DHOMST, 5,6-dimethoxysterigmatocystin, 5-methoxysterigmatocystin, or sterigmatin was incubated with the cell extract. Treatment of the cell extract with N-ethylmaleimide inhibited O-methyltransferase I activity but not that of O-methyltransferase II. Furthermore, these O-methyltransferases were different in their protein molecules and were involved in both the reactions from DMST to OMST and DHDMST to DHOMST. The reactions described in this paper were not observed when the same mold had been cultured in YEP medium.

Enzymes in aflatoxin B1 biosynthesis: strategies for identifying pertinent genes

Recent work on the aflatoxin biosynthetic pathway is reviewed, with special emphasis on the enzymes of the late stages of the pathway involving conversion of sterigmatocystin (ST) to aflatoxin B1 (AFB1) through an O-methylsterigmatocystin intermediate. Two enzyme activities were discovered in subcellular fractions of cell-free extracts of a mutant strain of Aspergillus parasiticus (SRRC 163): 1) A post-microsomal methyltransferase (MT) catalyzed conversion of ST to OMST, and 2) a microsomal-associated activity (oxido-reductase) converted OMST to AFB1. The 168 KDa, anionic MT was purified to homogeneity and characterized (two subunits, 110 KDa and 58 KDa). Preliminary evidence indicated the presence of a cationic isozyme of the MT in mycelial extracts. The oxido-reductase has been partially purified and characterized. Polyclonal antibodies were prepared to the anionic MT and the enzyme's amino acid composition determined. A cDNA library has been constructed from mRNA isolated from Aspergillus parasiticus mycelia during the onset of AFB1 biosynthesis for the purpose of identifying the genes responsible for aflatoxin biosynthesis.