The biosynthesis of the aflatoxins

Aflatoxins are a family of toxic, acetate-derived decaketides that arise biosynthetically through polyhydroxyanthraquinone intermediates. Most studies have assumed that aflatoxin B1 is the biosynthetic precursor of the other aflatoxins. We used a strain of Aspergillusflavus which accumulates aflatoxin B2 to investigate the later stages of afiatoxin biosynthesis. This strain produced aflatoxins B2 and M2 but no detectable aflatoxin B1 when grown over 12 days in a low-salt, defined growth medium containing asparagine. Addition of dichlorvos to this growth medium inhibited aflatoxin production with concomitant accumulation of versiconal hemiacetal acetate. When mycelial pellets were grown for 24, 48, and 72 h in growth medium and then transferred to a replacement medium, only aflatoxins B2 and M2 were recovered after 96 h of incubation. Addition of sterigmatocystin to the replacement medium led to the recovery of higher levels of aflatoxins B2 and M2 than were detected in control cultures, as well as to the formation of aflatoxins B1 and M, and O-methylsterigmatocystip. These results support the hypothesis that aflatoxins B1 and B2 can arise independently via a branched pathway.

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“…1), rather than sterigmatocystin (ST; Fig. 1), has been shown to be a precursor of AFB2 (6) support this view.…”

mentioning

confidence: 78%

Biosynthetic relationship among aflatoxins B1, B2, M1, and M2

Dutton¹,

Ehrlich²,

Bennett³

1985

Appl Environ Microbiol

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“…Averufin (5), a yellow pigment, as well as three other metabolites of Aspergillus spp. (3,4,8), have been shown to be involved in the biosynthesis of AT.…”

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confidence: 99%

High Production of Kojic Acid Crystals by Aspergillus parasiticus UNBF A12 in Liquid Medium

Lin

Mahajan

Dianese

et al. 1976

Appl Environ Microbiol

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The crystalline compound produced in large quantity in liquid medium by Aspergillus parasiticus UNBF A12, a high aflatoxin-producing strain isolated from the air in the Federal District of Brazil, was identified as kojic acid. The effect of pH on the production of crystalline kojic acid and aflatoxins by the strain was studied. Fourteen single spore isolates were evaluated for their capacity to produce kojic acid crystals and aflatoxins.

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“…Unfortunately, the timing of deoxygenation in conversion of VA to ST is still ambiguous because no putative intermediates have been isolated from aflatoxin-producing strains. 6-Deoxyversicolorin A (6-deoxy VA), however, has been identified to be a metabolite of Aspergillus versicolor which produces ST (15). This prompted us to propose that the ver-1-encoded pro- tein is involved in the deoxygenation of VA to form 6-deoxy VA.…”

Section: Vol 62 1996mentioning

confidence: 99%

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

Liang

Skory

Linz

1996

Appl Environ Microbiol

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The ver-1A gene was cloned and its nucleotide sequence was determined as part of a previous study on aflatoxin B 1 (AFB 1) 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 AFB 1 synthesis, a plasmid (pDV-VA) was designed to disrupt ver-1A and/or ver-1B by transformation of the AFB 1 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 AFB 1 , 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 AFB 1 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 AFB 1 synthesis. A similar gene duplication was also identified in several other strains of A. parasiticus.

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The biosynthesis of the aflatoxins

Cited by 32 publications

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Biosynthetic relationship among aflatoxins B1, B2, M1, and M2

Biosynthetic relationship among aflatoxins B1, B2, M1, and M2

High Production of Kojic Acid Crystals by Aspergillus parasiticus UNBF A12 in Liquid Medium

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

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