Submitted to: Gene
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2000
Publication Date: N/A
Interpretive Summary: Aflatoxins are notorious carcinogenic secondary metabolites produced by the fungi Aspergillus parasiticus and A. flavus. Aflatoxin contamination of agricultural commodities is, therefore, not only a potential threat to human health, but also a cause of significant economic losses. Gaining basic knowledge of the toxin formation will lead to strategies for controlling, even eliminating, aflatoxin contamination in food and feed. This study has accomplished the cloning and characterization of two of the important aflatoxin biosynthetic pathway genes. The similarities of these two genes among the toxin-producing and non-producing fungi have also been evaluated in this study. The knowledge of aflatoxin biosynthesis at the genetic and biochemical level will lead to the prevention and elimination of aflatoxin contamination of food and feed by developing appropriate control strategies.
Technical Abstract: Aflatoxins (B1, G1, B2, and G2) biosynthesis is a multi-enzyme process controlled genetically by over 20 genes. In this report, we have identified two genes in Aspergillus parasiticus: omtB, encoding O-methyltransferase B for the conversion of demethylsterigmatocystin (DMST) to sterigmatocystin (ST) and dihydrodemethylsterigmatocystin (DHDMST) to dihydrosterigmatocystin (DHST); and avfA, involved in the conversion of averufin (AVF) to versiconal hemiacetal acetate (VHA). Those genes are also present in other Aspergilli. Both the omtB and avfA genes from A. parasiticus were demonstrated to be expressed in the aflatoxin-conducive media (GMS), but not in non-aflatoxin-conducive media (PMS). The A. parasiticus omtB gene was found to be highly homologous to stcP in A. nidulans involved in the enzymatic steps for the sterigmatocystin formation in that species. Sequence comparisons among the Aspergilli species demonstrated that the in DNA region including omtB and avfA genes in A. parasiticus are highly homologous to the counterpart genes in A. flavus, A. sojae, and A. nidulans (stcP and stcO)with overall identities about 96%, 99%, and 55%-75% respectively. Complementation of an averufin-accumulating mutant strain of A. parasiticus SRRC 165 with the avfA counterpart gene from A. flavus, restored the ability to convert AVF to VHA and to produce aflatoxins B1, G1, B2 and G2. Sequence analysis revealed that a single amino acid replacement from aspartic acid to asparagine disable the function of the enzyme in the mutant strain SRRC 165. The A. parasiticus avfA counterpart gene in A. nidulans was identified to be stcO due to its high homology both at the nucleotide and at the amino acid level.