Submitted to: Applied Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 18, 1999
Publication Date: N/A
Interpretive Summary: Aflatoxins are natural poisons produced by two common fungi, Aspergillus flavus and A. parasiticus. When the fungi invade crops and produce aflatoxins, they render the crops unsalable. The carcinogenic property of aflatoxins has stimulated many scientists to study in detail how these toxins are synthesized. Our efforts are focused on the understanding of the biosynthetic pathway by characterization of the genes encoding the enzymes involved in the biosynthesis of these important toxins. In this report, we present the results of two newly identified transcripts, cypX and moxY, which are corresponding to the stcB and stcW in A. nidulans sterigmatocystin pathway gene cluster. The two transcripts are 3' to the omtA gene and define the boundary of the aflatoxin pathway gene cluster in omtA orientation. This study will provide a tool with which we can better devise strategies for eliminating aflatoxin contamination of food and feed crops.
Technical Abstract: The identification of overlapping cosmids resulted in the discovery that the aflatoxin biosynthetic pathway genes are clustered in Aspergillus flavus and A. parasiticus. This finding led to the cloning and characterization of 16 genes involved in aflatoxin biosynthesis including the most recent report on the gene, ordA, which has been identified to be involved in the formation of four aflatoxin (B1, B2, G1 and G2). However, these genes do not account for all of the identified chemical/biochemical steps in aflatoxin synthesis. In an effort to identify the genes for the other steps and the outer boundaries of the aflatoxin pathway gene cluster in the Aspergillus genome, we extended sequencing on both directions of the existing (60 kb) aflatoxin pathway gene cluster beyond the pksA gene on one end and the omtA gene on the other. Within the 25kb genomic DNA sequence determined at the omtA end of the cluster, several new gene sequences were identified. The recently reported genes, vbs and ordA, were found within this 25kb region. We also found in this region two additional genes, a cytochrome P450 monooxygenase encoding gene, tentatively named cypX, and a monooxygenase encoding gene, tentatively named moxY, which are reported in this study. These genes, based on Northern blot analysis and RT-PCR studies, appear to be expressed only with other genes involved in aflatoxin biosynthesis. Sequencing beyond these showed a 5kb non-coding region of DNA followed by the presence of a cluster of genes probably involved in sugar metabolism. Therefore, the sequences of the two putative aflatoxin pathway genes and a 5kb non-coding region of DNA define the boundary of the aflatoxin pathway gene cluster at one end in A. parasiticus.