COMPARATIVE MAPPING OF AFLATOXIN PATHWAY GENE CLUSTERS IN ASPERGILLUS-PARASITICUS AND ASPERGILLUS-FLAVUS

Aflatoxins are toxic and carcinogenic secondary metabolites produced b y the fungi Aspergillus flavus and A. parasiticus. Aflatoxins are synt hesized by condensation of acetate units; their synthesis is estimated to involve at least 16 different enzymes. In this study we have shown that at least nine genes involved in the aflatoxin biosynthetic pathw ay are located within a 60-kb DNA fragment. Four of these genes, nor-1 , aflR, ver-1, and omtA (previously named omt-1), have been cloned in A. flavus and A. parasiticus. In addition, five other genes, pksA, uvm 8, aad, ord-1, and ord-2 have been recently cloned in A. parasiticus. The pksA, aad, and uvm8 genes exhibit sequence homologies to polyketid e synthase, aryl-alcohol dehydrogenase, and fatty acid synthase genes, respectively. The cDNA sequences of ord-1 and ord-2 genes, which may be involved in later steps of aflatoxin biosynthesis, have been determ ined; the ord-1 gene prc,duct exhibits homology to cytochrome P-450-ty pe enzymes. By characterizing the overlapping regions of the DNA inser ts in different cosmid and lambda DNA clones, we have determined the o rder of these aflatoxin pathway genes within this 60-kb DNA region to be pksA, nor-1, uvm8, aflR, aad, ver-1, ord-1, ord-2, and omtA in A. p arasiticus and nor-1, aflR, ver-1, ord-1, ord-2, and omtA in A. flavus . The order is related to the order in enzymatic steps required for af latoxin biosynthesis. The physical distances (in kilobase pairs) and t he directions of transcription of these genes have been determined for both aflatoxigenic species.