Aflatoxin Biosynthesis Cluster GenecypAIs Required for G Aflatoxin Formation

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Abstract
Aspergillus flavusisolates produce only aflatoxins B1 and B2, whileAspergillus parasiticusandAspergillus nomiusproduce aflatoxins B1, B2, G1, and G2. Sequence comparison of the aflatoxin biosynthesis pathway gene cluster upstream from the polyketide synthase gene,pksA, revealed thatA. flavusisolates are missing portions of genes (cypAandnorB) predicted to encode, respectively, a cytochrome P450 monooxygenase and an aryl alcohol dehydrogenase. Insertional disruption ofcypAinA. parasiticusyielded transformants that lack the ability to produce G aflatoxins but not B aflatoxins. The enzyme encoded bycypAhas highest amino acid identity toGibberella zeaeTri4 (38%), a P450 monooxygenase previously shown to be involved in trichodiene epoxidation. The substrate for CypA may be an intermediate formed by oxidative cleavage of the A ring ofO-methylsterigmatocystin by OrdA, the P450 monooxygenase required for formation of aflatoxins B1 and B2.