adhA in Aspergillus parasiticus is involved in conversion of 5 '-hydroxyaverantin to averufin

Two routes for the conversion of 5'-hydroxyaverantin (HAVN) to averufin (AV F) in the synthesis of aflatoxin have been proposed. One involves the dehyd ration of HAVN to the lactone averufanin (AVNN), which is then oxidized to AVP. Another requires dehydrogenation of HAVN to 5'-ketoaverantin, the open -chain form of AVF, which then cyclizes spontaneously to AVF, We isolated a gene, adhA, from the aflatoxin gene cluster of Aspergillus parasiticus SU- 1. The deduced ADHA amino acid sequence contained two conserved motifs foun d in short-chain alcohol dehydrogenases-a glycine-rich loop (GXXXGXG) that is necessary for interaction with NAD(+)-NADP(+), and the motif YXXXK, whic h is found at the active site, A. parasiticus SU-1, which produces aflatoxi ns, has two copies of adhA (adhA1), whereas A. parasiticus SRRC 2043, a str ain that accumulates O-methylsterigmatocystin (OMST), has only one copy. Di sruption of adhA in SRRC 2043 resulted in a strain that accumulates predomi nantly HAVN, This result suggests that ADHA is involved in the dehydrogenat ion of HAVN to AVF. Those adhA disruptants that still made small amounts of OMST also accumulated other metabolites, including AVNN, after prolonged c ulture.