Conversion of 11-hydroxy-O-methylsterigmatocystin to aflatoxin G1 in Aspergillus parasiticus

Abstract

In aflatoxin biosynthesis, aflatoxins G(1) (AFG(1)) and B(1) (AFB(1)) are independently produced from a common precursor, O-methylsterigmatocystin (OMST). Recently, 11-hydroxy-O-methylsterigmatocystin (HOMST) was suggested to be a later precursor involved in the conversion of OMST to AFB(1), and conversion of HOMST to AFB(1) was catalyzed by OrdA enzyme. However, the involvement of HOMST in AFG(1) formation has not been determined. In this work, HOMST was prepared by incubating OrdA-expressing yeast with OMST. Feeding Aspergillus parasiticus with HOMST allowed production of AFG(1) as well as AFB(1). In cell-free systems, HOMST was converted to AFG(1) when the microsomal fraction, the cytosolic fraction from A. parasiticus, and yeast expressing A. parasiticus OrdA were added. These results demonstrated (1) HOMST is produced from OMST by OrdA, (2) HOMST is a precursor of AFG(1) as well as AFB(1), and (3) three enzymes, OrdA, CypA, and NadA, and possibly other unknown enzymes are involved in conversion of HOMST to AFG(1).

Aflatoxins: Mechanisms of Inhibition by Antagonistic Plants and Microorganisms

“…This separates AFG 1 formation from AFB 1 formation; the latter requires a second cleavage of the 370 Da intermediate to give an open-ring form (3, 388) followed by demethylation, reclosure, a decarboxylation/dehydration step (Udwary et al, 2002). Most recently, Zeng et al (2011) confirmed in A. parasiticus that HOMST was converted to AFG 1 and AFB 1 . Cai et al, 2008 showed that the nadA gene previously assigned to the adjacent sugar utilization gene cluster instead is required for the formation of AFG 1 and AFG 2 .…”

“…This cysteine heme-iron ligand signature, F[S/G/E] XGXRXCXG, is present at the N terminal portion of the six P450s involved in AF biosynthesis. It is believed that OrdA and CypA may correspond to the microsomal enzymes and NadA the cytosol enzyme that are involved in the formation of AFG 1 and AFG 2 (Yabe et al, 1999;Zeng et al, 2011). The CypA gene encodes a P450 monooxygenase, and its knockout in A. parasiticus abolished production of AFG 1 and AFG 2 but not AFB 1 and AFB 2 .…”

Enhanced diversity and aflatoxigenicity in interspecific hybrids ofAspergillus flavusandAspergillus parasiticus

“…This indicates that the loss-of-function mutation in these OMST-pro-ducing A. parasiticus strains is somewhere in the aflE-nadA segment of the aflatoxin biosynthetic pathway. A previous study reported that ordA (=aflQ), cypA (=aflU) and nadA are important in the conversion of OMST to G aflatoxins (Zeng et al 2011). As aflU is in the A. parasiticus subtelomeric segment, this suggests that aflU is functional in IC1612, IC1622 and IC1630 and that a possible loss-of-function mutation resides in aflO, nadA or both.…”

“…A previous study reported that ordA (= aflQ ), cypA (= aflU ) and nadA are important in the conversion of OMST to G aflatoxins (Zeng et al . ). As aflU is in the A. parasiticus subtelomeric segment, this suggests that aflU is functional in IC1612, IC1622 and IC1630 and that a possible loss‐of‐function mutation resides in aflO, nadA or both.…”

Aflatoxin Biosynthesis and Genetic Regulation: A Review

“…This gene is involved in the conversion of OMST into AFB1 by the oxidation of the A-ring of OMST [24,71]. This reaction leads to an AFB1 precursor named 11-hydroxy-O-methylsterigmatocystin (HOMST) [55].…”