Two distinct O-methyltransferases in aflatoxin biosynthesis

Abstract

The substances belonging to the sterigmatocystin group bear a close structural relationship to aflatoxins. When demethylsterigmatocystin (DMST) was fed to Aspergillus parasiticus NIAH-26, which endogenously produces neither aflatoxins nor precursors in YES medium, aflatoxins B1 and G, were produced. When dihydrodemethylsterigmatocystin (DHDMST) was fed to this mutant, aflatoxins B2 and G2 were produced. Results of the cell-free experiment with S-adenosyl-[methyl-3H]methionine showed that first the C-6-OH groups of DMST and DHDMST are methylated to produce sterigmatocystin and dihydrosterigmatocystin (0methyltransferase I) and then the C-7-OH groups are methylated to produce 0-methylsterigmatocystin (OMST) and dihydro-0-methylsterigmatocystin (DHOMST) (O-methyltransferase II). However, no methyltransferase activity was observed when either OMST, DHOMST, 5,6-dimethoxysterigmatocystin, 5-methoxysterigmatocystin, or sterigmatin was incubated with the cell extract. Treatment of the cell extract with N-ethylmaleimide inhibited 0-methyltransferase I activity but not that of 0-methyltransferase II. Furthermore, these 0-methyltransferases were different in their protein molecules and were involved in both the reactions from DMST to OMST and DHDMST to DHOMST. The reactions described in this paper were not observed when the same mold had been cultured in YEP medium.

Purification of a 40-kilodalton methyltransferase active in the aflatoxin biosynthetic pathway

“…It was observed that the ratio of total methyltransferase activity attributed to the 40-and 168-kDa proteins varied, sometimes depending on mycelial age and growth conditions, but not in any consistent manner; this was in agreement with previous studies (5). Several methyltransferase activities pertaining to aflatoxin biosynthesis have been identified; three of them catalyze the reaction of ST to OMST (this report; 8,35), and one catalyzes a step proposed to immediately precede the conversion of ST to OMST (35). The relationship between these different methyltransferases and their participation in aflatoxin biosynthesis will become clearer in future purification studies aimed at (i) further characterization and sequencing of the enzymes, (ii) identification and sequencing of the genes that encode these enzymes, and (iii) site-directed mutagenesis and/or gene disruption.…”

“…1). In two separate studies, methyltransferases which catalyze these conversions have been identified (10,35). Bhatnagar et al (8) purified a 168-kDa methyltransferase to homogeneity from A. parasiticus SRRC 163.…”

“…Bhatnagar et al (8) purified a 168-kDa methyltransferase to homogeneity from A. parasiticus SRRC 163. Yabe et al (35) identified two distinct methyltransferase activities in cell extracts ofA. parasiticus NRRL 2999 which migrated with the 180-and 210-kDa fractions on a gel filtration column.…”

Substrate-induced lipase gene expression and aflatoxin production in Aspergillus parasiticus and Aspergillus flavus

“…1995; Brown et al. 1996) led to a rapid identification of the majority of genes involved in aflatoxin biosynthesis (Yabe et al. 1989, 1998; Bhatnagar et al.…”

A red pigment synthesized by an Aspergillus parasiticus mutant as a possible new intermediate in the aflatoxin biosynthetic pathway

“…1987), dihydrosterigmatocystin (Yabe et a/. 1988), dihydro-0-methylsterigmatocystin (Yabe et al 1988), dihydrodemethylsterigmatocystin (Yabe et al 1989), demethylsterigmatocystin (Yabe et al 1989), and 5hydroxyaverantin (Yabe et al 1991 b). Other compounds have also been described as possible intermediates in the aflatoxin biosynthetic pathway, e.g.…”