ASPERGILLUS-NIDULANS VERA IS REQUIRED FOR PRODUCTION OF THE MYCOTOXINSTERIGMATOCYSTIN

Aspergillus nidulans produces the carcinogenic mycotoxin sterigmatocys tin (ST), the next-to-last precursor in the aflatoxin (AF) biosyntheti c pathway found in the closely related fungi Aspergillus flavus and As pergillus parasiticus. We identified and characterized an A. nidulans gene, verA, that is required for converting the AF precursor versicolo rin A to ST. verA is closely related to several polyketide biosyntheti c genes involved in polyketide production in Streptomyces spp. and exh ibits extended sequence similarity to A. parasiticus ver-1, a gene pro posed to encode an enzyme involved in converting versicolorin A to ST. By performing a sequence analysis of the region 3' to verA, we identi fied two additional open reading frames, designated ORF1 and ORF2. ORF 2 is closely related to a number of cytochrome P-450 monooxygenases, w hile ORF1 shares identity with the gamma subunit of translation elonga tion factor 1. Given that several steps in the ST-AF pathway may requi re monooxygenase activity and that AF biosynthetic genes are clustered in A. flavus and A. parasiticus, we suggest that verA may be part of a cluster of genes required for ST biosynthesis. We disrupted the verA coding region by inserting the A. nidulans argB gene into the center of the coding region and transformed an A. nidulans argB2 mutant to ar ginine prototrophy. Seven transformants that produced DNA patterns ind icative of a verA disruption event were grown under ST-inducing condit ions, and all of the transformants produced versicolorin A but negligi ble amounts of ST (200-fold to almost 1,000-fold less than the wild ty pe), confirming the hypothesis that verA encodes an enzyme necessary f or converting versicolorin A to ST.