abstr: Aspergillus nidulans mycotoxin gene

[Nancy Keller]

Abstract 

        Aspergillus nidulans produces the carcinogenic mycotoxin
sterigmatocystin (ST), the next to last precursor in the aflatoxin (AF)
biosynthetic pathway found in the closely related fungi Aspergillus flavus
and Aspergillus parasiticus.  We have identified and characterized an A.
nidulans gene, verA, that is required for converting the AF precursor
versicolorin A to ST.  verA is closely related to several polyketide
biosynthetic genes involved in polyketide production in Streptomyces spp.
and has extended sequence similarity to A. parasiticus ver-1, a gene
proposed to encode an enzyme involved in converting versicolorin A to ST. 
Sequence analysis of the region 3' to verA identified two additional open
reading frames, designated ORF1 and ORF2.  ORF2 is closely related to a
number of cytochrome P450 monooxygenases while ORF1 shares identity with
the gamma subunit of translation elongation factor 1.  Given that several
steps in the ST/AF pathway have been proposed to require monooxygenase
activity, and AF biosynthetic genes are clustered in A. flavus and A.
parasiticus, we suggest that verA is 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 arginine prototrophy.  Seven transformants
exhibiting DNA patterns indicative of a verA disruption event were grown
under ST-inducing conditions and all the transformants produced
versicolorin A but negligible ST (200 - near 1000 fold less than wild type)
confirming the prediction that verA encodes an enzyme necessary for
converting versicolorin A to ST.


Nancy P. Keller
Dept. Plant Pathology and Microbiology
Texas A&M University
College Station, TX 77843-2132
409-845-0963 P
409-845-8463 F
npk3325 at venus.tamu.edu