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J. Bacteriol., 08 1995, 4792-4800, Vol 177, No. 16
JH Yu and TJ Leonard
A filamentous fungus, Aspergillus nidulans, produces the carcinogenic
mycotoxin sterigmatocystin (ST), which is a polyketide-derived secondary
metabolite. A gene (pksST) encoding the ST polyketide synthase (PKSst) in
A. nidulans was cloned, sequenced, and characterized. Large induced
deletion mutants, which did not make ST or any ST intermediates, were used
to identify genes associated with ST biosynthesis. Among the transcripts
detected within the deletion region, which showed developmental expression
with ST production, was a 7.2-kb transcript. Functional inactivation of the
gene encoding the 7.2- kb transcript blocked production of ST and all ST
intermediate substrates but did not affect transcription of the pathway
genes, indicating that this gene was involved in a very early step of ST
biosynthesis. These results also indicate that PKSst was not associated
with activation of other ST genes. Sequencing of the region spanning this
gene revealed that it encoded a polypeptide with a deduced length of 2,181
amino acids that had high levels of similarity to many of the known
polyketide synthases and FASs. This gene, pksST, encodes a multifunctional
novel type I polyketide synthase which has as active sites a beta-ketoacyl
acyl carrier protein synthase, an acyltransferase, duplicated acyl carrier
proteins, and a thioesterase, all of these catalytic sites may be multiply
used. In addition, a 1.9- kb transcript, which also showed developmental
expression, was mapped adjacent to pksST, and the sequence of this gene
revealed that it encoded a cytochrome P-450 monooxygenase-like peptide.
Copyright © 1995, American Society for Microbiology
Sterigmatocystin biosynthesis in Aspergillus nidulans requires a novel type I polyketide synthase
Department of Genetics, University of Wisconsin-Madison 53706, USA.
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