STERIGMATOCYSTIN BIOSYNTHESIS IN ASPERGILLUS-NIDULANS REQUIRES A NOVEL TYPE-I POLYKETIDE SYNTHASE

A filamentous fungus, Aspergillus nidulans, produces the carcinogenic mycotoxin sterigmatocystin (ST), which is a polyketide-derived seconda ry metabolite. A gene (pksST) encoding the ST polyketide synthase (PKS st) in A. nidulans was cloned, sequenced, and characterized. Large ind uced 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 develop mental expression with ST production, was a 7.2-kb transcript. Functio nal inactivation of the gene encoding the 7.2-kb transcript blocked pr oduction of ST and all ST intermediate substrates but did not affect t ranscription of the pathway genes, indicating that this gene was invol ved in a very early step of ST biosynthesis. These results also indica te that PKSst was not associated with activation of other ST genes. Se quencing of the region spanning this gene revealed that it encoded a p olypeptide with a deduced length of 2,181 amino acids that had high le vels of similarity to many of the known polyketide synthases of FASs. This gene, pksST, encodes a multifunctional novel type I polyketide sy nthase which has as active a beta-ketoacyl acyl carrier protein syntha se, an acyltransferase, duplicated acyl carrier proteins, and a thioes terase; 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.