| Aflatoxin (AF) and sterigmatocystin (ST) are potent polyketide-derived carcinogens and present an agricultural, health and economical threat worldwide. A better understanding of regulation of AF/ST biosynthesis may provide vital clues for controlling AF/ST biosynthesis. A previous study identified a locus required for ST biosynthesis in A. nidulans. The studies presented in this thesis revealed that this locus encodes 2-methylcitrate synthase. The overall goal of this research is to elucidate the mechanism underlying the connection of 2-methylcitrate synthase to ST biosynthesis in A. nidulans.; Disruption of mcsA impairs biosynthesis of two polyketides: ST and asexual spore pigment. Feeding with downstream products of 2-methylcitrate synthase did not restore ST biosynthesis. However, propionate, an upstream substrate of 2-methylcitrate synthase, inhibited biosynthesis of ST, conidiospore pigment and the polyketide sexual spore pigment whereas over-expression of mcsA remediated inhibition of ST production by propionate. These data suggest accumulation of propionyl-CoA is likely the cause of defect in polyketide biosynthesis in the DeltamcsA strain. Transcription analyses revealed that disruption of mcsA does not affect expression of the specialized fatty acid synthase genes (stcJ and stcK) or polyketide synthase gene (stcA) required for formation of norsolorinic acid (NOR), the first stable intermediate in the ST biosynthetic pathway. Norsolorinic acid, but not hexanoic acid or malonate, restored ST production in the DeltamcsA strain, suggesting that the polyketide synthase StcA is likely the inhibition target.; Propionyl-CoA-forming amino acids and odd chain fatty acid inhibited ST biosynthesis and conidial pigmentation. A pcsA gene, encoding a putative propionyl-CoA synthase, was identified and disrupted. Introduction of the DeltapcsA and/or facA303 (a mutant allele of facA encoding acetyl-CoA synthase) in the Delta mcsA background restored ST biosynthesis and relieved inhibition of conidial pigmentation by propionate or propionyl-CoA-forming compounds. These data support that propionyl-CoA has an inhibitory effect on polyketide biosynthesis in A. nidulans. |
