Zhang Yong-Qiang, Keller Nancy P
Department of Plant Pathology, University of Wisconsin, 1630 Linden Drive, Madison, WI 53706, USA.
Mol Microbiol. 2004 Apr;52(2):541-50. doi: 10.1111/j.1365-2958.2004.03994.x.
Aspergillus nidulans produces the polyketide toxin sterigmatocystin (ST) of which the biosynthetic and pathway specific regulatory genes compose a stc gene cluster. A previous mutagenesis screen identified 23 mutants defective in production of ST. Five mutants constitute a single locus. Genetic complementation and sequencing analysis revealed the mutant locus to be mcsA encoding methylcitrate synthase that converts propionyl-CoA to methylcitrate. Feeding downstream products of methylcitrate synthase, methylcitrate and pyruvate, did not restore ST production in mcsA mutants, indicating that loss of methylcitrate cycle products is not the cause of the ST defect. However, propionate, a precursor for propionyl-CoA, inhibited ST production and induced transcription of mcsA in the wild type. Furthermore, propionate impaired formation of two polyketide spore pigments whereas overexpression of mcsA relieved inhibition of ST production by propionate. Transcription analyses revealed that disruption of mcsA did 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. Feeding studies showed that NOR but not hexanoic acid (the fatty acid produced by StcJ/StcK and primer unit of StcA) or malonate (source of the extender unit of StcA) restored ST production in the mcsA mutant. We hypothesize that excess buildup of propionyl-CoA in mcsA mutants interferes with polyketide synthase activity.
构巢曲霉产生聚酮化合物毒素柄曲霉素（ST），其生物合成和途径特异性调控基因组成一个stc基因簇。先前的诱变筛选鉴定出23个在ST产生方面有缺陷的突变体。其中五个突变体构成一个单一基因座。遗传互补和测序分析表明，该突变基因座是mcsA，其编码将丙酰辅酶A转化为甲基柠檬酸的甲基柠檬酸合酶。给mcsA突变体喂食甲基柠檬酸合酶的下游产物甲基柠檬酸和丙酮酸，并不能恢复ST的产生，这表明甲基柠檬酸循环产物的缺失不是ST缺陷的原因。然而，丙酰辅酶A的前体丙酸抑制了野生型中ST的产生并诱导了mcsA的转录。此外，丙酸损害了两种聚酮化合物孢子色素的形成，而mcsA的过表达减轻了丙酸对ST产生的抑制作用。转录分析表明，mcsA的破坏并不影响ST生物合成途径中第一个稳定中间体诺索洛林酸（NOR）形成所需的特殊脂肪酸合酶基因（stcJ和stcK）或聚酮合酶基因（stcA）的表达。喂食研究表明，NOR而非己酸（由StcJ/StcK产生的脂肪酸和StcA的引物单元）或丙二酸（StcA延伸单元的来源）可恢复mcsA突变体中ST的产生。我们推测，mcsA突变体中丙酰辅酶A的过量积累会干扰聚酮合酶的活性。
