Polyketide synthases from poison hemlock (Conium maculatum L.).

Coniine is a toxic alkaloid， the biosynthesis of which is not well understood. A possible route， supported by evidence from labelling experiments， involves a polyketide formed by the condensation of one acetyl-CoA and three malonyl-CoAs catalysed by a polyketide synthase (PKS). We isolated PKS genes or their fragments from poison hemlock (Conium maculatum L.) by using random amplification of cDNA ends (RACE) and transcriptome analysis， and characterized three full-length enzymes by feeding different starter-CoAs in vitro. On the basis of our in vitro experiments， two of the three characterized PKS genes in poison hemlock encode chalcone synthases (CPKS1 and CPKS2)， and one encodes a novel type of PKS (CPKS5). We show that CPKS5 kinetically favours butyryl-CoA as a starter-CoA in vitro. Our results suggest that CPKS5 is responsible for the initiation of coniine biosynthesis by catalysing the synthesis of the carbon backbone from one butyryl-CoA and two malonyl-CoAs.