Give Me a Reductase! Where Do Plant Polyketide Synthases Get Their Accessory Activities?

Microbial type I and type II polyketide synthases (PKSs) are large multienzyme complexes that resemble fatty acid synthases. In fatty acid biosynthesis, each newly introduced β-keto group is fully reduced to an alkane. In contrast, in type I and type II PKSs this reduction step is optional and tightly programmed, generating a wide diversity of products. Plant type III PKSs are evolutionarily related to type I and II PKSs, but they are simple homodimeric enzymes that lack intrinsic reductive activity. The type III PKS G2PS1 from the ornamental plant Gerbera hybrida is responsible for the accumulation of high levels of the bitter glucosidic lactones gerberin and parasorboside in all aerial tissues. The biosynthesis of these metabolites requires reduction reactions that were previously unknown. Inspired by the biosynthesis of 6-methylsalicylic acid by a fungal type I PKS in Penicillium patulum , we demonstrated that the key reduction step in Gerbera occurs at the triketide-CoA thioester stage, prior to lactonization. This reaction directly parallels β-keto reduction in fatty acid biosynthesis. Given that many plant polyketide pathways require analogous reduction steps, we examine here whether β-keto reduction represents a general mechanism in plant polyketide biosynthesis. Although such a step is theoretically possible in many pathways, it has been unequivocally demonstrated only for parasorboside biosynthesis in Gerbera so far.