Biosynthesis of 3-hydroxybenzoate-derived polyketides in Gentianaceae

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The Gentianaceae include well-known medicinal plants, such as Centaurium erythraea and Swertia chirata. Their profile of constituents involves xanthones, the skeleton of which is formed by a type III polyketide synthase, called benzophenone synthase (BPS). In contrast to BPSs from Hypericaceae, which use benzoyl-CoA as starter substrate, the enzymes from Gentianaceae prefer the 3-hydroxylated starter unit. As recently observed, the CoA ester with two 3-hydroxybenzoyl residues, i.e. 3-(3-hydroxybenzoyloxy)benzoyl-CoA is even the best substrate. This is also true for phenylpyrone synthase (PPS), for which cDNAs encoding both an active and an inactive isoform have been cloned. These isoenzymes differ in only four amino acids located at the surface. Mutagenesis of each of these residues in the inactive isoform yields active enzyme protein. Both BPSs and PPSs will be characterized with respect to biochemical and structural properties. Crystallization and structure analysis will be carried out in cooperation with the Prof. Blankenfeldt group (Helmholtz Center for Infection Research, Braunschweig). To address the physiological relevance of 3-(3-hydroxybenzoyloxy)benzoyl-CoA, detection and cDNA isolation of 3-hydroxybenzoyl transferase will be tackled, using PCR-based cloning and transcriptome mining.