Benzophenone synthase from Hypericum calycinum cell cultures: cDNA cloning and functional expression
Hypericum is a medicinally important genus (Clusiaceae). H. perforatum (St. John's wort) is the best-known member of the genus and widely used as an antidepressant agent. Cell suspension cultures of the related species, H. calycinum, form 1,3,6,7-tetrahydroxy-8-prenylxanthone upon elicitation with yeast extract. Xanthones thus appear to serve as phytoalexins in Hypericum species, as reported previously. In addition, they exhibit antitumour, anti-HIV, and antimicrobial activities .
The carbon skeleton of xanthones is formed by benzophenone synthase (BPS), which catalyses the condensation of benzoyl-CoA and three molecules of malonyl-CoA followed by intramolecular cyclization. Time-course changes in BPS activity and xanthone formation were studied. Maximum product formation and enzyme activity were found at 12 and 9h, respectively, after addition of the elicitor. The BPS cDNA was cloned using primers derived from H. androsaemum cDNA  and the open-reading frame was functionally expressed in E. coli as 6xHis-tagged protein. The enzymatic product after incubation with benzoyl-CoA and malonyl-CoA was identified as 2,4,6-trihydroxybenzophenone. Characterization of the recombinant enzyme is underway.
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