Pyrone polyketides synthesized by a type III polyketide synthase from Drosophyllum lusitanicum

A polyketide synthase cDNA was isolated from callus cultures of Drosophyllum lusitanicum Link. The corresponding recombinant enzyme DluHKS accepted acetyl-CoA as starter substrate and carried out sequential decarboxylative condensations with a malonyl-CoA extender unit. The main product was identified as the hexaketide 6-(2’,4’-dihydroxy-6’-methylphenyl)-4-hydroxy-2-pyrone. In addition, α-pyrones derived from three to five acetate units were detected. To isolate cDNAs involved in the biosynthesis of acetate-derived naphthoquinones in Drosophyllum lusitanicum, an expressed sequence tag analysis was performed. RNA from callus cultures was used to create a cDNA library from which 2004 expressed sequence tags were generated. One cDNA with similarity to known type III polyketide synthases was isolated as full-length sequence and termed DluHKS. The translated polypeptide sequence of DluHKS showed 51–67% identity with other plant type III PKSs. Recombinant DluHKS expressed in Escherichia coli accepted acetyl-coenzyme A (CoA) as starter and carried out sequential decarboxylative condensations with malonyl-CoA yielding α-pyrones from three to six acetate units. However, naphthalenes, the expected products, were not isolated. Since the main compound produced by DluHKS is a hexaketide α-pyrone, and the naphthoquinones in D. lusitanicum are composed of six acetate units, we propose that the enzyme provides the backbone of these secondary metabolites. An involvement of accessory proteins in this biosynthetic pathway is discussed.