Characterization of a non-reducing polyketide synthase gene from lichen Dirinaria applanata

Lichens are known to produce a variety of secondary metabolites including polyketides that have diverse biological role(s). The biosynthesis of fungal polyketides is governed by type I polyketide synthases (PKS), enzymes with a multidomain structure, including the beta-ketoacyl synthase (KS), acyl transferase (AT), ketoreductase (KR), dehydratase (DH), enoyl reductase (ER) and acyl carrier protein (ACP) domains. Established soredial cultures of Dirinaria applanata (Fée) producing atranorin and divaricatic acid were used to characterize a polyketide synthase gene (DnPKS). A 743bp fragment corresponding to the ketosynthase domain (KS) was isolated using degenerate primers. Complete sequence information for DnPKS (8162bp) was obtained by walking in the 5'and 3' directions of the isolated KS domain using TAIL PCR. A translation of the DnPKS sequence identified the presence of KS, AT, two ACP and TE domains with eight intervening introns. TBLASTX analysis and comparison with other PKS sequences suggest that the coding region of DnPKS sequence is complete with the identification of putative start and stop codons and a stretch of 1226 upstream of the start codon corresponding to the putative promoter. This sequence shows the presence of putative binding sites for fungal transcription factors such as AflR, AreA and PacC. Southern blot analysis suggests that additional DnPKS-like genes may be present in the D. applanata genome. Additionally, expression of a DnPKS-like transcript was examined under different culture conditions and found to be down-regulated by sucrose and up-regulated by mannitol, UV and neutral pH.