title: Insights from the first putative biosynthetic gene cluster for a lichen depside and depsidone.
abstract: The genes for polyketide synthases (PKSs), enzymes that assemble the carbon backbones of many secondary metabolites, often cluster with other secondary pathway genes. We describe here the first lichen PKS cluster likely to be implicated in the biosynthesis of a depside and a depsidone, compounds in a class almost exclusively produced by lichen fungi (mycobionts). With degenerate PCR with primers biased toward presumed PKS genes for depsides and depsidones we identified among the many PKS genes in Cladonia grayi four (CgrPKS13-16) potentially responsible for grayanic acid (GRA), the orcinol depsidone characteristic of this lichen. To single out a likely GRA PKS we compared mRNA and GRA induction in mycobiont cultures using the four candidate PKS genes plus three controls; only CgrPKS16 expression closely matched GRA induction. CgrPKS16 protein domains were compatible with orcinol depside biosynthesis. Phylogenetically CgrPKS16 fell in a new subclade of fungal PKSs uniquely producing orcinol compounds. In the C. grayi genome CgrPKS16 clustered with a CytP450 and an o-methyltransferase gene, appropriately matching the three compounds in the GRA pathway. Induction, domain organization, phylogeny and cluster pathway correspondence independently indicated that the CgrPKS16 cluster is most likely responsible for GRA biosynthesis. Specifically we propose that (i) a single PKS synthesizes two aromatic rings and links them into a depside, (ii) the depside to depsidone transition requires only a cytochrome P450 and (iii) lichen compounds evolved early in the radiation of filamentous fungi.
authors.affiliation: Department of Biology, Duke University, North Carolina 27708, USA. darmaleo@duke.edu
authors.forename: Daniele
authors.initials: D
authors.lastname: Armaleo
authors.forename: Xiameng
authors.initials: X
authors.lastname: Sun
authors.forename: Chicita
authors.initials: C
authors.lastname: Culberson
mesh_terms: D000595:Amino Acid Sequence; D001203:Ascomycota / Q000201:enzymology; D001483:Base Sequence; D053630:Depsides / Q000378:metabolism*; D005656:Fungal Proteins / Q000235:genetics* / Q000378:metabolism; D015966:Gene Expression Regulation, Fungal; D007783:Lactones / Q000378:metabolism*; D008011:Lichens / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism / Q000382:microbiology; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000235:genetics / Q000378:metabolism; D005810:Multigene Family; D009657:North Carolina; D048630:Polyketide Synthases / Q000235:genetics* / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D017386:Sequence Homology, Amino Acid; D013559:Symbiosis / Q000502:physiology
chemical_list: D053630:Depsides; D005656:Fungal Proteins; D007783:Lactones; D009097:Multienzyme Complexes; D012333:RNA, Messenger; C000875:depsidone; D048630:Polyketide Synthases