Type III polyketide synthases in lichen mycobionts.

Lichenized and non-lichenized fungi produce a wide range of secondary metabolites. So far, type I polyketide synthases (PKSs) are the suggested catalysts for the biosynthesis of lichen compounds. We were interested whether lichen mycobionts also contain type III PKSs, representing a class that was only recently discovered in fungi. With an alignment of known type III CHS-like genes we applied the CODEHOP strategy to design degenerate PCR primers. We further screened available fungal genomes for type III PKS genes and aligned these sequences for a phylogenetic analysis. Type III-like genes from lichen mycobionts are closely related to those known from non-lichenized fungi, but not to those of bacteria and/or plants. We conclude that type III PKS genes are ubiquitous in fungi. They are present in diverse unrelated lichen mycobionts, but their function in lichens is so far unclear.

Muggia L ，Grube M 《Fungal Biology》

6-MSAS-like polyketide synthase genes occur in lichenized ascomycetes.

Lichenized and non-lichenized filamentous ascomycetes produce a great variety of polyketide secondary metabolites. Some polyketide synthase (PKS) genes from non-lichenized fungi have been characterized, but the function of PKS genes from lichenized species remains unknown. Phylogenetic analysis of keto synthase (KS) domains allows prediction of the presence or absence of particular domains in the PKS gene. In the current study we screened genomic DNA from lichenized fungi for the presence of non-reducing and 6-methylsalicylic acid synthase (6-MSAS)-type PKS genes. We developed new degenerate primers in the acyl transferase (AT) region to amplify a PKS fragment spanning most of the KS region, the entire linker between KS and AT, and half of the AT region. Phylogenetic analysis shows that lichenized taxa possess PKS genes of the 6-MSAS-type. The extended alignment confirms overall phylogenetic relationships between fungal non-reducing, 6-MSAS-type and bacterial type I PKS genes.

Schmitt I ，Kautz S ，Lumbsch HT 《mycological research》

Insights from the first putative biosynthetic gene cluster for a lichen depside and depsidone.

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.

Armaleo D ，Sun X ，Culberson C 《MYCOLOGIA》

Purifying selection is a prevailing motif in the evolution of ketoacyl synthase domains of polyketide synthases from lichenized fungi.

We analysed ketoacyl synthase domains of type I polyketide synthase (PKS) gene fragments of 163 lichenized and 51 non-lichenized fungi in a Bayesian phylogenetic framework. Lichenized taxa from several unrelated taxonomic groups, some of which produce identical secondary metabolites, were included. We found 12 clades of non-reducing PKS genes, which represent monophyletic PKS paralogues. PAML and SELECTON analyses indicated that purifying selection is the prevailing selective force in the evolution of the keto synthase domain of these paralogues. We detected no unambiguous correlation between PKS clades and the distribution of lichen substances. Together with the strong evidence for purifying selection, the wide distribution of certain paralogues in ascomycetes suggested early gene duplication events in the evolutionary history of this gene family in the Ascomycota.

Muggia L ，Schmitt I ，Grube M 《mycological research》

Cloning and heterologous transcription of a polyketide synthase gene from the lichen Solorina crocea.

Lichens and most ascomycete fungi produce polyketide secondary metabolites often with valuable biological activities. Their biosynthesis is primarily governed by large iterative multifunctional type I polyketide synthases. Although there has been good progress studying filamentous non-lichenized fungi, there is limited information on polyketide biosynthesis in lichens and their mycobionts, due to their slow growth, difficulties in establishing pure cultures, and the absence of methods for direct genetic manipulation. However, heterologous expression in a surrogate host offers an alternative approach for exploring lichen polyketide biosynthesis. Here, we report cloning of a type I polyketide synthase gene from the foliose lichen Solorina crocea and its heterologous transcription in the filamentous fungus Aspergillus oryzae, including processing of the transcript. No new polyketide product was detected. The lichen polyketide synthase showed greatest homology with uncharacterized genes from filamentous fungi and lower homology with proteins catalysing biosynthesis of the decaketide alternapyrone and the tetraketide side-chain of squalestatin. The technology platform utilized here presents a useful tool for functional characterization of fungal biosynthetic genes and provides a means for novel production of valuable compounds.

Gagunashvili AN ，Davídsson SP ，Jónsson ZO ，Andrésson OS ... -  《-》