Isolation and characterization of a non-reducing polyketide synthase gene from the lichen-forming fungus Usnea longissima

Biosynthetic Potential of Hypogymnia Holobionts: Insights into Secondary Metabolite Pathways

Lichens are symbiotic associations consisting of a photobiont (algae or cyanobacteria) and a mycobiont (fungus). They are known to produce a variety of unique secondary metabolites. To access this biosynthetic potential for biotechnological applications, deeper insights into the biosynthetic pathways and corresponding gene clusters are necessary. Here we provide a comprehensive view of the biosynthetic gene clusters of all organisms comprising a lichen thallus: fungi, green algae, and bacteria. We present two high-quality PacBio metagenomes, in which we identified a total of 460 biosynthetic gene clusters. Lichen mycobionts yielded 73-114 clusters, other lichen associated ascomycetes 8-40, green algae of the genus Trebouxia 14-19, and lichen-associated bacteria 101-105 clusters. The mycobionts contained mainly T1PKSs, followed by NRPSs, and terpenes; Trebouxia reads harbored mainly clusters linked to terpenes, followed by NRPSs and T3PKSs. Other lichen-associated ascomycetes and bacteria contained a mix of diverse biosynthetic gene clusters. In this study, we identified for the first time the biosynthetic gene clusters of entire lichen holobionts. The yet untapped biosynthetic potential of two species of the genus Hypogymnia is made accessible for further research.

A Candidate Gene Cluster for the Bioactive Natural Product Gyrophoric Acid in Lichen-Forming Fungi

Natural products of lichen-forming fungi are structurally diverse and have a variety of medicinal properties. Despite this, they have limited implementation in industry mostly because the corresponding genes are unknown for most of their natural products. Here, we implement a long-read sequencing and bioinformatic approach to identify the putative biosynthetic gene cluster of the bioactive natural product gyrophoric acid (GA). Using 15 high-quality genomes representing nine GA-producing species of the lichen-forming fungal genus Umbilicaria, we identify the most likely GA cluster and investigate the cluster gene organization and composition across the nine species. Our results show that GA clusters are promiscuous within Umbilicaria, and only three genes are conserved across species, including the polyketide synthase (PKS) gene. In addition, our results suggest that the same cluster codes for different, but structurally similar compounds, namely, GA, umbilicaric-, and hiascic acid, bringing new evidence that lichen metabolite diversity is also generated through regulatory mechanisms at the molecular level. Ours is the first study to identify the most likely GA cluster and, thus, provides essential information to open new avenues for biotechnological approaches to producing and modifying GA and similar lichen-derived compounds. GA PKS is the first tridepside PKS to be identified.

IMPORTANCE The implementation of natural products in the pharmaceutical industry relies on the possibility of modifying the natural product (NP) pathway to optimize yields and pharmacological effects. Characterization of genes and pathways underlying natural product biosynthesis is a major bottleneck for exploiting the medicinal properties of the natural products. Genome mining is a promising and relatively cost- and time-effective approach to utilize unexplored NP resources for drug discovery. In this study, we identify the most likely gene cluster for the lichen-forming fungal depside gyrophoric acid in nine Umbilicaria species. This compound shows cytotoxic and antiproliferative properties against several cancer cell lines and is also a broad-spectrum antimicrobial agent. This information paves the way for generating GA analogs with modified properties by selective activation/deactivation of genes.

A comprehensive catalogue of polyketide synthase gene clusters in lichenizing fungi

Lichens are fungi that form symbiotic partnerships with algae. Although lichens produce diverse polyketides, difficulties in establishing and maintaining lichen cultures have prohibited detailed studies of their biosynthetic pathways. Creative, albeit non-definitive, methods have been developed to assign function to biosynthetic gene clusters in lieu of techniques such as gene knockout and heterologous expressions that are commonly applied to easily cultivatable organisms. We review a total of 81 completely sequenced polyketide synthase (PKS) genes from lichenizing fungi, comprising to our best efforts all complete and reported PKS genes in lichenizing fungi to date. This review provides an overview of the approaches used to locate and sequence PKS genes in lichen genomes, current approaches to assign function to lichen PKS gene clusters, and what polyketides are proposed to be biosynthesized by these PKS. We conclude with remarks on prospects for genomics-based natural products discovery in lichens. We hope that this review will serve as a guide to ongoing research efforts on polyketide biosynthesis in lichenizing fungi.

Identification of a putative polyketide synthase gene involved in usnic acid biosynthesis in the lichen Nephromopsis pallescens

Usnic acid is a unique polyketide produced by lichens. To characterize usnic acid biosynthesis, the transcriptome of the usnic-acid-producing lichen-forming fungus Nephromopsis pallescens was sequenced using Illumina NextSeq technology. Seven complete non-reducing polyketide synthase genes and nine highly-reducing polyketide synthase genes were obtained through transcriptome analysis. Gene expression results obtained by qPCR and usnic acid detection with LCMS-IT-TOF showed that Nppks7 is probably involved in usnic acid biosynthesis in N. pallescens. Nppks7 is a non-reducing polyketide synthase with a MeT domain that also possesses beta-ketoacyl-ACP synthase, acyl transferase, product template, acyl carrier protein, C-methyltransferase, and Claisen cyclase domains. Phylogenetic analysis shows that Nppks7and other polyketide synthases from lichens form a unique monophyletic clade. Taken together, our data indicate that Nppks7 is a novel PKS in N. pallescens that is likely involved in usnic acid biosynthesis.

Lichen Biosynthetic Gene Clusters. Part I. Genome Sequencing Reveals a Rich Biosynthetic Potential

Lichens are symbionts of fungi and algae that produce diverse secondary metabolites with useful properties. Little is known of lichen natural product biosynthesis because of the challenges of working with lichenizing fungi. We describe the first attempt to comprehensively profile the genetic secondary metabolome of a lichenizing fungus. An Illumina platform combined with the Antibiotics and Secondary Metabolites Analysis Shell (FungiSMASH, version 4.0) was used to sequence and annotate assembled contigs of the fungal partner of Cladonia uncialis. Up to 48 putative gene clusters are described comprising type I and type III polyketide synthases (PKS), nonribosomal peptide synthetases (NRPS), hybrid PKS-NRPS, and terpene synthases. The number of gene clusters revealed by this work dwarfs the number of known secondary metabolites from C. uncialis, suggesting that lichenizing fungi have an unexplored biosynthetic potential.

Algal carbohydrates affect polyketide synthesis of the lichen-forming fungus Cladonia rangiferina

Lichen secondary metabolites (polyketides) are produced by the fungal partner, but the role of algal carbohydrates in polyketide biosynthesis is not clear. This study examined whether the type and concentration of algal carbohydrate explained differences in polyketide production and gene transcription by a lichen fungus (Cladonia rangiferina). The carbohydrates identified from a free-living cyanobacterium (Spirulina platensis; glucose), a lichen-forming alga (Diplosphaera chodatii; sorbitol) and the lichen alga that associates with C. rangiferina (Asterochloris sp.; ribitol) were used in each of 1%, 5% and 10% concentrations to enrich malt yeast extract media for culturing the mycobiont. Polyketides were determined by high performance liquid chromatography (HPLC), and polyketide synthase (PKS) gene transcription was measured by quantitative PCR of the ketosynthase domain of four PKS genes. The lower concentrations of carbohydrates induced the PKS gene expression where ribitol up-regulated CrPKS1 and CrPKS16 gene transcription and sorbitol up-regulated CrPKS3 and CrPKS7 gene transcription. The HPLC results revealed that lower concentrations of carbon sources increased polyketide production for three carbohydrates. One polyketide from the natural lichen thallus (fumarprotocetraric acid) also was produced by the fungal culture in ribitol supplemented media only. This study provides a better understanding of the role of the type and concentration of the carbon source in fungal polyketide biosynthesis in the lichen Cladonia rangiferina.

Genome characteristics reveal the impact of lichenization on lichen-forming fungus Endocarpon pusillum Hedwig (Verrucariales, Ascomycota)

BACKGROUND

Lichen is a classic mutualistic organism and the lichenization is one of the fungal symbioses. The lichen-forming fungus Endocarpon pusillum is living in symbiosis with the green alga Diplosphaera chodatii Bialsuknia as a lichen in the arid regions.

RESULTS

454 and Illumina technologies were used to sequence the genome of E. pusillum. A total of 9,285 genes were annotated in the 37.5 Mb genome of E. pusillum. Analyses of the genes provided direct molecular evidence for certain natural characteristics, such as homothallic reproduction and drought-tolerance. Comparative genomics analysis indicated that the expansion and contraction of some protein families in the E. pusillum genome reflect the specific relationship with its photosynthetic partner (D. chodatii). Co-culture experiments using the lichen-forming fungus E. pusillum and its algal partner allowed the functional identification of genes involved in the nitrogen and carbon transfer between both symbionts, and three lectins without signal peptide domains were found to be essential for the symbiotic recognition in the lichen; interestingly, the ratio of the biomass of both lichen-forming fungus and its photosynthetic partner and their contact time were found to be important for the interaction between these two symbionts.

CONCLUSIONS

The present study lays a genomic analysis of the lichen-forming fungus E. pusillum for demonstrating its general biological features and the traits of the interaction between this fungus and its photosynthetic partner D. chodatii, and will provide research basis for investigating the nature of its drought resistance and symbiosis.