Depside and Depsidone Synthesis in Lichenized Fungi Comes into Focus through a Genome-Wide Comparison of the Olivetoric Acid and Physodic Acid Chemotypes of Pseudevernia furfuracea.

Garima Singh,Imke Schmitt,Francesco Dal Grande,Daniele Armaleo

doi:10.3390/biom11101445

Abstract

Primary biosynthetic enzymes involved in the synthesis of lichen polyphenolic compounds depsides and depsidones are non-reducing polyketide synthases (NR-PKSs), and cytochrome P450s. However, for most depsides and depsidones the corresponding PKSs are unknown. Additionally, in non-lichenized fungi specific fatty acid synthases (FASs) provide starters to the PKSs. Yet, the presence of such FASs in lichenized fungi remains to be investigated. Here we implement comparative genomics and metatranscriptomics to identify the most likely PKS and FASs for olivetoric acid and physodic acid biosynthesis, the primary depside and depsidone defining the two chemotypes of the lichen Pseudevernia furfuracea. We propose that the gene cluster PF33-1_006185, found in both chemotypes, is the most likely candidate for the olivetoric acid and physodic acid biosynthesis. This is the first study to identify the gene cluster and the FAS likely responsible for olivetoric acid and physodic acid biosynthesis in a lichenized fungus. Our findings suggest that gene regulation and other epigenetic factors determine whether the mycobiont produces the depside or the depsidone, providing the first direct indication that chemotype diversity in lichens can arise through regulatory and not only through genetic diversity. Combining these results and existing literature, we propose a detailed scheme for depside/depsidone synthesis.

Highlights

Depsides and depsidones, the polyphenolic polyketides mostly synthesized by lichenized fungi, are of significant pharmaceutical interest [1,2,3]
In this study we describe the putative biosynthetic gene cluster (BGC) for the biosynthesis of olivetoric acid and physodic acid in the lichen-forming fungus P. furfuracea from high-quality long-read genome assemblies of the two chemotypes
The polyketide synthases (PKSs) of cluster4 we identified in this study as most likely PKS associated with the biosynthesis of olivetoric acid and physodic acid,as is most the same as Pfur33-1_006185

Summary

Introduction

The polyphenolic polyketides mostly synthesized by lichenized fungi, are of significant pharmaceutical interest [1,2,3]. Depsides consist of two or sometimes three orcinol or ß-orcinol-derived aromatic rings joined by ester linkages; depsidones have an additional ether linkage between the rings (Figure 1). Depending on the starters used by the polyketide synthases (PKSs) assembling their backbones, 3–7 carbon side chains may be linked to the 6 and 6’ carbons of the orcinol-derived rings. Together with other ring modifications, side chains constitute the distinguishing features of different depsides and depsidones. Chemical proposals for depside and depsidone biosynthesis go back many decades [4,5], the precise enzymatic steps of depside and depsidone synthesis still need to be elucidated. For most of the depside and depsidone metabolites of lichens, the corresponding genes remain uncharacterized

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