Transcriptome Analysis Identifies a Gene Cluster for the Biosynthesis of Biruloquinone, a Rare Phenanthraquinone, in a Lichen-Forming Fungus Cladonia macilenta.

Wonyong Kim,Min-Hye Jeong,Sung-Hwan Yun,Jae-Seoun Hur

doi:10.3390/jof7050398

Wonyong Kim, Min-Hye Jeong + Show 2 more

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https://doi.org/10.3390/jof7050398

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Abstract

Lichens are prolific producers of natural products of polyketide origin. We previously described a culture of lichen-forming fungus (LFF) Cladonia macilenta that produces biruloquinone, a purple pigment that is a phenanthraquinone rarely found in nature. However, there was no genetic information on the biosynthesis of biruloquinone. To identify a biosynthetic gene cluster for biruloquinone, we mined polyketide synthase (PKS) genes from the genome sequence of a LFF isolated from thalli of C. macilenta. The 38 PKS in C. macilenta are highly diverse, many of which form phylogenetic clades with PKS previously characterized in non-lichenized fungi. We compared transcriptional profiles of the 38 PKS genes in two chemotypic variants, one producing biruloquinone and the other producing no appreciable metabolite in vitro. We identified a PKS gene (hereafter PKS21) that was highly upregulated in the LFF that produces biruloquinone. The boundaries of a putative biruloquinone gene cluster were demarcated by co-expression patterns of six clustered genes, including the PKS21. Biruloquinone gene clusters exhibited a high degree of synteny between related species. In this study we identified a novel PKS family responsible for the biosynthesis of biruloquinone through whole-transcriptome analysis.

Highlights

On the basis of the chemical structure, biruloquinone is proposed to be biosynthesized from an octaketide that is likely biosynthesized by an NR-polyketide synthase (PKS) (Figure 1B) [19]
non-reducing PKS (NR-PKS) suggested that a pyrone ring can be formed when the polyketide intermediate is released by a hydrolytic activity of the TE domain of NR-PKSs, as in the case of the cercosporin biosynthetic pathway [40]
Despite huge genome-encoded metabolic potentials of lichen-forming fungi [43,44], little is known about biosynthetic genes responsible for lichen metabolite production

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. More than a thousand polyketide-derived secondary metabolites (SMs) have been isolated from lichen thalli, many of which are exclusively found in lichens [1,2,3]. Several polyketide-derived SMs, such as barbatic acid, didymic acid, rhodocladonic acid, and thamnolic acid, have been extracted from lichen thalli of Cladonia macilenta Hoffm. A polyketide, biruloquinone, was discovered in an isolated mycobiont of Cladonia macilenta [6]. Biruloquinone is a phenanthraquinone rarely found in nature. It was first discovered in lichen thalli of Parmelia birulae [7], and later in a plant pathogenic fungus

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