A Novel Class of Plant Type III Polyketide Synthase Involved in Orsellinic Acid Biosynthesis from Rhododendron dauricum.

Futoshi Taura,Fumiya Kurosaki,Hiroyuki Morita,Hiromichi Kenmoku,Haruna Saeki,Miu Iijima,Eriko Yamanaka,Yoshinori Asakawa,Satoshi Morimoto,Hironobu Takahashi

doi:10.3389/fpls.2016.01452

Futoshi Taura, Fumiya Kurosaki + Show 8 more

Open Access

https://doi.org/10.3389/fpls.2016.01452

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Abstract

Rhododendron dauricum L. produces daurichromenic acid, the anti-HIV meroterpenoid consisting of sesquiterpene and orsellinic acid (OSA) moieties. To characterize the enzyme responsible for OSA biosynthesis, a cDNA encoding a novel polyketide synthase (PKS), orcinol synthase (ORS), was cloned from young leaves of R. dauricum. The primary structure of ORS shared relatively low identities to those of PKSs from other plants, and the active site of ORS had a unique amino acid composition. The bacterially expressed, recombinant ORS accepted acetyl-CoA as the preferable starter substrate, and produced orcinol as the major reaction product, along with four minor products including OSA. The ORS identified in this study is the first plant PKS that generates acetate-derived aromatic tetraketides, such as orcinol and OSA. Interestingly, OSA production was clearly enhanced in the presence of Cannabis sativa olivetolic acid cyclase, suggesting that the ORS is involved in OSA biosynthesis together with an unidentified cyclase in R. dauricum.

Highlights

Rhododendron dauricum L. (Ericaceae) produces daurichromenic acid (DCA), the unique meroterpenoid composed of sesquiterpene and orsellinic acid (OSA) moieties (Kashiwada et al, 2001) (Figure 1A)
As compared with chalcone synthase (CHS), the primary structure of orcinol synthase (ORS) contained simultaneous amino acid changes in the CHS’s conserved active site residues, it conserved the CysHis-Asn catalytic triad commonly found in the type III polyketide synthase (PKS)
In vitro enzyme assays revealed that the recombinant ORS did not accept p-coumaroyl-CoA, the typical starter molecule for plant type III PKSs, but preferred acetyl-CoA as the starter to produce five reaction products via two to three condensations with malonyl-CoA

Summary

Introduction

Rhododendron dauricum L. (Ericaceae) produces daurichromenic acid (DCA), the unique meroterpenoid composed of sesquiterpene and orsellinic acid (OSA) moieties (Kashiwada et al, 2001) (Figure 1A). We presumed that DCA is biosynthesized via a pathway similar to that for cannabinoid biosynthesis in Cannabis sativa L. (Taura et al, 2007a; Vickery et al, 2016), because of the structural similarity between DCA and cannabinoids (Figure 1). The proposed DCA pathway and the cannabinoid biosynthesis consist of three reaction steps: (1) polyketide formation, (2) prenylation, and (3) cyclization, to synthesize cyclic meroterpenoids as final metabolites (Figure 1). In contrast to the cannabinoid pathway, in which all of the biosynthetic enzymes have been identified (Taura et al, 2007a; Vickery et al, 2016), the biosynthetic mechanism leading to grifolic acid in the DCA pathway has remained elusive. We focused on the polyketide synthase (PKS) involved in the OSA biosynthesis in R. dauricum

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