The Discovery and Characterization of 1,4-Dihydroxy-2-naphthoic Acid Prenyltransferase Involved in the Biosynthesis of Anthraquinones in Rubia cordifolia

Anthraquinones constitute the largest group of natural quinones, which are used as safe natural dyes and have many pharmaceutical applications. In plants, anthraquinones are biosynthesized through two main routes: the polyketide pathway and the shikimate pathway. The shikimate pathway primarily forms alizarin-type anthraquinones, and the prenylation of 1,4-dihydroxy-2-naphthoic acid is the first pathway-specific step. However, the prenyltransferase responsible for this key step remains uncharacterized. In this study, the cell suspension culture of Rubia cordifolia, a plant rich in alizarin-type anthraquinones, was used in target prenyltransferase mining. The microsomal protein prepared from the cell suspension culture prenylated 1,4-dihydroxy-2-naphthoic acid to form 2-carboxyl-3-prenyl-1,4-naphthoquinone and 3-prenyl-1,4-naphthoquinone. Then a candidate gene belonging to UbiA superfamily, RcDT1, was discovered to account for the prenylation activity. Substrate specificity studies revealed that the recombinant RcDT1 recognized naphthoic acids primarily, followed by 4-hydroxyl benzoic acids. The prenylation activities of R. cordifolia microsomes and the recombinant RcDT1 were both strongly inhibited by 1,2- and 1,4-dihydroxynaphthalene. The plastid localization and root-specific expression further confirmed the participation of RcDT1 in anthraquinone biosynthesis. The phylogenetic analyses of RcDT1 and its rubiaceous homologs indicated that DHNA-prenylation activity evolved convergently in Rubiaceae via recruitment from the ubiquinone biosynthetic pathway. The discovery and evolutionary studies of RcDT1 provide useful guidance for identifying additional and evolutionarily varied prenyltransferases which enable entry into quinones derived from shikimate pathway. Moreover, these findings will have profound implications for understanding the biosynthetic process of the anthraquinone/naphthoquinone ring derived from shikimate pathway.