Intertwined Biosynthesis of Skyrin and Rugulosin A Underlies the Formation of Cage-Structured Bisanthraquinones

Skyrin and rugulosin A are bioactive bisanthraquinones found in many fungi, with the former suggested as a precursor of hypericin (a diversely bioactive phytochemical) and the latter characterized by its distinct cage-like structure. However, their biosynthetic pathways remain mysterious, although they have been characterized for over six decades. Here, we present the rug gene cluster that governs simultaneously the biosynthesis of skyrin and rugulosin A in Talaromyces sp. YE3016, a fungal endophyte residing in Aconitum carmichaeli. A combination of genome sequencing, gene inactivation, heterologous expression, and biotransformation tests allowed the identification of the gene function, biosynthetic precursor, and enzymatic sets involved in their molecular architecture constructions. In particular, skyrin was demonstrated to form from the 5,5'-dimerization of emodin radicals catalyzed by RugG, a cytochrome P450 monooxygenase evidenced to be potentially applicable for the (chemo)enzymatic synthesis of dimeric polyphenols. The fungal aldo-keto reductase RugH was shown to be capable of hijacking the closest skyrin precursor (CSP) immediately after the emodin radical coupling, catalyzing the ketone reduction of CSP to inactivate its tautomerization into skyrin and thus allowing for the spontaneous intramolecular Michael addition to cyclize the ketone-reduced form of CSP into rugulosin A, a representative of diverse cage-structured bisanthraquinones. Collectively, the work updates our understanding of bisanthraquinone biosynthesis and paves the way for synthetic biology accesses to skyrin, rugulosin A, and their siblings.