Reconstitution of a Highly Reducing Type II PKS System Reveals 6π-Electrocyclization Is Required for o‑Dialkylbenzene Biosynthesis

• Published in: Journal of the American Chemical Society 2021-02, Vol.143 (7), p.2962-2969
• Main Authors: Zhang, Jie, Yuzawa, Satoshi, Thong, Wei Li, Shinada, Tetsuro, Nishiyama, Makoto, Kuzuyama, Tomohisa
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.0c13378

Natural products containing an o-dialkylbenzene moiety exhibit a wide variety of bioactivities, including antibacterial, antifungal, antitumor, and antiangiogenic activities. However, the biosynthetic scheme of the o-dialkylbenzene moiety remains unclear. In this study, we identified the biosynthetic gene cluster (BGC) of compounds 1 and 2 in Streptomyces sp. SANK 60404, which contains a rare o-dialkylbenzene moiety, and successfully reconstituted the biosynthesis of 1 using 22 recombinant enzymes in vitro. Our study established a biosynthetic route for the o-tolyl group within the o-dialkylbenzene moiety, where the triene intermediate 3 loaded onto a unique acyl carrier protein (ACP) is elongated by a specific ketosynthase–chain length factor pair of a type II polyketide synthase system with the aid of a putative isomerase to be termed “electrocyclase” and a thioesterase-like enzyme in the BGC. The C2-elongated all-trans diketo–triene intermediate is subsequently isomerized to the 6Z configuration by the electrocyclase to allow intramolecular 6π-electrocyclization, followed by coenzyme FAD/FMN-dependent dehydrogenation. Bioinformatics analysis showed that the key genes are all conserved in BGCs of natural products containing an o-dialkylbenzene moiety, suggesting that the proposed biosynthetic scheme is a common strategy to form o-dialkylbenzenes in nature.