The 6,6-quinolone scaffold of the viridicatin-type of fungal alkaloids are found in various quinolone alkaloids which often exhibit useful biological activities. Thus, it is of interest to identify viridicatin-forming enzymes and understand how such alkaloids are biosynthesized. Here an Aspergillal gene cluster responsible for the biosynthesis of 4'-methoxyviridicatin was identified. Detailed in vitro studies led to the discovery of the dioxygenase AsqJ which performs two distinct oxidations: first desaturation to form a double bond and then monooxygenation of the double bond to install an epoxide. Interestingly, the epoxidation promotes non-enzymatic rearrangement of the 6,7-bicyclic core of 4'-methoxycyclopenin into the 6,6-quinolone viridicatin scaffold to yield 4'-methoxyviridicatin. The finding provides new insight into the biosynthesis of the viridicatin scaffold and suggests dioxygenase as a potential tool for 6,6-quinolone synthesis by epoxidation of benzodiazepinediones.
Keywords: alkaloids; enzymes; gene expression; natural products; oxidations.
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