Analysis and refactoring of the A-74528 biosynthetic pathway.

A-74528 is a C-30 polyketide natural product that functions as an inhibitor of 2′,5′-oligoadenylate phosphodiesterase (2′-PDE), a key regulatory enzyme of the interferon pathway. Modulation of 2′-PDE represents a unique therapeutic approach to regulate viral infections. The gene cluster responsible for biosynthesis of A-74528 yields minute amounts of this natural product together with considerably larger quantities of a structurally dissimilar C-30 cytotoxic agent, fredericamycin. Through construction and analysis of a series of knockout mutants, we identified the necessary genes for A-74528 biosynthesis. Remarkably, the formation of six stereocenters and the regiospecific formation of six rings in A- 74528 appears to be catalyzed by only two tailoring enzymes, a cyclase and an oxygenase, in addition to the core polyketide synthase. The inferred pathway was genetically refactored in a heterologous host, Streptomyces coelicolor CH999, to produce 3 mg/L A-74528 in the absence of fredericamycin.