Insights into radicicol biosynthesis via heterologous synthesis of intermediates and analogs

Resorcylic acid lactones are fungal polyketides that display diverse biological activities, with the potent Hsp90 inhibitor radicicol being an important representative member. Two fungal iterative polyketide synthases (IPKSs), Rdc5, the highly reducing IPKS, and Rdc1, the nonreducing IPKS, are required for the biosynthesis of radicicol in Pochonia chlamydosporia. In this study, the complete reconstitution of Rdc5 and Rdc1 activities both in vitro and in Saccharomyces cerevisiae uncovered the earliest resorcylic acid lactone intermediate of the radicicol biosynthetic pathway, (R)-monocillin II. The enzymatic synthesis of (R)-monocillin II confirmed the exquisite timing of the Rdc5 enoyl reductase domain. Using precursor-directed biosynthesis, the chemical modularity of the dual IPKS system was determined. Rdc1 readily accepted an N-acetylcysteamine thioester mimic of the reduced pentaketide product of Rdc5 to synthesize (R)-monocillin II with four additional iterations of polyketide elongation, indicating the C2' ketone group found in (R)-monocillin II is incorporated via the functions of Rdc1 instead of Rdc5. The involvement of the thioesterase domain in Rdc1 in macrolactonization was confirmed through both site-directed mutagenesis and domain deletion. The Rdc1 thioesterase domain was also shown to be tolerant of the opposite stereochemistry of the terminal hydroxyl nucleophile, demonstrated in the precursor-directed synthesis of the enantiomeric (S)-monocillin II. Finally, reconstitution of the halogenase Rdc2 was demonstrated both in vivo and in vitro in the synthesis of pochonin D and a new halogenated analog 6-chloro, 7',8'-dehydrozearalenol.