Mechanism Studies on Fungal Type I Highly-Reducing Polyketide Synthases and Polyketide Synthase-Nonribosomal Peptide Synthetase Hybrids

Filamentous fungi are known as promising sources for bio-active natural products, some of which are blockbuster drugs, such as penicillin and lovastatin. Fungal polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs) are large highly complex multidomain megasynthetases with complex programming rules. Different from their well-studied bacterial counterparts, the mechanisms of these megasynthases are not well understood to date. The thesis focuses on mechanism study of two fungal highly reducing PKSs involved in lovastatin biosynthesis and a PKS-NRPS hybrid from Apsergillus sp.Using the developed expression system in S. cerevisiae, we were able to probe the property of these enzymes under different conditions with in vivo experiment in the heterologous host and in vitro assays. In the lovastatin project, we observed and studied interaction between highly-reducing polyketide synthase (HR-PKS) and its product releasing partner. These PKS/releasing-enzyme pairs could widely exist in the HR-PKS systems. It also shows applications to drug industry because the efficiency of product releasing from PKSs could directly determine the yield of the natural products. In the study related to PKS-NRPS hybrid, we were able to be the first group to reconstitute full function of this type of megasynthetase in vitro and probe the programming on both PKS and NRPS module in this hybrid. More importantly, we also observed interaction between these two modules.