Characterization of the Biosynthetic Pathway of Fungal Aromatic Polyketides
Nature has endowed microbes and plants with enormous power to synthesize complex natural products from simple building blocks. Compounds derived from natural sources that show biological activities are defined as natural products. Over the past century, natural products have played an indispensible role in the pharmaceutical discovery, and have become an important element of modern therapy. Aromatic polyketides are an important group of natural products that exhibit extraordinary medicinal activities. Actinomycetes and filamentous fungi are the two major producers of aromatic polyketides, especially polycyclic aromatic polyketides; and they employ distinct enzymatic system to synthesize these complex structures from malonyl-CoA. Fungi utilize iterative multidomain megasynthase of type I nonreducing polyketide synthases (NRPKSs), while actinomycetes employ bacterial type II PKSs that comprise a group of dissociated enzymes. Compared to the well-studied type II PKS, NRPKS has been much less understood.
In this study, we first identified the unique product template (PT) domain of NRPKS that is not shared among the other PKS families as the first ring aldol cyclase. Based on the understanding of PT domain function, we performed phylogenetic analysis of PT domains, which classified the NRPKSs into five major groups. Subsequently, we proposed a sequence-function correlation of PT domain that allowed us to predict the activities of unknown PTs uncovered from genome sequencing. These target PT domains were assayed with the PKS4 (a previously identified NRPKS) minimal PKS domains in trans or in cis, and the proposed PT functions were confirmed through such product based assays. According to the established sequence-function relationship, we conducted genome mining of A. niger for the biosynthetic locus of TAN-1612, which contains an interesting tetracyclic naphthacenedione core structure. Subsequent investigations of the TAN-1612 biosynthetic pathway uncovered a novel alpha-hydroxylation-dependent Claisen cyclization cascade that involves a bifunctional dimanganese metallo-beta-lactamase-type thioesterase (MbL-TE). Also, the identification of the TAN-1612 biosynthetic pathway revealed a decaketide synthase AdaA, which enabled us to realize the in vitro enzymatic total synthesis of several important aromatic decaketide intermediates including rabelomycin and tetracenomycin D1.