A Discrete, Iterative Type I Polyketide Synthase System Catalyzing the Formation of Unusual Fatty Acid Chains

Polyketide synthase (PKS) is an ingenious core machine that produces structurally diverse polyketides. Here, we report an unusual iterative type I PKS encoded in the anb biosynthetic gene cluster of the uridine-derived liponucleoside antibiotic A-94964. While sequence analysis classifies this system as a type I PKS, its domain architecture diverges from that of canonical systems with the individual domains dispersed across four separate proteins. Functional assignments of individual domains were established by heterologous coexpression and in vitro reconstitution. These analyses revealed that this PKS iteratively catalyzes multiple reactions to construct the complex acyl side chain of A-94964, which remains tethered to the ACP domain. The ACP-bound acyl side chain is subsequently transferred to the nucleoside core by an atypical N-acetyltransferase, Anb3, which, in complex with Anb9, recognizes ACP-tethered acyl intermediates rather than free acyl-CoA substrates. Phylogenetic analysis suggests that AnbPKSs are related to noncanonical PKS systems such as polyunsaturated fatty acid (PUFA) synthases and enediyne synthases. Based on the distinctive domain architecture and reaction features, we describe this system as a "discrete iterative type I PKS system", thereby expanding the architectural and mechanistic diversity of type I PKSs.