Orchestration of discoid polyketide cyclization in the resistomycin pathway

Resistomycin is a bacterial polyphenolic metabolite from Streptomyces resistomycificus with a unique pentacyclic "discoid" ring system that clearly differs from the typical linear or angular architectures of aromatic polyketides. The first comprehensive cyclase amino acid sequence-function correlation revealed that the enzymes directing the nascent polyketide chain into a peri-fused system clearly differ from canonical linear and angular cyclases. All genes that are required and sufficent for resistomycin (rem) biosynthesis were identified through systematic dissection and reconstitution of the type II polyketide synthase (PKS) complex. The minimal rem PKS and the first cyclase were successfully cross-complemented with orthologues from the linear tetracenomycin polyketide pathway, indicating that both dekaketide pathways share early biosynthetic steps. In total three cyclases that are involved in discoid cyclization (RemI, RemF, and RemL) were identified by mutational analyses and in vivo pathway reconstitution. Analyses of the metabolic profiles of mutants expressing incomplete gene sets led to the discovery of a novel tetracenomycin derivative, TcmR1. The most surprising finding is that only the concerted action of the PKS and all three cyclases leads to the discoid ring structure. These results provide strong support for a model according to which the multienzyme complex forms a cage in which the polyketide is shaped, rather than a sequential cyclization of the polyketide chain by individual enzymes.