Unconventional origin and hybrid system for construction of pyrrolopyrrole moiety in kosinostatin biosynthesis.

Kosinostatin (KST), an antitumor antibiotic, features a pyrrolopyrrole moiety spirally jointed to a five-membered ring of an anthraquinone framework glycosylated with a γ-branched octose. By a combination of in silico analysis, genetic characterization, biochemical assay, and precursor feeding experiments, a biosynthetic pathway for KST was proposed, which revealed (1) the pyrrolopyrrole moiety originates from nicotinic acid and ribose, (2) the bicyclic amidine is constructed by a process similar to the tryptophan biosynthetic pathway, and (3) a discrete adenylation enzyme and a peptidyl carrier protein (PCP) are responsible for producing a PCP-tethered building block parallel to type II polyketide synthase (PKS) rather than for the PKS priming step by providing the starter unit. These findings provide an opportunity to further explore the inexplicable enzymatic logic that governs the formation of pyrrolopyrrole moiety and the spirocyclic skeleton.