Fig. 2. Proposed biosynthetic pathway for zeamine (2) and zeamine I (3). The Zmn16–18 hybrid NRPS/PKS catalyzes the sequential incorporation of γ-Asp, His, Asn, Asn, Thr and Val, followed by one or two elongations of the resulting pentapeptide with a malonyl extender unit. The stand-alone condensation domain Zmn19 condenses the ACP-bound thioesters (a) and (b) to the primary amino group of 1 via chain-terminating amide bond formation, giving rise to prezeamine I (5) and prezeamine (4), respectively. Zmn22 then cleaves off the N-terminal pentapeptide in a post-assembly processing step to form 3 and 2. The precise timing of the cyclization reaction between D-Asn and Thr remains unclear. The configurations of the stereocenters depicted here were bioinformatically predicted based on the presence of an epimerization domain in Zmn16 and sequence analysis of the KR domain in Zmn18. Domains are abbreviated as follows: C = condensation, A = adenylation, T = thiolation, KS = ketosynthase, AT = acyltransferase, KR = ketoreductase, ACP = acyl carrier protein.