Figure 1. The docking of ketosynthase (KS) and upstream acyl carrier protein (ACP) domains within a modular polyketide synthase (PKS). The Pikromycin precursor, Narbonolide, is biosynthesized through the actions of 23 enzymatic domains and 7 ACP domains. As each ACP carries out its reaction cycle, it docks and undocks each of its cognate enzymes. The final reaction in the reaction cycle of the first 6 modules in the Pikromycin PKS is transacylation, where an upstream ACP presents intermediates to KS and only the fully processed species is permitted onto its reactive cysteine. These checkpoints are exemplified by PikMod6, in which PikACP6 and PikKS6 associate to enable the transacylation of the hexaketide intermediate reduced by PikKR6. KSQ, generates propionyl priming units; AT, acyltransferase; KR, ketoreductase; KR0, epimerase; DH, dehydratase; ER, enoylreductase; TE, thioesterase; unlabeled circles, ACPs; unlabeled half-circles, docking domains.