Abstract
The assemblyβline architecture of polyketide synthases (PKSs) provides an opportunity to rationally reprogram polyketide biosynthetic pathways to produce novel antibiotics. A fundamental challenge toward this goal is to identify the factors that control the unidirectional channeling of reactive biosynthetic intermediates through these enzymatic assembly lines. Within the catalytic cycle of every PKS module, the acyl carrier protein (ACP) first collaborates with the ketosynthase (KS) domain of the paired subunit in its own homodimeric module so as to elongate the growing polyketide chain and then with the KS domain of the next module to translocate the newly elongated polyketide chain. Using NMR spectroscopy, we investigated the features of a structurally characterized ACP domain of the 6βdeoxyerythronolide B synthase that contribute to its association with its KS translocation partner. Not only were we able to visualize selective proteinβprotein interactions between the two partners, but also we detected a significant influence of the acyl chain substrate on this interaction. A novel reagent, CF~3~βSβACP, was developed as a ^19^F NMR spectroscopic probe of proteinβprotein interactions. The implications of our findings for understanding intermodular chain translocation are discussed.