Condensation-Incompetent Ketosynthase Inhibits trans-Acyltransferase Activity

Nonelongating modules with condensation-incompetent ketosynthase (KS 0 ) are frequently found in many trans -acyltransferase polyketide synthases ( trans -AT PKS). KS 0 catalyzes translocation of carbon chain without decarboxylative condensation. Unlike typical elongating modules where malonylation of acyl carrier protein (ACP) precedes elongation, the malonylation of ACP downstream of KS 0 is assumed to be prevented. In this study, the regulation mechanism(s) of ACP malonylation in a non-elongating module of difficidin biosynthase was investigated. In vitro reconstitution, protein mass spectrometry, and enzyme kinetics demonstrated that KS 0 controls the pathway by inhibiting the trans -AT activity. Protein–protein interactions of the surrounding domains also contribute to the regulation. Enzyme kinetics further identified the DfnKS 0 5 as an allosteric inhibitor of trans -AT. The principle and knowledge discovered from this study will enhance the understanding of this unusual PKS system.