This study describes the functional identification of a pair of mechanistically diverse enzymes that catalyze the successive dehydration (CurE ECH1) and decarboxylation (CurF ECH2) of (S)-HMG-ACP to generate a 3-methylcrotonyl-ACP intermediate, the presumed precursor of the cyclopropyl ring in curacin A. The reactions catalyzed by ECH1 and ECH2 are found in a broad cross-section of microbial natural product gene clusters and participate in the introduction of carbon chain branch points and functional group diversity as key steps in the HMG-CoA synthase mediated addition of C-2 from acetate to the beta-carbonyl group of polyketide chains.