The biosynthesis of the coronamic acid fragment of the pseudomonal phytotoxin coronatine involves construction of the cyclopropane ring from a gamma-chloro-L-allo-Ile intermediate while covalently tethered as a phosphopantetheinyl thioester to the carrier protein CmaD. The cyclopropane-forming catalyst is CmaC, catalyzing an intramolecular displacement of the gamma-Cl group by the alpha carbon. CmaC can be isolated as a Zn2+ protein with about 10-fold higher activity over the apo form. CmaC will not cyclize free gamma-chloro amino acids or their S-N-acetylcysteamine (NAC) thioester derivatives but will recognize some other carrier protein scaffolds. Turnover numbers of 5 min-1 are observed for Zn-CmaC, acting on gamma-chloro-L-aminobutyryl-S-CmaD, generating 1-aminocyclopropane-1-carbonyl (ACC)-S-CmaD. Products were detected either while still tethered to the phosphopantetheinyl prosthetic arm by mass spectrometry or after thioesterase-mediated release and derivatization of the free amino acid. In D2O, CmaC catalyzed exchange of one deuterium into the aminobutyryl moiety of the gamma-Cl-aminoacyl-S-CmaD, whereas the product ACC-S-CmaD lacked the deuterium, consistent with a competition for a gamma-Cl-aminobutyryl alpha-carbanion between reprotonation and cyclization. CmaC-mediated cyclization yielded solely ACC, resulting from C-C bond formation and no azetidine carboxylate from an alternate N-C cyclization. CmaC could cyclize gamma,gamma-dichloroaminobutyryl to the Cl-ACC product but did not cyclize delta- or epsilon-chloroaminoacyl-S-CmaD substrates.