INVESTIGATIONS OF THE BIOSYNTHESIS OF THE PHYTOTOXIN CORONATINE

The biosynthesis of the phytotoxin coronatine has been investigated by administration of isotopically labeled precursors to Pseudomonas syri ngae pv. glycinea. The structure of coronatine contains two moieties o f distinct biosynthetic origin, a bicyclic, hydrindanone carboxylic ac id (coronafacic acid) and a cyclopropyl alpha-amino acid (coronamic ac id). Investigations of coronafacic acid biosynthesis have shown that t his compound is a polyketide derived from three acetate units, one but yrate unit, and one pyruvate unit. The two carbonyl oxygen atoms of co ronafacic acid were found to be derived from the oxygen atoms of aceta te. Additional experiments are described that rule out some possible m odes for assembly of the polyketide chain. Coronamic acid is shown to be derived from L-isoleucine via the intermediacy of L-alloisoleucine. Examination of the mechanism of the cyclization of L-alloisoleucine t o coronamic acid revealed that the formation of the cyclopropane ring takes place with the removal of only two hydrogen atoms from the amino acid, one at C-2 and the other at C-6. The nitrogen atom at C-2 of L- alloisoleucine is shown to be retained. On the basis of these observat ions, a mechanism is postulated for the cyclization reaction that invo lves the diversion of an enzymatic hydroxylation reaction into an oxid ative cyclization. Finally, a precursor incorporation experiment with deuterium-labeled coronamic acid demonstrated that free coronamic acid can be efficiently incorporated into coronatine. This observation ind icates that the cyclization of L-alloisoleucine to coronamic acid can occur before formation of the amide bond between coronafacic acid and coronamic acid.