Biosynthesis of coronatine, a thermoregulated phytotoxin produced by the phytopathogen Pseudomonas syringae

Coronatine (COR) is a non-host-specific phytotoxin that is produced by several different pathovars in the species Pseudomonas syringae. COR consists of two distinct components: coronafacic acid (CFA), which is synthesized via the polyketide pathway, and coronamic acid (CMA), a cyclized derivative of isoleucine. Both CFA and CMA function as intermediates in the pathway to COR and must be joined together by an amide bond to form the phytotoxin. Although the mode of action for COR remains obscure, the CFA moiety is a structural and functional analogue of jasmonic acid, a compound that is produced in a variety of plants in response to stress. The COR biosynthetic gene cluster generally occurs on large plasmids in P. syringae, an observation that helps to explain the production of COR by multiple pathovars. Mutagenesis, feeding studies, and complementation analyses have been used to divide the COR biosynthetic gene cluster into functional regions. Nucleotide sequencing of the regions involved in CFA and CMA biosynthesis has revealed relatedness to genes encoding polyketide and peptide synthetases, respectively. The deduced amino acid sequence of the gene responsible for catalyzing amide bond formation between CMA and CFA shows relatedness to enzymes that activate cyclic carboxylic acids by adenylation. Coronatine biosynthesis has been shown to be temperature-sensitive and regulated by a modified two-component regulatory system.