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 cy clized derivative of isoleucine. Both CFA and CMA function as intermed iates in the pathway to COR and must be joined together by an amide bo nd to form the phytotoxin. Although the mode of action for COR remains obscure, the CFA moiety is a structural and functional analogue of ja smonic acid, a compound that is produced in a variety of plants in res ponse to stress. The COR biosynthetic gene cluster generally occurs on large plasmids in P. syringae, an observation that helps to explain t he production of COR by multiple pathovars. Mutagenesis, feeding studi es, and complementation analyses have been used to divide the COR bios ynthetic gene cluster into functional regions. Nucleotide sequencing o f the regions involved in CFA and CMA biosynthesis has revealed relate dness to genes encoding polyketide and peptide synthetases, respective ly. The deduced amino acid sequence of the gene responsible for cataly zing amide bond formation between CMA and CFA shows relatedness to enz ymes 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.