Characterization and transcriptional analysis of the gene cluster for coronafacic acid, the polyketide component of the phytotoxin coronatine

Abstract

Coronafacic acid (CFA), the polyketide component of the phytotoxin coronatine (COR), is activated and coupled to coronamic acid via amide bond formation, a biosynthetic step presumably catalyzed by the CFA ligase (cfl) gene product. The COR biosynthetic gene cluster in Pseudomonas syringae pv. glycinea PG4180 is located within a 32-kb region of a 90-kb plasmid designated p4180A. In the present study, a cloned region of p4180A complemented all CFA2 mutants spanning an 18.8-kb region of the COR biosynthetic cluster. The genetic evidence presented in this study indicates that cfl and the CFA biosynthetic gene cluster are encoded by a single transcript and that transcription of all of the genes in this operon is directed by the cfl promoter. The cfl promoter was localized to a 0.37-kb region upstream of the transcriptional start site by progressive subcloning in pRG960sd, a vector containing a promoterless glucuronidase gene. Transcription of the cfl/CFA operon was temperature sensitive and showed maximal glucuronidase activity at 18&C. Furthermore, transcription of the cfl/CFA operon was dependent on the functional activity of a modified two-component regulatory system located within the COR biosynthetic gene cluster. Thermoregulation of the cfl/CFA operon and the coronamic acid biosynthetic gene cluster via the modified two-component regulatory system is discussed.