CHARACTERIZATION OF CORR, A TRANSCRIPTIONAL ACTIVATOR WHICH IS REQUIRED FOR BIOSYNTHESIS OF THE PHYTOTOXIN CORONATINE

Coronatine (COR) is a plasmid-encoded phytotoxin synthesized by severa l pathovars of phytopathogenic Pseudomonas syringae. The COR biosynthe tic gene cluster in P. syringae pv. glycinea PG4180 is encoded by a 32 -kb region which contains both the structural and regulatory genes nee ded for COR synthesis, The regulatory region contains three genes: cor P, corS, and corR. corS is thought to function as a histidine protein kinase, whereas corP and corR show relatedness to response regulators of the two-component regulatory paradigm, In the present study, we inv estigated whether CorR is a positive activator of COR gene expression. We also studied whether CorR specifically binds the DNA region locate d upstream of cfl, a gene located at the 5' end of the gene cluster en coding coronafacic acid, the polyketide portion of COR, Complementatio n analysis with a corR mutant, FG4180.P2, and transcriptional fusions to a promoterless glucuronidase gene (uidA) indicated that Corp functi ons as a positive regulator of COR gene expression. Deletion analysis of the 5' end of the cfl upstream region was used to define the minima l region required for COR gene expression. A 360-bp DNA fragment locat ed over 500 bp upstream from the cfl transcriptional start site was us ed in DNase I protection assays to define the specific bases bound by CorR. An area extending from -704 to -650 with respect to the cfl tran scriptional start site was protected by DNase I footprinting, indicati ng a rather large area of protection. This area was also conserved in the promoter region for cmaA, which encodes a transcript containing ge nes for coronamic acid synthesis, another intermediate in the COR bios ynthetic pathway. The results obtained in the current study suggest th at both the coronafacic acid and the coronamic acid structural genes a re controlled by CorR, a positive activator of COR gene expression.