Occurrence of thermoregulation of genes involved in coronatine biosynthesis among various Pseudomonas syringae strains.

Several pathovars of Pseudomonas syringae produce the polyketide phytotoxin coronatine (COR). In the bacterial blight pathogen of soybean, P. syringae pv. glycinea PG4180, COR is produced at high levels at 18 degrees C whereas no toxin is synthesized at 28 degrees C. Previously, activation of three promoters inside the COR biosynthetic gene cluster by a modified two-component regulatory system was shown to influence thermoregulation of COR biosynthesis. Using phenotypic determination of COR synthesis, a transcriptional reporter gene fusion, and Western blot analysis, we screened a representative number of natural isolates of P. syringae for effects of temperature on expression of cmaA, cmaB, and cmaT, which encode enzymes involved in the biosynthesis of COR. Thermoregulation of cmaABT expression was frequent among the tested strains. However, intensities of the temperature effects varied widely. Coronatine synsthesis was found to differ at up to six-fold among COR producing strains. There was no strain which synthesized COR at 28 degrees C although some of them showed increased basal cmaABT promoter activities at this temperature. Transcriptional fusions between the cmaABT promoter and a promoterless reporter gene were found to be down regulated at 28 degrees C only in COR producing strains but not in the non-producing strains tested. The geographic origin of the bacterial strains did not influence the occurrence of temperature-dependent gene expression.