GROWTH-PHASE AND TEMPERATURE INFLUENCE PROMOTER ACTIVITY, TRANSCRIPT ABUNDANCE, AND PROTEIN STABILITY DURING BIOSYNTHESIS OF THE PSEUDOMONAS-SYRINGAE PHYTOTOXIN CORONATINE

The plant-pathogenic bacterium Pseudomonas syringae pv. glycinea PC418 0.N9 synthesizes high le, cls of the polyketide phytotoxin coronatine (COR) at 18 degrees C, whereas no detectable toxin is produced at 28 d egrees C. Previously, we reported that the temperature sensitive activ ation of three promoters within the COR biosynthetic gene cluster migh t explain thermoregulation of COR biosynthesis. The present study was aimed at furthering our understanding of the transcriptional as web as the posttranslational effects of temperature on expression of cmaB, w hich encodes an enzyme involved in COR biosynthesis. Transcriptional f usions using a promoterless glucuronidase gene and Northern blot analy ses were used to monitor promoter activities and transcript abundance for the cmaABT operon during bacterial growth at 18 and 28 degrees C. Promoter activity and transcription rates were maximal when cells mere incubated at 18 degrees C and sampled at mid-logarithmic phase. Trans cription declined moderately during the transition to stationary phase but remained higher at 18 degrees C than at 28 degrees C. Western blo t analysis indicated that CmaB accumulated in the late stationary phas e of P. syringae cultures grown at 18 degrees C but not in cultures in cubated at 28 degrees C. Temperature shift experiments indicated that CmaB stability was more pronounced at 18 degrees C than at 28 degrees C. Although temperature has a strong impact on transcription of COR bi osynthetic genes, we propose that thermoregulation of protein stabilit y might also control COR synthesis.