I BUDDE¹, B ROHDE¹, C BENDER² and M ULLRICH¹

¹Max-Planck-Institut für terrestrische Mikrobiologie, Marburg, Germany; ²Oklahoma State University, Stillwater, OK, USA

Background and objectives
The plant pathogenic bacterium Pseudomonas syringae pv. glycinea PG4180.N9 synthesizes high levels of the polyketide phytotoxin coronatine (COR) at 18°C, whereas no detectable toxin is produced at 28°C. Previously, we reported that the temperature-sensitive activation of three promoters inside the COR biosynthetic gene cluster might explain thermoregulation of COR biosynthesis [1]. The present study was aimed at furthering our understanding of the transcriptional as well as the post-translational effects of temperature on expression of cmaB, which encodes an enzyme involved in COR biosynthesis.

Results and conclusions
Transcriptional fusions using a promoterless glucuronidase gene and Northern blot analyses were used to monitor promoter activities and transcript abundance for the cmaABT operon during bacterial growth at 18 and 28°C. Promoter activity and transcription rates were maximal when cells were incubated at 18°C and sampled at mid-logarithmic phase. Transcription declined moderately during the transition to stationary phase but remained high at 18°C as compared to 28°C. Western blot analysis indicated that CmaB accumulated in the late stationary phase of P. syringae cultures grown at 18°C but not in cultures incubated at 28°C. Temperature-shift experiments indicated that CmaB stability was more pronounced at 18 than at 28°C. Although temperature has a strong impact on transcription of COR biosynthetic genes, we propose that thermoregulation of protein stability might also control COR synthesis.