Fig 4.

The level of hcp2 expression shows variation depending on the culture conditions and growth phase. (A) After 24 h, luciferase expression under the control of the hcp2 promoter was significantly higher in the minimal medium (MM) than in KB (P < 0.0001, Student's t test). After 48 h, luminescence counts per unit of OD₆₀₀ were significantly higher in MM plus fructose than in the two other media (P < 0.0001 in comparison with KB, P < 0.01 in comparison with MM plus mannose), and after 72 h, both growth and luminescence were declining in the minimal medium while they were still maintained in KB. Three parallel cultures were started in each medium, KB and MM with 10 mM fructose or 10 mM mannitol at an OD₆₀₀ of 0.3, and incubated with shaking for 3 days at 22°C. Error bars indicate SDs, and statistically significant differences are indicated by different letters. (B) The activity of the hcp2 promoter was higher on solid medium than in liquid medium. The P. syringae pv. tomato/phcp2-LUC strain and the promoterless control P. syringae pv. tomato/pPP strain were precultured on KB agar and inoculated at an OD₆₀₀ of 0.1 into fresh KB liquid medium or spread as a lawn on KB agar. The relative luminescence of the control strain stayed at a constant low level below 100. In agar cultures, luciferase activity could not be reliably determined before 48 h because the cell numbers were too low. Two separate cultures of each kind were sampled, and error bars indicate SDs. Results that are different at a confidence level of P < 0.05 (Student's t test) are marked with different letters. (C) The activity of the hcp2 promoter changed with the growth phase. Five parallel liquid cultures of the P. syringae pv. tomato/phcp2-LUC strain were started at an OD₆₀₀ of 0.2 in KB and grown overnight at 22°C. n = 5; error bars indicate SDs. The arrow points to the peak of hcp2 promoter activity detected in the mid-logarithmic growth phase.