FIG. 6.

Induction of argK expression by carbamoylphosphate under nonpermissive conditions for the synthesis of phaseolotoxin. (A) Northern blot of total RNA from P. syringae pv. phaseolicola (Psph) grown at 28°C in M9 medium supplemented with different concentrations of carbamoylphosphate (CP) (none, 1 mM, 1 μM, and 10 nM) and induced for 30 min. P. syringae pv. phaseolicola grown in rich medium (KB) at 18°C was used as a positive control. The blot was hybridized with a 1-kbp PCR product of argK labeled with [α-³²P]dCTP. 23S rRNA stained with ethidium bromide is shown as a loading control. The signal that has the size expected for a monocistronic argK message is marked. Asterisk indicates the position of a secondary transcript of ≈2.3 kbp. (B) Phaseolotoxin synthesis in the supernatant was evaluated by using the E. coli growth inhibition assay. +, presence of phaseolotoxin; −, absence of phaseolotoxin. (C) Densitometric analysis of the Northern blot. The relative transcript level is expressed as a percentage of the level of the positive control, which was defined as 100%.