In Pseudomonas syringae pv. phaseolicola the Synthesis of Phaseolotoxin and the Concurrent Expression of the argK Gene Coding for the Phaseolotoxin-Resistant Ornithyl-Carbamoyl Transferase Occur Independent of the Global Arginine Regulator ArgR

Abstract

Pseudomonas syringae pv. phaseolicola, the causal agent of halo blight in beans produces a non-host specific toxin capable of inhibiting the enzyme ornithine carbamoyltransferase (OCTase) of plant, bacterial and mammalian origin. When P. s. pv. phaseolicola grown under conditions leading to the synthesis of phaseolotoxin, synthesises a phaseolotoxin-resistant OCTase, the product of the argK gene. Under any other growth conditions, the synthesis of arginine is mediated by a phaseolotoxin-sensitive OCTase encoded by the argF gene. In P. aeruginosa genes involved in the catabolism of arginine, the synthesis of carbamoylphosphate and the expression of argF are regulated by arginine and the ArgR protein. We isolated and characterised the argR gene of P. s. pv phaseolicola, and constructed a null argR mutant. Northern blot analysis using this mutant showed that in this bacterium regulated expression of argF is dependent on the ArgR product, but argK expression and phaseolotoxin synthesis occur independent of this regulatory protein. Also, we have shown that exogenous arginine has an effect on the expression of argF but not on argK. These results strongly indicate a lack of metabolic coordination between the synthesis of phaseolotoxin and the system regulating arginine metabolism mediated by ArgR