Mechanism of Action of Pseudomonas syringae Phytotoxin, Syringomycin

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Abstract
Syringomycin, a peptide toxin produced by the phytopathogen Pseudomonas syringae pv syringae preferentially stimulated (2-fold) the vanadate-sensitive ATPase activity associated with the plasma membrane of red beet storage tissue. The toxin had a very slight effect on the tonoplast ATPase and had no detectable effect on the mitochondrial ATPase. Optimal stimulation was achieved with 10 to 50 micrograms of syringomycin per 25 micrograms of membrane protein. Treatment of membranes with 0.1% (weight/volume) deoxycholate eliminated the activation effect, and enzyme solubilized with Zwittergent 3-14 was not affected by syringomycin. ATPase activity was activated to the same extent at KCl concentrations ranging from 0 to 50 millimolar. Valinomycin, nigericin, carbonylcyanide p-trifluoromethoxyphenylhydrazone, and gramicidin did not increase the plasma membrane ATPase activity. However, these ionophores did not hinder the ability of syringomycin to stimulate the activity. We suggest that syringomycin does not increase ATPase activity by altering membrane ion gradients nor directly interacting with the enzyme, but possibly through regulatory effectors or covalent modification of the enzyme.