title: Self-protection of Pseudomonas syringae pv. "tabaci" from its toxin, tabtoxinine-beta-lactam.
abstract: An extracellular toxin, tabtoxinine-beta-lactam (T beta L), is produced by Pseudomonas syringae pv. "tabaci." This toxin irreversibly inhibits its target, glutamine synthetase; yet P. syringae pv. "tabaci" retains significant amounts of glutamine synthetase activity during toxin production in culture. As part of our investigation of the self-protection of P. syringae pv. "tabaci," we compared the effects of T beta L on Tox+ (T beta L-producing, insensitive to T beta L) and Tox- (T beta L nonproducing, sensitive to T beta L) strains. The extent of protection afforded to the Tox- strain when induced to adenylylate glutamine synthetase was tested. We concluded that an additional protection mechanism was required. A detoxification activity was found in the Tox+ strain which opens the beta-lactam ring of T beta L to produce the inactive, open-chain form, tabtoxinine. Whole cells of the Tox+ strain incubated for 24 h with [14C]T beta L (0.276 mumol/3 X 10(10) cells) contained [14C]tabtoxinine (0.056 mumol), and the medium contained T beta L (0.226 mumol). Extracts of spheroplasts of the Tox+ stain also converted T beta L to tabtoxinine, whereas extracts of the Tox- strain did not alter T beta L. The conversion was time dependent and stoichiometric and was destroyed by boiling for 30 min or by the addition of 5 mM EDTA. Penicillin, a possible substrate and competitive inhibitor of this lactamase activity, inhibited the conversion of T beta L to tabtoxinine. Periplasmic fluid did not catalyze the conversion of T beta L.
mesh_terms: D000249:Adenosine Monophosphate / Q000378:metabolism; D001384:Azetidines / Q000633:toxicity*; D001385:Azetines / Q000633:toxicity*; D001692:Biological Transport; D004352:Drug Resistance, Microbial; D005981:Glutathione Synthase / Q000378:metabolism; D008658:Inactivation, Metabolic; D008717:Methionine Sulfoximine / Q000494:pharmacology; D011549:Pseudomonas / Q000187:drug effects*
chemical_list: D001384:Azetidines; D001385:Azetines; D008717:Methionine Sulfoximine; D000249:Adenosine Monophosphate; C051368:tabtoxinine beta-lactam; D005981:Glutathione Synthase