Inactivation of Glutamine Synthetase by Tabtoxinine-β-lactam

Abstract

The inactivation of glutamine synthetase by tabtoxinine-.beta.-lactam, a phytotoxin produced by Pseudomonas syringae pathovar tabaci, was irreversible. The chloroplast and cytosolic forms of the enzyme from pea leaves (Pisum sativum L.) were separated, purified and found kinetically similar with Km values for glutamate of 6.7 and 4.3 mM and for ATP of 2.0 and 1.3 mM, respectively. Both forms were irreversibly inactivated by the toxin at equal rates. Using the chloroplast form, inactivation by tabtoxinine-.beta.-lactam required ATP. Glutamate and low levels of ammonia (< 2 mM) slowed the rate of inactivation, whereas high levels of ammonia (5, 20 and 50 mM) accelerated it. The inactivation proceeded at a faster rate as the pH was increased from pH 6.5 to 7.5. The role which cellular compartmentalization could play in the inactivation is discussed.