Plant Physiology 76:71-74 (1984)
© 1984 American Society of Plant Biologists
Articles
Inactivation of Glutamine Synthetase by Tabtoxinine- -lactam 1
Effects of Substrates and pH
Pat L. Langston-Unkefer2,
Patrick A. Macy and
Richard D. Durbin
United States Department of Agriculture, Agricultural Research Service, University of Wisconsin, Madison, Wisconsin 53706,
Department of Agronomy, University of Wisconsin, Madison, Wisconsin 53706,
Department of Plant Pathology, University of Wisconsin, Madison, Wisconsin 53706
The inactivation of glutamine synthetase by tabtoxinine- -lactam, a phytotoxin produced by Pseudomonas syringae pv. tabaci, was shown to be irreversible. The chloroplast and cytosolic forms of the enzyme from pea leaves (Pisum sativum L.) were separated, purified, and found to be kinetically similar with Km values for glutamate of 6.7 and 4.3 millimolar and for ATP of 2.0 and 1.3 millimolar, respectively. Both forms were irreversibly inactivated by the toxin at equal rates. Using the chloroplast form, it was found that inactivation by tabtoxinine--lactam required ATP. Glutamate and low levels of ammonia (<2 millimolar) slowed the rate of inactivation, whereas high levels of ammonia (5, 20, and 50 millimolar) 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.
2 Present address: Los Alamos National Laboratory, INC-4 MS C345, Los Alamos, NM 87545.
1 Supported by United States Department of Agriculture Competitive Research Grant Program, Grant No. 83-CRCR-1-1201; the United States Department of Agriculture, Agricultural Research Service; the University of Wisconsin General National Institute of Health Biomedical Sciences Support Grant; and the Wisconsin Agricultural Experiment Station.
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