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Plant Physiology 96:255-261 (1991)
© 1991 American Society of Plant Biologists

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Metabolism and Enzymology

Herbicide Resistance in Datura innoxia 1

Kinetic Characterization of Acetolactate Synthase from Wild-Type and Sulfonylurea-Resistant Cell Variants

Bala Rathinasabapathi2 and John King

Department of Biology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 0W0, Canada

Acetolactate synthase (ALS, EC 4. 1.3. 18), the first enzyme in the biosynthesis of branched-chain amino acids, was isolated from wild-type and sulfonylurea-resistant Datura innoxia cell variants and characterized. Apparent Km values of the ALS for pyruvate from three sulfonylurea-resistant variants (CSR2, CSR6, and CSR10) were manyfold greater than that of the wild type. The inhibition of wild-type and herbicide-resistant ALS activity by chlorsulfuron (CS), a sulfonylurea herbicide, and L-leucine (L-Leu), one of the feedback inhibitors of the enzyme, was examined. ALS from two CS-resistant variants exhibited severalfold greater resistance to CS than did the wild-type enzyme. Inhibition of ALS by L-Leu fitted a partially competitive pattern most closely. It is proposed that the herbicide resistance mutation accentuated the partial inhibition characteristics of ALS by L-Leu. ALS from one of the two CS-resistant variants (CSR6) had a Ki for L-Leu an order of magnitude greater than that of the wild-type enzyme. The alterations in kinetic properties observed in the ALS from sulfonylurea-resistant variants are discussed in relation to the possible evolutionary significance of the herbicide binding site of this enzyme, the physiological effects of such biochemical alterations, and their practical utility in genetic studies.

2 Present address: Michigan State University—Department of Energy Plant Research Laboratory, Michigan State University, East Lansing, MI 48824-1312.

1 Supported by an operating grant from the Natural Sciences and Engineering Research Council of Canada to J. K. and a graduate scholarship from the University of Saskatchewan to B. R.






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Copyright © 1991 by the American Society of Plant Biologists