Microbiological Identification and Characterization of an Amino Acid Biosynthetic Enzyme as the Site of Sulfonylurea Herbicide Action

Abstract

The inability to synthesize many amino acids distinguishes animal cells from those of plants, fungi and bacteria. Potential herbicidal targets include the plant enzymes responsible for synthesizing these amino acids since their selective inhibition might result in control of weed growth and low animal toxicity. Microbiological studies have recently indicated that several herbicides interfere with amino acid biosynthesis. Elucidation of the enzymic target of a sulfonylurea herbicide, sulfometuron methyl (SM), illustrates the utility of a microbial approach to studies of herbicide action. Specific reversal of SM-mediated inhibition of bacterial growth by isoleucine, methionine and pantothenate first suggested that the site of action was acetolactate synthase (ALS), a branched chain amino acid biosynthetic enzyme. Results confirming this predicition include the potent in vitro inhibition of microbial and plant ALS activity by SM, the mapping of both bacterial and yeast SM-resistant mutations to genes encoding ALS and the altered, SM-resistant forms of ALS found in microbial and plant mutants. Comparison of the DNA sequence of wild type and mutant structural genes encoding ALS from both bacteria and yeast has established the molecular bases for the herbicide-resistant phenotypes. The detailed understanding of branched chain amino acid biosynthesis in microbes has thus provided both tools and insights into the mode of SM action.