SUMMARY: The herbicide suphometuron methyl inhibits the utilization of pyruvate and 2-ketobutyrate by the branched-chain amino acid biosynthetic enzyme acetolactate synthase. Eighteen insertions of the transposon Tn into the genome LT2 caused hypersensitivity to this herbicide. Five of these insertions conferred a partial auxotrophic requirement. Concurrent herbicide sensitivity and heat-labile pantothenate auxotrophy was due to ::Tn mutations, while coincident sulphometuron methyl sensitivity and thiamin auxotrophy was attributable to ::Tn mutations. The phenotypes of these mutations suggested that coenzyme A and thiamin pyrophosphate availability modulated the cells’ response to sulphometuron methyl. A model suggesting a key role for 2-ketobutyrate accumulation in herbicide action is supported by the function of thiamin pyrophosphate in 2-ketoacid metabolism and the known role of a 2-ketoacid in coenzyme A synthesis.


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