Acetohydroxyacid synthase (EC 4.1.3.18) has been extracted from leaves of three valine-resistant (Valr) tobacco (Nicotiana tabacum) mutants, and compared with the enzyme from the wild-type. The enzyme from all three mutants is appreciably less sensitive to inhibition by leucine and valine than the wild-type. Two of the mutants, Valr-1 and Valr-6, have very similar enzymes, which under all conditions are inhibited by less than half that found for the wild-type. The other mutant, Valr-7, has an enzyme that only displays appreciably different characteristics from the wild-type at high pyruvate or inhibitor concentrations. Enzyme from Valr-7 also has a higher apparent Kmfor pyruvate, threefold greater than the value determined for the wild-type and the other mutants. The sulphonylurea herbicides strongly inhibit the enzyme from all the lines, though the concentrations required for half-maximal inhibition of enzyme from Valr-1 and Valr-6 are higher than for Valr-7 or the wildtype. No evidence has been found for multiple isoforms of acetohydroxyacid synthase, and it is suggested that the valine-resistance of these mutant lines is the result of two different mutations affecting a single enzyme, possibly involving different subunits.
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