SUMMARY: Isoleucine hydroxamate-resistant mutants of have been isolated. Biochemical examination of the isoleucine-valine biosynthetic enzymes allowed the mutants to be grouped into three classes: I, mutants with increased enzyme levels; II, mutants with feedback inhibition-insensitive L-threonine dehydratase; III, mutants without any alteration in either feedback inhibition or enzyme levels. Of these mutants, class II accumulated isoleucine outside the cells in medium containing L-threonine. Strain IHr313, whose L-threonine dehydratase was about 500-fold less sensitive to feedback inhibition when compared with wild-type enzyme, accumulated up to 7 mg. of isoleucine per ml. of medium.


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