Acetohydroxyacid synthase (EC 4.1.3.18, also known as acetolactate synthase) isoenzyme II from Escherichia coli is inhibited by sulphonylurea and imidazolinone herbicides, although it is much less sensitive than the plant enzyme. This isoenzyme is also unusual in that it is not inhibited by valine. Mutating S100 (Ser100 in one-letter amino acid notation) of the catalytic subunit to proline increases its sensitivity to sulphonylureas, but not to imidazolinones. Mutating P536 to serine, as found in the plant enzyme, had little effect on the properties of the enzyme. Mutating E14 of the regulatory subunit to glycine, either alone or in combination with the H29N (His29 → Asn) change, did not affect valine-sensitivity.

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