Imidazolinone herbicides are active against the enzyme acetohydroxyacid synthase (AHAS), also known as acetolactate synthase (ALS).<br>AHAS is an enzyme found in bacteria, certain other micro-organisms and plants. This enzyme catalyzes the first step in the biosynthesis of the essential branched chain amino acids isoleucine leucine and valine. Herbicide induced AHAS inhibition results in a lethal decrease in protein synthesis. Unmodified rice is not tolerant to imidazolinone herbicides.<br>A single amino acid substitution in the AHAS gene, sufficient to alter the binding site for imdazolinone herbicides such that Imazethapyr no longer inhibits the AHAS enzyme, resulted in a herbicide tolerant phenotype.<br>The novel Imazethapyr tolerance is under the control of the native AHAS promoter and is believed to be constitutively expressed. Sequence information from the modified AHAS gene was submitted for PWC16.<br>The tolerance to Imazethapyr was demonstrated by comparison of the activity of the AHAS enzyme extracted from PWC16 rice plants to that of the unmodified parental rice plants. The levels of valine, leucine and isoleucine produced in rice are regulated by feedback inhibition. BASF provided data to support the claim that the modified AHAS does not affect feedback inhibition and hence, the regulation and levels of these amino acids.