AN ACETOHYDROXY ACID SYNTHASE MUTANT REVEALS A SINGLE-SITE INVOLVED IN MULTIPLE HERBICIDE RESISTANCE

Acetohydroxy acid synthase (AHAS) is an essential enzyme for many orga nisms as it catalyzes the first step in the biosynthesis of the branch ed-chain amino acids valine, isoleucine, and leucine. The enzyme is un der allosteric control by these amino acids. It is also inhibited by s everal classes of herbicides, such as the sulfonylureas, imidazolinone s and triazolopyrimidines, that are believed to bind to a relic quinon e-binding site. In this study, a mutant allele of AHAS3 responsible fo r sulfonylurea resistance in a Brassica napus cell line was isolated. Sequence analyses predicted a single amino acid change (557 Trp-->Leu) within a conserved region of AHAS. Expression in transgenic plants co nferred strong resistance to the three classes of herbicides, revealin g a single site essential for the binding of all the herbicide classes . The mutation did not appear to affect feedback inhibition by the bra nched-chain amino acids in plants.