Systematic characterization of mutations in yeast acetohydroxyacid synthase

Interpretation of herbicide-resistance data

Authors

  • Ronald G. Duggleby,

    1. Department of Biochemistry and Molecular Biology, The University of Queensland, Brisbane, QLD 4072, Australia
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  • Siew Siew Pang,

    1. Department of Biochemistry and Molecular Biology, The University of Queensland, Brisbane, QLD 4072, Australia
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  • Hongqi Yu,

    1. Department of Biochemistry and Molecular Biology, The University of Queensland, Brisbane, QLD 4072, Australia
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    • Present address: Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA.

  • Luke W. Guddat

    1. Department of Biochemistry and Molecular Biology, The University of Queensland, Brisbane, QLD 4072, Australia
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  • Enzymes: acetohydroxyacid synthase (EC 4.1.3.18); ketol-acid reductoisomerase (EC 1.1.1.86); dihydroxy-acid dehydratase (EC 4.2.1.9); branched-chain amino acid transaminase (EC 2.6.1.42); isopropylmalate synthase (EC 4.1.3.12); isopropylmalate isomerase (EC 4.2.1.33); isopropylmalate dehydrogenase (EC 1.1.1.85).

  • Note: a web site is available at http://smms.uq.edu.au/duggleby

R. G. Duggleby, Department of Biochemistry and Molecular Biology, The University of Queensland, Brisbane, QLD 4072, Australia. Fax: + 617 33654699, Tel.: + 617 33654615, E-mail: Ronald.Duggleby@mailbox.uq.edu.au

Abstract

Acetohydroxyacid synthase (AHAS, EC 4.1.3.18) catalyses the first step in branched-chain amino acid biosynthesis and is the target for sulfonylurea and imidazolinone herbicides, which act as potent and specific inhibitors. Mutants of the enzyme have been identified that are resistant to particular herbicides. However, the selectivity of these mutants towards various sulfonylureas and imidazolinones has not been determined systematically. Now that the structure of the yeast enzyme is known, both in the absence and presence of a bound herbicide, a detailed understanding of the molecular interactions between the enzyme and its inhibitors becomes possible. Here we construct 10 active mutants of yeast AHAS, purify the enzymes and determine their sensitivity to six sulfonylureas and three imidazolinones. An additional three active mutants were constructed with a view to increasing imidazolinone sensitivity. These three variants were purified and tested for their sensitivity to the imidazolinones only. Substantial differences are observed in the sensitivity of the 13 mutants to the various inhibitors and these differences are interpreted in terms of the structure of the herbicide-binding site on the enzyme.

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