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The target site of glyphosate [N-(phosphonomethyl)glycine] inhibition in plants and bacteria is 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase. Our strategy for developing glyphosate-resistant crops has been to genetically engineer plants with a gene that codes for EPSP synthase with low sensitivity in glyphosate. We cloned such a gene from thearoAlocus of a glyphosate-resistant mutagenized strain ofSalmonella typhimurium.The enzyme encoded by this gene has a single amino acid change resulting in lower affinity for glyphosate and higher affinity for substrates than either plant or wild-type bacterial counterpart. A chimaeric gene containing the mutantaroAgene behind the octopine synthase promoter was constructed and integrated intoAgrobacteriumT-DNA vectors. Analysis of gall tissue fromBrassica campestrisL. (turnip rape) infected withA. tumefaciensK12 containing this chimaera showed mRNA and protein expressed from the bacterial gene; 50% of the total EPSP synthase activity present had kinetic properties of the mutant bacterial enzyme. Tobacco (Nicotiana tabacumL. ‘Xanthi′) plants have been regenerated from cocultivation withA. rhizogenescontaining the same construct; analysis indicates expression of the gene and enhanced tolerance to glyphosate.
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Thompson et al. (1987) studied this question.
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