Herbicide resistance can be genetically engineered by several principles: (1) an endogenous plant gene encoding the target enzyme can be modified (mutated) causing amino acid exchange(s) which may be made at the substrate-reaction site or at the regulatory site (in case of an allosteric enzyme). Examples are the D-1 protein of the photosynthetic electron transport or the acetolactate synthase (ALS). All typical photosynthesis inhibitors bind within a 60 amino-acid stretch of the D-1 protein of photosystem II and apparently compete with the redoxactive plastoquinone. Inhibitors of acetolactate synthase bind at or close to its regulatory site.
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