Practice question

Bialaphos tripeptide herbicide consists of phosphinothricin plus two alanine residues. In plant tissue peptidases release active phosphinothricin, structural analog of glutamate that inhibits glutamine synthetase essential for assimilation of ammonia into glutamine. Inhibition causes ammonia accumulation, photosynthesis disruption, and rapid cell death. Bacterium Streptomyces hygroscopicus produces bialaphos and protects itself via bar gene encoding phosphinothricin acetyltransferase. Enzyme transfers acetyl group from acetyl-CoA to free amino group of phosphinothricin, producing N-acetyl phosphinothricin unable to bind glutamine synthetase active site. Expression of bar under control of CaMV 35S promoter in transformed crops confers detoxification mediated resistance to glufosinate, ammonium salt of phosphinothricin used as Liberty herbicide. Method offers alternative weed management option, widely used in transgenic canola, corn, cotton, and as selectable marker in transformation laboratories thanks to efficient selection of resistant callus on phosphinothricin containing medium. Formulation of glufosinate herbicide marketed as Liberty 200 g per liter glufosinate ammonium provides broadleaf weed control in tolerant crops. Resistance mechanism enzymatic acetylation prevents binding to glutamine synthetase active site, maintaining ammonia assimilation via GS/GOGAT cycle. Transgenic plants accumulate N-acetyl glufosinate sequestered in vacuole without toxic effects. Gene bar widely used as selectable marker in transformation due to low escape rate and