title: Import of a precursor protein into chloroplasts is inhibited by the herbicide glyphosate.
abstract: Import of the precursor to 5-enolpyruvylshikimate-3-phosphate synthase (pEPSPS) into chloroplasts is inhibited by the herbicide glyphosate. Inhibition of import is maximal at glyphosate concentrations of >/=10 mum and occurs only when pEPSPS is present as a ternary complex of enzyme-shikimate-3-phosphate-glyphosate. Glyphosate alone had no effect on the import of pEPSPS since it is not known to interact with the enzyme in the absence of shikimate-3-phosphate. Experiments with wild-type and glyphosate-resistant mutant forms of pEPSPS show that inhibition of import is directly proportional to the binding constants for glyphosate. Inhibition of import is thus a direct consequence of glyphosate binding to the enzyme-shikimate-3-phosphate complex. The potential for non-specific effects of glyphosate on the chloroplast transport mechanism has been discounted by showing that import of another chloroplast-designated protein was unaffected by high concentrations of glyphosate and shikimate-3-phosphate. The mechanism of import inhibition by glyphosate is consistent with a precursor unfolding/refolding model.
authors.affiliation: Monsanto Company, 700 Chesterfield Village Parkway, St Louis, MO 63198, USA.
authors.forename: G
authors.initials: G
authors.lastname: Della-Cioppa
authors.forename: G M
authors.initials: GM
authors.lastname: Kishore