THE RAPIDLY METABOLIZED 32,000-DALTON POLYPEPTIDE OF THE CHLOROPLAST IS THE PROTEINACEOUS SHIELD REGULATING PHOTOSYSTEM-II ELECTRON-TRANSPORT AND MEDIATING DIURON HERBICIDE SENSITIVITY

Mild trypsin treatment of Spirodela oligorrhiza thylakoid membranes leads to partial digestion of the rapidly metabolized, surface-exposed, 32,000 dalton protein. Under these conditions, photoreduction of ferricyanide becomes insensitive to diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], an inhibitor of photosystem [PS] II electron transport. Preincubation of thylakoids with diuron leads to a conformational change in the 32,000 dalton protein, modifying its trypsin digestion and preventing expression of diuron insensitivity. Light affects the susceptibility of the 32,000 dalton protein to digestion by trypsin. In other experiments, thylakoids specifically depleted in the 32,000 dalton protein were deficient in electron transport at the reducing side of PS II, but not at the oxidizing side or in PS I activities. The rapidly metabolized 32,000 dalton thylakoid protein in Spirodela chloroplasts fulfills the requirements of the hypothesized proteinaceous shield [Renger, G.] regulating electron flow through PS II and mediating diuron sensitivity.