The roles of weak light, oxygen and dithiothreitol in the photoreactivation of the oxygen evolving center of tris-washed and 2, 6-dichlorophenol indophenol-treated chloroplasts : Evidence of the weak light needed for stimulating chloroplast electron transport in the reactivation of the O2-evolving center.

The inactivated O2-evolving center of Tris-washed chloroplasts was reactivated by DCPIP-treatment and photoreactivation in the presence of Mn(2+), Ca(2+), DTT and weak light. Many electron donors (Asc and reduced DCPIP, etc.) were found to be suitable substitutes for DTT. By studying the anaerobic inhibition of the reactivation, the electron acceptors O2, NADP(+), etc. were also found to be essential factors in photoreactivation. Weak light stimulated the chloroplast electron transport from the above-mentioned electron donors to the electron acceptor and effected the photoreactivation. More than 280 electrons were transported to NADP(+) in the anaerobic photoreactivation of one unit of an O2-evolving center with 400 Chl. Electron transport in the reactivation was inhibited by omitting DTT or Mn(2+) ion, and by adding DCMU. The photoreactivated chloroplasts incorporated about 2 Mn by 400 Chl. Omission of DTT in the reactivation caused chloroplasts in the weak light to bind large amounts of excess Mn.