ACIDIC INACTIVATION AND CA-DEPENDENT REACTIVATION OF THE CHLOROPLAST ELECTRON-TRANSFER CHAIN

Incubation of pea (Pisum sativum L.) chloroplasts in citrate buffer of pH 3.3 leads to inhibition of electron transfer, which is restituted at pH 7.6 in the presence of Ca2+ but not Mg2+, Mn2+, or Ba2+ ions. Th e extent of inactivation and reactivation varies with different accept ers: the activity declines to 27% of the initial with dichlorophenol-i ndophenol, to 62% with ferricyanide, and 16% with phenyl-p-benzoquinon e. Donor-acceptor analysis and thermoluminescence assays suggest that Ca-depleted chloroplasts are inhibited in the acceptor part of photosy stem II. Reactivation is suppressed by a water-soluble carbodiimide ED C but not by a lipid-soluble DCCD. Phenylbenzoquinone inhibits ferricy anide reduction in Ca-depleted chloroplasts but not in reactivated one s. The possibility of calcium control of the Q(B) binding site is disc ussed.