Inactivation and calcium-dependent reactivation of oxygen evolution in photosystem II preparations treated at pH 3.0 or with high concentrations of NaCl

A brief treatment at pH 3.0 of Photosystem II (PS II) membranes containing two bound Ca²⁺ from rice resulted in strong suppression of oxygen evolution concomitant with extraction of one Ca²⁺ and the lost activity was restored on addition of 50 mM Ca²⁺. However, inactivation of oxygen evolution by low pH-treatment of oxygen-evolving PS II complexes containing only one Ca²⁺ from a rice chlorophyll b-deficient mutant was not associated with extraction of the bound Ca²⁺, although oxygen evolution was markedly enhanced by the addition of Ca²⁺ to the treated complexes. Thus, the acid-inactivation of oxygen evolution cannot be related to extraction of Ca²⁺. On the other hand, low pH-treatment was found to share the following common features with NaCl-treatment which also causes a Ca²⁺-reversible inactivation of oxygen evolution. (1) Exposure of PS II membranes to pH 3.0 resulted in solubilization of the 23 and 17 kDa extrinsic proteins, although the released proteins rebound to the membranes when pH was raised to 6.5. (2) There was an apparent heterogeneity in the binding affinity of Ca²⁺ effective in restoration of the oxygen-evolving activity. (3) Low pH-treated preparations required a higher concentration of Ca²⁺ for the maximum reactivation of oxygen evolution than did NaCl-washed preparations. This was also the case with Sr²⁺, which stimulated oxygen evolution of both low pH-treated and NaCl-washed PS II membranes to smaller extents. When the extrinsic 23 and 17 kDa proteins had been removed, however, Ca²⁺ concentration dependence of oxygen evolution in low pH-treated membranes became similar to that in NaCl-washed PS II preparations and the changes were largely reversed by rebinding of the two proteins. These results strongly suggest that low pH-treatment and NaCl-wash involve similar mechanisms of Ca²⁺-dependent reactivation.