Studies on calcium depletion of PS II by pH 8.3 treatment

Abstract

We have investigated by EPR the hypothesis that the S 1 state of the oxygen-evolving complex (OEC) becomes unstable at pH 8.3 and is converted to the S 0 state in the dark (Plitjer et al. (1986) FEBS Lett. 195, 313–318). Treatment at pH 8.3 removed the ability to generate the S 2 multiline EPR signal, but this was restored by returning the sample to pH 6.3. The depletion of calcium by an NaCl-wash procedure at pH 8.3 in the dark inhibited oxygen evolution and the formation of the multiline EPR signal, even on returning to pH 6.3. The characteristic 25% reduction of the electron donor, D +, over 4 h in untreated samples was absent in pH 8.3 NaCl-washed samples. Calcium and vanadyl ions, VO 2+, restore the normal dark stable S 1 state, as measured by the large decrease in D + and ability to form the S 2 state multiline EPR signal after 200 K illumination. Calcium, strontium and vanadyl ions also partially restore oxygen evolution in calcium-depleted PS II. These results are interpreted as indicating the loss of the S 1 state at pH 8.3 in the dark and that there is a calcium requirement for S 0 to S 1 at pH 6.3.