Function of PsbO, the photosystem II manganese-stabilizing protein: probing the role of aspartic acid 157

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The D157N, D157E, and D157K mutations in the psbO gene encoding the photosystem II (PSII) manganese-stabilizing protein from spinach, exhibit near-wild-type PSII binding but are significantly impaired in O(2) evolution activity and Cl(-) retention by PSII [Popelkova et al. (2009) Biochemistry 48, 11920-11928]. To better characterize the role of PsbO-Asp157 in eukaryotic PSII, the effect of mutations in Asp157 on heat-induced changes in PsbO solution structure, O(2) release kinetics, and PSII redox reactions both within and outside the oxygen-evolving complex (OEC) have been examined. The data presented here show that Asn, Glu, or Lys mutations in PsbO-Asp157 modify PsbO thermostability in solution, which is consistent with the previously reported perturbation of the functional assembly of PsbO-Asp157 mutants into PSII that caused inefficient Cl(-) retention by PSII. Fluorescence decay signals from PSII reconstituted with Asp157 mutants indicate that that the Q(A)(-) to Q(B) transition on the PSII reducing side is unaffected, but complex alterations are detected on the PSII oxidizing side that affect the recombination of Q(A)(-) with the O(2)-evolving complex. In addition, oxygen yield on the first flash is increased, which indicates an impaired ability of mutant-reconstituted PSII samples to decay back to the S(1) state in the dark.

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