Documentation of significant electron transport defects on the reducing side of photosystem II upon removal of the PsbP and PsbQ extrinsic proteins.

The Photosystem II extrinsic proteins PsbO, PsbP, and PsbQ are required for efficient oxygen-evolving activity under physiological conditions. In this study, we have used fluorescence decay kinetics to quantitatively probe Photosystem II electron transport upon depletion of these components by standard salt washing protocols. Our results indicate that in addition to the expected oxidizing-side defects, removal of PsbP and PsbQ with 2 M NaCl significantly slows the rate of electron transfer from Q(A)(-) to Q(B). Electron transfer from Q(A)(-) to Q(B) in Photosystem II reaction centers with an occupied Q(B) site was slowed by a factor of 12, while electron transport from Q(A)(-) to Q(B) in centers with an unoccupied Q(B) site was slowed by a factor of 6. Subsequent removal of the PsbO protein by treatment with 200 mM NaCl and 2.6 M urea did not induce further reducing-side alterations. Our results demonstrate that studies attributing defects observed upon PsbP and PsbQ removal solely to the oxidizing side must be viewed with caution.