Bicarbonate Activation of the Monomeric Photosystem II-PsbS/Psb27 Complex

In thylakoid membranes, Photosystem II monomers from the stromal lamellae contain the subunits PsbS and Psb27 (PSIIm-S/27), while Photosystem II monomers from granal regions (PSIIm) lack these subunits. Here, we have isolated and characterised these two types of Photosystem II complexes. The PSIIm-S/27 showed enhanced fluorescence, the near-absence of oxygen evolution, as well as limited and slow electron transfer from Q A to Q B compared to the near-normal activities in the granal PSIIm. However, when bicarbonate was added to the PSIIm-S/27, water splitting and Q A to Q B electron transfer rates were comparable to those in granal PSIIm. The findings suggest that the binding of PsbS and/or Psb27 inhibits forward electron transfer and lowers the binding affinity for the bicarbonate. This can be rationalized in terms of the recently discovered photoprotection role played by bicarbonate binding via the redox tuning of the Q A /Q A •− couple, which controls the charge recombination route, and this limits chlorophyll triplet mediated 1 O 2 formation (Brinkert K et al. (2016) Proc Natl Acad Sci U S A. 113(43):12144-12149). These findings suggest that PSIIm-S/27 is an intermediate in the assembly of PSII in which PsbS and/or Psb27 restrict PSII activity while in transit, by using a bicarbonate-mediated switch and protective mechanism. One sentence summary A photosystem II monomer with PsbS and Psb27 as additional subunits, is inactive as isolated but activated by bicarbonate, and is attributed to be a late-stage intermediate in photoassembly.