Bicarbonate activation of the monomeric photosystem II-PsbS/Psb27 complex

In thylakoid membranes, photosystem II (PSII) monomers from the stromal lamellae contain the subunits PsbS and Psb27 (PSIIm-S/27), while PSII monomers (PSIIm) from granal regions lack these subunits. Here, we have isolated and characterized these 2 types of PSII complexes in tobacco (Nicotiana tabacum). PSIIm-S/27 showed enhanced fluorescence, the near absence of oxygen evolution, and 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 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 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)(center dot-) couple, which controls the charge recombination route, and this limits chlorophyll triplet-mediated O-1(2) formation. 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 using a bicarbonate-mediated switch and protective mechanism. Bicarbonate ions switch on a nonfunctional photosystem II monomer modified by the binding of 2 extra subunits (PsbS and Psb27) that may be a late-stage intermediate in photoassembly.

Automated summary

PSIIm-S/27 contains PsbS and Psb27 subunits, while PSIIm lacks these subunits. PSIIm-S/27 shows limited oxygen evolution and slow electron transfer compared to PSIIm, but bicarbonate can activate PSIIm-S/27. The binding of PsbS and/or Psb27 inhibits forward electron transfer and lowers bicarbonate binding affinity, and bicarbonate switch plays a protective role in PSII assembly.