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

Abstract

In thylakoid membranes, Photosystem II (PSII) monomers from the stromal lamellae contain the subunits PsbS and Psb27 (PSIIm-S/27), while PSII monomers from granal regions (PSIIm) lack these subunits. Here, we have isolated and characterised these two types of Photosystem II complexes in tobacco (Nicotiana tabacum). PSIIm-S/27 showed enhanced fluorescence, the near-absence of oxygen evolution, as well as limited and slow electron transfer from QA to QB compared to the near-normal activities in the granal PSIIm. However, when bicarbonate was added to PSIIm-S/27, water splitting and QA to QB 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 QA/QA•– couple, which controls the charge recombination route, and this limits chlorophyll triplet mediated 1O2 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.

The Psb27 Assembly Factor and Histidine‐252 of the D1 Protein Modify Energy Transfer to Photosystem II in Synechocystis sp. PCC 6803

“…PCC 6803 ΔPsb27 mutants have also been shown to grow poorly under high light and intermittent light/dark periods on plates (Bečková et al, 2017). In addition, a Psb27-containing monomer, found in the stromal fraction of thylakoid membranes of Nicotiana tabacum, transitioned to an active PS II upon addition of bicarbonate (Fantuzzi et al, 2023). Hence, in addition to assisting in the formation of the binding site for the OEC, additional physiological roles for the Psb27 subunit exist.…”

Glycogen synthesis prevents metabolic imbalance and disruption of photosynthetic electron transport from photosystem II during transition to photomixotrophy in Synechocystis sp. PCC 6803

“…PsbQ' from the red algae Cyanidium caldarium modulates the reduction potential of Q A making the formation of triplet Chls less likely, thereby providing photoprotection (Yamada et al, 2018). In Nicotiana tabacum L, PSII inactivation caused by PsbS/Psb27 was relieved by bicarbonate addition, restoring normal Q A reoxidation (Fantuzzi et al, 2023). The promptness of the glucose-induced alterations of photosynthetic performance suggests that the adjustments in PSII are probably post-translational and induced by the metabolic changes.…”

A step closer to fully understand how the engine of life is repaired from damages caused by its fuel

“…In a recent article published in Plant Physiology , Fantuzzi et al (2023) purified an assembly intermediate from the stromal lamella region of the thylakoid membrane using a fractional solubilization method ( Fey et al 2008 ; Haniewicz et al 2015 ). Fully assembled PSII in the granal regions are usually homodimeric; however, there is a small fraction of PSII that is functional and monomeric (PSIIm).…”