The PsbU Subunit of Photosystem II Stabilizes Energy Transfer and Primary Photochemistry in the Phycobilisome−Photosystem II Assembly of Synechocystis sp. PCC 6803

Abstract

The PsbU subunit of photosystem II (PSII) is one of three extrinsic polypeptides associated with stabilizing the oxygen evolving machinery of photosynthesis in cyanobacteria. We investigated the influence of PsbU on excitation energy transfer and primary photochemistry by spectroscopic analysis of a PsbU-less (or deltaPsbU) mutant. The absence of PsbU was found to have multiple effects on the excited state dynamics of the phycobilisome and PSII. DeltaPsbU cells exhibited decreased variable fluorescence when excited with light absorbed primarily by allophycocyanin but not when excited with light absorbed primarily by chlorophyll a. Fluorescence emission spectra at 77 K showed evidence for impaired energy transfer from the allophycocyanin terminal phycobilisome emitters to PSII. Picosecond fluorescence decay kinetics revealed changes in both allophycocyanin and PSII associated decay components. These changes were consistent with a decrease in the coupling of phycobilisomes to PSII and an increase in the number of closed PSII reaction centers in the dark-adapted deltaPsbU mutant. Our results are consistent with the assumption that PsbU stabilizes both energy transfer and electron transport in the PBS/PSII assembly.

Operon flv4-flv2 Provides Cyanobacterial Photosystem II with Flexibility of Electron Transfer

“…We speculate, however, that the interaction between PBS and PSII, as influenced by Flv2/Flv4, is rather indirect and due to perturbation of the integrity of PSII dimers. Such perturbation also results from mutagenesis of specific subunits of PSII leading to a decrease in energy transfer efficiency from PBS to PSII (Funk et al, 1998;Veerman et al, 2005).…”

The role of the orange carotenoid protein and PsbU protein in photoprotection in cyanobacteria

“…It has been previously shown that the deletion of the PsbU protein in Synechocystis cells results in the accumulation of excitonically decoupled phycobilisomes, which leads to disruption of energy transfer from phycobilisomes to PSII (Veerman et al, 2005). If similar effect (disconnected phycobilisomes) exists also in Synechococcus then the decreased PSII fluorescence in the PsbU mutant could be explained by the lack of saturation of PSII, which happens when we use the red actinic and measuring flashes.…”

“…The removal of PsbU affects not only the lumenal side and the core but also the stromal side of PSII. It has been reported that removal of PsbU altered primary photochemical processes and led to the accumulation of uncoupled phycobilisomes decreasing the energy transfer from PBs to PSII (Veerman et al, 2005). Recently, flash-induced Chl fluorescence decay and thermoluminescence (TL) measurements performed with a PsbU mutant of Synechococcus PCC 7942 showed that the rate of charge recombination from the S 2 Q A -state is slowed down, as shown by the retarded rate of fluorescence decay and the upshifted peak position of the Q band in the presence of the electron transport inhibitor DCMU (Balint et al, 2006).…”

“…The removal of PsbU affects not only the lumenal side and the core but also the stromal side of PSII. It has been reported that removal of PsbU altered primary photochemical processes and led to the accumulation of uncoupled phycobilisomes decreasing the energy transfer from PBs to PSII (Veerman et al, 2005).…”

“…A fehérje eltávolítása nemcsak a lumenális oldalra, hanem a PSII sztróma felöli oldalára is hatással van. Kimutatták, hogy a PsbU hiánya változásokat okoz az elsődleges fotokémiai folyamatokban, miközben szabadfikobiliszóma felhalmozódás tapasztalható és csökken a fikobiliszómák és a PSII közti energiaátadás (Veerman et al, 2005). A közelmúltban kimutatták azt is, hogy a PsbU-val nem rendelkező Synechococcus PCC7942 sejtek magasabb érzékenységet mutatnak oxidatív stresszel szemben, mint a vad típusú sejtek (Balint et al, 2006).…”