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Fluorescence yield relaxation following a light pulse was studied in various cyanobacteria under aerobic and microaerobic conditions. In Synechocystis PCC 6803 fluorescence yield decays in a monotonous fashion under aerobic conditions. However, under microaerobic conditions the decay exhibits a wave feature showing a dip at 30-50 ms after the flash followed by a transient rise, reaching maximum at ~1s, before decaying back to the initial level. The wave phenomenon can also be observed under aerobic conditions in cells preilluminated with continuous light. Illumination preconditions cells for the wave phenomenon transiently: for few seconds in Synechocystis PCC 6803, but up to one hour in Thermosynechocystis elongatus BP-1. The wave is eliminated by inhibition of plastoquinone binding either to the QB site of Photosystem-II or the Qo site of cytochrome b6f complex by 3-(3',4'-dichlorophenyl)-1,1-dimethylurea or 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, respectively. The wave is also absent in mutants, which lack either Photosystem-I or the NAD(P)H-quinone oxidoreductase (NDH-1) complex. Monitoring the redox state of the plastoquinone pool revealed that the dip of the fluorescence wave corresponds to transient oxidation, whereas the following rise to re-reduction of the plastoquinone pool. It is concluded that the unusual wave feature of fluorescence yield relaxation reflects transient oxidation of highly reduced plastoquinone pool by Photosystem-I followed by its re-reduction from stromal components via the NDH-1 complex, which is transmitted back to the fluorescence yield modulator primary quinone electron acceptor via charge equilibria. Potential applications of the wave phenomenon in studying photosynthetic and respiratory electron transport are discussed. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Oxygenic photosynthesis relies on accessory factors to promote the assembly and maintenance of the photosynthetic apparatus in the thylakoid membranes. The highly conserved membrane-bound rubredoxin-like protein RubA has previously been implicated in the accumulation of both PSI and PSII, but its mode of action remains unclear. Here, we show that RubA in the cyanobacterium Synechocystis sp PCC 6803 is required for photoautotrophic growth in fluctuating light and acts early in PSII biogenesis by promoting the formation of the heterodimeric D1/D2 reaction center complex, the site of primary photochemistry. We find that RubA, like the accessory factor Ycf48, is a component of the initial D1 assembly module as well as larger PSII assembly intermediates and that the redox-responsive rubredoxin-like domain is located on the cytoplasmic surface of PSII complexes. Fusion of RubA to Ycf48 still permits normal PSII assembly, suggesting a spatiotemporal proximity of both proteins during their action. RubA is also important for the accumulation of PSI, but this is an indirect effect stemming from the downregulation of light-dependent chlorophyll biosynthesis induced by PSII deficiency. Overall, our data support the involvement of RubA in the redox control of PSII biogenesis.