Photosystem activity and state transitions of the photosynthetic apparatus in cyanobacterium Synechocystis PCC 6803 mutants with different redox state of the plastoquinone pool

To better understand how photosystem (PS) activity is regulated during state transitions in cyanobacteria, we studied photosynthetic parameters of photosystem II (PSII) and photosystem I (PSI) in Synechocystis PCC 6803 wild type (WT) and its mutants deficient in oxidases (Ox−) or succinate dehydrogenase (SDH−). Dark-adapted Ox− mutant, lacking the oxidation agents, is expected to have a reduced PQ pool, while in SDH− mutant the PQ pool after dark adaptation will be more oxidized due to partial inhibition of the respiratory chain electron carriers. In this work, we tested the hypothesis that control of balance between linear and cyclic electron transport by the redox state of the PQ pool will affect PSII photosynthetic activity during state transition. We found that the PQ pool was reduced in Ox− mutant, but oxidized in SDH− mutant after prolonged dark adaptation, indicating different states of the photosynthetic apparatus in these mutants. Analysis of variable fluorescence and 77K fluorescence spectra revealed that the WT and SDH− mutant were in State 1 after dark adaptation, while the Ox− mutant was in State 2. State 2 was characterized by ∼1.5 time lower photochemical activity of PSII, as well as high rate of P700 reduction and the low level of P700 oxidation, indicating high activity of cyclic electron transfer around PSI. Illumination with continuous light 1 (440 nm) along with flashes of light 2 (620 nm) allowed oxidation of the PQ pool in the Ox− mutant, thus promoting it to State 1, but it did not affect PSII activity in dark adapted WT and SDH− mutant. State 1 in the Ox− mutant was characterized by high variable fluorescence and P700+ levels typical for WT and the SDH− mutant, indicating acceleration of linear electron transport. Thus, we show that PSII of cyanobacteria has a higher photosynthetic activity in State 1, while it is partially inactivated in State 2. This process is controlled by the redox state of PQ in cyanobacteria through enhancement/inhibition of electron transport on the acceptor side of PSII.