A transient replacement of the photosystem II reaction center protein D1:1 with D1:2 during low temperature stress of Synechococcus sp. PCC 7942 lowers the redox potential of QB

Upon high excitation stress, the cyanobacterium Synechococcus sp. PCC 7942 replaces the photosystem II reaction center D1 form 1 (D1:1) with D1 form 2 (D1:2). This replacement is only transient and after acclimation, the cells revert back to D1:1, which seems to be the preferred form in acclimated cells. In the present work we use thermoluminescence to study charge recombination events between the acceptor and donor sides of photosystem II in relation to D1 replacement. The data are interpreted to show that in stress induced D1:2, the redox potential of QB becomes lowered approaching that of QA. The recombination of QA- with the S2 and S3 states did not show any change in their redox characteristics upon D1 shift. It is suggested that the change in redox properties of QB result in altered charge equilibrium in favour of QA. This would increase the probability of recombinations to occur in photosystem II such as of reduced QA and P680+. This would result in non-photochemical quenching by recombining reaction centers thus protecting the cells from photodamage upon excessive excitation. It has been shown before that cells with D1:2 are intrinsically more resistant to excessive excitation than cells with D1:1. We suggest that the lowering of the redox potential of QB gives the reaction centers with D1:2 an increased resistance to high light stress, allowing the necessary time for the cells to acclimate to overcome the stress, e.g. chilling stress through lipid adjustments.