Two forms of the Photosystem II D1 protein alter energy dissipation and state transitions in the cyanobacterium Synechococcus sp. PCC 7942 |
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Authors: | Douglas Campbell Doug Bruce Christene Carpenter Petter Gustafsson Gunnar Öquist |
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Affiliation: | (1) Department of Plant Physiology, University of Ume, S-901 87 Ume, Sweden;(2) Department of Biological Sciences, Brock University, L2S 3A1 St. Catharine's, Ontario, Canada |
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Abstract: | Synechococcus sp. PCC 7942 (Anacystis nidulans R2) contains two forms of the Photosystem II reaction centre protein D1, which differ in 25 of 360 amino acids. D1: 1 predominates under low light but is transiently replaced by D1:2 upon shifts to higher light. Mutant cells containing only D1:1 have lower photochemical energy capture efficiency and decreased resistance to photoinhibition, compared to cells containing D1:2. We show that when dark-adapted or under low to moderate light, cells with D1:1 have higher non-photochemical quenching of PS II fluorescence (higher qN) than do cells with D1:2. This is reflected in the 77 K chlorophyll emission spectra, with lower Photosystem II fluorescence at 697–698 nm in cells containing D1:1 than in cells with D1:2. This difference in quenching of Photosystem II fluorescence occurs upon excitation of both chlorophyll at 435 nm and phycobilisomes at 570 nm. Measurement of time-resolved room temperature fluorescence shows that Photosystem II fluorescence related to charge stabilization is quenched more rapidly in cells containing D1:1 than in those with D1:2. Cells containing D1:1 appear generally shifted towards State II, with PS II down-regulated, while cells with D1:2 tend towards State I. In these cyanobacteria electron transport away from PS II remains non-saturated even under photoinhibitory levels of light. Therefore, the higher activity of D1:2 Photosystem II centres may allow more rapid photochemical dissipation of excess energy into the electron transport chain. D1:1 confers capacity for extreme State II which may be of benefit under low and variable light.Abbreviations D1 the atrazine-binding 32 kDa protein of the PS II reaction centre core - D1:1 the D1 protein constitutively expressed during acclimated growth in Synechococcus sp. PCC 7942 - D1:2 an alternate form of the D1 protein induced under excess excitation in Synechococcus sp. PCC 7942 - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - Fo minimal fluorescence in the dark-adapted state - Fo minimal fluorescence in a light-adapted state - FM maximum fluorescence with all quenching mechanisms at a minimum, measured in presence of DCMU - FM maximal fluorescence in a light-adapted state, measured with a saturating flash - FMdark maximal fluorescence in the dark-adapted state - FV variable fluorescence in a light-adapted state (FM-Fo) - PAM pulse amplitude modulated fluorometer - qN non-photochemical quenching of PS II fluorescence - qN (dark) qN in the dark adapted state - qP photochemical quenching of fluorescence |
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Keywords: | chlorophyll fluorescence electron transport light-acclimation light-harvesting photosynthesis quenching analysis |
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