Abstract
Upon exposure to low temperature under constant light conditions, the cyanobacterium Synechococcus sp. PCC 7942 exchanges the photosystem II reaction center D1 protein form 1 (D1:1) with D1 protein form 2 (D1:2). This exchange is only transient, and after acclimation to low temperature the cells revert back to D1:1, which is the preferred form in acclimated cells (Campbell, D., Zhou, G., Gustafsson, P., Oquist, G., and Clarke, A. K. (1995) EMBO J. 14, 5457-5466). 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 indicate that in cold-stressed cells exhibiting D1:2, the redox potential of Q(B) becomes lower approaching that of Q(A). This was confirmed by examining the Synechococcus sp. PCC 7942 inactivation mutants R2S2C3 and R2K1, which possess only D1:1 or D1:2, respectively. In contrast, the recombination of Q(A)(-) with the S(2) and S(3) states did not show any change in their redox characteristics upon the shift from D1:1 to D1:2. We suggest that the change in redox properties of Q(B) results in altered charge equilibrium in favor of Q(A). This would significantly increase the probability of Q(A)(-) and P680(+) recombination. The resulting non-radiative energy dissipation within the reaction center of PSII may serve as a highly effective protective mechanism against photodamage upon excessive excitation. The proposed reaction center quenching is an important protective mechanism because antenna and zeaxanthin cycle-dependent quenching are not present in cyanobacteria. We suggest that lowering the redox potential of Q(B) by exchanging D1:1 for D1:2 imparts the increased resistance to high excitation pressure induced by exposure to either low temperature or high light.
Highlights
The responses of Synechococcus and other unicellular cyanobacteria to various abiotic stresses have been extensively investigated and have provided useful information in understanding the mechanisms employed by cyanobacteria to over
We suggest that lowering the redox potential of QB by exchanging D1:1 for D1:2 imparts the increased resistance to high excitation pressure induced by exposure to either low temperature or high light
Earlier studies have shown that when cells of Synechococcus are exposed to an increased excitation pressure by either increasing the light [6] or by lowering the temperature under constant irradiance [2], there is a rapid and transient exchange of the D1:1 with the D1:2 polypeptide in the reaction centers of PSII
Summary
The responses of Synechococcus and other unicellular cyanobacteria to various abiotic stresses have been extensively investigated and have provided useful information in understanding the mechanisms employed by cyanobacteria to over-. This exchange is only transient, and when cells have acclimated to the new growth conditions they revert to D1:1 [10] These shifts are governed by changes in the relative expression of the psbAI gene encoding for D1:1 and the psbAII/III genes encoding for D1:2 [2, 4]. Cells with D1:2 appear to be more stress-resistant than those possessing D1:1 under conditions when the excitation pressure on PSII increases due to either increased irradiance or decreased temperature [2, 3, 6] This is partly due to a high rate of D1 synthesis and expression of the psbAII/III genes forming D1:2 under high excitation pressure [3, 4] and partly due to a higher intrinsic resistance of PSII reaction centers with D1:2 to photoinhibition [5, 13, 14]. The back reactions of QA and QB with the different S states, which reflect the charge stabilization on both acceptors and donors of PSII, were assessed by using the technique of thermoluminescence [15, 16]
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