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• W2024426642 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., O¨quist, 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 QB becomes lower approaching that of QA. This was confirmed by examining theSynechococcus 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 S2 and S3 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 QB results in altered charge equilibrium in favor of QA. 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 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. 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., O¨quist, 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 QB becomes lower approaching that of QA. This was confirmed by examining theSynechococcus 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 S2 and S3 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 QB results in altered charge equilibrium in favor of QA. 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 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. photosystem II thermoluminescence 3-(3,4-dichlorophenyl)-1,1-dimethylurea primary electron-accepting quinone in PSII secondary electron-accepting quinone in PSII temperature of maximum thermoluminescence emission 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 overcome unfavorable environmental conditions such as chilling temperatures (1Murata N. Wada H. Biochem. J. 1995; 308: 1-8Crossref PubMed Scopus (290) Google Scholar,2Campbell D. Zhou G. Gustafsson P. O¨quist G. Clarke A.K. EMBO J. 1995; 14: 5457-5466Crossref PubMed Scopus (100) Google Scholar), excess light (3Krupa Z. O¨quist G. Gustafsson P. Plant Physiol. 1990; 93: 1-6Crossref PubMed Scopus (34) Google Scholar, 4Clarke A.K. Soitamo A. Gustafsson P. O¨quist G. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 9973-9977Crossref PubMed Scopus (101) Google Scholar, 5Clarke A.K. Hurry V.M. Gustafsson P. O¨quist G. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 11985-11989Crossref PubMed Scopus (73) Google Scholar, 6Clarke A.K. Campbell D. Gustafsson P. O¨quist G. Planta. 1995; 197: 553-562Crossref Scopus (49) Google Scholar, 7Komenda J. Koblizzek M. Masojidek J. J. Photochem. Photobiol. 1999; 48: 114-119Crossref Scopus (14) Google Scholar, 8Komenda J. Biochim. Biophys. Acta. 2000; 1457: 243-252Crossref PubMed Scopus (20) Google Scholar), and UV-B (290–320 nm) exposure (9Campbell D. Eriksson M.-J. O¨quist G. Gustafsson P. Clarke A.K. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 364-369Crossref PubMed Scopus (154) Google Scholar). It has been shown that during environmental stress and acclimationSynechococcus has the ability to shift between two different forms of the D1 polypeptide of the PSII1 reaction center complex (4Clarke A.K. Soitamo A. Gustafsson P. O¨quist G. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 9973-9977Crossref PubMed Scopus (101) Google Scholar). D1 form 1 (D1:1) is the preferred form in a cell acclimated to its normal growth environment, but when stressed under either high light or low temperatures D1:1 is exchanged for another form of the D1 protein called D1 form 2 (D1:2) (2Campbell D. Zhou G. Gustafsson P. O¨quist G. Clarke A.K. EMBO J. 1995; 14: 5457-5466Crossref PubMed Scopus (100) Google Scholar, 6Clarke A.K. Campbell D. Gustafsson P. O¨quist G. Planta. 1995; 197: 553-562Crossref Scopus (49) Google Scholar). However, this exchange is only transient, and when cells have acclimated to the new growth conditions they revert to D1:1 (10O¨quist G. Campbell D. Clarke A.K. Gustafsson P. Photosynth. Res. 1995; 46: 151-158Crossref PubMed Scopus (53) Google Scholar). 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 (2Campbell D. Zhou G. Gustafsson P. O¨quist G. Clarke A.K. EMBO J. 1995; 14: 5457-5466Crossref PubMed Scopus (100) Google Scholar, 4Clarke A.K. Soitamo A. Gustafsson P. O¨quist G. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 9973-9977Crossref PubMed Scopus (101) Google Scholar).The D1:1 product of the psbAI gene is different from the D1:2 product of the psbAII/III genes (11Golden S.S. Brusslan J. Haselhorn R. EMBO J. 1986; 5: 2789-2798Crossref PubMed Scopus (226) Google Scholar). Of the 25 different amino acids, 13 reside in the N-terminal part, four in helix B, three in helix C, two in helix E, and the remaining three in the C-terminal region. Both D1:1 and D1:2 have a total of 360 amino acids. Gene-inactivation mutant strains expressing either only D1:1 (R2S2C3) or only D1:2 (R2K1) (11Golden S.S. Brusslan J. Haselhorn R. EMBO J. 1986; 5: 2789-2798Crossref PubMed Scopus (226) Google Scholar) have proven to be very useful in studies of differential psbA expression in response to environmental changes (10O¨quist G. Campbell D. Clarke A.K. Gustafsson P. Photosynth. Res. 1995; 46: 151-158Crossref PubMed Scopus (53) Google Scholar, 12Campbell D. Bruce D. Carpenter C. Gustafsson P. O¨quist G. Photosynth. Res. 1996; 47: 131-144Crossref PubMed Scopus (40) Google Scholar).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 (2Campbell D. Zhou G. Gustafsson P. O¨quist G. Clarke A.K. EMBO J. 1995; 14: 5457-5466Crossref PubMed Scopus (100) Google Scholar, 3Krupa Z. O¨quist G. Gustafsson P. Plant Physiol. 1990; 93: 1-6Crossref PubMed Scopus (34) Google Scholar, 6Clarke A.K. Campbell D. Gustafsson P. O¨quist G. Planta. 1995; 197: 553-562Crossref Scopus (49) Google Scholar). 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 (3Krupa Z. O¨quist G. Gustafsson P. Plant Physiol. 1990; 93: 1-6Crossref PubMed Scopus (34) Google Scholar, 4Clarke A.K. Soitamo A. Gustafsson P. O¨quist G. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 9973-9977Crossref PubMed Scopus (101) Google Scholar) and partly due to a higher intrinsic resistance of PSII reaction centers with D1:2 to photoinhibition (5Clarke A.K. Hurry V.M. Gustafsson P. O¨quist G. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 11985-11989Crossref PubMed Scopus (73) Google Scholar,13Krupa Z. O¨quist G. Gustafsson P. Physiol. Plant. 1991; 82: 1-8Crossref Scopus (27) Google Scholar, 14Campbell D. Clarke A.K. Gustafsson P. O¨quist G. Plant Sci. 1996; 115: 183-190Crossref Scopus (11) Google Scholar).Because the replacement of D1:1 by D1:2 is expected to modify the functional characteristics of PSII at the molecular level, we hypothesized that the D1:1 to D1:2 exchange affects PSII charge stabilization and charge recombination events as a consequence of alteration in the redox behavior of QA and QBas well as the water oxidation complex (the S states). 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 (15Sane P.V. Rutherford A.W. Govindjee Amesz J. Fork D.C. Light Emission by Plants and Bacteria. Academic Press, Orlando, FL1986: 329-360Google Scholar, 16Inoue Y. Amesz J. Hoff A. Biophysical Techniques in Photosynthesis. Kluwer Academic Publishers, Dordrecht, The Netherlands1996: 93-107Google Scholar).We exploited the fact that a reversible D1:1 to D1:2 exchange can be induced in wild type Synechococcus sp. PCC 7942 through a temperature shift from 36 to 25 °C (2Campbell D. Zhou G. Gustafsson P. O¨quist G. Clarke A.K. EMBO J. 1995; 14: 5457-5466Crossref PubMed Scopus (100) Google Scholar). In addition, the use of two mutants, one of which expresses only D1:2 while the other expresses only D1:1 (11Golden S.S. Brusslan J. Haselhorn R. EMBO J. 1986; 5: 2789-2798Crossref PubMed Scopus (226) Google Scholar), allowed us to confirm whether the changed photochemical behavior observed in wild type cells under conditions that permit expression of either D1:1 or D1:2 are indeed due to the type of D1 polypeptide present in the PSII complex.RESULTSRepresentative TL curves of control and DCMU-treated wild typeSynechococcus cells are presented in Fig.1. The samples were illuminated for 30 s with white light during cooling to liquid nitrogen temperature (19Sane P.V. Desai T.S. Rane S.S. Tatake V.G. Ind. J. Exp. Biol. 1983; 21: 401-404Google Scholar). Deconvolution of the TL glow signal of control cells grown at 36 °C resolved three peaks exhibiting characteristicTM at 0 °C, 24 °C, and 37 °C (Fig.1A, Table I). The major peak around 24 °C accounted for about 85% of the total luminescence, while the peak around 37 °C accounted for most of the remaining luminescence with the peak appearing near 0 °C contributing to less than 0.5% (Table I).Table IPeak emission temperatures (T °C) and contribution of different glow peaks to the total area under the TL curve of wild type Synechococcus sp. PCC 7942 cellsSample (Time in min)Control+ DCMUPeak T (°C)A, %Peak T (°C)A, %Control at 36 °C (0 min)0.4 ± 0.10.4 ± 0−2.67 ± 0.112.5 ± 0.123.7 ± 0.0184.7 ± 0.117.49 ± 0.0666.2 ± 1.237.1 ± 0.0114.8 ± 0.128.61 ± 0.2831.3 ± 1.236 °C → 25 °C (180 min)−1.01 ± 0.011.7 ± 0.1−2.24 ± 0.083.6 ± 0.120.85 ± 0.1279.2 ± 5.517.70 ± 0.1267.3 ± 2.930.44 ± 2.2419.1 ± 5.527.73 ± 0.7227.0 ± 2.925 °C → 36 °C (180 min)3.00 ± 0.065.9 ± 0.12.31 ± 0.141.9 ± 0.224.86 ± 0.0193.1 ± 0.119.84 ± 0.0860.2 ± 11.642.37 ± 0.071.0 ± 0.126.90 ± 2.1437.9 ± 11.8The contribution of glow peaks represented by characteristic sub-bands was estimated by a nonlinear least squares fitting of the experimental glow curves obtained after illumination with continuous white light, and the data are presented as a percentage of the total area (A, %). Mean values ± S.E. were calculated from six to eight measurements in three to five independent experiments. The measurements were performed in the presence and absence of DCMU following the temperature shift from 36 °C to 25 °C (180 min) and back to 36 °C. Open table in a new tab To identify the recombining redox species responsible for each of the peaks as documented in the literature (16Inoue Y. Amesz J. Hoff A. Biophysical Techniques in Photosynthesis. Kluwer Academic Publishers, Dordrecht, The Netherlands1996: 93-107Google Scholar, 22Gleiter H.M. Ohad N. Koike H. Hirschberg J. Renger G. Inoue Y. Biochim. Biophys. Acta. 1992; 1140: 135-143Crossref PubMed Scopus (19) Google Scholar), the glow curve patterns were determined in the presence of DCMU. In agreement with previous reports (16Inoue Y. Amesz J. Hoff A. Biophysical Techniques in Photosynthesis. Kluwer Academic Publishers, Dordrecht, The Netherlands1996: 93-107Google Scholar, 21Vass I. Govindjee Photosynth. Res. 1996; 48: 117-126Crossref PubMed Scopus (118) Google Scholar), DCMU caused a complete loss of the 37 °C peak and appearance of a new peak at 28 °C (Fig. 1B; Table I). In an earlier study of Synechococcus sp. PCC 7942, the peak appearing around 40 °C was assigned to the recombination of S2 and Q B− (22Gleiter H.M. Ohad N. Koike H. Hirschberg J. Renger G. Inoue Y. Biochim. Biophys. Acta. 1992; 1140: 135-143Crossref PubMed Scopus (19) Google Scholar), while the appearance of a DCMU or an atrazine-resistant peak around 30 °C was ascribed to the recombination of S2 and Q A− (16Inoue Y. Amesz J. Hoff A. Biophysical Techniques in Photosynthesis. Kluwer Academic Publishers, Dordrecht, The Netherlands1996: 93-107Google Scholar, 21Vass I. Govindjee Photosynth. Res. 1996; 48: 117-126Crossref PubMed Scopus (118) Google Scholar). More precise identification of the characteristic temperatures of S2Q A− and S2Q B− peaks was obtained from flash-induced TL measurements. It has been demonstrated earlier that the TL band produced after two saturating flashes is related to both S2Q B− and S3Q B− recombinations (36Meyers S.R. Dubbs J.M. Vass I. Hideg E. Nagy L. Barber J. Biochemistry. 1993; 32: 1454-1465Crossref PubMed Scopus (82) Google Scholar, 37Ma¨enpa¨a¨ P. Miranda T. Tyystjarvi E. Govindjee Ducruet J.-M Kirilovsky D. Plant Physiol. 1995; 107: 187-197Crossref PubMed Scopus (45) Google Scholar). In fact, exposure of the control wild type sample to two consecutive flashes of white saturating light also yielded a glow curve pattern that could be deconvoluted in two peaks, the major one appearing at 38 °C and the smaller one at 25 °C, respectively (Fig. 2A). As expected, addition of DCMU drastically reduced the peak at 38 °C with the appearance of a new peak with a TM of 32 °C (Fig. 2B). Hence, because DCMU treatment causes the inhibition of the electron flow between QA and QB and concomitant conversion of S2Q B− to S2Q A− (16Inoue Y. Amesz J. Hoff A. Biophysical Techniques in Photosynthesis. Kluwer Academic Publishers, Dordrecht, The Netherlands1996: 93-107Google Scholar), the data presented above clearly indicates that the peaks appearing around 38 °C and 28 °C in our study originate from S2Q B− and S2Q A− recombinations, respectively. The smaller peak appearing at 23.7 °C is due to S3Q B− recombination, and the peak at 17.5 °C from an S3Q A− recombination (TableI). We emphasize that flash excitation at 0 °C of a fully relaxed sample in the presence of DCMU could not generate S3 unless S2 is quite stable in the dark. Because, in these cells, S2Q B− appears at higher temperatures, we believe that S2 is also relatively more stable and can generate S3 in the presence of DCMU. The situation is different in experiments using continuous illumination during freezing (as was done in the present study). It is possible that, in the presence of DCMU, S2 to S3conversion can occur with concomitant reduction of an acceptor like C550 at very low temperatures. Although the characteristicTM assigned for S3Q A− and S2Q A− recombinations seem higher than those reported for higher plants (16Inoue Y. Amesz J. Hoff A. Biophysical Techniques in Photosynthesis. Kluwer Academic Publishers, Dordrecht, The Netherlands1996: 93-107Google Scholar), they are well within the range observed in cyanobacteria (23Sugiura M. Inoue Y. Plant Cell Physiol. 1999; 40: 1219-1231Crossref PubMed Scopus (149) Google Scholar, 34Minagawa J. Narusaka Y. Iniue Y. Satoh K. Biochemistry. 1999; 38: 770-775Crossref PubMed Scopus (35) Google Scholar, 36Meyers S.R. Dubbs J.M. Vass I. Hideg E. Nagy L. Barber J. Biochemistry. 1993; 32: 1454-1465Crossref PubMed Scopus (82) Google Scholar). In addition, different growth temperatures and the use of different organisms could result in shifting the peak positions to higher or lower temperatures (22Gleiter H.M. Ohad N. Koike H. Hirschberg J. Renger G. Inoue Y. Biochim. Biophys. Acta. 1992; 1140: 135-143Crossref PubMed Scopus (19) Google Scholar, 23Sugiura M. Inoue Y. Plant Cell Physiol. 1999; 40: 1219-1231Crossref PubMed Scopus (149) Google Scholar). It is interesting to note that the observed temperature difference between S2Q B−and S2Q A− inSynechococcus cells is rather small compared with that in higher plant chloroplasts and suggests that the redox potential difference between QA and QB is much lower than in higher plants.Figure 2Thermoluminescence glow curves and mathematical decomposition in Gaussian sub-bands of control (36 °C) (A) and DCMU-treated (10 μm) (B)Synechococcus sp. PCC 7942 wild type cells after illumination with two single turnover flashes of white saturating light are shown. The presented glow curves are averages from five to seven measurements in three independent experiments.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Shifting Synechococcus cells grown at 36 °C to a temperature of 25 °C induced a gradual low temperature shift of the two major glow peaks, and after exposure to 25 °C for 180 min the peaks exhibited a TM of about 21 °C and 30 °C, respectively. The resultant peak temperatures were very close to those observed in 36 °C-acclimated cells treated with DCMU (Fig.1B, Table I). Most surprising, the addition of DCMU caused only minor effects on the TL glow peak positions in cells shifted to 25 °C (Table I). It appears that exposing Synechococcuscells to 25 °C shifted the recombination temperatures of S2Q B− and S3Q B− pairs closer to those of S2Q A− and S3Q A− pairs. The time course of the reversible changes of S2Q B−, S2Q A−, S3Q B−, and S3Q A− recombinations during the temperature shift is presented in Fig.3. The characteristicTM of S2Q B− decreased gradually to 30 °C after 180 min of incubation at low temperature (Table I, Fig. 3A). Interestingly, transferring the cells back to the normal growth temperature of 36 °C caused a very rapid shift of the S2Q B− peak to higher temperatures. A similar trend was observed for S3Q B− recombinations (Fig.3B). In contrast, the temperatures of S2Q A− and S3Q A− recombinations remained fairly constant during the temperature shifts (Table I, Fig.3, A and B). In addition, the relative TL yield measured as the integrated area under the glow curves also decreased by 30% after 180 min of exposure of the cells to low temperature (Fig.3C). This effect was reversible, and the TL yield fully recovered after shifting the cultures back to the normal growth temperature of 36 °C.Figure 3Time course of S2Q B− and S2Q A− (A) and S3Q B− and S3Q A− (B) characteristic peaks in wild type Synechococcus sp.PCC 7942 cells during the temperature shift from the growth temperature of 36 °C to 25 °C for the first 180 min and back to 36 °C for the second part of the curve are shown. C, relative TL yield measured as the total area under the experimental glow curves. The peak positions were estimated by decomposition analysis of the experimental TL curves after illumination with continuous white light. The presented mean values ± S.E. are calculated from six to eight measurements in three to five independent experiments. ↑ = shift from 25 °C back to 36 °C.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Consistent with previous reports (2Campbell D. Zhou G. Gustafsson P. O¨quist G. Clarke A.K. EMBO J. 1995; 14: 5457-5466Crossref PubMed Scopus (100) Google Scholar, 24Porankiewicz J. Selstam E. Campbell D. O¨quist G. Physiol. Plant. 1998; 104: 405-412Crossref Scopus (9) Google Scholar), immunoblot analysis of the relative polypeptide abundance of the PSII reaction center D1:1 and D1:2 forms clearly indicated that the transfer ofSynechococcus sp. PCC 7942 cells from 36 °C to 25 °C caused a gradual appearance of D1:2 polypeptide and a concomitant decrease in the D1:1 polypeptide within 180 min (Fig.4). Interestingly, shifting the culture back to the normal growth temperature of 36 °C induced the opposite effect. The amount of D1:1 increased in parallel with the reduction in D1:2 abundance within the same time frame (Fig. 4). The finding that the changes in glow peak positions during the temperature shift from 36 °C to 25 °C occurred in parallel with the transient appearance D1:2 suggests that the recombination patterns between the S states and Q B− are controlled by the different D1 forms. Regression analysis of the experimental data yielded a strong linear correlation between the relative abundance of D1:2 polypeptide and the characteristic TM of S2Q B− TL-band (r2 = 0.851) (Fig.5). Furthermore, the observation that cells acclimated to 36 °C in the presence of DCMU and cells acclimated to 25 °C ± DCMU show approximately similar glow peak signatures suggests that the S2Q B− and S3Q B− recombinations in cells shifted to 25 °C have almost similar activation energies as the S2Q A− and S3Q A− recombinations in 36 °C grown cells.Figure 4Representative immunoblots (A) and densitometric analysis (B) of D1:1 and D1:2 polypeptides of PSII during the temperature shift ofSynechococcus sp. PCC 7942 cells from 36 °C to 25 °C for 180 min and back to 36 °C are shown. Polypeptide abundance was detected by immunoblotting after SDS-PAGE with D1:1-and D1:2-specific antibodies. Mean values ± S.E. were calculated from five to seven independent experiments. The data for relative abundance of D1:1 was normalized to its maximal values in control non-treated cells and for D1:2 to its maximal values after 180 min at 25 °C.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 5Correlation between the characteristic peak temperature of S2Q B−TL-band and the relative abundance of D1:2 polypeptide inSynechococcus sp. PCC 7942 cells during the temperature shift from 36 °C to 25 °C for 180 min and back to 36 °C are shown. All values represent means ± S.E. from five to eight independent measurements.View Large Image Figure ViewerDownload Hi-res image Download (PPT)The observed relationship between the transient low temperature-induced appearance of D1:2 form and the corresponding changes in the redox potential of QB was further tested by examining the TL pattern of Synechococcus sp. PCC 7942 mutants possessing only D1:1 (mutant R2S2C3) or only D1:2 (mutant R2K1) (11Golden S.S. Brusslan J. Haselhorn R. EMBO J. 1986; 5: 2789-2798Crossref PubMed Scopus (226) Google Scholar). As expected, the mutant expressing only D1:2 showed similar glow peak characteristics as wild type cells shifted to 25 °C. The characteristic TM of the major glow peak around 22 °C and the overall TL yield were not significantly affected by a low temperature shift to 25 °C (Fig. 6). It should be pointed out, however, that the TL yield of control R2K1 cells was 2-fold lower (TLAREA = 274.5 ± 41.0) than in wild type (TLAREA = 666.9 ± 67.2) and R2S2C3 mutant (TLAREA = 586.6 ± 37.6) cells. Furthermore, the mutant expressing only D1:1 showed at the growth temperature a major peak at 27 °C accounting for a little over 53% of the total luminescence and another appearing at 36 °C accounting for around 46% of the total luminescence (Fig. 6). This is qualitatively similar to what we found in the wild type cells grown at 36 °C (Fig. 1, Table I) This pattern is expected because at normal growth temperature the R2S2C3 mutant is similar to the wild type, which under the same conditions also contains only D1: 1.Figure 6Thermoluminescence glow curves of R2K1 mutant cells grown at 36 °C in the absence and in the presence of DCMU and the R2K1 mutant cells subjected to 25 °C for 180 min. The TL curve of the R2S2C3 mutant at 37 °C is also presented. All TL curves were obtained after illumination with continuous white light, and the presented traces are averages from five to seven measurements in three independent experiments.View Large Image Figure ViewerDownload Hi-res image Download (PPT)DISCUSSIONEarlier studies have shown that when cells ofSynechococcus are exposed to an increased excitation pressure by either increasing the light (6Clarke A.K. Campbell D. Gustafsson P. O¨quist G. Planta. 1995; 197: 553-562Crossref Scopus (49) Google Scholar) or by lowering the temperature under constant irradiance (2Campbell D. Zhou G. Gustafsson P. O¨quist G. Clarke A.K. EMBO J. 1995; 14: 5457-5466Crossref PubMed Scopus (100) Google Scholar), there is a rapid and transient exchange of the D1:1 with the D1:2 polypeptide in the reaction centers of PSII. In full agreement with these reports, a very rapid differential response of D1:1 and D1:2 forms to low temperature shift was also observed in our study (Fig. 4). Furthermore, it should be pointed out that shifting the growth temperature back to 36 °C completely reversed the polypeptide abundance in only 3 h (Fig.4). It has also been shown earlier that cells with D1:2 are much more resistant to high light stress than cells possessing D1:1 because of both a sustained high rate of D1:2 turnover and associated PSII repair (4Clarke A.K. Soitamo A. Gustafsson P. O¨quist G. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 9973-9977Crossref PubMed Scopus (101) Google Scholar) and an intrinsically higher resistance of cells with D1:2 (3Krupa Z. O¨quist G. Gustafsson P. Plant Physiol. 1990; 93: 1-6Crossref PubMed Scopus (34) Google Scholar, 13Krupa Z. O¨quist G. Gustafsson P. Physiol. Plant. 1991; 82: 1-8Crossref Scopus (27) Google Scholar). The TL analyses of the present work indicate that the reversible shift between the D1:1 and D1:2 forms in Synechococcus has a major influence on the redox potential of QB (Figs. 1 and 3, and Table I). The finding that the S2Q B− peak in 25 °C shifted cells (containing D1:2) appears at temperature that is similar to S2Q A− peak suggests that the redox potential of QB in the presence of D1:2 is similar to the redox potential of QA. It has been demonstrated that the temperature at which the TL peak appears is proportional to the changes in free energy and therefore to the changes in the midpoint potential of the reactive species (25de Vault D. Govindjee Photosynth. Res. 1990; 24: 175-181PubMed Google Scholar). Because there is little, if any, change in the temperature at which the recombination reaction S2Q A− and S3Q A− occurs (Fig. 3), it is obvious that neither the redox potential of S2/S3, the oxidizing species participating in TL, nor the redox potential of QA are affected by a shift from the D1:1 to the D1:2 form. In contrast, it is evident that the redox potential of QB has changed significantly upon transition of the cells to 25 °C toward a more negative midpoint potential approaching that of QA. These differential effects of the low temperature stress on the redox properties of QA and QB are consistent with the fact that the QB site resides on the D1 polypeptide, whereas the QA site resides on the D2 polypeptide (26Xiong J. Subramaniam S. Govindjee Photosynth. Res. 1998; 56: 229-254Crossref Scopus (84) Google Scholar) of PSII reaction centers, which does not undergo a change under stress conditions (2Campbell D. Zhou G. Gustafsson P. O¨quist G. Clarke A.K. EMBO J. 1995; 14: 5457-5466Crossref PubMed Scopus (100) Google Scholar).Changes in the TL band assigned to S2Q B− recombination to lower temperature have been well documented to occur in herbicide-resistant mutants of many species (27Demeter S. Vass I. Hideg E. Sallai A. Biochim. Biophys. Acta. 1985; 806: 16-27Crossref Scopus (45) Google Scholar, 28Etienne A.-L. Ducruet J.-M. Ajlani G. Vernotte C. Biochim. Biophys" @default.
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• W2024426642 date "2002-09-01" @default.
• W2024426642 modified "2023-09-30" @default.
• W2024426642 title "A Transient Exchange of the Photosystem II Reaction Center Protein D1:1 with D1:2 during Low Temperature Stress ofSynechococcus sp. PCC 7942 in the Light Lowers the Redox Potential of QB" @default.
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