Biomedical subjects

E M Aro

Publications and source records attributed to E M Aro.

49 records · Page 3Linked to original sources

PubMed1994

Changes of amino acid sequence in PEST-like area and QEEET motif affect degradation rate of D1 polypeptide in photosystem II.

The degradation rate of the D1 polypeptide was measured in three Synechocystis PCC 6803 mutants in vivo. Mutations were introduced into a putative cleavage area of the D1 polypeptide (QEEET motif) and into the PEST-like area. PEST sequences are often found in proteins with a high turnover rate. The QEEET-motif mutants are CA1 [delta (E242-E244);Q241H] and E243K, and the third mutation, E229D, was directed to the PEST-like area. During high-light illumination (1500 mumol photons m-2 s-1) that induced photoinhibition of photosystem II (PSII), the half-life time of the D1 polypeptide in mutant E229D (t 1/2 = 35 min) was about twice as long as in AR (control strain) cells (t 1/2 = 19 min). In growth light (40 mumol photons m-2 s-1), the degradation rate of the D1 polypeptide in E229D and AR strains was the same (t 1/2 approximately 5 h). In growth light the D1 polypeptide was degraded faster in both QEEET-motif mutants than in the AR strain, but in photoinhibitory light the degradation rates were similar. According to these results, the highly conservative QEEET motif as such is not required for the proteolytic cut of the D1 polypeptide, but it does affect the rate of degradation. No simple correlation existed between the degradation rate of the D1 polypeptide and the susceptibility of PSII to photoinhibition in mutant and AR cells under our experimental conditions.

Amino Acid Sequence↗

PubMed1993

Reduced content of the quinone acceptor QA in photosystem II complexes isolated from thylakoid membranes after prolonged photoinhibition under anaerobic conditions.

The plastoquinone content of photosystem II complexes isolated from spinach photosystem II-enriched membranes subjected to strong photoinhibitory illumination under anaerobic conditions was determined by HPLC. A pronounced decrease of the plastoquinone content was found in the photoinactivated complexes. These results corroborate earlier models in which photoinhibitory illumination is suggested to eventually lead to release of doubly reduced and protonated QA from its site on the D2-protein.

Binding Sites↗

PubMed1993

Photoinhibition of Photosystem II. Inactivation, protein damage and turnover.

Even though light is the source of energy for photosynthesis, it can also be harmful to plants. Light-induced damage is targetted mainly to Photosystem II and leads to inactivation of electron transport and subsequent oxidative damage of the reaction centre, in particular to the D1 protein. Inactivation and protein damage can be induced by two different mechanisms, either from the acceptor side or from donor side of P680. The damaged D1 protein is triggered for degradation and digested by at least one serine-type proteinase that is tightly associated with the Photosystem II complex itself. The damaged Photosystem II complex dissociates from the light-harvesting antenna and migrates from appressed to non-appressed thylakoid regions where a new D1 protein is co-translationally inserted into the partially disassembled Photosystem II complex. D1 protein phosphorylation probably allows for coordinated biodegradation and biosynthesis of the D1 protein. After religation of cofactors and assembly of subunits, the repaired Photosystem II complex can again be found in the appressed membrane regions. Various protective mechanisms and an efficient repair cycle of Photosystem II allow plants to survive light stress.

Amino Acid Sequence↗

PubMed1993

Site-specific mutations in the D1 polypeptide affect the susceptibility of Synechocystis 6803 cells to photoinhibition.

Photoinhibition of photosystem II in the cyanobacterium Synechocystis 6803 was followed after site-specific mutagenesis of the D1 polypeptide. Mutations were created in the stromal/cytosolic loop connecting helices D and E. Two mutations E243K and CA1, a deletion of the three glutamates 242-244 and a substitution Q241H, were made in the putative cleavage area of the D1 polypeptide. A third mutation E229D was made in the PEST-like sequence. Mutants and control cells were illuminated and FV/FM was recorded. Compared to the control, the mutants were less photoinhibited. Fluorescence relaxation after a single flash was delayed in CA1. Restoration of FV/FM after photoinhibition in the mutants was totally dependent on protein synthesis while control cells were able to recover partially also when protein synthesis was inhibited. In addition, the protein synthesis-dependent recovery of CA1 was slowed down. Our results indicate a correlation between the mutated amino acids and photoinhibition of photosystem II.

Amino Acid Sequence↗

PubMed1992

ATP and light regulate D1 protein modification and degradation. Role of D1* in photoinhibition.

We have recently shown that during in vivo photoinhibition the D1 protein is degraded via a modified form, designated D1*. Depending on light conditions, the amount of D1* varies in leaves between 0 and 50% of total D1 content. By isolating thylakoids from leaves acclimated to different light levels, and performing photoinhibition experiments on these thylakoids, the following results on D1 protein degradation were obtained: (i) the protease involved in D1 degradation requires activation by light; (ii) neither acceptor nor donor side photoinhibition of PSII induces formation of D1* in vitro; (iii) in isolated thylakoids, the transformation of D1 to D1* can be induced in low light in the presence of ATP, which suggests that D1* is a phosphorylated form of the D1 protein; (iv) D1*, induced either in vivo or in vitro, is much less susceptible to degradation during illumination of isolated thylakoids than the original D1 protein. We suggest that the modification to D1* is a means to prevent disassembly of photodamaged photosystem II complex in appressed membranes.

Adenosine Triphosphate↗

PubMed1992

Slow degradation of the d1 protein is related to the susceptibility of low-light-grown pumpkin plants to photoinhibition.

Photoinhibition of photosystem II (PSII) electron transport and subsequent degradation of the D1 protein were studied in pumpkin (Cucurbita pepo L.) leaves developed under high (1000 mumol m(-2) s(-1)) and low (80 mumol m(-2) s(-1)) photon flux densities. The low-light leaves were more susceptible to high light. This difference was greatly diminished when illumination was performed in the presence of chloramphenicol, indicating that a poor capacity to repair photodamaged PSII centers is decisive in the susceptibility of low-light leaves to photoinhibition. In fact, the first phases of the repair cycle, degradation and removal of photodamaged D1 protein from the reaction center complex, occurred slowly in low-light leaves, whereas in high-light leaves the degradation of the D1 protein more readily followed photoinhibition of PSII electron transport. A modified form of the D1 protein, with slightly slower electrophoretic mobility than the original D1, accumulated in the appressed thylakoid membranes of low-light leaves during illumination and was subsequently degraded only slowly.

Journal Article↗

PubMed1991

D1 protein degradation during photoinhibition of intact leaves. A modification of the D1 protein precedes degradation.

Illumination of intact pumpkin leaves with high light led to severe photoinhibition of photosystem II with no net degradation of the D1 protein. Instead, however, a modified form of D1 protein with slightly slower electrophoretic mobility was induced with corresponding loss in the original form of the D1 protein. When the leaves were illuminated in the presence of chloramphenicol the modified form was degraded, which led to a decrease in the total amount of the D1 protein. Subfractionation of the thylakoid membranes further supported the conclusion that the novel form of the D1 protein was not a precursor but a high-light modified form that was subsequently degraded.

Blotting, Western↗

PubMed1991

Small light-harvesting antenna does not protect from photoinhibition.

High-light-induced decrease in photosystem II (PSII) electron transfer activity was studied in high- and low-light-grown pumpkin (Cucurbita pepo L.) plants in vivo and in vitro. The PSII light-harvesting antenna of the low-light leaves was estimated to be twice as big as that of the high-light leaves. The low-light leaves were more susceptible to photoinhibition in vivo. However, thylakoids isolated from these two plant materials were equally sensitive to photoinhibition when illuminated in the absence of external electron acceptors. Only the intensity of the photoinhibitory light and the chlorophyll concentration of the sample, not the size of the light-harvesting antenna, determined the rate of PSII photoinhibition in vitro. Because excitation of the reaction center and not only the antenna chlorophylls is a prerequisite for photoinhibition of PSII activity, independence of photoinhibition on antenna size provides support for the hypothesis (Schatz EH, Brock H, Holzwarth AR [1988] Biophys J 54: 397-405) that the excitations of the antenna chlorophylls are in equilibrium with the excitations of the reaction centers. Better tolerance of the high-light leaves in vivo was due to a more active repair process and more powerful protective mechanisms, including photosynthesis. Apparently, some protective mechanism of the high-light-grown plants is at least partially active at low temperature. The protective mechanisms do not appear to function in vitro.

Journal Article↗

PubMed1990

Restoration of light induced photosystem II inhibition without de novo protein synthesis.

Illumination of isolated spinach thylakoid membranes under anaerobic conditions gave rise to severe inhibition of photosystem II electron transport but did not result in D1-protein degradation. When these photoinhibited thylakoids were incubated in total darkness the photosystem II activity could be fully restored in vitro in a process that required 1-2 h for completion.

Chlorophyll↗

PubMed1990

Distribution of Chlorophyll-Protein Complexes during Chilling in the Light Compared with Heat-Induced Modifications.

The effects of chilling in the light (4 days at 5 degrees C and 100-200 micromoles of photons per square meter per second) on the distribution of chlorophyll (Chl) protein complexes between appressed and nonappressed thylakoid regions of pumpkin (Cucurbita pepo L.) chloroplasts were studied and compared with the changes occurring during in vitro heat treatment (5 minutes at 40 degrees C) of isolated thylakoids. Both treatments induced an increase (18 and 65%, respectively) in the relative amount of the antenna Chl a protein complexes (CP47 + CP43) of photosystem II (PSII) in stroma lamellae vesicles. Freeze-fracture replicas of light-chilled material revealed an increase in the particle density on the exoplasmic fracture face of unstacked membrane regions. These two treatments differed markedly, however, in respect to comigration of the light-harvesting Chl a/b protein complex (LHCII) of PSII. The LHCII/PSII ratio in stroma lamellae vesicles remained fairly constant during chilling in the light, whereas it dropped during the heat treatment. Moreover, it was a minor light-harvesting Chl a/b protein complex of PSII, CP29, that increased most in stroma lamellae vesicles during light-chilling. Changes in the organization of LHCII during chilling were suggested by a shift to particles of smaller sizes on the protoplasmic fracture face of stacked membrane regions and a decrease in the amount of trans-Delta(3)-hexadecenoic acid in the phosphatidyldiacylglycerol fraction.

Journal Article↗

PubMed1989

The Nature of Light-Induced Inhibition of Photosystem II in Pumpkin (Cucurbita pepo L.) Leaves Depends on Temperature.

Attached leaves of pumpkin (Cucurbita pepo L.) were treated in high or moderate light at room temperature or a 1 degrees C. The symptoms of photoinhibition appearing during light treatments at room temperature could be attributed to a decrease in the primary activity of PSII. However, when the light treatment was given at 1 degrees C, the quantum yield of photosynthetic oxygen evolution decreased much more than would be expected from the decrease in the ratio of variable to maximum fluorescence at 77 degrees K. Also, light treatment at 1 degrees C lowered the chloroplast wholechain electron transfer capacity much more than it affected PSII electron transport (H(2)O to paraphenylbenzoquinone). Light treatments at both room temperature and 1 degrees C led to an increase in B(max), which indicates an increase in the proportion of PSII(beta) centers. PSI was not affected by the light treatments, and the treatments in the dark at 1 degrees C caused only minor changes in the measured properties of the leaves. We conclude that high light always inhibits the primary activity of PSII, but at low temperature there is greater inhibition of electron transfer from primary electron accepting plastoquinone of PSII to the plastoquinone pool, which leads to a drastic decrease in the quantum yield of oxygen evolution in the chilling-sensitive pumpkin.

Journal Article↗

PubMed1988

Rearrangement of the chloroplast thylakoid at chilling temperature in the light.

The leaves of chilling-sensitive pumpkin (Cucurbita pepo L.) showed symptoms reminiscent of photoinhibition when kept for 4 days at 5 degrees C in moderate light. A decrease was observed in the variable part of chlorophyll alpha fluorescence, apparent quantum yield, and maximum rate of O(2) evolution. Chloroplast whole-chain electron transport activity measured from chloroplast thylakoids had decreased to 51% of the control value. Photosystem II (PSII) activity decreased by only 9%, suggesting that photoinhibition was not responsible for the loss of electron transport activity. An increase in the proportion of PSII(beta) (measured as a beta(max) value) was observed after the chilling treatment. Fractionation of thylakoid membranes showed a 42% increase in PSII activity in the nonappressed region while that in the appressed region decreased slightly. This was accompanied by a decrease in the ratio of the length of appressed to nonappressed thylakoid membranes. Leaf photosynthesis largely recovered within 24 hours of returning to the original growth conditions. We suggest that the increase in the proportion of PSII(beta) during chilling in light plays a role in protecting PSII from photoinhibitory damage.

Journal Article↗

PubMed1984

CO(2) and O(2) Exchange in Two Mosses, Hypnum cupressiforme and Dicranum scoparium.

Photosynthetic CO(2) and O(2) exchange was studied in two moss species, Hypnum cupressiforme Hedw. and Dicranum scoparium Hedw. Most experiments were made during steady state of photosynthesis, using (18)O(2) to trace O(2) uptake. In standard experimental conditions (photoperiod 12 h, 135 micromoles photons per square meter per second, 18 degrees C, 330 microliters per liter CO(2), 21% O(2)) the net photosynthetic rate was around 40 micromoles CO(2) per gram dry weight per hour in H. cupressiforme and 50 micromoles CO(2) per gram dry weight per hour in D. scoparium. The CO(2) compensation point lay between 45 and 55 microliters per liter CO(2) and the enhancement of net photosynthesis by 3% O(2)versus 21% O(2) was 40 to 45%. The ratio of O(2) uptake to net photosynthesis was 0.8 to 0.9 irrespective of the light intensity. The response of net photosynthesis to CO(2) showed a high apparent K(m) (CO(2)) even in nonsaturating light. On the other hand, O(2) uptake in standard conditions was not far from saturation. It could be enhanced by only 25% by increasing the O(2) concentration (saturating level as low as 30% O(2)), and by 65% by decreasing the CO(2) concentration to the compensation point. Although O(2) is a competitive inhibitor of CO(2) uptake it could not replace CO(2) completely as an electron acceptor, and electron flow, expressed as gross O(2) production, was inhibited by both high O(2) and low CO(2) levels. At high CO(2), O(2) uptake was 70% lower than the maximum at the CO(2) compensation point. The remaining activity (30%) can be attributed to dark respiration and the Mehler reaction.

Journal Article↗