Thylakoid Polypeptides Research Articles

The "Additional Subunit" CF0II of the Photosynthetic ATP-Synthase and the Thylakoid Polypeptide, Binding Ferredoxin NADP Reductase: Are they Different?

Evidence is presented to support the notion that the 16 kDa thylakoid polypeptide, called CF0II, is an essential subunit of the photosynthetic ATP-synthase complex CF0CF1: It is co-isolated with the other subunits of CF0CF1 in preparations either using octylglucoside/cholate or Triton X-100. It is co-precipitated by antibodies together with the other CF0CF1 subunits. It is immunochemically not related to thylakoid polypeptides of higher molecular weight nor to some thylakoid polypeptides with similar apparent molecular weight between 16 and 18 kDa: CF1 epsilon, CF0I, subunit IV of the b6f complex, the 16.5 kDa peripheral polypeptide of the oxygen evolving complex of PS II, and the intrinsic ferredoxin NADP reductase binding protein. The N-terminal amino acid sequences of CF0II and the reductase binding protein is determined by Edman degradation and compared: The two sequences are different and not identical to other characterized thylakoid polypeptides. Monospecific antibodies against CF0II inhibit rebinding of CF1 to EDTA treated thylakoid membranes, H+ efflux from EDTA treated membranes and cyclic photophosphorylation. Thus the additional polypeptide CF0II qualifies for a functional subunit of the photosynthetic ATP-synthase.

Lipid Compositions of Thylakoids with Different Contents of Carotenoids and Apoproteins of the Light-Harvesting Complex of Photosystem II

Summary Grains of wheat ( Triticum aestivum ) were soaked in 0–28 mgL -1 SAN-9789, a herbicide which inhibits carotenoid synthesis, and grown under weak red light (16mWm- 2 ). Untreated grains were also grown in a greenhouse. Western blot analysis showed that the polypeptides of the light-harvesting complex of photosystem II (LHCII) were present in thylakoids of plants treated with 0–0.28mgL -1 SAN-9789, while treatment with 2.8 or 28mgL -1 resulted in detection of only the 27kDa polypeptide of LHCII and in significantly reduced amounts compared to the other treatments. Phosphatidylglycerol acylated with trans-hexadecenoic acid (PG-16: 1- trans ) was present in all treatments. In control plants grown in a greenhouse, LHCII proteins constituted a larger part of the thylakoid polypeptides compared to the red light-grown plants, and PG-16: Itrans made up twice as large a proportion of the thylakoid lipids as in the other treatments. In the plants treated with 2.8 or 28mgL -1 SAN-9789, there was no correlation between presence of LHC II polypeptides and PG-16:1 trans . The results show that presence of PG-16:1 trans did not regulate accumulation of LHC II proteins or thylakoid appression when the carotenoid content was very low.

Solution Effects on the Thermostability of Bean Chloroplast Thylakoids

Knowledge of the physicochemical nature of the strains imposed by heat stress on biological membranes requires a definition of other stresses that impose similar strains. To that end, this paper defines several solution properties that alter the effects of heat on isolated chloroplast thylakoid membranes. Isolated bean (Phaseolus vulgaris L.) ‘Oregon 1604’ thylakoids in various milieus were exposed to 25 or 45 °C, and the repercussion(s) of heat were determined. Raffinose (0.4–0.5 M) was slightly more thermostabilizing than 2 M sucrose as evidenced by an amelioration of the: (i) heat‐induced decline electron transport, (ii) heat‐induced alteration in the susceptibility of thylakoid polypeptides to trypsin digestion, and (iii) heat‐induced decline in the maximum fluorescence transient measured at 685 rim. Equimolar glucose + fructose appeared to be equivalent in effectiveness to sucrose. Solutions with a pH of 5.5 or below and 7.0 or above appeared to increase the susceptibility of thylakoid electron transport to heat. Thermoprotection of electron transport to heat was afforded by aspartate at pH 6 and glutamate at pH 7. Dimethylsulfoxide and ethylene glycol appeared to provide some thermostability at concentrations tested (1 M and 100 mL/L, respectively). Urea appeared to mimic heat‐induced injury. Exposure of thylakoids to 45 °C for 2 min appeared to cause the same degree of impairment as ca. 0.8 M urea, while exposure for 4 min appeared to cause the same as ca. 0.9 M urea. Finally, low concentrations (0.05‐0.1 M) of mercaptoethanol appeared to exacerbate heat‐induced thylakoid malfunction.

Influence of exogenous sucrose on the greening of oat

The greening of roots and leaves has been studied in whole oat seedlings grown on White's medium either with or without 2% sucrose. The added nutrient promotes chlorophyll synthesis and chloroplast differentiation in the roots. Yet it manifests a negative effect in the foliar tissues where it accelerates the decline in chlorophyll as well as the chloroplast ultrastructural alterations usually associated with senescence. The negative effect of the nutrient in the leaves is probably a consequence of the addition of exogenous sucrose to the endogenous sugars produced by photosynthesis. The foliar tissues would therefore be in the presence of high sucrose concentrations, which are known to be harmful for the photosynthetic apparatus. SDS-PAGE analysis of thylakoid polypeptides from root and leaf chloroplasts has revealed organ-specific differences in the electrophoretic patterns.

Isolation and characterization of cDNA clones encoding photosystem I subunits with molecular masses 11.0, 10.0 and 8.4 kDa from Chlamydomonas reinhardtii.

cDNA clones encoding three photosystem I subunits of Chlamydomonas reinhardtii with apparent molecular masses 13, 5 and 3 kDa (thylakoid polypeptides 28, 35 and 37; P28, P35 and P37, respectively) were isolated using gene specific oligonucleotides as probes. The sequences of these oligonucleotides were deduced from the N-terminal amino acid sequences of the proteins. The cDNAs were sequenced and used to probe Southern and Northern blots. The Southern blot analysis indicates that the proteins are encoded by single-copy genes. The mRNA sizes of the three components are 960 (P28), 1120 (P35) and 790 (P37) nucleotides. Comparison between the open reading frames of the cDNAs and the N-terminal amino acid sequences of the proteins indicates that the nascent polypeptides possess N-terminal transit sequences that are removed to give mature proteins of 11.0 (P28), 10.0 (P35) and 8.4 (P37) kDa. Analysis of the deduced protein sequences suggests that P28 and P35 are extrinsic membrane proteins and that P37 spans the thylakoid membrane. All three proteins have short transit peptides that probably route them to the stromal side of the thylakoid membrane.

Isolation and characterization of cDNA clones encoding the 17.9 and 8.1 kDa subunits of Photosystem I from Chlamydomonas reinhardtii

cDNA clones encoding two Photosystem I subunits of Chlamydomonas reinhardtii with apparent molecular masses of 18 and 11 kDa (thylakoid polypeptides 21 and 30; P21 and P30 respectively) were isolated using oligonucleotides, the sequences of which were deduced from the N-terminal amino acid sequences of the proteins. The cDNAs were sequenced and used to probe Southern and Northern blots. The Southern blot analysis indicates that both proteins are encoded by single-copy genes. The mRNA sizes of the two components are 1400 and 740 nucleotides, respectively. Comparison between the open reading frames of the cDNAs and the N-terminal amino acid sequences of the proteins indicates that the molecular masses of the mature proteins are 17.9 (P21) and 8.1 kDa (P30). Analysis of the deduced protein sequences predicts that both subunits are extrinsic membrane proteins with net positive charges. The amino acid sequences of the transit peptides suggest that P21 and P30 are routed towards the lumenal and stromal sides of the thylakoid membranes, respectively.

Thermal Acclimation of Photosynthetic Electron Transport Activity by Thylakoids of Saxifraga cernua

Thermal acclimation by Saxifraga cernua to low temperatures results in a change in the optimum temperature for gross photosynthetic activity and may directly involve the photosynthetic apparatus. In order to test this hypothesis photosynthetic electron transport activity of S. cernua thylakoids acclimated to growth temperatures of 20 degrees C and 10 degrees C was measured in vitro. Both populations exhibited optimum temperatures for whole chain and PSII electron transport activity at temperatures close to those at which the plants were grown. Chlorophyll a fluorescence transients from 10 degrees C-acclimated leaves showed higher rates in the rise and subsequent quenching of variable fluorescence at low measuring temperatures; 20 degrees C-acclimated leaves showed higher rates of fluorescence rise at higher measuring temperatures. At these higher temperatures, fluorescence quenching rates were similar in both populations. The kinetics of State 1-State 2 transitions in 20 degrees C- and 10 degrees C-acclimated leaf discs were measured as changes in the magnitude of the fluorescence emission maxima measured at 77K. Leaves acclimated at 10 degrees C showed a larger F730/F695 ratio at low temperatures, while at higher temperatures, 20 degrees C-acclimated leaves showed a higher F730/F695 ratio after the establishment of State 2. High incubation temperatures also resulted in a decrease in the F695/F685 ratio for 10 degrees C-acclimated leaves, suggesting a reduction in the excitation transfer from the light-harvesting complex of photosystem II to photosystem II reaction centers. The relative amounts of chlorophyll-protein complexes and thylakoid polypeptides separated electro-phoretically were similar for both 20 degrees C- and 10 degrees C-acclimated leaves. Thus, photosynthetic acclimation to low temperatures by S. cernua is correlated with an increase in photosynthetic electron transport activity but does not appear to be accompanied by major structural changes or different relative amounts in thylakoid protein composition.

Amino acid sequence of the 9-kDa iron-sulfur protein of photosystem I in barley.

The 9-kDa thylakoid polypeptide which in vivo carries the iron-sulfur centers A and B of photosystem I was isolated from barley (Hordeum vulgare L.) and the complete amino acid sequence determined. The polypeptide shows a very high degree of homology with the corresponding polypeptides in other plant species. The polypeptide is not post-translationally processed except for the removal of the N-terminal formyl-methionine and the insertion of the iron-sulfur centers.

Modulation of the Light-Stimulated Loss of Polypeptides from Isolated Wheat Thylakoids in vitro

Exposure of thylakoids free of vacuolar proteases to white light causes the loss of several thylakoid bound polypeptides. At a light intensity of 1,500 μE m -2 s -1, such loss is apparent within 5 min although this light intensity does not saturate the reaction. This degradation of thylakoid polypeptides proceeds most rapidly at a pH of 9.0. The rate of polypeptide degradation can be increased by incubation of thylakoids with low concentrations of the detergents Triton X-100 or SDS. Inclusion of an electron transport inhibitor or an uncoupler Of photosynthetic phosphorylation in the assay had no effect on the loss of thylakoid polypeptides in the light. Pre-digestion of thylakoids with trypsin or denaturing thylakoid proteins in a buffered solution of 2 % SDS, 6 M urea at 100 °C for five min prior to the assay did not prevent the loss of thylakoid polypeptides. These data strongly suggest that the light-stimulated loss of polypeptides is not mediated by a protease. The loss of thylakoid polypeptides could be prevented by a variety of reducing agents or by maintaining thylakoids in an anaerobic environment. These data suggest that a species of activated oxygen, probably singlet oxygen, is responsible for the loss of thylakoid polypeptides in the light.

Correction of chlorophyll‐defective male‐sterile winter oilseed rape (Brassica napus) through organelle exchange: Characterization of the chlorophyll deficiency

Jarl, C. I., Ljungberg, U. K. and Bornman, C. H. 1988. Correction of chlorophyll‐defective male‐sterile winter oilseed rape (Brassica napus) through organelle exchange: Characterization of the chlorophyll deficiency. ‐ Physiol. Plant. 72: 505–510.As is known, the introduction of male‐sterile Raphanus sativus L. cytoplasm into Brassica napus L. results in male‐sterile oilseed rape plants, which display a temperature‐related chlorophyll defect. The influences of temperature and irradiance on this defect were investigated. Compared to a line of normal (green phenotype) male‐fertile oilseed rape, the male‐sterile line had reduced chlorophyll content, fewer chloroplasts per cell, an altered ultrastructure of the chloroplasts and reduced activities of both photosystems, although the relative amounts of the photosystems and the chlorophyll a/b ratio were similar. The lower activity of the photosystems is explained by a decreased functional antennae size and a reduced efficiency in the interactions between the nuclear‐encoded light‐harvesting proteins and the reaction centres coded for by the plastome. Some thylakoid polypeptides differed in proportion between the male‐fertile line with green phenotype and the male‐sterile line with chlorotic phenotype. Characters, in which the two lines exhibited differences, are ascribed to difficulties in molecular communication between the oilseed rape nucleus and the radish cytoplasm, which are combined in the deficient male‐sterile line.

Chlorophyll proteins of the prymnesiophyte Pavlova lutherii (Droop) comb. nov.: Identification of the major light-harvesting complex

A chlorophyll ac-fucoxanthin light-harvesting protein has been separated by SDS-polyacrylamide gel electrophoresis and by digitonin-sucrose density centrifigation from thylakoids of Pavlova lutherii. It contains a single major polypeptide of 21 kDa, comprises 69% of the total chlorophyll a and is enriched in chlorophyll c compared to the thylakoids. Energy transfer from chlorophyll c and fucoxanthin to chlorophyll a was demonstrated within the protein complex. Antibodies to the 21 kDa apoprotein showed cross-reactivity with the 26–28 kDa apoproteins of higher plant light-harvesting chlorophyll a b protein and with the 19 kDa apoprotein of the light-harvesting complex of diatoms, but much reduced or no cross-reactivity with the major thylakoid polypeptides of dinoflagellates and cryptophytes.

Structural properties of the D1 and surrounding photosystem II polypeptides as revealed by their interaction with cross-linking reagents.

Treatment of Chlamydomonas reinhardtii thylakoids with cross-linking reagents including glutaraldehyde causes polymerization of all thylakoid polypeptides, but not of the reaction center II polypeptide D1 unless the thylakoids are presolubilized by octyl beta-D-glucoside (Adir, N., and Ohad, I. (1986) Biochim. Biophys. Acta 850, 264-274). The results presented here show that this is a general property of D1 as it can be demonstrated in thylakoids of cyanophytes, Dasicladaceae, green algae, and C3 and C4 plants. Solubilization of the membranes by ionic detergents, deoxycholate, lauryl sucrose, or dodecyl beta-D-maltoside is not effective in inducing cross-linking of the D1 polypeptides by glutaraldehyde. The most effective alkyl glucosides were those with 7-9 carbon alkyl chains. The same behavior toward glutaraldehyde was exhibited by the unprocessed D1 precursor and by the palmitoylated D1 protein. Based on the refractility of the D1 protein to cross-linking reagents, a procedure was developed for its isolation from cross-linked thylakoids by lithium dodecyl sulfate-polyacrylamide gel electrophoresis. Isolated D1 retained its behavior toward cross-linking by glutaraldehyde and generated tryptic fragments similar to those obtained following trypsin treatment of intact thylakoids. Denaturation of isolated D1 protein by acetone facilitates cross-linking by glutaraldehyde and extensive degradation by trypsin. The photosystem II polypeptides are differentially cross-linked with increasing concentrations of glutaraldehyde, the most susceptible being the 28- and 23-kDa components of the light-harvesting chlorophyll a-b protein complex and the core complex 44- and 51-kDa polypeptides, and the least affected being the cytochrome b559, the D2 protein, and a 24-kDa component of the light-harvesting chlorophyll a-b protein complex. These results reflect the relative position and interaction of the photosystem II polypeptides within the complex and suggest that strong and specific hydrophobic interactions may be responsible for the tight and stable conformation of D1. This may be based mostly on the conserved amino acid sequences of D1 and possibly plays a role in the process of D1 integration and removal from the reaction center during its light-dependent turnover.

Studies on the limitations to photosynthesis in leaves of the atrazine-resistant mutant ofSenecio vulgaris L.

In leaves of an atrazine-resistant mutant ofSenecio vulgaris the quantum efficiency of CO2 assimilation was reduced by 21% compared to the atrazine-susceptible wild type, and at a light level twice that required to saturate photosynthesis in the wild type the CO2 fixation rate in the mutant was decreased by 15%. In leaves at steady-state photosynthesis there was a measurable increase in the reduction state of the photosystem II (PSII) primary quinone acceptor,Q A. Although this would lead to a decreased rate of PSII electron transport and may thus explain the decrease in quantum efficiency, this cannot account for the fall in the maximum rate of CO2 fixation. The atrazine-resistant mutant showed an appreciably longer photosynthetic induction time which indicates an effect on carbon metabolism; however, the response of CO2-fixation rate to intercellular CO2 concentration revealed no differences in carboxylation efficiency. There were also no differences in the ability to perform a State 1-State 2 transition between the atrazine-resistant and susceptible biotypes and no difference in the profiles of phosphorylated thylakoid polypeptides. It is concluded that the alteration of the redox equilibrium between PSII quinone electron acceptors in the atrazine-resistant biotype limits appreciably the photosynthetic efficiency in non-saturating light. Additionally, there is a further, as yet unidentified, limitation which decreases photosynthesis in the resistant mutant under light-saturating conditions.

Evidence for protection by heat-shock proteins against photoinhibition during heat-shock

The nuclear-coded 22 kd heat-shock protein (HSP-22) which is transported into the chloroplast and localized in the thylakoids was further characterized and found to be located in the grana lamellae (stacked thylakoids) as an extrinsic protein in the green alga Chlamydomonas reinhardtii. Inhibition of photosynthetic electron flow during heat-shock of Chlamydomonas cells was light-dependent, occurring at low-light intensities (<100 W/m) as compared with photoinhibition at 25 degrees C (>1000 W/m). The site of the damage was localized at the photosystem II (PS II) reaction center. The damage was drastically increased when heat-shock treatment was carried out in the presence of the 80S ribosomal translation inhibitor, cycloheximide (CHI). Pre-incubation of Chlamydomonas cells at 42 degrees C resulted in partial protection against photoinhibition during heat-shock, as compared with cells pre-incubated at 42 degrees C in the presence of CHI which, therefore, did not translate the heat-shock proteins. Analysis of the thylakoid polypeptides' pattern by SDS-PAGE revealed that during heat-shock in the light, thylakoid proteins became aggregated proportionally to the light intensity. Heat-shock in the presence of CHI enhanced the aggregation process which, at low light intensities, was specific to the PS II reaction center D1-protein. The results suggest that the chloroplasts HSPs prevent damage to the PS II reaction center during heat-shock in the light.

Polypeptide patterns obtained by polyacrylamide gel electrophoresis of thylakoid membranes isolated from light and dark grown strains of Chlorogloea fritschii

Sodium dodecyl sulphate-polyacrylamide gel electrophoresis has been used to determine the difference between thylakoid polypeptide patterns of light and dark grown strains of the cyanobacterium Chlorogloea fritschii. There were only 2 prominent bands present in the dark grown strains, polypeptide Mr50,000 and polypeptide Mr90,000, also five fainter bands in the Mr range 45,000-66,200 corresponding to photosystem one, compared with the 32 bands present in the light grown strains. There was no obvious indication of the Mr 33,000 3-(3,4-chlorophenyl)-1,1-dimethyl-urea binding protein. In addition the progressive daily development of the various photosystem components in the light and their relationship in photosynthesis was determined. It was observed that the increase of the relative concentrations of the photosystem two and phycocyanin components indicated their developments are mutually synchronized. The effect of light to dark and dark to light transfer on established strains was investigated. Appreciable loss of photosystem two components and the presence of an additional band Mr22,500 of unknown function in the light to dark transfer, and little reactivation of the photosynthetic capabilities in the dark to light transfer was observed.

Specific loss of LHCII phosphorylation in the Lemna mutant 1073 lacking the cytochrome b6/ f complex

The thylakoid protein kinase(s) activity of Lemna perpusilla strain 6746 (wild type, WT) and the cytochrome (cyt) b 6/ f-less mutant 1073 was compared. Isolated thylakoids of both WT and mutant phosphorylated the polypeptides of 9–15, 29, 32–34 and 40–45 kDa. This kinase(s) activity was light-dependent and could be elicited by addition of duroquinol in the dark. Thylakoids from both WT and mutant phosphorylated histone III-S at comparable rates. However, the redox-controlled phosphorylation of the LHCII polypeptide which could be demonstrated in vitro and in vivo in the WT thylakoids could not be detected under any experimental condition in the cyt b 6/ f-less thylakoids. Halogenated quinone analogues known to inhibit reduction of the cyt b 6/ f complex inhibited both the electron flow and duroquinol-activated LHCII phosphorylation, but had no effect on the duroquinol-dependent phosphorylation of the other thylakoid polypeptides. These results indicate that the Lemna thylakoids contain at least two redox-activated protein kinase(s). A quinone-binding site is involved in the activation of the LHCII kinase system which is rendered inactive in the absence of the cyt b 6/ f complex.

STUDY ON THE FUNCTIONAL SITES OF Mg 2+ -INDUCED Chl α FLUORESCENCE AND SURFACE CHARGE CHANGES OF THYLAKOID MEMBRANES

Thylakoid polypeptide components, Mg~(2+) -induced Chl a fluorescence at 77K and surface chargechanges were measured to investigate the functional sites of Mg~(2+) in the regulation of energy distri-bution between two photosystems in completely and incompletely developed barley chloroplasts. It wasfound that in contrast to the completely developed chloroplasts, the incompletely developed chloroplastslacked Chl b and did not contain the 23KDa and 25KDa polypeptide components of LHC-PSII. In themeantime, these membranes did nor present Mg~(2+) -induced Cbl a fluorescence and surface charge changesof thylakoids. These results provided strong evidence that the 23KDa and 25KDa polypeptides ofLHC-PSII are the specific acting sites of the cation that induced these two phenomena. It is suggested in this paper that Mg~(2+) -induced change of excitation energy distribution betweentwo photosystems is produced by the mechanism due to electrostatic neutralization of LHC-PSII bythe cation to cause a structural or conformational alteration of thylakoids.

The Phenotype of a Virescent Chloroplast Mutation in Tobacco Is Associated with the Absence of a 37.5 kD Thylakoid Polypeptide

The Vir-c mutation is a virescent chloroplast mutation found in a line of plants derived from protoplast fusions between a Nicotina tabacum line and a line containing N. tabacum nuclei with Nicotiana suaveolens cytoplasm. Vir-c displays a lag period in chlorophyll accumulation and granal stack formation in young leaves. We examined total chloroplast protein in young leaves and showed the mutant contains 1.3 to 2.1 times less stromal protein, and 2.9 to 4.3 times less thylakoid protein when compared to the N. tabacum var "Turkish Samsun" control. Electrophoretic patterns of total thylakoid proteins indicated three polypeptides were specifically decreased in amount within the context of the overall reduction in thylakoid protein. Electrophoresis of thylakoid proteins synthesized by chloroplasts isolated from half-expanded leaves demonstrated that mutant chloroplasts did not synthesize a 37.5 kilodalton polypeptide which was synthesized by "Samsun" chloroplasts. A polypeptide of this molecular weight was synthesized by Vir-c chloroplasts isolated from mature leaves which had recovered the normal phenotype. Restriction digestion and electrophoresis of the mutant's chloroplast DNA produced a pattern of restriction fragments different from either N. tabacum or N. suaveolens chloroplast DNA.

Development of chloroplast membranes during light induced greening ofChlorella fusca g-2

The development of thylakoid protein complexes during light induced greening of a mutant ofChlorella fusca was studied. Separation of chlorophyll-protein complexes and thylakoid polypeptides by LDS-polyacrylamide gel electrophoresis show that cells grown in the dark contain proteins belonging to coupling factor and cytochrome f/b6 complex. Parts of both reaction centers are present also. The antennae complexes are specifically lost in yellow cells. The changes in polypeptide pattern at different stages of development in the light are related to ultrastructural changes. The beginning of membrane appression can be correlated with the appearance of the light-harvesting complex II. While the average diameter of EF-particles increases throughout the greening process, their densitiesapart from the rearrangement due to membrane stacking-remain fairly constant. The kinetics of EFu-particle enlargement are different from those of EFs-particles.

Phosphorylation of Thylakoid Proteins of Oryza sativa

The phosphorylation of thylakoid proteins of rice (Oryza sativa L.) was studied in vitro using [gamma-(32)P]ATP. Several thylakoid proteins are labeled, including the light-harvesting complex of photosystem II. Protein phosphorylation is sensitive to temperature, pH, and ADP, ATP, and divalent cation concentrations. In the range pH 7 to 8.2, phosphorylation of the light-harvesting polypeptides declines above pH 7.5, whereas labeling of several other thylakoid polypeptides increases. Increasing divalent cation concentration from 3 to 20 millimolar results in a decrease in phosphorylation of the 26 kilodalton light-harvesting complex polypeptide and increased phosphorylation of several other polypeptides. ADP has an inhibitory effect on the phosphorylation of the light-harvesting complex polypeptides. Phosphorylation of the 26 kilodalton light-harvesting polypeptide requires 0.45 millimolar ATP for half-maximal phosphorylation, compared to 0.3 millimolar for the 32 kilodalton phosphoprotein. Low temperature inhibits the phosphorylation of thylakoid proteins in chilling-sensitive rice. However, phosphorylation of histones by thylakoid-bound kinase(s) is independent of temperature in the range of 25 to 5 degrees C, suggesting that the effect of low temperature is on accessibility of the substrate, rather than on the activity of the kinase.