Catabolite repression of chloroplast development in Euglena

Two-dimensional gel electrophoresis resolved total cellular protein from Euglena gracilis Klebs var. bacillaris Cori into 640 polypeptides detectable by silver staining. The addition of 84 mM ethanol to dark-grown resting carbon-starved cells increased the relative amounts of 6 polypeptides and decreased the relative amounts of 3 polypeptides. The addition of 84 mM malate to resting cells increased the relative amounts of 3 of the ethanol-induced polypeptides, suggesting that the induction of these polypeptides represents a generalized response to the provision of a utilizable carbon source, a nutritional shift up, rather than a specific response to ethanol addition. Exposure of dark-grown resting Euglena to light increases the relative amounts of 79 polypeptides encoded by the nuclear as well as the chloroplast genome and decreases the relative amounts of 72 polypeptides. Ethanol but not malate specifically inhibited all of the light-dependent changes in polypeptide levels, indicating that chloroplast development in Euglena is a catabolite-sensitive process.

Regulation of a wheat actin-depolymerizing factor during cold acclimation

We have previously shown that the wheat (Triticum aestivum) TaADF gene expression level is correlated with the plants capacity to tolerate freezing. Sequence analysis revealed that this gene encodes a protein homologous to members of the actin-depolymerizing factor (ADF)/cofilin family. We report here on the characterization of the recombinant TaADF protein. Assays for ADF activity showed that TaADF is capable of sequestering actin, preventing nucleotide exchange, and inducing actin depolymerization. In vitro phosphorylation studies showed that TaADF is a substrate for a wheat 52-kD kinase. The activity of this kinase is modulated by low temperature during the acclimation period. Western-blot analyses revealed that TaADF is expressed only in cold-acclimated Gramineae species and that the accumulation level is much higher in the freezing-tolerant wheat cultivars compared with the less tolerant ones. This accumulation was found to be regulated by a factor(s) encoded by a gene(s) located on chromosome 5A, the chromosome most often found to be associated with cold hardiness. The induction of an active ADF during cold acclimation and the correlation with an increased freezing tolerance suggest that the protein may be required for the cytoskeletal rearrangements that may occur upon low temperature exposure. These remodelings might be important for the enhancement of freezing tolerance.

The induction of kin genes in cold-acclimating Arabidopsis thaliana. Evidence of a role for calcium

The involvement of calcium signaling during cold-induction of the kin genes of Arabidopsis thaliana (L.) Heynh. was examined. Treatments with chemicals which either chelate extracellular calcium (EGTA) or block the plasma-membrane calcium channels (La3+, Gd3+) inhibited cold acclimation as well as kin gene expression. Ruthenium red, an inhibitor of calcium release from intracellular stores partially inhibited kin gene expression and development of freezing tolerance. An inhibitor of calcium-dependent protein kinases (CDPKs) and calmodulin prevented cold acclimation as well as the cold induction of kin genes. Using restriction fragment length polymorphism-coupled domain-directed differential display, five CDPK clones were identified which showed differential regulation by cold. The amplified fragments showed homology to known plant CDPKs. The involvement of calcium and calcium-binding proteins in cold acclimation of A. thaliana is discussed.

Low temperature perception in plants: effects of cold on protein phosphorylation in cell-free extracts

Activities of prevalent protein phosphatases decreased by nearly 95% and those of individual protein kinases were differentially reduced at low temperature. Inhibition of phosphatase activity at temperatures below 12 degrees C resulted in marked hyperphosphorylation of a 58-kDa protein (PP58). The temperature threshold for hyperphosphorylation of PP58 coincided with the known threshold for cold-induced calcium influx. Since calcium influx is triggered by several environmental stresses, we propose that the observed direct effects of cold on the phosphorylation of specific proteins enable cells to couple a shared calcium signal to a cold-specific transduction pathway.

Vacuolar H(+)-translocating pyrophosphatase is induced by anoxia or chilling in seedlings of rice

The present study was undertaken to determine whether vacuolar H(+)-pyrophosphatase (V-PPase) might replace vacuolar H(+)-ATPase under energy stress due to anoxia or chilling in anoxia-tolerant species such as rice (Oryza sativa L.) and corn (Zea mays L.). The relative transcript level of V-PPase in rice seedlings, like that of alcohol dehydrogenase 1, increased greatly under anoxia and declined again when the seedlings were returned to air. However, the distribution of transcripts in root, shoot, and seed differed somewhat from that of alcohol dehydrogenase 1. Immunoreactive V-PPase protein and V-PPase enzyme specific activity in a tonoplast fraction from rice seedlings increased progressively with time of anoxia or chilling at 10 degrees C, showing a 75-fold increase after 6 d of anoxia, compared with a 2-fold increase of vacuolar H(+)-ATPase activity. When the seedlings were returned to air, the specific activity returned to its initial level within 2 d. After 6 d of chilling at 10 degrees C, V-PPase specific activity reached a level 20-fold of that at 25 degrees C. In microsomes of corn roots, V-PPase specific activity did not respond to anoxia but was constitutively high. It is proposed that V-PPase can be an important element in the survival strategies of plants under hypoxic or chilling stress.

Low-temperature signal transduction: induction of cold acclimation-specific genes of alfalfa by calcium at 25 degrees C

To study the role of calcium in cold acclimation, we examined the relationship between calcium influx and accumulation of transcripts of two cas (cold acclimation-specific) genes of alfalfa, cas15 and cas18. Whereas a decline in temperature from 25 to 15 degrees C had little effect on the influx of extracellular 45Ca2+, an increasing influx was observed when the temperature was lowered further. The influx of 45Ca2+ at 4 degrees C was nearly 15 times greater than at 25 degrees C. The addition of calcium chelators or of calcium channel blockers, which have been shown to prevent cold acclimation, inhibited the influx of extracellular 45Ca2+ as well as the expression of cas genes at 4 degrees C. The addition of a calcium ionophore or a calcium channel agonist to nonacclimated cells caused the influx of extracellular 45Ca2+ and induced the expression of cas genes at 25 degrees C. These results suggest that a cold-induced calcium influx plays an essential role in cold acclimation. To further study the role of calcium, we isolated two sequences corresponding to calcium-dependent protein kinases. The transcript level of one of them was markedly upregulated at 4 degrees C. We propose a sequence of signaling events that is likely to occur early during cold acclimation and leads to the expression of cas genes and the development of freezing tolerance.

Low-temperature signal transduction: induction of cold acclimation-specific genes of alfalfa by calcium at 25 degrees C

To study the role of calcium in cold acclimation, we examined the relationship between calcium influx and accumulation of transcripts of two cas (cold acclimation-specific) genes of alfalfa, cas15 and cas18. Whereas a decline in temperature from 25 to 15 degrees C had little effect on the influx of extracellular 45Ca2+, an increasing influx was observed when the temperature was lowered further. The influx of 45Ca2+ at 4 degrees C was nearly 15 times greater than at 25 degrees C. The addition of calcium chelators or of calcium channel blockers, which have been shown to prevent cold acclimation, inhibited the influx of extracellular 45Ca2+ as well as the expression of cas genes at 4 degrees C. The addition of a calcium ionophore or a calcium channel agonist to nonacclimated cells caused the influx of extracellular 45Ca2+ and induced the expression of cas genes at 25 degrees C. These results suggest that a cold-induced calcium influx plays an essential role in cold acclimation. To further study the role of calcium, we isolated two sequences corresponding to calcium-dependent protein kinases. The transcript level of one of them was markedly upregulated at 4 degrees C. We propose a sequence of signaling events that is likely to occur early during cold acclimation and leads to the expression of cas genes and the development of freezing tolerance.

Expression of mitochondrial genes in the germinating conidia of Neurospora crassa

The germinating asexual spores (conidia) of Neurospora crassa were employed to study steps in the accumulation of transcripts of groups of mitochondrial genes, including those for peptide subunits of cytochrome c oxidase (CO), ATPase (ATP), and apocytochrome b (COB). Physically clustered groups of genes were expressed as cohorts: transcripts of the ATP8-ATP6-mtATP9-CO2 genes were almost undetectable in the dormant spores, and they accumulated rapidly as a group immediately after spore activation. Transcripts of COB and the adjacent CO1 were abundant in the dormant spores, and the dormant and germinating spores contained size forms of the COB transcripts that were not evident in vegetative cells. Polyribosomes were prepared from mitochondrial lysates, and the polyribosomal RNA was probed to identify the mRNAs of specific genes; in several instances polycistronic mRNAs were present in the polyribosomes as were the smaller end-products of the inferred transcript processing pathways. The expression of the physically dispersed genes for subunit peptides of cytochrome c oxidase appears to be regulated to the level of translation; these transcripts are accumulated in the total mitochondrial RNA with sharply different kinetics, but they appeared in the polyribosomes uniformly, their appearance correlating with the uniform synthesis of the subunit peptides. Transcripts for a previously reported non-functional mitochondrial gene, homologous to the functional nuclear gene for ATPase subunit 9, were found in the germinating spores, but were not detected in vegetative cells. These mtATP9 transcripts were also present in the polyribosomes and were apparently translated into a protein in vivo whose synthesis was insensitive to cycloheximide and detectable with an anti-ATP9 subunit antibody. Transcripts for two nuclear genes for mitochondrially localized proteins, ATP9 and CO5, were accumulated in unison and especially rapidly during spore germination.

Cold-Induced Changes in Freezing Tolerance, Protein Phosphorylation, and Gene Expression (Evidence for a Role of Calcium)

The role of Ca2+ in cold-induced changes in protein phosphorylation, gene expression, and development of freezing tolerance has been studied in cell-suspension cultures of a freezing-tolerant cultivar of alfalfa (Medicago sativa spp. falcata cv Anik). Chemical treatments to block Ca2+ channels, antagonize calmodulin action, or inhibit protein kinases markedly inhibited the cellular capacity to develop cold-induced freezing tolerance but had little effect on cell viability. An analysis of phosphoprotein profile by two-dimensional polyacrylamide gel electrophoresis revealed that at low temperature the relative level of phosphorylation of several proteins increased, whereas that of several others decreased. When cold acclimation was carried out in the presence of N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide hydrochloride, an antagonist of calmodulin and Ca2+-dependent protein kinases, or the Ca2+ channel blocker La3+, the cold-induced changes in protein phosphorylation were strongly inhibited, cells lost their capacity to develop freezing tolerance, and accumulation of transcripts of cold acclimation-specific genes was substantially reduced. An inhibitor of protein kinases, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride, had less pronounced effects on the cold-induced protein phosphorylation and caused only a partial inhibition of the cold-induced development of freezing tolerance and accumulation of the transcripts. The level of phosphorylation of one protein, of about 15 kD, increased more than 10-fold at low temperature and showed a strong positive correlation with cold-induced freezing tolerance and gene expression even when the latter were altered with various chemical treatments. These results suggest that Ca2+ and protein phosphorylation, or perhaps a coupling of the two, play an important role during the acquisition of freezing tolerance during cold acclimation.

A new cold-induced alfalfa gene is associated with enhanced hardening at subzero temperature

When alfalfa (Medicago sativa L. cv Apica) plants grown at room temperature are transferred to 2 degrees C, the temperature at which 50% of the plants fail to survive (LT50) decreases from -6 to -14 degrees C during the first 2 weeks but then increases to -9 degrees C during the subsequent 2 weeks. However, when plants are kept for 2 weeks at 2 degrees C and then transferred to -2 degrees C for another two weeks, the LT50 declines to -16 degrees C. These changes in freezing tolerance are paralleled by changes in transcript levels of cas15 (cold acclimation-specific gene encoding a 14.5-kD protein), a cold-induced gene. Cold-activation of cas15 occurs even when protein synthesis is inhibited by more than 90%, suggesting that cold-initiated events up to and including the accumulation of cas15 transcripts depend on preexisting gene products. cas15 shows little homology to any known gene at the nucleotide or amino acid level. The deduced polypeptide (CAS15) of 14.5 kD contains four repeats of a decapeptide motif and possesses a bipartite sequence domain at the carboxy terminus with homology to the reported nuclear-targeting signal sequences. Although the relative amount of cas15 DNA as a fraction of the total genomic DNA is similar in cultivars with different degrees of freezing tolerance, its organization in the genome is different. The possible role of cas15 in the development of cold-induced freezing tolerance is discussed.

Photocontrol of organelle and cell type specific changes in the polypeptide composition of Euglena and sorghum

Two-dimensional (2-D) gel electrophoresis has been used to follow changes in cell type specific and organelle localized polypeptides upon exposure of etiolated sorghum shoots and dark-grown resting Euglena to light. Total protein extracted from isolated bundle sheath strands and mesophyll protoplasts was resolved by 2-D gel electrophoresis. The cell type specific polypeptides were localized on the whole shoot 2-D gel map in order to determine changes in the levels of these polypeptides upon light exposure. An image analyzer was used to analyze fluorographs of 2-D gels of total Euglena protein pulse-labeled with [35S]sulfate in the dark, immediately upon light exposure and 1, 4, 14, 24, 48 and 72 h after light exposure. The subset of polypeptides whose relative rate of synthesis changes more than threefold immediately upon light exposure was identified. The different patterns of changes in the rate of synthesis of this subset of polypeptides were followed.

Photocontrol of chloroplast and mitochondrial polypeptide levels in euglena

Two dimensional polyacrylamide gel electrophoresis resolved protein from intact chloroplasts of wild type Euglena gracilis Klebs var. bacillaris Cori into 185 polypeptides of which 55 were localized on the whole cell polypeptide map. Of these chloroplast polypeptides, the relative amounts of 49 increased, the relative amounts of two decreased, and the relative amounts of four polypeptides were unaltered by exposure of dark grown resting cells to light for 72 hours. Proteins from intact purified mitochondria obtained from a bleached mutant (W(10)BSmL) lacking plastids were resolved into 193 polypeptides of which 44 were localized on the whole cell polypeptide map from wild type cells. Of these mitochondrial polypeptides, the relative amount of one increased, the relative amounts of 12 were unaltered, and the relative amounts of 31 decreased after exposure of the dark grown resting cells to light. Since it is known that the development of the chloroplast in Euglena occurs without a net increase in total cellular protein and without a change in the size of the cellular amino acid pools, the degradation of mitochondrial polypeptides represents a major source of amino acids for the synthesis of chloroplast polypeptides.

Influence of photosynthesis and chlorophyll synthesis on polypeptide accumulation in greening euglena

Two-dimensional gel electrophoresis resolves total cellular protein from Euglena gracilis klebs var bacillaris Cori into 640 polypeptides, 79 of which are induced by light exposure. The inhibition of chloroplast translation by streptomycin, the direct inhibition of photosynthesis as well as the indirect inhibition of chlorophyll synthesis by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and the specific inhibition of photosynthesis but not chlorophyll synthesis by DCMU in the presence of 17 millimolar ethanol failed to inhibit the accumulation of 40 polypeptides. These polypeptides appear to be synthesized on cytoplasmic ribosomes and their accumulation is independent of the developmental status of the chloroplast. Streptomycin but not DCMU completely inhibited the accumulation of six polypeptides which are undetectable in mutants lacking chloroplast DNA suggesting that these polypeptides are translated on chloroplast ribosomes. The accumulation of seven polypeptides which are detectable in mutants lacking chloroplast DNA was also inhibited by streptomycin but not by DCMU suggesting that the accumulation of these polypeptides is dependent upon stabilization by a chloroplast translation product. The accumulation of 12 polypeptides was inhibited by streptomycin and by DCMU under conditions in which chlorophyll synthesis was inhibited, but not under conditions in which chlorophyll synthesis was unaffected by DCMU. The inhibition by DCMU of the accumulation of these polypeptides appears to be due to the inhibition of chlorophyll synthesis suggesting that they are components of pigment protein complexes. The accumulation of six polypeptides was inhibited under all conditions in which photosynthesis was inhibited suggesting that the accumulation of these polypeptides is dependent upon a product of photosynthesis.

Photocontrol of the polypeptide composition ofEuglena : Analysis by two-dimensional gel electrophoresis

Two-dimensional gel electrophoresis resolved total cellular protein fromEuglena gracilis Klebs var.bacillaris Cori into 650 polypeptides detectable by silver staining. Exposure of dark-grown restingEuglena to light for 72 h increased the relative amounts of 79 polypeptides and decreased the relative amounts of 72 polypeptides. Four polypeptides whose level increased upon light exposure were undetectable in the bleached mutant W3BUL. Since W3BUL has lost most if not all of its chloroplast genome, these polypeptides may be coded by the chloroplast genome. Although the majority of the polypeptides studied appear to be coded by the nuclear or mitochondrial genomes, seven polypeptides which were present in W3BUL were not detectable in another chloroplast-DNA-deficient, bleached mutant, W10BSmL. It appears that a number of nonchloroplast genes are no longer expressed in this mutant. Exposure of dark-grown resting cells of the bleached mutant, W3BUL, to light increased the relative amounts of 12 polypeptides and decreased the relative amounts of 14 polypeptides. Since W3BUL lacks detectable protochlorophyll(ide), the chloroplast photoreceptor, the levels of these polypeptides are regulated by light acting through a nonchloroplast photoreceptor. Exposure of cells to light had no detectable effect on the relative amounts of those polypeptides which were present in W10BSmL, even though W10BSmL has a nonchloroplast photoreceptor. EitherEuglena contains multiple nonchloroplast photoreceptors, some of which are absent from W10BSmL, or the nonchloroplast photoreceptor present in W10BSmL is uncoupled from some of the events normally controlled by that photoreceptor.