Regulation of chloroplast protein import through a protochlorophyllide-responsive transit peptide

NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR) is the key enzyme of chlorophyll biosynthesis in angiosperms. In barley, two POR enzymes, termed PORA and PORB, exist. Both are nucleus-encoded plastid proteins that must be imported posttranslationally from the cytosol. Whereas the import of the precursor of PORA, pPORA, previously has been shown to depend on Pchlide, the import of pPORB occurred constitutively. To study this striking difference, chimeric precursor proteins were constructed in which the transit sequences of the pPORA and pPORB were exchanged and fused to either their cognate polypeptides or to a cytosolic dihydrofolate reductase (DHFR) reporter protein of mouse. As shown here, the transit peptide of the pPORA (transA) conferred the Pchlide requirement of import onto both the mature PORB and the DHFR. By contrast, the transit peptide of the pPORB directed the reporter protein into both chloroplasts that contained or lacked translocation-active Pchlide. In vitro binding studies further demonstrated that the transit peptide of the pPORA, but not of the pPORB, is able to bind Pchlide. We conclude that the import of the authentic pPORA and that of the transA-PORB and transA-DHFR fusion proteins is regulated by a direct transit peptide-Pchlide interaction, which is likely to occur in the plastid envelope, a major site of porphyrin biosynthesis.

Chloroplast DNA footprints of postglacial recolonization by oaks

Recolonization of Europe by forest tree species after the last glaciation is well documented in the fossil pollen record. This spread may have been achieved at low densities by rare events of long-distance dispersal, rather than by a compact wave of advance, generating a patchy genetic structure through founder effects. In long-lived oak species, this structure could still be discernible by using maternally transmitted genetic markers. To test this hypothesis, a fine-scale study of chloroplast DNA (cpDNA) variability of two sympatric oak species was carried out in western France. The distributions of six cpDNA length variants were analyzed at 188 localities over a 200 × 300 km area. A cpDNA map was obtained by applying geostatistics methods to the complete data set. Patches of several hundred square kilometers exist which are virtually fixed for a single haplotype for both oak species. This local systematic interspecific sharing of the maternal genome strongly suggests that long-distance seed dispersal events followed by interspecific exchanges were involved at the time of colonization, about 10,000 years ago.

Addition of destabilizing poly(A)-rich sequences to endonuclease cleavage sites during the degradation of chloroplast mRNA

In this work, we report the posttranscriptional addition of poly(A)-rich sequences to mRNA in chloroplasts of higher plants. Several sites in the coding region and the mature end of spinach chloroplast psbA mRNA, which encodes the D1 protein of photosystem II, are detected as polyadenylylated sites. In eukaryotic cells, the addition of multiple adenosine residues to the 3′ end of nuclear RNA plays a key role in generating functional mRNAs and in regulating mRNA degradation. In bacteria, the adenylation of several RNAs greatly accelerates their decay. The poly(A) moiety in the chloroplast, in contrast to that in eukaryotic nuclear encoded and bacterial RNAs, is not a ribohomopolymer of adenosine residues, but clusters of adenosines bounded mostly by guanosines and rarely by cytidines and uridines; it may be as long as several hundred nucleotides. Further analysis of the initial steps of chloroplast psbA mRNA decay revealed specific endonuclease cleavage sites that perfectly matched the sites where poly(A)-rich sequences were added. Our results suggest a mechanism for the degradation of psbA mRNA in which endonucleolytic cleavages are followed by the addition of poly(A)-rich sequences to the upstream cleavage products, which target these RNAs for rapid decay.

Constitutive expression of the cold-regulated Arabidopsis thaliana COR15a gene affects both chloroplast and protoplast freezing tolerance

Cold acclimation in plants is associated with the expression of COR (cold-regulated) genes that encode polypeptides of unknown function. It has been widely speculated that products of these genes might have roles in freezing tolerance. Here we provide direct evidence in support of this hypothesis. We show that constitutive expression of COR15a, a cold-regulated gene of Arabidopsis thaliana that encodes a chloroplast-targeted polypeptide, enhances the in vivo freezing tolerance of chloroplasts in nonacclimated plants by almost 2°C, nearly one-third of the increase that occurs upon cold acclimation of wild-type plants. Significantly, constitutive expression of COR15a also affects the in vitro freezing tolerance of protoplasts. At temperatures between −5 and −8°C, the survival of protoplasts isolated from leaves of nonacclimated transgenic plants expressing COR15a was greater than that of protoplasts isolated from leaves of nonacclimated wild-type plants. At temperatures between −2 and −4°C, constitutive expression of COR15a had a slight negative effect on survival. The implications of these data regarding possible modes of COR15a action are discussed.

Chloroplast small heat shock proteins: Evidence for atypical evolution of an organelle-localized protein

Knowledge of the origin and evolution of gene families is critical to our understanding of the evolution of protein function. To gain a detailed understanding of the evolution of the small heat shock proteins (sHSPs) in plants, we have examined the evolutionary history of the chloroplast (CP)-localized sHSPs. Previously, these nuclear-encoded CP proteins had been identified only from angiosperms. This study reveals the presence of the CP sHSPs in a moss, Funaria hygrometrica. Two clones for CP sHSPs were isolated from a F. hygrometrica heat shock cDNA library that represent two distinct CP sHSP genes. Our analysis of the CP sHSPs reveals unexpected evolutionary relationships and patterns of sequence conservation. Phylogenetic analysis of the CP sHSPs with other plant CP sHSPs and eukaryotic, archaeal, and bacterial sHSPs shows that the CP sHSPs are not closely related to the cyanobacterial sHSPs. Thus, they most likely evolved via gene duplication from a nuclear-encoded cytosolic sHSP and not via gene transfer from the CP endosymbiont. Previous sequence analysis had shown that all angiosperm CP sHSPs possess a methionine-rich region in the N-terminal domain. The primary sequence of this region is not highly conserved in the F. hygrometrica CP sHSPs. This lack of sequence conservation indicates that sometime in land plant evolution, after the divergence of mosses from the common ancestor of angiosperms but before the monocot–dicot divergence, there was a change in the selective constraints acting on the CP sHSPs.

Chloroplast protein and centrosomal genes, a tRNA intron, and odd telomeres in an unusually compact eukaryotic genome, the cryptomonad nucleomorph

Cells of several major algal groups are evolutionary chimeras of two radically different eukaryotic cells. Most of these “cells within cells” lost the nucleus of the former algal endosymbiont. But after hundreds of millions of years cryptomonads still retain the nucleus of their former red algal endosymbiont as a tiny relict organelle, the nucleomorph, which has three minute linear chromosomes, but their function and the nature of their ends have been unclear. We report extensive cryptomonad nucleomorph sequences (68.5 kb), from one end of each of the three chromosomes of Guillardia theta. Telomeres of the nucleomorph chromosomes differ dramatically from those of other eukaryotes, being repeats of the 23-mer sequence (AG)7AAG6A, not a typical hexamer (commonly TTAGGG). The subterminal regions comprising the rRNA cistrons and one protein-coding gene are exactly repeated at all three chromosome ends. Gene density (one per 0.8 kb) is the highest for any cellular genome. None of the 38 protein-coding genes has spliceosomal introns, in marked contrast to the chlorarachniophyte nucleomorph. Most identified nucleomorph genes are for gene expression or protein degradation; histone, tubulin, and putatively centrosomal ranbpm genes are probably important for chromosome segregation. No genes for primary or secondary metabolism have been found. Two of the three tRNA genes have introns, one in a hitherto undescribed location. Intergenic regions are exceptionally short; three genes transcribed by two different RNA polymerases overlap their neighbors. The reported sequences encode two essential chloroplast proteins, FtsZ and rubredoxin, thus explaining why cryptomonad nucleomorphs persist.

GTP bound to chloroplast thylakoid membranes is required for light-induced, multienzyme degradation of the photosystem II D1 protein

Even though light is the driving force in photosynthesis, it also can be harmful to plants. The water-splitting photosystem II is the main target for this light stress, leading to inactivation of photosynthetic electron transport and photooxidative damage to its reaction center. The plant survives through an intricate repair mechanism involving proteolytic degradation and replacement of the photodamaged reaction center D1 protein. Based on experiments with isolated chloroplast thylakoid membranes and photosystem II core complexes, we report several aspects concerning the rapid turnover of the D1 protein. (i) The primary cleavage step is a GTP-dependent process, leading to accumulation of a 23-kDa N-terminal fragment. (ii) Proteolysis of the D1 protein is inhibited below basal levels by nonhydrolyzable GTP analogues and apyrase treatment, indicating the existence of endogenous GTP tightly bound to the thylakoid membrane. This possibility was corroborated by binding studies. (iii) The proteolysis of the 23-kDa primary degradation fragment (but not of the D1 protein) is an ATP- and zinc-dependent process. (iv) D1 protein degradation is a multienzyme event involving a strategic (primary) protease and a cleaning-up (secondary) protease. (v) The chloroplast FtsH protease is likely to be involved in the secondary degradation steps. Apart from its significance for understanding the repair of photoinhibition, the discovery of tightly bound GTP should have general implications for other regulatory reactions and signal transduction pathways associated with the photosynthetic membrane.

A single gene of chloroplast origin codes for mitochondrial and chloroplastic methionyl–tRNA synthetase in Arabidopsis thaliana

One-fifth of the tRNAs used in plant mitochondrial translation is coded for by chloroplast-derived tRNA genes. To understand how aminoacyl–tRNA synthetases have adapted to the presence of these tRNAs in mitochondria, we have cloned an Arabidopsis thaliana cDNA coding for a methionyl–tRNA synthetase. This enzyme was chosen because chloroplast-like elongator tRNAMet genes have been described in several plant species, including A. thaliana. We demonstrate here that the isolated cDNA codes for both the chloroplastic and the mitochondrial methionyl–tRNA synthetase (MetRS). The protein is transported into isolated chloroplasts and mitochondria and is processed to its mature form in both organelles. Transient expression assays using the green fluorescent protein demonstrated that the N-terminal region of the MetRS is sufficient to address the protein to both chloroplasts and mitochondria. Moreover, characterization of MetRS activities from mitochondria and chloroplasts of pea showed that only one MetRS activity exists in each organelle and that both are indistinguishable by their behavior on ion exchange and hydrophobic chromatographies. The high degree of sequence similarity between A. thaliana and Synechocystis MetRS strongly suggests that the A. thaliana MetRS gene described here is of chloroplast origin.

Origin of a chloroplast protein importer

During evolution, chloroplasts have relinquished the majority of their genes to the nucleus. The products of transferred genes are imported into the organelle with the help of an import machinery that is distributed across the inner and outer plastid membranes. The evolutionary origin of this machinery is puzzling because, in the putative predecessors, the cyanobacteria, the outer two membranes, the plasma membrane, and the lipopolysaccharide layer lack a functionally similar protein import system. A 75-kDa protein-conducting channel in the outer envelope of pea chloroplasts, Toc75, shares ≈22% amino acid identity to a similarly sized protein, designated SynToc75, encoded in the Synechocystis PCC6803 genome. Here we show that SynToc75 is located in the outer membrane (lipopolysaccharide layer) of Synechocystis PCC6803 and that SynToc75 forms a voltage-gated, high conductance channel with a high affinity for polyamines and peptides in reconstituted liposomes. These findings suggest that a component of the chloroplast protein import system, Toc75, was recruited from a preexisting channel-forming protein of the cyanobacterial outer membrane. Furthermore, the presence of a protein in the chloroplastic outer envelope homologous to a cyanobacterial protein provides support for the prokaryotic nature of this chloroplastic membrane.

Chlorophyll precursors are signals of chloroplast origin involved in light induction of nuclear heat-shock genes

Coordination between the activities of organelles and the nucleus requires the exchange of signals. Using Chlamydomonas, we provide evidence that plastid-derived chlorophyll precursors may replace light in the induction of two nuclear heat-shock genes (HSP70A and HSP70B) and thus qualify as plastidic signal. Mutants defective in the synthesis of Mg-protoporphyrin IX were no longer inducible by light. Feeding of Mg-protoporphyrin IX or its dimethyl ester to wild-type or mutant cells in the dark resulted in induction. The analysis of HSP70A promoter mutants that do or do not respond to light revealed that these chlorophyll precursors specifically activate the light signaling pathway. Activation of gene expression was not observed when protoporphyrin IX, protochlorophyllide, or chlorophyllide were added. A specific interaction of defined chlorophyll precursors with factor(s) that regulate nuclear gene expression is suggested.

Plant growth in elevated CO2 alters mitochondrial number and chloroplast fine structure

With increasing interest in the effects of elevated atmospheric CO2 on plant growth and the global carbon balance, there is a need for greater understanding of how plants respond to variations in atmospheric partial pressure of CO2. Our research shows that elevated CO2 produces significant fine structural changes in major cellular organelles that appear to be an important component of the metabolic responses of plants to this global change. Nine species (representing seven plant families) in several experimental facilities with different CO2-dosing technologies were examined. Growth in elevated CO2 increased numbers of mitochondria per unit cell area by 1.3–2.4 times the number in control plants grown in lower CO2 and produced a statistically significant increase in the amount of chloroplast stroma (nonappressed) thylakoid membranes compared with those in lower CO2 treatments. There was no observable change in size of the mitochondria. However, in contrast to the CO2 effect on mitochondrial number, elevated CO2 promoted a decrease in the rate of mass-based dark respiration. These changes may reflect a major shift in plant metabolism and energy balance that may help to explain enhanced plant productivity in response to elevated atmospheric CO2 concentrations.

14-3-3 protein is a regulator of the mitochondrial and chloroplast ATP synthase

Mitochondrial and chloroplast ATP synthases are key enzymes in plant metabolism, providing cells with ATP, the universal energy currency. ATP synthases use a transmembrane electrochemical proton gradient to drive synthesis of ATP. The enzyme complexes function as miniature rotary engines, ensuring energy coupling with very high efficiency. Although our understanding of the structure and functioning of the synthase has made enormous progress in recent years, our understanding of regulatory mechanisms is still rather preliminary. Here we report a role for 14-3-3 proteins in the regulation of ATP synthases. These 14-3-3 proteins are highly conserved phosphoserine/phosphothreonine-binding proteins that regulate a wide range of enzymes in plants, animals, and yeast. Recently, the presence of 14-3-3 proteins in chloroplasts was illustrated, and we show here that plant mitochondria harbor 14-3-3s within the inner mitochondrial-membrane compartment. There, the 14-3-3 proteins were found to be associated with the ATP synthases, in a phosphorylation-dependent manner, through direct interaction with the F1 β-subunit. The activity of the ATP synthases in both organelles is drastically reduced by recombinant 14-3-3. The rapid reduction in chloroplast ATPase activity during dark adaptation was prevented by a phosphopeptide containing the 14-3-3 interaction motif, demonstrating a role for endogenous 14-3-3 in the down-regulation of the CFoF1 activity. We conclude that regulation of the ATP synthases by 14-3-3 represents a mechanism for plant adaptation to environmental changes such as light/dark transitions, anoxia in roots, and fluctuations in nutrient supply.

A single alteration 20 nt 5′ to an editing target inhibits chloroplast RNA editing in vivo

Transcripts of typical dicot plant plastid genes undergo C→U RNA editing at approximately 30 locations, but there is no consensus sequence surrounding the C targets of editing. The cis-acting elements required for editing of the C located at tobacco rpoB editing site II were investigated by introducing translatable chimeric minigenes containing sequence –20 to +6 surrounding the C target of editing. When the –20 to +6 sequence specified by the homologous region present in the black pine chloroplast genome was incorporated, virtually no editing of the transcripts occurred in transgenic tobacco plastids. Nucleotides that differ between the black pine and tobacco sequence were tested for their role in C→U editing by designing chimeric genes containing one or more of these divergent nucleotides. Surprisingly, the divergent nucleotide that had the strongest negative effect on editing of the minigene transcript was located –20 nt 5′ to the C target of editing. Expression of transgene transcripts carrying the 27 nt sequence did not affect the editing extent of the endogenous rpoB transcripts, even though the chimeric transcripts were much more abundant than those of the endogenous gene. In plants carrying a 93 nt rpoB editing site sequence, transgene transcripts accumulated to a level three times greater than transgene transcripts in the plants carrying the 27 nt rpoB editing sites and resulted in editing of the endogenous transcripts from 100 to 50%. Both a lower affinity of the 27 nt site for a trans-acting factor and lower abundance of the transcript could explain why expression of minigene transcripts containing the 27 nt sequence did not affect endogenous editing.

Chloroplast-Targeted ERD1 Protein Declines but Its mRNA Increases during Senescence in Arabidopsis1

Arabidopsis ERD1 is a ClpC-like protein that sequence analysis suggests may interact with the chloroplast-localized ClpP protease to facilitate proteolysis. The mRNA encoded by the ERD1 gene has previously been shown to accumulate in response to senescence and to a variety of stresses and hormones. Here we show that the ERD1 protein, in contrast to the ERD1 mRNA, strongly declines in abundance with age, becoming undetectable in fully expanded leaves. Sequence analysis also suggests that ERD1 is chloroplast targeted, and we show in an in vitro system that the native protein is properly imported, processed, and present within the soluble fraction of the chloroplast, presumably the stroma. We show that ClpP protein, which is also present in the stroma, declines with age in parallel with ERD1. These results are consistent with the interaction of ERD1 and ClpP, but they suggest that it is unlikely that either plays a major role during senescence. Certain other chloroplast proteins decline with age coordinately with ERD1 and ClpP, suggesting that these declines are markers of an early age-mediated change that occurs within the chloroplast.

Identification of a Ca2+/H+ Antiport in the Plant Chloroplast Thylakoid Membrane1

To assess the availability of Ca2+ in the lumen of the thylakoid membrane that is required to support the assembly of the oxygen-evolving complex of photosystem II, we have investigated the mechanism of 45Ca2+ transport into the lumen of pea (Pisum sativum) thylakoid membranes using silicone-oil centrifugation. Trans-thylakoid Ca2+ transport is dependent on light or, in the dark, on exogenously added ATP. Both light and ATP hydrolysis are coupled to Ca2+ transport through the formation of a transthylakoid pH gradient. The H+-transporting ionophores nigericin/K+ and carbonyl cyanide 3-chlorophenylhydrazone inhibit the transport of Ca2+. Thylakoid membranes are capable of accumulating up to 30 nmol Ca2+ mg−1 chlorophyll from external concentrations of 15 μm over the course of a 15-min reaction. These results are consistent with the presence of an active Ca2+/H+ antiport in the thylakoid membrane. Ca2+ transport across the thylakoid membrane has significant implications for chloroplast and plant Ca2+ homeostasis. We propose a model of chloroplast Ca2+ regulation whereby the activity of the Ca2+/H+ antiporter facilitates the light-dependent uptake of Ca2+ by chloroplasts and reduces stromal Ca2+ levels.