Localization and function of SulP, a nuclear-encoded chloroplast sulfate permease in Chlamydomonas reinhardtii

A chloroplast sulphate transporter modulates glutathione-mediated redox cycling to regulate cell division

Glutathione redox cycling is important for cell cycle regulation, but its mechanisms are not well understood. We previously identified a small-sized mutant, suppressor of mat3 15-1 (smt15-1) that has elevated cellular glutathione. Here, we demonstrated that SMT15 is a chloroplast sulphate transporter. Reducing expression of γ-GLUTAMYLCYSTEINE SYNTHETASE, encoding the rate-limiting enzyme required for glutathione biosynthesis, corrected the size defect of smt15-1 cells. Overexpressing GLUTATHIONE SYNTHETASE (GSH2) recapitulated the small-size phenotype of smt15-1 mutant, confirming the role of glutathione in cell division. Hence, SMT15 may regulate chloroplast sulphate concentration to modulate cellular glutathione levels. In wild-type cells, glutathione and/or thiol-containing molecules (GSH/thiol) accumulated in the cytosol at the G1 phase and decreased as cells entered the S/M phase. While the cytosolic GSH/thiol levels in the small-sized mutants, smt15-1 and GSH2 overexpressors, mirrored those of wild-type cells (accumulating during G1 and declining at early S/M phase), GSH/thiol was specifically accumulated in the basal bodies at early S/M phase in the small-sized mutants. Therefore, we propose that GSH/thiol-mediated redox signalling in the basal bodies may regulate mitotic division number in Chlamydomonas reinhardtii. Our findings suggest a new mechanism by which glutathione regulates the multiple fission cell cycle in C. reinhardtii.

The relationship between sulfur metabolism and tolerance of hexavalent chromium in Scenedesmus acutus (Spheropleales): Role of ATP sulfurylase

Sulfur availability and the end products of its metabolism, cysteine, glutathione and phytochelatins, play an important role in heavy metal tolerance, chromium included. Sulfate and chromate not only compete for the transporters but also for assimilation enzymes and chromium tolerance in various organisms has been associated to differences in this pathway. We investigated the mechanisms of Cr(VI)-tolerance increase induced by S-starvation focusing on the role of ATP sulfurylase (ATS) in two strains of Scenedesmus acutus with different chromium sensitivity. S-starvation enhances the defence potential by increasing sulfate uptake/assimilation and decreasing chromium uptake, thus suggesting a change in the transport system. We isolated two isoforms of the enzyme, SaATS1 and SaATS2, with different sensitivity to sulfur availability, and analysed them in S-sufficient and S-replete condition both in standard and in chromium supplemented medium. SaATS2 expression is different in the two strains and presumably marks a different sulfur perception/exploitation in the Cr-tolerant. Its induction and silencing are compatible with a role in the transient tolerance increase induced by S-starvation. This enzyme can however hardly be responsible for the large cysteine production of the Cr-tolerant strain after starvation, suggesting that cytosolic rather than chloroplastic cysteine production is differently regulated in the two strains.

Increase of chromium tolerance in Scenedesmus acutus after sulfur starvation: Chromium uptake and compartmentalization in two strains with different sensitivities to Cr(VI)

In photosynthetic organisms sulfate constitutes the main sulfur source for the biosynthesis of GSH and its precursor Cys. Hence, sulfur availability can modulate the capacity to cope with environmental stresses, a phenomenon known as SIR/SED (Sulfur Induced Resistance or Sulfur Enhanced Defence). Since chromate may compete for sulfate transport into the cells, in this study chromium accumulation and tolerance were investigated in relation to sulfur availability in two strains of the unicellular green alga Scenedesmus acutus with different Cr-sensitivities. Paradoxically, sulfur deprivation has been demonstrated to induce a transient increase of Cr-tolerance in both strains. Sulfur deprivation is known to enhance the sulfate uptake/assimilation pathway leading to important consequences on Cr-tolerance: (i) reduced chromate uptake due to the induction of high affinity sulfate transporters (ii) higher production of cysteine and GSH which can play a role both through the formation of unsoluble complexes and their sequestration in inert compartments. To investigate the role of the above mentioned mechanisms, Cr accumulation in total cells and in different cell compartments (cell wall, membranes, soluble and miscellaneous fractions) was analyzed in both sulfur-starved and unstarved cells. Both strains mainly accumulated chromium in the soluble fraction, but the uptake was higher in the wild-type. In this type a short period of sulfur starvation before Cr(VI) treatment lowered chromium accumulation to the level observed in the unstarved Cr-tolerant strain, in which Cr uptake seems instead less influenced by S-starvation, since no significant decrease was observed. The increase in Cr-tolerance following S-starvation seems thus to rely on different mechanisms in the two strains, suggesting the induction of a mechanism constitutively active in the Cr-tolerant strain, maybe a high affinity sulfate transporter also in the wild-type. Changes observed in the cell wall and membrane fractions suggest a strong involvement of these compartments in Cr-tolerance increase following S-starvation.

SULTR3;1 is a chloroplast-localized sulfate transporter in Arabidopsis thaliana

Plants play a prominent role as sulfur reducers in the global sulfur cycle. Sulfate, the major form of inorganic sulfur utilized by plants, is absorbed and transported by specific sulfate transporters into plastids, especially chloroplasts, where it is reduced and assimilated into cysteine before entering other metabolic processes. How sulfate is transported into the chloroplast, however, remains unresolved; no plastid-localized sulfate transporters have been previously identified in higher plants. Here we report that SULTR3;1 is localized in the chloroplast, which was demonstrated by SULTR3;1-GFP localization, Western blot analysis, protein import as well as comparative analysis of sulfate uptake by chloroplasts between knockout mutants, complemented transgenic plants, and the wild type. Loss of SULTR3;1 significantly decreases the sulfate uptake of the chloroplast. Complementation of the sultr3;1 mutant phenotypes by expression of a 35S-SULTR3;1 construct further confirms that SULTR3;1 is one of the transporters responsible for sulfate transport into chloroplasts.

Proteomic analysis of hydrogen photoproduction in sulfur-deprived Chlamydomonas cells

The green alga Chlamydomonas reinhardtii is a model organism to study H(2) metabolism in photosynthetic eukaryotes. To understand the molecular mechanism of H(2) metabolism, we used 2-DE coupled with MALDI-TOF and MALDI-TOF/TOF-MS to investigate proteomic changes of Chlamydomonas cells that undergo sulfur-depleted H(2) photoproduction process. In this report, we obtained 2-D PAGE soluble protein profiles of Chlamydomonas at three time points representing different phases leading to H(2) production. We found over 105 Coomassie-stained protein spots, corresponding to 82 unique gene products, changed in abundance throughout the process. Major changes included photosynthetic machinery, protein biosynthetic apparatus, molecular chaperones, and 20S proteasomal components. A number of proteins related to sulfate, nitrogen and acetate assimilation, and antioxidative reactions were also changed significantly. Other proteins showing alteration during the sulfur-depleted H(2) photoproduction process were proteins involved in cell wall and flagella metabolisms. In addition, among these differentially expressed proteins, 11 were found to be predicted proteins without functional annotation in the Chlamydomonas genome database. The results of this proteomic analysis provide new insight into molecular basis of H(2) photoproduction in Chlamydomonas under sulfur depletion.

Hydrogen photo-evolution upon S deprivation stepwise: an illustration of microalgal photosynthetic and metabolic flexibility and a step stone for future biotechnological methods of renewable H(2) production

The metabolic flexibility of some photosynthetic microalgae enables them to survive periods of anaerobiosis in the light by developing a particular photofermentative metabolism. The latter entails compounds of the photosynthetic electron transfer chain and an oxygen-sensitive hydrogenase in order to reoxidize reducing equivalents and to generate ATP for maintaining basal metabolic function. This pathway results in the photo-evolution of hydrogen gas by the algae. A decade ago, Melis and coworkers managed to reproduce such a condition in a laboratory context by depletion of sulfur in the algal culture media, making the photo-evolution by the algae sustainable for several days (Melis et al. in Plant Physiol 122:127-136, 2000). This observation boosted research in algal H(2) evolution. A feature, which due to its transient nature was long time considered as a curiosity of algal photosynthesis suddenly became a phenomenon with biotechnological potential. Although the Melis procedure has not been developed into a biotechnological process of renewable H(2) generation so far, it has been a useful tool for studying microalgal metabolic and photosynthetic flexibility and a possible step stone for future H(2) production procedures. Ten years later most of the critical steps and limitations of H(2) production by this protocol have been studied from different angles particularly with the model organism Chlamydomonas reinhardtii, by introducing various changes in culture conditions and making use of mutants issued from different screens or by reverse genomic approaches. A synthesis of these observations with the most important conclusions driven from recent studies will be presented in this review.

Characterization of Nda2, a plastoquinone-reducing type II NAD(P)H dehydrogenase in chlamydomonas chloroplasts

Electron transfer pathways associated to oxygenic photosynthesis, including cyclic electron flow around photosystem I and chlororespiration, rely on non-photochemical reduction of plastoquinones (PQs). In higher plant chloroplasts, a bacterial-like NDH complex homologous to complex I is involved in PQ reduction, but such a complex is absent from Chlamydomonas plastids where a type II NAD(P)H dehydrogenase activity has been proposed to operate. With the aim to elucidate the nature of the enzyme-supporting non-photochemical reduction of PQs, one of the type II NAD(P)H dehydrogenases identified in the Chlamydomonas reinhardtii genome (Nda2) was produced as a recombinant protein in Escherichia coli and further characterized. As many type II NAD(P)H dehydrogenases, Nda2 uses NADH as a preferential substrate, but in contrast to the eukaryotic enzymes described so far, contains non-covalently bound FMN as a cofactor. When expressed at a low level, Nda2 complements growth of an E. coli lacking both NDH-1 and NDH-2, but is toxic at high expression levels. Using an antibody raised against the recombinant protein and based on its mass spectrometric identification, we show that Nda2 is localized in thylakoid membranes. Chlorophyll fluorescence measurements performed on thylakoid membranes show that Nda2 is able to interact with thylakoid membranes of C. reinhardtii by reducing PQs from exogenous NADH or NADPH. We discuss the possible involvement of Nda2 in cyclic electron flow around PSI, chlororespiration, and hydrogen production.

The chloroplast sulfate transport system in the green alga Chlamydomonas reinhardtii

The genome of the model unicellular green alga Chlamydomonas reinhardtii contains four distinct genes, SulP, SulP2, Sbp and Sabc, which together are postulated to encode a chloroplast envelope-localized sulfate transporter holocomplex. In this work, evidence is presented that regulation of expression of SulP2, Sbp and Sabc is specifically modulated by sulfur availability to the cells. Induction of transcription and higher steady-state levels of the respective mRNAs are reported under S-deprivation conditions. No such induction could be observed under N or P deprivation conditions. Expression, localization, and complex-association of the Sabc protein was specifically investigated using cellular and chloroplast fractionations, BN-PAGE, SDS-PAGE and Western blot analyses. It is shown that Sabc protein levels in the cells increased under S-deprivation conditions, consistent with the observed induction of Sabc gene transcription. It is further shown that the Sabc protein co-localizes with SulP to the chloroplast envelope. Blue-native PAGE followed by Western blot analysis revealed the presence of an apparent 380 kDa complex in C. reinhardtii, specifically recognized by polyclonal antibodies against SulP and Sabc. These results suggest the presence and function in C. reinhardtii of a Sbp-SulP-SulP2-Sabc chloroplast envelope sulfate transporter holocomplex.

Phosphate and sulfur limitation responses in the chloroplast of Chlamydomonas reinhardtii

Phosphorus (P) and sulfur (S) are two macronutrients that photosynthetic organisms require in relatively large amounts despite their levels in the environment often being limited. Accordingly, to adapt to random changes in macronutrient concentrations, plants and algae must sense and respond in a coordinated fashion. The unicellular green alga Chlamydomonas reinhardti is a widely used model organism for the study of P and S stress responses. Herein, we review the current knowledge of P and S nutrient stress responses, highlighting the roles of P and S key global-regulator proteins in mediating signals that link P and S detection to different chloroplast nutrient stress responses.

A rapid method for chloroplast isolation from the green alga Chlamydomonas reinhardtii

This method has been developed to yield highly purified intact chloroplasts from Chlamydomonas reinhardtii. This procedure involves breaking cell-wall-deficient cells by passage through a narrow-bore syringe needle and purifying the intact chloroplasts by differential centrifugation and Percoll gradient centrifugation. This procedure can be completed in less than 3 h and is capable of generating relatively high yields of chloroplasts that should be useful for researchers studying the biochemistry and cell biology of C. reinhardtii chloroplasts.

Photosynthetic H2 metabolism in Chlamydomonas reinhardtii (unicellular green algae)

Unicellular green algae have the ability to operate in two distinctly different environments (aerobic and anaerobic), and to photosynthetically generate molecular hydrogen (H2). A recently developed metabolic protocol in the green alga Chlamydomonas reinhardtii permitted separation of photosynthetic O2-evolution and carbon accumulation from anaerobic consumption of cellular metabolites and concomitant photosynthetic H2-evolution. The H2 evolution process was induced upon sulfate nutrient deprivation of the cells, which reversibly inhibits photosystem-II and O2-evolution in their chloroplast. In the absence of O2, and in order to generate ATP, green algae resorted to anaerobic photosynthetic metabolism, evolved H2 in the light and consumed endogenous substrate. This study summarizes recent advances on green algal hydrogen metabolism and discusses avenues of research for the further development of this method. Included is the mechanism of a substantial tenfold starch accumulation in the cells, observed promptly upon S-deprivation, and the regulated starch and protein catabolism during the subsequent H2-evolution. Also discussed is the function of a chloroplast envelope-localized sulfate permease, and the photosynthesis-respiration relationship in green algae as potential tools by which to stabilize and enhance H2 metabolism. In addition to potential practical applications of H2, approaches discussed in this work are beginning to address the biochemistry of anaerobic H2 photoproduction, its genes, proteins, regulation, and communication with other metabolic pathways in microalgae. Photosynthetic H2 production by green algae may hold the promise of generating a renewable fuel from nature's most plentiful resources, sunlight and water. The process potentially concerns global warming and the question of energy supply and demand.

Utilization of a chloroplast membrane sulfolipid as a major internal sulfur source for protein synthesis in the early phase of sulfur starvation inChlamydomonas reinhardtii

Information is limited on sulfur (S)-sources inside plant cells for synthesis of the proteins for acclimation to S-starvation. We found that a green alga, Chlamydomonas reinhardtii, when transferred to S-starved conditions, degrades 85% of a chloroplast membrane lipid, sulfoquinovosyl diacylglycerol (SQDG), to redistribute its S to a large part of protein fraction as early as by 6h. Furthermore, the degradation of SQDG preceded that of proteins such as ribulose bisphosphate carboxylase/oxygenase, the candidates of internal S-sources. SQDG was thus demonstrated to yield a major internal S-source for protein synthesis during the early phase of acclimation process to S-starvation.

Chloroplast sulfate transport in green algae--genes, proteins and effects

This review summarizes evidence at the molecular genetic, protein and regulatory levels concerning the existence and function of a putative ABC-type chloroplast envelope-localized sulfate transporter in the model unicellular green alga Chlamydomonas reinhardtii. From the four nuclear genes encoding this sulfate permease holocomplex, two are coding for chloroplast envelope-targeted transmembrane proteins (SulP and SulP2), a chloroplast stroma-targeted ATP-binding protein (Sabc) and a substrate (sulfate)-binding protein (Sbp) that is localized on the cytosolic side of the chloroplast envelope. The sulfate permease holocomplex is postulated to consist of a SulP-SulP2 chloroplast envelope transmembrane heterodimer, flanked by the Sabc and the Sbp proteins on the stroma side and the cytosolic side of the inner envelope, respectively. The mature SulP and SulP2 proteins contain seven transmembrane domains and one or two large hydrophilic loops, which are oriented toward the cytosol. The corresponding prokaryotic-origin genes (SulP and SulP2) probably migrated from the chloroplast to the nuclear genome during the evolution of Chlamydomonas reinhardtii. These genes, or any of its homologues, have not been retained in vascular plants, e.g. Arabidopsis thaliana, although they are encountered in the chloroplast genome of a liverwort (Marchantia polymorpha). The function of the SulP protein was probed in antisense transformants of C. reinhardtii having lower expression levels of the SulP gene. Results showed that cellular sulfate uptake capacity was lowered as a consequence of attenuated SulP gene expression in the cell, directly affecting rates of de novo protein biosynthesis in the chloroplast. The antisense transformants exhibited phenotypes of sulfate-deprived cells, displaying slow rates of light-saturated oxygen evolution, low levels of Rubisco in the chloroplast and low steady-state levels of the Photosystem II D1 reaction center protein. The role of the chloroplast sulfate transport in the uptake and assimilation of sulfate in Chlamydomonas reinhardtii is discussed along with its impact on the repair of Photosystem II from a frequently occurring photo-oxidative damage and H2-evolution related metabolism in this green alga.

Insights into the acclimation of Chlamydomonas reinhardtii to sulfur deprivation

During sulfur deprivation, the photosynthetic green alga Chlamydomonas reinhardtii develops a high-affinity sulfate uptake system and increases the expression of genes encoding proteins involved in sulfur assimilation. Although two regulatory elements, SAC1 and SAC3, have been shown to be required for normal acclimation of C. reinhardtii to sulfur deprivation, a number of other regulatory elements appear to also be involved. The molecular mechanisms by which these regulatory elements function are largely unknown. This manuscript presents our current knowledge of sulfur deprivation responses and the regulation of these responses in C. reinhardtii. In addition, we present preliminary results of a sub-saturation screen for novel sulfur acclimation mutants of C. reinhardtii. A speculative model, incorporating the activities of established regulatory elements with putative novel components of the signal transduction pathway(s) is discussed.

RNA silencing in Chlamydomonas: mechanisms and tools

The generation of a comprehensive EST library and the sequencing of its genome set the stage for reverse genetics approaches in Chlamydomonas reinhardtii. However, these also require tools for the specific downregulation of target gene expression. Consequently, a large number of diverse constructs were developed aimed at reducing target gene expression in Chlamydomonas via the stable expression of antisense or inverted repeat-containing RNA. Double-stranded RNA (dsRNA) generated by the annealing of antisense and sense RNAs or by hairpin formation of an inverted repeat, feeds into the RNA silencing pathway. In this pathway, dsRNA is cleaved into approximately 25-bp small interfering RNAs (siRNAs) by the endonuclease Dicer. One of the two complementary strands of a siRNA is then loaded onto an Argonaute-like protein present as core component within larger complexes. Guided by this single-stranded RNA, the Argonaute-like protein either detects homologous transcripts and cleaves these endonucleolytically, or initiates transcriptional gene silencing. This article summarizes current information derived mainly from the Chlamydomonas genome project on components that are assumed to be involved in RNA silencing mechanisms in Chlamydomonas. Furthermore, all approaches employed in Chlamydomonas to date to downregulate target gene expression by antisense or inverted repeat constructs are reviewed and discussed critically.

Role of SulP, a nuclear-encoded chloroplast sulfate permease, in sulfate transport and H2 evolution in Chlamydomonas reinhardtii

Antisense technology was applied to the green alga Chlamydomonas reinhardtiito probe the function of a novel nuclear gene encoding a chloroplast-envelope localized sulfate permease (SulP; GenBank Accession Numbers AF467891 and AF481828). Analysis showed that antiSulP transformants are impaired in sulfate uptake, a consequence of repression in the SulP gene expression. Antisense antiSulP transformants exhibited a sulfur-deprivation phenotype, strong induction of arylsulfatase activity, and global induction of sulfate assimilation gene expression. In sealed cultures, opposite to the wild-type control, antiSulP strains photo-evolved H2, underlining the notion of sulfate uptake limitation by the chloroplast, a slow-down in the rate of oxygen evolution, establishment of anaerobiosis due to internal respiration and spontaneous expression of the [Fe]-hydrogenase in these strains. It is concluded that antiSulP strains are promising as tools to limit the supply of sulfates to the chloroplast, leading to a down-regulation of H2O-oxidation and O2-evolution activity, to the constitutive expression of the [Fe]-hydrogenase and continuous H2-photoproduction in Chlamydomonas reinhardtii.Thus, antisulPstrains might permit a study of the biochemistry of H2 metabolism in this green alga under constitutive anaerobic oxygenic photosynthesis conditions.

Solute transporters of the plastid envelope membrane

Plastids are metabolically extraordinarily active and versatile organelles that are found in all plant cells with the exception of angiosperm pollen grains. Many of the plastid-localized biochemical pathways depend on precursors from the cytosol and, in turn, many cytosolic pathways depend on the supply of precursor molecules from the plastid stroma. Hence, a massive traffic of metabolites occurs across the permeability barrier between plastids and cytosol that is called the plastid envelope membrane. Many of the known plastid envelope solute transporters have been identified by biochemical purification and peptide sequencing. This approach is of limited use for less abundant proteins and for proteins of plastid subtypes that are difficult to isolate in preparative amounts. Hence, the majority of plastid envelope membrane transporters are not yet identified at the molecular level. The availability of fully sequenced plant genomes, the progress in bioinformatics to predict membrane transporters localized in plastids, and the development of highly sensitive proteomics techniques open new avenues toward identifying additional, to date unknown, plastid envelope membrane transporters.