Binding of cysteine synthase to the STAS domain of sulfate transporter and its regulatory consequences

SLC26 Anion Transporters

Solute carrier family 26 (SLC26) is a family of functionally diverse anion transporters found in all kingdoms of life. Anions transported by SLC26 proteins include chloride, bicarbonate, and sulfate, but also small organic dicarboxylates such as fumarate and oxalate. The human genome encodes ten functional homologs, several of which are causally associated with severe human diseases, highlighting their physiological importance. Here, we review novel insights into the structure and function of SLC26 proteins and summarize the physiological relevance of human members.

Genome-Wide Characterization of the Sulfate Transporter Gene Family in Oilseed Crops: Camelina sativa and Brassica napus

Sulfate transporters (SULTRs) are responsible for the uptake of sulfate (SO₄^2-) ions in the rhizosphere by roots and their distribution to plant organs. In this study, SULTR family members in the genomes of two oilseed crops (Camelina sativa and Brassica napus) were identified and characterized based on their sequence structures, duplication events, phylogenetic relationships, phosphorylation sites, and expression levels. In total, 36 and 45 putative SULTR genes were recognized in the genomes of C. sativa and B. napus, respectively. SULTR proteins were predicted to be basophilic proteins with low hydrophilicity in both studied species. According to the observed phylogenetic relationships, we divided the SULTRs into five groups, out of which the SULTR 3 group showed the highest variation. Additionally, several duplication events were observed between the SULTRs. The first duplication event occurred approximately five million years ago between three SULTR 3.1 genes in C. sativa. Furthermore, two subunits were identified in the 3D structures of the SULTRs, which demonstrated that the active binding sites differed between C. sativa and B. napus. According to the available RNA-seq data, the SULTRs showed diverse expression levels in tissues and diverse responses to stimuli. SULTR 3 was expressed in all tissues. SULTR 3.1 was more upregulated in response to abiotic stresses in C. sativa, while SULTR 3.3 and SULTR 2.1 were upregulated in B. napus. Furthermore, SULTR 3 and SULTR 4.1 were upregulated in response to biotic stresses in B. napus. Additionally, the qPCR data showed that the SULTRs in C. sativa were involved in the plant's response to salinity. Based on the distribution of cis-regulatory elements in the promoter region, we speculated that SULTRs might be controlled by phytohormones, such as ABA and MeJA. Therefore, it seems likely that SULTR genes in C. sativa have been more heavily influenced by evolutionary processes and have acquired further diversity. The results reveal new insights of the structures and functions of SULTRs in oilseed crops. However, further analyses, related to functional studies, are needed to uncover the role of SULTRs in the plants' development and growth processes, as well as in their response to stimuli.

Insights into the Sulfate Transporter Gene Family and Its Expression Patterns in Durum Wheat Seedlings under Salinity

Sulfate transporters (SULTRs) are an essential plant transporter class responsible for the absorption and distribution of sulfur, an essential plant growth element. SULTRs are also involved in processes related to growth and development and in response to environmental stimuli. In the present study, 22 TdSULTR family members were identified and characterized in the genome of Triticum turgidum L. ssp. durum (Desf.) using available bioinformatics tools. The expression levels of candidate TdSULTR genes were investigated under salt treatments of 150 and 250 mM NaCl after several different exposure times. TdSULTRs showed diversity in terms of physiochemical properties, gene structure, and pocket sites. TdSULTRs and their orthologues were classified into the known five main plant groups of highly diverse subfamilies. In addition, it was noted that segmental duplication events could lengthen TdSULTR family members under evolutionary processes. Based on pocket site analysis, the amino acids leucine (L), valine (V), and serine (S) were most often detected in TdSULTR protein binding sites. Moreover, it was predicted that TdSULTRs have a high potential to be targeted by phosphorylation modifications. According to promoter site analysis, the plant bioregulators ABA and MeJA were predicted to affect TdSULTR expression patterns. Real-time PCR analysis revealed TdSULTR genes are differentially expressed at 150 mM NaCl but show similar expression in response to 250 mM NaCl. TdSULTR reached a maximum level of expression 72 h after the 250 mM salt treatment. Overall, we conclude that TdSULTR genes are involved in the response to salinity in durum wheat. However, additional studies of functionality are needed to determine their precise function and linked-interaction pathways.

Sulfur signaling and starvation response in Arabidopsis

As sessile organisms, plants have developed sophisticated mechanism to sense and utilize nutrients from the environment, and modulate their growth and development according to the nutrient availability. Research in the past two decades revealed that nutrient assimilation is not occurring spontaneously, but nutrient signaling networks are complexly regulated and integrate sensing and signaling, gene expression, and metabolism to ensure homeostasis and coordination with plant energy conversion and other processes. Here, we review the importance of the macronutrient sulfur (S) and compare the knowledge of S signaling with other important macronutrients, such as nitrogen (N) and phosphorus (P). We focus on key advances in understanding sulfur sensing and signaling, uptake and assimilation, and we provide new analysis of published literature, to identify core genes regulated by the key transcriptional factor in S starvation response, SLIM1/EIL3, and compare the impact on other nutrient deficiency and stresses on S-related genes.

Posttranslational regulation of transporters important for symbiotic interactions

Coordinated sharing of nutritional resources is a central feature of symbiotic interactions, and, despite the importance of this topic, many questions remain concerning the identification, activity, and regulation of transporter proteins involved. Recent progress in obtaining genome and transcriptome sequences for symbiotic organisms provides a wealth of information on plant, fungal, and bacterial transporters that can be applied to these questions. In this update, we focus on legume-rhizobia and mycorrhizal symbioses and how transporters at the symbiotic interfaces can be regulated at the protein level. We point out areas where more research is needed and ways that an understanding of transporter mechanism and energetics can focus hypotheses. Protein phosphorylation is a predominant mechanism of posttranslational regulation of transporters in general and at the symbiotic interface specifically. Other mechanisms of transporter regulation, such as protein-protein interaction, including transporter multimerization, polar localization, and regulation by pH and membrane potential are also important at the symbiotic interface. Most of the transporters that function in the symbiotic interface are members of transporter families; we bring in relevant information on posttranslational regulation within transporter families to help generate hypotheses for transporter regulation at the symbiotic interface.

Role of Sulfate Transporters in Chromium Tolerance in Scenedesmus acutus M. (Sphaeropleales)

Sulfur (S) is essential for the synthesis of important defense compounds and in the scavenging potential of oxidative stress, conferring increased capacity to cope with biotic and abiotic stresses. Chromate can induce a sort of S-starvation by competing for uptake with SO₄²⁻ and causing a depletion of cellular reduced compounds, thus emphasizing the role of S-transporters in heavy-metal tolerance. In this work we analyzed the sulfate transporter system in the freshwater green algae Scenedesmus acutus, that proved to possess both H⁺/SO₄²⁻ (SULTRs) and Na⁺/SO₄²⁻ (SLTs) plasma membrane sulfate transporters and a chloroplast-envelope localized ABC-type holocomplex. We discuss the sulfate uptake system of S. acutus in comparison with other taxa, enlightening differences among the clade Sphaeropleales and Volvocales/Chlamydomonadales. To define the role of S transporters in chromium tolerance, we analyzed the expression of SULTRs and SULPs components of the chloroplast ABC transporter in two strains of S. acutus with different Cr(VI) sensitivity. Their differential expression in response to Cr(VI) exposure and S availability seems directly linked to Cr(VI) tolerance, confirming the role of sulfate uptake/assimilation pathways in the metal stress response. The SULTRs up-regulation, observed in both strains after S-starvation, may directly contribute to enhancing Cr-tolerance by limiting Cr(VI) uptake and increasing sulfur availability for the synthesis of sulfur-containing defense molecules.

Genome-Wide Identification of Sultr Genes in Malus domestica and Low Sulfur-Induced MhSultr3;1a to Increase Cysteine-Improving Growth

Sulfur is an essential nutrient for plant growth and development. Sulfate transporters (Sultrs) are critical for sulfate (SO 4 2 - ) uptake from the soil by the roots in higher plants. However, knowledge about Sultrs in apples (Malus domestica) is scarce. Here, nine putative MdSultrs were identified and classified into two groups according to the their phylogenetic relationships, gene structures, and conserved motifs. Various cis-regulatory elements related to abiotic stress and plant hormone responsiveness were found in the promoter regions of MdSultrs. These MdSultrs exhibited tissue-specific expression patterns and responded to low sulfur (S), abscisic acid (ABA), indole-3-acetic acid (IAA), and methyl jasmonate (MeJA), wherein MdSultr3;1a was especially expressed in the roots and induced by low S. The uptake ofSO 4 2 - in cultivated apples depends on the roots of its rootstock, and MhSultr3;1a was isolated from Malus hupehensis roots used as a rootstock. MhSultr3;1a shared 99.85% homology with MdSultr3;1a and localized on the plasma membrane and nucleus membrane. Further function characterization revealed that MhSultr3;1a complemented anSO 4 2 - transport-deficient yeast mutant and improved the growth of yeast and apple calli under low S conditions. The MhSultr3;1a-overexpressing apple calli had a higher fresh weight compared with the wild type (WT) under a low-S treatment because of the increasedSO 4 2 - and cysteine (Cys) content. These results demonstrate that MhSultr3;1a may increase the content ofSO 4 2 - and Cys to meet the demands of S-containing compounds and improve their growth under S-limiting conditions.

Micrografting Provides Evidence for Systemic Regulation of Sulfur Metabolism between Shoot and Root

The uptake of sulfate by roots and its reductive assimilation mainly in the leaves are not only essential for plant growth and development but also for defense responses against biotic and abiotic stresses. The latter functions result in stimulus-induced fluctuations of sulfur demand at the cellular level. However, the maintenance and acclimation of sulfur homeostasis at local and systemic levels is not fully understood. Previous research mostly focused on signaling in response to external sulfate supply to roots. Here we apply micrografting of Arabidopsis wildtype knock-down sir1-1 mutant plants that suffer from an internally lowered reductive sulfur assimilation and a concomitant slow growth phenotype. Homografts of wildtype and sir1-1 confirm the hallmarks of non-grafted sir1-1 mutants, displaying substantial induction of sulfate transporter genes in roots and sulfate accumulation in shoots. Heterografts of wildtype scions and sir1-1 rootstocks and vice versa, respectively, demonstrate a dominant role of the shoot over the root with respect to sulfur-related gene expression, sulfate accumulation and organic sulfur metabolites, including the regulatory compound O-acetylserine. The results provide evidence for demand-driven control of the shoot over the sulfate uptake system of roots under sulfur-sufficient conditions, allowing sulfur uptake and transport to the shoot for dynamic responses.

Sulfate transport mutants affect hydrogen sulfide and sulfite production during alcoholic fermentation

Hydrogen sulfide is a common wine fault, with a rotten-egg odour, which is directly related to yeast metabolism in response to nitrogen and sulfur availability. In grape juice, sulfate is the most abundant inorganic sulfur compound, which is taken up by yeast through two high-affinity sulfate transporters, Sul1p and Sul2p, and a low affinity transporter, Soa1p. Sulfate contributes to H₂ S production under nitrogen limitation, by being reduced via the Sulfur Assimilation Pathway (SAP). Therefore, yeast strains with limited H₂ S are highly desirable. We report on the use of toxic analogues of sulfate following ethyl methane sulfate treatment, to isolate six wine yeast mutants that produce no or reduced H₂ S and SO₂ during fermentation in synthetic and natural juice. Four amino acid substitutions (A99V, G380R, N588K and E856K) in Sul1p were found in all strains except D25-1 which had heterozygous alleles. Two changes were also identified in Sul2p (L268S and A470T). The Sul1p (G380R) and Sul2p (A470T) mutations were chosen for further investigation as these residues are conserved amongst SLC26 membrane proteins (including sulfate permeases). The mutations were introduced into EC1118 using Crispr cas9 technology and shown to reduce accumulation of H₂ S and do not result in increased SO₂ production during fermentation of model medium (chemically defined grape juice) or Riesling juice. The Sul1p (G380R) and Sul2p (A470T) mutations are newly reported as causal mutations. Our findings contribute to knowledge of the genetic basis of H₂ S production as well as the potential use of these strains for winemaking and in yeast breeding programmes.

Hydrogen sulfide (H2S) signaling in plant development and stress responses

ABSTRACT

Hydrogen sulfide (H₂S) was initially recognized as a toxic gas and its biological functions in mammalian cells have been gradually discovered during the past decades. In the latest decade, numerous studies have revealed that H₂S has versatile functions in plants as well. In this review, we summarize H₂S-mediated sulfur metabolic pathways, as well as the progress in the recognition of its biological functions in plant growth and development, particularly its physiological functions in biotic and abiotic stress responses. Besides direct chemical reactions, nitric oxide (NO) and hydrogen peroxide (H₂O₂) have complex relationships with H₂S in plant signaling, both of which mediate protein post-translational modification (PTM) to attack the cysteine residues. We also discuss recent progress in the research on the three types of PTMs and their biological functions in plants. Finally, we propose the relevant issues that need to be addressed in the future research.

GRAPHIC ABSTRACT

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s42994-021-00035-4.

Coordinating Sulfur Pools under Sulfate Deprivation

Plants display manifold metabolic changes on sulfate deficiency (S deficiency) with all sulfur-containing pools of primary and secondary metabolism affected. O-Acetylserine (OAS), whose levels are rapidly altered on S deficiency, is correlated tightly with novel regulators of plant sulfur metabolism that have key roles in balancing plant sulfur pools, including the Sulfur Deficiency Induced genes (SDI1 and SDI2), More Sulfur Accumulation1 (MSA1), and GGCT2;1. Despite the importance of OAS in the coordination of S pools under stress, mechanisms of OAS perception and signaling have remained elusive. Here, we put particular focus on the general OAS-responsive genes but also elaborate on the specific roles of SDI1 and SDI2 genes, which downregulate the glucosinolate (GSL) pool size. We also highlight the key open questions in sulfur partitioning.

Cysteine and Methionine Biosynthetic Enzymes Have Distinct Effects on Seed Nutritional Quality and on Molecular Phenotypes Associated With Accumulation of a Methionine-Rich Seed Storage Protein in Rice

Staple crops in human and livestock diets suffer from deficiencies in certain "essential" amino acids including methionine. With the goal of increasing methionine in rice seed, we generated a pair of "Push × Pull" double transgenic lines, each containing a methionine-dense seed storage protein (2S albumin from sunflower, HaSSA) and an exogenous enzyme for either methionine (feedback desensitized cystathionine gamma synthase from Arabidopsis, AtD-CGS) or cysteine (serine acetyltransferase from E. coli, EcSAT) biosynthesis. In both double transgenic lines, the total seed methionine content was approximately 50% higher than in their untransformed parental line, Oryza sativa ssp. japonica cv. Taipei 309. HaSSA-containing rice seeds were reported to display an altered seed protein profile, speculatively due to insufficient sulfur amino acid content. However, here we present data suggesting that this may result from an overloaded protein folding machinery in the endoplasmic reticulum rather than primarily from redistribution of limited methionine from endogenous seed proteins to HaSSA. We hypothesize that HaSSA-associated endoplasmic reticulum stress results in redox perturbations that negatively impact sulfate reduction to cysteine, and we speculate that this is mitigated by EcSAT-associated increased sulfur import into the seed, which facilitates additional synthesis of cysteine and glutathione. The data presented here reveal challenges associated with increasing the methionine content in rice seed, including what may be relatively low protein folding capacity in the endoplasmic reticulum and an insufficient pool of sulfate available for additional cysteine and methionine synthesis. We propose that future approaches to further improve the methionine content in rice should focus on increasing seed sulfur loading and avoiding the accumulation of unfolded proteins in the endoplasmic reticulum. Oryza sativa ssp. japonica: urn:lsid:ipni.org:names:60471378-2.

Plant Sulfate Transporters in the Low Phytic Acid Network: Some Educated Guesses

A few new papers report that mutations in some genes belonging to the group 3 of plant sulfate transporter family result in low phytic acid phenotypes, drawing novel strategies and approaches for engineering the low-phytate trait in cereal grains. Here, we shortly review the current knowledge on phosphorus/sulfur interplay and sulfate transport regulation in plants, to critically discuss some hypotheses that could help in unveiling the physiological links between sulfate transport and phosphorus accumulation in seeds.

Structural mechanism of the active bicarbonate transporter from cyanobacteria

Bicarbonate transporters play essential roles in pH homeostasis in mammals and photosynthesis in aquatic photoautotrophs. A number of bicarbonate transporters have been characterized, among which is BicA-a low-affinity, high-flux SLC26-family bicarbonate transporter involved in cyanobacterial CO₂-concentrating mechanisms (CCMs) that accumulate CO₂ and improve photosynthetic carbon fixation. Here, we report the three-dimensional structure of BicA from Synechocystis sp. PCC6803. Crystal structures of the transmembrane domain (BicA^TM) and the cytoplasmic STAS domain (BicA^STAS) of BicA were solved. BicA^TM was captured in an inward-facing HCO₃^--bound conformation and adopts a '7+7' fold monomer. HCO₃^- binds to a cytoplasm-facing hydrophilic pocket within the membrane. BicA^STAS is assembled as a compact homodimer structure and is required for the dimerization of BicA. The dimeric structure of BicA was further analysed using cryo-electron microscopy and physiological analysis of the full-length BicA, and may represent the physiological unit of SLC26-family transporters. Comparing the BicA^TM structure with the outward-facing transmembrane domain structures of other bicarbonate transporters suggests an elevator transport mechanism that is applicable to the SLC26/4 family of sodium-dependent bicarbonate transporters. This study advances our knowledge of the structures and functions of cyanobacterial bicarbonate transporters, and will inform strategies for bioengineering functional BicA in heterologous organisms to increase assimilation of CO₂.

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.

Sulfate transport systems in plants: functional diversity and molecular mechanisms underlying regulatory coordination

Sulfate transporters are integral membrane proteins controlling the flux of sulfate (SO42-) entering the cells and subcellular compartments across the membrane lipid bilayers. Sulfate uptake is a dynamic biological process that occurs in multiple cell layers and organs in plants. In vascular plants, sulfate ions are taken up from the soil environment to the outermost cell layers of roots and horizontally transferred to the vascular tissues for further distribution to distant organs. The amount of sulfate ions being metabolized in the cytosol and chloroplast/plastid or temporarily stored in the vacuole depends on expression levels and functionalities of sulfate transporters bound specifically to the plasma membrane, chloroplast/plastid envelopes, and tonoplast membrane. The entire system for sulfate homeostasis, therefore, requires different types of sulfate transporters to be expressed and coordinately regulated in specific organs, cell types, and subcellular compartments. Transcriptional and post-transcriptional regulatory mechanisms control the expression levels and functions of sulfate transporters to optimize sulfate uptake and internal distribution in response to sulfate availability and demands for synthesis of organic sulfur metabolites. This review article provides an overview of sulfate transport systems and discusses their regulatory aspects investigated in the model plant species Arabidopsis thaliana.

Transcriptome-wide comparison of selenium hyperaccumulator and nonaccumulator Stanleya species provides new insight into key processes mediating the hyperaccumulation syndrome

To obtain better insight into the mechanisms of selenium hyperaccumulation in Stanleya pinnata, transcriptome-wide differences in root and shoot gene expression levels were investigated in S. pinnata and related nonaccumulator Stanleya elata grown with or without 20 μm selenate. Genes predicted to be involved in sulphate/selenate transport and assimilation or in oxidative stress resistance (glutathione-related genes and peroxidases) were among the most differentially expressed between species; many showed constitutively elevated expression in S. pinnata. A number of defence-related genes predicted to mediate synthesis and signalling of defence hormones jasmonic acid (JA, reported to induce sulphur assimilatory and glutathione biosynthesis genes), salicylic acid (SA) and ethylene were also more expressed in S. pinnata than S. elata. Several upstream signalling genes that up-regulate defence hormone synthesis showed higher expression in S. pinnata than S. elata and might trigger these selenium-mediated defence responses. Thus, selenium hyperaccumulation and hypertolerance in S. pinnata may be mediated by constitutive, up-regulated JA, SA and ethylene-mediated defence systems, associated with elevated expression of genes involved in sulphate/selenate uptake and assimilation or in antioxidant activity. Genes pinpointed in this study may be targets of genetic engineering of plants that may be employed in biofortification or phytoremediation.

The E. coli dicarboxylic acid transporters DauA act as a signal transducer by interacting with the DctA uptake system

The Slc26A/SulP family of ions transporter is ubiquitous and widpsread in all kingdon of life. In E. coli, we have demonstrated that the Slc26 protein DauA is a C₄-dicarboxilic acids (C₄-diC) transporter active at acidic pH. The main C₄-diC transporter active at pH7 is DctA and is induced by C₄-diC via the DcuS/R two component system. DctA interacts with DcuS, the membrane embedded histidine kinase, to transfers DcuS to the responsive state, i.e. in the absence of DctA, DcuS is permanently "on", but its activity is C₄-diC-dependent when in complex with DctA. Using phenotypic characterization, transport assays and protein expression studies, we show that at pH7 full DctA production depends on the presence of DauA. A Bacterial Two Hybrid system indicates that DauA and the sensor complex DctA/DcuS physically interact at the membrane. Pull down experiments completed by co-purification study prove that DauA and DctA interact physically at the membrane. These data open a completely new aspect of the C₄-diC metabolism in E. coli and reveals how the bacterial Slc26A uptake systems participate in multiple cellular functions. This constitutes a new example of a bacterial transporter that acts as a processor in a transduction pathway.

Genetic, metabolomic and transcriptomic analyses of the de novo L-cysteine biosynthetic pathway in the enteric protozoan parasite Entamoeba histolytica

The de novo L-cysteine biosynthetic pathway is critical for the growth, antioxidative stress defenses, and pathogenesis of bacterial and protozoan pathogens, such as Salmonella typhimurium and Entamoeba histolytica. This pathway involves two key enzymes, serine acetyltransferase (SAT) and cysteine synthase (CS), which are absent in mammals and therefore represent rational drug targets. The human parasite E. histolytica possesses three SAT and CS isozymes; however, the specific roles of individual isoforms and significance of such apparent redundancy remains unclear. In the present study, we generated E. histolytica cell lines in which CS and SAT expression was knocked down by transcriptional gene silencing. The strain in which CS1, 2 and 3 were simultaneously silenced and the SAT3 gene-silenced strain showed impaired growth when cultured in a cysteine lacking BI-S-33 medium, whereas silencing of SAT1 and SAT2 had no effects on growth. Combined transcriptomic and metabolomic analyses revealed that, CS and SAT3 are involved in S-methylcysteine/cysteine synthesis. Furthermore, silencing of the CS1-3 or SAT3 caused upregulation of various iron-sulfur flavoprotein genes. Taken together, these results provide the first direct evidence of the biological importance of SAT3 and CS isoforms in E. histolytica and justify the exploitation of these enzymes as potential drug targets.

Metabolic changes sustain the plant life in low-sulfur environments

Plants assimilate inorganic sulfate into various organic sulfur (S) compounds, which contributes to the global sulfur cycle in the environment as well as the nutritional supply of this essential element to animals. Plants, to sustain their lives, adapt the flow of their S metabolism to respond to external S status by activating S assimilation and catabolism of stored S compounds, and by repressing the synthesis of secondary S metabolites like glucosinolates. The molecular mechanism of this response has been gradually revealed, including the discovery of several regulatory proteins and enzymes involved in S deficiency responses. Recent progress in this research area and the remaining issues are reviewed here.

Multilevel Regulation of Abiotic Stress Responses in Plants

The sessile lifestyle of plants requires them to cope with stresses in situ. Plants overcome abiotic stresses by altering structure/morphology, and in some extreme conditions, by compressing the life cycle to survive the stresses in the form of seeds. Genetic and molecular studies have uncovered complex regulatory processes that coordinate stress adaptation and tolerance in plants, which are integrated at various levels. Investigating natural variation in stress responses has provided important insights into the evolutionary processes that shape the integrated regulation of adaptation and tolerance. This review primarily focuses on the current understanding of how transcriptional, post-transcriptional, post-translational, and epigenetic processes along with genetic variation orchestrate stress responses in plants. We also discuss the current and future development of computational tools to identify biologically meaningful factors from high dimensional, genome-scale data and construct the signaling networks consisting of these components.

Human SLC26A4/Pendrin STAS domain is a nucleotide-binding protein: Refolding and characterization for structural studies

Mutations in the human SLC26A4/Pendrin polypeptide (hPDS) cause Pendred Syndrome /DFNB4, syndromic deafness with enlargement of the vestibular aqueduct and low-penetrance goiter. Here we present data on cloning, protein overexpression and purification, refolding, and biophysical characterization of the recombinant hPDS STAS domain lacking its intrinsic variable sequence (STAS-ΔIVS). We report a reproducible protein refolding protocol enabling milligram scale expression and purification of uniformly ¹⁵N- and ¹³C^/15N-enriched hPDS STAS-ΔIVS domain suitable for structural characterization by solution NMR. Circular dichroism, one-dimensional ¹H, two-dimensional ¹H–¹⁵N HSQC, and ¹H–¹³C HSQC NMR spectra confirmed the well-folded state of purified hPDS STAS-ΔIVS in solution. Heteronuclear NMR chemical shift perturbation of select STAS-ΔIVS residues by GDP was observed at fast-to-intermediate NMR time scales. Intrinsic tryptophan fluorescence quench experiments demonstrated GDP binding to hPDS STAS-ΔIVS with K_d of 178 μM. These results are useful for structure/function characterization of hPDS STAS, the cytoplasmic subdomain of the congenital deafness protein, pendrin, as well as for studies of other mammalian STAS domains.

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Reproducible protein refolding protocol for human pendrin STAS domain.

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Milligram scale purification of uniformly ¹⁵N- and ¹³C^/15N-enriched protein.

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Protein adopts folded conformation in solution.

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Heteronuclear NMR and fluorescence demonstrate protein binding to GDP.

Identification and functional characterization of the sulfate transporter gene GmSULTR1;2b in soybean

BACKGROUND

Soybean is a major source of oil and protein in the human diet and in animal feed. However, as soybean is deficient in sulfur-containing amino acids, its nutritional value is limited. Increasing sulfate assimilation and utilization efficiency is a valuable approach to augment the concentration of sulfur-containing amino acids in soybean seeds, and sulfate transporters play important roles in both sulfate uptake and translocation within plants.

RESULTS

In this study, we isolated and characterized a soybean sulfate transporter gene: GmSULTR1;2b. The gene was found to be specifically expressed in root tissues and induced by low-sulfur stress. In addition, GmSULTR1;2b expression in yeast could complement deficiency in the sulfate transporter genes SUL1 and SUL2. Under +S conditions, GmSULTR1;2b-overexpressing tobacco plants accumulated higher levels of organic matter and exhibited enhanced biomass and seed weight compared to control plants. Under -S conditions, acclimation of GmSULTR1;2b-overexpressing plants was much better than control plants. GmSULTR1;2b-overexpressing tobacco seedlings showed better tolerance to drought stress than the control plants, but no significant difference was observed under salt stress. Transcriptome analysis revealed 515 genes with at least a 2-fold change in expression in the leaves of tobacco plants overexpressing GmSULTR1;2b. Of these, 227 gene annotations were classified into 12 functional categories associated with 123 relevant pathways, including biosynthesis and metabolism-related proteins, stress-related proteins, and transporters.

CONCLUSIONS

The findings reported here indicate that the increased biomass and seed yield observed in transgenic tobacco plants could have resulted from greater nutrient uptake and transport capability as well as enhanced development and accumulation of organic matter. Taken together, our results indicate that GmSULTR1;2b plays an important role in sulfur uptake and could alter the sulfur status of plants. Our study suggests that overexpressing GmSULTR1;2b may enhance plant yield under +S conditions, reduce plant production loss under -S conditions, and improve plant tolerance to sulfur deficiency stress.

Compartmentalization and Regulation of Sulfate Assimilation Pathways in Plants

Plants utilize sulfate to synthesize primary and secondary sulfur-containing metabolites required for growth and survival in the environment. Sulfate is taken up into roots from the soil and distributed to various organs through the functions of membrane-bound sulfate transporters, while it is utilized as the primary substrate for synthesizing sulfur-containing metabolites in the sulfate assimilation pathways. Transporters and enzymes for the assimilative conversion of sulfate are regulated in highly organized manners depending on changes in sulfate supply from the environment and demand for biosynthesis of reduced sulfur compounds in the plant systems. Over the past few decades, the effect of sulfur nutrition on gene expression of sulfate transporters and assimilatory enzymes has been extensively studied with the aim of understanding the full landscape of regulatory networks.

β-Substituting alanine synthases: roles in cysteine metabolism and abiotic and biotic stress signalling in plants

Cysteine is required for the synthesis of proteins and metabolites, and is therefore an indispensable compound for growth and development. The β-substituting alanine synthase (BSAS) gene family encodes enzymes known as O-acetylserine thiol lyases (OASTLs), which carry out cysteine biosynthesis in plants. The functions of the BSAS isoforms have been reported to be crucial in assimilation of S and cysteine biosynthesis, and homeostasis in plants. In this review we explore the functional variation in this classic pyridoxal-phosphate-dependent enzyme family of BSAS isoforms. We discuss how specialisation and divergence in BSAS catalytic activities makes a more dynamic set of biological routers that integrate cysteine metabolism and abiotic and biotic stress signalling in Arabidopsis thaliana (L.) Heynh. and also other species. Our review presents a universal scenario in which enzymes modulating cysteine metabolism promote survival and fitness of the species by counteracting internal and external stress factors.

Exploring the importance of sulfate transporters and ATP sulphurylases for selenium hyperaccumulation-a comparison of Stanleya pinnata and Brassica juncea (Brassicaceae)

Selenium (Se) hyperaccumulation, the capacity of some species to concentrate Se to levels upwards of 0.1% of dry weight, is an intriguing phenomenon that is only partially understood. Questions that remain to be answered are: do hyperaccumulators have one or more Se-specific transporters? How are these regulated by Se and sulfur (S)? In this study, hyperaccumulator Stanleya pinnata was compared with related non-hyperaccumulator Brassica juncea with respect to S-dependent selenate uptake and translocation, as well as for the expression levels of three sulfate/selenate transporters (Sultr) and three ATP sulphurylases (APS). Selenium accumulation went down ~10-fold with increasing sulfate supply in B. juncea, while S. pinnata only had a 2-3-fold difference in Se uptake between the highest (5 mM) and lowest sulfate (0 mM) treatments. The Se/S ratio was generally higher in the hyperaccumulator than the non-hyperaccumulator, and while tissue Se/S ratio in B. juncea largely reflected the ratio in the growth medium, S. pinnata enriched itself up to 5-fold with Se relative to S. The transcript levels of Sultr1;2 and 2;1 and APS1, 2, and 4 were generally much higher in S. pinnata than B. juncea, and the species showed differential transcript responses to S and Se supply. These results indicate that S. pinnata has at least one transporter with significant selenate specificity over sulfate. Also, the hyperaccumulator has elevated expression levels of several sulfate/selenate transporters and APS enzymes, which likely contribute to the Se hyperaccumulation and hypertolerance phenotype.

N-glycosylation and topology of the human SLC26 family of anion transport membrane proteins

The human solute carrier (SLC26) family of anion transporters consists of 10 members (SLCA1-11, SLCA10 being a pseudogene) that encode membrane proteins containing ~12 transmembrane (TM) segments with putative N-glycosylation sites (-NXS/T-) in extracellular loops and a COOH-terminal cytosolic STAS domain. All 10 members of the human SLC26 family, FLAG-tagged at the NH2 terminus, were transiently expressed in HEK-293 cells. While most proteins were observed to contain both high-mannose and complex oligosaccharides, SLC26A2 was mainly in the complex form, SLC26A4 in the high-mannose form, and SLC26A8 was not N-glycosylated. Mutation of the putative N-glycosylation sites showed that most members contain multiple N-glycosylation sites in the second extracytosolic (EC) loop, except SLC26A11, which was N-glycosylated in EC loop 4. Immunofluorescence staining of permeabilized cells localized the proteins to the plasma membrane and the endoplasmic reticulum, with SLC26A2 highly localized to the plasma membrane. N-glycosylation was not a necessary requirement for cell surface expression as the localization of nonglycosylated proteins was similar to their wild-type counterparts, although a lower level of cell-surface biotinylation was observed. No immunostaining of intact cells was observed for any SLC26 members, demonstrating that the NH2-terminal FLAG tag was located in the cytosol. Topological models of the SLC26 proteins that contain an even number of transmembrane segments with both the NH2 and COOH termini located in the cytosol and utilized N-glycosylation sites defining the positions of two EC loops are presented.

Transporters in plant sulfur metabolism

Sulfur is an essential nutrient, necessary for synthesis of many metabolites. The uptake of sulfate, primary and secondary assimilation, the biosynthesis, storage, and final utilization of sulfur (S) containing compounds requires a lot of movement between organs, cells, and organelles. Efficient transport systems of S-containing compounds across the internal barriers or the plasma membrane and organellar membranes are therefore required. Here, we review a current state of knowledge of the transport of a range of S-containing metabolites within and between the cells as well as of their long distance transport. An improved understanding of mechanisms and regulation of transport will facilitate successful engineering of the respective pathways, to improve the plant yield, biotic interaction and nutritional properties of crops.

Aberrant gene expression in the Arabidopsis SULTR1;2 mutants suggests a possible regulatory role for this sulfate transporter in response to sulfur nutrient status

Sulfur is required for the biosynthesis of cysteine, methionine and numerous other metabolites, and thus is critical for cellular metabolism and various growth and developmental processes. Plants are able to sense their physiological state with respect to sulfur availability, but the sensor remains to be identified. Here we report the isolation and characterization of two novel allelic mutants of Arabidopsis thaliana, sel1-15 and sel1-16, which show increased expression of a sulfur deficiency-activated gene β-glucosidase 28 (BGLU28). The mutants, which represent two different missense alleles of SULTR1;2, which encodes a high-affinity sulfate transporter, are defective in sulfate transport and as a result have a lower cellular sulfate level. However, when treated with a very high dose of sulfate, sel1-15 and sel1-16 accumulated similar amounts of internal sulfate and its metabolite glutathione (GSH) to wild-type, but showed higher expression of BGLU28 and other sulfur deficiency-activated genes than wild-type. Reduced sensitivity to inhibition of gene expression was also observed in the sel1 mutants when fed with the sulfate metabolites Cys and GSH. In addition, a SULTR1;2 knockout allele also exhibits reduced inhibition in response to sulfate, Cys and GSH, consistent with the phenotype of sel1-15 and sel1-16. Taken together, the genetic evidence suggests that, in addition to its known function as a high-affinity sulfate transporter, SULTR1;2 may have a regulatory role in response to sulfur nutrient status. The possibility that SULTR1;2 may function as a sensor of sulfur status or a component of a sulfur sensory mechanism is discussed.

Molecular mechanisms of regulation of sulfate assimilation: first steps on a long road

The pathway of sulfate assimilation, which provides plants with the essential nutrient sulfur, is tightly regulated and coordinated with the demand for reduced sulfur. The responses of metabolite concentrations, enzyme activities and mRNA levels to various signals and environmental conditions have been well described for the pathway. However, only little is known about the molecular mechanisms of this regulation. To date, nine transcription factors have been described to control transcription of genes of sulfate uptake and assimilation. In addition, other levels of regulation contribute to the control of sulfur metabolism. Post-transcriptional regulation has been shown for sulfate transporters, adenosine 5'phosphosulfate reductase, and cysteine synthase. Several genes of the pathway are targets of microRNA miR395. In addition, protein-protein interaction is increasingly found in the center of various regulatory circuits. On top of the mechanisms of regulation of single genes, we are starting to learn more about mechanisms of adaptation, due to analyses of natural variation. In this article, the summary of different mechanisms of regulation will be accompanied by identification of the major gaps in knowledge and proposition of possible ways of filling them.

Transceptors at the boundary of nutrient transporters and receptors: a new role for Arabidopsis SULTR1;2 in sulfur sensing

Plants have evolved a sophisticated mechanism to sense the extracellular sulfur (S) status so that sulfate transport and S assimilation/metabolism can be coordinated. Genetic, biochemical, and molecular studies in Arabidopsis over the past 10 years have started to shed some light on the regulatory mechanism of the S response. Key advances in transcriptional regulation (SLIM1, MYB, and miR395), involvement of hormones (auxin, cytokinin, and abscisic acid) and identification of putative sensors (OASTL and SULTR1;2) are highlighted here. Although our current view of S nutrient sensing and signaling remains fragmented, it is anticipated that through further studies a sensing and signaling network will be revealed in the near future.

The SLC26 gene family of anion transporters and channels

The phylogenetically ancient SLC26 gene family encodes multifunctional anion exchangers and anion channels transporting a broad range of substrates, including Cl(-), HCO3(-), sulfate, oxalate, I(-), and formate. SLC26 polypeptides are characterized by N-terminal cytoplasmic domains, 10-14 hydrophobic transmembrane spans, and C-terminal cytoplasmic STAS domains, and appear to be homo-oligomeric. SLC26-related SulP proteins of marine bacteria likely transport HCO3(-) as part of oceanic carbon fixation. SulP genes present in antibiotic operons may provide sulfate for antibiotic biosynthetic pathways. SLC26-related Sultr proteins transport sulfate in unicellular eukaryotes and in plants. Mutations in three human SLC26 genes are associated with congenital or early onset Mendelian diseases: chondrodysplasias for SLC26A2, chloride diarrhea for SLC26A3, and deafness with enlargement of the vestibular aqueduct for SLC26A4. Additional disease phenotypes evident only in mouse knockout models include oxalate urolithiasis for Slc26a6 and Slc26a1, non-syndromic deafness for Slc26a5, gastric hypochlorhydria for Slc26a7 and Slc26a9, distal renal tubular acidosis for Slc26a7, and male infertility for Slc26a8. STAS domains are required for cell surface expression of SLC26 proteins, and contribute to regulation of the cystic fibrosis transmembrane regulator in complex, cell- and tissue-specific ways. The protein interactomes of SLC26 polypeptides are under active investigation.

Activation of R-mediated innate immunity and disease susceptibility is affected by mutations in a cytosolic O-acetylserine (thiol) lyase in Arabidopsis

O-acetylserine (thiol) lyases (OASTLs) are evolutionarily conserved proteins among many prokaryotes and eukaryotes that perform sulfur acquisition and synthesis of cysteine. A mutation in the cytosolic OASTL-A1 protein ONSET OF LEAF DEATH3 (OLD3) was previously shown to reduce the OASTL activity of the old3-1 protein in vitro and cause auto-necrosis in specific Arabidopsis accessions. Here we investigated why a mutation in this protein causes auto-necrosis in some but not other accessions. The auto-necrosis was found to depend on Recognition of Peronospora Parasitica 1 (RPP1)-like disease resistance R gene(s) from an evolutionarily divergent R gene cluster that is present in Ler-0 but not the reference accession Col-0. RPP1-like gene(s) show a negative epistatic interaction with the old3-1 mutation that is not linked to reduced cysteine biosynthesis. Metabolic profiling and transcriptional analysis further indicate that an effector triggered-like immune response and metabolic disorder are associated with auto-necrosis in old3-1 mutants, probably activated by an RPP1-like gene. However, the old3-1 protein in itself results in largely neutral changes in primary plant metabolism, stress defence and immune responses. Finally, we showed that lack of a functional OASTL-A1 results in enhanced disease susceptibility against infection with virulent and non-virulent Pseudomonas syringae pv. tomato DC3000 strains. These results reveal an interaction between the cytosolic OASTL and components of plant immunity.

Membrane transport metabolons

In this review evidence from a wide variety of biological systems is presented for the genetic, functional, and likely physical association of membrane transporters and the enzymes that metabolize the transported substrates. This evidence supports the hypothesis that the dynamic association of transporters and enzymes creates functional membrane transport metabolons that channel substrates typically obtained from the extracellular compartment directly into their cellular metabolism. The immediate modification of substrates on the inner surface of the membrane prevents back-flux through facilitated transporters, increasing the efficiency of transport. In some cases products of the enzymes are themselves substrates for the transporters that efflux the products in an exchange or antiport mechanism. Regulation of the binding of enzymes to transporters and their mutual activities may play a role in modulating flux through transporters and entry of substrates into metabolic pathways. Examples showing the physical association of transporters and enzymes are provided, but available structural data is sparse. Genetic and functional linkages between membrane transporters and enzymes were revealed by an analysis of Escherichia coli operons encoding polycistronic mRNAs and provide a list of predicted interactions ripe for further structural studies. This article supports the view that membrane transport metabolons are important throughout Nature in organisms ranging from bacteria to humans.

Molecular Biology, Biochemistry and Cellular Physiology of Cysteine Metabolism in Arabidopsis thaliana

Cysteine is one of the most versatile molecules in biology, taking over such different functions as catalysis, structure, regulation and electron transport during evolution. Research on Arabidopsis has contributed decisively to the understanding of cysteine synthesis and its role in the assimilatory pathways of S, N and C in plants. The multimeric cysteine synthase complex is present in the cytosol, plastids and mitochondria and forms the centre of a unique metabolic sensing and signaling system. Its association is reversible, rendering the first enzyme of cysteine synthesis active and the second one inactive, and vice-versa. Complex formation is triggered by the reaction intermediates of cysteine synthesis in response to supply and demand and gives rise to regulation of genes of sulfur metabolism to adjust cellular sulfur homeostasis. Combinations of biochemistry, forward and reverse genetics, structural- and cell-biology approaches using Arabidopsis have revealed new enzyme functions and the unique pattern of spatial distribution of cysteine metabolism in plant cells. These findings place the synthesis of cysteine in the centre of the network of primary metabolism.

STAS domain structure and function

Pendrin shares with nearly all SLC26/SulP anion transporters a carboxy-terminal cytoplasmic segment organized around a Sulfate Transporter and Anti-Sigma factor antagonist (STAS) domain. STAS domains of divergent amino acid sequence exhibit a conserved fold of 4 β strands interspersed among 5 α helices. The first STAS domain proteins studied were single-domain anti-sigma factor antagonists (anti-anti-σ). These anti-anti-σ indirectly stimulate bacterial RNA polymerase by inactivating inhibitory anti-σ kinases, liberating σ factors to direct specific transcription of target genes or operons. Some STAS domains are nucleotide-binding phosphoproteins or nucleotidases. Others are interaction/transduction modules within multidomain sensors of light, oxygen and other gasotransmitters, cyclic nucleotides, inositol phosphates, and G proteins. Additional multidomain STAS protein sequences suggest functions in sensing, metabolism, or transport of nutrients such as sugars, amino acids, lipids, anions, vitamins, or hydrocarbons. Still other multidomain STAS polypeptides include histidine and serine/threonine kinase domains and ligand-activated transcription factor domains. SulP/SLC26 STAS domains and adjacent sequences interact with other transporters, cytoskeletal scaffolds, and with enzymes metabolizing transported anion substrates, forming putative metabolons. STAS domains are central to membrane targeting of many SulP/SLC26 anion transporters, and STAS domain mutations are associated with at least three human recessive diseases. This review summarizes STAS domain structure and function.

Identification of an Arabidopsis solute carrier critical for intracellular transport and inter-organ allocation of molybdate

Plants represent an important source of molybdenum in the human diet. Recently, MOT1 has been identified as a transport protein responsible for molybdate import in Arabidopsis thaliana L.; however, the function of the homologous protein MOT2 has not been resolved. Interestingly, MOT2-GFP analysis indicated a vacuolar location of this carrier protein. By site directed mutagenesis at the N-terminal end of MOT2, we identified a di-leucine motif that is essential for driving the protein into the vacuolar membrane. Molybdate quantification in isolated vacuoles showed that this organelle serves as an important molybdate store in Arabidopsis cells. When grown on soil, leaves from mot2 T-DNA mutants contained more molybdate, whereas mot2 seeds contained significantly less molybdate than corresponding wild-type (Wt) tissues. Remarkably, MOT2 mRNA accumulates in senescing leaves and mot2 leaves from plants that had finished their life cycle had 15-fold higher molybdate levels than Wt leaves. Reintroduction of the endogenous MOT2 gene led to a Wt molybdate phenotype. Thus, mot2 mutants exhibit impaired inter-organ molybdate allocation. As total concentrations of the molybdenum cofactor (Moco) and its precursor MPT correlates with leaf molybdate levels, we present novel evidence for an adjustment of Moco biosynthesis in response to cellular MoO₄²⁻ levels. We conclude that MOT2 is important for vacuolar molybdate export, an N-terminal di-leucine motif is critical for correct subcellular localisation of MOT2 and activity of this carrier is required for accumulation of molybdate in Arabidopsis seeds. MOT2 is a novel element in inter-organ translocation of an essential metal ion.

The multifaceted pyridoxal 5'-phosphate-dependent O-acetylserine sulfhydrylase

Cysteine is the final product of the reductive sulfate assimilation pathway in bacteria and plants and serves as the precursor for all sulfur-containing biological compounds, such as methionine, S-adenosyl methionine, iron-sulfur clusters and glutathione. Moreover, in several microorganisms cysteine plays a role as a reducing agent, eventually counteracting host oxidative defense strategies. Cysteine is synthesized by the PLP-dependent O-acetylserine sulfhydrylase, a dimeric enzyme belonging to the fold type II, catalyzing a beta-replacement reaction. In this review, the spectroscopic properties, catalytic mechanism, three-dimensional structure, conformational changes accompanying catalysis, determinants of enzyme stability, role of selected amino acids in catalysis, and the regulation of enzyme activity by ligands and interaction with serine acetyltransferase, the preceding enzyme in the biosynthetic pathway, are described. Given the key biological role played by O-acetylserine sulfhydrylase in bacteria, inhibitors with potential antibiotic activity have been developed. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.

Sulfur assimilation in photosynthetic organisms: molecular functions and regulations of transporters and assimilatory enzymes

Sulfur is required for growth of all organisms and is present in a wide variety of metabolites having distinctive biological functions. Sulfur is cycled in ecosystems in nature where conversion of sulfate to organic sulfur compounds is primarily dependent on sulfate uptake and reduction pathways in photosynthetic organisms and microorganisms. In vascular plant species, transport proteins and enzymes in this pathway are functionally diversified to have distinct biochemical properties in specific cellular and subcellular compartments. Recent findings indicate regulatory processes of sulfate transport and metabolism are tightly connected through several modes of transcriptional and posttranscriptional mechanisms. This review provides up-to-date knowledge in functions and regulations of sulfur assimilation in plants and algae, focusing on sulfate transport systems and metabolic pathways for sulfate reduction and synthesis of downstream metabolites with diverse biological functions.

Solution structure of the guanine nucleotide-binding STAS domain of SLC26-related SulP protein Rv1739c from Mycobacterium tuberculosis

The structure and intrinsic activities of conserved STAS domains of the ubiquitous SulP/SLC26 anion transporter superfamily have until recently remained unknown. Here we report the heteronuclear, multidimensional NMR spectroscopy solution structure of the STAS domain from the SulP/SLC26 putative anion transporter Rv1739c of Mycobacterium tuberculosis. The 0.87-Å root mean square deviation structure revealed a four-stranded β-sheet with five interspersed α-helices, resembling the anti-σ factor antagonist fold. Rv1739c STAS was shown to be a guanine nucleotide-binding protein, as revealed by nucleotide-dependent quench of intrinsic STAS fluorescence and photoaffinity labeling. NMR chemical shift perturbation analysis partnered with in silico docking calculations identified solvent-exposed STAS residues involved in nucleotide binding. Rv1739c STAS was not an in vitro substrate of mycobacterial kinases or anti-σ factors. These results demonstrate that Rv1739c STAS binds guanine nucleotides at physiological concentrations and undergoes a ligand-induced conformational change but, unlike anti-σ factor antagonists, may not mediate signals via phosphorylation.

Identification and regulation of plasma membrane sulfate transporters in Chlamydomonas

Chlamydomonas (Chlamydomonas reinhardtii) exhibits several responses following exposure to sulfur (S)-deprivation conditions, including an increased efficiency of import and assimilation of the sulfate anion (SO(4)(2-)). Aspects of SO(4)(2-) transport during S-replete and S-depleted conditions were previously studied, although the transporters had not been functionally identified. We employed a reverse genetics approach to identify putative SO(4)(2-) transporters, examine their regulation, establish their biogenesis and subcellular locations, and explore their functionality. Upon S starvation of wild-type Chlamydomonas cells, the accumulation of transcripts encoding the putative SO(4)(2-) transporters SLT1 (for SAC1-like transporter 1), SLT2, and SULTR2 markedly increased, suggesting that these proteins function in high-affinity SO(4)(2-) transport. The Chlamydomonas sac1 and snrk2.1 mutants (defective for acclimation to S deprivation) exhibited much less of an increase in the levels of SLT1, SLT2, and SULTR2 transcripts and their encoded proteins in response to S deprivation compared with wild-type cells. All three transporters were localized to the plasma membrane, and their rates of turnover were significantly impacted by S availability; the turnover of SLT1 and SLT2 was proteasome dependent, while that of SULTR2 was proteasome independent. Finally, mutants identified for each of the S-deprivation-responsive transporters were used to establish their critical role in the transport of SO(4)(2-) into S-deprived cells.