Chloroplast HCF101 is a scaffold protein for [4Fe-4S] cluster assembly

Evidence of SUFBC2D directly deliver Fe-S cluster to apo- NITRITE REDUCTASE1 (NIR1)

Fe-S clusters are ancient, ubiquitous, and highly essential prosthetic groups of many proteins. According to current understanding, Fe-S clusters are biosynthesized on scaffold, and transferred to carrier proteins, thereafter, they will be inserted into apo-proteins. However, it was also suggested that the scaffold complex SUFBC2D can directly deliver Fe-S cluster to apo-proteins. In this study, we combined biotin ligase-based proximity labeling with Yeast two-hybrid (Y2H) assay, Bimolecular fluorescence complementation (BiFC) assay and pull-down assay, identified that chloroplast Fe-S protein NIR1 directly interact with SUFC, suggesting that NIR1 may directly receive the Fe-S clusters from SUFBC2D. Thus, we provided evidence to support the exist of a new Fe-S clusters delivery pathway in chloroplast.

The HCF101 protein is an important component of the cytosolic iron-sulfur synthesis pathway in Toxoplasma gondii

Several key cellular functions depend on proteins harboring an iron-sulfur (Fe-S) cofactor. As these Fe-S proteins localize to several subcellular compartments, they require a dedicated machinery for cofactor assembly. For instance, in plants and algae there are Fe-S cluster synthesis pathways localizing to the cytosol, but also present in the mitochondrion and in the chloroplast, 2 organelles of endosymbiotic origin. Toxoplasma gondii is a plastid-bearing parasitic protist responsible for a pathology affecting humans and other warm-blooded vertebrates. We have characterized the Toxoplasma homolog of HCF101, originally identified in plants as a protein transferring Fe-S clusters to photosystem I subunits in the chloroplast. Contrarily to plants, we have shown that HCF101 does not localize to the plastid in parasites, but instead is an important component of the cytosolic Fe-S assembly (CIA) pathway which is vital for Toxoplasma. While the CIA pathway is widely conserved in eukaryotes, it is the first time the involvement of HCF101 in this pan-eukaryotic machinery is established. Moreover, as this protein is essential for parasite viability and absent from its mammalian hosts, it constitutes a novel and promising potential drug target.

Cuticle development and the underlying transcriptome-metabolome associations during early seedling establishment

The plant cuticle is a complex extracellular lipid barrier that has multiple protective functions. This study investigated cuticle deposition by integrating metabolomics and transcriptomics data gathered from six different maize seedling organs of four genotypes, the inbred lines B73 and Mo17, and their reciprocal hybrids. These datasets captured the developmental transition of the seedling from heterotrophic skotomorphogenic growth to autotrophic photomorphogenic growth, a transition that is highly vulnerable to environmental stresses. Statistical interrogation of these data revealed that the predominant determinant of cuticle composition is seedling organ type, whereas the seedling genotype has a smaller effect on this phenotype. Gene-to-metabolite associations assessed by integrated statistical analyses identified three gene networks associated with the deposition of different elements of the cuticle: cuticular waxes; monomers of lipidized cell wall biopolymers, including cutin and suberin; and both of these elements. These gene networks reveal three metabolic programs that appear to support cuticle deposition, including processes of chloroplast biogenesis, lipid metabolism, and molecular regulation (e.g. transcription factors, post-translational regulators, and phytohormones). This study demonstrates the wider physiological metabolic context that can determine cuticle deposition and lays the groundwork for new targets for modulating the properties of this protective barrier.

Structure, function, and assembly of PSI in thylakoid membranes of vascular plants

The photosynthetic apparatus is formed by thylakoid membrane-embedded multiprotein complexes that carry out linear electron transport in oxygenic photosynthesis. The machinery is largely conserved from cyanobacteria to land plants, and structure and function of the protein complexes involved are relatively well studied. By contrast, how the machinery is assembled in thylakoid membranes remains poorly understood. The complexes participating in photosynthetic electron transfer are composed of many proteins, pigments, and redox-active cofactors, whose temporally and spatially highly coordinated incorporation is essential to build functional mature complexes. Several proteins, jointly referred to as assembly factors, engage in the biogenesis of these complexes to bring the components together in a step-wise manner, in the right order and time. In this review, we focus on the biogenesis of the terminal protein supercomplex of the photosynthetic electron transport chain, PSI, in vascular plants. We summarize our current knowledge of the assembly process and the factors involved and describe the challenges associated with resolving the assembly pathway in molecular detail.

Iron‑sulfur cluster synthesis in plastids by the SUF system: A mechanistic and structural perspective

About 50 proteins expressed in plastids of photosynthetic eukaryotes ligate iron‑sulfur (Fe-S) clusters and ensure vital functions in photosynthesis, sulfur and nitrogen assimilation, but also in the synthesis of pigments, vitamins and hormones. The synthesis of these Fe-S clusters, which are co- or post-translationally incorporated into these proteins, relies on several proteins belonging to the so-called sulfur mobilization (SUF) machinery. An Fe-S cluster is first de novo synthesized on a scaffold protein complex before additional late-acting maturation factors act in the specific transfer, possible conversion and insertion of this cluster into target recipient proteins. In this review, we will summarize what is known about the molecular mechanisms responsible for both the synthesis and transfer steps, focusing in particular on the structural aspects that allow the formation of the required protein complexes.

Research progress on the biosynthesis and delivery of iron-sulfur clusters in the plastid

Iron-sulfur (Fe-S) clusters are ancient protein cofactors ubiquitously exist in organisms. They are involved in many important life processes. Plastids are semi-autonomous organelles with a double membrane and it is believed to originate from a cyanobacterial endosymbiont. By learning form the research in cyanobacteria, a Fe-S cluster biosynthesis and delivery pathway has been proposed and partly demonstrated in plastids, including iron uptake, sulfur mobilization, Fe-S cluster assembly and delivery. Fe-S clusters are essential for the downstream Fe-S proteins to perform their normal biological functions. Because of the importance of Fe-S proteins in plastid, researchers have made a lot of research progress on this pathway in recent years. This review summarizes the detail research progress made in recent years. In addition, the scientific problems remained in this pathway are also discussed.

NADP+ supply adjusts the synthesis of photosystem I in Arabidopsis chloroplasts

In oxygenic photosynthesis, NADP+ acts as the final acceptor of the photosynthetic electron transport chain and receives electrons via the thylakoid membrane complex photosystem I (PSI) to synthesize NAPDH by the enzyme ferredoxin:NADP+ oxidoreductase. The NADP+/NADPH redox couple is essential for cellular metabolism and redox homeostasis. However, how the homeostasis of these two dinucleotides is integrated into chloroplast biogenesis remains largely unknown. Here, we demonstrate the important role of NADP+ supply for the biogenesis of PSI by examining the nad kinase 2 (nadk2) mutant in Arabidopsis (Arabidopsis thaliana), which demonstrates disrupted synthesis of NADP+ from NAD+ in chloroplasts. Although the nadk2 mutant is highly sensitive to light, the reaction center of photosystem II (PSII) is only mildly and likely only secondarily affected compared to the wild-type. Our studies revealed that the primary limitation of photosynthetic electron transport, even at low light intensities, occurs at PSI rather than at PSII in the nadk2 mutant. Remarkably, this primarily impairs the de novo synthesis of the two PSI core subunits PsaA and PsaB, leading to the deficiency of the PSI complex in the nadk2 mutant. This study reveals an unexpected molecular link between NADK activity and mRNA translation of psaA/B in chloroplasts that may mediate a feedback mechanism to adjust de novo biosynthesis of the PSI complex in response to a variable NADPH demand. This adjustment may be important to protect PSI from photoinhibition under conditions that favor acceptor side limitation.

Mycobacterium tuberculosis requires SufT for Fe-S cluster maturation, metabolism, and survival in vivo

Iron-sulfur (Fe-S) cluster proteins carry out essential cellular functions in diverse organisms, including the human pathogen Mycobacterium tuberculosis (Mtb). The mechanisms underlying Fe-S cluster biogenesis are poorly defined in Mtb. Here, we show that Mtb SufT (Rv1466), a DUF59 domain-containing essential protein, is required for the Fe-S cluster maturation. Mtb SufT homodimerizes and interacts with Fe-S cluster biogenesis proteins; SufS and SufU. SufT also interacts with the 4Fe-4S cluster containing proteins; aconitase and SufR. Importantly, a hyperactive cysteine in the DUF59 domain mediates interaction of SufT with SufS, SufU, aconitase, and SufR. We efficiently repressed the expression of SufT to generate a SufT knock-down strain in Mtb (SufT-KD) using CRISPR interference. Depleting SufT reduces aconitase's enzymatic activity under standard growth conditions and in response to oxidative stress and iron limitation. The SufT-KD strain exhibited defective growth and an altered pool of tricarboxylic acid cycle intermediates, amino acids, and sulfur metabolites. Using Seahorse Extracellular Flux analyzer, we demonstrated that SufT depletion diminishes glycolytic rate and oxidative phosphorylation in Mtb. The SufT-KD strain showed defective survival upon exposure to oxidative stress and nitric oxide. Lastly, SufT depletion reduced the survival of Mtb in macrophages and attenuated the ability of Mtb to persist in mice. Altogether, SufT assists in Fe-S cluster maturation and couples this process to bioenergetics of Mtb for survival under low and high demand for Fe-S clusters.

The DnaJ proteins DJA6 and DJA5 are essential for chloroplast iron-sulfur cluster biogenesis

Fe-S clusters are ancient, ubiquitous and highly essential prosthetic groups for numerous fundamental processes of life. The biogenesis of Fe-S clusters is a multistep process including iron acquisition, sulfur mobilization, and cluster formation. Extensive studies have provided deep insights into the mechanism of the latter two assembly steps. However, the mechanism of iron utilization during chloroplast Fe-S cluster biogenesis is still unknown. Here we identified two Arabidopsis DnaJ proteins, DJA6 and DJA5, that can bind iron through their conserved cysteine residues and facilitate iron incorporation into Fe-S clusters by interactions with the SUF (sulfur utilization factor) apparatus through their J domain. Loss of these two proteins causes severe defects in the accumulation of chloroplast Fe-S proteins, a dysfunction of photosynthesis, and a significant intracellular iron overload. Evolutionary analyses revealed that DJA6 and DJA5 are highly conserved in photosynthetic organisms ranging from cyanobacteria to higher plants and share a strong evolutionary relationship with SUFE1, SUFC, and SUFD throughout the green lineage. Thus, our work uncovers a conserved mechanism of iron utilization for chloroplast Fe-S cluster biogenesis.

The iron-sulfur scaffold protein HCF101 unveils the complexity of organellar evolution in SAR, Haptista and Cryptista

Background

Nbp35-like proteins (Nbp35, Cfd1, HCF101, Ind1, and AbpC) are P-loop NTPases that serve as components of iron-sulfur cluster (FeS) assembly machineries. In eukaryotes, Ind1 is present in mitochondria, and its function is associated with the assembly of FeS clusters in subunits of respiratory Complex I, Nbp35 and Cfd1 are the components of the cytosolic FeS assembly (CIA) pathway, and HCF101 is involved in FeS assembly of photosystem I in plastids of plants (chHCF101). The AbpC protein operates in Bacteria and Archaea. To date, the cellular distribution of these proteins is considered to be highly conserved with only a few exceptions.

Results

We searched for the genes of all members of the Nbp35-like protein family and analyzed their targeting sequences. Nbp35 and Cfd1 were predicted to reside in the cytoplasm with some exceptions of Nbp35 localization to the mitochondria; Ind1was found in the mitochondria, and HCF101 was predicted to reside in plastids (chHCF101) of all photosynthetically active eukaryotes. Surprisingly, we found a second HCF101 paralog in all members of Cryptista, Haptista, and SAR that was predicted to predominantly target mitochondria (mHCF101), whereas Ind1 appeared to be absent in these organisms. We also identified a few exceptions, as apicomplexans possess mHCF101 predicted to localize in the cytosol and Nbp35 in the mitochondria. Our predictions were experimentally confirmed in selected representatives of Apicomplexa (Toxoplasma gondii), Stramenopila (Phaeodactylum tricornutum, Thalassiosira pseudonana), and Ciliophora (Tetrahymena thermophila) by tagging proteins with a transgenic reporter. Phylogenetic analysis suggested that chHCF101 and mHCF101 evolved from a common ancestral HCF101 independently of the Nbp35/Cfd1 and Ind1 proteins. Interestingly, phylogenetic analysis supports rather a lateral gene transfer of ancestral HCF101 from bacteria than its acquisition being associated with either α-proteobacterial or cyanobacterial endosymbionts.

Conclusion

Our searches for Nbp35-like proteins across eukaryotic lineages revealed that SAR, Haptista, and Cryptista possess mitochondrial HCF101. Because plastid localization of HCF101 was only known thus far, the discovery of its mitochondrial paralog explains confusion regarding the presence of HCF101 in organisms that possibly lost secondary plastids (e.g., ciliates, Cryptosporidium) or possess reduced nonphotosynthetic plastids (apicomplexans).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01777-x.

Functional relationships of three NFU proteins in the biogenesis of chloroplastic iron-sulfur clusters

Iron-sulfur clusters are required in a variety of biological processes. Biogenesis of iron-sulfur clusters includes assembly of iron-sulfur clusters on scaffold complexes and transfer of iron-sulfur clusters to recipient apoproteins by iron-sulfur carriers, such as nitrogen-fixation-subunit-U (NFU)-type proteins. Arabidopsis thaliana has three plastid-targeted NFUs: NFU1, NFU2, and NFU3. We previously discovered that nfu2 -/- nfu3 -/- mutants are embryo lethal. The lack of viable nfu2 -/- nfu3 -/- mutants posed a serious challenge. To overcome this problem, we characterized nfu2-1 -/- nfu3-2+/- and nfu2-1+/- nfu3-2 -/- sesquimutants. Simultaneous loss-of-function mutations in NFU2 and NFU3 have an additive effect on the declines of 4Fe-4S-containing PSI core subunits. Consequently, the sesquimutants had much lower PSI and PSII activities, much less chlorophyll, and much smaller plant sizes, than nfu2-1 and nfu3-2 single mutants. These observations are consistent with proposed roles of NFU3 and NFU2 in the biogenesis of chloroplastic 4Fe-4S. By performing spectroscopic and in vitro reconstitution experiments, we found that NFU1 may act as a carrier for chloroplastic 4Fe-4S and 3Fe-4S clusters. In line with this hypothesis, loss-of-function mutations in NFU1 resulted in significant declines in 4Fe-4S- and 3Fe-4S-containing chloroplastic proteins. The declines of PSI activity and 4Fe-4S-containing PSI core subunits in nfu1 mutants indicate that PSI is the main target of NFU1 action. The reductions in 4Fe-4S-containing PSI core proteins and PSI activity in nfu3-2, nfu2-1, and nfu1 single mutants suggest that all three plastid-targeted NFU proteins contribute to the biogenesis of chloroplastic 4Fe-4S clusters. Although different insertion sites of T-DNA lines may cause variations in phenotypic results, mutation severity could be an indicator of the relative importance of the gene product. Our results are consistent with the hypothesis that NFU3 contributes more than NFU2 and NFU2 contributes more than NFU1 to the production of 4Fe-4S-containing PSI core subunits.

Pentatricopeptide repeat protein PHOTOSYSTEM I BIOGENESIS FACTOR2 is required for splicing of ycf3

To gain a better understanding of the molecular mechanisms of photosystem I (PSI) biogenesis, we characterized the Arabidopsis thaliana photosystem I biogenesis factor 2 (pbf2) mutant, which lacks PSI complex. PBF2 encodes a P-class pentatricopeptide repeat (PPR) protein. In the pbf2 mutants, we observed a striking decrease in the transcript level of only one gene, the chloroplast gene ycf3, which is essential for PSI assembly. Further analysis of ycf3 transcripts showed that PBF2 is specifically required for the splicing of ycf3 intron 1. Computational prediction of binding sequences and electrophoretic mobility shift assays reveal that PBF2 specifically binds to a sequence in ycf3 intron 1. Moreover, we found that PBF2 interacted with two general factors for group II intron splicing CHLOROPLAST RNA SPLICING2-ASSOCIATED FACTOR1 (CAF1) and CAF2, and facilitated the association of these two factors with ycf3 intron 1. Our results suggest that PBF2 is specifically required for the splicing of ycf3 intron 1 through cooperating with CAF1 and CAF2. Our results also suggest that additional proteins are required to contribute to the specificity of CAF-dependent group II intron splicing.

Iron deficiency and the loss of chloroplast iron-sulfur cluster assembly trigger distinct transcriptome changes in Arabidopsis rosettes

Regulation of mRNA abundance revealed a genetic program for plant leaf acclimation to iron (Fe) limitation. The transcript for SUFB, a key component of the plastid iron-sulfur (Fe-S) assembly pathway is down-regulated early after Fe deficiency, and prior to down-regulation of mRNAs encoding abundant chloroplast Fe containing proteins, which should economize the use of Fe. What controls this system is unclear. We utilized RNA-seq. aimed to identify differentially expressed transcripts that are co-regulated with SUFB after Fe deficiency in leaves. To distinguish if lack of Fe or lack of Fe-S cofactors and associated loss of enzymatic and photosynthetic activity trigger transcriptome reprogramming, WT plants on low Fe were compared with an inducible sufb-RNAi knockdown. Fe deficiency targeted a limited set of genes and predominantly affected transcripts for chloroplast localized proteins. A set of glutaredoxin transcripts was concertedly down-regulated early after Fe deficiency, however when these same genes were down-regulated by RNAi the effect on known chloroplast Fe deficiency marker proteins was minimal. In promoters of differentially expressed genes, binding motifs for AP2/ERF transcription factors were most abundant and three AP2/ERF transcription factors were also differentially expressed early after low Fe treatment. Surprisingly, Fe deficiency in a WT on low Fe and a sufb-RNAi knockdown presented very little overlap in differentially expressed genes. sufb-RNAi produced expression patterns expected for Fe excess and up-regulation of a transcript for another Fe-S assembly component not affected by low Fe. These findings indicate that Fe scarcity, not Fe utilization, triggers reprogramming of the transcriptome in leaves.

Mechanistic concepts of iron-sulfur protein biogenesis in Biology

Iron-sulfur (Fe/S) proteins are present in virtually all living organisms and are involved in numerous cellular processes such as respiration, photosynthesis, metabolic reactions, nitrogen fixation, radical biochemistry, protein synthesis, antiviral defense, and genome maintenance. Their versatile functions may go back to the proposed role of their Fe/S cofactors in the origin of life as efficient catalysts and electron carriers. More than two decades ago, it was discovered that the in vivo synthesis of cellular Fe/S clusters and their integration into polypeptide chains requires assistance by complex proteinaceous machineries, despite the fact that Fe/S proteins can be assembled chemically in vitro. In prokaryotes, three Fe/S protein biogenesis systems are known; ISC, SUF, and the more specialized NIF. The former two systems have been transferred by endosymbiosis from bacteria to mitochondria and plastids, respectively, of eukaryotes. In their cytosol, eukaryotes use the CIA machinery for the biogenesis of cytosolic and nuclear Fe/S proteins. Despite the structural diversity of the protein constituents of these four machineries, general mechanistic concepts underlie the complex process of Fe/S protein biogenesis. This review provides a comprehensive and comparative overview of the various known biogenesis systems in Biology, and summarizes their common or diverging molecular mechanisms, thereby illustrating both the conservation and diverse adaptions of these four machineries during evolution and under different lifestyles. Knowledge of these fundamental biochemical pathways is not only of basic scientific interest, but is important for the understanding of human 'Fe/S diseases' and can be used in biotechnology.

Fe-S cluster biogenesis by the bacterial Suf pathway

Biogenesis of iron-sulfur (FeS) clusters in an essential process in living organisms due to the critical role of FeS cluster proteins in myriad cell functions. During biogenesis of FeS clusters, multi-protein complexes are used to drive the mobilization and protection of reactive sulfur and iron intermediates, regulate assembly of various FeS clusters on an ATPase-dependent, multi-protein scaffold, and target nascent clusters to their downstream protein targets. The evolutionarily ancient sulfur formation (Suf) pathway for FeS cluster assembly is found in bacteria and archaea. In Escherichia coli, the Suf pathway functions as an emergency pathway under conditions of iron limitation or oxidative stress. In other pathogenic bacteria, such as Mycobacterium tuberculosis and Enterococcus faecalis, the Suf pathway is the sole source for FeS clusters and therefore is a potential target for the development of novel antibacterial compounds. Here we summarize the considerable progress that has been made in characterizing the first step of mobilization and protection of reactive sulfur carried out by the SufS-SufE or SufS-SufU complex, FeS cluster assembly on SufBC₂D scaffold complexes, and the downstream trafficking of nascent FeS clusters to A-type carrier (ATC) proteins. Cell Biology of Metals III edited by Roland Lill and Mick Petris.

Biosynthesis of Sulfur-Containing Small Biomolecules in Plants

Sulfur is an essential element required for plant growth. It can be found as a thiol group of proteins or non-protein molecules, and as various sulfur-containing small biomolecules, including iron-sulfur (Fe/S) clusters, molybdenum cofactor (Moco), and sulfur-modified nucleotides. Thiol-mediated redox regulation has been well investigated, whereas biosynthesis pathways of the sulfur-containing small biomolecules have not yet been clearly described. In order to understand overall sulfur transfer processes in plant cells, it is important to elucidate the relationships among various sulfur delivery pathways as well as to investigate their interactions. In this review, we summarize the information from recent studies on the biosynthesis pathways of several sulfur-containing small biomolecules and the proteins participating in these processes. In addition, we show characteristic features of gene expression in Arabidopsis at the early stage of sulfate depletion from the medium, and we provide insights into sulfur transfer processes in plant cells.

Regulation of Iron Homeostasis and Use in Chloroplasts

Iron (Fe) is essential for life because of its role in protein cofactors. Photosynthesis, in particular photosynthetic electron transport, has a very high demand for Fe cofactors. Fe is commonly limiting in the environment, and therefore photosynthetic organisms must acclimate to Fe availability and avoid stress associated with Fe deficiency. In plants, adjustment of metabolism, of Fe utilization, and gene expression, is especially important in the chloroplasts during Fe limitation. In this review, we discuss Fe use, Fe transport, and mechanisms of acclimation to Fe limitation in photosynthetic lineages with a focus on the photosynthetic electron transport chain. We compare Fe homeostasis in Cyanobacteria, the evolutionary ancestors of chloroplasts, with Fe homeostasis in green algae and in land plants in order to provide a deeper understanding of how chloroplasts and photosynthesis may cope with Fe limitation.

Iron-Sulfur Cluster Biogenesis and Iron Homeostasis in Cyanobacteria

Iron–sulfur (Fe–S) clusters are ancient and ubiquitous cofactors and are involved in many important biological processes. Unlike the non-photosynthetic bacteria, cyanobacteria have developed the sulfur utilization factor (SUF) mechanism as their main assembly pathway for Fe–S clusters, supplemented by the iron–sulfur cluster and nitrogen-fixing mechanisms. The SUF system consists of cysteine desulfurase SufS, SufE that can enhance SufS activity, SufBC₂D scaffold complex, carrier protein SufA, and regulatory repressor SufR. The S source for the Fe–S cluster assembly mainly originates from L-cysteine, but the Fe donor remains elusive. This minireview mainly focuses on the biogenesis pathway of the Fe–S clusters in cyanobacteria and its relationship with iron homeostasis. Future challenges of studying Fe–S clusters in cyanobacteria are also discussed.

The plastidial Arabidopsis thaliana NFU1 protein binds and delivers [4Fe-4S] clusters to specific client proteins

Proteins incorporating iron-sulfur (Fe-S) co-factors are required for a plethora of metabolic processes. Their maturation depends on three Fe-S cluster assembly machineries in plants, located in the cytosol, mitochondria, and chloroplasts. After de novo formation on scaffold proteins, transfer proteins load Fe-S clusters onto client proteins. Among the plastidial representatives of these transfer proteins, NFU2 and NFU3 are required for the maturation of the [4Fe-4S] clusters present in photosystem I subunits, acting upstream of the high-chlorophyll fluorescence 101 (HCF101) protein. NFU2 is also required for the maturation of the [2Fe-2S]-containing dihydroxyacid dehydratase, important for branched-chain amino acid synthesis. Here, we report that recombinant Arabidopsis thaliana NFU1 assembles one [4Fe-4S] cluster per homodimer. Performing co-immunoprecipitation experiments and assessing physical interactions of NFU1 with many [4Fe-4S]-containing plastidial proteins in binary yeast two-hybrid assays, we also gained insights into the specificity of NFU1 for the maturation of chloroplastic Fe-S proteins. Using bimolecular fluorescence complementation and in vitro Fe-S cluster transfer experiments, we confirmed interactions with two proteins involved in isoprenoid and thiamine biosynthesis, 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate synthase and 4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate synthase, respectively. An additional interaction detected with the scaffold protein SUFD enabled us to build a model in which NFU1 receives its Fe-S cluster from the SUFBC2D scaffold complex and serves in the maturation of specific [4Fe-4S] client proteins. The identification of the NFU1 partner proteins reported here more clearly defines the role of NFU1 in Fe-S client protein maturation in Arabidopsis chloroplasts among other SUF components.

Iron-sulphur cluster biogenesis via the SUF pathway

Iron-sulphur (Fe-S) clusters are versatile cofactors, which are essential for key metabolic processes in cells, such as respiration and photosynthesis, and which may have also played a crucial role in establishing life on Earth. They can be found in almost all living organisms, from unicellular prokaryotes and archaea to multicellular animals and plants, and exist in diverse forms. This review focuses on the most ancient Fe-S cluster assembly system, the sulphur utilization factor (SUF) mechanism, which is crucial in bacteria for cell survival under stress conditions such as oxidation and iron starvation, and which is also present in the chloroplasts of green microalgae and plants, where it is responsible for plastidial Fe-S protein maturation. We explain the SUF Fe-S cluster assembly process, the proteins involved, their regulation and provide evolutionary insights. We specifically focus on examples from Fe-S cluster synthesis in the model organisms Escherichia coli and Arabidopsis thaliana and discuss in an in vivo context the assembly of the [FeFe]-hydrogenase H-cluster from Chlamydomonas reinhardtii.

Iron-sulfur protein NFU2 is required for branched-chain amino acid synthesis in Arabidopsis roots

Numerous proteins require a metallic co-factor for their function. In plastids, the maturation of iron-sulfur (Fe-S) proteins necessitates a complex assembly machinery. In this study, we focused on Arabidopsis thaliana NFU1, NFU2, and NFU3, which participate in the final steps of the maturation process. According to the strong photosynthetic defects observed in high chlorophyll fluorescence 101 (hcf101), nfu2, and nfu3 plants, we determined that NFU2 and NFU3, but not NFU1, act immediately upstream of HCF101 for the maturation of [Fe4S4]-containing photosystem I subunits. An additional function of NFU2 in the maturation of the [Fe2S2] cluster of a dihydroxyacid dehydratase was obvious from the accumulation of precursors of the branched-chain amino acid synthesis pathway in roots of nfu2 plants and from the rescue of the primary root growth defect by supplying branched-chain amino acids. The absence of NFU3 in roots precluded any compensation. Overall, unlike their eukaryotic and prokaryotic counterparts, which are specific to [Fe4S4] proteins, NFU2 and NFU3 contribute to the maturation of both [Fe2S2] and [Fe4S4] proteins, either as a relay in conjunction with other proteins such as HCF101 or by directly delivering Fe-S clusters to client proteins. Considering the low number of Fe-S cluster transfer proteins relative to final acceptors, additional targets probably await identification.

Comparative Proteomic Analysis of Coregulation of CIPK14 and WHIRLY1/3 Mediated Pale Yellowing of Leaves in Arabidopsis

Pale yellowing of leaf variegation is observed in the mutant Arabidopsis lines Calcineurin B-Like-Interacting Protein Kinase14 (CIPK14) overexpression (oeCIPK14) and double-knockout WHIRLY1/WHIRLY3 (why1/3). Further, the relative distribution of WHIRLY1 (WHY1) protein between plastids and the nucleus is affected by the phosphorylation of WHY1 by CIPK14. To elucidate the coregulation of CIPK14 and WHIRLY1/WHIRLY3-mediated pale yellowing of leaves, a differential proteomic analysis was conducted between the oeCIPK14 variegated (oeCIPK14-var) line, why1/3 variegated (why1/3-var) line, and wild type (WT). More than 800 protein spots were resolved on each gel, and 67 differentially abundant proteins (DAPs) were identified by matrix-assisted laser desorption ionization-time of flight/time of flight mass spectrometry (MALDI-TOF/TOF-MS). Of these 67 proteins, 34 DAPs were in the oeCIPK14-var line and 33 DAPs were in the why1/3-var line compared to the WT. Five overlapping proteins were differentially expressed in both the oeCIPK14-var and why1/3-var lines: ATP-dependent Clp protease proteolytic subunit-related protein 3 (ClpR3), Ribulose bisphosphate carboxylase large chain (RBCL), Beta-amylase 3 (BAM3), Ribosome-recycling factor (RRF), and Ribulose bisphosphate carboxylase small chain (RBCS). Bioinformatics analysis showed that most of the DAPs are involved in photosynthesis, defense and antioxidation pathways, protein metabolism, amino acid metabolism, energy metabolism, malate biosynthesis, lipid metabolism, and transcription. Thus, in the why1/3-var and oeCIPK14-var lines, there was a decrease in the photosystem parameters, including the content of chlorophyll, the photochemical efficiency of photosystem (PS II) (Fv/Fm), and electron transport rates (ETRs), but there was an increase in non-photochemical quenching (NPQ). Both mutants showed high sensitivity to intense light. Based on the annotation of the DAPs from both why1/3-var and oeCIPK14-var lines, we conclude that the CIPK14 phosphorylation-mediated WHY1 deficiency in plastids is related to the impairment of protein metabolism, leading to chloroplast dysfunction.

Recent Advances in the [Fe-S] Cluster Biogenesis (SUF) Pathway Functional in the Apicoplast of Plasmodium

Iron-sulfur [Fe-S] clusters are one of the most ancient, ubiquitous, structurally and functionally versatile natural biosynthetic prosthetic groups required by various proteins involved in important metabolic processes. Genome mining and localization studies in Plasmodium have shown two evolutionarily distinct biogenesis pathways: the ISC pathway in mitochondria and the SUF pathway in the apicoplast. In recent years, the myriad efforts made to elucidate the SUF pathway have deciphered the role of various proteins involved in the pathway and their importance for the parasite life cycle in both asexual and sexual stages. This review aims to discuss recent research in the apicoplast [Fe-S] biogenesis pathway from Plasmodium to enhance our current understanding of parasite biology with an overall aim to identify gaps to strengthen our fight against malaria.

Roles and maturation of iron-sulfur proteins in plastids

One reason why iron is an essential element for most organisms is its presence in prosthetic groups such as hemes or iron–sulfur (Fe–S) clusters, which are notably required for electron transfer reactions. As an organelle with an intense metabolism in plants, chloroplast relies on many Fe–S proteins. This includes those present in the electron transfer chain which will be, in fact, essential for most other metabolic processes occurring in chloroplasts, e.g., carbon fixation, nitrogen and sulfur assimilation, pigment, amino acid, and vitamin biosynthetic pathways to cite only a few examples. The maturation of these Fe–S proteins requires a complex and specific machinery named SUF (sulfur mobilisation). The assembly process can be split in two major steps, (1) the de novo assembly on scaffold proteins which requires ATP, iron and sulfur atoms, electrons, and thus the concerted action of several proteins forming early acting assembly complexes, and (2) the transfer of the preformed Fe–S cluster to client proteins using a set of late-acting maturation factors. Similar machineries, having in common these basic principles, are present in the cytosol and in mitochondria. This review focuses on the currently known molecular details concerning the assembly and roles of Fe–S proteins in plastids.

Approaches to Interrogate the Role of Nucleotide Hydrolysis by Metal Trafficking NTPases: The Nbp35-Cfd1 Iron-Sulfur Cluster Scaffold as a Case Study

Nucleotide hydrolases play integral yet poorly understood roles in several metallocluster biosynthetic pathways. For example, the cytosolic iron-sulfur cluster assembly (CIA) is initiated by the CIA scaffold, an ATPase which builds new iron-sulfur clusters for proteins localized to the cytosol and the nucleus in eukaryotic organisms. While in vivo studies have demonstrated the scaffold's nucleotide hydrolase domain is vital for its function, in vitro approaches have not revealed tight allosteric coupling between the cluster scaffolding site and the ATPase site. Thus, the role of ATP hydrolysis has been hard to pinpoint. Herein, we describe methods to probe the nucleotide affinity and hydrolysis activity of the CIA scaffold from yeast, which is comprised of two homologous polypeptides called Nbp35 and Cfd1. In particular, we report two different equilibrium binding assays that make use of commercially available fluorescent nucleotide analogs. Importantly, these assays can be applied to probe nucleotide affinity of both the apo- and holo-forms of the CIA scaffold. Generally, these fluorescent nucleotide analogs have been underutilized to probe metal trafficking NTPase because one of the most commonly used probes, mantATP, which is labeled with the methylanthraniloyl probe via the 2' or 3' sugar hydroxyls, has an absorption which overlaps with the UV-Vis features of many metal-binding proteins. However, by exploiting analogs like BODIPY-FL and trinitrophenyl-labeled nucleotides which have better photophysical properties for metalloprotein applications, these approaches have the potential to reveal the mechanistic underpinnings of NTPases required for metallocluster biosynthesis.

Assembly and Transfer of Iron-Sulfur Clusters in the Plastid

Iron-Sulfur (Fe-S) clusters and proteins are essential to many growth and developmental processes. In plants, they exist in the plastids, mitochondria, cytosol, and nucleus. Six types of Fe-S clusters are found in the plastid: classic 2Fe-2S, NEET-type 2Fe-2S, Rieske-type 2Fe-2S, 3Fe-4S, 4Fe-4S, and siroheme 4Fe-4S. Classic, NEET-type, and Rieske-type 2Fe-2S clusters have the same 2Fe-2S core; similarly, common and siroheme 4Fe-4S clusters have the same 4Fe-4S core. Plastidial Fe-S clusters are assembled by the sulfur mobilization (SUF) pathway, which contains cysteine desulfurase (EC 2.8.1.7), sulfur transferase (EC 2.8.1.3), Fe-S scaffold complex, and Fe-S carrier proteins. The plastidial cysteine desulfurase-sulfur transferase-Fe-S-scaffold complex system is responsible for de novo assembly of all plastidial Fe-S clusters. However, different types of Fe-S clusters are transferred to recipient proteins via respective Fe-S carrier proteins. This review focuses on recent discoveries on the molecular functions of different assembly and transfer factors involved in the plastidial SUF pathway. It also discusses potential points for regulation of the SUF pathway, relationships among the plastidial, mitochondrial, and cytosolic Fe-S assembly and transfer pathways, as well as several open questions about the carrier proteins for Rieske-type 2Fe-2S, NEET-type 2Fe-2S, and 3F-4S clusters.

The CIA Targeting Complex Is Highly Regulated and Provides Two Distinct Binding Sites for Client Iron-Sulfur Proteins

The cytoplasmic iron-sulfur assembly (CIA) targeting complex is required for the transfer of an iron-sulfur (Fe-S) cluster to cytoplasmic and nuclear proteins, but how it engages with client proteins is unknown. Here, we show that the complex members MIP18 and CIAO1 associate with the C terminus of MMS19. By doing so, they form a docking site for Fe-S proteins that is disrupted in the absence of either MMS19 or MIP18. The Fe-S helicase XPD seems to be the only exception, since it can interact with MMS19 independently of MIP18 and CIAO1. We further show that the direct interaction between MMS19 and MIP18 is required to protect MIP18 from proteasomal degradation. Taken together, these data suggest a remarkably regulated interaction between the CIA targeting complex and client proteins and raise the possibility that Fe-S cluster transfer is controlled, at least in part, by the stability of the CIA targeting complex itself.

PALE CRESS binds to plastid RNAs and facilitates the biogenesis of the 50S ribosomal subunit

The plant-specific PALE CRESS (PAC) protein has previously been shown to be essential for photoautotrophic growth. Here we further investigated the molecular function of the PAC protein. PAC localizes to plastid nucleoids and forms large proteinaceous and RNA-containing megadalton complexes. It co-immunoprecipitates with a specific subset of chloroplast RNAs including psbK-psbI, ndhF, ndhD, and 23S ribosomal RNA (rRNA), as demonstrated by RNA immunoprecipitation in combination with high throughput RNA sequencing (RIP-seq) analyses. Furthermore, it co-migrates with premature 50S ribosomal particles and specifically binds to 23S rRNA in vitro. This coincides with severely reduced levels of 23S rRNA in pac leading to translational deficiencies and related alterations of plastid transcript patterns and abundance similar to plants treated with the translation inhibitor lincomycin. Thus, we conclude that deficiency in plastid ribosomes accounts for the pac phenotype. Moreover, the absence or reduction of PAC levels in the corresponding mutants induces structural changes of the 23S rRNA, as demonstrated by in vivo RNA structure probing. Our results indicate that PAC binds to the 23S rRNA to promote the biogenesis of the 50S subunit.

Drought and salinity induced changes in ecophysiology and proteomic profile of Parthenium hysterophorus

Parthenium hysterophorus is a plant that tolerates drought and salinity to an extremely high degree. Higher expression of stress-responsive proteome contributes for greater defence against abiotic stresses. Thus, P. hysterophorus could be a rich source of genes that encode stress-imparting mechanisms and systems. The present study utilizes comparative physiological and proteomic approaches for identification of key proteins involved in stress-defence of P. hysterophorus. Thirty-days-old plants were exposed to drought (10% PEG 6000) and salinity (160 mM NaCl) for 10 days duration. Both stresses induced oxidative stress estimated in terms of TBARS and H2O2. Levels of both enzymatic and non-enzymatic antioxidants were elevated, more by drought than salinity. Particularly, SOD, GR, CAT and GST proved to be assisting as very commendable defence under drought, as well as salinity. Levels of ascorbate, glutathione and proline were also increased by both stresses, more in response to drought. Comparative proteomics analysis revealed a significant change in relative abundance of 72 proteins under drought and salinity. Drought and salinity increased abundance of 45 and 41 proteins and decreased abundance of 24 and 26 proteins, respectively. Drought and salinity increased and decreased abundance of 31 and 18 proteins, respectively. The functions of identified proteins included those related to defence response (26%), signal transduction (13%), transcription and translation (10%), growth and development (8.5%), photosynthesis (8.5%), metabolism (7%), terpenoid biosynthesis (5.5%), protein modification and transport (7%), oxido-reductase (4%) and Miscellaneous (11%). Among the defence related proteins, antioxidants and HSPs constituted 26% and 21%, respectively. Present study suggests a potential role of defence proteins. Proteins involved in molecular stabilization, formation of osmolytes and wax and contributing to stress-avoiding anatomical features emerged as key and complex mechanisms for imparting stress tolerance to P. hysterophorus.

Investigating the role(s) of SufT and the domain of unknown function 59 (DUF59) in the maturation of iron-sulfur proteins

Comprehending biology at the molecular and systems levels is predicated upon understanding the functions of proteins. Proteins are typically composed of one or more functional moieties termed domains. Members of Bacteria, Eukarya, and Archaea utilize proteins containing a domain of unknown function (DUF) 59. Proteins requiring iron-sulfur (FeS) clusters containing cofactors are necessary for nearly all organisms making the assembly of functional FeS proteins essential. Recently, studies in eukaryotic and bacterial organisms have shown that proteins containing a DUF59, or those composed solely of DUF59, function in FeS protein maturation and/or intracellular Fe homeostasis. Herein, we review the current literature, discuss potential roles for DUF59, and address future studies that will help advance the field.

The SUFBC2 D complex is required for the biogenesis of all major classes of plastid Fe-S proteins

Iron-sulfur (Fe-S) proteins play crucial roles in plastids, participating in photosynthesis and other metabolic pathways. Fe-S clusters are thought to be assembled on a scaffold complex composed of SUFB, SUFC and SUFD proteins. However, several additional proteins provide putative scaffold functions in plastids, and, therefore, the contribution of SUFB, C and D proteins to overall Fe-S assembly still remains unclear. In order to gain insights regarding Fe-S cluster biosynthesis in plastids, we analyzed the complex composed of SUFB, C and D in Arabidopsis by blue native-polyacrylamide gel electrophoresis. Using this approach, a major complex of 170 kDa containing all subunits was detected, indicating that these proteins constitute a SUFBC₂ D complex similar to their well characterized bacterial counterparts. The functional effects of SUFB, SUFC or SUFD depletion were analyzed using an inducible RNAi silencing system to specifically target the aforementioned components; resulting in a decrease of various plastidic Fe-S proteins including the PsaA/B and PsaC subunits of photosystem I, ferredoxin and glutamine oxoglutarate aminotransferase. In contrast, the knockout of potential Fe-S scaffold proteins, NFU2 and HCF101, resulted in a specific decrease in the PsaA/B and PsaC levels. These results indicate that the functions of SUFB, SUFC and SUFD for Fe-S cluster biosynthesis cannot be replaced by other scaffold proteins and that SUFBC₂ D, NFU2 and HCF101 are involved in the same pathway for the biogenesis of PSI. Taken together, our results provide in vivo evidence supporting the hypothesis that SUFBC₂ D is the major, and possibly sole scaffold in plastids.

Staphylococcus aureus SufT: an essential iron-sulphur cluster assembly factor in cells experiencing a high-demand for lipoic acid

Staphylococcus aureus SufT is composed solely of the domain of unknown function 59 (DUF59) and has a role in the maturation of iron-sulphur (Fe-S) proteins. We report that SufT is essential for S. aureus when growth is heavily reliant upon lipoamide-utilizing enzymes, but dispensable when this reliance is decreased. LipA requires Fe-S clusters for lipoic acid (LA) synthesis and a ΔsufT strain had phenotypes suggestive of decreased LA production and decreased activities of lipoamide-requiring enzymes. Fermentative growth, a null clpC allele, or decreased flux through the TCA cycle diminished the demand for LA and rendered SufT non-essential. Abundance of the Fe-S cluster carrier Nfu was increased in a ΔclpC strain and a null clpC allele was unable to suppress the LA requirement of a ΔsufT Δnfu strain. Over-expression of nfu suppressed the LA requirement of the ΔsufT strain. We propose a model wherein SufT, and by extension the DUF59, is essential for the maturation of holo-LipA in S. aureus cells experiencing a high demand for lipoamide-dependent enzymes. The findings presented suggest that the demand for products of Fe-S enzymes is a factor governing the usage of one Fe-S cluster assembly factor over another in the maturation of apo-proteins.

The DUF59 Containing Protein SufT Is Involved in the Maturation of Iron-Sulfur (FeS) Proteins during Conditions of High FeS Cofactor Demand in Staphylococcus aureus

Proteins containing DUF59 domains have roles in iron-sulfur (FeS) cluster assembly and are widespread throughout Eukarya, Bacteria, and Archaea. However, the function(s) of this domain is unknown. Staphylococcus aureus SufT is composed solely of a DUF59 domain. We noted that sufT is often co-localized with sufBC, which encode for the Suf FeS cluster biosynthetic machinery. Phylogenetic analyses indicated that sufT was recruited to the suf operon, suggesting a role for SufT in FeS cluster assembly. A S. aureus ΔsufT mutant was defective in the assembly of FeS proteins. The DUF59 protein Rv1466 from Mycobacterium tuberculosis partially corrected the phenotypes of a ΔsufT mutant, consistent with a widespread role for DUF59 in FeS protein maturation. SufT was dispensable for FeS protein maturation during conditions that imposed a low cellular demand for FeS cluster assembly. In contrast, the role of SufT was maximal during conditions imposing a high demand for FeS cluster assembly. SufT was not involved in the repair of FeS clusters damaged by reactive oxygen species or in the physical protection of FeS clusters from oxidants. Nfu is a FeS cluster carrier and nfu displayed synergy with sufT. Furthermore, introduction of nfu upon a multicopy plasmid partially corrected the phenotypes of the ΔsufT mutant. Biofilm formation and exoprotein production are critical for S. aureus pathogenesis and vancomycin is a drug of last-resort to treat staphylococcal infections. Defective FeS protein maturation resulted in increased biofilm formation, decreased production of exoproteins, increased resistance to vancomycin, and the appearance of phenotypes consistent with vancomycin-intermediate resistant S. aureus. We propose that SufT, and by extension the DUF59 domain, is an accessory factor that functions in the maturation of FeS proteins. In S. aureus, the involvement of SufT is maximal during conditions of high demand for FeS proteins.

A Novel Redoxin in the Thylakoid Membrane Regulates the Titer of Photosystem I

In photosynthetic organisms like cyanobacteria and plants, the main engines of oxygenic photosynthesis are the pigment-protein complexes photosystem I (PSI) and photosystem II (PSII) located in the thylakoid membrane. In the cyanobacterium Synechocystis sp. PCC 6803, the slr1796 gene encodes a single cysteine thioredoxin-like protein, orthologs of which are found in multiple cyanobacterial strains as well as chloroplasts of higher plants. Targeted inactivation of slr1796 in Synechocystis 6803 resulted in compromised photoautotrophic growth. The mutant displayed decreased chlorophyll a content. These changes correlated with a decrease in the PSI titer of the mutant cells, whereas the PSII content was unaffected. In the mutant, the transcript levels of genes for PSI structural and accessory proteins remained unaffected, whereas the levels of PSI structural proteins were severely diminished, indicating that Slr1796 acts at a posttranscriptional level. Biochemical analysis indicated that Slr1796 is an integral thylakoid membrane protein. We conclude that Slr1796 is a novel regulatory factor that modulates PSI titer.

The Yeast Nbp35-Cfd1 Cytosolic Iron-Sulfur Cluster Scaffold Is an ATPase

Nbp35 and Cfd1 are prototypical members of the MRP/Nbp35 class of iron-sulfur (FeS) cluster scaffolds that function to assemble nascent FeS clusters for transfer to FeS-requiring enzymes. Both proteins contain a conserved NTPase domain that genetic studies have demonstrated is essential for their cluster assembly activity inside the cell. It was recently reported that these proteins possess no or very low nucleotide hydrolysis activity in vitro, and thus the role of the NTPase domain in cluster biogenesis has remained uncertain. We have reexamined the NTPase activity of Nbp35, Cfd1, and their complex. Using in vitro assays and site-directed mutagenesis, we demonstrate that the Nbp35 homodimer and the Nbp35-Cfd1 heterodimer are ATPases, whereas the Cfd1 homodimer exhibited no or very low ATPase activity. We ruled out the possibility that the observed ATP hydrolysis activity might result from a contaminating ATPase by showing that mutation of key active site residues reduced activity to background levels. Finally, we demonstrate that the fluorescent ATP analog 2'/3'-O-(N'-methylanthraniloyl)-ATP (mantATP) binds stoichiometrically to Nbp35 with a KD = 15.6 μM and that an Nbp35 mutant deficient in ATP hydrolysis activity also displays an increased KD for mantATP. Together, our results demonstrate that the cytosolic iron-sulfur cluster assembly scaffold is an ATPase and pave the way for interrogating the role of nucleotide hydrolysis in cluster biogenesis by this large family of cluster scaffolding proteins found across all domains of life.

Hsp90 is involved in the regulation of cytosolic precursor protein abundance in tomato

Cytosolic chaperones are involved in the regulation of cellular protein homeostasis in general. Members of the families of heat stress proteins 70 (Hsp70) and 90 (Hsp90) assist the transport of preproteins to organelles such as chloroplasts or mitochondria. In addition, Hsp70 was described to be involved in the degradation of chloroplast preproteins that accumulate in the cytosol. Because a similar function has not been established for Hsp90, we analyzed the influences of Hsp90 and Hsp70 on the protein abundance in the cellular context using an in vivo system based on mesophyll protoplasts. We observed a differential behavior of preproteins with respect to the cytosolic chaperone-dependent regulation. Some preproteins such as pOE33 show a high dependence on Hsp90, whereas the abundance of preproteins such as pSSU is more strongly dependent on Hsp70. The E3 ligase, C-terminus of Hsp70-interacting protein (Chip), appears to have a more general role in the control of cytosolic protein abundance. We discuss why the different reaction modes are comparable with the cytosolic unfolded protein response.

Molecular mechanism of photosystem I assembly in oxygenic organisms

Photosystem I, an integral membrane and multi-subunit complex, catalyzes the oxidation of plastocyanin and the reduction of ferredoxin by absorbed light energy. Photosystem I participates in photosynthetic acclimation processes by being involved in cyclic electron transfer and state transitions for sustaining efficient photosynthesis. The photosystem I complex is highly conserved from cyanobacteria to higher plants and contains the light-harvesting complex and the reaction center complex. The assembly of the photosystem I complex is highly complicated and involves the concerted assembly of multiple subunits and hundreds of cofactors. A suite of regulatory factors for the assembly of photosystem I subunits and cofactors have been identified that constitute an integrative network regulating PSI accumulation. This review aims to discuss recent findings in the field relating to how the photosystem I complex is assembled in oxygenic organisms. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Recent advances in the Suf Fe-S cluster biogenesis pathway: Beyond the Proteobacteria

Fe-S clusters play critical roles in cellular function throughout all three kingdoms of life. Consequently, Fe-S cluster biogenesis systems are present in most organisms. The Suf (sulfur formation) system is the most ancient of the three characterized Fe-S cluster biogenesis pathways, which also include the Isc and Nif systems. Much of the first work on the Suf system took place in Gram-negative Proteobacteria used as model organisms. These early studies led to a wealth of biochemical, genetic, and physiological information on Suf function. From those studies we have learned that SufB functions as an Fe-S scaffold in conjunction with SufC (and in some cases SufD). SufS and SufE together mobilize sulfur for cluster assembly and SufA traffics the complete Fe-S cluster from SufB to target apo-proteins. However, recent progress on the Suf system in other organisms has opened up new avenues of research and new hypotheses about Suf function. This review focuses primarily on the most recent discoveries about the Suf pathway and where those new models may lead the field. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

A thylakoid membrane protein harboring a DnaJ-type zinc finger domain is required for photosystem I accumulation in plants

Photosystem I (PSI) is a large pigment-protein complex and one of the two photosystems that drive electron transfer in oxygenic photosynthesis. We identified a nuclear gene required specifically for the accumulation of PSI in a forward genetic analysis of chloroplast biogenesis in maize. This gene, designated psa2, belongs to the "GreenCut" gene set, a group of genes found in green algae and plants but not in non-photosynthetic organisms. Disruption of the psa2 ortholog in Arabidopsis likewise resulted in the specific loss of PSI proteins. PSA2 harbors a conserved domain found in DnaJ chaperones where it has been shown to form a zinc finger and to have protein-disulfide isomerase activity. Accordingly, PSA2 exhibited protein-disulfide reductase activity in vitro. PSA2 localized to the thylakoid lumen and was found in a ∼250-kDa complex harboring the peripheral PSI protein PsaG but lacking several core PSI subunits. PSA2 mRNA is coexpressed with mRNAs encoding various proteins involved in the biogenesis of the photosynthetic apparatus with peak expression preceding that of genes encoding structural components. PSA2 protein abundance was not decreased in the absence of PSI but was reduced in the absence of the PSI assembly factor Ycf3. These findings suggest that a complex harboring PSA2 and PsaG mediates thiol transactions in the thylakoid lumen that are important for the assembly of PSI.

Chloroplast evolution, structure and functions

In this review, we consider a selection of recent advances in chloroplast biology. These include new findings concerning chloroplast evolution, such as the identification of Chlamydiae as a third partner in primary endosymbiosis, a second instance of primary endosymbiosis represented by the chromatophores found in amoebae of the genus Paulinella, and a new explanation for the longevity of captured chloroplasts (kleptoplasts) in sacoglossan sea slugs. The controversy surrounding the three-dimensional structure of grana, its recent resolution by tomographic analyses, and the role of the CURVATURE THYLAKOID1 (CURT1) proteins in supporting grana formation are also discussed. We also present an updated inventory of photosynthetic proteins and the factors involved in the assembly of thylakoid multiprotein complexes, and evaluate findings that reveal that cyclic electron flow involves NADPH dehydrogenase (NDH)- and PGRL1/PGR5-dependent pathways, both of which receive electrons from ferredoxin. Other topics covered in this review include new protein components of nucleoids, an updated inventory of the chloroplast proteome, new enzymes in chlorophyll biosynthesis and new candidate messengers in retrograde signaling. Finally, we discuss the first successful synthetic biology approaches that resulted in chloroplasts in which electrons from the photosynthetic light reactions are fed to enzymes derived from secondary metabolism.

Structure and dynamics of thylakoids in land plants

Thylakoids of land plants have a bipartite structure, consisting of cylindrical grana stacks, made of membranous discs piled one on top of the other, and stroma lamellae which are helically wound around the cylinders. Protein complexes predominantly located in the stroma lamellae and grana end membranes are either bulky [photosystem I (PSI) and the chloroplast ATP synthase (cpATPase)] or are involved in cyclic electron flow [the NAD(P)H dehydrogenase (NDH) and PGRL1-PGR5 heterodimers], whereas photosystem II (PSII) and its light-harvesting complex (LHCII) are found in the appressed membranes of the granum. Stacking of grana is thought to be due to adhesion between Lhcb proteins (LHCII or CP26) located in opposed thylakoid membranes. The grana margins contain oligomers of CURT1 proteins, which appear to control the size and number of grana discs in a dosage- and phosphorylation-dependent manner. Depending on light conditions, thylakoid membranes undergo dynamic structural changes that involve alterations in granum diameter and height, vertical unstacking of grana, and swelling of the thylakoid lumen. This plasticity is realized predominantly by reorganization of the supramolecular structure of protein complexes within grana stacks and by changes in multiprotein complex composition between appressed and non-appressed membrane domains. Reversible phosphorylation of LHC proteins (LHCPs) and PSII components appears to initiate most of the underlying regulatory mechanisms. An update on the roles of lipids, proteins, and protein complexes, as well as possible trafficking mechanisms, during thylakoid biogenesis and the de-etiolation process complements this review.

Genes for iron-sulphur cluster assembly are targets of abiotic stress in rice, Oryza sativa

Iron-sulphur (Fe-S) cluster assembly occurs in chloroplasts, mitochondria and cytosol, involving dozens of genes in higher plants. In this study, we have identified 41 putative Fe-S cluster assembly genes in rice (Oryza sativa) genome, and the expression of all genes was verified. To investigate the role of Fe-S cluster assembly as a metabolic pathway, we applied abiotic stresses to rice seedlings and analysed Fe-S cluster assembly gene expression by qRT-PCR. Our data showed that genes for Fe-S cluster assembly in chloroplasts of leaves are particularly sensitive to heavy metal treatments, and that Fe-S cluster assembly genes in roots were up-regulated in response to iron toxicity, oxidative stress and some heavy metal assault. The effect of each stress treatment on the Fe-S cluster assembly machinery demonstrated an unexpected tissue or organelle specificity, suggesting that the physiological relevance of the Fe-S cluster assembly is more complex than thought. Furthermore, our results may reveal potential candidate genes for molecular breeding of rice.

Iron cofactor assembly in plants

Iron is an essential element for all photosynthetic organisms. The biological use of this transition metal is as an enzyme cofactor, predominantly in electron transfer and catalysis. The main forms of iron cofactor are, in order of decreasing abundance, iron-sulfur clusters, heme, and di-iron or mononuclear iron, with a wide functional range. In plants and algae, iron-sulfur cluster assembly pathways of bacterial origin are localized in the mitochondria and plastids, where there is a high demand for these cofactors. A third iron-sulfur cluster assembly pathway is present in the cytosol that depends on the mitochondria but not on plastid assembly proteins. The biosynthesis of heme takes place mainly in the plastids. The importance of iron-sulfur cofactors beyond photosynthesis and respiration has become evident with recent discoveries of novel iron-sulfur proteins involved in epigenetics and DNA metabolism. In addition, increased understanding of intracellular iron trafficking is opening up research into how iron is distributed between iron cofactor assembly pathways and how this distribution is regulated.

Evolution of the cytosolic iron-sulfur cluster assembly machinery in Blastocystis species and other microbial eukaryotes

The cytosolic iron/sulfur cluster assembly (CIA) machinery is responsible for the assembly of cytosolic and nuclear iron/sulfur clusters, cofactors that are vital for all living cells. This machinery is uniquely found in eukaryotes and consists of at least eight proteins in opisthokont lineages, such as animals and fungi. We sought to identify and characterize homologues of the CIA system proteins in the anaerobic stramenopile parasite Blastocystis sp. strain NandII. We identified transcripts encoding six of the components-Cia1, Cia2, MMS19, Nbp35, Nar1, and a putative Tah18-and showed using immunofluorescence microscopy, immunoelectron microscopy, and subcellular fractionation that the last three of them localized to the cytoplasm of the cell. We then used comparative genomic and phylogenetic approaches to investigate the evolutionary history of these proteins. While most Blastocystis homologues branch with their eukaryotic counterparts, the putative Blastocystis Tah18 seems to have a separate evolutionary origin and therefore possibly a different function. Furthermore, our phylogenomic analyses revealed that all eight CIA components described in opisthokonts originated before the diversification of extant eukaryotic lineages and were likely already present in the last eukaryotic common ancestor (LECA). The Nbp35, Nar1 Cia1, and Cia2 proteins have been conserved during the subsequent evolutionary diversification of eukaryotes and are present in virtually all extant lineages, whereas the other CIA proteins have patchy phylogenetic distributions. Cia2 appears to be homologous to SufT, a component of the prokaryotic sulfur utilization factors (SUF) system, making this the first reported evolutionary link between the CIA and any other Fe/S biogenesis pathway. All of our results suggest that the CIA machinery is an ubiquitous biosynthetic pathway in eukaryotes, but its apparent plasticity in composition raises questions regarding how it functions in nonmodel organisms and how it interfaces with various iron/sulfur cluster systems (i.e., the iron/sulfur cluster, nitrogen fixation, and/or SUF system) found in eukaryotic cells.

The evolutionarily conserved iron-sulfur protein INDH is required for complex I assembly and mitochondrial translation in Arabidopsis [corrected]

The assembly of respiratory complexes is a multistep process, requiring coordinate expression of mitochondrial and nuclear genes and cofactor biosynthesis. We functionally characterized the iron-sulfur protein required for NADH dehydrogenase (INDH) in the model plant Arabidopsis thaliana. An indh knockout mutant lacked complex I but had low levels of a 650-kD assembly intermediate, similar to mutations in the homologous NUBPL (nucleotide binding protein-like) in Homo sapiens. However, heterozygous indh/+ mutants displayed unusual phenotypes during gametogenesis and resembled mutants in mitochondrial translation more than mutants in complex I. Gradually increased expression of INDH in indh knockout plants revealed a significant delay in reassembly of complex I, suggesting an indirect role for INDH in the assembly process. Depletion of INDH protein was associated with decreased (35)S-Met labeling of translation products in isolated mitochondria, whereas the steady state levels of several mitochondrial transcripts were increased. Mitochondrially encoded proteins were differentially affected, with near normal levels of cytochrome c oxidase subunit2 and Nad7 but little Nad6 protein in the indh mutant. These data suggest that INDH has a primary role in mitochondrial translation that underlies its role in complex I assembly.

Identification of a cyanobacterial CRR6 protein, Slr1097, required for efficient assembly of NDH-1 complexes in Synechocystis sp. PCC 6803

Despite significant progress in clarifying the subunit compositions and functions of the multiple NADPH dehydrogenase (NDH-1) complexes in cyanobacteria, the subunit maturation and assembly of their NDH-1 complexes are poorly understood. By transformation of wild-type cells with a transposon-tagged library, we isolated three mutants of Synechocystis sp. PCC 6803 defective in NDH-1-mediated cyclic electron transfer and unable to grow under high light conditions. All the mutants were tagged in the same slr1097 gene, encoding an unknown protein that shares significant homology with the Arabidopsis protein chlororespiratory reduction 6 (CRR6). The slr1097 product was localized in the cytoplasm and was required for efficient assembly of NDH-1 complexes. Analysis of the interaction of Slr1097 with 18 subunits of NDH-1 complexes using a yeast two-hybrid system indicated a strong interaction with NdhI but not with other Ndh subunits. Absence of Slr1097 resulted in a significant decrease of NdhI in the cytoplasm, but not of other Ndh subunits including NdhH, NdhK and NdhM; the decrease was more evident in the cytoplasm than in the thylakoid membranes. In the ∆slr1097 mutant, NdhH, NdhI, NdhK and NdhM were hardly detectable in the NDH-1M complex, whereas almost half the wild-type levels of these subunits were present in NDH-1L complex; similar results were observed in the NdhI-less mutant. These results suggest that Slr1097 is involved in the maturation of NdhI, and that assembly of the NDH-1M complex is strongly dependent on this factor. Maturation of NdhI appears not to be crucial to assembly of the NDH-1L complex.

Interaction with Cfd1 increases the kinetic lability of FeS on the Nbp35 scaffold

P-loop NTPases of the ApbC/Nbp35 family are involved in FeS protein maturation in nearly all organisms and are proposed to function as scaffolds for initial FeS cluster assembly. In yeast and animals, Cfd1 and Nbp35 are homologous P-loop NTPases that form a heterotetrameric complex essential for FeS protein maturation through the cytosolic FeS cluster assembly (CIA) pathway. Cfd1 is conserved in animals, fungi, and several archaeal species, but in many organisms, only Nbp35 is present, raising the question of the unique roles played by Cfd1 and Nbp35. To begin to investigate this issue, we examined Cfd1 and Nbp35 function in budding yeast. About half of each protein was detected in a heterocomplex in logarithmically growing yeast. Nbp35 readily bound (55)Fe when fed to cells, whereas (55)Fe binding by free Cfd1 could not be detected. Rapid (55)Fe binding to and release from Nbp35 was impaired by Cfd1 deficiency. A Cfd1 mutation that caused a defect in heterocomplex stability supported iron binding to Nbp35 but impaired iron release. Our results suggest a model in which Cfd1-Nbp35 interaction increases the lability of assembled FeS on the Nbp35 scaffold for transfer to target apo-FeS proteins.

Regulatory factors for the assembly of thylakoid membrane protein complexes

Major multi-protein photosynthetic complexes, located in thylakoid membranes, are responsible for the capture of light and its conversion into chemical energy in oxygenic photosynthetic organisms. Although the structures and functions of these photosynthetic complexes have been explored, the molecular mechanisms underlying their assembly remain elusive. In this review, we summarize current knowledge of the regulatory components involved in the assembly of thylakoid membrane protein complexes in photosynthetic organisms. Many of the known regulatory factors are conserved between prokaryotes and eukaryotes, whereas others appear to be newly evolved or to have expanded predominantly in eukaryotes. Their specific features and fundamental differences in cyanobacteria, green algae and land plants are discussed.

How to build functional thylakoid membranes: from plastid transcription to protein complex assembly

Chloroplasts are the endosymbiotic descendants of cyanobacterium-like prokaryotes. Present genomes of plant and green algae chloroplasts (plastomes) contain ~100 genes mainly encoding for their transcription-/translation-machinery, subunits of the thylakoid membrane complexes (photosystems II and I, cytochrome b (6) f, ATP synthase), and the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase. Nevertheless, proteomic studies have identified several thousand proteins in chloroplasts indicating that the majority of the plastid proteome is not encoded by the plastome. Indeed, plastid and host cell genomes have been massively rearranged in the course of their co-evolution, mainly through gene loss, horizontal gene transfer from the cyanobacterium/chloroplast to the nucleus of the host cell, and the emergence of new nuclear genes. Besides structural components of thylakoid membrane complexes and other (enzymatic) complexes, the nucleus provides essential factors that are involved in a variety of processes inside the chloroplast, like gene expression (transcription, RNA-maturation and translation), complex assembly, and protein import. Here, we provide an overview on regulatory factors that have been described and characterized in the past years, putting emphasis on mechanisms regulating the expression and assembly of the photosynthetic thylakoid membrane complexes.