A Nitrogen-Fixing Subunit Essential for Accumulating 4Fe-4S-Containing Photosystem I Core Proteins

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.

Light signaling-dependent regulation of plastid RNA processing in Arabidopsis

Light is a vital environmental signal that regulates the expression of plastid genes. Plastids are crucial organelles that respond to light, but the effects of light on plastid RNA processing following transcription remain unclear. In this study, we systematically examined the influence of light exposure on plastid RNA processing, focusing on RNA splicing and RNA editing. We demonstrated that light promotes the splicing of transcripts from the plastid genes rps12, ndhA, atpF, petB, and rpl2. Additionally, light increased the editing rate of the accD transcript at nucleotide 794 (accD-794) and the ndhF transcript at nucleotide 290 (ndhF-290), while decreasing the editing rate of the clpP transcript at nucleotide 559 (clpP-559). We have identified key regulators of signaling pathways, such as CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1), ELONGATED HYPOCOTYL 5 (HY5), and PHYTOCHROME-INTERACTING FACTORs (PIFs), as important players in the regulation of plastid RNA splicing and editing. Notably, COP1 was required for GENOMES UNCOUPLED1 (GUN1)-dependent repression of clpP-559 editing in the light. We showed that HY5 and PIF1 bind to the promoters of nuclear genes encoding plastid-localized RNA processing factors in a light-dependent manner. This study provides insight into the mechanisms underlying light-mediated post-transcriptional regulation of plastid gene expression.

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.

A homolog of GuidedEntry of Tail-anchored proteins3 functions in membrane-specific protein targeting in chloroplasts of Arabidopsis

The chloroplasts and mitochondria of photosynthetic eukaryotes contain proteins that are closely related to cytosolic Guided Entry of Tail-anchored proteins3 (Get3). Get3 is a targeting factor that efficiently escorts tail-anchored (TA) proteins to the ER. Because other components of the cytosolic-targeting pathway appear to be absent in organelles, previous investigators have asserted that organellar Get3 homologs are unlikely to act as targeting factors. However, we show here both that the Arabidopsis thaliana chloroplast homolog designated as GET3B is structurally similar to cytosolic Get3 proteins and that it selectively binds a thylakoid-localized TA protein. Based on genetic interactions between a get3b mutation and mutations affecting the chloroplast signal recognition particle-targeting pathway, as well as changes in the abundance of photosynthesis-related proteins in mutant plants, we propose that GET3B acts primarily to direct proteins to the thylakoids. Furthermore, through molecular complementation experiments, we show that function of GET3B depends on its ability to hydrolyze ATP, and this is consistent with action as a targeting factor. We propose that GET3B and related organellar Get3 homologs play a role that is analogous to that of cytosolic Get3 proteins, and that GET3B acts as a targeting factor in the chloroplast stroma to deliver TA proteins in a membrane-specific manner.

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.

Protection of PSI and PSII complexes of wheat from toxic effect of anthracene by Bacillus subtilis (NCIM 5594)

Contamination of polycyclic aromatic hydrocarbons (PAHs) in environment indicates a serious problem to the present era. These are carcinogenic and mutagenic compounds and pose a potential risk to photosynthetic organisms. The present study illustrates the protection of Photosystem I and Photosystem II complexes of wheat plant by Bacillus subtilis (NCIM 5594) from toxic effects of anthracene (ANT). Initially, Chl a fluorescence induction curve measurement revealed declined J-I and I-P phase in ANT-treated plants. Efficiency of light absorption, trapping, and electron transport was reduced in ANT-treated plants, while in ANT + Bacillus subtilis (NCIM 5594)-treated plants value of these parameters was restored. Effect of ANT and ANT + Bacillus subtilis (NCIM 5594) on energy conversion of Photosystem I and Photosystem II was measured. Quantum yield of Photosystem I (YI) and Photosystem II (YII) was decreased in the presence of ANT, while these values were recovered in ANT + Bacillus subtilis (NCIM 5594)-treated plants. Reduction in Y(II) was associated with an increase in non-regulated energy dissipation NO. Likewise the reduction of Y(I) was induced due to donor-side and acceptor-side limitation of Photosystem I caused by toxic effect of ANT. Toxic effects of ANT on electron transport rate (ETR_I and ETR_II) were found to be reduced in ANT + Bacillus subtilis (NCIM 5594)-treated plants. Activation of Cyclic electron flow around Photosystem I in ANT-treated plants was recovered by bacteria. It was concluded that toxic effect of ANT on Photosystem I and Photosystem II complexes was recovered by Bacillus subtilis (NCIM 5594) strain, and thus it is useful strain for crop improvement in ANT-polluted soil.

A Global Proteomic Approach Sheds New Light on Potential Iron-Sulfur Client Proteins of the Chloroplastic Maturation Factor NFU3

Iron-sulfur (Fe-S) proteins play critical functions in plants. Most Fe-S proteins are synthetized in the cytosol as apo-proteins and the subsequent Fe-S cluster incorporation relies on specific protein assembly machineries. They are notably formed by a scaffold complex, which serves for the de novo Fe-S cluster synthesis, and by transfer proteins that insure cluster delivery to apo-targets. However, scarce information is available about the maturation pathways of most plastidial Fe-S proteins and their specificities towards transfer proteins of the associated SUF machinery. To gain more insights into these steps, the expression and protein localization of the NFU1, NFU2, and NFU3 transfer proteins were analyzed in various Arabidopsis thaliana organs and tissues showing quite similar expression patterns. In addition, quantitative proteomic analysis of an nfu3 loss-of-function mutant allowed to propose novel potential client proteins for NFU3 and to show that the protein accumulation profiles and thus metabolic adjustments differ substantially from those established in the nfu2 mutant. By clarifying the respective roles of the three plastidial NFU paralogs, these data allow better delineating the maturation process of plastidial Fe-S proteins.

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.

Chloroplast Transition Metal Regulation for Efficient Photosynthesis

Plants require sunlight, water, CO₂, and essential nutrients to drive photosynthesis and fulfill their life cycle. The photosynthetic apparatus resides in chloroplasts and fundamentally relies on transition metals as catalysts and cofactors. Accordingly, chloroplasts are particularly rich in iron (Fe), manganese (Mn), and copper (Cu). Owing to their redox properties, those metals need to be carefully balanced within the cell. However, the regulation of transition metal homeostasis in chloroplasts is poorly understood. With the availability of the arabidopsis genome information and membrane protein databases, a wider catalogue for searching chloroplast metal transporters has considerably advanced the study of transition metal regulation. This review provides an updated overview of the chloroplast transition metal requirements and the transporters involved for efficient photosynthesis in higher plants.

Identification of client iron-sulfur proteins of the chloroplastic NFU2 transfer protein in Arabidopsis thaliana

Iron-sulfur (Fe-S) proteins have critical functions in plastids, notably participating in photosynthetic electron transfer, sulfur and nitrogen assimilation, chlorophyll metabolism, and vitamin or amino acid biosynthesis. Their maturation relies on the so-called SUF (sulfur mobilization) assembly machinery. Fe-S clusters are synthesized de novo on a scaffold protein complex and then delivered to client proteins via several transfer proteins. However, the maturation pathways of most client proteins and their specificities for transfer proteins are mostly unknown. In order to decipher the proteins interacting with the Fe-S cluster transfer protein NFU2, one of the three plastidial representatives found in Arabidopsis thaliana, we performed a quantitative proteomic analysis of shoots, roots, and seedlings of nfu2 plants, combined with NFU2 co-immunoprecipitation and binary yeast two-hybrid experiments. We identified 14 new targets, among which nine were validated in planta using a binary bimolecular fluorescence complementation assay. These analyses also revealed a possible role for NFU2 in the plant response to desiccation. Altogether, this study better delineates the maturation pathways of many chloroplast Fe-S proteins, considerably extending the number of NFU2 clients. It also helps to clarify the respective roles of the three NFU paralogs NFU1, NFU2, and NFU3.

Two organelle RNA recognition motif proteins affect distinct sets of RNA editing sites in the Arabidopsis thaliana plastid

Plastid and mitochondrial RNAs in vascular plants are subjected to cytidine-to-uridine editing. The model plant species Arabidopsis thaliana (Arabidopsis) has two nuclear-encoded plastid-targeted organelle RNA recognition motif (ORRM) proteins: ORRM1 and ORRM6. In the orrm1 mutant, 21 plastid RNA editing sites were affected but none are essential to photosynthesis. In the orrm6 mutants, two plastid RNA editing sites were affected: psbF-C77 and accD-C794. Because psbF encodes the β subunit of cytochrome b 559 in photosystem II, which is essential to photosynthesis, the orrm6 mutants were much smaller than the wild type. In addition, the orrm6 mutants had pale green leaves and reduced photosynthetic efficiency. To investigate the functional relationship between ORRM1 and ORRM6, we generated orrm1 orrm6 double homozygous mutants. Morphological and physiological analyses showed that the orrm1 orrm6 double mutants had a smaller plant size, reduced chlorophyll contents, and decreased photosynthetic efficiency, similar to the orrm6 single mutants. Although the orrm1 orrm6 double mutants adopted the phenotype of the orrm6 single mutants, the total number of plastid RNA editing sites affected in the orrm1 orrm6 double mutants was the sum of the sites affected in the orrm1 and orrm6 single mutants. These data suggest that ORRM1 and ORRM6 are in charge of distinct sets of plastid RNA editing sites and that simultaneous mutations in ORRM1 and ORRM6 genes do not cause additional reduction in editing extent at other plastid RNA editing sites.

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.

Function and maturation of the Fe-S center in dihydroxyacid dehydratase from Arabidopsis

Dihydroxyacid dehydratase (DHAD) is the third enzyme required for branched-chain amino acid biosynthesis in bacteria, fungi, and plants. DHAD enzymes contain two distinct types of active-site Fe-S clusters. The best characterized examples are Escherichia coli DHAD, which contains an oxygen-labile [Fe₄S₄] cluster, and spinach DHAD, which contains an oxygen-resistant [Fe₂S₂] cluster. Although the Fe-S cluster is crucial for DHAD function, little is known about the cluster-coordination environment or the mechanism of catalysis and cluster biogenesis. Here, using the combination of UV-visible absorption and circular dichroism and resonance Raman and electron paramagnetic resonance, we spectroscopically characterized the Fe-S center in DHAD from Arabidopsis thaliana (At). Our results indicated that AtDHAD can accommodate [Fe₂S₂] and [Fe₄S₄] clusters. However, only the [Fe₂S₂] cluster-bound form is catalytically active. We found that the [Fe₂S₂] cluster is coordinated by at least one non-cysteinyl ligand, which can be replaced by the thiol group(s) of dithiothreitol. In vitro cluster transfer and reconstitution reactions revealed that [Fe₂S₂] cluster-containing NFU2 protein is likely the physiological cluster donor for in vivo maturation of AtDHAD. In summary, AtDHAD binds either one [Fe₄S₄] or one [Fe₂S₂] cluster, with only the latter being catalytically competent and capable of substrate and product binding, and NFU2 appears to be the physiological [Fe₂S₂] cluster donor for DHAD maturation. This work represents the first in vitro characterization of recombinant AtDHAD, providing new insights into the properties, biogenesis, and catalytic role of the active-site Fe-S center in a plant DHAD.

Switching off photoprotection of photosystem I - a novel tool for gradual PSI photoinhibition

Photosystem I (PSI) has evolved in anaerobic atmospheric conditions and until today remains susceptible to oxygen. To minimize the probability of damaging side reactions, plants have evolved sophisticated mechanisms to control electron transfer, and PSI becomes inhibited only when malfunctions of these regulatory mechanisms occur. Because of the complicated induction of PSI photoinhibition, a detailed investigation into the process and following reactions are still largely missing. Here, we introduce the theoretical framework and a novel method for an easy and controlled induction of PSI photoinhibition in vivo. The method mimics the PSI damage mechanisms of fluctuating light-sensitive mutant plants (stn7, pgr5) which cannot control electron donation to PSI. Because PSII and PSI have different light absorption properties, electrons accumulate in the intersystem electron transfer chain (ETC), if PSII is preferentially excited. A saturating light pulse given upon an over-reduced ETC leads to the saturation of PSI electron acceptors, ultimately leading to the production of reactive oxygen species and photoinhibition of PSI. By adjusting the time of the light treatment, PSI can be gradually photoinhibited, providing a novel tool to holistically investigate the PSI photoinhibition phenomenon.

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.

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.

An Organelle RNA Recognition Motif Protein Is Required for Photosystem II Subunit psbF Transcript Editing

Loss-of-function mutations in ORGANELLE RNA RECOGNITION MOTIF PROTEIN6 (ORRM6) result in the near absence of RNA editing of psbF-C77 and the reduction in accD-C794 editing in Arabidopsis (Arabidopsis thaliana). The orrm6 mutants have decreased levels of photosystem II (PSII) proteins, especially PsbF, lower PSII activity, pale green pigmentation, smaller leaf and plant sizes, and retarded growth. Stable expression of ORRM6 rescues the orrm6 editing defects and mutant phenotype. Unlike ORRM1, the other known ORRM plastid editing factor, ORRM6, does not contain RNA editing interacting protein/multiple organellar RNA editing factor (RIP/MORF) boxes, which are required for ORRM1 to interact with site-specific pentatricopeptide repeat protein editing factors. ORRM6 interacts with RIP1/MORF8, RIP2/MORF2, and RIP9/MORF9, known components of RNA editosomes. While some plastid RRM proteins are involved in other forms of RNA processing and translation, the primary function of ORRM6 is evidently to mediate psbF-C77 editing, like the essential site-specific pentatricopeptide repeat protein LOW PSII ACCUMULATION66. Stable expression in the orrm6 mutants of a nucleus-encoded, plastid-targeted PsbF protein from a psbF gene carrying a T at nucleotide 77 significantly increases leaf and plant sizes, chlorophyll content, and PSII activity. These transformants demonstrate that plastid RNA editing can be bypassed through the expression of nucleus-encoded, edited forms of plastid genes.

Chloroplastic iron-sulfur scaffold protein NFU3 is essential to overall plant fitness

A previous study showed that Nitrogen-Fixing-subunit-U-type protein NFU3 may act an iron-sulfur scaffold protein in the assembly and transfer of 4Fe-4S and 3Fe-4S clusters in the chloroplast. Examples of 4Fe-4S and 3Fe-4S-requiring proteins and complexes include Photosystem I (PSI), NAD(P)H dehydrogenase, and ferredoxin-dependent glutamine oxoglutarate aminotransferases. In this paper, the authors provided additional evidence for the role of NFU3 in 4Fe-4S and 3Fe-4S cluster assembly and transfer, as well as its role in overall plant fitness. Confocal microscopic analysis of the fluorescently-tagged NFU3 protein confirmed the chloroplast localization of the NFU3 protein. Detailed analysis of chlorophyll fluorescence data revealed that a substantial increase in minimal fluorescence is the primary contributor to the decrease in PSII maximum photochemical efficiency observed in the nfu3 mutants. The substantial increase in minimal fluorescence in the nfu3 mutants is probably the result of an impaired PSI function, blockage of electron flow from PSII to PSI, and over-accumulation of reduced plastoquinone at the acceptor side of PSII. Analyses of seed morphology and germination showed that NFU3 is essential to seed development and germination, in addition to plant growth, development, and flowering. In summary, NFU3 has wide-ranging effects on many biologic processes and is therefore important to overall plant fitness. NFU3 may exert these effects by modulating the availability of 4Fe-4S and 3Fe-4S clusters to 4Fe-4S and 3Fe-4S-requiring proteins and complexes involved in various biologic processes.