Expression of the chlI, chlD, and chlH genes from the Cyanobacterium synechocystis PCC6803 in Escherichia coli and demonstration that the three cognate proteins are required for magnesium-protoporphyrin chelatase activity

Consumer acceptance of microalgae as a novel food - Where are we now? And how to get further

Microalgae provide a potential new food resource for sustainable human nutrition. Many microalgae species can produce a high content of total protein with a balanced composition of essential amino acids, healthy oils rich in polyunsaturated fatty acids, carotenoids, fibers, and vitamins. These components can be made available via unprocessed microalgae or refined as individual ingredients. In either case, if added to foods, microalgae may affect taste, smell, texture, and appearance. This review focuses on how consumer acceptance of new foods - such as microalgae - can be accessed in the world of sensory science by bringing together examples from recent consumer surveys. The main aim is to obtain an overview of the attitude towards microalgae as a food ingredient in Europe. The overarching finding suggests that European consumers generally find microalgae acceptable as ingredients in food products. However, there is a prevailing preference for keeping inclusion levels low, primarily attributed to the vivid green color that algae impart to food items upon addition. Additionally, consumers tend to favor the taste of freshwater algae over marine species, often finding the latter's pronounced fishy flavor less appealing.

QTL mapping and candidate gene analysis reveal two major loci regulating green leaf color in non-heading Chinese cabbage

KEY MESSAGE

Two major-effect QTL GlcA07.1 and GlcA09.1 for green leaf color were fine mapped into 170.25 kb and 191.41 kb intervals on chromosomes A07 and A09, respectively, and were validated by transcriptome analysis. Non-heading Chinese cabbage (NHCC) is a leafy vegetable with a wide range of green colors. Understanding the genetic mechanism behind broad spectrum of green may facilitate the breeding of high-quality NHCC. Here, we used F2 and F7:8 recombination inbred line (RIL) population from a cross between Wutacai (dark-green) and Erqing (lime-green) to undertake the genetic analysis and quantitative trait locus (QTL) mapping in NHCC. The genetic investigation of the F2 population revealed that the variation of green leaf color was controlled by two recessive genes. Six pigments associated with green leaf color, including total chlorophyll, chlorophyll a, chlorophyll b, total carotenoids, lutein, and carotene were quantified and applied for QTL mapping in the RIL population. A total of 7 QTL were detected across the whole genome. Among them, two major-effect QTL were mapped on chromosomes A07 (GlcA07.1) and A09 (GlcA09.1) corresponding to two QTL identified in the F2 population. The QTL GlcA07.1 and GlcA09.1 were further fine mapped into 170.25 kb and 191.41 kb genomic regions, respectively. By comparing gene expression level and gene annotation, BraC07g023810 and BraC07g023970 were proposed as the best candidates for GlcA07.1, while BraC09g052220 and BraC09g052270 were suggested for GlcA09.1. Two InDel molecular markers (GlcA07.1-BcGUN4 and GlcA09.1-BcSG1) associated with BraC07gA023810 and BraC09g052220 were developed and could effectively identify leaf color in natural NHCC accessions, suggesting their potential for marker-assisted leaf color selection in NHCC breeding.

Translation Fidelity and Respiration Deficits in CLPP-Deficient Tissues: Mechanistic Insights from Mitochondrial Complexome Profiling

The mitochondrial matrix peptidase CLPP is crucial during cell stress. Its loss causes Perrault syndrome type 3 (PRLTS3) with infertility, neurodegeneration, and a growth deficit. Its target proteins are disaggregated by CLPX, which also regulates heme biosynthesis via unfolding ALAS enzymes, providing access for pyridoxal-5'-phosphate (PLP). Despite efforts in diverse organisms with multiple techniques, CLPXP substrates remain controversial. Here, avoiding recombinant overexpression, we employed complexomics in mitochondria from three mouse tissues to identify endogenous targets. A CLPP absence caused the accumulation and dispersion of CLPX-VWA8 as AAA+ unfoldases, and of PLPBP. Similar changes and CLPX-VWA8 co-migration were evident for mitoribosomal central protuberance clusters, translation factors like GFM1-HARS2, the RNA granule components LRPPRC-SLIRP, and enzymes OAT-ALDH18A1. Mitochondrially translated proteins in testes showed reductions to <30% for MTCO1-3, the mis-assembly of the complex IV supercomplex, and accumulated metal-binding assembly factors COX15-SFXN4. Indeed, heavy metal levels were increased for iron, molybdenum, cobalt, and manganese. RT-qPCR showed compensatory downregulation only for Clpx mRNA; most accumulated proteins appeared transcriptionally upregulated. Immunoblots validated VWA8, MRPL38, MRPL18, GFM1, and OAT accumulation. Co-immunoprecipitation confirmed CLPX binding to MRPL38, GFM1, and OAT, so excess CLPX and PLP may affect their activity. Our data mechanistically elucidate the mitochondrial translation fidelity deficits which underlie progressive hearing impairment in PRLTS3.

Characterization and fine mapping of a yellow leaf gene regulating chlorophyll biosynthesis and chloroplast development in cotton (Gossypium arboreum)

Chlorophyll biosynthesis and chloroplast development are essential for photosynthesis and plant growth. Gossypium arboreum, a valuable source of genetic variation for cotton improvement, remains poorly studied for the mechanisms regulating chlorophyll biosynthesis and chloroplast development. Here we created a G. arboreum etiolated leaf and stuntedness (els) mutant that displayed a distinct yellow color of leaves, bracts and stems throughout the whole growth, where chlorophyll accumulation in leaves was reduced and chloroplast development was delayed. The GaCHLH gene, which encodes the H subunit of magnesium chelatase (Mg-chelatase), was screened by MutMap and KASP analysis. Compared to GaCHLH, the gene Gachlh of the mutant had a single nucleotide transition (G to A) at 1549 bp, which causes the substitution of a glycine (G) by a serine (S) at the 517th amino acid, resulting in an abnormal secondary structure of the Gachlh protein. GaCHLH-silenced SXY1 and ZM24 plants exhibited a lower GaCHLH expression level, a lower chlorophyll content, and the yellow-leaf phenotype. Gachlh expression affected the expression of key genes in the tetrapyrrole pathway. GaCHLH and Gachlh were located in the chloroplasts and that alteration of the mutation site did not affect the final target position. The BiFC assay result indicated that Gachlh could not bind to GaCHLD properly, which prevented the assembly of Mg-chelatase and thus led to the failure of chlorophyll synthesis. In this study, the Gachlh gene of G. arboreum els was finely localized and identified for the first time, providing new insights into the chlorophyll biosynthesis pathway in cotton.

Conformational variability of cyanobacterial ChlI, the AAA+ motor of magnesium chelatase involved in chlorophyll biosynthesis

IMPORTANCE

Photosynthesis is an essential life process that relies on chlorophyll. In photosynthetic organisms, chlorophyll synthesis involves multiple steps and depends on magnesium chelatase. This enzyme complex is responsible for inserting magnesium into the chlorophyll precursor, but the molecular mechanism of this process is not fully understood. By using cryogenic electron microscopy and conducting functional analyses, we have discovered that the motor subunit ChlI of magnesium chelatase undergoes conformational changes in the presence of ATP. Our findings offer new insights into how energy is transferred from ChlI to the other components of magnesium chelatase. This information significantly contributes to our understanding of the initial step in chlorophyll biosynthesis and lays the foundation for future studies on the entire process of chlorophyll production.

Joint toxicity mechanisms of binary emerging PFAS mixture on algae (Chlorella pyrenoidosa) at environmental concentration

Since traditional Per- and polyfluoroalkyl substances (PFAS) were banned in 2009 due to their bioaccumulation, persistence and biological toxicity, the emerging PFAS have been widely used as their substitutes and entered the aquatic environment in the form of mixtures. However, the joint toxicity mechanisms of these emerging PFAS mixtures to aquatic organisms remain largely unknown. Then, based on the testing of growth inhibition, cytotoxicity, photosynthesis and oxidative stress, and the toxicity mechanism of PFAS mixture (Perfluorobutane sulfonate and Perfluorobutane sulfonamide) to algae was explored using the Gene set enrichment analysis (GSEA). The results revealed that all three emerging PFAS treatments had a certain growth inhibitory effect on Chlorella pyrenoidosa (C. pyrenoidosa), but the toxicity of PFAS mixture was stronger than that of individual PFAS and showed a significant synergistic effect at environmental concentration. The joint toxicity mechanisms of binary PFAS mixture to C. pyrenoidosa were related to the damage of photosynthetic system, obstruction of ROS metabolism, and inhibition of DNA replication. Our findings are conductive to adding knowledge in understanding the joint toxicity mechanisms and provide a basis for assessing the environmental risk of emerging PFAS.

Chlorophyll decomposition is accelerated in banana leaves after the long-term magnesium deficiency according to transcriptome analysis

Magnesium (Mg) is an essential macronutrient for plant growth and development. Physiological and transcriptome analyses were conducted to elucidate the adaptive mechanisms to long-term Mg deficiency (MD) in banana seedlings at the 6-leaf stage. Banana seedlings were irrigated with a Mg-free nutrient solution for 42 days, and a mock control was treated with an optimum Mg supply. Leaf edge chlorosis was observed on the 9th leaf, which gradually turned yellow from the edge to the interior region. Accordingly, the total chlorophyll content was reduced by 47.1%, 47.4%, and 53.8% in the interior, center and edge regions, respectively, and the net photosynthetic rate was significantly decreased in the 9th leaf. Transcriptome analysis revealed that MD induced 9,314, 7,425 and 5,716 differentially expressed genes (DEGs) in the interior, center and edge regions, respectively. Of these, the chlorophyll metabolism pathway was preferentially enriched according to Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. The expression levels of the five candidate genes in leaves were consistent with what is expected during chlorophyll metabolism. Our results suggest that changes in the expression of genes related to chlorophyll synthesis and decomposition result in the yellowing of banana seedling leaves, and these results are helpful for understanding the banana response mechanism to long-term MD.

Comparison of Auxenochlorella protothecoides and Chlorella spp. Chloroplast Genomes: Evidence for Endosymbiosis and Horizontal Virus-like Gene Transfer

Resequencing of the chloroplast genome (cpDNA) of Auxenochlorella protothecoides UTEX 25 was completed (GenBank Accession no. KC631634.1), revealing a genome size of 84,576 base pairs and 30.8% GC content, consistent with features reported for the previously sequenced A. protothecoides 0710, (GenBank Accession no. KC843975). The A. protothecoides UTEX 25 cpDNA encoded 78 predicted open reading frames, 32 tRNAs, and 4 rRNAs, making it smaller and more compact than the cpDNA genome of C. variabilis (124,579 bp) and C. vulgaris (150,613 bp). By comparison, the compact genome size of A. protothecoides was attributable primarily to a lower intergenic sequence content. The cpDNA coding regions of all known Chlorella species were found to be organized in conserved colinear blocks, with some rearrangements. The Auxenochlorella and Chlorella species genome structure and composition were similar, and of particular interest were genes influencing photosynthetic efficiency, i.e., chlorophyll synthesis and photosystem subunit I and II genes, consistent with other biofuel species of interest. Phylogenetic analysis revealed that Prototheca cutis is the closest known A. protothecoides relative, followed by members of the genus Chlorella. The cpDNA of A. protothecoides encodes 37 genes that are highly homologous to representative cyanobacteria species, including rrn16, rrn23, and psbA, corroborating a well-recognized symbiosis. Several putative coding regions were identified that shared high nucleotide sequence identity with virus-like sequences, suggestive of horizontal gene transfer. Despite these predictions, no corresponding transcripts were obtained by RT-PCR amplification, indicating they are unlikely to be expressed in the extant lineage.

Open Issues for Protein Function Assignment in Haloferax volcanii and Other Halophilic Archaea

Background: Annotation ambiguities and annotation errors are a general challenge in genomics. While a reliable protein function assignment can be obtained by experimental characterization, this is expensive and time-consuming, and the number of such Gold Standard Proteins (GSP) with experimental support remains very low compared to proteins annotated by sequence homology, usually through automated pipelines. Even a GSP may give a misleading assignment when used as a reference: the homolog may be close enough to support isofunctionality, but the substrate of the GSP is absent from the species being annotated. In such cases, the enzymes cannot be isofunctional. Here, we examined a variety of such issues in halophilic archaea (class Halobacteria), with a strong focus on the model haloarchaeon Haloferax volcanii. Results: Annotated proteins of Hfx. volcanii were identified for which public databases tend to assign a function that is probably incorrect. In some cases, an alternative, probably correct, function can be predicted or inferred from the available evidence, but this has not been adopted by public databases because experimental validation is lacking. In other cases, a probably invalid specific function is predicted by homology, and while there is evidence that this assigned function is unlikely, the true function remains elusive. We listed 50 of those cases, each with detailed background information, so that a conclusion about the most likely biological function can be drawn. For reasons of brevity and comprehension, only the key aspects are listed in the main text, with detailed information being provided in a corresponding section of the Supplementary Materials. Conclusions: Compiling, describing and summarizing these open annotation issues and functional predictions will benefit the scientific community in the general effort to improve the evaluation of protein function assignments and more thoroughly detail them. By highlighting the gaps and likely annotation errors currently in the databases, we hope this study will provide a framework for experimentalists to systematically confirm (or disprove) our function predictions or to uncover yet more unexpected functions.

Bilin-dependent regulation of chlorophyll biosynthesis by GUN4

Biosyntheses of chlorophyll and heme in oxygenic phototrophs share a common trunk pathway that diverges with insertion of magnesium or iron into the last common intermediate, protoporphyrin IX. Since both tetrapyrroles are pro-oxidants, it is essential that their metabolism is tightly regulated. Here, we establish that heme-derived linear tetrapyrroles (bilins) function to stimulate the enzymatic activity of magnesium chelatase (MgCh) via their interaction with GENOMES UNCOUPLED 4 (GUN4) in the model green alga Chlamydomonas reinhardtii A key tetrapyrrole-binding component of MgCh found in all oxygenic photosynthetic species, CrGUN4, also stabilizes the bilin-dependent accumulation of protoporphyrin IX-binding CrCHLH1 subunit of MgCh in light-grown C. reinhardtii cells by preventing its photooxidative inactivation. Exogenous application of biliverdin IXα reverses the loss of CrCHLH1 in the bilin-deficient heme oxygenase (hmox1) mutant, but not in the gun4 mutant. We propose that these dual regulatory roles of GUN4:bilin complexes are responsible for the retention of bilin biosynthesis in all photosynthetic eukaryotes, which sustains chlorophyll biosynthesis in an illuminated oxic environment.

Current knowledge and recent advances in understanding metabolism of the model cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria are key organisms in the global ecosystem, useful models for studying metabolic and physiological processes conserved in photosynthetic organisms, and potential renewable platforms for production of chemicals. Characterizing cyanobacterial metabolism and physiology is key to understanding their role in the environment and unlocking their potential for biotechnology applications. Many aspects of cyanobacterial biology differ from heterotrophic bacteria. For example, most cyanobacteria incorporate a series of internal thylakoid membranes where both oxygenic photosynthesis and respiration occur, while CO2 fixation takes place in specialized compartments termed carboxysomes. In this review, we provide a comprehensive summary of our knowledge on cyanobacterial physiology and the pathways in Synechocystis sp. PCC 6803 (Synechocystis) involved in biosynthesis of sugar-based metabolites, amino acids, nucleotides, lipids, cofactors, vitamins, isoprenoids, pigments and cell wall components, in addition to the proteins involved in metabolite transport. While some pathways are conserved between model cyanobacteria, such as Synechocystis, and model heterotrophic bacteria like Escherichia coli, many enzymes and/or pathways involved in the biosynthesis of key metabolites in cyanobacteria have not been completely characterized. These include pathways required for biosynthesis of chorismate and membrane lipids, nucleotides, several amino acids, vitamins and cofactors, and isoprenoids such as plastoquinone, carotenoids, and tocopherols. Moreover, our understanding of photorespiration, lipopolysaccharide assembly and transport, and degradation of lipids, sucrose, most vitamins and amino acids, and haem, is incomplete. We discuss tools that may aid our understanding of cyanobacterial metabolism, notably CyanoSource, a barcoded library of targeted Synechocystis mutants, which will significantly accelerate characterization of individual proteins.

The active site of magnesium chelatase

The insertion of magnesium into protoporphyrin initiates the biosynthesis of chlorophyll, the pigment that underpins photosynthesis. This reaction, catalysed by the magnesium chelatase complex, couples ATP hydrolysis by a ChlID motor complex to chelation within the ChlH subunit. We probed the structure and catalytic function of ChlH using a combination of X-ray crystallography, computational modelling, mutagenesis and enzymology. Two linked domains of ChlH in an initially open conformation of ChlH bind protoporphyrin IX, and the rearrangement of several loops envelops this substrate, forming an active site cavity. This induced fit brings an essential glutamate (E660), proposed to be the key catalytic residue for magnesium insertion, into proximity with the porphyrin. A buried solvent channel adjacent to E660 connects the exterior bulk solvent to the active site, forming a possible conduit for the delivery of magnesium or abstraction of protons.

Heterologous Expression of the Barley (Hordeum vulgare L.) Xantha-f, -g and -h Genes that Encode Magnesium Chelatase Subunits

Biosynthesis of chlorophyll involves several enzymatic reactions of which many are shared with the heme biosynthesis pathway. Magnesium chelatase is the first specific enzyme in the chlorophyll pathway. It catalyzes the formation of Mg-protoporphyrin IX from the insertion of Mg²⁺ into protoporphyrin IX. The enzyme consists of three subunits encoded by three genes. The three genes are named Xantha-h, Xantha-g and Xantha-f in barley (Hordeum vulgare L.). The products of the genes have a molecular weight of 38, 78 and 148 kDa, respectively, as mature proteins in the chloroplast. Most studies on magnesium chelatase enzymes have been performed using recombinant proteins of Rhodobacter capsulatus, Synechocystis sp. PCC6803 and Thermosynechococcus elongatus, which are photosynthetic bacteria. In the present study we established a recombinant expression system for barley magnesium chelatase with the long-term goal to obtain structural information of this enigmatic enzyme complex from a higher plant. The genes Xantha-h, -g and -f were cloned in plasmid pET15b, which allowed the production of the three subunits as His-tagged proteins in Escherichia coli BL21(DE3)pLysS. The purified subunits stimulated magnesium chelatase activity of barley plastid extracts and produced activity in assays with only recombinant proteins. In preparation for future structural analyses of the barley magnesium chelatase, stability tests were performed on the subunits and activity assays were screened to find an optimal buffer system and pH.

Single Nucleotide Mutagenesis of the TaCHLI Gene Suppressed Chlorophyll and Fatty Acid Biosynthesis in Common Wheat Seedlings

Wheat (Triticum aestivum L.) is one of the most important crops in the world. Chlorophyll plays a vital role in plant development and crop improvement and further determines the crop productivity to a certain extent. The biosynthesis of chlorophyll remains a complex metabolic process, and fundamental biochemical discoveries have resulted from studies of plant mutants with altered leaf color. In this study, we identified a chlorophyll-deficiency mutant, referred to as chli, from the wheat cultivar Shaannong33 that exhibited an obvious pale-green leaf phenotype at the seedling stage, with significantly decreased accumulation of chlorophyll and its precursors, protoporphyrin IX and Mg-protoporphyrin IX. Interestingly, a higher protoporphyrin IX to Mg-protoporphyrin IX ratio was observed in chli. Lipid biosynthesis in chli leaves and seeds was also affected, with the mutant displaying significantly reduced total lipid content relative to Shaanong33. Genetic analysis indicated that the pale-green leaf phenotype was controlled by a single pair of recessive nuclear genes. Furthermore, sequence alignment revealed a single-nucleotide mutation (G664A) in the gene TraesCS7A01G480700.1, which encodes subunit I of the Mg-chelatase in plants. This single-nucleotide mutation resulted in an amino acid substitution (D221N) in the highly conserved domain of subunit I. As a result, mutant protein Tachli-7A lost the ability to interact with the normal protein TaCHLI-7A, as assessed by yeast two-hybrid assay. Meanwhile, Tachli-7A could not recover the chlorophyll deficiency phenotype of the Arabidopsis thaliana SALK_050029 mutant. Furthermore, we found that in Shaannong33, the protoporphyrin IX to Mg-protoporphyrin IX ratio was growth state-dependent and insensitive to environmental change. Overall, the mutation in Tachli-7A impaired the function of Mg-chelatase and blocked the conversion of protoporphyrin IX to Mg- protoporphyrin IX. Based on our results, the chli mutant represents a potentially useful resource for better understanding chlorophyll and lipid biosynthetic pathways in common wheat.

Hexameric structure of the ATPase motor subunit of magnesium chelatase in chlorophyll biosynthesis

Magnesium chelatase (MgCh) is a heterotrimeric enzyme complex, composed of two AAA+ family subunits that can assembly into a double ring structure and a large catalytic subunit. The small AAA+ subunit has ATPase activity and can self-oligomerize into a ring structure, while the other AAA+ subunit lacks independent ATPase activity. Previous structural studies of the ATPase motor subunit of MgCh from a bacteriochlorophyll-synthesizing bacterium have identified a unique ATPase clade, but the model of oligomeric assembly is unclear. Here we present the hexameric structure of the MgCh ATPase motor subunit from the chlorophyll-synthesizing cyanobacterium Synechocystis sp. PCC 6803. This structure reveals details of how the hexameric ring is assembled, and thus provides a basis for further studying the heterotrimeric complex.

Structure of sirohydrochlorin ferrochelatase SirB: the last of the structures of the class II chelatase family

The crystal structure of Bacillus subtilis SirB, which catalyses the insertion of Fe2+ into the substrate sirohydrochlorin (SHC) in siroheme biosynthesis, is reported herein as the last of the structures of class II chelatases. The structure of SirB with Co2+ showed that the active site of SirB is located at the N-terminal domain with metal-binding amino acid residues His10, Glu43, and His76, which was also predicted for CbiX, but is distinct from the C-terminal active sites of CbiK and HemH. The biosynthetic model reactions using SirB, Co2+ and uroporphyrin I or protoporphyrin IX as a SHC analogue revealed that SirB showed chelatase activity for uroporphyrin I, but not for protoporphyrin IX. Simulations of tetrapyrroles docking to SirB provided an insight into its tetrapyrrole substrate recognition: SHC and uroporphyrin I were suitably bound beside the Co2+ ion-binding site at the active site cavity; protoporphyrin IX was also docked to the active site but its orientation was different from those of the other two tetrapyrroles. Summarizing the present data, it was proposed that the key structural features for substrate recognition of SirB could be the hydrophobic area at the active site as well as the substituents of the tetrapyrroles.

Role of Galactolipids in Plastid Differentiation Before and After Light Exposure

Galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are the predominant lipid classes in the thylakoid membrane of chloroplasts. These lipids are also major constituents of internal membrane structures called prolamellar bodies (PLBs) and prothylakoids (PTs) in etioplasts, which develop in the cotyledon cells of dark-grown angiosperms. Analysis of Arabidopsis mutants defective in the major galactolipid biosynthesis pathway revealed that MGDG and DGDG are similarly and, in part, differently required for membrane-associated processes such as the organization of PLBs and PTs and the formation of pigment–protein complexes in etioplasts. After light exposure, PLBs and PTs in etioplasts are transformed into the thylakoid membrane, resulting in chloroplast biogenesis. During the etioplast-to-chloroplast differentiation, galactolipids facilitate thylakoid membrane biogenesis from PLBs and PTs and play crucial roles in chlorophyll biosynthesis and accumulation of light-harvesting proteins. These recent findings shed light on the roles of galactolipids as key facilitators of several membrane-associated processes during the development of the internal membrane systems in plant plastids.

The ChlD subunit links the motor and porphyrin binding subunits of magnesium chelatase

Magnesium chelatase initiates chlorophyll biosynthesis, catalysing the MgATP²⁻-dependent insertion of a Mg²⁺ ion into protoporphyrin IX. The catalytic core of this large enzyme complex consists of three subunits: Bch/ChlI, Bch/ChlD and Bch/ChlH (in bacteriochlorophyll and chlorophyll producing species, respectively). The D and I subunits are members of the AAA⁺ (ATPases associated with various cellular activities) superfamily of enzymes, and they form a complex that binds to H, the site of metal ion insertion. In order to investigate the physical coupling between ChlID and ChlH in vivo and in vitro, ChlD was FLAG-tagged in the cyanobacterium Synechocystis sp. PCC 6803 and co-immunoprecipitation experiments showed interactions with both ChlI and ChlH. Co-production of recombinant ChlD and ChlH in Escherichia coli yielded a ChlDH complex. Quantitative analysis using microscale thermophoresis showed magnesium-dependent binding (K_d 331 ± 58 nM) between ChlD and H. The physical basis for a ChlD–H interaction was investigated using chemical cross-linking coupled with mass spectrometry (XL–MS), together with modifications that either truncate ChlD or modify single residues. We found that the C-terminal integrin I domain of ChlD governs association with ChlH, the Mg²⁺ dependence of which also mediates the cooperative response of the Synechocystis chelatase to magnesium. The interaction site between the AAA⁺ motor and the chelatase domain of magnesium chelatase will be essential for understanding how free energy from the hydrolysis of ATP on the AAA⁺ ChlI subunit is transmitted via the bridging subunit ChlD to the active site on ChlH.

In situ metabolomic- and transcriptomic-profiling of the host-associated cyanobacteria Prochloron and Acaryochloris marina

The tropical ascidian Lissoclinum patella hosts two enigmatic cyanobacteria: (1) the photoendosymbiont Prochloron spp., a producer of valuable bioactive compounds and (2) the chlorophyll-d containing Acaryochloris spp., residing in the near-infrared enriched underside of the animal. Despite numerous efforts, Prochloron remains uncultivable, restricting the investigation of its biochemical potential to cultivation-independent techniques. Likewise, in both cyanobacteria, universally important parameters on light-niche adaptation and in situ photosynthetic regulation are unknown. Here we used genome sequencing, transcriptomics and metabolomics to investigate the symbiotic linkage between host and photoendosymbiont and simultaneously probed the transcriptional response of Acaryochloris in situ. During high light, both cyanobacteria downregulate CO2 fixing pathways, likely a result of O2 photorespiration on the functioning of RuBisCO, and employ a variety of stress-quenching mechanisms, even under less stressful far-red light (Acaryochloris). Metabolomics reveals a distinct biochemical modulation between Prochloron and L. patella, including noon/midnight-dependent signatures of amino acids, nitrogenous waste products and primary photosynthates. Surprisingly, Prochloron constitutively expressed genes coding for patellamides, that is, cyclic peptides of great pharmaceutical value, with yet unknown ecological significance. Together these findings shed further light on far-red-driven photosynthesis in natural consortia, the interplay of Prochloron and its ascidian partner in a model chordate photosymbiosis and the uncultivability of Prochloron.

Complete enzyme set for chlorophyll biosynthesis in Escherichia coli

Chlorophylls are essential cofactors for photosynthesis, which sustains global food chains and oxygen production. Billions of tons of chlorophylls are synthesized annually, yet full understanding of chlorophyll biosynthesis has been hindered by the lack of characterization of the Mg-protoporphyrin IX monomethyl ester oxidative cyclase step, which confers the distinctive green color of these pigments. We demonstrate cyclase activity using heterologously expressed enzyme. Next, we assemble a genetic module that encodes the complete chlorophyll biosynthetic pathway and show that it functions in Escherichia coli. Expression of 12 genes converts endogenous protoporphyrin IX into chlorophyll a, turning E. coli cells green. Our results delineate a minimum set of enzymes required to make chlorophyll and establish a platform for engineering photosynthesis in a heterotrophic model organism.

Molecular Characterization of Magnesium Chelatase in Soybean [Glycine max (L.) Merr.]

Soybean (Glycine max) seed yields rely on the efficiency of photosynthesis, which is poorly understood in soybean. Chlorophyll, the major light harvesting pigment, is crucial for chloroplast biogenesis and photosynthesis. Magnesium chelatase catalyzes the insertion of Mg²⁺ into protoporphyrin IX in the first committed and key regulatory step of chlorophyll biosynthesis. It consists of three types of subunits, ChlI, ChlD, and ChlH. To gain a better knowledge of chlorophyll biosynthesis in soybean, we analyzed soybean Mg-chelatase subunits and their encoding genes. Soybean genome harbors 4 GmChlI genes, 2 GmChlD genes, and 3 GmChlH genes, likely evolved from two rounds of gene duplication events. The qRT-PCR analysis revealed that GmChlI, GmChlD, and GmChlH genes predominantly expressed in photosynthetic tissues, but the expression levels among paralogs are different. In silicon promoter analyses revealed these genes harbor different cis-regulatory elements in their promoter regions, suggesting they could differentially respond to various environmental and developmental signals. Subcellular localization analyses illustrated that GmChlI, GmChlD, and GmChlH isoforms are all localized in chloroplast, consistent with their functions. Yeast two hybrid and bimolecular fluorescence complementation (BiFC) assays showed each isoform has a potential to be assembled into the Mg-chelatase holocomplex. We expressed each GmChlI, GmChlD, and GmChlH isoform in Arabidopsis corresponding mutants, and results showed that 4 GmChlI and 2 GmChlD isoforms and GmChlH1 could rescue the severe phenotype of Arabidopsis mutants, indicating that they maintain normal biochemical functions in vivo. However, GmChlH2 and GmChlH3 could not completely rescue the chlorotic phenotype of Arabidopsis gun5-2 mutant, suggesting that the functions of these two proteins could be different from GmChlH1. Considering the differences shown on primary sequences, biochemical functions, and gene expression profiles, we conclude that the paralogs of each soybean Mg-chelatase subunit have diverged more or less during evolution. Soybean could have developed a complex regulatory mechanism to control chlorophyll content to adapt to different developmental and environmental situations.

Fine mapping and candidate gene analysis of the virescent gene v 1 in Upland cotton (Gossypium hirsutum)

The young leaves of virescent mutants are yellowish and gradually turn green as the plants reach maturity. Understanding the genetic basis of virescent mutants can aid research of the regulatory mechanisms underlying chloroplast development and chlorophyll biosynthesis, as well as contribute to the application of virescent traits in crop breeding. In this study, fine mapping was employed, and a recessive gene (v ₁) from a virescent mutant of Upland cotton was narrowed to an 84.1-Kb region containing ten candidate genes. The GhChlI gene encodes the cotton Mg-chelatase I subunit (CHLI) and was identified as the candidate gene for the virescent mutation using gene annotation. BLAST analysis showed that the GhChlI gene has two copies, Gh_A10G0282 and Gh_D10G0283. Sequence analysis indicated that the coding region (CDS) of GhChlI is 1269 bp in length, with three predicted exons and one non-synonymous nucleotide mutation (G1082A) in the third exon of Gh_D10G0283, with an amino acid (AA) substitution of arginine (R) to lysine (K). GhChlI-silenced TM-1 plants exhibited a lower GhChlI expression level, a lower chlorophyll content, and the virescent phenotype. Analysis of upstream regulatory elements and expression levels of GhChlI showed that the expression quantity of GhChlI may be normal, and with the development of the true leaf, the increase in the Gh_A10G0282 dosage may partially make up for the deficiency of Gh_D10G0283 in the v ₁ mutant. Phylogenetic analysis and sequence alignment revealed that the protein sequence encoded by the third exon of GhChlI is highly conserved across diverse plant species, in which AA substitutions among the completely conserved residues frequently result in changes in leaf color in various species. These results suggest that the mutation (G1082A) within the GhChlI gene may cause a functional defect of the GhCHLI subunit and thus the virescent phenotype in the v₁ mutant. The GhChlI mutation not only provides a tool for understanding the associations of CHLI protein function and the chlorophyll biosynthesis pathway but also has implications for cotton breeding.

1-N-histidine phosphorylation of ChlD by the AAA+ ChlI2 stimulates magnesium chelatase activity in chlorophyll synthesis

Magnesium chelatase (Mg-chelatase) inserts magnesium into protoporphyrin during the biosynthesis of chlorophyll and bacteriochlorophyll. Enzyme activity is reconstituted by forming two separate preactivated complexes consisting of a GUN4/ChlH/protoporphyrin IX substrate complex and a ChlI/ChlD enzyme 'motor' complex. Formation of the ChlI/ChlD complex in both Chlamydomonas reinhardtii and Oryza sativa is accompanied by phosphorylation of ChlD by ChlI, but the orthologous protein complex from Rhodobacter capsulatus, BchI/BchD, gives no detectable phosphorylation of BchD. Phosphorylation produces a 1-N-phospho-histidine within ChlD. Proteomic analysis indicates that phosphorylation occurs at a conserved His residue in the C-terminal integrin I domain of ChlD. Comparative analysis of the ChlD phosphorylation with enzyme activities of various ChlI/ChlD complexes correlates the phosphorylation by ChlI2 with stimulation of Mg-chelatase activity. Mutation of the H641 of CrChlD to E641 prevents both phosphorylation and stimulation of Mg-chelatase activity, confirming that phosphorylation at H641 stimulates Mg-chelatase. The properties of ChlI2 compared with ChlI1 of Chlamydomonas and with ChlI of Oryza, shows that ChlI2 has a regulatory role in Chlamydomonas.

Nanomechanical and Thermophoretic Analyses of the Nucleotide-Dependent Interactions between the AAA(+) Subunits of Magnesium Chelatase

In chlorophyll biosynthesis, the magnesium chelatase enzyme complex catalyzes the insertion of a Mg(2+) ion into protoporphyrin IX. Prior to this event, two of the three subunits, the AAA(+) proteins ChlI and ChlD, form a ChlID-MgATP complex. We used microscale thermophoresis to directly determine dissociation constants for the I-D subunits from Synechocystis, and to show that the formation of a ChlID-MgADP complex, mediated by the arginine finger and the sensor II domain on ChlD, is necessary for the assembly of the catalytically active ChlHID-MgATP complex. The N-terminal AAA(+) domain of ChlD is essential for complex formation, but some stability is preserved in the absence of the C-terminal integrin domain of ChlD, particularly if the intervening polyproline linker region is retained. Single molecule force spectroscopy (SMFS) was used to determine the factors that stabilize formation of the ChlID-MgADP complex at the single molecule level; ChlD was attached to an atomic force microscope (AFM) probe in two different orientations, and the ChlI subunits were tethered to a silica surface; the probability of subunits interacting more than doubled in the presence of MgADP, and we show that the N-terminal AAA(+) domain of ChlD mediates this process, in agreement with the microscale thermophoresis data. Analysis of the unbinding data revealed a most probable interaction force of around 109 pN for formation of single ChlID-MgADP complexes. These experiments provide a quantitative basis for understanding the assembly and function of the Mg chelatase complex.

Mutation of Gly195 of the ChlH Subunit of Mg-chelatase Reduces Chlorophyll and Further Disrupts PS II Assembly in a Ycf48-Deficient Strain of Synechocystis sp. PCC 6803

Biogenesis of the photosystems in oxygenic phototrophs requires co-translational insertion of chlorophyll a. The first committed step of chlorophyll a biosynthesis is the insertion of a Mg(2+) ion into the tetrapyrrole intermediate protoporphyrin IX, catalyzed by Mg-chelatase. We have identified a Synechocystis sp. PCC 6803 strain with a spontaneous mutation in chlH that results in a Gly195 to Glu substitution in a conserved region of the catalytic subunit of Mg-chelatase. Mutant strains containing the ChlH Gly195 to Glu mutation were generated using a two-step protocol that introduced the chlH gene into a putative neutral site in the chromosome prior to deletion of the native gene. The Gly195 to Glu mutation resulted in strains with decreased chlorophyll a. Deletion of the PS II assembly factor Ycf48 in a strain carrying the ChlH Gly195 to Glu mutation did not grow photoautotrophically. In addition, the ChlH-G195E:ΔYcf48 strain showed impaired PS II activity and decreased assembly of PS II centers in comparison to a ΔYcf48 strain. We suggest decreased chlorophyll in the ChlH-G195E mutant provides a background to screen for the role of assembly factors that are not essential under optimal growth conditions.

Five glutamic acid residues in the C-terminal domain of the ChlD subunit play a major role in conferring Mg(2+) cooperativity upon magnesium chelatase

Magnesium chelatase catalyzes the first committed step in chlorophyll biosynthesis by inserting a Mg(2+) ion into protoporphyrin IX in an ATP-dependent manner. The cyanobacterial (Synechocystis) and higher-plant chelatases exhibit a complex cooperative response to free magnesium, while the chelatases from Thermosynechococcus elongatus and photosynthetic bacteria do not. To investigate the basis for this cooperativity, we constructed a series of chimeric ChlD proteins using N-terminal, central, and C-terminal domains from Synechocystis and Thermosynechococcus. We show that five glutamic acid residues in the C-terminal domain play a major role in this process.

Porphyrin Binding to Gun4 Protein, Facilitated by a Flexible Loop, Controls Metabolite Flow through the Chlorophyll Biosynthetic Pathway

In oxygenic phototrophs, chlorophylls, hemes, and bilins are synthesized by a common branched pathway. Given the phototoxic nature of tetrapyrroles, this pathway must be tightly regulated, and an important regulatory role is attributed to magnesium chelatase enzyme at the branching between the heme and chlorophyll pathway. Gun4 is a porphyrin-binding protein known to stimulate in vitro the magnesium chelatase activity, but how the Gun4-porphyrin complex acts in the cell was unknown. To address this issue, we first performed simulations to determine the porphyrin-docking mechanism to the cyanobacterial Gun4 structure. After correcting crystallographic loop contacts, we determined the binding site for magnesium protoporphyrin IX. Molecular modeling revealed that the orientation of α6/α7 loop is critical for the binding, and the magnesium ion held within the porphyrin is coordinated by Asn-211 residue. We also identified the basis for stronger binding in the Gun4-1 variant and for weaker binding in the W192A mutant. The W192A-Gun4 was further characterized in magnesium chelatase assay showing that tight porphyrin binding in Gun4 facilitates its interaction with the magnesium chelatase ChlH subunit. Finally, we introduced the W192A mutation into cells and show that the Gun4-porphyrin complex is important for the accumulation of ChlH and for channeling metabolites into the chlorophyll biosynthetic pathway.

Crystal structure of the catalytic subunit of magnesium chelatase

Tetrapyrroles, including haem and chlorophyll, play vital roles for various biological processes, such as respiration and photosynthesis, and their biosynthesis is critical for virtually all organisms. In photosynthetic organisms, magnesium chelatase (MgCh) catalyses insertion of magnesium into the centre of protoporphyrin IX, the branch-point precursor for both haem and chlorophyll, leading tetrapyrrole biosynthesis into the magnesium branch(1,2). This reaction needs a cooperated action of the three subunits of MgCh: the catalytic subunit ChlH and two AAA(+) subunits, ChlI and ChlD (refs 3-5). To date, the mechanism of MgCh awaits further elucidation due to a lack of high-resolution structures, especially for the ∼150 kDa catalytic subunit. Here we report the crystal structure of ChlH from the photosynthetic cyanobacterium Synechocystis PCC 6803, solved at 2.5 Å resolution. The active site is buried deeply inside the protein interior, and the surrounding residues are conserved throughout evolution. This structure helps to explain the loss of function reported for the cch and gun5 mutations of the ChlH subunit, and to provide the molecular basis of substrate channelling during the magnesium-chelating process. The structure advances our understanding of the holoenzyme of MgCh, a metal chelating enzyme other than ferrochelatase.

Structure of GUN4 from Chlamydomonas reinhardtii

The genomes uncoupled 4 (GUN4) protein stimulates chlorophyll biosynthesis by increasing the activity of Mg-chelatase, the enzyme that inserts magnesium into protoporphyrin IX (PPIX) in the chlorophyll biosynthesis pathway. One of the roles of GUN4 is in binding PPIX and Mg-PPIX. In eukaryotes, GUN4 also participates in plastid-to-nucleus signalling, although the mechanism for this is unclear. Here, the first crystal structure of a eukaryotic GUN4, from Chlamydomonas reinhardtii, is presented. The structure is in broad agreement with those of previously solved cyanobacterial structures. Most interestingly, conformational divergence is restricted to several loops which cover the porphyrin-binding cleft. The conformational dynamics suggested by this ensemble of structures lend support to the understanding of how GUN4 binds PPIX or Mg-PPIX.

Salt-stress induced modulation of chlorophyll biosynthesis during de-etiolation of rice seedlings

Chlorophyll biosynthesis in plants is subjected to modulation by various environmental factors. To understand the modulation of the chlorophyll (Chl) biosynthesis during greening process by salt, 100-200 mM NaCl was applied to the roots of etiolated rice seedlings 12 h prior to the transfer to light. Application of 200 mM NaCl to rice seedlings that were grown in light for further 72 h resulted in reduced dry matter production (-58%) and Chl accumulation (-66%). Ionic imbalance due to salinity stress resulted in additional downregulation (41-45%) of seedling dry weight, Chl and carotenoid contents over and above that of similar osmotic stress induced by polyethylene glycol. Downregulation of Chl biosynthesis may be attributed to decreased activities of Chl biosynthetic pathway enzymes, i.e. 5-aminolevulinic acid (ALA) dehydratase (EC-2.4.1.24), porphobilinogen deaminase (EC-4.3.1.8), coproporphyrinogen III oxidase (EC-1.3.3.3), protoporphyrinogen IX oxidase (EC-1.3.3.4), Mg-protoporphyrin IX chelatase (EC-6.6.1.1) and protochlorophyllide oxidoreductase (EC-1.3.33.1). Reduced enzymatic activities were due to downregulation of their protein abundance and/or gene expression in salt-stressed seedlings. The extent of downregulation of ALA biosynthesis nearly matched with that of protochlorophyllide and Chl to prevent the accumulation of highly photosensitive photodynamic tetrapyrroles that generates singlet oxygen under stress conditions. Although, ALA synthesis decreased, the gene/protein expression of glutamyl-tRNA reductase (EC-1.2.1.70) increased suggesting it may play a role in acclimation to salt stress. The similar downregulation of both early and late Chl biosynthesis intermediates in salt-stressed seedlings suggests a regulatory network of genes involved in tetrapyrrole biosynthesis.

Structural and functional consequences of removing the N-terminal domain from the magnesium chelatase ChlH subunit of Thermosynechococcus elongatus

Magnesium chelatase (MgCH) initiates chlorophyll biosynthesis by catalysing the ATP-dependent insertion of Mg²⁺ into protoporphyrin. This large enzyme complex comprises ChlH, I and D subunits, with I and D involved in ATP hydrolysis, and H the protein that handles the substrate and product. The 148 kDa ChlH subunit has a globular N-terminal domain attached by a narrow linker to a hollow cage-like structure. Following deletion of this ~18 kDa domain from the Thermosynechoccus elongatus ChlH, we used single particle reconstruction to show that the apo- and porphyrin-bound forms of the mutant subunit consist of a hollow globular protein with three connected lobes; superposition of the mutant and native ChlH structures shows that, despite the clear absence of the N-terminal ‘head’ region, the rest of the protein appears to be correctly folded. Analyses of dissociation constants shows that the ΔN159ChlH mutant retains the ability to bind protoporphyrin and the Gun4 enhancer protein, although the addition of I and D subunits yields an extremely impaired active enzyme complex. Addition of the Gun4 enhancer protein, which stimulates MgCH activity significantly especially at low Mg²⁺ concentrations, partially reactivates the ΔN159ChlH–I–D mutant enzyme complex, suggesting that the binding site or sites for Gun4 on H do not wholly depend on the N-terminal domain.

The N-terminal domain of the 148 kDa ChlH is essential for normal activity of the ChlH–I–D magnesium chelatase complex. Deleting this 18 kDa domain retains the hollow cage-like structure and porphyrin binding. Chelatase activity is partially restored by the Gun4 protein.

Metal preferences and metallation

The metal binding preferences of most metalloproteins do not match their metal requirements. Thus, metallation of an estimated 30% of metalloenzymes is aided by metal delivery systems, with ∼ 25% acquiring preassembled metal cofactors. The remaining ∼ 70% are presumed to compete for metals from buffered metal pools. Metallation is further aided by maintaining the relative concentrations of these pools as an inverse function of the stabilities of the respective metal complexes. For example, magnesium enzymes always prefer to bind zinc, and these metals dominate the metalloenzymes without metal delivery systems. Therefore, the buffered concentration of zinc is held at least a million-fold below magnesium inside most cells.

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.

Characterization of the magnesium chelatase from Thermosynechococcus elongatus

The first committed step in chlorophyll biosynthesis is catalysed by magnesium chelatase (E.C. 6.6.1.1), which uses the free energy of ATP hydrolysis to insert an Mg(2+) ion into the ring of protoporphyrin IX. We have characterized magnesium chelatase from the thermophilic cyanobacterium Thermosynechococcus elongatus. This chelatase is thermostable, with subunit melting temperatures between 55 and 63°C and optimal activity at 50°C. The T. elongatus chelatase (kcat of 0.16 μM/min) shows a Michaelis-Menten-type response to both Mg(2+) (Km of 2.3 mM) and MgATP(2-) (Km of 0.8 mM). The response to porphyrin is more complex; porphyrin inhibits at high concentrations of ChlH, but when the concentration of ChlH is comparable with the other two subunits the response is of a Michaelis-Menten type (at 0.4 μM ChlH, Km is 0.2 μM). Hybrid magnesium chelatases containing a mixture of subunits from the mesophilic Synechocystis and Thermosynechococcus enzymes are active. We generated all six possible hybrid magnesium chelatases; the hybrid chelatase containing Thermosynechococcus ChlD and Synechocystis ChlI and ChlH is not co-operative towards Mg(2+), in contrast with the Synechocystis magnesium chelatase. This loss of co-operativity reveals the significant regulatory role of Synechocystis ChlD.

The allosteric role of the AAA+ domain of ChlD protein from the magnesium chelatase of synechocystis species PCC 6803

Magnesium chelatase is an AAA⁺ ATPase that catalyzes the first step in chlorophyll biosynthesis, the energetically unfavorable insertion of a magnesium ion into a porphyrin ring. This enzyme contains two AAA⁺ domains, one active in the ChlI protein and one inactive in the ChlD protein. Using a series of mutants in the AAA⁺ domain of ChlD, we show that this site is essential for magnesium chelation and allosterically regulates Mg²⁺ and MgATP²⁻ binding.

Catalytic turnover triggers exchange of subunits of the magnesium chelatase AAA+ motor unit

The ATP-dependent insertion of Mg(2+) into protoporphyrin IX is the first committed step in the chlorophyll biosynthetic pathway. The reaction is catalyzed by magnesium chelatase, which consists of three gene products: BchI, BchD, and BchH. The BchI and BchD subunits belong to the family of AAA+ proteins (ATPases associated with various cellular activities) and form a two-ring complex with six BchI subunits in one layer and six BchD subunits in the other layer. This BchID complex is a two-layered trimer of dimers with the ATP binding site located at the interface between two neighboring BchI subunits. ATP hydrolysis by the BchID motor unit fuels the insertion of Mg(2+) into the porphyrin by the BchH subunit. In the present study, we explored mutations that were originally identified in semidominant barley (Hordeum vulgare L.) mutants. The resulting recombinant BchI proteins have marginal ATPase activity and cannot contribute to magnesium chelatase activity although they apparently form structurally correct complexes with BchD. Mixing experiments with modified and wild-type BchI in various combinations showed that an exchange of BchI subunits in magnesium chelatase occurs during the catalytic cycle, which indicates that dissociation of the complex may be part of the reaction mechanism related to product release. Mixing experiments also showed that more than three functional interfaces in the BchI ring structure are required for magnesium chelatase activity.

Retrograde signaling and photoprotection in a gun4 mutant of Chlamydomonas reinhardtii

GUN4 is a regulatory subunit of Mg-chelatase involved in the control of tetrapyrrole synthesis in plants and cyanobacteria. Here, we report the first characterization of a gun4 insertion mutant of the unicellular green alga Chlamydomonas reinhardtii. The mutant contains 50% of chlorophyll as compared to wild-type and accumulates ProtoIX. In contrast to the increase in LHC transcription, the accumulation of most LHC proteins is drastically diminished, implying posttranscriptional down-regulation in the absence of transcriptional coordination. We found that 803 genes change their expression level in gun4 as compared to wild-type, by RNA-Seq, and this wide-ranging effect on transcription is apparent under physiological conditions. Besides LHCs, we identified transcripts encoding enzymes of the tetrapyrrole pathway and factors involved in signal transduction, transcription, and chromatin remodeling. Moreover, we observe perturbations in electron transport with a strongly decreased PSI-to-PSII ratio. This is accompanied by an enhanced activity of the plastid terminal oxidase (PTOX) that could have a physiological role in decreasing photosystem II excitation pressure.

Retrograde signaling and photoprotection in a gun4 mutant of Chlamydomonas reinhardtii

GUN4 is a regulatory subunit of Mg-chelatase involved in the control of tetrapyrrole synthesis in plants and cyanobacteria. Here, we report the first characterization of a gun4 insertion mutant of the unicellular green alga Chlamydomonas reinhardtii. The mutant contains 50% of chlorophyll as compared to wild-type and accumulates ProtoIX. In contrast to the increase in LHC transcription, the accumulation of most LHC proteins is drastically diminished, implying posttranscriptional down-regulation in the absence of transcriptional coordination. We found that 803 genes change their expression level in gun4 as compared to wild-type, by RNA-Seq, and this wide-ranging effect on transcription is apparent under physiological conditions. Besides LHCs, we identified transcripts encoding enzymes of the tetrapyrrole pathway and factors involved in signal transduction, transcription, and chromatin remodeling. Moreover, we observe perturbations in electron transport with a strongly decreased PSI-to-PSII ratio. This is accompanied by an enhanced activity of the plastid terminal oxidase (PTOX) that could have a physiological role in decreasing photosystem II excitation pressure.

Modulation of chlorophyll biosynthesis by water stress in rice seedlings during chloroplast biogenesis

To understand the impact of water stress on the greening process, water stress was applied to 6-day-old etiolated seedlings of a drought-sensitive cultivar of rice (Oryza sativa), Pusa Basmati-1 by immersing their roots in 40 mm polyethylene glycol (PEG) 6000 (-0.69 MPa) or 50 mm PEG 6000 (-1.03 MPa) dissolved in half-strength Murashige and Skoog (MS)-nutrient-solution, 16 h prior to transfer to cool-white-fluorescent + incandescent light. Chlorophyll (Chl) accumulation substantially declined in developing water-stressed seedlings. Reduced Chl synthesis was due to decreased accumulation of chlorophyll biosynthetic intermediates, that is, glutamate-1-semialdehyde (GSA), 5-aminolevulinic acid, Mg-protoporphyrin IX monomethylester and protochlorophyllide. Although 5-aminolevulinic acid synthesis decreased, the gene expression and protein abundance of the enzyme responsible for its synthesis, GSA aminotransferase, increased, suggesting its crucial role in the greening process in stressful environment. The biochemical activities of Chl biosynthetic enzymes, that is, 5-aminolevulinic acid dehydratase, porphobilinogen deaminase, coproporphyrinogen III oxidase, porphyrinogen IX oxidase, Mg-chelatase and protochlorophyllide oxidoreductase, were down-regulated due to their reduced protein abundance/gene expression in water-stressed seedlings. Down-regulation of protochlorophyllide oxidoreductase resulted in impaired Shibata shift. Our results demonstrate that reduced synthesis of early intermediates, that is, GSA and 5-aminolevulinic acid, could modulate the gene expression of later enzymes of Chl biosynthesis pathway.

Thioredoxin redox regulates ATPase activity of magnesium chelatase CHLI subunit and modulates redox-mediated signaling in tetrapyrrole biosynthesis and homeostasis of reactive oxygen species in pea plants

The chloroplast thioredoxins (TRXs) function as messengers of redox signals from ferredoxin to target enzymes. In this work, we studied the regulatory impact of pea (Pisum sativum) TRX-F on the magnesium (Mg) chelatase CHLI subunit and the enzymatic activation of Mg chelatase in vitro and in vivo. In vitro, reduced TRX-F activated the ATPase activity of pea CHLI and enhanced the activity of Mg chelatase reconstituted from the three recombinant subunits CHLI, CHLD, and CHLH in combination with the regulator protein GENOMES UNCOUPLED4 (GUN4). Yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated that TRX-F physically interacts with CHLI but not with either of the other two subunits or GUN4. In vivo, virus-induced TRX-F gene silencing (VIGS-TRX-F) in pea plants did not result in an altered redox state of CHLI. However, simultaneous silencing of the pea TRX-F and TRX-M genes (VIGS-TRX-F/TRX-M) resulted in partially and fully oxidized CHLI in vivo. VIGS-TRX-F/TRX-M plants demonstrated a significant reduction in Mg chelatase activity and 5-aminolevulinic acid synthesizing capacity as well as reduced pigment content and lower photosynthetic capacity. These results suggest that, in vivo, TRX-M can compensate for a lack of TRX-F and that both TRXs act as important redox regulators of Mg chelatase. Furthermore, the silencing of TRX-F and TRX-M expression also affects gene expression in the tetrapyrrole biosynthesis pathway and leads to the accumulation of reactive oxygen species, which may also serve as an additional signal for the transcriptional regulation of photosynthesis-associated nuclear genes.

FERROCHELATASE: THE CONVERGENCE OF THE PORPHYRIN BIOSYNTHESIS AND IRON TRANSPORT PATHWAYS

Ferrochelatase (also known as PPIX ferrochelatase; Enzyme Commission number 4.9.9.1.1) catalyzes the insertion of ferrous iron into PPIX to form heme. This reaction unites the biochemically synchronized pathways of porphyrin synthesis and iron transport in nearly all living organisms. The ferrochelatases are an evolutionarily diverse family of enzymes with no more than six active site residues known to be perfectly conserved. The availability of over thirty different crystal structures, including many with bound metal ions or porphyrins, has added tremendously to our understanding of ferrochelatase structure and function. It is generally believed that ferrous iron is directly channeled to ferrochelatase in vivo, but the identity of the suspected chaperone remains uncertain despite much recent progress in this area. Identification of a conserved metal ion binding site at the base of the active site cleft may be an important clue as to how ferrochelatases acquire iron, and catalyze desolvation during transport to the catalytic site to complete heme synthesis.

Structure of the cyanobacterial Magnesium Chelatase H subunit determined by single particle reconstruction and small-angle X-ray scattering

The biosynthesis of chlorophyll, an essential cofactor for photosynthesis, requires the ATP-dependent insertion of Mg(2+) into protoporphyrin IX catalyzed by the multisubunit enzyme magnesium chelatase. This enzyme complex consists of the I subunit, an ATPase that forms a complex with the D subunit, and an H subunit that binds both the protoporphyrin substrate and the magnesium protoporphyrin product. In this study we used electron microscopy and small-angle x-ray scattering to investigate the structure of the magnesium chelatase H subunit, ChlH, from the thermophilic cyanobacterium Thermosynechococcus elongatus. Single particle reconstruction of negatively stained apo-ChlH and Chl-porphyrin proteins was used to reconstitute three-dimensional structures to a resolution of ∼30 Å. ChlH is a large, 148-kDa protein of 1326 residues, forming a cage-like assembly comprising the majority of the structure, attached to a globular N-terminal domain of ∼16 kDa by a narrow linker region. This N-terminal domain is adjacent to a 5 nm-diameter opening in the structure that allows access to a cavity. Small-angle x-ray scattering analysis of ChlH, performed on soluble, catalytically active ChlH, verifies the presence of two domains and their relative sizes. Our results provide a basis for the multiple regulatory and catalytic functions of ChlH of oxygenic photosynthetic organisms and for a chaperoning function that sequesters the enzyme-bound magnesium protoporphyrin product prior to its delivery to the next enzyme in the chlorophyll biosynthetic pathway, magnesium protoporphyrin methyltransferase.

Post-translational control of tetrapyrrole biosynthesis in plants, algae, and cyanobacteria

The tetrapyrrole biosynthetic pathway provides the vital cofactors and pigments for photoautotrophic growth (chlorophyll), several essential redox reactions in electron transport chains (haem), N- and S-assimilation (sirohaem), and photomorphogenic processes (phytochromobilin). While the biochemistry of the pathway is well understood and almost all genes encoding enzymes of tetrapyrrole biosynthesis have been identified in plants, the post-translational control and organization of the pathway remains to be clarified. Post-translational mechanisms controlling metabolic activities are of particular interest since tetrapyrrole biosynthesis needs adaptation to environmental challenges. This review surveys post-translational mechanisms that have been reported to modulate metabolic activities and organization of the tetrapyrrole biosynthesis pathway.

C-terminal residues of oryza sativa GUN4 are required for the activation of the ChlH subunit of magnesium chelatase in chlorophyll synthesis

Oryza sativa GUN4 together with the magnesium chelatase subunits ChlI, ChlD, and ChlH have been heterologously expressed and purified to reconstitute magnesium chelatase activity in vitro. Maximum magnesium chelatase activity requires pre-activation of OsChlH with OsGUN4, Mg(2+) and protoporphyrin-IX. OsGUN4 and OsChlH preincubated without protoporphyrin-IX yields magnesium chelatase activity similar to assays without OsGUN4, suggesting formation of a dead-end complex. Either 9 or 10 C-terminal amino acids of OsGUN4 are slowly hydrolyzed to yield a truncated OsGUN4. These truncated OsGUN4 still bind protoporphyrin-IX and Mg-protoporphyrin-IX but are unable to activate OsChlH. This suggests the mechanism of GUN4 activation of magnesium chelatase is different in eukaryotes compared to cyanobacteria as the orthologous cyanobacterial GUN4 proteins lack this C-terminal extension.

A BchD (magnesium chelatase) mutant of rhodobacter sphaeroides synthesizes zinc bacteriochlorophyll through novel zinc-containing intermediates

Heme and bacteriochlorophyll a (BChl) biosyntheses share the same pathway to protoporphyrin IX, which then branches as follows. Fe(2+) chelation into the macrocycle by ferrochelatase results in heme formation, and Mg(2+) addition by Mg-chelatase commits the porphyrin to BChl synthesis. It was recently discovered that a bchD (Mg-chelatase) mutant of Rhodobacter sphaeroides produces an alternative BChl in which Mg(2+) is substituted by Zn(2+). Zn-BChl has been found in only one other organism before, the acidophilic Acidiphilium rubrum. Our objectives in this work on the bchD mutant were to 1) elucidate the Zn-BChl biosynthetic pathway in this organism and 2) understand causes for the low amounts of Zn-BChl produced. The bchD mutant was found to contain a Zn-protoporphyrin IX pool, analogous to the Mg-protoporphyrin IX pool found in the wild type strain. Inhibition of ferrochelatase with N-methylprotoporphyrin IX caused Zn-protoporphyrin IX and Zn-BChl levels to decline by 80-90% in the bchD mutant, whereas in the wild type strain, Mg-protoporphyrin IX and Mg-BChl levels increased by 170-240%. Two early metabolites of the Zn-BChl pathway were isolated from the bchD mutant and identified as Zn-protoporphyrin IX monomethyl ester and divinyl-Zn-protochlorophyllide. Our data support a model in which ferrochelatase synthesizes Zn-protoporphyrin IX, and this metabolite is acted on by enzymes of the BChl pathway to produce Zn-BChl. Finally, the low amounts of Zn-BChl in the bchD mutant may be due, at least in part, to a bottleneck upstream of the step where divinyl-Zn-protochlorophyllide is converted to monovinyl-Zn-protochlorophyllide.

Functional assignments for the carboxyl-terminal domains of the ferrochelatase from Synechocystis PCC 6803: the CAB domain plays a regulatory role, and region II is essential for catalysis

Ferrochelatase (FeCH) catalyzes the insertion of Fe(2+) into protoporphyrin, forming protoheme. In photosynthetic organisms, FeCH and magnesium chelatase lie at a biosynthetic branch point where partitioning down the heme and chlorophyll (Chl) pathways occurs. Unlike their mammalian, yeast, and other bacterial counterparts, cyanobacterial and algal FeCHs as well as FeCH2 isoform from plants possess a carboxyl-terminal Chl a/b-binding (CAB) domain with a conserved Chl-binding motif. The CAB domain is connected to the FeCH catalytic core by a proline-rich linker sequence (region II). In order to dissect the regulatory, catalytic, and structural roles of the region II and CAB domains, we analyzed a FeCH ΔH347 mutant that retains region II but lacks the CAB domain and compared it with the ΔH324-FeCH mutant that lacks both these domains. We found that the CAB domain is not required for catalytic activity but is essential for dimerization of FeCH; its absence causes aberrant accumulation of Chl-protein complexes under high light accompanied by high levels of the Chl precursor chlorophyllide. Thus, the CAB domain appears to serve mainly a regulatory function, possibly in balancing Chl biosynthesis with the synthesis of cognate apoproteins. Region II is essential for the catalytic function of the plastid-type FeCH enzyme, although the low residual activity of the ΔH324-FeCH is more than sufficient to furnish the cellular demand for heme. We propose that the apparent surplus of FeCH activity in the wild type is critical for cell viability under high light due to a regulatory role of FeCH in the distribution of Chl into apoproteins.

Redox regulation of chlorophyll biosynthesis

Chlorophyll captures and redirects light-energy and is thus essential for photosynthetic organisms. The demand for chlorophyll differs throughout the day and night and in response to changing light conditions. Moreover, the chlorophyll biosynthesis pathway is up to certain points shared between the different tetrapyrroles; chlorophyll, heme, siroheme and phytochromobilin, for which the cell has different requirements at different time points. Combined with the phototoxic properties of tetrapyrroles which, if not properly protected, can lead to formation of reactive oxygen species (ROS), the need for a strict regulation of the chlorophyll biosynthetic pathway is obvious. Here we describe the current knowledge on regulation of chlorophyll biosynthesis in plants by the chloroplast redox state with emphasis on the Mg-chelatase situated at the branch point between the heme and the chlorophyll pathway. We discuss the proposed role of the Mg-chelatase as a key regulator of the tetrapyrrole pathway by its effect on enzymes both up- and downstream in the pathway and we specifically describe how redox state might regulate the Mg-branch. Finally, we propose that a recently identified NADPH-dependent thioredoxin reductase (NTRC) could be involved in redox regulation or protection of chlorophyll biosynthetic enzymes and describe the possible modes of action by this enzyme.