Plastids and protein targeting

Soil Cd bioavailability response characteristics to microbes in paddy fields with co-incorporation of milk vetch, rice straw and amendments

Excessive heavy metals in soils can threaten food security and soil health. New practical technology is urgently needed to remediate cadmium (Cd) contaminated paddies in many parts of the world. Chinese milk vetch (M), rice straw (R), and soil amendments can reduce Cd activity in soil; however, the mechanism underlying this reduction is not well understood. This study explored the impact of co-incorporation of milk vetch, rice straw, and either lime (L), sesbania biochar (B), or sepiolite on soil Cd bioavailability through field experiments. The results indicated that the rice grain Cd concentrations in soil treated with milk vetch + rice straw + fertilizer (MRF, 16.6 %), milk vetch + rice straw + fertilizer + sesbania biochar (MRFB, 50.1 %), and milk vetch + rice straw + fertilizer + lime (MRFL, 48.3 %) were significantly lower than those in soil treated with fertilizer (F). The acid-soluble Cd concentrations influenced rice grain Cd uptake and were 33.9 % and 47.5 % lower for the MRFB and MRFL treatments, respectively, than for F alone. A decrease in acid-soluble Cd (AciCd) was accompanied by a decrease in Eh and increases in pH, Fe2+, cation exchange capacity, and dissolved organic carbon. The MRFB treatment promoted iron plaque (IP) formation on the rice root surface. The relative abundances of Desulfobacterota and Verrucomicrobiota were higher for the MRFB treatment than for the other treatments. A partial least squares path model confirmed that Aci-Cd and low-crystalline IP (IP-Feh) influenced the rice grain Cd concentration.

Distinct non-synonymous mutations in cytochrome b highly correlate with decoquinate resistance in apicomplexan parasite Eimeria tenella

BACKGROUND

Protozoan parasites of the genus Eimeria are the causative agents of chicken coccidiosis. Parasite resistance to most anticoccidial drugs is one of the major challenges to controlling this disease. There is an urgent need for a molecular marker to monitor the emergence of resistance against anticoccidial drugs, such as decoquinate.

METHODS

We developed decoquinate-resistant strains by successively exposing the Houghton (H) and Xinjiang (XJ) strains of E. tenella to incremental concentrations of this drug in chickens. Additionally, we isolated a decoquinate-resistant strain from the field. The resistance of these three strains was tested using the criteria of weight gain, relative oocyst production and reduction of lesion scores. Whole-genome sequencing was used to identify the non-synonymous mutations in coding genes that were highly associated with the decoquinate-resistant phenotype in the two laboratory-induced strains. Subsequently, we scrutinized the missense mutation in a field-resistant strain for verification. We also employed the AlphaFold and PyMOL systems to model the alterations in the binding affinity of the mutants toward the drug molecule.

RESULTS

We obtained two decoquinate-resistant (DecR) strains, DecR_H and XJ, originating from the original H and XJ strains, respectively, as well as a decoquinate-resistant E. tenella strain from the field (DecR_SC). These three strains displayed resistance to 120 mg/kg decoquinate administered through feed. Through whole-genome sequencing analysis, we identified the cytochrome b gene (cyt b; ETH2_MIT00100) as the sole mutated gene shared between the DecR_H and XJ strains and also detected this gene in the DecR_SC strain. Distinct non-synonymous mutations, namely Gln131Lys in DecR_H, Phe263Leu in DecR_XJ, and Phe283Leu in DecR_SC were observed in the three resistant strains. Notably, these mutations were located in the extracellular segments of cyt b, in close proximity to the ubiquinol oxidation site Qo. Drug molecular docking studies revealed that cyt b harboring these mutants exhibited varying degrees of reduced binding ability to decoquinate.

CONCLUSIONS

Our findings emphasize the critical role of cyt b mutations in the development of decoquinate resistance in E. tenella. The strong correlation observed between cyt b mutant alleles and resistance indicates their potential as valuable molecular markers for the rapid detection of decoquinate resistance.

Using Diatom and Apicomplexan Models to Study the Heme Pathway of Chromera velia

Heme biosynthesis is essential for almost all living organisms. Despite its conserved function, the pathway’s enzymes can be located in a remarkable diversity of cellular compartments in different organisms. This location does not always reflect their evolutionary origins, as might be expected from the history of their acquisition through endosymbiosis. Instead, the final subcellular localization of the enzyme reflects multiple factors, including evolutionary origin, demand for the product, availability of the substrate, and mechanism of pathway regulation. The biosynthesis of heme in the apicomonad Chromera velia follows a chimeric pathway combining heme elements from the ancient algal symbiont and the host. Computational analyses using different algorithms predict complex targeting patterns, placing enzymes in the mitochondrion, plastid, endoplasmic reticulum, or the cytoplasm. We employed heterologous reporter gene expression in the apicomplexan parasite Toxoplasma gondii and the diatom Phaeodactylum tricornutum to experimentally test these predictions. 5-aminolevulinate synthase was located in the mitochondria in both transfection systems. In T. gondii, the two 5-aminolevulinate dehydratases were located in the cytosol, uroporphyrinogen synthase in the mitochondrion, and the two ferrochelatases in the plastid. In P. tricornutum, all remaining enzymes, from ALA-dehydratase to ferrochelatase, were placed either in the endoplasmic reticulum or in the periplastidial space.

Dually localised proteins found in both the apicoplast and mitochondrion utilize the Golgi-dependent pathway for apicoplast targeting in Toxoplasma gondii

BACKGROUND INFORMATION

Like other apicomplexan parasites, Toxoplasma gondii harbours a four-membraned endosymbiotic organelle - the apicoplast. Apicoplast proteins are nuclear encoded and trafficked to the organelle through the endoplasmic reticulum (ER). From the ER to the apicoplast, two distinct protein trafficking pathways can be used. One such pathway is the cell's secretory pathway involving the Golgi, whereas the other is a unique Golgi-independent pathway. Using different experimental approaches, many apicoplast proteins have been shown to utilize the Golgi-independent pathway, whereas a handful of reports show that a few proteins use the Golgi-dependent pathway. This has led to an emphasis towards the unique Golgi-independent pathway when apicoplast protein trafficking is discussed in the literature. Additionally, the molecular features that drive proteins to each pathway are not known.

RESULTS

In this report, we systematically test eight apicoplast proteins, using a C-terminal HDEL sequence to assess the role of the Golgi in their transport. We demonstrate that dually localised proteins of the apicoplast and mitochondrion (TgSOD2, TgTPx1/2 and TgACN/IRP) are trafficked through the Golgi, whereas proteins localised exclusively to the apicoplast are trafficked independent of the Golgi. Mutants of the dually localised proteins that localised exclusively to the apicoplast also showed trafficking through the Golgi. Phylogenetic analysis of TgSOD2, TgTPx1/2 and TgACN/IRP suggested that the evolutionary origins of TgSOD2 and TgTPx1/2 lie in the mitochondrion, whereas TgACN/IRP appears to have originated from the apicoplast.

CONCLUSIONS AND SIGNIFICANCE

Collectively, with these results, for the first time, we establish that the driver of the Golgi-dependent trafficking route to the apicoplast is the dual localisation of the protein to the apicoplast and the mitochondrion.

Curcumin@metal organic frameworks nano-composite for treatment of chronic toxoplasmosis

Toxoplasmosis is a zoonotic protozoal disease caused by Toxoplasma gondii, an intracellular opportunistic protozoan parasite that can infect any warm-blooded vertebrate cell. In this study, zirconium, and iron-based metal-organic framework was prepared according to the solvothermal method. New nanocomposite (Curcumin@MOFs) was prepared by reacting curcumin with amino-functionalized metal-organic frameworks (Fe-MOF and UiO-66-NH₂). Besides characterizations of the composite by powder X-ray diffraction and scanning electron microscope, nano-Curcumin@MOFs was used as a new novel structure as atrial for treatment of chronic toxoplasmosis. Results showed a reduced number of brain cysts, high levels of serum Toxo IgG, and normal histo-morphology with preserved parenchymal, and stromal tissues in rats groups treated with curcumin and Curcumin@MOFs nanocomposite.

Fatty Acid Biosynthesis in Chromerids

Fatty acids are essential components of biological membranes, important for the maintenance of cellular structures, especially in organisms with complex life cycles like protozoan parasites. Apicomplexans are obligate parasites responsible for various deadly diseases of humans and livestock. We analyzed the fatty acids produced by the closest phototrophic relatives of parasitic apicomplexans, the chromerids Chromera velia and Vitrella brassicaformis, and investigated the genes coding for enzymes involved in fatty acids biosynthesis in chromerids, in comparison to their parasitic relatives. Based on evidence from genomic and metabolomic data, we propose a model of fatty acid synthesis in chromerids: the plastid-localized FAS-II pathway is responsible for the de novo synthesis of fatty acids reaching the maximum length of 18 carbon units. Short saturated fatty acids (C14:0-C18:0) originate from the plastid are then elongated and desaturated in the cytosol and the endoplasmic reticulum. We identified giant FAS I-like multi-modular enzymes in both chromerids, which seem to be involved in polyketide synthesis and fatty acid elongation. This full-scale description of the biosynthesis of fatty acids and their derivatives provides important insights into the reductive evolutionary transition of a phototropic algal ancestor to obligate parasites.

Kingdom Chromista and its eight phyla: a new synthesis emphasising periplastid protein targeting, cytoskeletal and periplastid evolution, and ancient divergences

In 1981 I established kingdom Chromista, distinguished from Plantae because of its more complex chloroplast-associated membrane topology and rigid tubular multipartite ciliary hairs. Plantae originated by converting a cyanobacterium to chloroplasts with Toc/Tic translocons; most evolved cell walls early, thereby losing phagotrophy. Chromists originated by enslaving a phagocytosed red alga, surrounding plastids by two extra membranes, placing them within the endomembrane system, necessitating novel protein import machineries. Early chromists retained phagotrophy, remaining naked and repeatedly reverted to heterotrophy by losing chloroplasts. Therefore, Chromista include secondary phagoheterotrophs (notably ciliates, many dinoflagellates, Opalozoa, Rhizaria, heliozoans) or walled osmotrophs (Pseudofungi, Labyrinthulea), formerly considered protozoa or fungi respectively, plus endoparasites (e.g. Sporozoa) and all chromophyte algae (other dinoflagellates, chromeroids, ochrophytes, haptophytes, cryptophytes). I discuss their origin, evolutionary diversification, and reasons for making chromists one kingdom despite highly divergent cytoskeletons and trophic modes, including improved explanations for periplastid/chloroplast protein targeting, derlin evolution, and ciliary/cytoskeletal diversification. I conjecture that transit-peptide-receptor-mediated 'endocytosis' from periplastid membranes generates periplastid vesicles that fuse with the arguably derlin-translocon-containing periplastid reticulum (putative red algal trans-Golgi network homologue; present in all chromophytes except dinoflagellates). I explain chromist origin from ancestral corticates and neokaryotes, reappraising tertiary symbiogenesis; a chromist cytoskeletal synapomorphy, a bypassing microtubule band dextral to both centrioles, favoured multiple axopodial origins. I revise chromist higher classification by transferring rhizarian subphylum Endomyxa from Cercozoa to Retaria; establishing retarian subphylum Ectoreta for Foraminifera plus Radiozoa, apicomonad subclasses, new dinozoan classes Myzodinea (grouping Colpovora gen. n., Psammosa), Endodinea, Sulcodinea, and subclass Karlodinia; and ranking heterokont Gyrista as phylum not superphylum.

De novo transcriptome of the cosmopolitan dinoflagellate Amphidinium carterae to identify enzymes with biotechnological potential

Dinoflagellates are phytoplanktonic organisms found in both freshwater and marine habitats. They are often studied because related to harmful algal blooms but they are also known to produce bioactive compounds for the treatment of human pathologies. The aim of this study was to sequence the full transcriptome of the dinoflagellate Amphidinium carterae in both nitrogen-starved and -replete culturing conditions (1) to evaluate the response to nitrogen starvation at the transcriptional level, (2) to look for possible polyketide synthases (PKSs) in the studied clone (genes that may be involved in the synthesis of bioactive compounds), (3) if present, to evaluate if nutrient starvation can influence PKS expression, (4) to look for other possible enzymes of biotechnological interest and (5) to test strain cytotoxicity on human cell lines. Results showed an increase in nitrogen metabolism and stress response in nitrogen-starved cells and confirmed the presence of a type I β-ketosynthase. In addition, L-asparaginase (used for the treatment of Leukemia and for acrylamide reduction in food industries) and cellulase (useful for biofuel production and other industrial applications) have been identified for the first time in this species, giving new insights into possible biotechnological applications of dinoflagellates.

Role of Modular Polyketide Synthases in the Production of Polyether Ladder Compounds in Ciguatoxin-Producing Gambierdiscus polynesiensis and G. excentricus (Dinophyceae)

Gambierdiscus, a benthic dinoflagellate, produces ciguatoxins that cause the human illness Ciguatera. Ciguatoxins are polyether ladder compounds that have a polyketide origin, indicating that polyketide synthases (PKS) are involved in their production. We sequenced transcriptomes of Gambierdiscus excentricus and Gambierdiscus polynesiensis and found 264 contigs encoding single domain ketoacyl synthases (KS; G. excentricus: 106, G. polynesiensis: 143) and ketoreductases (KR; G. excentricus: 7, G. polynesiensis: 8) with sequence similarity to type I PKSs, as reported in other dinoflagellates. In addition, 24 contigs (G. excentricus: 3, G. polynesiensis: 21) encoding multiple PKS domains (forming typical type I PKSs modules) were found. The proposed structure produced by one of these megasynthases resembles a partial carbon backbone of a polyether ladder compound. Seventeen contigs encoding single domain KS, KR, s-malonyltransacylase, dehydratase and enoyl reductase with sequence similarity to type II fatty acid synthases (FAS) in plants were found. Type I PKS and type II FAS genes were distinguished based on the arrangement of domains on the contigs and their sequence similarity and phylogenetic clustering with known PKS/FAS genes in other organisms. This differentiation of PKS and FAS pathways in Gambierdiscus is important, as it will facilitate approaches to investigating toxin biosynthesis pathways in dinoflagellates.

Replication and maintenance of the Plasmodium falciparum apicoplast genome

Members of the phylum Apicomplexa are responsible for many devastating diseases including malaria (Plasmodium spp.), toxoplasmosis (Toxoplasma gondii), babesiosis (Babesia bovis), and cyclosporiasis (Cyclospora cayetanensis). Most Apicomplexans contain a unique and essential organelle called the apicoplast. Derived from an ancient chloroplast, the apicoplast replicates and maintains a 35 kilobase (kb) circular genome. Due to its essential nature within the parasite, drugs targeted to proteins involved in DNA replication and repair of the apicoplast should be potent and specific. This review summarizes the current knowledge surrounding the replication and repair of the Plasmodium falciparum apicoplast genome and identifies several putative proteins involved in replication and repair pathways.

Evolutionary distinctiveness of fatty acid and polyketide synthesis in eukaryotes

Fatty acids, which are essential cell membrane constituents and fuel storage molecules, are thought to share a common evolutionary origin with polyketide toxins in eukaryotes. While fatty acids are primary metabolic products, polyketide toxins are secondary metabolites that are involved in ecologically relevant processes, such as chemical defence, and produce the adverse effects of harmful algal blooms. Selection pressures on such compounds may be different, resulting in differing evolutionary histories. Surprisingly, some studies of dinoflagellates have suggested that the same enzymes may catalyse these processes. Here we show the presence and evolutionary distinctiveness of genes encoding six key enzymes essential for fatty acid production in 13 eukaryotic lineages for which no previous sequence data were available (alveolates: dinoflagellates, Vitrella, Chromera; stramenopiles: bolidophytes, chrysophytes, pelagophytes, raphidophytes, dictyochophytes, pinguiophytes, xanthophytes; Rhizaria: chlorarachniophytes, haplosporida; euglenids) and 8 other lineages (apicomplexans, bacillariophytes, synurophytes, cryptophytes, haptophytes, chlorophyceans, prasinophytes, trebouxiophytes). The phylogeny of fatty acid synthase genes reflects the evolutionary history of the organism, indicating selection to maintain conserved functionality. In contrast, polyketide synthase gene families are highly expanded in dinoflagellates and haptophytes, suggesting relaxed constraints in their evolutionary history, while completely absent from some protist lineages. This demonstrates a vast potential for the production of bioactive polyketide compounds in some lineages of microbial eukaryotes, indicating that the evolution of these compounds may have played an important role in their ecological success.

Prospective function of FtsZ proteins in the secondary plastid of chlorarachniophyte algae

BACKGROUND

Division of double-membraned plastids (primary plastids) is performed by constriction of a ring-like division complex consisting of multiple plastid division proteins. Consistent with the endosymbiotic origin of primary plastids, some of the plastid division proteins are descended from cyanobacterial cell division machinery, and the others are of host origin. In several algal lineages, complex plastids, the "secondary plastids", have been acquired by the endosymbiotic uptake of primary plastid-bearing algae, and are surrounded by three or four membranes. Although homologous genes for primary plastid division proteins have been found in genome sequences of secondary plastid-bearing organisms, little is known about the function of these proteins or the mechanism of secondary plastid division.

RESULTS

To gain insight into the mechanism of secondary plastid division, we characterized two plastid division proteins, FtsZD-1 and FtsZD-2, in chlorarachniophyte algae. FtsZ homologs were encoded by the nuclear genomes and carried an N-terminal plastid targeting signal. Immunoelectron microscopy revealed that both FtsZD-1 and FtsZD-2 formed a ring-like structure at the midpoint of bilobate plastids with a projecting pyrenoid in Bigelowiella natans. The ring was always associated with a shallow plate-like invagination of the two innermost plastid membranes. Furthermore, gene expression analysis confirmed that transcripts of ftsZD genes were periodically increased soon after cell division during the B. natans cell cycle, which is not consistent with the timing of plastid division.

CONCLUSIONS

Our findings suggest that chlorarachniophyte FtsZD proteins are involved in partial constriction of the inner pair of plastid membranes, but not in the whole process of plastid division. It is uncertain how the outer pair of plastid membranes is constricted, and as-yet-unknown mechanism is required for the secondary plastid division in chlorarachniophytes.

Insights into transcriptional changes that accompany organelle sequestration from the stolen nucleus of Mesodinium rubrum

Background

Organelle retention is a form of mixotrophy that allows organisms to reap metabolic benefits similar to those of photoautotrophs through capture of algal prey and sequestration of their plastids. Mesodinium rubrum is an abundant and broadly distributed photosynthetic marine ciliate that steals organelles from cryptophyte algae, such as Geminigera cryophila. M. rubrum is unique from most other acquired phototrophs because it also steals a functional nucleus that facilitates genetic control of sequestered plastids and other organelles. We analyzed changes in G. cryophila nuclear gene expression and transcript abundance after its incorporation into the cellular architecture of M. rubrum as an initial step towards understanding this complex system.

Methods

We compared Illumina-generated transcriptomes of the cryptophyte Geminigera cryophila as a free-living cell and as a sequestered nucleus in M. rubrum to identify changes in protein abundance and gene expression. After KEGG annotation, proteins were clustered by functional categories, which were evaluated for over- or under-representation in the sequestered nucleus. Similarly, coding sequences were grouped by KEGG categories/pathways, which were then evaluated for over- or under-expression via read count strategies.

Results

At the time of sampling, the global transcriptome of M. rubrum was dominated (~58–62 %) by transcription from its stolen nucleus. A comparison of transcriptomes from free-living G. cryophila cells to those of the sequestered nucleus revealed a decrease in gene expression and transcript abundance for most functional protein categories within the ciliate. However, genes coding for proteins involved in photosynthesis, oxidative stress reduction, and several other metabolic pathways revealed striking exceptions to this general decline.

Conclusions

Major changes in G. cryophila transcript expression after sequestration by M. rubrum and the ciliate’s success as a photoautotroph imply some level of control or gene regulation by the ciliate and at the very least reflect a degree of coordination between host and foreign organelles. Intriguingly, cryptophyte genes involved in protein transport are significantly under-expressed in M. rubrum, implicating a role for the ciliate’s endomembrane system in targeting cryptophyte proteins to plastid complexes. Collectively, this initial portrait of an acquired transcriptome within a dynamic and ecologically successful ciliate highlights the remarkable cellular and metabolic chimerism of this system.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2052-9) contains supplementary material, which is available to authorized users.

Genomic perspectives on the birth and spread of plastids

The endosymbiotic origin of plastids from cyanobacteria was a landmark event in the history of eukaryotic life. Subsequent to the evolution of primary plastids, photosynthesis spread from red and green algae to unrelated eukaryotes by secondary and tertiary endosymbiosis. Although the movement of cyanobacterial genes from endosymbiont to host is well studied, less is known about the migration of eukaryotic genes from one nucleus to the other in the context of serial endosymbiosis. Here I explore the magnitude and potential impact of nucleus-to-nucleus endosymbiotic gene transfer in the evolution of complex algae, and the extent to which such transfers compromise our ability to infer the deep structure of the eukaryotic tree of life. In addition to endosymbiotic gene transfer, horizontal gene transfer events occurring before, during, and after endosymbioses further confound our efforts to reconstruct the ancient mergers that forged multiple lines of photosynthetic microbial eukaryotes.

The mitochondrial and chloroplast genomes of the haptophyte Chrysochromulina tobin contain unique repeat structures and gene profiles

BACKGROUND

Haptophytes are widely and abundantly distributed in both marine and freshwater ecosystems. Few genomic analyses of representatives within this taxon have been reported, despite their early evolutionary origins and their prominent role in global carbon fixation.

RESULTS

The complete mitochondrial and chloroplast genome sequences of the haptophyte Chrysochromulina tobin (Prymnesiales) provide insight into the architecture and gene content of haptophyte organellar genomes. The mitochondrial genome (~34 kb) encodes 21 protein coding genes and contains a complex, 9 kb tandem repeat region. Similar to other haptophytes and rhodophytes, but not cryptophytes or stramenopiles, the mitochondrial genome has lost the nad7, nad9 and nad11 genes. The ~105 kb chloroplast genome encodes 112 protein coding genes, including ycf39 which has strong structural homology to NADP-binding nitrate transcriptional regulators; a divergent 'CheY-like' two-component response regulator (ycf55) and Tic/Toc (ycf60 and ycf80) membrane transporters. Notably, a zinc finger domain has been identified in the rpl36 ribosomal protein gene of all chloroplasts sequenced to date with the exception of haptophytes and cryptophytes--algae that have gained (via lateral gene transfer) an alternative rpl36 lacking the zinc finger motif. The two C. tobin chloroplast ribosomal RNA operon spacer regions differ in tRNA content. Additionally, each ribosomal operon contains multiple single nucleotide polymorphisms (SNPs)--a pattern observed in rhodophytes and cryptophytes, but few stramenopiles. Analysis of small (<200 bp) chloroplast encoded tandem and inverted repeats in C. tobin and 78 other algal chloroplast genomes show that repeat type, size and location are correlated with gene identity and taxonomic clade.

CONCLUSION

The Chrysochromulina tobin organellar genomes provide new insight into organellar function and evolution. These are the first organellar genomes to be determined for the prymnesiales, a taxon that is present in both oceanic and freshwater systems and represents major primary photosynthetic producers and contributors to global ecosystem stability.

Proteomics reveals plastid- and periplastid-targeted proteins in the chlorarachniophyte alga Bigelowiella natans

Chlorarachniophytes are unicellular marine algae with plastids (chloroplasts) of secondary endosymbiotic origin. Chlorarachniophyte cells retain the remnant nucleus (nucleomorph) and cytoplasm (periplastidial compartment, PPC) of the green algal endosymbiont from which their plastid was derived. To characterize the diversity of nucleus-encoded proteins targeted to the chlorarachniophyte plastid, nucleomorph, and PPC, we isolated plastid–nucleomorph complexes from the model chlorarachniophyte Bigelowiella natans and subjected them to high-pressure liquid chromatography-tandem mass spectrometry. Our proteomic analysis, the first of its kind for a nucleomorph-bearing alga, resulted in the identification of 324 proteins with 95% confidence. Approximately 50% of these proteins have predicted bipartite leader sequences at their amino termini. Nucleus-encoded proteins make up >90% of the proteins identified. With respect to biological function, plastid-localized light-harvesting proteins were well represented, as were proteins involved in chlorophyll biosynthesis. Phylogenetic analyses revealed that many, but by no means all, of the proteins identified in our proteomic screen are of apparent green algal ancestry, consistent with the inferred evolutionary origin of the plastid and nucleomorph in chlorarachniophytes.

The number, speed, and impact of plastid endosymbioses in eukaryotic evolution

Plastids (chloroplasts) have long been recognized to have originated by endosymbiosis of a cyanobacterium, but their subsequent evolutionary history has proved complex because they have also moved between eukaryotes during additional rounds of secondary and tertiary endosymbioses. Much of this history has been revealed by genomic analyses, but some debates remain unresolved, in particular those relating to secondary red plastids of the chromalveolates, especially cryptomonads. Here, I examine several fundamental questions and assumptions about endosymbiosis and plastid evolution, including the number of endosymbiotic events needed to explain plastid diversity, whether the genetic contribution of the endosymbionts to the host genome goes far beyond plastid-targeted genes, and whether organelle origins are best viewed as a singular transition involving one symbiont or as a gradual transition involving a long line of transient food/symbionts. I also discuss a possible link between transporters and the evolution of protein targeting in organelle integration.

Plasmodium falciparum SSB tetramer wraps single-stranded DNA with similar topology but opposite polarity to E. coli SSB

Single-stranded DNA binding (SSB) proteins play central roles in genome maintenance in all organisms. Plasmodium falciparum, the causative agent of malaria, encodes an SSB protein that localizes to the apicoplast and likely functions in the replication and maintenance of its genome. P. falciparum SSB (Pf-SSB) shares a high degree of sequence homology with bacterial SSB proteins but differs in the composition of its C-terminus, which interacts with more than a dozen other proteins in Escherichia coli SSB (Ec-SSB). Using sedimentation methods, we show that Pf-SSB forms a stable homo-tetramer alone and when bound to single-stranded DNA (ssDNA). We also present a crystal structure at 2.1 Å resolution of the Pf-SSB tetramer bound to two (dT)(35) molecules. The Pf-SSB tetramer is structurally similar to the Ec-SSB tetramer, and ssDNA wraps completely around the tetramer with a "baseball seam" topology that is similar to Ec-SSB in its "65 binding mode". However, the polarity of the ssDNA wrapping around Pf-SSB is opposite to that observed for Ec-SSB. The interactions between the bases in the DNA and the amino acid side chains also differ from those observed in the Ec-SSB-DNA structure, suggesting that other differences may exist in the DNA binding properties of these structurally similar proteins.

In vitro anti-Toxoplasma gondii activity of root extract/fractions of Eurycoma longifolia Jack

Background

Toxoplasma gondii infection causes toxoplasmosis, an infectious disease with worldwide prevalence. The limited efficiency of drugs against this infection, their side effects and the potential appearance of resistant strains make the search of novel drugs an essential need. We examined Eurycoma longifolia root extract and fractions as potential sources of new compounds with high activity and low toxicity. The main goal of this study was to investigate the anti-T. gondii activity of crude extract (TACME) and four fractions (TAF 273, TAF 355, TAF 191 and TAF 401) from E. longifolia, with clindamycin as the positive control.

Methods

In vitro toxoplasmacidal evaluation was performed using Vero cells as host for T. gondii. Light microscopy technique was used to study in situ antiparasitic activity.

Results

Significant anti-T. gondii activity was observed with clindamycin (EC50 = 0.016 μg/ml), follow by TAF 355 (EC50 = 0.369 μg/ml) and TAF 401 (EC50 = 0.882 μg/ml). Light microscopy revealed that most Vero cells were infected after 3 h of exposure to T. gondii. After 36 h of exposure to the E. longifolia fraction, the host Vero cells showed no visible intracellular parasite and no remarkable morphological changes.

Conclusions

Our study demonstrated that TAF 355 and TAF401 fractions may be the sources of new anti-T. gondii compounds.

Overproduction, purification and crystallization of PfTic22, a component of the import apparatus from the apicoplast of Plasmodium falciparum

Tic22 is a component of the protein-import apparatus of the chloroplasts of plants and algae and the apicoplasts of the Apicomplexa, a large group of organisms that includes the parasites that cause malaria. Tic22 is important for protein import into these organelles and for organelle biogenesis. It lies between the two membranes of chloroplasts, making interactions with components of both the TIC and TOC complexes. In the apicoplast, it is predicted to be located between the inner two membranes and to play a similar role in import. Although Tic22 is ubiquitous, its function is as yet uncertain. Tic22 from Plasmodium falciparum was therefore overproduced, purified and crystallized. A data set extending to 2.15 Å resolution has been collected from a crystal containing selenomethionine-labelled protein and structure determination is under way.

A chemical genomic analysis of decoquinate, a Plasmodium falciparum cytochrome b inhibitor

Decoquinate has single-digit nanomolar activity against in vitro blood stage Plasmodium falciparum parasites, the causative agent of human malaria. In vitro evolution of decoquinate-resistant parasites and subsequent comparative genomic analysis to the drug-sensitive parental strain revealed resistance was conferred by two nonsynonymous single nucleotide polymorphisms in the gene encoding cytochrome b. The resultant amino acid mutations, A122T and Y126C, reside within helix C in the ubiquinol-binding pocket of cytochrome b, an essential subunit of the cytochrome bc₁ complex. As with other cytochrome bc₁ inhibitors, such as atovaquone, decoquinate has low nanomolar activity against in vitro liver stage P. yoelii and provides partial prophylaxis protection when administered to infected mice at 50 mg kg^–1. In addition, transgenic parasites expressing yeast dihydroorotate dehydrogenase are >200-fold less sensitive to decoquinate, which provides additional evidence that this drug inhibits the parasite’s mitochondrial electron transport chain. Importantly, decoquinate exhibits limited cross-resistance to a panel of atovaquone-resistant parasites evolved to harbor various mutations in cytochrome b. The basis for this difference was revealed by molecular docking studies, in which both of these inhibitors were shown to have distinctly different modes of binding within the ubiquinol-binding site of cytochrome b.

Tetrapyrrole synthesis of photosynthetic chromerids is likely homologous to the unusual pathway of apicomplexan parasites

Most photosynthetic eukaryotes synthesize both heme and chlorophyll via a common tetrapyrrole biosynthetic pathway starting from glutamate. This pathway was derived mainly from cyanobacterial predecessor of the plastid and differs from the heme synthesis of the plastid-lacking eukaryotes. Here, we show that the coral-associated alveolate Chromera velia, the closest known photosynthetic relative to Apicomplexa, possesses a tetrapyrrole pathway that is homologous to the unusual pathway of apicomplexan parasites. We also demonstrate that, unlike other eukaryotic phototrophs, Chromera synthesizes chlorophyll from glycine and succinyl-CoA rather than glutamate. Our data shed light on the evolution of the heme biosynthesis in parasitic Apicomplexa and photosynthesis-related biochemical processes in their ancestors.

Genome evolution of a tertiary dinoflagellate plastid

The dinoflagellates have repeatedly replaced their ancestral peridinin-plastid by plastids derived from a variety of algal lineages ranging from green algae to diatoms. Here, we have characterized the genome of a dinoflagellate plastid of tertiary origin in order to understand the evolutionary processes that have shaped the organelle since it was acquired as a symbiont cell. To address this, the genome of the haptophyte-derived plastid in Karlodinium veneficum was analyzed by Sanger sequencing of library clones and 454 pyrosequencing of plastid enriched DNA fractions. The sequences were assembled into a single contig of 143 kb, encoding 70 proteins, 3 rRNAs and a nearly full set of tRNAs. Comparative genomics revealed massive rearrangements and gene losses compared to the haptophyte plastid; only a small fraction of the gene clusters usually found in haptophytes as well as other types of plastids are present in K. veneficum. Despite the reduced number of genes, the K. veneficum plastid genome has retained a large size due to expanded intergenic regions. Some of the plastid genes are highly diverged and may be pseudogenes or subject to RNA editing. Gene losses and rearrangements are also features of the genomes of the peridinin-containing plastids, apicomplexa and Chromera, suggesting that the evolutionary processes that once shaped these plastids have occurred at multiple independent occasions over the history of the Alveolata.

More membranes, more proteins: complex protein import mechanisms into secondary plastids

Plastids are found across the tree of life in a tremendous diversity of life forms. Surprisingly they are not limited to photosynthetic organisms but also found in numerous predators and parasites. An important reason for the pervasiveness of plastids has been their ability to move laterally and to jump from one branch of the tree of life to the next through secondary endosymbiosis. Eukaryotic algae have entered endosymbiotic relationships with other eukaryotes on multiple independent occasions. The descendants of these endosymbiotic events now carry complex plastids, organelles that are bound by three or even four membranes. As in all endosymbiotic organelles most of the symbiont's genes have been transferred to the host and their protein products have to be imported into the organelle. As four membranes might suggest, this is a complex process. The emerging mechanisms display a series of translocons that mirror the divergent ancestry of the membranes they cross. This review is written from the viewpoint of a parasite biologist and seeks to provide a brief overview of plastid evolution in particular for readers not already familiar with plant and algal biology and then focuses on recent molecular discoveries using genetically tractable Apicomplexa and diatoms.

Malaria, Plasmodium falciparum and its apicoplast

Malaria, which is caused by species of the parasite genus Plasmodium, remains a major global health problem. A vestigial plastid homologous with the chloroplasts of plants and algae was discovered in malaria and related parasites from the phylum Apicomplexa and has radically changed our view of the evolutionary origins of these disease-causing protists. We now recognize that this large group of parasites had a photosynthetic ancestry and were converted into parasitism early in the evolution of animals. Apicomplexans have probably been parasitizing the animal kingdom for more than 500 million years. The relic plastid persists in most apicomplexans and is an essential component. Perturbation of apicoplast function or inheritance results in parasite death, making the organelle a promising target for chemotherapy. Plastids, including those of malaria parasites, are essentially reduced endosymbiotic bacteria living inside a eukaryotic host. This means that plastids have bacterial-type metabolic pathways and housekeeping processes, all of which are vulnerable to antibacterial compounds. Indeed, many antibacterials kill malaria parasites by blocking essential processes in the plastid. Furthermore, a range of herbicides that target plastid metabolism of undesired plants are also parasiticidal, making them potential new leads for antimalarial drugs. In the present review, we examine the evolutionary origins of the malaria parasite's plastid by endosymbiosis and outline the recent findings on how the organelle imports nuclear-encoded proteins through a set of translocation machineries in the membranes that bound the organelle.

A phylogenetic mosaic plastid proteome and unusual plastid-targeting signals in the green-colored dinoflagellate Lepidodinium chlorophorum

Background

Plastid replacements through secondary endosymbioses include massive transfer of genes from the endosymbiont to the host nucleus and require a new targeting system to enable transport of the plastid-targeted proteins across 3-4 plastid membranes. The dinoflagellates are the only eukaryotic lineage that has been shown to have undergone several plastid replacement events, and this group is thus highly relevant for studying the processes involved in plastid evolution. In this study, we analyzed the phylogenetic origin and N-terminal extensions of plastid-targeted proteins from Lepidodinium chlorophorum, a member of the only dinoflagellate genus that harbors a green secondary plastid rather than the red algal-derived, peridinin-containing plastid usually found in photosynthetic dinoflagellates.

Results

We sequenced 4,746 randomly picked clones from a L. chlorophorum cDNA library. 22 of the assembled genes were identified as genes encoding proteins functioning in plastids. Some of these were of green algal origin. This confirms that genes have been transferred from the plastid to the host nucleus of L. chlorophorum and indicates that the plastid is fully integrated as an organelle in the host. Other nuclear-encoded plastid-targeted protein genes, however, are clearly not of green algal origin, but have been derived from a number of different algal groups, including dinoflagellates, streptophytes, heterokonts, and red algae. The characteristics of N-terminal plastid-targeting peptides of all of these genes are substantially different from those found in peridinin-containing dinoflagellates and green algae.

Conclusions

L. chlorophorum expresses plastid-targeted proteins with a range of different origins, which probably arose through endosymbiotic gene transfer (EGT) and horizontal gene transfer (HGT). The N-terminal extension of the genes is different from the extensions found in green alga and other dinoflagellates (peridinin- and haptophyte plastids). These modifications have likely enabled the mosaic proteome of L. chlorophorum.

The endosymbiotic origin, diversification and fate of plastids

Plastids and mitochondria each arose from a single endosymbiotic event and share many similarities in how they were reduced and integrated with their host. However, the subsequent evolution of the two organelles could hardly be more different: mitochondria are a stable fixture of eukaryotic cells that are neither lost nor shuffled between lineages, whereas plastid evolution has been a complex mix of movement, loss and replacement. Molecular data from the past decade have substantially untangled this complex history, and we now know that plastids are derived from a single endosymbiotic event in the ancestor of glaucophytes, red algae and green algae (including plants). The plastids of both red algae and green algae were subsequently transferred to other lineages by secondary endosymbiosis. Green algal plastids were taken up by euglenids and chlorarachniophytes, as well as one small group of dinoflagellates. Red algae appear to have been taken up only once, giving rise to a diverse group called chromalveolates. Additional layers of complexity come from plastid loss, which has happened at least once and probably many times, and replacement. Plastid loss is difficult to prove, and cryptic, non-photosynthetic plastids are being found in many non-photosynthetic lineages. In other cases, photosynthetic lineages are now understood to have evolved from ancestors with a plastid of different origin, so an ancestral plastid has been replaced with a new one. Such replacement has taken place in several dinoflagellates (by tertiary endosymbiosis with other chromalveolates or serial secondary endosymbiosis with a green alga), and apparently also in two rhizarian lineages: chlorarachniophytes and Paulinella (which appear to have evolved from chromalveolate ancestors). The many twists and turns of plastid evolution each represent major evolutionary transitions, and each offers a glimpse into how genomes evolve and how cells integrate through gene transfers and protein trafficking.

Role of horizontal gene transfer in the evolution of photosynthetic eukaryotes and their plastids

Plastids are the organelles derived from a cyanobacterium through endosymbiosis. Unlike mitochondria, plastids are not found in all eukaryotes, but their evolution has an added layer of complexity since plastids have moved between eukaryotic lineages by secondary and tertiary endosymbiotic events. This complex history, together with the genetic integration between plastids and their host, has led to many opportunities for gene flow between phylogenetically distinct lineages. Some intracellular transfers do not lead to a protein functioning in a new environment, but many others do and the protein makeup of many plastids appears to have been influenced by exogenous sources as well. Here, different evolutionary sources and cellular destinations of gene flow that has affected the plastid lineage are reviewed. Most horizontal gene transfer (HGT) affecting the modern plastid has taken place via the host nucleus, in the form of genes for plastid-targeted proteins. The impact of this varies greatly from lineage to lineage, but in some cases such transfers can be as high as one fifth of analyzed genes. More rarely, genes have also been transferred to the plastid genome itself, and plastid genes have also been transferred to other non-plant, non-algal lineages. Overall, the proteome of many plastids has emerged as a mosaic of proteins from many sources, some from within the same cell (e.g., cytosolic genes or genes left over from the replacement of an earlier plastid), some from the plastid of other algal lineages, and some from completely unrelated sources.

Cloning, expression and purification of cytochrome c(6) from the brown alga Hizikia fusiformis and complete X-ray diffraction analysis of the structure

The primary sequence of cytochrome c(6) from the brown alga Hizikia fusiformis has been determined by cDNA cloning and the crystal structure has been solved at 1.6 A resolution. The crystal belonged to the tetragonal space group P4(1)2(1)2, with unit-cell parameters a = b = 84.58, c = 232.91 A and six molecules per asymmetric unit. The genome code, amino-acid sequence and crystal structure of H. fusiformis cytochrome c(6) were most similar to those of red algal cytochrome c(6). These results support the hypothesis that brown algae acquired their chloroplasts via secondary endosymbiosis involving a red algal endosymbiont and a eukaryote host.

Organelle evolution: what's in a name?

Plastids are organelles derived from cyanobacterial endosymbionts and the evolutionary process that gave rise to them is well understood. Or is it? The complete genome sequence of a recently evolved photosynthetic body in Paulinella chromatophora is cause for reflection on the distinction between 'endosymbiont' and 'organelle', and how the boundaries between these terms can blur.

Nucleus-encoded periplastid-targeted EFL in chlorarachniophytes

Chlorarachniophytes are cercozoan amoeboflagellates that acquired photosynthesis by enslaving a green alga, which has retained a highly reduced nucleus called a nucleomorph. The nucleomorph lacks many genes necessary for its own maintenance and expression, suggesting that some genes have been moved to the host nucleus and their products are now targeted back to the periplastid compartment (PPC), the reduced eukaryotic cytoplasm of the endosymbiont. Protein trafficking in chlorarachniophytes is therefore complex, including nucleus-encoded plastid-targeted proteins, nucleomorph-encoded plastid-targeted proteins, and nucleus-encoded periplastid-targeted proteins. A major gap in our understanding of this system is the PPC-targeted proteins because none have been described in any chlorarachniophytes. Here we describe the first such protein, the GTPase EFL. EFL was characterized from 7 chlorarachniophytes, and 2 distinct types were found. One is related to foraminiferan EFL and lacks an amino-terminal extension. The second, distantly related, type encodes an amino-terminal extension consisting of a signal peptide followed by sequence sharing many characteristics with transit peptides from nucleus-encoded plastid-targeted proteins and which we conclude is most likely PPC targeted. Western blotting with antibodies specific to putative host and PPC-targeted EFL from the chlorarachniophytes Bigelowiella natans and Gymnochlora stellata is consistent with posttranslational cleavage of the leaders from PPC-targeted proteins. Immunolocalization of both proteins in B. natans confirmed the cytosolic location of the leaderless EFL and a distinct localization pattern for the PPC-targeted protein but could not rule out a plastid location (albeit very unlikely). We sought other proteins with a similar leader and identified a eukaryotic translation initiation factor 1 encoding a bipartite extension with the same properties. Transit peptide sequences were characterized from all 3 classes of targeted protein by comparing all examples of each class from expressed sequence tag surveys of B. natans and G. stellata. No recognizable difference between plastid- and PPC-targeted proteins was observed, but nucleomorph-encoded transit peptides differ, likely reflecting high AT content of nucleomorph genomes. Taken together, the data suggest that the system that directs proteins to the PPC in chlorarachniophytes uses a bipartite targeting sequence, as does the PPC-targeting system that evolved independently in cryptomonads.

Protein targeting to the malaria parasite plastid

The relict plastid, or apicoplast, of the malaria parasite Plasmodium falciparum is an essential organelle and a promising drug target. Most apicoplast proteins are nuclear encoded and post-translationally targeted into the organelle using a bipartite N-terminal extension, consisting of a typical endomembrane signal peptide and a plant-like transit peptide. Apicoplast protein targeting commences through the parasite's secretory pathway. We review recent experimental evidence suggesting that the apicoplast resides in the mainstream endomembrane system proximal to the Golgi. Further, we explore possible mechanisms for translocation of nuclear-encoded apicoplast proteins across the four bounding membranes. Recent insights into the composition of the transit peptide and how it is cleaved and degraded after use are also examined. Characterization of apicoplast targeting has not only shed light on how this group of parasites mediate intracellular protein trafficking events but also it has helped identify new targets for therapeutics. The distinctive leader sequences of apicoplast proteins make them readily identifiable, allowing assembly of a virtual organelle metabolome from the genome. Such analysis has lead to the identification of several biochemical pathways that are absent from the human host and thus represent novel therapeutic targets for parasitic infection.

Tryptophan biosynthesis in stramenopiles: eukaryotic winners in the diatom complex chloroplast

Tryptophan is an essential amino acid that, in eukaryotes, is synthesized either in the plastids of photoautotrophs or in the cytosol of fungi and oomycetes. Here we present an in silico analysis of the tryptophan biosynthetic pathway in stramenopiles, based on analysis of the genomes of the oomycetes Phytophthora sojae and P. ramorum and the diatoms Thalassiosira pseudonana and Phaeodactylum tricornutum. Although the complete pathway is putatively located in the complex chloroplast of diatoms, only one of the involved enzymes, indole-3-glycerol phosphate synthase (InGPS), displays a possible cyanobacterial origin. On the other hand, in P. tricornutum this gene is fused with the cyanobacteria-derived hypothetical protein COG4398. Anthranilate synthase is also fused in diatoms. This fusion gene is almost certainly of bacterial origin, although the particular source of the gene cannot be resolved. All other diatom enzymes originate from the nucleus of the primary host (red alga) or secondary host (ancestor of chromalveolates). The entire pathway is of eukaryotic origin and cytosolic localization in oomycetes; however, one of the enzymes, anthranilate phosphoribosyl transferase, was likely transferred to the oomycete nucleus from the red algal nucleus during secondary endosymbiosis. This suggests possible retention of the complex plastid in the ancestor of stramenopiles and later loss of this organelle in oomycetes.

Phylogeny and Nucleomorph Karyotype Diversity of Chlorarachniophyte Algae

Chlorarachniophytes are flagellated and/or reticulopod-forming marine algae with chlorophyll a- and b-containing plastids of secondary endosymbiotic origin. They are one of only two algal groups known to possess a "nucleomorph" (i.e. the remnant nucleus of the eukaryotic endosymbiont that donated the plastid). Apart from the recently sequenced nucleomorph genome of Bigelowiella natans, little is known about the size, structure, and composition of chlorarachniophyte nucleomorph genomes. Toward the goal of better understanding nucleomorph genome diversity, as well as establishing a phylogenetic framework with which to interpret variation in chlorarachniophyte morphology, ultrastructure, and life cycle, we are studying a wide range of chlorarachniophyte strains from public culture collections and natural habitats. We have obtained 22 new chlorarachniophyte nuclear and nucleomorph 18S rRNA gene (18S rDNA) sequences and nucleomorph genome size estimates for 14 different strains. Consistent with previous studies, all of the chlorarachniophytes examined appear to possess three nucleomorph chromosomes. However, our results suggest considerable variation in nucleomorph genome size and structure, with individual chromosome sizes ranging from approximately 90 to approximately 210 kbp, and total genome sizes between approximately 330 kbp in Lotharella amoebiformis and approximately 610 kbp in unidentified chlorarachniophyte strain CCMP622. The significance of these phylogenetic and nucleomorph karyotype data is discussed.

Protein targeting into complex diatom plastids: functional characterisation of a specific targeting motif

Plastids of diatoms and related algae evolved by secondary endocytobiosis, the uptake of a eukaryotic alga into a eukaryotic host cell and its subsequent reduction into an organelle. As a result diatom plastids are surrounded by four membranes. Protein targeting of nucleus encoded plastid proteins across these membranes depends on N-terminal bipartite presequences consisting of a signal and a transit peptide-like domain. Diatoms and cryptophytes share a conserved amino acid motif of unknown function at the cleavage site of the signal peptides (ASAFAP), which is particularly important for successful plastid targeting. Screening genomic databases we found that in rare cases the very conserved phenylalanine within the motif may be replaced by tryptophan, tyrosine or leucine. To test such unusual presequences for functionality and to better understand the role of the motif and putative receptor proteins involved in targeting, we constructed presequence:GFP fusion proteins with or without modifications of the "ASAFAP"-motif and expressed them in the diatom Phaeodactylum tricornutum. In this comprehensive mutational analysis we found that only the aromatic amino acids phenylalanine, tryptophan, tyrosine and the bulky amino acid leucine at the +1 position of the predicted signal peptidase cleavage site allow plastid import, as expected from the sequence comparison of native plastid targeting presequences of P. tricornutum and the cryptophyte Guillardia theta. Deletions within the signal peptide domains also impaired plastid import, showing that the presence of F at the N-terminus of the transit peptide together with a cleavable signal peptide is crucial for plastid import.

Nucleomorph genomes: structure, function, origin and evolution

The cryptomonads and chlorarachniophytes are two unicellular algal lineages with complex cellular structures and fascinating evolutionary histories. Both groups acquired their photosynthetic abilities through the assimilation of eukaryotic endosymbionts. As a result, they possess two distinct cytosolic compartments and four genomes--two nuclear genomes, an endosymbiont-derived plastid genome and a mitochondrial genome derived from the host cell. Like mitochondrial and plastid genomes, the genome of the endosymbiont nucleus, or 'nucleomorph', of cryptomonad and chlorarachniophyte cells has been greatly reduced through the combined effects of gene loss and intracellular gene transfer. This article focuses on the structure, function, origin and evolution of cryptomonad and chlorarachniophyte nucleomorph genomes in light of recent comparisons of genome sequence data from both groups. It is now possible to speculate on the reasons that nucleomorphs persist in cryptomonads and chlorarachniophytes but have been lost in all other algae with plastids of secondary endosymbiotic origin.

Algal Genomics: Exploring the Imprint of Endosymbiosis

The nuclear genomes of photosynthetic eukaryotes are littered with genes derived from the cyanobacterial progenitor of modern-day plastids. A genomic analysis of Cyanophora paradoxa - a deeply diverged unicellular alga - suggests that the abundance and functional diversity of nucleus-encoded genes of cyanobacterial origin differs in plants and algae.

New comprehension of the apicoplast of Sarcocystis by transmission electron tomography

BACKGROUND INFORMATION

Apicomplexan parasites (like Plasmodium, Toxoplasma, Eimeria and Sarcocystis) contain a distinctive organelle, the apicoplast, acquired by a secondary endosymbiotic process analogous to chloroplasts and mitochondria. The apicoplast is essential for long-term survival of the parasite. This prokaryotic origin implies that molecular and metabolic processes in the apicoplast differ from those of the eukaryotic host cells and therefore offer options for specific chemotherapeutic treatment. We studied the apicoplast in high-pressure frozen and freeze-substituted cysts of Sarcocystis sp. from roe deer (Capreolus capreolus) to get better insight in apicoplast morphology.

RESULTS AND CONCLUSIONS

We observed that the apicoplast contains four continuous membranes. The two inner membranes have a circular shape with a constant distance from each other and large-sized protein complexes are located between them. The two outer membranes have irregular shapes. The periplastid membrane also contains large-sized protein complexes, while the outer membrane displays protuberances into the parasite cytoplasm. In addition, it is closely associated with the endoplasmic reticulum by 'contact sites'.

Analysis of Euglena gracilis plastid-targeted proteins reveals different classes of transit sequences

The plastid of Euglena gracilis was acquired secondarily through an endosymbiotic event with a eukaryotic green alga, and as a result, it is surrounded by a third membrane. This membrane complexity raises the question of how the plastid proteins are targeted to and imported into the organelle. To further explore plastid protein targeting in Euglena, we screened a total of 9,461 expressed sequence tag (EST) clusters (derived from 19,013 individual ESTs) for full-length proteins that are plastid localized to characterize their targeting sequences and to infer potential modes of translocation. Of the 117 proteins identified as being potentially plastid localized whose N-terminal targeting sequences could be inferred, 83 were unique and could be classified into two major groups. Class I proteins have tripartite targeting sequences, comprising (in order) an N-terminal signal sequence, a plastid transit peptide domain, and a predicted stop-transfer sequence. Within this class of proteins are the lumen-targeted proteins (class IB), which have an additional hydrophobic domain similar to a signal sequence and required for further targeting across the thylakoid membrane. Class II proteins lack the putative stop-transfer sequence and possess only a signal sequence at the N terminus, followed by what, in amino acid composition, resembles a plastid transit peptide. Unexpectedly, a few unrelated plastid-targeted proteins exhibit highly similar transit sequences, implying either a recent swapping of these domains or a conserved function. This work represents the most comprehensive description to date of transit peptides in Euglena and hints at the complex routes of plastid targeting that must exist in this organism.

Endosymbiosis: Double-Take on Plastid Origins

Plastids--the light-harvesting machines of plant and algal cells--evolved from cyanobacteria inside a eukaryotic host more than a billion years ago. New data reveal that a mysterious unicellular alga acquired its photosynthetic apparatus much more recently than other eukaryotes, affording a second look at the primary endosymbiotic origin of plastids.

N-terminal positively charged amino acids, but not their exact position, are important for apicoplast transit peptide fidelity in Toxoplasma gondii

The non-photosynthetic plastid - or apicoplast - of Toxoplasma gondii and other apicomplexan parasites is an essential organelle and promising drug target. Most apicoplast proteins are encoded in the nucleus and targeted into the organelle through the apicoplast's four membranes courtesy of a bipartite N-terminal leader sequence comprising of an endomembrane signal peptide followed by a plastid transit peptide. Apicoplast transit peptides, like plant plastid transit peptides, have no primary consensus, are variable in length and may be distinguishable only by a relative depletion of negative charged residues and consequent enrichment in basic residues. In this study we examine the role of charged residues within an apicoplast transit peptide in T. gondii by point mutagenesis. We demonstrate that positive charged residues, combined with the absence of negatively charged amino acids, are essential for apicoplast transit peptide fidelity, as also observed in P. falciparum. Furthermore, we show that positive charge is more important at the transit peptide's N-terminus than its C-terminus, and that the nature of the positive residue and the exact position of the N-terminal positive charge are not important. These results suggest that a simple, rule-based prediction for T. gondii transit peptides, similar to that successfully implemented for P. falciparum should help to identify apicoplast proteins and facilitate the identification of drug targets in this important human pathogen.

Metabolic maps and functions of the Plasmodium mitochondrion

The mitochondrion of Plasmodium species is a validated drug target. However, very little is known about the functions of this organelle. In this review, we utilize data available from the Plasmodium falciparum genome sequencing project to piece together putative metabolic pathways that occur in the parasite, comparing this with the existing biochemical and cell biological knowledge. The Plasmodium mitochondrion contains both conserved and unusual features, including an active electron transport chain and many of the necessary enzymes for coenzyme Q and iron-sulphur cluster biosynthesis. It also plays an important role in pyrimidine metabolism. The mitochondrion participates in an unusual hybrid haem biosynthesis pathway, with enzymes localizing in both the mitochondrion and plastid organelles. The function of the tricarboxylic acid cycle in the mitochondrion is unclear. We discuss directions for future research into this fascinating, yet enigmatic, organelle.

Comparative rates of evolution in endosymbiotic nuclear genomes

BACKGROUND

The nucleomorphs associated with secondary plastids of cryptomonads and chlorarachniophytes are the sole examples of organelles with eukaryotic nuclear genomes. Although not as widespread as their prokaryotic equivalents in mitochondria and plastids, nucleomorph genomes share similarities in terms of reduction and compaction. They also differ in several aspects, not least in that they encode proteins that target to the plastid, and so function in a different compartment from that in which they are encoded.

RESULTS

Here, we test whether the phylogenetically distinct nucleomorph genomes of the cryptomonad, Guillardia theta, and the chlorarachniophyte, Bigelowiella natans, have experienced similar evolutionary pressures during their transformation to reduced organelles. We compared the evolutionary rates of genes from nuclear, nucleomorph, and plastid genomes, all of which encode proteins that function in the same cellular compartment, the plastid, and are thus subject to similar selection pressures. Furthermore, we investigated the divergence of nucleomorphs within cryptomonads by comparing G. theta and Rhodomonas salina.

CONCLUSION

Chlorarachniophyte nucleomorph genes have accumulated errors at a faster rate than other genomes within the same cell, regardless of the compartment where the gene product functions. In contrast, most nucleomorph genes in cryptomonads have evolved faster than genes in other genomes on average, but genes for plastid-targeted proteins are not overly divergent, and it appears that cryptomonad nucleomorphs are not presently evolving rapidly and have therefore stabilized. Overall, these analyses suggest that the forces at work in the two lineages are different, despite the similarities between the structures of their genomes.