Primary Endosymbiotic Event Research Articles

How Did Thylakoids Emerge in Cyanobacteria, and How Were the Primary Chloroplast and Chromatophore Acquired?

The emergence of thylakoid membranes in cyanobacteria is a key event in the evolution of all oxygenic photosynthetic cells, from prokaryotes to eukaryotes. Recent analyses show that they could originate from a unique lipid phase transition rather than from a supposed vesicular budding mechanism. Emergence of thylakoids coincided with the great oxygenation event, more than two billion years ago. The acquisition of semi-autonomous organelles, such as the mitochondrion, the chloroplast, and, more recently, the chromatophore, is a critical step in the evolution of eukaryotes. They resulted from primary endosymbiotic events that seem to share general features, i.e., an acquisition of a bacterium/cyanobacteria likely via a phagocytic membrane, a genome reduction coinciding with an escape of genes from the organelle to the nucleus, and, finally, the appearance of an active system translocating nuclear-encoded proteins back to the organelles. An intense mobilization of foreign genes of bacterial origin, via horizontal gene transfers, plays a critical role. Some third partners, like Chlamydia, might have facilitated the transition from cyanobacteria to the early chloroplast. This chapter further details our current understanding of primary endosymbiosis, focusing on primary chloroplasts, thought to have appeared over a billion years ago, and the chromatophore, which appeared around a hundred years ago.

Evolution of light-independent protochlorophyllide oxidoreductase.

The nonhomologous enzymes, the light-independent protochlorophyllide reductase (DPOR) and the light-dependent protochlorophyllide oxidoreductase (LPOR), catalyze the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) in the penultimate step of biosynthesis of chlorophyll (Chl) required for photosynthetic light absorption and energy conversion. The two enzymes differ with respect to the requirement of light for catalysis and oxygen sensitivity. DPOR and LPOR initially evolved in the ancestral prokaryotic genome perhaps at different times. DPOR originated in the anoxygenic environment of the Earth from nitrogenase-like enzyme of methanogenic archaea. Due to the transition from anoxygenic to oxygenic photosynthesis in the prokaryote, the DPOR was mostly inactivated in the daytime by photosynthetic O2 leading to the evolution of oxygen-insensitive LPOR that could function in the light. The primary endosymbiotic event transferred the DPOR and LPOR genes to the eukaryotic phototroph; the DPOR remained in the genome of the ancestor that turned into the plastid, whereas LPOR was transferred to the host nuclear genome. From an evolutionary point of view, several compelling theories that explain the disappearance of DPOR from several species cutting across different phyla are as follows: (i) pressure of the oxygenic environment; (ii) change in the light conditions and temperature; and (iii) lineage-specific gene losses, RNA editing, and nonsynonymous substitution. Certain primary amino acid sequence and the physiochemical properties of the ChlL subunit of DPOR have similarity with that of LPOR suggesting a convergence of these two enzymes in certain evolutionary event. The newly obtained sequence data from different phototrophs will further enhance the width of the phylogenetic information on DPOR.

Primary Endosymbiosis: Emergence of the Primary Chloroplast and the Chromatophore, Two Independent Events.

The emergence of semiautonomous organelles, such as the mitochondrion, the chloroplast, and more recently, the chromatophore, are critical steps in the evolution of eukaryotes. They resulted from primary endosymbiotic events that seem to share general features, i.e., an acquisition of a bacterium/cyanobacteria likely via a phagocytic membrane, a genome reduction coinciding with an escape of genes from the organelle to the nucleus, and finally the appearance of an active system translocating nuclear-encoded proteins back to the organelles. An intense mobilization of foreign genes of bacterial origin, via horizontal gene transfers, plays a critical role. Some third partners, like Chlamydia, might have facilitated the transition from cyanobacteria to the early chloroplast. This chapter describes our current understanding of primary endosymbiosis, with a specific focus on primary chloroplasts considered to have emerged more than one billion years ago, and on the chromatophore, having emerged about one hundred millionyears ago.

Early photosynthetic eukaryotes inhabited low-salinity habitats

The early evolutionary history of the chloroplast lineage remains an open question. It is widely accepted that the endosymbiosis that established the chloroplast lineage in eukaryotes can be traced back to a single event, in which a cyanobacterium was incorporated into a protistan host. It is still unclear, however, which Cyanobacteria are most closely related to the chloroplast, when the plastid lineage first evolved, and in what habitats this endosymbiotic event occurred. We present phylogenomic and molecular clock analyses, including data from cyanobacterial and chloroplast genomes using a Bayesian approach, with the aim of estimating the age for the primary endosymbiotic event, the ages of crown groups for photosynthetic eukaryotes, and the independent incorporation of a cyanobacterial endosymbiont by Paulinella Our analyses include both broad taxon sampling (119 taxa) and 18 fossil calibrations across all Cyanobacteria and photosynthetic eukaryotes. Phylogenomic analyses support the hypothesis that the chloroplast lineage diverged from its closet relative Gloeomargarita, a basal cyanobacterial lineage, ∼2.1 billion y ago (Bya). Our analyses suggest that the Archaeplastida, consisting of glaucophytes, red algae, green algae, and land plants, share a common ancestor that lived ∼1.9 Bya. Whereas crown group Rhodophyta evolved in the Mesoproterozoic Era (1,600-1,000 Mya), crown groups Chlorophyta and Streptophyta began to radiate early in the Neoproterozoic (1,000-542 Mya). Stochastic mapping analyses indicate that the first endosymbiotic event occurred in low-salinity environments. Both red and green algae colonized marine environments early in their histories, with prasinophyte green phytoplankton diversifying 850-650 Mya.

Impact of light intensity and quality on chromatophore and nuclear gene expression in Paulinella chromatophora, an amoeba with nascent photosynthetic organelles.

Plastid evolution has been attributed to a single primary endosymbiotic event that occurred about 1.6 billion years ago (BYA) in which a cyanobacterium was engulfed and retained by a eukaryotic cell, although early steps in plastid integration are poorly understood. The photosynthetic amoeba Paulinella chromatophora represents a unique model for the study of plastid evolution because it contains cyanobacterium-derived photosynthetic organelles termed 'chromatophores' that originated relatively recently (0.09-0.14BYA). The chromatophore genome is about a third the size of the genome of closely related cyanobacteria, but 10-fold larger than most plastid genomes. Several genes have been transferred from the chromatophore genome to the host nuclear genome through endosymbiotic gene transfer (EGT). Some EGT-derived proteins could be imported into chromatophores for function. Two photosynthesis-related genes (psaI and csos4A) are encoded by both the nuclear and chromatophore genomes, suggesting that EGT in Paulinella chromatophora is ongoing. Many EGT-derived genes encode proteins that function in photosynthesis and photoprotection, including an expanded family of high-light-inducible (ncHLI) proteins. Cyanobacterial hli genes are high-light induced and required for cell viability under excess light. We examined the impact of light on Paulinella chromatophora and found that this organism is light sensitive and lacks light-induced transcriptional regulation of chromatophore genes and most EGT-derived nuclear genes. However, several ncHLI genes have reestablished light-dependent regulation, which appears analogous to what is observed in cyanobacteria. We postulate that expansion of the ncHLI gene family and its regulation may reflect the light/oxidative stress experienced by Paulinella chromatophora as a consequence of the as yet incomplete integration of host and chromatophore metabolisms.

Metabolic constraints for a novel symbiosis.

Ancient evolutionary events are difficult to study because their current products are derived forms altered by millions of years of adaptation. The primary endosymbiotic event formed the first photosynthetic eukaryote resulting in both plants and algae, with vast consequences for life on Earth. The evolutionary time that passed since this event means the dominant mechanisms and changes that were required are obscured. Synthetic symbioses such as the novel interaction between Paramecium bursaria and the cyanobacterium Synechocystis PC6803, recently established in the laboratory, permit a unique window on the possible early trajectories of this critical evolutionary event. Here, we apply metabolic modelling, using flux balance analysis (FBA), to predict the metabolic adaptations necessary for this previously free-living symbiont to transition to the endosymbiotic niche. By enforcing reciprocal nutrient trading, we are able to predict the most efficient exchange nutrients for both host and symbiont. During the transition from free-living to obligate symbiosis, it is likely that the trading parameters will change over time, which leads in our model to discontinuous changes in the preferred exchange nutrients. Our results show the applicability of FBA modelling to ancient evolutionary transitions driven by metabolic exchanges, and predict how newly established endosymbioses, governed by conflict, will differ from a well-developed one that has reached a mutual-benefit state.

Phylogenetic Analysis of Nucleus-Encoded Acetyl-CoA Carboxylases Targeted at the Cytosol and Plastid of Algae.

The understanding of algal phylogeny is being impeded by an unknown number of events of horizontal gene transfer (HGT), and primary and secondary/tertiary endosymbiosis. Through these events, previously heterotrophic eukaryotes developed photosynthesis and acquired new biochemical pathways. Acetyl-CoA carboxylase (ACCase) is a key enzyme in the fatty acid synthesis and elongation pathways in algae, where ACCase exists in two locations (cytosol and plastid) and in two forms (homomeric and heteromeric). All algae contain nucleus-encoded homomeric ACCase in the cytosol, independent of the origin of the plastid. Nucleus-encoded homomeric ACCase is also found in plastids of algae that arose from a secondary/tertiary endosymbiotic event. In contrast, plastids of algae that arose from a primary endosymbiotic event contain heteromeric ACCase, which consists of three nucleus-encoded and one plastid-encoded subunits. These properties of ACCase provide the potential to inform on the phylogenetic relationships of hosts and their plastids, allowing different hypothesis of endosymbiotic events to be tested. Alveolata (Dinoflagellata and Apicomplexa) and Chromista (Stramenopiles, Haptophyta and Cryptophyta) have traditionally been grouped together as Chromalveolata, forming the red lineage. However, recent genetic evidence groups the Stramenopiles, Alveolata and green plastid containing Rhizaria as SAR, excluding Haptophyta and Cryptophyta. Sequences coding for plastid and cytosol targeted homomeric ACCases were isolated from Isochrysis aff. galbana (TISO), Chromera velia and Nannochloropsis oculata, representing three taxonomic groups for which sequences were lacking. Phylogenetic analyses show that cytosolic ACCase strongly supports the SAR grouping. Conversely, plastidial ACCase groups the SAR with the Haptophyta, Cryptophyta and Prasinophyceae (Chlorophyta). These two ACCase based, phylogenetic relationships suggest that the plastidial homomeric ACCase was acquired by the Haptophyta, Cryptophyta and SAR, before the photosynthetic Rhizaria acquired their green plastid. Additionally, plastidial ACCase was derived by HGT from an ancestor or relative of the Prasinophyceae and not by duplication of cytosolic ACCase.

Overview of the evolutionary history and the role in citric acid production of alternative oxidase

All organisms are exposed to countless environmental effects, which influence in a disadvantageous way their life processes. They continuously adapt to the changing conditions and respond to the stress impacts by defence mechanisms. Through different signal transduction pathways they are able to increase or decrease the expression of their genes and consequently modify their metabolic processes. My interest focuses on alternative oxidase (AOX) enzyme whose expression is often increased under biotic and abiotic stress. The so far proven and putative functions of the AOX play a role in the ability of organisms to adapt to different conditions, such as heavy metals accumulation, pathogenic infection, oxidative stress and lack of oxygen or nutrients.
 AOX is a member of the di-iron carboxylate protein family. Members of the di-iron carboxylate protein family are present in all kingdoms of life. They are considered to have ancient origin. It is believed that their sulfide-resistant and oxygen-reducing ability played a role in the survival of organisms during the transition between the anaerobic and the aerobic world. It is assumed that the AOX arose in eukaryotes through a primary endosymbiotic event, and this event made possible the development of mitochondria. Afterwords, vertical inheritance, and secondary and tertiary endosimbiotic events led to its spread among eukaryotes. It is assumed that bacteria obtained AOX by horizontal gene transfer from plants.
 AOX-catalyzed alternative respiration plays an important role in the operation of energy-producing and biosynthesizing system of microorganisms. In these cases, the regeneration of reduced cofactors is an essential condition, and therefore may be rate-limiting for biotechnological processes, including the citric acid production.

Special issue on evolution and biogenesis of chloroplasts and mitochondria

Mitochondria and chloroplasts are both of endosymbiotic origin. In an initial step about 1.5–2.0 billion years ago an a-proteobacterium was taken up into an undisclosed host cell and enslaved to become mitochondrion. Later a mitochondria containing cell took up an ancestral photosynthetic cyanobacterium, which became a chloroplast. Both primary endosymbiotic events occurred successfully only once in evolution. During the endosymbiotic process, massive gene transfer to the host nucleus and gene loss occurred. The arising eukaryotic cell took the chance from combining three genomes to establish a division of labour principle in the newly compartmentalised cell. Though mitochondria and chloroplasts have retained a remnant genome and the capacity for transcription and translation, the control lies with the nucleus. Organellar genomes have been retained throughout all eukaryotic life forms with very few exceptions. The reasons for this are not completely understood. Anabolic and catabolic biosynthetic pathways are in many cases distributed between different cellular compartments and endosymbiotic organelles forming an integrated highly sophisticated network. For example, the process of photorespiration involves chloroplasts, mitochondria, peroxisomes and the cytoplasm, thus requiring multiple membrane transporters and metabolic crosstalk to coordinate the process. In October 2012 an international symposium at the University of Munich in Germany entitled ‘‘Endosymbiosis: From Prokaryotes to Eukaryotic Organelles’’ highlighted many of the fundamental aspects surrounding the endosymbiotic process as well as mitochondrial and plastid function. Many colleagues who attended the conference contributed to this special volume in ‘‘Planta’’ on ‘‘Biogenesis and Function of Chloroplasts and Mitochondria’’. It provides a number of fascinating peer-reviewed articles on how these organelles perform today, how their interplay works and what challenges we face in the future. I like to thank everybody involved in making this special volume possible.

An Assessment of Vertical Inheritance versus Endosymbiont Transfer of Nucleus-encoded Genes for Mitochondrial Proteins Following Tertiary Endosymbiosis in Karlodinium micrum

Most photosynthetic dinoflagellates harbour a red alga-derived secondary plastid. In the dinoflagellate Karlodinium micrum, this plastid was replaced by a subsequent endosymbiosis, resulting in a tertiary plastid derived from a haptophyte. Evolution of endosymbionts entails substantial relocation of endosymbiont genes to the host nucleus: a process called endosymbiotic gene transfer (EGT). In K. micrum, numerous plastid genes from the haptophyte nucleus are found in the host nucleus, providing evidence for EGT in this system. In other cases of endosymbiosis, notably ancient primary endosymbiotic events, EGT has been inferred to contribute to remodeling of other cell functions by expression of proteins in compartments other than the endosymbiont from which they derived. K. micrum provides a more recently derived endosymbiotic system to test for evidence of EGT and gain of function in non-plastid compartments. In this study, we test for gain of haptophyte-derived proteins for mitochondrial function in K. micrum. Using molecular phylogenies we have analysed whether nucleus-encoded mitochondrial proteins were inherited by EGT from the haptophyte endosymbiont, or vertically inherited from the dinoflagellate host lineage. From this dataset we found no evidence of haptophyte-derived mitochondrial genes, and the only cases of non-vertical inheritance were genes derived from lateral gene transfer events.

Connecting the Plastid: Transporters of the Plastid Envelope and Their Role in Linking Plastidial with Cytosolic Metabolism

Plastids have a multitude of functions in eukaryotic cells, ranging from photosynthesis to storage, and a role in essential biosynthetic pathways. All plastids are of either primary or higher-order endosymbiotic origin. That is, either a photosynthetic cyanobacterium was integrated into a mitochondriate eukaryotic host cell (primary endosymbiosis) or a plastid-bearing eukaryotic cell merged with another eukaryotic cell (secondary or higher-order endosymbioses), thereby passing on the plastid between various eukaryotic lineages. For all of these endosymbioses to become functional, it was essential to establish metabolic connections between organelle and host cell. Here, we review the present understanding of metabolite exchange between plastids and the surrounding cytosol in the context of the endosymbiotic origin of plastids in various eukaryotic lineages. We show that only a small number of transporters that can be traced down to the primary endosymbiotic event are conserved between plastids of diverse origins.

Evolutionary inheritance of elemental stoichiometry in phytoplankton

The elemental composition of phytoplankton is a fusion of the evolutionary history of the host and plastid, resulting in differences in genetic constraints and selection pressures associated with environmental conditions. The evolutionary inheritance hypothesis predicts similarities in elemental composition within related taxonomic lineages of phytoplankton. To test this hypothesis, we measured the elemental composition (C, N, P, S, K, Mg, Ca, Sr, Fe, Mn, Zn, Cu, Co, Cd and Mo) of 14 phytoplankton species and combined these with published data from 15 more species from both marine and freshwater environments grown under nutrient-replete conditions. The largest differences in the elemental profiles of the species distinguish between the prokaryotic Cyanophyta and primary endosymbiotic events that resulted in the green and red plastid lineages. Smaller differences in trace element stoichiometry within the red and green plastid lineages are consistent with changes in trace elemental stoichiometry owing to the processes associated with secondary endosymbioses and inheritance by descent with modification.

Eukaryotic algae: where lies the diversity of oxygenic photosynthesis

Due to their fast growth, homogeneity as cell populations and easy handling, microalgae attracted plant biologists as laboratory organisms for the study of the metabolism and physiology of photosynthetic cells. This led, for example, to the extensive use of the green alga Chlamydomonas reinhardtii for studying photosynthesis, to such a degree that this alga was nicknamed the green yeast (e.g. Goodenough 1992). Reinforcing the dominant position of Chlamydomonas, the availability of its nuclear genome sequence (Merchant et al. 2007) made also possible the identification of a minimal set of proteins (designated the GreenCut) that were likely involved specifically in chloroplast function within the green lineage. Recent advances in approaching the functions of these proteins are highlighted in this special issue (Grossman et al. 2010). Many pioneering works and more recent findings on the regulation of photosynthetic electron transport arose from studies on eukaryotic microalgae, such as chlororespiration (Bennoun 1982) and cyclic electron flow (Iwai et al. 2010) in Chlamydomonas, or state transitions in the green alga Chlorella pyrenoidosa (Bonaventura and Meyers 1969). Recent developments concerned with state transitions and auxiliary electron transfer pathways are reviewed in this issue (Alric 2010; Lemeille and Rochaix 2010; Peltier et al. 2010). Oxygenic photosynthesis in eukaryotes is not restricted to terrestrial plants and plant-model algal systems (mainly green algae). Indeed photosynthesis in eukaryotic cell was acquired laterally through a primary endosymbiotic event with a cyanobacteria and this gave rise to plants, green algae, red algae and glaucophytes (e.g. Rodriguez-Ezpeleta et al. 2005). As examples, two contributions to this issue highlight the unique architecture of the photosynthetic apparatus in red algae (Neilson and Durnford 2010; Su et al. 2010). Photosynthesis then spread throughout different eukaryotic kingdoms laterally via secondary endosymbiosis, most commonly through the engulfment by a nonphotosynthetic host of a red alga or green alga, giving rise for example to diatoms and euglena, respectively (e.g. Archibald 2009). Among eukaryotic algae, diatoms play a considerable role in the primary productivity of oceans and thus in biogeochemical carbon cycle, comparable to that of cyanobacteria. The acquisition of these so-called secondary plastids also accounts for much of the photosynthetic diversity on the planet, i.e. it was associated with a variety of adaptation strategies involving the photosynthetic process. Some of these peculiarities are dealt with here in reviews on carotenoid biosynthesis in diatoms (Bertrand 2010), light-harvesting processes (Neilson and Durnford 2010), photoprotective mechanisms (Goss and Jakob 2010), and inorganic carbon acquisition (Raven 2010). At a time when human societies are facing major challenges in terms of climate control, renewable energy production, and nutrition of populations across the planet, the understanding of photosynthetic processes and their features in different groups of algae forms a basis for the development of algal biotechnology. The availability of suitable algal strains and the optimization of the mass P. Cardol (&) Laboratory of Genetics of Microorganisms, Department of Life Sciences, Institute of Plant Biology, B22, University of Liege, 27 Boulevard du Rectorat, 4000 Liege, Belgium e-mail: pierre.cardol@ulg.ac.be

Genomic Insights into Marine Microalgae

Marine eukaryotic photosynthesis is dominated by a diverse group of unicellular organisms collectively called microalgae. Microalgae include cells derived from a primary endosymbiotic event (similar to land plants) and cells derived from subsequent secondary and/or tertiary endosymbiotic events. These latter cells are chimeras of several genomes and dominate primary production in the marine environment. Two consequences of multiple endosymbiotic events include complex targeting mechanisms to allow nuclear-encoded proteins to be imported into the plastid and coordination of enzymes, potentially from disparate originator cells, to form complete metabolic pathways. In this review, we discuss the forces that shaped the genomes of marine microalgae and then discuss some of the metabolic consequences of such a complex evolutionary history. We focus our metabolic discussion on carbon, nitrogen, and iron. We then discuss biomineralization and new evidence for programmed cell death in microalgae. We conclude with a short summary on advances in genetic manipulation of microalgae and thoughts on the future directions of marine algal genomics.

Chromatophore Genome Sequence of Paulinella Sheds Light on Acquisition of Photosynthesis by Eukaryotes

It is commonly accepted that a single primary endosymbiosis gave rise to the photosynthetic organelles of plants, the plastids. Recently, we presented evidence that photosynthetic inclusions, termed "chromatophores," present in the filose thecamoeba Paulinella chromatophora originated from an independent, more recent primary endosymbiotic event. To clarify metabolic capabilities of the chromatophore and its state of integration into the host, we present here the complete genome sequence of the chromatophore. Our data reveal a fundamental reduction of the chromatophore genome. The single, circular chromosome of 1.02 Mb encodes 867 protein-coding genes and is, therewith, the smallest cyanobacterial genome reported to date. Compared to Synechococcus WH5701, a free-living relative of the chromatophore, only 26% of the genes were retained. Eleven putative pseudogenes were identified, indicating that reductive genome evolution is ongoing. Although the chromatophore genome contains a complete set of photosynthesis genes, it lacks not only genes thought to be dispensable for an intracellular lifestyle but also genes of essential pathways for amino acid and cofactor synthesis. Our data characterize the chromatophore as a photosynthetic entity that is absolutely dependent on its host for growth and survival. Thus, the chromatophores of P. chromatophora are the only known cyanobacterial descendants besides plastids with a significantly reduced genome that confer photosynthesis to their eukaryotic host. Their comparison with plastids and bacterial endosymbionts of invertebrates sheds light on early steps of the integration of a photosynthetic prokaryote into a eukaryotic cell.

Holy alliances?

Holy alliances?

Homologous protein import machineries in chloroplasts and cyanelles1

The cyanelles of the glaucocystophyte alga Cyanophora paradoxa resemble endosymbiotic cyanobacteria, especially in the presence of a peptidoglycan wall between the inner and outer envelope membranes. However, it is now clear that cyanelles are in fact primitive plastids. Phylogenetic analyses of plastid, nuclear and mitochondrial genes support a single primary endosymbiotic event. In this scenario, cyanelles and all other plastid types are derived from an ancestral photosynthetic organelle combining the high gene content of rhodoplasts and the peptidoglycan wall of cyanelles. This means that the import apparatuses of all primary plastids, i.e. those from glaucocystophytes, red algae, green algae and higher plants, should be homologous. If this is the case, then transit sequences should be similar and heterologous import experiments feasible. Thus far, heterologous in vitro import has been shown in one direction only: precursors from C. paradoxa were imported into isolated pea or spinach chloroplasts. Cyanelle transit sequences differ from chloroplast stroma targeting peptides in containing in their N-terminal domain an invariant phenylalanine residue which is shown here to be crucial for import. In addition, we now demonstrate that heterologous precursors are readily imported into isolated cyanelles, provided that the essential phenylalanine residue is engineered into the N-terminal part of chloroplast transit peptides. The cyanelle and likely also the rhodoplast import apparatus can be envisaged as prototypes with a single receptor/channel showing this requirement for N-terminal phenylalanine. In chloroplasts, multiple receptors with overlapping and less stringent specificities have evolved, explaining the efficient heterologous import of native precursors from C. paradoxa.

Mosaic Origin of the Heme Biosynthesis Pathway in Photosynthetic Eukaryotes

Heme biosynthesis represents one of the most essential metabolic pathways in living organisms, providing the precursors for cytochrome prosthetic groups, photosynthetic pigments, and vitamin B(12). Using genomic data, we have compared the heme pathway in the diatom Thalassiosira pseudonana and the red alga Cyanidioschyzon merolae to those of green algae and higher plants, as well as to those of heterotrophic eukaryotes (fungi, apicomplexans, and animals). Phylogenetic analyses showed the mosaic character of this pathway in photosynthetic eukaryotes. Although most of the algal and plant enzymes showed the expected plastid (cyanobacterial) origin, at least one of them (porphobilinogen deaminase) appears to have a mitochondrial (alpha-proteobacterial) origin. Another enzyme, glutamyl-tRNA synthase, obviously originated in the eukaryotic nucleus. Because all the plastid-targeted sequences consistently form a well-supported cluster, this suggests that genes were either transferred from the primary endosymbiont (cyanobacteria) to the primary host nucleus shortly after the primary endosymbiotic event or replaced with genes from other sources at an equally early time, i.e., before the formation of three primary plastid lineages. The one striking exception to this pattern is ferrochelatase, the enzyme catalyzing the first committed step to heme and bilin pigments. In this case, two red algal sequences do not cluster either with the other plastid sequences or with cyanobacterial sequences and appear to have a proteobacterial origin like that of the apicomplexan parasites Plasmodium and Toxoplasma. Although the heterokonts also acquired their plastid via secondary endosymbiosis from a red alga, the diatom has a typical plastid-cyanobacterial ferrochelatase. We have not found any remnants of the plastidlike heme pathway in the nonphotosynthetic heterokonts Phytophthora ramorum and Phytophthora sojae.

Protein translocation into and within cyanelles (Review)

The cyanelles of the glaucocystophyte alga Cyanophora paradoxa resemble endosymbiotic cyanobacteria in morphology, pigmentation and, especially, in the presence of a peptidoglycan wall situated between the inner and outer envelope membranes. However, it is now clear that cyanelles in fact are primitive plastids. Phylogenetic analyses of plastid, nuclear and mitochondrial genes support a single primary endosymbiotic event. In this scenario cyanelles and all other plastid types are derived from an ancestral photosynthetic organelle combining the high plastid gene content of the Porphyra purpurea rhodoplast and the peptidoglycan wall of glaucocystophyte cyanelles. This means that the import apparatus of all primary plastids should be homologous. Indeed, heterologous in vitro import can now be shown in both directions, provided a phenylalanine residue essential for cyanelle import is engineered into the N-terminal part of chloroplast transit peptides. The cyanelle and likely also the rhodoplast import apparatus can be envisaged as prototypes with a single receptor showing this requirement for N-terminal phenylalanine. In chloroplasts, multiple receptors with overlapping and less stringent specificities have evolved explaining the efficient heterologous import of native precursors from C. paradoxa. With respect to conservative sorting in cyanelles, both the Sec and Tat pathways could be demonstrated. Another cyanobacterial feature, the dual location of the Sec translocase in thylakoid and inner envelope membranes, is also unique to cyanelles. For the first time, protease protection of internalized lumenal proteins could be shown for cyanobacteria-like, phycobilisome-bearing thylakoid membranes after import into isolated cyanelles.

The Cell Walls that Bind the Tree of Life

Abstract Cell walls have evolved independently in many phyletically diverse clades, including the Eubacteria and Archaebacteria. However, a review of the available genetic and biochemical evidence indicates that the machinery responsible for synthesizing the cell walls of land plants, of their nearest algal relatives (the charophytes), and of some very ancient algal lineages (chlorophytes, rhodophytes, and phaeophytes) can be traced back to ancient primary endosymbiotic events involving Eubacteria (specifically cyanobacteria and proteobacteria). Lateral gene transfers attending secondary endosymbiotic events appear to be responsible for manufacturing the cell walls of more recently evolved photoautotrophic and heterotrophic lineages (e.g., euglenoids and tunicates). Recent research into the genetic basis of cell wall synthesis and chemical composition in bacteria, algae, and land plants continues to shed light on the phylogenetic relationships among a broad spectrum of evolutionarily and ecologically dive...