Is Monoglucosyldiacylglycerol a Precursor to Monogalactosyldiacylglycerol in All Cyanobacteria?

High microbiome and metabolome diversification in coexisting sponges with different bio-ecological traits

Marine Porifera host diverse microbial communities, which influence host metabolism and fitness. However, functional relationships between sponge microbiomes and metabolic signatures are poorly understood. We integrate microbiome characterization, metabolomics and microbial predicted functions of four coexisting Mediterranean sponges -Petrosia ficiformis, Chondrosia reniformis, Crambe crambe and Chondrilla nucula. Microscopy observations reveal anatomical differences in microbial densities. Microbiomes exhibit strong species-specific trends. C. crambe shares many rare amplicon sequence variants (ASV) with the surrounding seawater. This suggests important inputs of microbial diversity acquired by selective horizontal acquisition. Phylum Cyanobacteria is mainly represented in C. nucula and C. crambe. According to putative functions, the microbiome of P. ficiformis and C. reniformis are functionally heterotrophic, while C. crambe and C. nucula are autotrophic. The four species display distinct metabolic profiles at single compound level. However, at molecular class level they share a "core metabolome". Concurrently, we find global microbiome-metabolome association when considering all four sponge species. Within each species still, sets of microbe/metabolites are identified driving multi-omics congruence. Our findings suggest that diverse microbial players and metabolic profiles may promote niche diversification, but also, analogous phenotypic patterns of "symbiont evolutionary convergence" in sponge assemblages where holobionts co-exist in the same area.

Evolutionary implications from lipids in membrane bilayers and photosynthetic complexes in cyanobacteria and chloroplasts

In biomembranes, lipids form bilayer structures that serve as the fluid matrix for membrane proteins and other hydrophobic compounds. Additionally, lipid molecules associate with membrane proteins and impact their structures and functions. In both cyanobacteria and the chloroplasts of plants and algae, the lipid bilayer of the thylakoid membrane consists of four distinct glycerolipid classes: monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. These lipids are also integral components of photosynthetic complexes such as photosystem II and photosystem I. The lipid-binding sites within the photosystems, as well as the lipid composition in the thylakoid membrane, are highly conserved between cyanobacteria and photosynthetic eukaryotes, and each lipid class has specific roles in oxygenic photosynthesis. This review aims to shed light on the potential evolutionary implications of lipid utilization in membrane lipid bilayers and photosynthetic complexes in oxygenic photosynthetic organisms.

Acyl plastoquinol is a major cyanobacterial substance that co-migrates with triacylglycerol in thin-layer chromatography

Various studies have suggested the presence of triacylglycerol in cyanobacteria, but no convincing evidence exists. We purified a substance co-migrating with triacylglycerol in thin-layer chromatography and determined its structure using mass spectrometry, gas chromatography, and ¹H and ¹³C NMR. The major components were palmitoyl and stearoyl plastoquinols (acyl plastoquinol). Acyl plastoquinol has never been described before, although acyloxy derivative of plastoquione has been described as plastoquinone B. The level of acyl plastoquinol was 0.4% of the total lipids. We still do not have clear evidence for the presence of triacylglycerol. If present, the maximum triacylglycerol level must be at most 10% of acyl plastoquinol. The Synechocystis Slr2103 protein was suggested to synthesize triacylglycerol, but the product could be acyl plastoquinol. The possible roles of this novel compound in photosynthesis should be a new focus of research.

Origin of cyanobacterial thylakoids via a non-vesicular glycolipid phase transition and their impact on the Great Oxygenation Event

The appearance of oxygenic photosynthesis in cyanobacteria is a major event in evolution. It had an irreversible impact on the Earth, promoting the Great Oxygenation Event (GOE) ~2.4 billion years ago. Ancient cyanobacteria predating the GOE were Gloeobacter-type cells lacking thylakoids, which hosted photosystems in their cytoplasmic membrane. The driver of the GOE was proposed to be the transition from unicellular to filamentous cyanobacteria. However, the appearance of thylakoids expanded the photosynthetic surface to such an extent that it introduced a multiplier effect, which would be more coherent with an impact on the atmosphere. Primitive thylakoids self-organize as concentric parietal uninterrupted multilayers. There is no robust evidence for an origin of thylakoids via a vesicular-based scenario. This review reports studies supporting that hexagonal II-forming glucolipids and galactolipids at the periphery of the cytosolic membrane could be turned, within nanoseconds and without any external source of energy, into membrane multilayers. Comparison of lipid biosynthetic pathways shows that ancient cyanobacteria contained only one anionic lamellar-forming lipid, phosphatidylglycerol. The acquisition of sulfoquinovosyldiacylglycerol biosynthesis correlates with thylakoid emergence, possibly enabling sufficient provision of anionic lipids to trigger a hexagonal II-to-lamellar phase transition. With this non-vesicular lipid-phase transition, a framework is also available to re-examine the role of companion proteins in thylakoid biogenesis.

Lipids in photosynthetic protein complexes in the thylakoid membrane of plants, algae, and cyanobacteria

In the thylakoid membrane of cyanobacteria and chloroplasts, many proteins involved in photosynthesis are associated with or integrated into the fluid bilayer matrix formed by four unique glycerolipid classes, monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidylglycerol. Biochemical and molecular genetic studies have revealed that these glycerolipids play essential roles not only in the formation of thylakoid lipid bilayers but also in the assembly and functions of photosynthetic complexes. Moreover, considerable advances in structural biology have identified a number of lipid molecules within the photosynthetic complexes such as PSI and PSII. These data have provided important insights into the association of lipids with protein subunits in photosynthetic complexes and the distribution of lipids in the thylakoid membrane. Here, we summarize recent high-resolution observations of lipid molecules in the structures of photosynthetic complexes from plants, algae, and cyanobacteria, and evaluate the distribution of lipids among photosynthetic protein complexes and thylakoid lipid bilayers. By integrating the structural information into the findings from biochemical and molecular genetic studies, we highlight the conserved and differentiated roles of lipids in the assembly and functions of photosynthetic complexes among plants, algae, and cyanobacteria.

Are Cyanobacteria an Ancestor of Chloroplasts or Just One of the Gene Donors for Plants and Algae?

Chloroplasts of plants and algae are currently believed to originate from a cyanobacterial endosymbiont, mainly based on the shared proteins involved in the oxygenic photosynthesis and gene expression system. The phylogenetic relationship between the chloroplast and cyanobacterial genomes was important evidence for the notion that chloroplasts originated from cyanobacterial endosymbiosis. However, studies in the post-genomic era revealed that various substances (glycolipids, peptidoglycan, etc.) shared by cyanobacteria and chloroplasts are synthesized by different pathways or phylogenetically unrelated enzymes. Membranes and genomes are essential components of a cell (or an organelle), but the origins of these turned out to be different. Besides, phylogenetic trees of chloroplast-encoded genes suggest an alternative possibility that chloroplast genes could be acquired from at least three different lineages of cyanobacteria. We have to seriously examine that the chloroplast genome might be chimeric due to various independent gene flows from cyanobacteria. Chloroplast formation could be more complex than a single event of cyanobacterial endosymbiosis. I present the “host-directed chloroplast formation” hypothesis, in which the eukaryotic host cell that had acquired glycolipid synthesis genes as an adaptation to phosphate limitation facilitated chloroplast formation by providing glycolipid-based membranes (pre-adaptation). The origins of the membranes and the genome could be different, and the origin of the genome could be complex.

Dynamism of Metabolic Carbon Flow of Starch and Lipids in Chlamydomonas debaryana

Microalgae have the potential to recycle CO2 as starch and triacylglycerol (TAG), which provide alternative source of biofuel and high added-value chemicals. Starch accumulates in the chloroplast, whereas TAG accumulates in the cytoplasmic lipid droplets (LD). Preferential accumulation of starch or TAG may be achieved by switching intracellular metabolic carbon flow, but our knowledge on this control remains limited. Are these two products mutually exclusive? Or, does starch act as a precursor to TAG synthesis, or vice versa? To answer these questions, we analyzed carbon flow in starch and lipids using a stable isotope 13C in Chlamydomonas debaryana NIES-2212, which accumulates, without nutrient limitation, starch in the exponential growth phase and TAG in the stationary phase. Pulse labeling experiments as well as pulse labeling and chase experiments were conducted, and then, gas chromatography-mass spectrometry (GC-MS) analysis was performed on starch-derived glucose and lipid-bound fatty acids. We exploited the previously developed method of isotopomer analysis to estimate the proportion of various pools with different isotopic abundance. Starch turned over rapidly to provide carbon for the synthesis of fatty acids in the exponential phase cells. Most fatty acids showed rapid and slow components of metabolism, whereas oleic acid decayed according to a single exponential curve. Highly labeled population of fatty acids that accumulated during the initial labeling decreased rapidly, and replaced by low abundance population during the chase time, indicating that highly labeled fatty acids were degraded and the resulting carbons were re-used in the re-synthesis with about 9-fold unlabeled, newly fixed carbons. Elongation of C16-C18 acids in vivo was indicated by partially labeled C18 acids. The accumulation of TAG in the stationary growth phase was accounted for by both de novo synthesis and remodeling of membrane lipids. These results suggest that de novo synthesis of starch and TAG was rapid and transient, and also almost independent to each other, but there is a pool of starch quickly turning over for the synthesis of fatty acids. Fatty acids were also subject to re-synthesis. Evidence was also provided for remodeling of lipids, namely, re-use of acyl groups in polar lipids for TAG synthesis.

Isolation and purification of glycoglycerolipids to induce apoptosis in breast cancer cells

Monogalactosyldiacylglycerol (MGDG) is the most abundant type of glycoglycerolipid found in the plant cell membrane and mostly in the chloroplast thylakoid membrane. The amphiphilic nature of MGDG is attractive in pharmaceutical fields for interaction with other biological molecules and hence exerting therapeutic anti-cancer, anti-viral, and anti-inflammatory activities. In this study, we investigated the therapeutic efficacy of cyanobacteria derived MGDG to inhibit breast cancer cell growth. MGDG was extracted from a cyanobacteria Synechocystis sp. PCC 6803 followed by a subsequent fractionation by column chromatographic technique. The purity and molecular structure of MGDG were analyzed by nuclear magnetic resonance (NMR) spectroscopy analysis. The presence of MGDG in the extracted fraction was further confirmed and quantified by high-performance liquid chromatography (HPLC). The anti-proliferation activity of the extracted MGDG molecule was tested against BT-474 and MDA-MB-231 breast cancer cell lines. The in vitro study showed that MGDG extracted from Synechocystis sp. PCC 6803 induced apoptosis in (70 ± 8) % of BT-474 (p < 0.001) and (58 ± 5) % of MDA-MB-231 cells (p < 0.001) using ~ 60 and 200 ng/ml of concentrations, respectively. The half-maximal inhibitory concentration, IC₅₀ of MGDG extracted from Synechocystis sp. PCC 6803 were (27.2 ± 7.6) and (150 ± 70) ng/ml in BT-474 and MDA-MB-231 cell lines, respectively. Quantification of caspase-3/7 activity using flow cytometry showed (3.0 ± 0.4) and (2.1 ± 0.04)-fold (p < 0.001) higher protein expressions in the MGDG treated BT-474 and MDA-MB-231 cells, respectively than untreated controls conferring to the caspase-dependent apoptosis. The MGDG did not show any significant cytotoxic side effects in human dermal fibroblasts cells. A commercially available MGDG control did not induce any apoptotic cell death in cancer cells substantiating the potential of the MGDG extracted from Synechocystis sp. PCC 6803 for the treatment of breast cancer cells through the apoptosis-mediated pathway.

Characterization and Biosynthesis of Lipids in Paulinella micropora MYN1: Evidence for Efficient Integration of Chromatophores into Cellular Lipid Metabolism

The chromatophores found in the cells of photosynthetic Paulinella species, once believed to be endosymbiotic cyanobacteria, are photosynthetic organelles that are distinct from chloroplasts. The chromatophore genome is similar to the genomes of α-cyanobacteria and encodes about 1,000 genes. Therefore, the chromatophore is an intriguing model of organelle formation. In this study, we analyzed the lipids of Paulinella micropora MYN1 to verify that this organism is a composite of cyanobacterial descendants and a heterotrophic protist. We detected glycolipids and phospholipids, as well as a betaine lipid diacylglyceryl-3-O-carboxyhydroxymethylcholine, previously detected in many marine algae. Cholesterol was the only sterol component detected, suggesting that the host cell is similar to animal cells. The glycolipids, presumably present in the chromatophores, contained mainly C16 fatty acids, whereas other classes of lipids, presumably present in the other compartments, were abundant in C20 and C22 polyunsaturated fatty acids. This suggests that chromatophores are metabolically distinct from the rest of the cell. Metabolic studies using isotopically labeled substrates showed that different fatty acids are synthesized in the chromatophore and the cytosol, which is consistent with the presence of both type I and type II fatty acid synthases, supposedly present in the cytosol and the chromatophore, respectively. Nevertheless, rapid labeling of the fatty acids in triacylglycerol and phosphatidylcholine by photosynthetically fixed carbon suggested that the chromatophores efficiently provide metabolites to the host. The metabolic and ultrastructural evidence suggests that chromatophores are tightly integrated into the whole cellular metabolism.

Monoglucosyldiacylglycerol participates in phosphate stress adaptation in Synechococcus sp. PCC 7942

Cyanobacterial monoglucosyldiacylglycerol (MGlcDG) not only serves as a precursor for monogalactosyldiacylglycerol (MGDG) synthesis, but also participates in stress acclimation. Two genes (mgdA and mgdE) related to MGDG synthesis of Synechococcus sp. PCC 7942 were identified. The mgdE-suppressed mutant (AE) accumulated MGlcDG (4.2%) and showed better growth and photosynthetic activities compared with WT and other mutants (mgdA/mgdE-overexpressed and mgdA-suppressed strains), which suggested that MGlcDG was involved in phosphate stress adaptation for Synechococcus sp. PCC 7942. A notable increase in contents of 18:1 fatty acid (FA) of MGDG (127%), DGDG (68%), and SQDG (105%) in AE were found under phosphate starvation. However, the expression of △9 desaturase (desC) was not higher in AE than that in WT during phosphate-starved period. These results suggested that MGlcDG might be involved in the process of FA desaturation, which contributed to membrane fluidity and cell basic metabolism for stress acclimation in cyanobacteria. In complementary experiments of E. coli, although the expression of mgdA and desC in the mgdA and desC coexpressed strain (OEAC) reduced by 22% and 35% compared with that of the strains only overexpressing mgdA (OEA) or desC (OEC), the content of unsaturated FA in OEAC was the highest. This further implied that the accumulation of MGlcDG could prompt FA desaturation in E. coli. Therefore, we propose that an overproduction of MGlcDG is responsible for FA desaturation and participates in phosphate stress adaptation in cyanobacteria.

Complex origins of chloroplast membranes with photosynthetic machineries: multiple transfers of genes from divergent organisms at different times or a single endosymbiotic event?

The paradigm "cyanobacterial origin of chloroplasts" is currently viewed as an established fact. However, we may have to re-consider the origin of chloroplast membranes, because membranes are not replicated by their own. It is the genes for lipid biosynthetic enzymes that are inherited. In the current understandings, these enzymes became encoded by the nuclear genome as a result of endosymbiotic gene transfer from the endosymbiont. However, we previously showed that many enzymes involved in the synthesis of chloroplast peptidoglycan and glycolipids did not originate from cyanobacteria. Here I present results of comprehensive phylogenetic analysis of chloroplast enzymes involved in fatty acid and lipid biosynthesis, as well as additional chloroplast components related to photosynthesis and gene expression. Four types of phylogenetic relationship between chloroplast enzymes (encoded by the chloroplast and nuclear genomes) and cyanobacterial counterparts were found: type 1, chloroplast enzymes diverged from inside of cyanobacterial clade; type 2, chloroplast and cyanobacterial enzymes are sister groups; type 3, chloroplast enzymes originated from homologs of bacteria other than cyanobacteria; type 4, chloroplast enzymes diverged from eukaryotic homologs. Estimation of evolutionary distances suggested that the acquisition times of chloroplast enzymes were diverse, indicating that multiple gene transfers accounted for the chloroplast enzymes analyzed. Based on the results, I try to relax the tight logic of the endosymbiotic origin of chloroplasts involving a single endosymbiotic event by proposing alternative hypotheses. The hypothesis of host-directed chloroplast formation proposes that glycolipid synthesis ability had been acquired by the eukaryotic host before the acquisition of chloroplast ribosomes. Chloroplast membrane system could have been provided by the host, whereas cyanobacteria contributed to the genes for the genetic and photosynthesis systems, at various times, either before or after the formation of chloroplast membranes. The origin(s) of chloroplasts seems to be more complicated than the single event of primary endosymbiosis.

High myristic acid content in the cyanobacterium Cyanothece sp. PCC 8801 results from substrate specificity of lysophosphatidic acid acyltransferase

Analysis of fatty acids from the cyanobacterium Cyanothece sp. PCC 8801 revealed that this species contained high levels of myristic acid (14:0) and linoleic acid in its glycerolipids, with minor contributions from palmitic acid (16:0), stearic acid, and oleic acid. The level of 14:0 relative to total fatty acids reached nearly 50%. This 14:0 fatty acid was esterified primarily to the sn-2 position of the glycerol moiety of glycerolipids. This characteristic is unique because, in most of the cyanobacterial strains, the sn-2 position is esterified exclusively with C16 fatty acids, generally 16:0. Transformation of Synechocystis sp. PCC 6803 with the PCC8801_1274 gene for lysophosphatidic acid acyltransferase (1-acyl-sn-glycerol-3-phosphate acyltransferase) from Cyanothece sp. PCC 8801 increased the level of 14:0 from 2% to 17% in total lipids and the increase in the 14:0 content was observed in all lipid classes. These findings suggest that the high content of 14:0 in Cyanothece sp. PCC 8801 might be a result of the high specificity of this acyltransferase toward the 14:0-acyl-carrier protein.

"Prokaryotic Pathway" Is Not Prokaryotic: Noncyanobacterial Origin of the Chloroplast Lipid Biosynthetic Pathway Revealed by Comprehensive Phylogenomic Analysis

Lipid biosynthesis within the chloroplast, or more generally plastids, was conventionally called “prokaryotic pathway,” which produces glycerolipids bearing C18 acids at the sn-1 position and C16 acids at the sn-2 position, as in cyanobacteria such as Anabaena and Synechocystis. This positional specificity is determined during the synthesis of phosphatidate, which is a precursor to diacylglycerol, the acceptor of galactose for the synthesis of galactolipids. The first acylation at sn-1 is catalyzed by glycerol-3-phosphate acyltransferase (GPAT or GPT), whereas the second acylation at sn-2 is performed by lysophosphatidate acyltransferase (LPAAT, AGPAT, or PlsC). Here we present comprehensive phylogenomic analysis of the origins of various acyltransferases involved in the synthesis of phosphatidate, as well as phosphatidate phosphatases in the chloroplasts. The results showed that the enzymes involved in the two steps of acylation in cyanobacteria and chloroplasts are entirely phylogenetically unrelated despite a previous report stating that the chloroplast LPAAT (ATS2) and cyanobacterial PlsC were sister groups. Phosphatidate phosphatases were separated into eukaryotic and prokaryotic clades, and the chloroplast enzymes were not of cyanobacterial origin, in contrast with another previous report. These results indicate that the lipid biosynthetic pathway in the chloroplasts or plastids did not originate from the cyanobacterial endosymbiont and is not “prokaryotic” in the context of endosymbiotic theory of plastid origin. This is another line of evidence for the discontinuity of plastids and cyanobacteria, which has been suggested in the glycolipid biosynthesis.

Thermoacclimation and genome adaptation of the membrane lipidome in marine Synechococcus

The marine cyanobacteria of the genus Synechococcus are important primary producers, displaying a wide latitudinal distribution that is underpinned by diversification into temperature ecotypes. The physiological basis underlying these ecotypes is poorly known. In many organisms, regulation of membrane fluidity is crucial for acclimating to variations in temperature. Here, we reveal the detailed composition of the membrane lipidome of the model strain Synechococcus sp. WH7803 and its response to temperature variation. Unlike freshwater strains, membranes are almost devoid of C18, mainly containing C14 and C16 chains with no more than two unsaturations. In response to cold, we observed a rarely observed process of acyl chain shortening that likely induces membrane thinning, along with specific desaturation activities. Both of these mechanisms likely regulate membrane fluidity, facilitating the maintenance of efficient photosynthetic activity. A comprehensive examination of 53 Synechococcus genomes revealed clade-specific gene sets regulating membrane lipids. In particular, the genes encoding desaturase enzymes, which is a key to the temperature stress response, appeared to be temperature ecotype-specific, with some of them originating from lateral transfers. Our study suggests that regulation of membrane fluidity has been among the important adaptation processes for the colonization of different thermal niches by marine Synechococcus.

Separation and identification of lipids in the photosynthetic cousins of Apicomplexa Chromera velia and Vitrella brassicaformis

The alveolate algae Chromera velia and Vitrella brassicaformis (chromerids) are the closest known phototrophic relatives to apicomplexan parasites. Apicomplexans are responsible for fatal diseases of humans and animals and severe economic losses. Availability of the genome sequences of chromerids together with easy and rapid culturing of C. velia makes this alga a suitable model for investigating elementary biochemical principals potentially important for the apicomplexan pathogenicity. Such knowledge allows us to better understand processes during the evolutionary transition from a phototrophy to the parasitism in Apicomplexa. We explored lipidomes of both algae using high-performance liquid chromatography with mass spectrometry or gas chromatography with flame ionization detection. A single high-performance liquid chromatography with mass spectrometry analysis in both ionization modes was sufficient for the separation and semi-quantification of lipids in chromerid algae. We detected more than 250 analytes belonging to five structural lipid classes, two lipid classes of precursors and intermediates, and triacylglycerols as storage lipids. Identification of suggested structures was confirmed by high-resolution mass spectrometry with an Orbitrap mass analyzer. An outstandingly high accumulation of storage triacylglycerols was found in both species. All the investigated aspects make C. velia a prospective organism for further applications in biotechnology.

Lipid and carotenoid cooperation-driven adaptation to light and temperature stress in Synechocystis sp. PCC6803

Polyunsaturated lipids are important components of photosynthetic membranes. Xanthophylls are the main photoprotective agents, can assist in protection against light stress, and are crucial in the recovery from photoinhibition. We generated the xanthophyll- and polyunsaturated lipid-deficient ROAD mutant of Synechocystis sp. PCC6803 (Synechocystis) in order to study the little-known cooperative effects of lipids and carotenoids (Cars). Electron microscopic investigations confirmed that in the absence of xanthophylls the S-layer of the cellular envelope is missing. In wild-type (WT) cells, as well as the xanthophyll-less (RO), polyunsaturated lipid-less (AD), and the newly constructed ROAD mutants the lipid and Car compositions were determined by MS and HPLC, respectively. We found that, relative to the WT, the lipid composition of the mutants was remodeled and the Car content changed accordingly. In the mutants the ratio of non-bilayer-forming (NBL) to bilayer-forming (BL) lipids was found considerably lower. Xanthophyll to β-carotene ratio increased in the AD mutant. In vitro and in vivo methods demonstrated that saturated, monounsaturated lipids and xanthophylls may stabilize the trimerization of Photosystem I (PSI). Fluorescence induction and oxygen-evolving activity measurements revealed increased light sensitivity of RO cells compared to those of the WT. ROAD showed a robust increase in light susceptibility and reduced recovery capability, especially at moderate low (ML) and moderate high (MH) temperatures, indicating a cooperative effect of xanthophylls and polyunsaturated lipids. We suggest that both lipid unsaturation and xanthophylls are required for providing the proper structure and functioning of the membrane environment that protects against light and temperature stress.

Isotopic Combinatomer Analysis Provides in Vivo Evidence of the Direct Epimerization of Monoglucosyl Diacylglycerol in Cyanobacteria

Galactolipids constitute the majority of photosynthetic membranes called thylakoid membranes in cyanobacteria and chloroplasts of land plants and algae. The galactolipids, although identical in headgroup structure, are synthesized by significantly different pathways in cyanobacteria and chloroplasts. In the cyanobacterial pathway, monoglucosyl diacylglycerol (GlcDG) is synthesized first and then converted to monogalactosyl diacylglycerol (MGDG). On the basis of circumstantial evidence, the mechanism of conversion was thought to be epimerization at C-4, but no direct evidence has yet been provided, because there is no in vitro enzymatic system of the putative membrane-bound reaction. Labeling studies with ¹⁴C and ¹³C suggested that the labels in the headgroup and the acyl groups were kept at a reasonably constant ratio before and after the conversion. We then provide in vivo evidence of the direct epimerization based on detailed isotopomer analysis of the conversion, named "combinatomer analysis". The different types of molecules formed by the combination of labeled or unlabeled parts (sn-1 acyl, sn-2 acyl, glycerol, and hexose) are called here "combinatomers". Combinatomer analysis of the experiments with pulse labeling with ¹³C and chase in Anabaena sp. PCC 7118 indicated that the composition of combinatomers in the precursor GlcDG was kept unchanged in the product MGDG. Production of combinatomers resulting from exchange of hexose was minimal. This provides solid evidence of the epimerization of the glucose moiety of GlcDG, as well as the direct desaturation of acyl groups at the sn-1 position.

Role of membrane glycerolipids in photosynthesis, thylakoid biogenesis and chloroplast development

The lipid bilayer of the thylakoid membrane in plant chloroplasts and cyanobacterial cells is predominantly composed of four unique lipid classes; monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG). MGDG and DGDG are uncharged galactolipids that constitute the bulk of thylakoid membrane lipids and provide a lipid bilayer matrix for photosynthetic complexes as the main constituents. The glycolipid SQDG and phospholipid PG are anionic lipids with a negative charge on their head groups. SQDG and PG substitute for each other to maintain the amount of total anionic lipids in the thylakoid membrane, with PG having indispensable functions in photosynthesis. In addition to biochemical studies, extensive analyses of mutants deficient in thylakoid lipids have revealed important roles of these lipids in photosynthesis and thylakoid membrane biogenesis. Moreover, recent studies of Arabidopsis thaliana suggest that thylakoid lipid biosynthesis triggers the expression of photosynthesis-associated genes in both the nucleus and plastids and activates the formation of photosynthetic machineries and chloroplast development. Meanwhile, galactolipid biosynthesis is regulated in response to chloroplast functionality and lipid metabolism at transcriptional and post-translational levels. This review summarizes the roles of thylakoid lipids with their biosynthetic pathways in plants and discusses the coordinated regulation of thylakoid lipid biosynthesis with the development of photosynthetic machinery during chloroplast biogenesis.

Diversity in Biosynthetic Pathways of Galactolipids in the Light of Endosymbiotic Origin of Chloroplasts

Cyanobacteria and chloroplasts perform oxygenic photosynthesis, and share a common origin. Galactolipids are present in the photosynthetic membranes of both cyanobacteria and chloroplasts, but the biosynthetic pathways of the galactolipids are significantly different in the two systems. In this minireview, we explain the history of the discovery of the cyanobacterial pathway, and present a probable scenario of the evolution of the two pathways.