Our paper analyzes full plastid DNA sequence data of 202 wild and cultivated diploid potatoes, Solanum section Petota, to explore its phylogenetic utility compared to prior analyses of the same ...accessions using genome-wide nuclear SNPs, and plastid DNA restriction site data. The present plastid analysis discovered the same major clades as the nuclear data but with some substantial differences in topology within the clades. The considerably larger plastid and nuclear data sets add phylogenetic resolution within the prior plastid DNA restriction site data, highlight plastid/nuclear incongruence that supports hypotheses of hybridization/introgression to help explain the taxonomic difficulty in the section.
Solid semi-crystalline starch and hydrosoluble glycogen define two distinct physical states of the same type of storage polysaccharide. Appearance of semi-crystalline storage polysaccharides appears ...linked to the requirement of unicellular diazotrophic cyanobacteria to fuel nitrogenase and protect it from oxygen through respiration of vast amounts of stored carbon. Starch metabolism itself resulted from the merging of the bacterial and eukaryote pathways of storage polysaccharide metabolism after endosymbiosis of the plastid. This generated the three Archaeplastida lineages: the green algae and land plants (Chloroplastida), the red algae (Rhodophyceae), and the glaucophytes (Glaucophyta). Reconstruction of starch metabolism in the common ancestor of Archaeplastida suggests that polysaccharide synthesis was ancestrally cytosolic. In addition, the synthesis of cytosolic starch from the ADP-glucose exported from the cyanobacterial symbiont possibly defined the original metabolic flux by which the cyanobiont provided photosynthate to its host. Additional evidence supporting this scenario include the monophyletic origin of the major carbon translocators of the inner membrane of eukaryote plastids which are sisters to nucleotide-sugar transporters of the eukaryote endomembrane system. It also includes the extent of enzyme subfunctionalization that came as a consequence of the rewiring of this pathway to the chloroplasts in the green algae. Recent evidence suggests that, at the time of endosymbiosis, obligate intracellular energy parasites related to extant Chlamydia have donated important genes to the ancestral starch metabolism network.
Branching enzyme (BE) catalyzes the formation of α-1,6-glucosidic linkages in amylopectin and glycogen. The reaction products are variable, depending on the organism sources, and the mechanistic ...basis for these different outcomes is unclear. Although most cyanobacteria have only one BE isoform belonging to glycoside hydrolase family 13, Cyanothece sp. ATCC 51142 has three isoforms (BE1, BE2, and BE3) with distinct enzymatic properties, suggesting that investigations of these enzymes might provide unique insights into this system. Here, we report the crystal structure of ligand-free wild-type BE1 (residues 5–759 of 1–773) at 1.85 Å resolution. The enzyme consists of four domains, including domain N, carbohydrate-binding module family 48 (CBM48), domain A containing the catalytic site, and domain C. The central domain A displays a (β/α)8-barrel fold, whereas the other domains adopt β-sandwich folds. Domain N was found in a new location at the back of the protein, forming hydrogen bonds and hydrophobic interactions with CBM48 and domain A. Site-directed mutational analysis identified a mutant (W610N) that bound maltoheptaose with sufficient affinity to enable structure determination at 2.30 Å resolution. In this structure, maltoheptaose was bound in the active site cleft, allowing us to assign subsites −7 to −1. Moreover, seven oligosaccharide-binding sites were identified on the protein surface, and we postulated that two of these in domain A served as the entrance and exit of the donor/acceptor glucan chains, respectively. Based on these structures, we propose a substrate binding model explaining the mechanism of glycosylation/deglycosylation reactions catalyzed by BE.
Under the endosymbiont hypothesis, over a billion years ago a heterotrophic eukaryote entered into a symbiotic relationship with a cyanobacterium (the cyanobiont). This partnership culminated in the ...plastid that has spread to forms as diverse as plants and diatoms. However, why primary plastid acquisition has not been repeated multiple times remains unclear. Here, we report a possible answer to this question by showing that primary plastid endosymbiosis was likely to have been primed by the secretion in the host cytosol of effector proteins from intracellular Chlamydiales pathogens. We provide evidence suggesting that the cyanobiont might have rescued its afflicted host by feeding photosynthetic carbon into a chlamydia-controlled assimilation pathway.
The plastid originated 1.5 billion years ago through a primary endosymbiosis involving a heterotrophic eukaryote and an ancient cyanobacterium. Phylogenetic and biochemical evidence suggests that the ...incipient endosymbiont interacted with an obligate intracellular chlamydial pathogen that housed it in an inclusion. This aspect of the ménage-à-trois hypothesis (MATH) posits that Chlamydiales provided critical novel transporters and enzymes secreted by the pathogens in the host cytosol. This initiated the efflux of photosynthate to both the inclusion lumen and host cytosol. Here we review the experimental evidence supporting the MATH and focus on chlamydial genes that replaced existing cyanobacterial functions. The picture emerging from these studies underlines the importance of chlamydial host-pathogen interactions in the metabolic integration of the primary plastid.
Phylogenetic analysis has detected over 50 lateral gene transfers shared between Chlamydiales and the ancestor of all photosynthetic eukaryotes.
The ‘ménage-à-trois hypothesis’ proposes that Chlamydiales secreted enzymes required for assimilation of photosynthetic carbon in the cytosol of the eukaryote host of plastid endosymbiosis.
Recent evidence suggests that the chlamydial genes involved in photosynthate export from the plastid were donated by conjugation in a common inclusion vacuole.
Glycogen metabolism of extant Chlamydiaceae provides evidence that ancient Chlamydiales were preadapted to generate the carbohydrate flux detailed in the MATH.
Analysis of tryptophan metabolism genes in plants suggests that the tryptophan metabolism operon of Chlamydiales was donated through conjugation to the evolving plastid genome to ensure tryptophan export to the inclusion.
•Starch evolved from glycogen in the host cytosol after plastid endosymbiosis.•A chlamydial debranching enzyme was recruited for crystalline polysaccharide synthesis.•A GWD evolved to enable ...mobilization of amylopectin crystals.•The novel dikinase step built on preexisting glycogen phosphorylation in eukaryotes.•The dikinases coupled starch catabolism to cyanobacterial carbon supply.•Transition from glycogen to starch maximized cytosolic carbon-sink strength.
In this opinion article we propose a scenario detailing how two crucial components have evolved simultaneously to ensure the transition of glycogen to starch in the cytosol of the Archaeplastida last common ancestor: (i) the recruitment of an enzyme from intracellular Chlamydiae pathogens to facilitate crystallization of α-glucan chains; and (ii) the evolution of novel types of polysaccharide (de)phosphorylating enzymes from preexisting glycogen (de)phosphorylation host pathways to allow the turnover of such crystals. We speculate that the transition to starch benefitted Archaeplastida in three ways: more carbon could be packed into osmotically inert material; the host could resume control of carbon assimilation from the chlamydial pathogen that triggered plastid endosymbiosis; and cyanobacterial photosynthate export could be integrated in the emerging Archaeplastida.
Eukaryotes most often synthesize storage polysaccharides in the cytosol or vacuoles in the form of either alpha (glycogen/starch)- or beta-glucosidic (chrysolaminarins and paramylon) linked glucan ...polymers. In both cases, the glucose can be packed either in water-soluble (glycogen and chrysolaminarins) or solid crystalline (starch and paramylon) forms with different impacts, respectively, on the osmotic pressure, the glucose accessibility, and the amounts stored. Glycogen or starch accumulation appears universal in all free-living unikonts (metazoa, fungi, amoebozoa, etc.), as well as Archaeplastida and alveolata, while other lineages offer a more complex picture featuring both alpha- and beta-glucan accumulators. We now infer the distribution of these polymers in stramenopiles through the bioinformatic detection of their suspected metabolic pathways. Detailed phylogenetic analysis of key enzymes of these pathways correlated to the phylogeny of Stramenopila enables us to retrace the evolution of storage polysaccharide metabolism in this diverse group of organisms. The possible ancestral nature of glycogen metabolism in eukaryotes and the underlying source of its replacement by beta-glucans are discussed.
Most rhodophytes synthesize semi-amylopectin as a storage polysaccharide, whereas some species in the most primitive class (Cyanidiophyceae) make glycogen. To know the roles of isoamylases in ...semi-amylopectin synthesis, we investigated the effects of
isoamylase
gene (
CMI294C
and
CMS197C
)-deficiencies on semi-amylopectin molecular structure and starch granule morphology in
Cyanidioschyzon merolae
(Cyanidiophyceae). Semi-amylopectin content in a
CMS197C
-disruption mutant (
ΔCMS197C
) was not significantly different from that in the control strain, while that in a
CMI294C
-disruption mutant (
ΔCMI294C
) was much lower than those in the control strain, suggesting that CMI294C is essential for semi-amylopectin synthesis. Scanning electron microscopy showed that the
ΔCMI294C
strain contained smaller starch granules, while the
ΔCMS197C
strain had normal size, but donut-shaped granules, unlike those of the control strain. Although the chain length distribution of starch from the control strain displayed a semi-amylopectin pattern with a peak around degree of polymerization (DP) 11–13, differences in chain length profiles revealed that the
ΔCMS197C
strain has more short chains (DP of 3 and 4) than the control strain, while the
ΔCMI294C
strain has more long chains (DP ≥12). These findings suggest that CMI294C-type isoamylase, which can debranch a wide range of chains, probably plays an important role in semi-amylopectin synthesis unique in the Rhodophyta.
Haptophytes synthesize unique β-glucans containing more β-1,6-linkages than β-1,3 linkages, as a storage polysaccharide. To understand the mechanism of the synthesis, we investigated the roles of ...Kre6 (yeast 1,6-β-transglycosylase) homologs, PhTGS, in the haptophyte
Pleurochrysis haptonemofera
. RNAi of
PhTGS
repressed β-glucan accumulation and simultaneously induced lipid production, suggesting that PhTGS is involved in β-glucan synthesis and that the knockdown leads to the alteration of the carbon metabolic flow.
PhTGS
was expressed more in light, where β-glucan was actively produced by photosynthesis, than in the dark. The crude extract of
E
.
coli
expressing PhKre6 demonstrated its activity to incorporate
14
C-UDP-glucose into β-glucan of
P. haptonemofera
. These findings suggest that PhTGS functions in storage β-glucan synthesis specifically in light, probably by producing the β-1,6-branch.
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