Microscope image of Galdieria sulphuraria. In order to allow comparative analyses with other genomes sequenced by the Joint Genome Institute, a copy of this genome is incorporated into the JGI Genome Portal. Please note that this copy of the genome is not maintained by NCBI and is therefore not automatically updated. It is acido-thermophilic and can grow both autotrophically and heterotrophically in the dark.
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Search Menu Abstract Redox modulation is a general mechanism for enzyme regulation, particularly for the post-translational regulation of the Calvin cycle in chloroplasts of green plants.
Although red algae and photosynthetic protists that harbor plastids of red algal origin contribute greatly to global carbon fixation, relatively little is known about post-translational regulation of chloroplast enzymes in this important group of photosynthetic eukaryotes.
To address this question, we used biochemistry, phylogenetics and analysis of recently completed genome sequences. Despite high sequence similarity of G. However, PRK inactivation upon complex formation turned out to be incomplete.
G6PDH was redox modulated, but remained in its tetrameric form; FBPase was poorly redox regulated, despite conservation of the two redox-active cysteines. Photoautotrophic algae from the families of chlorophytes and rhodophytes also use the ferredoxin—thioredoxin system for redox modulation of Calvin cycle enzymes. However, the molecular mechanism and the redox-active structures of the target enzymes are often distinct from those of higher plants Martin et al.
These enzymes and their regulatory properties are rather well understood in higher plants and also in green algae. In our study, we focus on the redox regulation of the chloroplast enzymes phosphoribulokinase PRK, EC 2.
Upon illumination, these complexes are reversibly dissociated and the enzymes are thereby activated Lazaro et al. This protein is present in all photoautotrophic organisms, including cyanobacteria Pohlmeyer et al.
GapB has never been found in a unicellular organism. It appears first in the multicellular charophyte Chlorokybus atmophyticus R. Cerff and J. Petersen, personal communication. This redox regulation involves two of the three cysteines present on an amino acid insert of the enzyme Chiadmi et al. On the other hand, the key enzyme of the oxidative pentose phosphate cycle, G6PDH, proved responsive towards redox changes, in both higher plants and cyanobacteria, although the conserved cysteines are different in these organisms Wenderoth et al.
Still, the enzyme is inactive if export of reducing equivalents from the chloroplast is not required. However, this redox regulation is less strict than in higher plants Ocheretina et al. In contrast, no such enzyme is present in cyanobacteria, which comprise only a single compartment. In this study, we focus on the chloroplast enzymes from the red alga Galdieria sulphuraria Cyanidiaceae. Recent data identified the Cyanidiaceae as a highly conserved sister group of the rhodophytes and place them at the basis of secondary endosymbiosis Yoon et al.
It is therefore interesting to analyze how enzyme regulation in G. Incubation of extracts with GSSG prior to gel filtration led to formation of the high molecular weight complex Fig. Extracts from darkened cells contained only the complexed form of PRK and GAPDH, while untreated extracts, harvested and extracted in dim light, yielded both the aggregated and the dissociated forms of the enzymes Fig.
C The cell extract was prepared in dim light and without pre-incubation. Enzyme activities after gel filtration were determined after incubation of the fractions with DTT. PRK activity was only slightly decreased upon incubation with GSSG, suggesting that the complexed enzyme was almost fully active.
Enzymes in the DTT-treated profile eluted in their active state and already possessed their maximal activities which could not be stimulated further Fig. In the oxidized extract, immunostaining revealed a band at about 15 kDa Fig. Some signal in the first fractions results from highly complexed forms of a few thousand kiloDaltons containing membrane fragments and phycobilins, as could be concluded from the colored fractions.
Detection of CP12 protein was achieved with anti-G. GAPDH activity was determined in the fractions hatched bars. A column without bound CP12 was used as a control black bars. Redox-dependent activation of other chloroplast enzymes from G. Chloroplast FBPase eluted as a tetramer under all conditions, and the oxidized enzyme still exhibited significant activity Fig. Experimental conditions were as described in the legend of Fig. In contrast, G6PDH, the key enzyme of the oxidative pentose phosphate pathway, is clearly subject to redox modulation in G.
The tetrameric enzyme, obtained in its oxidized form when the crude cell extracts was pre-incubated with GSSG prior to gel filtration, was inactivated by incubation with 10 mM DTT Table 1 , Fig. A complete enzyme inactivation, however, could not be achieved due to the presence of a cytosolic isoform.
Table 1 Redox-dependent activities of chloroplast enzymes from Galdieria sulphuraria after gel filtration of crude cell extracts Incubation.
To address this issue, we assembled the mitochondrial and plastid genomes from a polyextremophilic red alga, Galdieria sulphuraria strain W, and performed a comparative genomic analysis with other red algae and more broadly across eukaryotes. The mitogenome is highly reduced in size and genetic content and exhibits the highest guanine—cytosine skew of any known genome and the fastest substitution rate among all red algae. The plastid genome contains a large number of intergenic stem-loop structures but is otherwise rather typical in size, structure, and content in comparison with other red algae. We suggest that these unique genomic modifications result not only from the harsh conditions in which Galdieria lives but also from its unusual capability to grow heterotrophically, endolithically, and in the dark. These conditions place additional mutational pressures on the mitogenome due to the increased reliance on the mitochondrion for energy production, whereas the decreased reliance on photosynthesis and the presence of numerous stem-loop structures may shield the plastome from similar genomic stress. Keywords: Galdieria sulphuraria, red algae, facultative heterotrophy, polyextremophily, GC skew, substitution rate Introduction Red algae Rhodophyta are one of the three ancient lineages of photosynthetic eukaryotes along with green plants and glaucophytes derived from the primary endosymbiosis event that established the plastid Reyes-Prieto et al.