Carbon dioxide assimilation in oxygenic and anoxygenic photosynthesis

This article represents a summary of our contemporary understanding of carbon dioxide assimilation in photosynthesis, including both the oxygen-evolving (oxygenic) type characteristic of cyanobacteria, algae and higher plants, and the non-oxygen-evolving (anoxygenic) type characteristic of other bacteria. Mechanisms functional in the regulation of the reductive pentose phosphate cycle of oxygenic photosynthesis are emphasized, as is the reductive carboxylic acid cycle-the photosynthetic carbon pathway functional in anoxygenic green sulfur bacteria. Thioredoxins, an ubiquitous group of low molecular weight proteins with catalytically active thiols, are also described in some detail, notably their role in regulating the reductive pentose phosphate cycle of oxygenic photosynthesis and their potential use as markers to trace the evolutionary development of photosynthesis.Abbreviations NADP-GAPDH-NADP glyceraldehyde 3-phosphate dehydrogenase - FBPase fructose 1,6-bisphosphatase - FTR ferredoxin-thioredoxin reductase - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - SBPase sedoheptulos 1,7-bisphosphatase - PRK phosphoribulokinase - NADP-MDH-NADP malate dehydrogenase - CF1-ATPase chloroplast coupling factor - G6PDH glucose 6-phosphate dehydrogenase Most of the references cited in this article are reviews. For references to specific material, readers should consult the appropriate review.     

The ferredoxin-thioredoxin system of a green alga,Chlamydomonas reinhardtii

The components of the ferredoxin-thioredoxin (FT) system of Chlamydomonas reinhardtii have been purified and characterized. The system resembled that of higher plants in consisting of a ferredoxin-thioredoxin reductase (FTR) and two types of thioredoxin, a single f and two m species, m1 and m2. The Chlamydomonas m and f thioredoxins were antigenically similar to their higher-plant counterparts, but not to one another. The m thioredoxins were recognized by antibodies to both higher-plant m and bacterial thioredoxins, whereas the thioredoxin f was not. Chlamydomonas thioredoxin f reacted, although weakly, with the antibody to spinach thioredoxin f. The algal thioredoxin f differed from thioredoxins studied previously in behaving as a basic protein on ion-exchange columns. Purification revealed that the algal thioredoxins had molecular masses (M_rs) typical of thioredoxins from other sources, m1 and m2 being 10700 and f 11 500. Chlamydomonas FTR had two dissimilar subunits, a feature common to all FTRs studied thus far. One, the 13-kDa (similar) subunit, resembled its counterpart from other sources in both size and antigenicity. The other, 10-kDa (variable) sub-unit was not recognized by antibodies to any FTR tested. When combined with spinach, (Spinacia oleracea L.) thylakoid membranes, the components of the FT system functioned in the light activation of the standard target enzymes from chloroplasts, corn (Zea mays L.) NADP-malate dehydrogenase (EC 1.1.1.82) and spinach fructose 1,6-bisphosphatase (EC 3.1.3.11) as well as the chloroplast-type fructose 1,6-bisphosphatase from Chlamydomonas. Activity was greatest if ferredoxin and other components of the FT system were from Chlamydomonas. The capacity of the Chlamydomonas FT system to activate autologous FBPase indicates that light regulates the photosynthetic carbon metabolism of green algae as in other oxygenic photosynthetic organisms.Abbreviations DEAE diethylaminoethyl - ELISA enzyme-linked immunosorption assay - FBPase fructose 1,6-bisphosphatase - Fd ferredoxin - FPLC fast protein liquid chromatography - FTR ferredoxin-thioredoxin reductase - FT system ferredoxin-thioredoxin system - kDa kilodaltons - M_r relative molecular mass - NADP-MDH NADP-malate dehydrogenase - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis This work was supported in part by a grant from the National Aeronautics and Space Administration. We would like to thank Don Carlson and Jacqueline Girard for their assistance with cell cultures.     

Increased activity of the tandem fructose-1,6-bisphosphate aldolase,triosephosphate isomerase and fructose-1,6-bisphosphatase enzymes in <Emphasis Type="Italic">Anabaena</Emphasis> sp. strain PCC 7120 stimulates photosynthetic yield

The regulation of photosynthetic yield at the genetic level has largely focused on manipulation of the catalytic enzymes in the Calvin cycle by genetic engineering. In order to investigate the contribution of increased enzymatic activity in the Calvin cycle on photosynthetic yield, the rice fructose-1,6-bisphosphate aldolase (FBA), spinach triosephosphate isomerase (TPI) and wheat fructose-1,6-bisphosphatase (FBPase) genes were cloned in tandem and co-overexpressed in cyanobacterium Anabaena sp. strain PCC 7120 cells. The enzymatic activities of FBA, TPI and FBPase, as well as sedoheptulose-1,7-bisphosphatase (SBPase), were remarkably increased in transgenic cells relative to the wild-type. The photosynthetic yield, as reflected by photosynthetic O₂ evolution and dry cellular weight, was also markedly increased in transgenic cells versus wide-type cells. The activity of SBPase is considered the most important factor for ribulose-1,5-bisphosphate (RuBP) regeneration in the Calvin cycle, and increased activity of TPI alone in transgenic cells does not stimulate photosynthetic yield. Thus, the increased activity of FBA and FBPase, but not TPI, significantly improved photosynthetic yield in transgenic cells by stimulating SBPase activity and consequently accelerating the RuBP regeneration rate.     

The diversity and complexity of the cyanobacterial thioredoxin systems

Cyanobacteria perform oxygenic photosynthesis, which makes them unique among the prokaryotes, and this feature together with their abundance and worldwide distribution renders them a central ecological role. Cyanobacteria and chloroplasts of plants and algae are believed to share a common ancestor and the modern chloroplast would thus be the remnant of an endosymbiosis between a eukaryotic cell and an ancestral oxygenic photosynthetic prokaryote. Chloroplast metabolic processes are coordinated with those of the other cellular compartments and are strictly controlled by means of regulatory systems that commonly involve redox reactions. Disulphide/dithiol exchange catalysed by thioredoxin is a fundamental example of such regulation and represents the molecular mechanism for light-dependent redox control of an ever-increasing number of chloroplast enzymatic activities. In contrast to chloroplast thioredoxins, the functions of the cyanobacterial thioredoxins have long remained elusive, despite their common origin. The sequenced genomes of several cyanobacterial species together with novel experimental approaches involving proteomics have provided new tools for re-examining the roles of the thioredoxin systems in these organisms. Thus, each cyanobacterial genome encodes between one and eight thioredoxins and all components necessary for the reduction of thioredoxins. Screening for thioredoxin target proteins in cyanobacteria indicates that assimilation and storage of nutrients, as well as some central metabolic pathways, are regulated by mechanisms involving disulphide/dithiol exchange, which could be catalysed by thioredoxins or related thiol-containing proteins.     

Enzyme Regulation in Crassulacean Acid Metabolism Photosynthesis : Studies on Thioredoxin-Linked Enzymes of KalanchoE daigremontiana

Fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase) were identified and purified from the Crassulacean acid metabolism (CAM) plant, Kalanchoë daigremontiana. FBPase and SBPase showed respective molecular weights of 180,000 and 76,000, and exhibited immunological cross-reactivity with their counterparts from chloroplasts of C₃ (spinach) and C₄ (corn) plants. Based on Western blot analysis, FBPase was composed of four identical 45,000-dalton subunits and SBPase of two identical 38,000-dalton subunits. Immunological evidence, together with physical properties, indicated that both enzymes were of chloroplast origin.

Kalanchoë FBPase and SBPase could be activated by thioredoxin f reduced chemically by dithiothreitol or photochemically by a reconstituted Kalanchoë ferredoxin/thioredoxin system. Both enzymes were activated synergistically by reduced thioredoxin f and thier respective substrates.

Kalanchoë FBPase could be partially activated by Mg²⁺ at concentrations greater than 10 millimolar; however, such activation was considerably less than that observed in the presence of reduced thioredoxin and Ca²⁺, especially in the pH range between 7.8 and 8.3. In contrast to FBPase, Kalanchoë SBPase exhibited an absolute requirement for a dithiol such as reduced thioredoxin irrespective of Mg²⁺ concentration. However, like FBPase, increased Mg²⁺ concentrations enhanced the thioredoxin-linked activation of this enzyme.

In conjunction with these studies, an NADP-linked malate dehydrogenase (NADP-MDH) was identified in cell-free preparations of Kalanchoë leaves which required reduced thioredoxin m for activity.

These results indicate that Kalanchoë FBPase, SBPase, and NADP-MDH share physical and regulatory properties with their equivalents in C₃ and C₄ plants. In contrast to previous evidence, all three enzymes appear to have the capacity to be photoregulated in chloroplasts of CAM plants, thereby providing a means for the functional segregation of glucan synthesis and degradation.

     

Contrasting evolutionary histories of chloroplast thioredoxins f and m

Fourteen thioredoxin sequences were used to construct a minimal phylogenetic tree by using parsimony. The bacterial thioredoxins clustered into three groups: one containing the photosynthetic purple bacteria, Escherichia and Corynebacterium; a second containing the photosynthetic green bacterium, Chlorobium; and a third containing cyanobacteria. These groupings are similar to those generated from earlier 16s RNA analyses. Animal thioredoxins formed a fourth group. The two thioredoxins of chloroplasts (f and m) showed contrasting phylogenetic patterns. As predicted from prior studies, spinach chloroplast thioredoxin m grouped with its counterparts from cyanobacteria and eukaryotic algae, but, unexpectedly, thioredoxin f grouped with the animal thioredoxins. The results indicate that, during evolution, thioredoxin m of contemporary photosynthetic eukaryotic cells was derived from a prokaryotic symbiont, whereas thioredoxin f descended from an ancestral eukaryote common to plants and animals. The findings illustrate the potential of thioredoxin as a phylogenetic marker and suggest a relationship between the animal and f-type thioredoxins.      

Partial sequence of ribulose-1,5-bisphosphate carboxylase/oxygenase and the phylogeny of Prochloron and Prochlorococcus (Prochlorales)

The prochlorophytes, oxygenic photosynthetic prokaryotes having no phycobiliprotein but possessing chlorophylls a and b, have been proposed to have a common ancestry with green chloroplasts, yet this is still controversal. We report here that partial sequence comparisons of the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, including sequence data from two prochlorophytes, Prochlorococcus and Prochloron, indicate that Prochlorococcus is more closely related to a photosynthetic bacterium, Chromatium vinosum (-purple bacteria), than to cyanobacteria, while Prochloron is closely related to the prochlorophyte Prochlorothrix and to cyanobacteria. The molecular phylogenetic tree indicates that a common ancestor of Prochlorococcus and -purple bacteria branched off from the land plant lineage earlier than Prochloron, Prochlorothrix, and cyanobacteria.Correspondence to: A. Shimada     

The form II fructose 1,6-bisphosphatase and phosphoribulokinase genes form part of a large operon in Rhodobacter sphaeroides: primary structure and insertional mutagenesis analysis

Fructose 1,6-bisphosphatase (FBPase) and phosphoribulokinase (PRK) are two key enzymes of the reductive pentose phosphate pathway or Calvin cycle of photosynthetic carbon dioxide assimilation. Early studies had indicated that the properties of enzymes isolated from photosynthetic bacteria were clearly distinct from those of enzymes obtained from the chloroplasts of higher plants [for a review, see Tabita (1988)]. The eucaryotic enzymes, which are light activated by the thioredoxin/ferredoxin system (Buchanan, 1980), were each shown to contain a putative regulatory amino acid sequence (Marcus et al., 1988; Porter et al., 1988). The enzymes from photosynthetic bacteria are not controlled by the thioredoxin/ferredoxin system but exhibit complex kinetic properties and, in the case of PRK, there is an absolute requirement of NADH for activity. In the photosynthetic bacterium Rhodobacter sphaeroides, the structural genes of the Calvin cycle, including the genes that encode FBPase (fbp) and PRK (prk), are found in two distinct clusters, and the fbp and prk genes are closely associated in each cluster. In the present investigation, we have determined the nucleotide sequence of the fbpB and prkB genes of the form II cluster and have compared the deduced amino acid sequences to previously determined sequences of light-activated enzymes from higher plants and from other eucaryotic and procaryotic sources. In the case of FBPase, there are several regions that are conserved in the R. sphaeroides enzymes, including a protease-sensitive area located in a region equivalent to residues 51-71 of mammalian FBPase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Discovery of the first light‐dependent protochlorophyllide oxidoreductase in anoxygenic phototrophic bacteria

In all photosynthetic organisms, chlorophylls function as light‐absorbing photopigments allowing the efficient harvesting of light energy. Chlorophyll biosynthesis recurs in similar ways in anoxygenic phototrophic proteobacteria as well as oxygenic phototrophic cyanobacteria and plants. Here, the biocatalytic conversion of protochlorophyllide to chlorophyllide is catalysed by evolutionary and structurally distinct protochlorophyllide reductases (PORs) in anoxygenic and oxygenic phototrophs. It is commonly assumed that anoxygenic phototrophs only contain oxygen‐sensitive dark‐operative PORs (DPORs), which catalyse protochlorophyllide reduction independent of the presence of light. In contrast, oxygenic phototrophs additionally (or exclusively) possess oxygen‐insensitive but light‐dependent PORs (LPORs). Based on this observation it was suggested that light‐dependent protochlorophyllide reduction first emerged as a consequence of increased atmospheric oxygen levels caused by oxygenic photosynthesis in cyanobacteria. Here, we provide experimental evidence for the presence of an LPOR in the anoxygenic phototrophic α‐proteobacterium Dinoroseobacter shibae DFL12^T. In vitro and in vivo functional assays unequivocally prove light‐dependent protochlorophyllide reduction by this enzyme and reveal that LPORs are not restricted to cyanobacteria and plants. Sequence‐based phylogenetic analyses reconcile our findings with current hypotheses about the evolution of LPORs by suggesting that the light‐dependent enzyme of D. shibae DFL12^T might have been obtained from cyanobacteria by horizontal gene transfer.     

Unexpected diversity of ferredoxin-dependent thioredoxin reductases in cyanobacteria

Thioredoxin reductases control the redox state of thioredoxins (Trxs)—ubiquitous proteins that regulate a spectrum of enzymes by dithiol–disulfide exchange reactions. In most organisms, Trx is reduced by NADPH via a thioredoxin reductase flavoenzyme (NTR), but in oxygenic photosynthetic organisms, this function can also be performed by an iron-sulfur ferredoxin (Fdx)-dependent thioredoxin reductase (FTR) that links light to metabolic regulation. We have recently found that some cyanobacteria, such as the thylakoid-less Gloeobacter and the ocean-dwelling green oxyphotobacterium Prochlorococcus, lack NTR and FTR but contain a thioredoxin reductase flavoenzyme (formerly tentatively called deeply-rooted thioredoxin reductase or DTR), whose electron donor remained undefined. Here, we demonstrate that Fdx functions in this capacity and report the crystallographic structure of the transient complex between the plant-type Fdx1 and the thioredoxin reductase flavoenzyme from Gloeobacter violaceus. Thereby, our data demonstrate that this cyanobacterial enzyme belongs to the Fdx flavin-thioredoxin reductase (FFTR) family, originally described in the anaerobic bacterium Clostridium pasteurianum. Accordingly, the enzyme hitherto termed DTR is renamed FFTR. Our experiments further show that the redox-sensitive peptide CP12 is modulated in vitro by the FFTR/Trx system, demonstrating that FFTR functionally substitutes for FTR in light-linked enzyme regulation in Gloeobacter. Altogether, we demonstrate the FFTR is spread within the cyanobacteria phylum and propose that, by substituting for FTR, it connects the reduction of target proteins to photosynthesis. Besides, the results indicate that FFTR acquisition constitutes a mechanism of evolutionary adaptation in marine phytoplankton such as Prochlorococcus that live in low-iron environments.     

Fellow travellers: a concordance of colonization patterns between mice and men in the North Atlantic region

Background

In the Calvin cycle of eubacteria, the dephosphorylations of both fructose-1, 6-bisphosphate (FBP) and sedoheptulose-1, 7-bisphosphate (SBP) are catalyzed by the same bifunctional enzyme: fructose-1, 6-bisphosphatase/sedoheptulose-1, 7-bisphosphatase (F/SBPase), while in that of eukaryotic chloroplasts by two distinct enzymes: chloroplastic fructose-1, 6-bisphosphatase (FBPase) and sedoheptulose-1, 7-bisphosphatase (SBPase), respectively. It was proposed that these two eukaryotic enzymes arose from the divergence of a common ancestral eubacterial bifunctional F/SBPase of mitochondrial origin. However, no specific affinity between SBPase and eubacterial FBPase or F/SBPase can be observed in the previous phylogenetic analyses, and it is hard to explain why SBPase and/or F/SBPase are/is absent from most extant nonphotosynthetic eukaryotes according to this scenario.

Results

Domain analysis indicated that eubacterial F/SBPase of two different resources contain distinct domains: proteobacterial F/SBPases contain typical FBPase domain, while cyanobacterial F/SBPases possess FBPase_glpX domain. Therefore, like prokaryotic FBPase, eubacterial F/SBPase can also be divided into two evolutionarily distant classes (Class I and II). Phylogenetic analysis based on a much larger taxonomic sampling than previous work revealed that all eukaryotic SBPase cluster together and form a close sister group to the clade of epsilon-proteobacterial Class I FBPase which are gluconeogenesis-specific enzymes, while all eukaryotic chloroplast FBPase group together with eukaryotic cytosolic FBPase and form another distinct clade which then groups with the Class I FBPase of diverse eubacteria. Motif analysis of these enzymes also supports these phylogenetic correlations.

Conclusions

There are two evolutionarily distant classes of eubacterial bifunctional F/SBPase. Eukaryotic FBPase and SBPase do not diverge from either of them but have two independent origins: SBPase share a common ancestor with the gluconeogenesis-specific Class I FBPase of epsilon-proteobacteria (or probably originated from that of the ancestor of epsilon-proteobacteria), while FBPase arise from Class I FBPase of an unknown kind of eubacteria. During the evolution of SBPase from eubacterial Class I FBPase, the SBP-dephosphorylation activity was acquired through the transition ??from specialist to generalist??. The evolutionary substitution of the endosymbiotic-origin cyanobacterial bifunctional F/SBPase by the two light-regulated substrate-specific enzymes made the regulation of the Calvin cycle more delicate, which contributed to the evolution of eukaryotic photosynthesis and even the entire photosynthetic eukaryotes.     

Electron transport in green photosynthetic bacteria

Green bacteria make up two of the four families of anoxygenic photosynthetic prokaryotes. The two families have similar pigment compositions and membrane fine structure, and both contain a specialized antenna structure known as a chlorosome. The primary photochemistry and electron transport pathways of the two groups are, however, quite distinct. The anaerobic green bacteria (Chlorobiaceae) contain low-potential iron-sulfur proteins as early electron acceptors and can directly reduce NAD⁺ in a manner reminiscent of Photosystem I of oxygenic organisms. The facultatively aerobic green bacteria (Chloroflexaceae) contain quinone-type acceptors and have an overall pattern of electron transport very similar to that found in purple bacteria. Many aspects of energy storage in green bacteria, especially photophosphorylation and the role of cytochrome b/c complexes in electron transport, remain poorly understood.     

Chlamydomonas reinhardtii thioredoxins: structure of the genes coding for the chloroplastic m and cytosolic h isoforms; expression in Escherichia coli of the recombinant proteins,purification and biochemical properties

Based on known amino acid sequences, probes have been generated by PCR and used for the subsequent isolation of cDNAs and genes coding for two thioredoxins (m and h) of Chlamydomonas reinhardtii. Thioredoxin m, a chloroplastic protein, is encoded as a preprotein of 140 amino acids (15 101 Da) containing a transit peptide of 34 amino acids with a very high content of Ala and Arg residues. The sequence for thioredoxin h codes for a 113 amino acid protein with a molecular mass of 11817 Da and no signal sequence. The thioredoxin m gene contains a single intron and seems to be more archaic in structure than the thioredoxin h gene, which is split into 4 exons. The cDNA sequences encoding C. reinhardtii thioredoxins m and h have been integrated into the pET-3d expression vector, which permits efficient production of proteins in Escherichia coli cells. A high expression level of recombinant thioredoxins was obtained (up to 50 mg/l culture). This has allowed us to study the biochemical/biophysical properties of the two recombinant proteins. Interestingly, while the m-type thioredoxin was found to have characteristics very close to the ones of prokaryotic thioredoxins, the h-type thioredoxin was quite different with respect to its kinetic behaviour and, most strikingly, its heat denaturation properties.Abbreviations DTT dithiothreitol - FBPase Fructose 1,6-biphosphate phosphatase - FTR ferredoxin-thioredoxin reductase - IPTG isopropyl thiogalactoside - NADP-MDH NADPH-dependent malate dehydrogenase - NMR nuclear magnetic resonance - NTR NADPH-dependent thioredoxin reductase Dedicated to the memory of Claude Crétin     

Oxidation-reduction properties of chloroplast thioredoxins, ferredoxin:thioredoxin reductase, and thioredoxin f-regulated enzymes

Oxidation-reduction midpoint potentials were determined, as a function of pH, for the disulfide/dithiol couples of spinach and pea thioredoxins f, for spinach and Chlamydomonas reinhardtii thioredoxins m, for spinach ferredoxin:thioredoxin reductase (FTR), and for two enzymes regulated by thioredoxin f, spinach phosphoribulokinase (PRK) and the fructose-1,6-bisphosphatases (FBPase) from pea and spinach. Midpoint oxidation-reduction potential (Em) values at pH 7.0 of -290 mV for both spinach and pea thioredoxin f, -300 mV for both C. reinhardtii and spinach thioredoxin m, -320 mV for spinach FTR, -290 mV for spinach PRK, -315 mV for pea FBPase, and -330 mV for spinach FBPase were obtained. With the exception of spinach FBPase, titrations showed a single two-electron component at all pH values tested. Spinach FBPase exhibited a more complicated behavior, with a single two-electron component being observed at pH values >/= 7.0, but with two components being present at pH values <7.0. The slopes of plots of Em versus pH were close to the -60 mV/pH unit value expected for a process that involves the uptake of two protons per two electrons (i. e., the reduction of a disulfide to two fully protonated thiols) for thioredoxins f and m, for FTR, and for pea FBPase. The slope of the Em versus pH profile for PRK shows three regions, consistent with the presence of pKa values for the two regulatory cysteines in the region between pH 7.5 and 9.0.     

Thioredoxin/fructose-1,6-bisphosphatase affinity in the enzyme activation by the ferredoxin-thioredoxin system

In this work we analyze the affinity relationship between photosynthetic fructose-1,6-bisphosphatase and ferredoxin and thioredoxin from spinach leaves, two components of the proposed light-activation system of this enzyme, using affinity techniques on ferredoxin- and thioredoxin-Sepharose columns. Oxidized and reduced ferredoxin did not show enzyme affinity, whereas thioredoxin, both the oxidized and the dithiothreitol-reduced form, exhibited a strong bisphosphatase affinity at pH 7.5; this thioredoxin/enzyme affinity appears diminished at pH 8.2. When the affinity experiments were performed in the presence of 5 mM Mg2+, only 30% and 12% of the bisphosphatase remained bound to the thioredoxin-Sepharose at pH 7.5 and 8.0, respectively; these percentages were reduced to 6% when the Mg2+ concentration increased to 10 mM. These results suggest that a rise of stromal pH and Mg2+ concentration can account for a loosening of the thioredoxin/bisphosphatase linkage, which could be of physiological significance in the dark-light transition. Studies on the nature of the chemical groups responsible for the affinity have shown that the thioredoxin/bisphosphatase linkage is concerned with the existence of hydrophobic clusters. We have found no difference in the behaviour of the chloroplastic thioredoxins f and m, and the cytoplasmic ones cf and cm. These results support the existence of an in vivo thioredoxin/fructose-1,6-bisphosphatase interaction, in accordance with the light-activation mechanism by the ferredoxin-thioredoxin system.     

Rhodospirillum centenum,sp. nov., a thermotolerant cyst-forming anoxygenic photosynthetic bacterium

A novel non-sulfur purple photosynthetic bacterium, designated Rhodospirillum centenum, was isolated from an enrichment culture designed to favor growth of anoxygenic photosynthetic N₂-fixing bacteria. R. centenum grows optimally at 40–42° C and has the capacity to produce cytoplasmic R bodies, refractile structures not observed hitherto in photosynthetic prokaryotes. The bacterium is also unusual among photosynthetic bacteria in that it forms desiccation-resistant cysts when grown aerobically in darkness with butyrate as the sole carbon source.     

Anoxygenic Photosynthesis Controls Oxygenic Photosynthesis in a Cyanobacterium from a Sulfidic Spring

Before the Earth''s complete oxygenation (0.58 to 0.55 billion years [Ga] ago), the photic zone of the Proterozoic oceans was probably redox stratified, with a slightly aerobic, nutrient-limited upper layer above a light-limited layer that tended toward euxinia. In such oceans, cyanobacteria capable of both oxygenic and sulfide-driven anoxygenic photosynthesis played a fundamental role in the global carbon, oxygen, and sulfur cycle. We have isolated a cyanobacterium, Pseudanabaena strain FS39, in which this versatility is still conserved, and we show that the transition between the two photosynthetic modes follows a surprisingly simple kinetic regulation controlled by this organism''s affinity for H₂S. Specifically, oxygenic photosynthesis is performed in addition to anoxygenic photosynthesis only when H₂S becomes limiting and its concentration decreases below a threshold that increases predictably with the available ambient light. The carbon-based growth rates during oxygenic and anoxygenic photosynthesis were similar. However, Pseudanabaena FS39 additionally assimilated NO₃⁻ during anoxygenic photosynthesis. Thus, the transition between anoxygenic and oxygenic photosynthesis was accompanied by a shift of the C/N ratio of the total bulk biomass. These mechanisms offer new insights into the way in which, despite nutrient limitation in the oxic photic zone in the mid-Proterozoic oceans, versatile cyanobacteria might have promoted oxygenic photosynthesis and total primary productivity, a key step that enabled the complete oxygenation of our planet and the subsequent diversification of life.     

Protein structure,electron transfer and evolution of prokaryotic photosynthetic reaction centers

Photosynthetic reaction centers from a variety of organisms have been isolated and characterized. The groups of prokaryotic photosynthetic organisms include the purple bacteria, the filamentous green bacteria, the green sulfur bacteria and the heliobacteria as anoxygenic representatives as well as the cyanobacteria and prochlorophytes as oxygenic representatives. This review focuses on structural and functional comparisons of the various groups of photosynthetic reaction centers and considers possible evolutionary scenarios to explain the diversity of existing photosynthetic organisms.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - Rb Rhodobacter - Rp Rhodopseudomonas     

<Emphasis Type="Italic">Bd</Emphasis> oxidase homologue of photosynthetic purple sulfur bacterium <Emphasis Type="Italic">Allochromatium vinosum</Emphasis> is co-transcribed with a nitrogen fixation related gene

Purple sulfur bacteria, which are known to be the most ancient among anoxygenic phototrophs, play an important role in the global sulfur cycle. Allochromatium vinosum oxidizes reduced sulfur compounds such as hydrogen sulfide, elemental sulfur and thiosulfide. At low oxygen concentrations, A. vinosum can grow chemotrophically using oxygen as the terminal electron acceptor. Being also a nitrogen fixer, A. vinosum is faced with the paradox of co-existence of aerobic metabolism and nitrogen fixation. Due to growth difficulties, only a few studies have dealt with the aerobic metabolism of the organism and, until now, there has been no information about the genes involved in the respiratory metabolism of purple sulfur bacteria. In this article we show the first terminal oxidase gene for A. vinosum. The presence of a Bd type of quinol oxidase is necessary to protect nitrogenases against the inhibitory effects of oxygen. In this case, a nitrogen fixation related gene is part of the cyd operon and this gene is co-transcribed with cydAB genes. Bd oxidase of A. vinosum may be the earliest form of oxidase where the function of the enzyme is to scavenge the contaminant oxygen during nitrogen fixation. This may be an important clue about the early evolution of oxygenic photosynthesis, perhaps as a protective mechanism for nitrogen fixation.     

Location of the redox-active cysteines in chloroplast sedoheptulose-1,7-bisphosphatase indicates that its allosteric regulation is similar but not identical to that of fructose-1,6-bisphosphatase

Sedoheptulose-1,7-bisphosphatase (SBPase) is a Calvin Cycle enzyme exclusive to chloroplasts and is involved in photosynthetic carbon fixation. The two cysteine residues involved in its redox regulation have been identified by site-directed mutagenesis. They are four residues apart in a predicted loop between two alpha helices and probably form a disulphide bond when oxidised. Three-dimensional modelling of SBPase has been performed using crystallographic data from the structurally homologous pig fructose-1,6-bisphosphatase (FBPase). The results suggest that formation of the disulphide bridge in SBPase is directly analogous to the allosteric regulation of pig FBPase by AMP in terms of the resulting structural changes. Similar changes are thought to occur in chloroplast FBPase, which like SBPase, is also redox regulated and involved in carbon fixation. From the results presented here it appears that the same basic mechanism for the allosteric regulation of enzymic activity operates in the FBPases and SBPase but that the sites at which the regulatory ligands (AMP or thioredoxin) exert their effects are different in each