Role of calcium in acclimation of the cyanobacterium Synechococcus elongatus PCC 7942 to nitrogen starvation

The small Ca2+-binding protein CSE links Ca2+ signalling with nitrogen metabolism and filament integrity in Anabaena sp. PCC 7120

Background

Filamentous cyanobacteria represent model organisms for investigating multicellularity. For many species, nitrogen-fixing heterocysts are formed from photosynthetic vegetative cells under nitrogen limitation. Intracellular Ca²⁺ has been implicated in the highly regulated process of heterocyst differentiation but its role remains unclear. Ca²⁺ is known to operate more broadly in metabolic signalling in cyanobacteria, although the signalling mechanisms are virtually unknown. A Ca²⁺-binding protein called the Ca²⁺ Sensor EF-hand (CSE) is found almost exclusively in filamentous cyanobacteria. Expression of asr1131 encoding the CSE protein in Anabaena sp. PCC 7120 was strongly induced by low CO₂ conditions, and rapidly downregulated during nitrogen step-down. A previous study suggests a role for CSE and Ca²⁺ in regulation of photosynthetic activity in response to changes in carbon and nitrogen availability.

Results

In the current study, a mutant Anabaena sp. PCC 7120 strain lacking asr1131 (Δcse) was highly prone to filament fragmentation, leading to a striking phenotype of very short filaments and poor growth under nitrogen-depleted conditions. Transcriptomics analysis under nitrogen-replete conditions revealed that genes involved in heterocyst differentiation and function were downregulated in Δcse, while heterocyst inhibitors were upregulated, compared to the wild-type.

Conclusions

These results indicate that CSE is required for filament integrity and for proper differentiation and function of heterocysts upon changes in the cellular carbon/nitrogen balance. A role for CSE in transmitting Ca²⁺ signals during the first response to changes in metabolic homeostasis is discussed.

A nitric oxide synthase-like protein from Synechococcus produces NO/NO3- from l-arginine and NADPH in a tetrahydrobiopterin- and Ca2+-dependent manner

Nitric oxide synthases (NOSs) are heme-based monooxygenases that convert l-Arg to l-citrulline and nitric oxide (NO), a key signaling molecule and cytotoxic agent in mammals. Bacteria also contain NOS proteins, but the role of NO production within these organisms, where understood, differs considerably from that of mammals. For example, a NOS protein in the marine cyanobacterium Synechococcus sp. PCC 7335 (syNOS) has recently been proposed to function in nitrogen assimilation from l-Arg. syNOS retains the oxygenase (NOSox) and reductase (NOSred) domains present in mammalian NOS enzymes (mNOSs), but also contains an N-terminal globin domain (NOSg) homologous to bacterial flavohemoglobin proteins. Herein, we show that syNOS functions as a dimer and produces NO from l-Arg and NADPH in a tetrahydrobiopterin (H4B)-dependent manner at levels similar to those produced by other NOSs but does not require Ca2+-calmodulin, which regulates NOSred-mediated NOSox reduction in mNOSs. Unlike other bacterial NOSs, syNOS cannot function with tetrahydrofolate and requires high Ca2+ levels (>200 μm) for its activation. NOSg converts NO to NO3- in the presence of O2 and NADPH; however, NOSg did not protect Escherichia coli strains against nitrosative stress, even in a mutant devoid of NO-protective flavohemoglobin. We also found that syNOS does not have NOS activity in E. coli (which lacks H4B) and that the recombinant protein does not confer growth advantages on l-Arg as a nitrogen source. Our findings indicate that syNOS has both NOS and NO oxygenase activities, requires H4B, and may play a role in Ca2+-mediated signaling.

A novel Ca2+-binding protein influences photosynthetic electron transport in Anabaena sp. PCC 7120

Ca²⁺ is a potent signalling molecule that regulates many cellular processes. In cyanobacteria, Ca²⁺ has been linked to cell growth, stress response and photosynthesis, and to the development of specialist heterocyst cells in certain nitrogen-fixing species. Despite this, the pathways of Ca²⁺ signal transduction in cyanobacteria are poorly understood, and very few protein components are known. The current study describes a previously unreported Ca²⁺-binding protein which was called the Ca²⁺ Sensor EF-hand (CSE), which is conserved in filamentous, nitrogen-fixing cyanobacteria. CSE is shown to bind Ca²⁺, which induces a conformational change in the protein structure. Poor growth of a strain of Anabaena sp. PCC 7120 overexpressing CSE was attributed to diminished photosynthetic performance. Transcriptomics, biophysics and proteomics analyses revealed modifications in the light-harvesting phycobilisome and photosynthetic reaction centre protein complexes.

Decomposition of cyanobacterial light harvesting complexes: NblA-dependent role of the bilin lyase homolog NblB

Phycobilisomes, the macromolecular light harvesting complexes of cyanobacteria are degraded under nutrient-limiting conditions. This crucial response is required to adjust light excitation to the metabolic status and avoid damage by excess excitation. Phycobilisomes are comprised of phycobiliproteins, apo-proteins that covalently bind bilin chromophores. In the cyanobacterium Synechococcus elongatus, the phycobiliproteins allophycocyanin and phycocyanin comprise the core and the rods of the phycobilisome, respectively. Previously, NblB was identified as an essential component required for phycocyanin degradation under nutrient starvation. This protein is homologous to bilin-lyases, enzymes that catalyze the covalent attachment of bilins to apo-proteins. However, the nblB-inactivated strain is not impaired in phycobiliprotein synthesis, but rather is characterized by aberrant phycocyanin degradation. Here, using a phycocyanin-deficient strain, we demonstrate that NblB is required for degradation of the core pigment, allophycocyanin. Furthermore, we show that the protein NblB is expressed under nutrient sufficient conditions, but during nitrogen starvation its level decreases about two-fold. This finding is in contrast to an additional component essential for degradation, NblA, the expression of which is highly induced under starvation. We further identified NblB residues required for phycocyanin degradation in vivo. Finally, we demonstrate phycocyanin degradation in a cell-free system, thereby providing support for the suggestion that NblB directly mediates pigment degradation by chromophore detachment. The dependence of NblB function on NblA revealed using this system, together with the results indicating presence of NblB under nutrient sufficient conditions, suggests a rapid mechanism for induction of pigment degradation, which requires only the expression of NblA.

The Rhizobium tropici CIAT 899 NodD2 protein regulates the production of Nod factors under salt stress in a flavonoid-independent manner

In the symbiotic associations between rhizobia and legumes, NodD promotes the expression of the nodulation genes in the presence of appropriate flavonoids. This set of genes is implied in the synthesis of Nodulation factors, which are responsible for launching the nodulation process. Rhizobium tropici CIAT 899 is the most successful symbiont of Phaseolus vulgaris and can nodulate a variety of legumes. This strain produces Nodulation factors under abiotic stress such as acidity or high concentration of salt. Genome sequencing of CIAT 899 allowed the identification of five nodD genes. Whereas NodD1 is essential to nodulate Leucaena leucocephala, Lotus japonicus and Macroptilium atropurpureum, symbiosis with P. vulgaris and Lotus burtii decreased the nodule number but did not abolish the symbiotic process when NodD1 is absent. Nodulation factor synthesis under salt stress is not regulated by NodD1. Here we confirmed that NodD2 is responsible for the activation of the CIAT 899 symbiotic genes under salt stress. We have demonstrated that NodD1 and NodD2 control the synthesis of the Nod factor necessary for a successful symbiosis with P. vulgaris and L. burtii. This is the first time that NodD is directly implied in the activation of the symbiotic genes under an abiotic stress.

Intracellular free Ca2+signals antibiotic exposure in cyanobacteria

Intracellular free Ca²⁺, [Ca²⁺]_i, is a key element of the cellular response to many abiotic and biotic stresses.

Extreme cellular adaptations and cell differentiation required by a cyanobacterium for carbonate excavation

Some cyanobacteria, known as euendoliths, excavate and grow into calcium carbonates, with their activity leading to significant marine and terrestrial carbonate erosion and to deleterious effects on coral reef and bivalve ecology. Despite their environmental relevance, the mechanisms by which they can bore have remained elusive and paradoxical, in that, as oxygenic phototrophs, cyanobacteria tend to alkalinize their surroundings, which will encourage carbonate precipitation, not dissolution. Therefore, cyanobacteria must rely on unique adaptations to bore. Studies with the filamentous euendolith, Mastigocoleus testarum, indicated that excavation requires both cellular energy and transcellular calcium transport, mediated by P-type ATPases, but the cellular basis for this phenomenon remains obscure. We present evidence that excavation in M. testarum involves two unique cellular adaptations. Long-range calcium transport is based on active pumping at multiple cells along boring filaments, orchestrated by the preferential localization of calcium ATPases at one cell pole, in a ring pattern, facing the cross-walls, and by repeating this placement and polarity, a pattern that breaks at branching and apical cells. In addition, M. testarum differentiates specialized cells we call calcicytes, that which accumulate calcium at concentrations more than 500-fold those found in other cyanobacteria, concomitantly and drastically lowering photosynthetic pigments and enduring severe cytoplasmatic alkalinization. Calcicytes occur commonly, but not exclusively, in apical parts of the filaments distal to the excavation front. We suggest that calcicytes allow for fast calcium flow at low, nontoxic concentrations through undifferentiated cells by providing buffering storage for excess calcium before final excretion to the outside medium.

The proteolysis adaptor, NblA, is essential for degradation of the core pigment of the cyanobacterial light-harvesting complex

The cyanobacterial light-harvesting complex, the phycobilisome, is degraded under nutrient limitation, allowing the cell to adjust light absorbance to its metabolic capacity. This large light-harvesting antenna comprises a core complex of the pigment allophycocyanin, and rod-shaped pigment assemblies emanating from the core. NblA, a low-molecular-weight protein, is essential for degradation of the phycobilisome. NblA mutants exhibit high absorbance of rod pigments under conditions that generally elicit phycobilisome degradation, implicating NblA in degradation of these pigments. However, the vast abundance of rod pigments and the substantial overlap between the absorbance spectra of rod and core pigments has made it difficult to directly associate NblA with proteolysis of the phycobilisome core. Furthermore, lack of allophycocyanin degradation in an NblA mutant may reflect a requirement for rod degradation preceding core degradation, and does not prove direct involvement of NblA in proteolysis of the core pigment. Therefore, in this study, we used a mutant lacking phycocyanin, the rod pigment of Synechococcus elongatusPCC7942, to examine whether NblA is required for allophycocyanin degradation. We demonstrate that NblA is essential for degradation of the core complex of the phycobilisome. Furthermore, fluorescence lifetime imaging microscopy provided in situ evidence for the interaction of NblA with allophycocyanin, and indicated that NblA interacts with allophycocyanin complexes that are associated with the photosynthetic membranes. Based on these data, as well as previous observations indicating interaction of NblA with phycobilisomes attached to the photosynthetic membranes, we suggest a model for sequential phycobilisome disassembly by NblA.

Calcium binding proteins and calcium signaling in prokaryotes

With the continued increase of genomic information and computational analyses during the recent years, the number of newly discovered calcium binding proteins (CaBPs) in prokaryotic organisms has increased dramatically. These proteins contain sequences that closely resemble a variety of eukaryotic calcium (Ca(2+)) binding motifs including the canonical and pseudo EF-hand motifs, Ca(2+)-binding β-roll, Greek key motif and a novel putative Ca(2+)-binding domain, called the Big domain. Prokaryotic CaBPs have been implicated in diverse cellular activities such as division, development, motility, homeostasis, stress response, secretion, transport, signaling and host-pathogen interactions. However, the majority of these proteins are hypothetical, and only few of them have been studied functionally. The finding of many diverse CaBPs in prokaryotic genomes opens an exciting area of research to explore and define the role of Ca(2+) in organisms other than eukaryotes. This review presents the most recent developments in the field of CaBPs and novel advancements in the role of Ca(2+) in prokaryotes.

Molecular complementarity between simple, universal molecules and ions limited phenotype space in the precursors of cells

Background

Fundamental problems faced by the protocells and their modern descendants include how to go from one phenotypic state to another; escape from a basin of attraction in the space of phenotypes; reconcile conflicting growth and survival strategies (and thereby live on ‘the scales of equilibria’); and create a coherent, reproducible phenotype from a multitude of constituents.

Presentation of the hypothesis

The solutions to these problems are likely to be found with the organic and inorganic molecules and inorganic ions that constituted protocells, which we term SUMIs for Simple Universal Molecules and Ions. These SUMIs probably included polyphosphate (PolyP) as a source of energy and of phosphate; poly-(R)-3-hydroxybutyrate (PHB) as a source of carbon and as a transporter in association with PolyP; polyamines as a source of nitrogen; lipids as precursors of membranes; as well as peptides, nucleic acids, and calcium. Here, we explore the hypothesis that the direct interactions between PHB, PolyP, polyamines and lipids – modulated by calcium – played a central role in solving the fundamental problems faced by early and modern cells.

Testing the hypothesis

We review evidence that SUMIs (1) were abundant and available to protocells; (2) are widespread in modern cells; (3) interact with one another and other cellular constituents to create structures with new functions surprisingly similar to those of proteins and RNA; (4) are essential to creating coherent phenotypes in modern bacteria. SUMIs are therefore natural candidates for reducing the immensity of phenotype space and making the transition from a “primordial soup” to living cells.

Implications of the hypothesis

We discuss the relevance of the SUMIs and their interactions to the ideas of molecular complementarity, composomes (molecular aggregates with hereditary properties based on molecular complementarity), and a prebiotic ecology of co-evolving populations of composomes. In particular, we propose that SUMIs might limit the initial phenotype space of composomes in a coherent way. As examples, we propose that acidocalcisomes arose from interactions and self-selection among SUMIs and that the phosphorylation of proteins in modern cells had its origin in the covalent modification of proteins by PHB.

Reviewers

This article was reviewed by Doron Lancet and Kepa Ruiz-Mirazo.

Survival strategies in the aquatic and terrestrial world: the impact of second messengers on cyanobacterial processes

Second messengers are intracellular substances regulated by specific external stimuli globally known as first messengers. Cells rely on second messengers to generate rapid responses to environmental changes and the importance of their roles is becoming increasingly realized in cellular signaling research. Cyanobacteria are photooxygenic bacteria that inhabit most of Earth's environments. The ability of cyanobacteria to survive in ecologically diverse habitats is due to their capacity to adapt and respond to environmental changes. This article reviews known second messenger-controlled physiological processes in cyanobacteria. Second messengers used in these systems include the element calcium (Ca2+), nucleotide-based guanosine tetraphosphate or pentaphosphate (ppGpp or pppGpp, represented as (p)ppGpp), cyclic adenosine 3',5'-monophosphate (cAMP), cyclic dimeric GMP (c-di-GMP), cyclic guanosine 3',5'-monophosphate (cGMP), and cyclic dimeric AMP (c-di-AMP), and the gaseous nitric oxide (NO). The discussion focuses on processes central to cyanobacteria, such as nitrogen fixation, light perception, photosynthesis-related processes, and gliding motility. In addition, we address future research trajectories needed to better understand the signaling networks and cross talk in the signaling pathways of these molecules in cyanobacteria. Second messengers have significant potential to be adapted as technological tools and we highlight possible novel and practical applications based on our understanding of these molecules and the signaling networks that they control.

Calcium signaling in plant endosymbiotic organelles: mechanism and role in physiology

Recent studies have demonstrated that chloroplasts and mitochondria evoke specific Ca(2+) signals in response to biotic and abiotic stresses in a stress-dependent manner. The identification of Ca(2+) transporters and Ca(2+) signaling molecules in chloroplasts and mitochondria implies that they play roles in controlling not only intra-organellar functions, but also extra-organellar processes such as plant immunity and stress responses. It appears that organellar Ca(2+) signaling might be more important to plant cell functions than previously thought. This review briefly summarizes what is known about the molecular basis of Ca(2+) signaling in plant mitochondria and chloroplasts.

The proteolysis adaptor, NblA, initiates protein pigment degradation by interacting with the cyanobacterial light-harvesting complexes

Degradation of the cyanobacterial protein pigment complexes, the phycobilisomes, is a central acclimation response that controls light energy capture. The small protein, NblA, is essential for proteolysis of these large complexes, which may reach a molecular mass of up to 4 MDa. Interactions of NblA in vitro supported the suggestion that NblA is a proteolysis adaptor that labels the pigment proteins for degradation. The mode of operation of NblA in situ, however, remained unresolved. Particularly, it was unclear whether NblA interacts with phycobilisome proteins while part of the large complex, or alternatively interaction with NblA, necessitates dissociation of pigment subunits from the assembly. Fluorescence intensity profiles demonstrated the preferential presence of NblA::GFP (green fluorescent protein) at the photosynthetic membranes, indicating co-localization with phycobilisomes. Furthermore, fluorescence lifetime imaging microscopy provided in situ evidence for interaction of NblA with phycobilisome protein pigments. Additionally, we demonstrated the role of NblA in vivo as a proteolysis tag based on the rapid degradation of the fusion protein NblA::GFP compared with free GFP. Taken together, these observations demonstrated in vivo the role of NblA as a proteolysis adaptor. Additionally, the interaction of NblA with phycobilisomes indicates that the dissociation of protein pigment subunits from the large complex is not a prerequisite for interaction with this adaptor and, furthermore, implicates NblA in the disassembly of the protein pigment complex. Thus, we suggest that, in the case of proteolysis of the phycobilisome, the adaptor serves a dual function: undermining the complex stability and designating the dissociated pigments for degradation.

A Pseudomonas aeruginosa EF-hand protein, EfhP (PA4107), modulates stress responses and virulence at high calcium concentration

Pseudomonas aeruginosa is a facultative human pathogen, and a major cause of nosocomial infections and severe chronic infections in endocarditis and in cystic fibrosis (CF) patients. Calcium (Ca2+) accumulates in pulmonary fluids of CF patients, and plays a role in the hyperinflammatory response to bacterial infection. Earlier we showed that P. aeruginosa responds to increased Ca2+ levels, primarily through the increased production of secreted virulence factors. Here we describe the role of putative Ca2+-binding protein, with an EF-hand domain, PA4107 (EfhP), in this response. Deletion mutations of efhP were generated in P. aeruginosa strain PAO1 and CF pulmonary isolate, strain FRD1. The lack of EfhP abolished the ability of P. aeruginosa PAO1 to maintain intracellular Ca2+ homeostasis. Quantitative high-resolution 2D-PAGE showed that the efhP deletion also affected the proteomes of both strains during growth with added Ca2+. The greatest proteome effects occurred when the pulmonary isolate was cultured in biofilms. Among the proteins that were significantly less abundant or absent in the mutant strains were proteins involved in iron acquisition, biosynthesis of pyocyanin, proteases, and stress response proteins. In support, the phenotypic responses of FRD1 ΔefhP showed that the mutant strain lost its ability to produce pyocyanin, developed less biofilm, and had decreased resistance to oxidative stress (H2O2) when cultured at high [Ca2+]. Furthermore, the mutant strain was unable to produce alginate when grown at high [Ca2+] and no iron. The effect of the ΔefhP mutations on virulence was determined in a lettuce model of infection. Growth of wild-type P. aeruginosa strains at high [Ca2+] causes an increased area of disease. In contrast, the lack of efhP prevented this Ca2+-induced increase in the diseased zone. The results indicate that EfhP is important for Ca2+ homeostasis and virulence of P. aeruginosa when it encounters host environments with high [Ca2+].

A battery of bioreporters of nitrogen bioavailability in aquatic ecosystems based on cyanobacteria

Cyanobacteria are the main primary producers and are responsible for the blooms and eutrophication processes caused by excess nutrients in surface waters. The aim of this paper is to use cyanobacteria to monitor the presence and bioavailability of different chemical species of nitrogen in freshwater. Cyanobacteria have mechanisms which can detect the presence of nutrients in their environment and can activate or repress specific genes, or operons, depending on nutrient bioavailability. Therefore, monitoring the expression of these genes can facilitate measurement of the availability of nutrients. To achieve this we have constructed self-bioluminescent reporter strains of the filamentous nitrogen-fixing cyanobacterium Nostoc sp. PCC 7120 expressing promoters of genes responsive to nitrogen fused to the luxCDABE operon. Three promoters were selected to direct the expression of luxCDABE: The first, the glnA promoter, is activated in the absence of combined nitrogen. We found that it responded linearly to the addition of known amounts of combined N in the range 50-500 μM NH₄(+) or NO₃(-). The second, the nirA operon promoter, turns on in the presence of nitrate being inhibited by ammonium. The bioreporter responded linearly in the range of 10-100 μM NO₃. The third, the gifA promoter, is activated in the presence of ammonium, responding linearly in the range 100-600 μM NH4(+). We also used a previously described strain of Synechococcus elongatus PCC 7942 expressing glnN (glutamine synthetase type III) fused to luxAB. We found that the glnN promoter responded linearly to the addition of known amounts of N in the range 50-500 μM NH₄(+) or NO₃(-). These cyanobacterial bioreporters were tested in real environmental samples (i.e. river waters) which confirmed their validity and showed a broad spectrum response. They are therefore useful in the detection of both total N-bioavailability and specific nitrogen species.

Evaluation of the ecotoxicity of pollutants with bioluminescent microorganisms

This chapter deals with the use of bioluminescent microorganisms in environmental monitoring, particularly in the assessment of the ecotoxicity of pollutants. Toxicity bioassays based on bioluminescent microorganisms are an interesting complement to classical toxicity assays, providing easiness of use, rapid response, mass production, and cost effectiveness. A description of the characteristics and main environmental applications in ecotoxicity testing of naturally bioluminescent microorganisms, covering bacteria and eukaryotes such as fungi and dinoglagellates, is reported in this chapter. The main features and applications of a wide variety of recombinant bioluminescent microorganisms, both prokaryotic and eukaryotic, are also summarized and critically considered. Quantitative structure-activity relationship models and hormesis are two important concepts in ecotoxicology; bioluminescent microorganisms have played a pivotal role in their development. As pollutants usually occur in complex mixtures in the environment, the use of both natural and recombinant bioluminescent microorganisms to assess mixture toxicity has been discussed. The main information has been summarized in tables, allowing quick consultation of the variety of luminescent organisms, bioluminescence gene systems, commercially available bioluminescent tests, environmental applications, and relevant references.

Calcium homeostasis in Pseudomonas aeruginosa requires multiple transporters and modulates swarming motility

Pseudomonas aeruginosa is an opportunistic human pathogen causing severe acute and chronic infections. Earlier we have shown that calcium (Ca(2+)) induces P. aeruginosa biofilm formation and production of virulence factors. To enable further studies of the regulatory role of Ca(2+), we characterized Ca(2+) homeostasis in P. aeruginosa PAO1 cells. By using Ca(2+)-binding photoprotein aequorin, we determined that the concentration of free intracellular Ca(2+) ([Ca(2+)]in) is 0.14±0.05μM. In response to external Ca(2+), the [Ca(2+)]in quickly increased at least 13-fold followed by a multi-phase decline by up to 73%. Growth at elevated Ca(2+) modulated this response. Treatment with inhibitors known to affect Ca(2+) channels, monovalent cations gradient, or P-type and F-type ATPases impaired [Ca(2+)]in response, suggesting the importance of the corresponding mechanisms in Ca(2+) homeostasis. To identify Ca(2+) transporters maintaining this homeostasis, bioinformatic and LC-MS/MS-based membrane proteomic analyses were used. [Ca(2+)]in homeostasis was monitored for seven Ca(2+)-affected and eleven bioinformatically predicted transporters by using transposon insertion mutants. Disruption of P-type ATPases PA2435, PA3920, and ion exchanger PA2092 significantly impaired Ca(2+) homeostasis. The lack of PA3920 and vanadate treatment abolished Ca(2+)-induced swarming, suggesting the role of the P-type ATPase in regulating P. aeruginosa response to Ca(2+).

Influence of the ADP/ATP ratio, 2-oxoglutarate and divalent ions on Azospirillum brasilense PII protein signalling

Proteins belonging to the P(II) family coordinate cellular nitrogen metabolism by direct interaction with a variety of enzymes, transcriptional regulators and transporters. The sensing function of P(II) relies on its ability to bind the nitrogen/carbon signalling molecule 2-oxoglutarate (2-OG). In Proteobacteria, P(II) is further subject to reversible uridylylation according to the intracellular levels of glutamine, which reflect the cellular nitrogen status. A number of P(II) proteins have been shown to bind ADP and ATP in a competitive manner, suggesting that P(II) might act as an energy sensor. Here, we analyse the influence of the ADP/ATP ratio, 2-OG levels and divalent metal ions on in vitro uridylylation of the Azospirillum brasilense P(II) proteins GlnB and GlnZ, and on interaction with their targets AmtB, DraG and DraT. The results support the notion that the cellular concentration of 2-OG is a key factor governing occupation of the GlnB and GlnZ nucleotide binding sites by ATP or ADP, with high 2-OG levels favouring the occupation of P(II) by ATP. Both P(II) uridylylation and interaction with target proteins responded to the ADP/ATP ratio within the expected physiological range, supporting the concept that P(II) proteins might act as cellular energy sensors.

Plant organellar calcium signalling: an emerging field

This review provides a comprehensive overview of the established and emerging roles that organelles play in calcium signalling. The function of calcium as a secondary messenger in signal transduction networks is well documented in all eukaryotic organisms, but so far existing reviews have hardly addressed the role of organelles in calcium signalling, except for the nucleus. Therefore, a brief overview on the main calcium stores in plants-the vacuole, the endoplasmic reticulum, and the apoplast-is provided and knowledge on the regulation of calcium concentrations in different cellular compartments is summarized. The main focus of the review will be the calcium handling properties of chloroplasts, mitochondria, and peroxisomes. Recently, it became clear that these organelles not only undergo calcium regulation themselves, but are able to influence the Ca(2+) signalling pathways of the cytoplasm and the entire cell. Furthermore, the relevance of recent discoveries in the animal field for the regulation of organellar calcium signals will be discussed and conclusions will be drawn regarding potential homologous mechanisms in plant cells. Finally, a short overview on bacterial calcium signalling is included to provide some ideas on the question where this typically eukaryotic signalling mechanism could have originated from during evolution.

Signalling by the global regulatory molecule ppGpp in bacteria and chloroplasts of land plants

The hyperphosphorylated guanine ribonucleotide ppGpp mediates the stringent response in bacteria. Biochemical and genetic studies of this response in Escherichia coli have shown that the biosynthesis of ppGpp is catalysed by two homologous enzymes, RelA and SpoT. RelA is activated in response to amino acid starvation, and SpoT responds to abiotic physical stress beside nutritional stress. All free-living bacteria, including Gram-positive firmicutes, contain RelA-SpoT homologues (RSH). Further, novel ppGpp biosynthetic enzymes, designated small alarmone synthetases (SASs), were recently identified in a subset of bacteria, including the Gram-positive organism Bacillus subtilis, and were shown to consist only of a ppGpp synthetase domain. Studies suggest that these SAS proteins contribute to ppGpp signalling in response to stressful conditions in a manner distinct from that of RelA-SpoT enzymes. SAS proteins currently appear to always occur in addition to RSH enzymes in various combinations but never alone. RSHs have also been identified in chloroplasts, organelles of photosynthetic eukaryotes that originated from endosymbiotic photosynthetic bacteria. These chloroplast RSHs are exclusively encoded in nuclear DNA and targeted into chloroplasts. The findings suggest that ppGpp may regulate chloroplast functions similar to those regulated in bacteria, including transcription and translation. In addition, a novel ppGpp synthetase that is regulated by Ca²⁺ as a result of the presence of two EF-hand motifs at its COOH terminus was recently identified in chloroplasts of land plants. This finding indicates the existence of a direct connection between eukaryotic Ca²⁺ signalling and prokaryotic ppGpp signalling in chloroplasts. The new observations with regard to ppGpp signalling in land plants suggest that such signalling contributes to the regulation of a wider range of cellular functions than previously anticipated.

Sulfate-driven elemental sparing is regulated at the transcriptional and posttranscriptional levels in a filamentous cyanobacterium

Sulfur is an essential nutrient that can exist at growth-limiting concentrations in freshwater environments. The freshwater cyanobacterium Fremyella diplosiphon (also known as Tolypothrix sp. PCC 7601) is capable of remodeling the composition of its light-harvesting antennae, or phycobilisomes, in response to changes in the sulfur levels in its environment. Depletion of sulfur causes these cells to cease the accumulation of two forms of a major phycobilisome protein called phycocyanin and initiate the production of a third form of phycocyanin, which possesses a minimal number of sulfur-containing amino acids. Since phycobilisomes make up approximately 50% of the total protein in these cells, this elemental sparing response has the potential to significantly influence the fitness of this species under low-sulfur conditions. This response is specific for sulfate and occurs over the physiological range of sulfate concentrations likely to be encountered by this organism in its natural environment. F. diplosiphon has two separate sulfur deprivation responses, with low sulfate levels activating the phycobilisome remodeling response and low sulfur levels activating the chlorosis or bleaching response. The phycobilisome remodeling response results from changes in RNA abundance that are regulated at both the transcriptional and posttranscriptional levels. The potential of this response, and the more general bleaching response of cyanobacteria, to provide sulfur-containing amino acids during periods of sulfur deprivation is examined.