Halotolerant cyanobacterium Aphanothece halophytica contains a betaine transporter active at alkaline pH and high salinity

Current insights into molecular mechanisms of environmental stress tolerance in Cyanobacteria

The photoautotrophic nature of cyanobacteria, coupled with their fast growth and relative ease of genetic manipulation, makes these microorganisms very promising factories for the sustainable production of bio-products from atmospheric carbon dioxide. However, both in nature and in cultivation, cyanobacteria go through different abiotic stresses such as high light (HL) stress, heavy metal stress, nutrient limitation, heat stress, salt stress, oxidative stress, and alcohol stress. In recent years, significant improvement has been made in identifying the stress-responsive genes and the linked pathways in cyanobacteria and developing genome editing tools for their manipulation. Metabolic pathways play an important role in stress tolerance; their modification is also a very promising approach to adapting to stress conditions. Several synthetic as well as systems biology approaches have been developed to identify and manipulate genes regulating cellular responses under different stresses. In this review, we summarize the impact of different stresses on metabolic processes, the small RNAs, genes and heat shock proteins (HSPs) involved, changes in the metabolome and their adaptive mechanisms. The developing knowledge of the adaptive behaviour of cyanobacteria may also be utilised to develop better stress-responsive strains for various applications.

Structural basis of a microbial trimethylamine transporter

Trimethylamine (TMA), a simple trace biogenic amine resulting from the decomposition of proteins and other macromolecules, is ubiquitous in nature. It is found in the human gut as well as in various terrestrial and marine ecosystems. While the role of TMA in promoting cardiovascular diseases and depolarizing olfactory sensory neurons in humans has only recently been explored, many microbes are well known for their ability to utilize TMA as a carbon, nitrogen, and energy source. Here, we report the first structure of a TMA transporter, TmaT, originally identified from a marine bacterium. TmaT is a member of the betaine-choline-carnitine transporter family, and we show that TmaT is an Na+/TMA symporter, which possessed high specificity and binding affinity toward TMA. Furthermore, the structures of TmaT and two TmaT-TMA complexes were solved by cryo-EM. TmaT forms a homotrimer structure in solution. Each TmaT monomer has 12 transmembrane helices, and the TMA transport channel is formed by a four-helix bundle. TMA can move between different aromatic boxes, which provides the structural basis of TmaT importing TMA. When TMA is bound in location I, residues Trp146, Trp151, Tyr154, and Trp326 form an aromatic box to accommodate TMA. Moreover, Met105 also plays an important role in the binding of TMA. When TMA is transferred to location II, it is bound in the aromatic box formed by Trp325, Trp326, and Trp329. Based on our results, we proposed the TMA transport mechanism by TmaT. This study provides novel insights into TMA transport across biological membranes.

IMPORTANCE

The volatile trimethylamine (TMA) plays an important role in promoting cardiovascular diseases and depolarizing olfactory sensory neurons in humans and serves as a key nutrient source for a variety of ubiquitous marine microbes. While the TMA transporter TmaT has been identified from a marine bacterium, the structure of TmaT and the molecular mechanism involved in TMA transport remain unclear. In this study, we elucidated the high-resolution cryo-EM structures of TmaT and TmaT-TMA complexes and revealed the TMA binding and transport mechanisms by structural and biochemical analyses. The results advance our understanding of the TMA transport processes across biological membranes.

Crosstalk between biological and chemical diversity with cytotoxic and cytostatic effects of Aphanothece halophytica in vitro

Different solvent extracts from Aphanothece halophytica (A. halophytica) were evaluated for their cytotoxic effects against four human cancer cell lines. The samples demonstrated different percentages of cyanobacteria species populations. The samples containing 100% A. halophytica and 90% A. halophytica showed a significant cytotoxic effect in human breast cancer cells MDA231. The cytostatic effect was demonstrated in MDA231 and human glioblastoma T98G cells regardless of the treatment, resulting in a significant cell cycle arrest in the S phase. The chemical profiles of the extracts were proven to be diverse in qualitative and quantitative compositions. This variability was dependent on the A. halophytica´s abundance in each extract. The 100% A. halophytica extract induced cytotoxic and cytostatic effects in breast cancer cells, and those could be associated with the predominance of fatty acids, hydrocarbons and phthalates, indicating that A. halophytica is an interesting source of novel compound with anticancer effect.

Virtual 2D map of cyanobacterial proteomes

Cyanobacteria are prokaryotic Gram-negative organisms prevalent in nearly all habitats. A detailed proteomics study of Cyanobacteria has not been conducted despite extensive study of their genome sequences. Therefore, we conducted a proteome-wide analysis of the Cyanobacteria proteome and found Calothrix desertica as the largest (680331.825 kDa) and Candidatus synechococcus spongiarum as the smallest (42726.77 kDa) proteome of the cyanobacterial kingdom. A Cyanobacterial proteome encodes 312.018 amino acids per protein, with a molecular weight of 182173.1324 kDa per proteome. The isoelectric point (pI) of the Cyanobacterial proteome ranges from 2.13 to 13.32. It was found that the Cyanobacterial proteome encodes a greater number of acidic-pI proteins, and their average pI is 6.437. The proteins with higher pI are likely to contain repetitive amino acids. A virtual 2D map of Cyanobacterial proteome showed a bimodal distribution of molecular weight and pI. Several proteins within the Cyanobacterial proteome were found to encode Selenocysteine (Sec) amino acid, while Pyrrolysine amino acids were not detected. The study can enable us to generate a high-resolution cell map to monitor proteomic dynamics. Through this computational analysis, we can gain a better understanding of the bias in codon usage by analyzing the amino acid composition of the Cyanobacterial proteome.

Antibiotic resistance genes alternation in soils modified with neutral and alkaline salts: interplay of salinity stress and response strategies of microbes

Growing evidence points to the pivotal roles of salt accumulation in mediating antibiotic resistance genes (ARGs) spread in soil, whereas how salt mediates ARGs dissemination remains unknown. Herein, the effects of neutral or alkaline (Ne/Al) salt at low, moderate and high levels (Ne/Al-L, Ne/Al-M, Ne/Al-H) on the dissemination of ten typical ARGs in soils were explored, by simultaneously considering the roles of salinity stress and response strategies of microbes. In the soils amended with Ne/Al-L and Al-M salt, the dissemination of ARGs was negligible and the relative abundances of ARGs and mobile genetic elements (MGEs) were decreased. However, Ne-M and Al-H salt contributed to the dissemination of ARGs in soils, with the significantly increased absolute and relative abundances of ARGs and MGEs. In Ne-H soil, although the absolute abundance of ARGs declined drastically due to serious oxidative damage, their relative abundances were promoted. The facilitated ARGs transfer was potentially related to the excessive generation of intracellular reactive oxygen species and increased activities of DNA repair enzymes involved in SOS system. In addition, the activated intracellular protective response including quorum sensing and energy metabolism largely provided essential factors for ARGs dissemination. The co-occurrence of ARGs and over-expressed salt-tolerant genes in specific halotolerant bacteria further suggested the selection of salt stress on ARGs. Moreover, less disturbance of alkaline salt than neutral salt on ARGs evolution was observed, due to the lower abiotic stress and selective pressure on microbes. This study highlights that soil salinity-sodicity could dose-dependently reshape the dissemination of ARGs and community structure of microbes, which may increase the ecological risks of ARGs in agricultural environment.

Proteomic analysis reveals the mechanism of different environmental stress-induced tolerance of Pseudomonas aeruginosa to monochloramine disinfection

Although drinking water disinfection proved to be an effective strategy to eliminate many pathogens, bacteria can still show disinfection tolerance in drinking water distribution systems. To date, the molecular mechanisms on how environmental stress affects the tolerance of Pseudomonas aeruginosa to monochloramine are not well understood. Here, we investigated how three stress conditions, namely starvation, low temperature, and starvation combined with low temperature, affected the monochloramine tolerance of Pseudomonas aeruginosa, an opportunistic pathogen commonly found in drinking water distribution systems. All stress conditions significantly promoted monochloramine tolerance, among which starvation had the most drastic effects. Proteomic analyses suggested that the three conditions not only triggered a positive antioxidant defense against oxidative damages but also prepared the bacteria to employ a passive defense mechanism against disinfectants via dormancy. Moreover, the expression of antioxidant enzymes reached the maximum under the starvation condition and further low temperature treatment had little effect on bacterial response to oxidative stress. Instead, we found further treatment of the starved cells with low temperature decreased the osmotic stress response and the stringent response, which generally play pivotal roles in disinfection tolerance. Taken together, these findings shed light on how abiotic factors influence the bacterial disinfection tolerance and will aid design of efficient strategies to eliminate Pseudomonas aeruginosa from drinking water.

Transporter engineering for the development of cyanobacteria as cell factories: A text analytics guided survey

Cyanobacteria are attractive candidates for photoautotrophic production of platform chemicals due to their inherent ability to utilize carbon dioxide as the sole carbon source. Metabolic pathways can be engineered more readily in cyanobacteria compared to higher photosynthetic organisms. Although significant progress has been made in pathway engineering, intracellular accumulation of the product is a potential bottleneck in large-scale production. Likewise, substrate uptake is known to limit growth and product formation. These limitations can potentially be addressed by targeted and controlled expression of transporter proteins in the metabolically engineered strains. This review focuses on the transporters that have been explored in cyanobacteria. To highlight the progress on characterization and application of cyanobacterial transporters, we applied text analytics to extract relevant information from over 1000 publications. We have categorized the transporters based on their source, their function and the solute they transport. Further, the review provides insights into the potential of transporters in the metabolic engineering of cyanobacteria for improved product titer.

Osmotic Adaptation and Compatible Solute Biosynthesis of Phototrophic Bacteria as Revealed from Genome Analyses

Osmotic adaptation and accumulation of compatible solutes is a key process for life at high osmotic pressure and elevated salt concentrations. Most important solutes that can protect cell structures and metabolic processes at high salt concentrations are glycine betaine and ectoine. The genome analysis of more than 130 phototrophic bacteria shows that biosynthesis of glycine betaine is common among marine and halophilic phototrophic Proteobacteria and their chemotrophic relatives, as well as in representatives of Pirellulaceae and Actinobacteria, but are also found in halophilic Cyanobacteria and Chloroherpeton thalassium. This ability correlates well with the successful toleration of extreme salt concentrations. Freshwater bacteria in general lack the possibilities to synthesize and often also to take up these compounds. The biosynthesis of ectoine is found in the phylogenetic lines of phototrophic Alpha- and Gammaproteobacteria, most prominent in the Halorhodospira species and a number of Rhodobacteraceae. It is also common among Streptomycetes and Bacilli. The phylogeny of glycine-sarcosine methyltransferase (GMT) and diaminobutyrate-pyruvate aminotransferase (EctB) sequences correlate well with otherwise established phylogenetic groups. Most significantly, GMT sequences of cyanobacteria form two major phylogenetic branches and the branch of Halorhodospira species is distinct from all other Ectothiorhodospiraceae. A variety of transport systems for osmolytes are present in the studied bacteria.

Structural and kinetic properties of serine hydroxymethyltransferase from the halophytic cyanobacterium Aphanothece halophytica provide a rationale for salt tolerance

Serine hydroxymethyltransferase (SHMT) is a pyridoxal 5'-phosphate-dependent enzyme that plays a pivotal role in cellular one‑carbon metabolism. In plants and cyanobacteria, this enzyme is also involved in photorespiration and confers salt tolerance, as in the case of SHMT from the halophilic cyanobacterium Aphanothece halophytica (AhSHMT). We have characterized the catalytic properties of AhSHMT in different salt and pH conditions. Although the kinetic properties of AhSHMT correlate with those of the mesophilic orthologue from Escherichia coli, AhSHMT appears more catalytically efficient, especially in presence of salt. Our studies also reveal substrate inhibition, previously unobserved in AhSHMT. Furthermore, addition of the osmoprotectant glycine betaine under salt conditions has a distinct positive effect on AhSHMT activity. The crystal structures of AhSHMT in three forms, as internal aldimine, as external aldimine with the l-serine substrate, and as a covalent complex with malonate, give structural insights on the possible role of specific amino acid residues implicated in the halophilic features of AhSHMT. Importantly, we observed that overexpression of the gene encoding SHMT, independently from its origin, increases the capability of E. coli to grow in high salt conditions, suggesting that the catalytic activity of this enzyme in itself plays a fundamental role in salt tolerance.

Functional characterization of aminotransferase involved in serine and aspartate metabolism in a halotolerant cyanobacterium, Aphanothece halophytica

Aminotransferases catalyze the reversible pyridoxal phosphate-dependent transfer of amino groups from amino acids to oxo acids and play important roles for the balance between carbon and nitrogen metabolism. In this report, four aminotransferases (Ap1-Ap4) from a halotolerant cyanobacterium Aphanothece halophytica were examined. The results revealed that Ap1 and Ap2 exhibited the aspartate:2-oxoglutarate aminotransferase (AspAT) activity whereas Ap2 catalyzed further aminotransferase activities with alanine (AlaAT) and LL-diaminopimelate (an intermediate for the synthesis of Lys/peptidoglycan) as amino donors. Ap4 exhibited bifunctional aminotransferase with phosphoserine (PSAT) and glycine (GGAT) as amino donors. No activity was observed for Ap3. We identified third gene encoding phosphoserine phosphatase (PSP) in phosphorylate serine biosynthetic pathway. The levels of mRNA for Ap2 and ApMurE encoding UDP-N-acetylmuramoyl-L-alanyl-D-glutamate-2,6-diaminopimelate ligase were increased after salt stress. These results suggest the link among photorespiratory metabolite (serine, glycine, glyoxylate), phosphorylate serine biosynthetic pathway and aspartate metabolism via aminotransferases for the synthesis of peptidoglycan and betaine under salt stress conditions.

Halophiles and Their Vast Potential in Biofuel Production

Global warming and the limitations of using fossil fuels are a main concern of all societies, and thus, the development of alternative fuel sources is crucial to improving the current global energy situation. Biofuels are known as the best alternatives of unrenewable fuels and justify increasing extensive research to develop new and less expensive methods for their production. The most frequent biofuels are bioethanol, biobutanol, biodiesel, and biogas. The production of these biofuels is the result of microbial activity on organic substrates like sugars, starch, oil crops, non-food biomasses, and agricultural and animal wastes. Several industrial production processes are carried out in the presence of high concentrations of NaCl and therefore, researchers have focused on halophiles for biofuel production. In this review, we focus on the role of halophilic microorganisms and their current utilization in the production of all types of biofuels. Also, the outstanding potential of them and their hydrolytic enzymes in the hydrolysis of different kind of biomasses and the production of biofuels are discussed.

State-of-the-Art Genetic Modalities to Engineer Cyanobacteria for Sustainable Biosynthesis of Biofuel and Fine-Chemicals to Meet Bio-Economy Challenges

In recent years, metabolic engineering of microorganisms has attained much research interest to produce biofuels and industrially pertinent chemicals. Owing to the relatively fast growth rate, genetic malleability, and carbon neutral production process, cyanobacteria has been recognized as a specialized microorganism with a significant biotechnological perspective. Metabolically engineering cyanobacterial strains have shown great potential for the photosynthetic production of an array of valuable native or non-native chemicals and metabolites with profound agricultural and pharmaceutical significance using CO₂ as a building block. In recent years, substantial improvements in developing and introducing novel and efficient genetic tools such as genome-scale modeling, high throughput omics analyses, synthetic/system biology tools, metabolic flux analysis and clustered regularly interspaced short palindromic repeats (CRISPR)-associated nuclease (CRISPR/cas) systems have been made for engineering cyanobacterial strains. Use of these tools and technologies has led to a greater understanding of the host metabolism, as well as endogenous and heterologous carbon regulation mechanisms which consequently results in the expansion of maximum productive ability and biochemical diversity. This review summarizes recent advances in engineering cyanobacteria to produce biofuel and industrially relevant fine chemicals of high interest. Moreover, the development and applications of cutting-edge toolboxes such as the CRISPR-cas9 system, synthetic biology, high-throughput "omics", and metabolic flux analysis to engineer cyanobacteria for large-scale cultivation are also discussed.

Betaine Improves Milk Yield in Grazing Dairy Cows Supplemented with Concentrates at High Temperatures

Simple Summary

Heat events during summer can result in dramatic reductions in milk production in grazing dairy cows as they attempt to reduce their accumulated heat load. Therefore, there is interest in dietary manipulations that can decrease heat production or increase heat dissipation. One of the actions of sugar beet-derived betaine is to act as an osmolyte and reduce intracellular ion pumping and heat production. Therefore, this study was conducted to investigate the effects of dietary betaine supplementation on milk and milk component production in grazing dairy cows during hot periods in summer.

Abstract

Betaine is an organic osmolyte sourced from sugar beet that accumulates in plant cells undergoing osmotic stress. Since the accumulation of betaine lowers the energy requirements of animals and, therefore, metabolic heat production, the aim of this experiment was to investigate if betaine supplementation improved milk yield in grazing dairy cows in summer. One hundred and eighteen Friesian × Holstein cows were paired on days in milk and, within each pair, randomly allocated to a containing treatment of either 0 or 2 g/kg natural betaine in their concentrate ration for approximately 3 weeks during February/March 2015 (summer in Australia). The mean maximum February temperature was 30 °C. Cows were allocated approximately 14 kg dry matter pasture and 7.5 kg of concentrate pellets (fed in the milking shed) per cow per day and were milked through an automatic milking system three times per day. Betaine supplementation increased average daily milk yield by over 6% (22.0 vs. 23.4 kg/day, p < 0.001) with the response increasing as the study progressed as indicated by the interaction (p < 0.001) between betaine and day. Milk fat % (p = 0.87), milk protein % (p = 0.90), and milk somatic cell count (p = 0.81) were unchanged by dietary betaine. However, betaine supplementation increased milk protein yield (677 vs. 719 g/day, p < 0.001) and fat yield (874 vs. 922 g/day, p < 0.001) with responses again being more pronounced as the study progressed. In conclusion, dietary betaine supplementation increased milk and component yield during summer in grazing dairy cows.

Evaluation of dietary betaine in lactating Holstein cows subjected to heat stress

Betaine (BET), a natural, organic osmolyte, improves cellular efficiency by acting as a chaperone, refolding denatured proteins. To test if dietary BET reduced the effect of heat stress (HS) in lactating dairy cows, multiparous, lactating Holstein cows (n=24) were blocked by days in milk (101.4±8.6 d) and randomly assigned to 1 of 3 daily intakes of dietary BET: the control (CON) group received no BET, mid intake (MID) received 57mg of BET/kg of body weight, and high dose (HI) received 114mg of BET/kg of body weight. Cows were fed twice daily and BET was top-dressed at each feeding. Cows were milked 2 times/d and milk samples were taken daily for analysis. Milk components, yield, feed intake, and water intake records were taken daily. Rectal temperature and respiration rate were taken 3 times/d at 0600, 1400, and 1800h. Cows were housed in environmentally controlled rooms and were allowed acclimation for 7d at thermoneutral (TN) conditions with a mean temperature-humidity index of 56.6. Cows were then exposed to 7d of TN followed by 7d of HS represented by a temperature-humidity index of 71.5 for 14d. This was followed by a recovery period of 3d at TN. Dietary BET increased milk yield during the TN period. No differences were found between BET and CON in total milk production or milk composition during HS. The increase in water intake during HS was not as great for cows fed BET compared with controls. The cows on CON diets had higher p.m. respiration rate than both MID and HI BET during HS, but lower rectal temperature compared with BET. No difference was found in serum glucose during TN, but cows given HI had elevated glucose levels during HS compared with CON. No differences were found in serum insulin levels between CON and BET but an intake by environment interaction was present with insulin increasing in HI-treated lactating dairy cows during HS. The heat shock response [heat shock protein (HSP) 27 and HSP70] was upregulated in bovine mammary epithelial cells in vitro. Blood leukocyte HSP27 was downregulated at the HI dose under TN conditions and HSP70 was upregulated at the HI dose and this effect was increased by HS. No effect was seen with the MID dose with HSP27 or HSP70. The lack of effect of BET at MID may be associated with uptake across the gut. We conclude that BET increased milk production under TN conditions and was associated with reduced feed and water intake and slightly increased body temperatures during HS of cows fed BET. The effect of BET on milk production was lost during HS with HI BET, whereas serum glucose levels increased during HS.

Metabolic potential and community structure of endophytic and rhizosphere bacteria associated with the roots of the halophyte Aster tripolium L

The submitted work assumes that the abundance and diversity of endophytic and rhizosphere microorganisms co-existing with the halophytic plant Aster tripolium L. growing in a salty meadow in the vicinity of a soda factory (central Poland) represent unique populations of cultivable bacterial strains. Endophytic and rhizosphere bacteria were (i) isolated and identified based on 16S rDNA sequences; (ii) screened for nifH and acdS genes; and (iii) analyzed based on selected metabolic properties. Moreover, total microbial biomass and community structures of the roots (endophytes), rhizosphere and soil were evaluated using a cultivation-independent technique (PLFA) to characterize plant-microbial interactions under natural salt conditions. The identification of the isolated strains showed domination by Gram-positive bacteria (mostly Bacillus spp.) both in the rhizosphere (90.9%) and roots (72.7%) of A. tripolium. Rhizosphere bacterial strains exhibited broader metabolic capacities, while endophytes exhibited higher specificities for metabolic activity. The PLFA analysis showed that the total bacterial biomass decreased in the following order (rhizosphere<soil<endophytes) and confirmed the dominance of Gram-positive endophytic bacteria in the roots of the halophyte. The described strain collection provides a valuable basis for a subsequent applications of bacteria in improvement of site adaptation of plants in saline soils.

Functional characterization of a member of alanine or glycine: cation symporter family in halotolerant cyanobacterium Aphanothece halophytica

Membrane proteins of amino acid–polyamine–organocation (APC) superfamily transport amino acids and amines across membranes and play important roles in the regulation of cellular processes. The alanine or glycine: cation symporter (AGCS) family belongs to APC superfamily and is found in prokaryotes, but its substrate specificity remains to be clarified. In this study, we found that a halotolerant cyanobacterium, Aphanothece halophytica has two putative ApagcS genes. The deduced amino acid sequence of one of genes, ApagcS1, exhibited high homology to Pseudomonas AgcS. The ApagcS1 gene was expressed in Escherichia coli JW4166 which is deficient in glycine uptake. Kinetics studies in JW4166 revealed that ApAgcS1 is a sodium-dependent glycine transporter. Competition experiments showed the significant inhibition by glutamine, asparagine, and glycine. The level of mRNA for ApagcS1 was induced by NaCl and nitrogen-deficient stresses. Uptake of glutamine by ApAgcS1 was also observed. Based on these data, the physiological role of ApAgcS1 was discussed.

Salt acclimation of cyanobacteria and their application in biotechnology

The long evolutionary history and photo-autotrophic lifestyle of cyanobacteria has allowed them to colonize almost all photic habitats on Earth, including environments with high or fluctuating salinity. Their basal salt acclimation strategy includes two principal reactions, the active export of ions and the accumulation of compatible solutes. Cyanobacterial salt acclimation has been characterized in much detail using selected model cyanobacteria, but their salt sensing and regulatory mechanisms are less well understood. Here, we briefly review recent advances in the identification of salt acclimation processes and the essential genes/proteins involved in acclimation to high salt. This knowledge is of increasing importance because the necessary mass cultivation of cyanobacteria for future use in biotechnology will be performed in sea water. In addition, cyanobacterial salt resistance genes also can be applied to improve the salt tolerance of salt sensitive organisms, such as crop plants.

Sucrose synthesis in the nitrogen-fixing Cyanobacterium Anabaena sp. strain PCC 7120 is controlled by the two-component response regulator OrrA

The filamentous, nitrogen-fixing cyanobacterium Anabaena sp. strain PCC 7120 accumulates sucrose as a compatible solute against salt stress. Sucrose-phosphate synthase activity, which is responsible for the sucrose synthesis, is increased by salt stress, but the mechanism underlying the regulation of sucrose synthesis remains unknown. In the present study, a response regulator, OrrA, was shown to control sucrose synthesis. Expression of spsA, which encodes a sucrose-phosphate synthase, and susA and susB, which encode sucrose synthases, was induced by salt stress. In the orrA disruptant, salt induction of these genes was completely abolished. The cellular sucrose level of the orrA disruptant was reduced to 40% of that in the wild type under salt stress conditions. Moreover, overexpression of orrA resulted in enhanced expression of spsA, susA, and susB, followed by accumulation of sucrose, without the addition of NaCl. We also found that SigB2, a group 2 sigma factor of RNA polymerase, regulated the early response to salt stress under the control of OrrA. It is concluded that OrrA controls sucrose synthesis in collaboration with SigB2.

Identification and upregulation of biosynthetic genes required for accumulation of Mycosporine-2-glycine under salt stress conditions in the halotolerant cyanobacterium Aphanothece halophytica

Mycosporine-like amino acids (MAAs) are valuable molecules that are the basis for important photoprotective constituents. Here we report molecular analysis of mycosporine-like amino acid biosynthetic genes from the halotolerant cyanobacterium Aphanothece halophytica, which can survive at high salinity and alkaline pH. This extremophile was found to have a unique MAA core (4-deoxygadusol)-synthesizing gene separated from three other genes. In vivo analysis showed accumulation of the mycosporine-2-glycine but not shinorine or mycosporine-glycine. Mycosporine-2-glycine accumulation was stimulated more under the stress condition of high salinity than UV-B radiation. The Aphanothece MAA biosynthetic genes also manifested a strong transcript level response to salt stress. Furthermore, the transformed Escherichia coli and Synechococcus strains expressing four putative Aphanothece MAA genes under the control of a native promoter were found to be capable of synthesizing mycosporine-2-glycine. The accumulation level of mycosporine-2-glycine was again higher under the high-salinity condition. In the transformed E. coli cells, its level was approximately 85.2 ± 0.7 μmol/g (dry weight). Successful production of a large amount of mycosporine in these cells provides a new opportunity in the search for an alternative natural sunscreen compound source.

Culture of a high-chlorophyll-producing and halotolerant Chlorella vulgaris

In order to increase the value of freshwater algae as raw ingredients for health foods and feed for seawater-based farmed fish, we sought to breed high-chlorophyll halotolerant Chlorella with the objective of generating strains with both high chlorophyll concentrations (≥ 5%) and halotolerance (up to 1% NaCl). We used the Chlorella vulgaris K strain in our research institute culture collection and induced mutations with UV irradiation and acriflavine which is known to effect mutations of mitochondrial DNA that are associated with chlorophyll production. Screenings were conducted on seawater-based "For Chlorella spp." (FC) agar medium, and dark-green-colored colonies were visually selected by macroscopic inspection. We obtained a high-chlorophyll halotolerant strain (designated C. vulgaris M-207A7) that had a chlorophyll concentration of 6.7% (d.m.), a level at least three-fold higher than that of K strain. This isolate also exhibited a greater survival rate in seawater that of K strain.

Distinct salt-dependent effects impair Fremyella diplosiphon pigmentation and cellular shape

Salt impairs cellular morphology and photosynthetic pigment accumulation in the cyanobacterium Fremyella diplosiphon. Recent findings indicated that the impact of salt on cellular morphology was attributable to salt-associated effects on osmotic regulation, as the impact on morphology was reversible when cells were treated with an osmoticum in the presence of salt. The impact of salt on photosynthetic pigment accumulation was associated with ionic effects of salt on the cells, as pigment levels remained low when salt-treated cells were incubated together with an osmoticum or an antioxidant, the latter to mitigate the impact of a salt-associated accumulation of reactive oxygen species. Here, we provide evidence that the transcripts for genes encoding the phycobiliproteins are not reduced in the presence of salt. These results suggest that the negative impact of salt-mediated changes on pigment accumulation occurs post-transcriptionally. A greater understanding of the mechanisms which impact growth of strains such as F. diplosiphon, which harbor pigments that allow low-light and shade-tolerated growth, may facilitate the development or adaptation of such strains as useful for remediation of salt-impacted soils or biofuel production.

Stress adaptation of Saccharomyces cerevisiae as monitored via metabolites using two-dimensional NMR spectroscopy

Many studies on yeast metabolism are focused on its response to specific stress conditions because the results can be extended to the human medical issues. Most of those works have been accomplished through functional genomics studies. However, these changes may not show a linear correlation with protein or metabolite levels. For many organisms including yeast, the number of metabolites is far fewer than that of genes or gene products. Thus, metabolic profiling can provide a simpler yet efficient snapshot of the system's physiology. Metabolites of Saccharomyces cerevisiae under various stresses were analyzed and compared with those under the normal, unstressed growth conditions by two-dimensional NMR spectroscopy. At least 31 metabolites were identified for most of the samples. The levels of many identified metabolites showed significant increase or decrease depending on the nature of the stress. The statistical analysis produced a holistic view: different stresses were clustered and isolated from one another with the exception of high pH, heat, and oxidative stresses. This work could provide a link between the metabolite profiles and mRNA or protein profiles under representative and well-studied stress conditions.

REGULATION OF BIOSYNTHESIS OF DIMETHYLSULFONIOPROPIONATE AND ITS UPTAKE IN STERILE MUTANT OF ULVA PERTUSA (CHLOROPHYTA)1

It has been shown that marine algae produce the compatible solute dimethylsulfoniopropionate (DMSP) from methionine (Met) via four enzymatic reactions in which the third step, synthesis of 4-dimethylsulfonio-2-hydroxy-butyrate (DMSHB) from 4-methylthio-2-hydroxybutyrate (MTHB), is the committing step. However, regulation of the biosynthetic pathways and transport properties of DMSP is largely unknown. Here, the effects of sulfur and sodium concentrations on the uptake and synthesis of DMSHB and DMSP were examined in a sterile mutant of Ulva pertusa Kjellm. Sulfur deficiency increased the activity of the sulfur assimilation enzyme O-acetyl serine sulfhydrylase but decreased the MTHB S-methyltransferase activity, suggesting the preferential utilization of sulfur atoms for Met metabolites other than DMSP. Uptake of DMSP and DMSHB was enhanced by S deficiency. High salinity enhanced the MTHB S-methyltransferase activity as well as the uptake of DMSHB. The MTHB S-methyltransferase activity was inhibited by its product DMSP. These data demonstrate the importance of MTHB S-methyltransferase activity and uptake of DMSHB for the regulation of DMSP.

Isolation and characterization of proline/betaine transporter gene from oil palm

Oil production from oil palm is adversely affected by drought and salt. Under drought and salt stress, proline content increases in oil palm; the mechanism for this is unknown. Here, an 8319-nucleotide sequence including cDNA, genomic DNA and the promoter region of proline transporter gene from oil palm Elaeis guineensis was determined. The transporter gene exhibited high similarity to Bet/ProT genes from several plants, but the highest homology was found with rice ProT1. The exon-intron structure of genomic DNA was unique, and numerous stress-response cis-elements were found in the promoter region. Expression of cDNA EgProT1 in Escherichia coli mutant exhibited uptake activities for glycinebetaine and choline as well as proline. Under salt-stressed conditions, exogenously applied glycinebetaine was taken up into the root more rapidly than the control. These data indicate that oil palm has a unique Pro/T1 gene. Nucleotide sequence data for the cDNA and genomic DNA of proline transporter gene from Elaeis guineensis are available in the DDJB database under accession numbers AB597035 and AB597036, respectively.

The BCCT family of carriers: from physiology to crystal structure

Increases in the environmental osmolarity are key determinants for the growth of microorganisms. To ensure a physiologically acceptable level of cellular hydration and turgor at high osmolarity, many bacteria accumulate compatible solutes. Osmotically controlled uptake systems allow the scavenging of these compounds from scarce environmental sources as effective osmoprotectants. A number of these systems belong to the BCCT family (betaine-choline-carnitine-transporter), sodium- or proton-coupled transporters (e.g. BetP and BetT respectively) that are ubiquitous in microorganisms. The BCCT family also contains CaiT, an L-carnitine/γ-butyrobetaine antiporter that is not involved in osmotic stress responses. The glycine betaine transporter BetP from Corynebacterium glutamicum is a representative for osmoregulated symporters of the BCCT family and functions both as an osmosensor and osmoregulator. The crystal structure of BetP in an occluded conformation in complex with its substrate glycine betaine and two crystal structures of CaiT in an inward-facing open conformation in complex with L-carnitine and γ-butyrobetaine were reported recently. These structures and the wealth of biochemical data on the activity control of BetP in response to osmotic stress enable a correlation between the sensing of osmotic stress by a transporter protein with the ensuing regulation of transport activity. Molecular determinants governing the high-affinity binding of the compatible solutes by BetP and CaiT, the coupling in symporters and antiporters, and the osmoregulatory properties are discussed in detail for BetP and various BCCT carriers.

Genomic structure of an economically important cyanobacterium, Arthrospira (Spirulina) platensis NIES-39

A filamentous non-N₂-fixing cyanobacterium, Arthrospira (Spirulina) platensis, is an important organism for industrial applications and as a food supply. Almost the complete genome of A. platensis NIES-39 was determined in this study. The genome structure of A. platensis is estimated to be a single, circular chromosome of 6.8 Mb, based on optical mapping. Annotation of this 6.7 Mb sequence yielded 6630 protein-coding genes as well as two sets of rRNA genes and 40 tRNA genes. Of the protein-coding genes, 78% are similar to those of other organisms; the remaining 22% are currently unknown. A total 612 kb of the genome comprise group II introns, insertion sequences and some repetitive elements. Group I introns are located in a protein-coding region. Abundant restriction-modification systems were determined. Unique features in the gene composition were noted, particularly in a large number of genes for adenylate cyclase and haemolysin-like Ca²⁺-binding proteins and in chemotaxis proteins. Filament-specific genes were highlighted by comparative genomic analysis.

Ecological genomics of marine picocyanobacteria

Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus numerically dominate the picophytoplankton of the world ocean, making a key contribution to global primary production. Prochlorococcus was isolated around 20 years ago and is probably the most abundant photosynthetic organism on Earth. The genus comprises specific ecotypes which are phylogenetically distinct and differ markedly in their photophysiology, allowing growth over a broad range of light and nutrient conditions within the 45 degrees N to 40 degrees S latitudinal belt that they occupy. Synechococcus and Prochlorococcus are closely related, together forming a discrete picophytoplankton clade, but are distinguishable by their possession of dissimilar light-harvesting apparatuses and differences in cell size and elemental composition. Synechococcus strains have a ubiquitous oceanic distribution compared to that of Prochlorococcus strains and are characterized by phylogenetically discrete lineages with a wide range of pigmentation. In this review, we put our current knowledge of marine picocyanobacterial genomics into an environmental context and present previously unpublished genomic information arising from extensive genomic comparisons in order to provide insights into the adaptations of these marine microbes to their environment and how they are reflected at the genomic level.

Identification of ligands affecting the activity of the transcriptional repressor CcpN from Bacillus subtilis

Carbon catabolite repression in Bacillus subtilis is mediated primarily by the major regulator CcpA. However, sugar-dependent repression of three genes, sr1 encoding a small nontranslated RNA and two genes coding for gluconeogenic enzymes, gapB and pckA, is carried out by the transcriptional repressor CcpN (control catabolite protein of gluconeogenic genes). It has previously been shown that ccpN is constitutively expressed, which leads to a constant occupation of all operators with CcpN. Since this would not allow for specific regulation, a ligand that modulates CcpN activity is required. In vitro transcription assays demonstrated that CcpN is able to specifically repress transcription to a small extent at the three mentioned promoters in the absence of an activating ligand. Upon testing of several ligands, including nucleotides and glycolysis intermediates, it could be shown that ATP is able to specifically enhance the repressing activity of CcpN, and this effect was more pronounced at a slightly acidic pH. Furthermore, ADP was found to specifically counteract the repressive effect of ATP. Circular dichroism measurements demonstrated a significant alteration of CcpN structure in the presence of ATP at acidic pH and in the presence of ADP. Electrophoretic mobility shift assays revealed that neither ATP nor ADP altered the affinity of CcpN for its operators. Therefore, we hypothesise that the effect of ligand-bound CcpN on the RNA polymerase might be due to a conformational switch that alters the interaction between the two proteins. Based on these results, a working model for CcpN action is discussed.

Presence of a Na+-stimulated P-type ATPase in the plasma membrane of the alkaliphilic halotolerant cyanobacterium Aphanothece halophytica

Aphanothece cells could take up Na(+) and this uptake was strongly inhibited by the protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Cells preloaded with Na(+) exhibited Na(+) extrusion ability upon energizing with glucose. Na(+) was also taken up by the plasma membranes supplied with ATP and the uptake was abolished by gramicidin D, monensin or Na(+)-ionophore. Orthovanadate and CCCP strongly inhibited Na(+) uptake, whereas N, N'-dicyclohexylcarbodiimide (DCCD) slightly inhibited the uptake. Plasma membranes could hydrolyse ATP in the presence of Na(+) but not with K(+), Ca(2+) and Li(+). The K(m) values for ATP and Na(+) were 1.66+/-0.12 and 25.0+/-1.8 mM, respectively, whereas the V(max) value was 0.66+/-0.05 mumol min(-1) mg(-1). Mg(2+) was required for ATPase activity whose optimal pH was 7.5. The ATPase was insensitive to N-ethylmaleimide, nitrate, thiocyanate, azide and ouabain, but was substantially inhibited by orthovanadate and DCCD. Amiloride, a Na(+)/H(+) antiporter inhibitor, and CCCP showed little or no effect. Gramicidin D and monensin stimulated ATPase activity. All these results suggest the existence of a P-type Na(+)-stimulated ATPase in Aphanothece halophytica. Plasma membranes from cells grown under salt stress condition showed higher ATPase activity than those from cells grown under nonstress condition.