Glutamine, glutamate, and alpha-glucosylglycerate are the major osmotic solutes accumulated by Erwinia chrysanthemi strain 3937

Strategies for the synthesis of the osmolyte glucosylglycerate and its precursor glycerate

Glycosidic osmolytes are widespread natural compounds that protect microorganisms and their macromolecules from the deleterious effects of various environmental stresses. Their protective properties have attracted considerable interest for industrial applications, especially as active ingredients in cosmetics and healthcare products. In that regard, the osmolyte glucosylglycerate is somewhat overlooked. Glucosylglycerate is typically accumulated by certain organisms when they are exposed to high salinity and nitrogen starvation, and its potent stabilizing effects have been demonstrated in vitro. However, the applications of this osmolyte have not been thoroughly explored due to the lack of a cost-efficient production process. Here, we present an overview of the progress that has been made in developing promising strategies for the synthesis of glucosylglycerate and its precursor glycerate, and discuss the remaining challenges. KEY POINTS: • Bacterial milking could be explored for fermentative production of glucosylglycerate • Glycoside phosphorylases of GH13_18 represent attractive alternatives for biocatalytic production • Conversion of glycerol with alditol oxidase is a promising strategy for generating the precursor glycerate.

Acidithiobacillia class members originating at sites within the Pacific Ring of Fire and other tectonically active locations and description of the novel genus 'Igneacidithiobacillus'

Recent studies have expanded the genomic contours of the Acidithiobacillia, highlighting important lacunae in our comprehension of the phylogenetic space occupied by certain lineages of the class. One such lineage is 'Igneacidithiobacillus', a novel genus-level taxon, represented by 'Igneacidithiobacillus copahuensis' VAN18-1T as its type species, along with two other uncultivated metagenome-assembled genomes (MAGs) originating from geothermally active sites across the Pacific Ring of Fire. In this study, we investigate the genetic and genomic diversity, and the distribution patterns of several uncharacterized Acidithiobacillia class strains and sequence clones, which are ascribed to the same 16S rRNA gene sequence clade. By digging deeper into this data and contributing to novel MAGs emerging from environmental studies in tectonically active locations, the description of this novel genus has been consolidated. Using state-of-the-art genomic taxonomy methods, we added to already recognized taxa, an additional four novel Candidate (Ca.) species, including 'Ca. Igneacidithiobacillus chanchocoensis' (mCHCt20-1TS), 'Igneacidithiobacillus siniensis' (S30A2T), 'Ca. Igneacidithiobacillus taupoensis' (TVZ-G3 TS), and 'Ca. Igneacidithiobacillus waiarikiensis' (TVZ-G4 TS). Analysis of published data on the isolation, enrichment, cultivation, and preliminary microbiological characterization of several of these unassigned or misassigned strains, along with the type species of the genus, plus the recoverable environmental data from metagenomic studies, allowed us to identify habitat preferences of these taxa. Commonalities and lineage-specific adaptations of the seven species of the genus were derived from pangenome analysis and comparative genomic metabolic reconstruction. The findings emerging from this study lay the groundwork for further research on the ecology, evolution, and biotechnological potential of the novel genus 'Igneacidithiobacillus'.

Osmoregulation in freshwater anaerobic methane-oxidizing archaea under salt stress

Climate change-driven sea level rise threatens freshwater ecosystems and elicits salinity stress in microbiomes. Methane emissions in these systems are largely mitigated by methane-oxidizing microorganisms. Here, we characterized the physiological and metabolic response of freshwater methanotrophic archaea to salt stress. In our microcosm experiments, inhibition of methanotrophic archaea started at 1%. However, during gradual increase of salt up to 3% in a reactor over 12 weeks, the culture continued to oxidize methane. Using gene expression profiles and metabolomics, we identified a pathway for salt-stress response that produces the osmolyte of anaerobic methanotrophic archaea: N(ε)-acetyl-β-L-lysine. An extensive phylogenomic analysis on N(ε)-acetyl-β-L-lysine-producing enzymes revealed that they are widespread across both bacteria and archaea, indicating a potential horizontal gene transfer and a link to BORG extrachromosomal elements. Physicochemical analysis of bioreactor biomass further indicated the presence of sialic acids and the consumption of intracellular polyhydroxyalkanoates in anaerobic methanotrophs during salt stress.

Amino Acid Starvation-Induced Glutamine Accumulation Enhances Pneumococcal Survival

Bacteria are known to cope with amino acid starvation by the stringent response signaling system, which is mediated by the accumulation of the (p)ppGpp alarmones when uncharged tRNAs stall at the ribosomal A site. While a number of metabolic processes have been shown to be regulatory targets of the stringent response in many bacteria, the global impact of amino acid starvation on bacterial metabolism remains obscure. This work reports the metabolomic profiling of the human pathogen Streptococcus pneumoniae under methionine starvation. Methionine limitation led to the massive overhaul of the pneumococcal metabolome. In particular, methionine-starved pneumococci showed a massive accumulation of many metabolites such as glutamine, glutamic acid, lactate, and cyclic AMP (cAMP). In the meantime, methionine-starved pneumococci showed a lower intracellular pH and prolonged survival. Isotope tracing revealed that pneumococci depend predominantly on amino acid uptake to replenish intracellular glutamine but cannot convert glutamine to methionine. Further genetic and biochemical analyses strongly suggested that glutamine is involved in the formation of a "prosurvival" metabolic state by maintaining an appropriate intracellular pH, which is accomplished by the enzymatic release of ammonia from glutamine. Methionine starvation-induced intracellular pH reduction and glutamine accumulation also occurred to various extents under the limitation of other amino acids. These findings have uncovered a new metabolic mechanism of bacterial adaptation to amino acid limitation and perhaps other stresses, which may be used as a potential therapeutic target for infection control. IMPORTANCE Bacteria are known to cope with amino acid starvation by halting growth and prolonging survival via the stringent response signaling system. Previous investigations have allowed us to understand how the stringent response regulates many aspects of macromolecule synthesis and catabolism, but how amino acid starvation promotes bacterial survival at the metabolic level remains largely unclear. This paper reports our systematic profiling of the methionine starvation-induced metabolome in S. pneumoniae. To the best of our knowledge, this represents the first reported bacterial metabolome under amino acid starvation. These data have revealed that the significant accumulation of glutamine and lactate enables S. pneumoniae to form a "prosurvival" metabolic state with a lower intracellular pH, which inhibits bacterial growth for prolonged survival. Our findings have provided insightful information on the metabolic mechanisms of pneumococcal adaptation to nutrient limitation during the colonization of the human upper airway.

Nitrogen Fixation and Ammonium Assimilation Pathway Expression of Geobacter sulfurreducens Changes in Response to the Anode Potential in Microbial Electrochemical Cells

Nitrogen gas (N2) fixation in the anode-respiring bacterium Geobacter sulfurreducens occurs through complex, multistep processes. Optimizing ammonium (NH4+) production from this bacterium in microbial electrochemical technologies (METs) requires an understanding of how those processes are regulated in response to electrical driving forces. In this study, we quantified gene expression levels (via RNA sequencing) of G. sulfurreducens growing on anodes fixed at two different potentials (-0.15 V and +0.15 V versus standard hydrogen electrode). The anode potential had a significant impact on the expression levels of N2 fixation genes. At -0.15 V, the expression of nitrogenase genes, such as nifH, nifD, and nifK, significantly increased relative to that at +0.15 V, as well as genes associated with NH4+ uptake and transformation, such as glutamine and glutamate synthetases. Metabolite analysis confirmed that both of these organic compounds were present in significantly higher intracellular concentrations at -0.15 V. N2 fixation rates (estimated using the acetylene reduction assay and normalized to total protein) were significantly larger at -0.15 V. Genes expressing flavin-based electron bifurcation complexes, such as electron-transferring flavoproteins (EtfAB) and the NADH-dependent ferredoxin:NADP reductase (NfnAB), were also significantly upregulated at -0.15 V, suggesting that these mechanisms may be involved in N2 fixation at that potential. Our results show that in energy-constrained situations (i.e., low anode potential), the cells increase per-cell respiration and N2 fixation rates. We hypothesize that at -0.15 V, they increase N2 fixation activity to help maintain redox homeostasis, and they leverage electron bifurcation as a strategy to optimize energy generation and use. IMPORTANCE Biological nitrogen fixation coupled with ammonium recovery provides a sustainable alternative to the carbon-, water-, and energy-intensive Haber-Bosch process. Aerobic biological nitrogen fixation technologies are hindered by oxygen gas inhibition of the nitrogenase enzyme. Electrically driving biological nitrogen fixation in anaerobic microbial electrochemical technologies overcomes this challenge. Using Geobacter sulfurreducens as a model exoelectrogenic diazotroph, we show that the anode potential in microbial electrochemical technologies has a significant impact on nitrogen gas fixation rates, ammonium assimilation pathways, and expression of genes associated with nitrogen gas fixation. These findings have important implications for understanding regulatory pathways of nitrogen gas fixation and will help identify target genes and operational strategies to enhance ammonium production in microbial electrochemical technologies.

RB-TnSeq identifies genetic targets for improved tolerance of Pseudomonas putida towards compounds relevant to lignin conversion

Lignin-derived mixtures intended for bioconversion commonly contain high concentrations of aromatic acids, aliphatic acids, and salts. The inherent toxicity of these chemicals places a significant bottleneck upon the effective use of microbial systems for the valorization of these mixtures. Pseudomonas putida KT2440 can tolerate stressful quantities of several lignin-related compounds, making this bacterium a promising host for converting these chemicals to valuable bioproducts. Nonetheless, further increasing P. putida tolerance to chemicals in lignin-rich substrates has the potential to improve bioprocess performance. Accordingly, we employed random barcoded transposon insertion sequencing (RB-TnSeq) to reveal genetic determinants in P. putida KT2440 that influence stress outcomes during exposure to representative constituents found in lignin-rich process streams. The fitness information obtained from the RB-TnSeq experiments informed engineering of strains via deletion or constitutive expression of several genes. Namely, ΔgacAS, ΔfleQ, ΔlapAB, ΔttgR::P_tac:ttgABC, P_tac:PP_1150:PP_1152, ΔrelA, and ΔPP_1430 mutants showed growth improvement in the presence of single compounds, and some also exhibited greater tolerance when grown using a complex chemical mixture representative of a lignin-rich chemical stream. Overall, this work demonstrates the successful implementation of a genome-scale screening tool for the identification of genes influencing stress tolerance against notable compounds within lignin-enriched chemical streams, and the genetic targets identified herein offer promising engineering targets for improving feedstock tolerance in lignin valorization strains of P. putida KT2440.

Ecologically coherent population structure of uncultivated bacterioplankton

Bacterioplankton are main drivers of biogeochemical cycles and important components of aquatic food webs. While sequencing-based studies have revealed how bacterioplankton communities are structured in time and space, relatively little is known about intraspecies diversity patterns and their ecological relevance. Here, we use the newly developed software POGENOM (POpulation GENomics from Metagenomes) to investigate genomic diversity and differentiation in metagenome-assembled genomes from the Baltic Sea, and investigate their genomic variation using metagenome data spanning a 1700 km transect and covering seasonal variation at one station. The majority of the investigated species, representing several major bacterioplankton clades, displayed population structures correlating significantly with environmental factors such as salinity and temperature. Population differentiation was more pronounced over spatial than temporal scales. We discovered genes that have undergone adaptation to different salinity regimes, potentially responsible for the populations' existence along with the salinity range. This in turn implies the broad existence of ecotypes that may remain undetected by rRNA gene sequencing. Our findings emphasize the importance of physiological barriers, and highlight the role of adaptive divergence as a structuring mechanism of bacterioplankton species.

Biotechnological and Ecological Potential of Micromonospora provocatoris sp. nov., a Gifted Strain Isolated from the Challenger Deep of the Mariana Trench

A Micromonospora strain, isolate MT25^T, was recovered from a sediment collected from the Challenger Deep of the Mariana Trench using a selective isolation procedure. The isolate produced two major metabolites, n-acetylglutaminyl glutamine amide and desferrioxamine B, the chemical structures of which were determined using 1D and 2D-NMR, including ¹H-¹⁵N HSQC and ¹H-¹⁵N HMBC 2D-NMR, as well as high resolution MS. A whole genome sequence of the strain showed the presence of ten natural product-biosynthetic gene clusters, including one responsible for the biosynthesis of desferrioxamine B. Whilst 16S rRNA gene sequence analyses showed that the isolate was most closely related to the type strain of Micromonospora chalcea, a whole genome sequence analysis revealed it to be most closely related to Micromonospora tulbaghiae 45142^T. The two strains were distinguished using a combination of genomic and phenotypic features. Based on these data, it is proposed that strain MT25^T (NCIMB 15245^T, TISTR 2834^T) be classified as Micromonospora provocatoris sp. nov. Analysis of the genome sequence of strain MT25^T (genome size 6.1 Mbp) revealed genes predicted to responsible for its adaptation to extreme environmental conditions that prevail in deep-sea sediments.

Genomic characterization of closely related species in the Rumoiensis clade infers ecogenomic signatures to non-marine environments

Members of the family Vibrionaceae are generally found in marine and brackish environments, playing important roles in nutrient cycling. The Rumoiensis clade is an unconventional group in the genus Vibrio, currently comprising six species from different origins including two species isolated from non-marine environments. In this study, we performed comparative genome analysis of all six species in the clade using their complete genome sequences. We found that two non-marine species, Vibrio casei and Vibrio gangliei, lacked the genes responsible for algal polysaccharide degradation, while a number of glycoside hydrolase genes were enriched in these two species. Expansion of insertion sequences was observed in V. casei and Vibrio rumoiensis, which suggests ongoing genomic changes associated with niche adaptations. The genes responsible for the metabolism of glucosylglycerate, a compound known to play a role as compatible solutes under nitrogen limitation, were conserved across the clade. These characteristics, along with genes encoding species-specific functions, may reflect the habit expansion which has led to the current distribution of Rumoiensis clade species. Genome analysis of all species in a single clade give us valuable insights into the genomic background of the Rumoiensis clade species and emphasize the genomic diversity and versatility of Vibrionaceae.

Adaptive Metabolism in Staphylococci: Survival and Persistence in Environmental and Clinical Settings

Staphylococci are highly successful at colonizing a variety of dynamic environments, both nonpathogenic and those of clinical importance, and comprise the list of pathogens of global public health significance. Their remarkable survival and persistence can be attributed to a host of strategies, one of which is metabolic versatility—their ability to rapidly alter their metabolism in the presence of transient or long-term bacteriostatic and bactericidal conditions and facilitate cellular homeostasis. These attributes contribute to their widespread dissemination and challenging eradication particularly from clinical settings. The study of microbial behaviour at the metabolite level provides insight into mechanisms of survival and persistence under defined environmental and clinical conditions. This paper reviews the range of metabolic modulations that facilitate staphylococcal acclimatization and persistence in varying terrestrial and host conditions, and their public health ramifications in these settings.

Discovery of a Kojibiose Phosphorylase in Escherichia coli K-12

The substrate profiles for three uncharacterized enzymes (YcjM, YcjT, and YcjU) that are expressed from a cluster of 12 genes ( ycjM-W and ompG) of unknown function in Escherichia coli K-12 were determined. Through a comprehensive bioinformatic and steady-state kinetic analysis, the catalytic function of YcjT was determined to be kojibiose phosphorylase. In the presence of saturating phosphate and kojibiose (α-(1,2)-d-glucose-d-glucose), this enzyme catalyzes the formation of d-glucose and β-d-glucose-1-phosphate ( k_cat = 1.1 s^-1, K_m = 1.05 mM, and k_cat/ K_m = 1.12 × 10³ M^-1 s^-1). Additionally, it was also shown that in the presence of β-d-glucose-1-phosphate, YcjT can catalyze the formation of other disaccharides using 1,5-anhydro-d-glucitol, l-sorbose, d-sorbitol, or l-iditol as a substitute for d-glucose. Kojibiose is a component of cell wall lipoteichoic acids in Gram-positive bacteria and is of interest as a potential low-calorie sweetener and prebiotic. YcjU was determined to be a β-phosphoglucomutase that catalyzes the isomerization of β-d-glucose-1-phosphate ( k_cat = 21 s^-1, K_m = 18 μM, and k_cat/ K_m = 1.1 × 10⁶ M^-1 s^-1) to d-glucose-6-phosphate. YcjU was also shown to exhibit catalytic activity with β-d-allose-1-phosphate, β-d-mannose-1-phosphate, and β-d-galactose-1-phosphate. YcjM catalyzes the phosphorolysis of α-(1,2)-d-glucose-d-glycerate with a k_cat = 2.1 s^-1, K_m = 69 μM, and k_cat/ K_m = 3.1 × 10⁴ M^-1 s^-1.

Stress-Adaptive Responses Associated with High-Level Carbapenem Resistance in KPC-Producing Klebsiella pneumoniae

Carbapenem-resistant Enterobacteriaceae (CRE) organisms have emerged to become a major global public health threat among antimicrobial resistant bacterial human pathogens. Little is known about how CREs emerge. One characteristic phenotype of CREs is heteroresistance, which is clinically associated with treatment failure in patients given a carbapenem. Through in vitro whole-transcriptome analysis we tracked gene expression over time in two different strains (BR7, BR21) of heteroresistant KPC-producing Klebsiella pneumoniae, first exposed to a bactericidal concentration of imipenem followed by growth in drug-free medium. In both strains, the immediate response was dominated by a shift in expression of genes involved in glycolysis toward those involved in catabolic pathways. This response was followed by global dampening of transcriptional changes involving protein translation, folding and transport, and decreased expression of genes encoding critical junctures of lipopolysaccharide biosynthesis. The emerged high-level carbapenem-resistant BR21 subpopulation had a prophage (IS1) disrupting ompK36 associated with irreversible OmpK36 porin loss. On the other hand, OmpK36 loss in BR7 was reversible. The acquisition of high-level carbapenem resistance by the two heteroresistant strains was associated with distinct and shared stepwise transcriptional programs. Carbapenem heteroresistance may emerge from the most adaptive subpopulation among a population of cells undergoing a complex set of stress-adaptive responses.

In a quest for engineering acidophiles for biomining applications: challenges and opportunities

Biomining with acidophilic microorganisms has been used at commercial scale for the extraction of metals from various sulfide ores. With metal demand and energy prices on the rise and the concurrent decline in quality and availability of mineral resources, there is an increasing interest in applying biomining technology, in particular for leaching metals from low grade minerals and wastes. However, bioprocessing is often hampered by the presence of inhibitory compounds that originate from complex ores. Synthetic biology could provide tools to improve the tolerance of biomining microbes to various stress factors that are present in biomining environments, which would ultimately increase bioleaching efficiency. This paper reviews the state-of-the-art tools to genetically modify acidophilic biomining microorganisms and the limitations of these tools. The first part of this review discusses resilience pathways that can be engineered in acidophiles to enhance their robustness and tolerance in harsh environments that prevail in bioleaching. The second part of the paper reviews the efforts that have been carried out towards engineering robust microorganisms and developing metabolic modelling tools. Novel synthetic biology tools have the potential to transform the biomining industry and facilitate the extraction of value from ores and wastes that cannot be processed with existing biomining microorganisms.

Glutamate uptake is important for osmoregulation and survival in the rice pathogen Burkholderia glumae

Bacteria exhibit an optimal growth rate in culture media with sufficient nutrients at an optimal temperature and pH. In addition, the concentration of solutes plays a critical role in bacterial growth and survival. Glutamate is known to be a major anionic solute involved in osmoregulation and the bacterial cell's response to changes in solute concentration. To determine how glutamate uptake is involved in osmoregulation in the rice bacterial pathogen Burkholderia glumae BGR1, we mutated the gltI gene encoding a periplasmic substrate binding protein of a glutamate transport system to abolish glutamate uptake, and monitored the growth of the gltI null mutant in Luria-Bertani medium. We found that the gltI null mutant showed a slower growth rate than the wild-type strain and experienced hyperosmotic stress resulting in water loss from the cytoplasm in stationary phase. When the incubation time was extended, the mutant population collapsed due to the hyperosmotic stress. The gltI null mutant exhibited loss of adaptability under both hypoosmotic and hyperosmotic stresses. The growth rate of the gltI null mutant was restored to the level of wild-type growth by exogenous addition of glycine betaine to the culture medium, indicating that glycine betaine is a compatible solute in B. glumae. These results indicate that glutamate uptake from the environment plays a key role in osmoregulation in B. glumae.

Growth arrest and a persister state enable resistance to osmotic shock and facilitate dissemination of Vibrio cholerae

Vibrio cholerae is a water-borne bacterial pathogen and causative agent of cholera. Although V. cholerae is a halophile, it can survive in fresh water, and this has a major role in cholera epidemics through consumption of contaminated water and subsequent fecal-oral spread. After dissemination from humans back into fresh water, V. cholerae encounters limited nutrient availability and an abrupt drop in conductivity but little is known about how V. cholerae adapts to, and survives in this environment. In this work, by abolishing or altering the expression of V. cholerae genes in a high-throughput manner, we observed that many osmotic shock tolerant mutants exhibited slowed or arrested growth, and/or generated a higher proportion of persister cells. In addition, we show that growth-arrested V. cholerae, including a persister subpopulation, are generated during infection of the intestinal tract and together allow for the successful dissemination to fresh water. Our results suggest that growth-arrested and persister subpopulations enable survival of V. cholerae upon shedding to the aquatic environment.

Comparison of the compatible solute pool of two slightly halophilic planctomycetes species, Gimesia maris and Rubinisphaera brasiliensis

Gimesia maris and Rubinisphaera brasiliensis are slightly halophilic representatives of the deep-branching phylum Planctomycetes. For osmoadaptation both species accumulated α-glutamate, sucrose, ectoine and hydroxyectoine. A major role was found for ectoine, hydroxyectoine as well as sucrose under hyper-osmotic shock conditions. Nevertheless, the levels of sucrose were up-regulated by the increased salinity levels and also by low nitrogen availability. Additionally, G. maris accumulated glucosylglycerate (GG) as major solute specifically under low nitrogen levels, which prompted us to analyse the transcript abundance of two homologues genes known for the biosynthesis of GG, namely glucosyl-3-phosphoglycerate synthase (GpgS) and glucosyl-3-phosphoglycerate phosphatase (GpgP). By qPCR using a suitable reference gene selected in this study, the transcript abundance of the biosynthetic genes was quantified in G. maris cells under hyper-osmotic shock or under low nitrogen conditions. The gpgS gene was induced under nitrogen-limiting conditions suggesting that GG synthesis is regulated primarily at the transcription level. Moreover, the expression of a gene coding for a putative sucrose-phosphorylase (Spase) located upstream the gpgS and gpgP genes was up-regulated, predicting a metabolic role of Spase probably related to GG synthesis.

Influence of osmotic stress on desiccation and irradiation tolerance of (hyper)-thermophilic microorganisms

This study examined the influence of prior salt adaptation on the survival rate of (hyper)-thermophilic bacteria and archaea after desiccation and UV or ionizing irradiation treatment. Survival rates after desiccation of Hydrogenothermus marinus and Archaeoglobus fulgidus increased considerably when the cells were cultivated at higher salt concentrations before drying. By doubling the concentration of NaCl, a 30 times higher survival rate of H. marinus after desiccation was observed. Under salt stress, the compatible solute diglycerol phosphate in A. fulgidus and glucosylglycerate in H. marinus accumulated in the cytoplasm. Several different compatible solutes were added as protectants to A. fulgidus and H. marinus before desiccation treatment. Some of these had similar effects as intracellularly produced compatible solutes. The survival rates of H. marinus and A. fulgidus after exposure to UV-C (254 nm) or ionizing X-ray/gamma radiation were irrespective of the salt-induced synthesis or the addition of compatible solutes.

Changes in the Cytoplasmic Composition of Amino Acids and Proteins Observed in Staphylococcus aureus during Growth under Variable Growth Conditions Representative of the Human Wound Site

Staphylococcus aureus is an opportunistic pathogen responsible for a high proportion of nosocomial infections. This study was conducted to assess the bacterial responses in the cytoplasmic composition of amino acids and ribosomal proteins under various environmental conditions designed to mimic those on the human skin or within a wound site: pH6-8, temperature 35–37°C, and additional 0–5% NaCl. It was found that each set of environmental conditions elicited substantial adjustments in cytoplasmic levels of glutamic acid, aspartic acid, proline, alanine and glycine (P< 0.05). These alterations generated characteristic amino acid profiles assessed by principle component analysis (PCA). Substantial alterations in cytoplasmic amino acid and protein composition occurred during growth under conditions of higher salinity stress implemented via additional levels of NaCl in the growth medium. The cells responded to additional NaCl at pH 6 by reducing levels of ribosomal proteins, whereas at pH 8 there was an upregulation of ribosomal proteins compared with the reference control. The levels of two ribosomal proteins, L32 and S19, remained constant across all experimental conditions. The data supported the hypothesis that the bacterium was continually responding to the dynamic environment by modifying the proteome and optimising metabolic homeostasis.

Virulence Program of a Bacterial Plant Pathogen: The Dickeya Model

The pectinolytic Dickeya spp. are Gram-negative bacteria causing severe disease in a wide range of plant species. Although the Dickeya genus was initially restricted to tropical and subtropical areas, two Dickeya species (D. dianthicola and D. solani) emerged recently in potato cultures in Europe. Soft-rot, the visible symptoms, is caused by plant cell wall degrading enzymes, mainly pectate lyases (Pels) that cleave the pectin polymer. However, an efficient colonization of the host requires many additional elements including early factors (eg, flagella, lipopolysaccharide, and exopolysaccharide) that allow adhesion of the bacteria and intermediate factors involved in adaptation to new growth conditions encountered in the host (eg, oxidative stress, iron starvation, and toxic compounds). To facilitate this adaptation, Dickeya have developed complex regulatory networks ensuring appropriate expression of virulence genes. This review presents recent advances in our understanding of the signals and genetic circuits impacting the expression of virulence determinants. Special attention is paid to integrated control of virulence functions by variations in the superhelical density of chromosomal DNA, and the global and specific regulators, making the regulation of Dickeya virulence an especially attractive model for those interested in relationships between the chromosomal dynamics and gene regulatory networks.

Global transcriptional response of Dickeya dadantii to environmental stimuli relevant to the plant infection

Dickeya species are soft rot disease-causing bacterial plant pathogens and an emerging agricultural threat in Europe. Environmental modulation of gene expression is critical for Dickeya dadantii pathogenesis. While the bacterium uses various environmental cues to distinguish between its habitats, an intricate transcriptional control system coordinating the expression of virulence genes ensures efficient infection. Understanding of this behaviour requires a detailed knowledge of expression patterns under a wide range of environmental conditions, which is currently lacking. To obtain a comprehensive picture of this adaptive response, we devised a strategy to examine the D. dadantii transcriptome in a series of 32 infection-relevant conditions encountered in the hosts. We propose a temporal map of the bacterial response to various stress conditions and show that D. dadantii elicits complex genetic behaviour combining common stress-response genes with distinct sets of genes specifically induced under each particular stress. Comparison of our dataset with an in planta expression profile reveals the combined impact of stress factors and enables us to predict the major stress confronting D. dadantii at a particular stage of infection. We provide a comprehensive catalog of D. dadantii genomic responses to environmentally relevant stimuli, thus facilitating future studies of this important plant pathogen.

Glutamate Dehydrogenase Is Required by Mycobacterium bovis BCG for Resistance to Cellular Stress

We recently reported on our success to generate deletion mutants of the genes encoding glutamate dehydrogenase (GDH) and glutamine oxoglutarate aminotransferase (GOGAT) in M. bovis BCG, despite their in vitro essentiality in M. tuberculosis. We could use these mutants to delineate the roles of GDH and GOGAT in mycobacterial nitrogen metabolism by using M. bovis BCG as a model for M. tuberculosis specifically. Here, we extended our investigation towards the involvement of GDH and GOGAT in other aspects of M. bovis BCG physiology, including the use of glutamate as a carbon source and resistance to known phagosomal stresses, as well as in survival inside macrophages. We find that gdh is indispensable for the utilization of glutamate as a major carbon source, in low pH environments and when challenged with nitric oxide. On the other hand, the gltBD mutant had increased viability under low pH conditions and was unaffected by a challenge with nitric oxide. Strikingly, GDH was required to sustain M. bovis BCG during infection of both murine RAW 264.7 and bone-marrow derived and macrophages, while GOGAT was not. We conclude that the catabolism of glutamate in slow growing mycobacteria may be a crucial function during infection of macrophage cells and demonstrate a novel requirement for M. bovis BCG GDH in the protection against acidic and nitrosative stress. These results provide strong clues on the role of GDH in intracellular survival of M. tuberculosis, in which the essentiality of the gdh gene complicates knock out studies making the study of the role of this enzyme in pathogenesis difficult.

A unique glyceryl diglycoside identified in the thermophilic, radiation-resistant bacterium Rubrobacter xylanophilus

The solute pool of the actinobacterium Rubrobacter xylanophilus has been investigated as a function of the growth temperature and concentration of NaCl in the medium (Empadinhas et al. Extremophiles 11: 667-673, 2007). Changing the carbon source from glucose to maltose in a minimal growth medium led to the accumulation of an unknown organic compound whose structure was investigated by NMR and confirmed by chemical synthesis in the present study as: (2R)-2-(1-O-α-D-mannopyranosyl)-3-(1-O-α-D-glucopyranosyl)-D-glycerate (MGlyG). In addition to this newly identified diglycoside, the solute pool of R. xylanophilus included trehalose, mannosylglycerate, di-myo-inositol phosphate and di-N-acetyl-glucosamine phosphate. The structure of MGlyG was established by NMR and confirmed by chemical synthesis. The availability of g-amounts of the synthetic material allowed us to perform stabilization tests on three model enzymes (malate dehydrogenase, staphylococcal nuclease, and lysozyme), and compare the efficacy of MGlyG with other natural glyceryl glycosides, such as α-D-mannosyl-D-glycerate, α-D-glucosyl-D-glycerate and α-D-glucosyl-(1 → 6)-α-D-glucosyl-(1 → 2)-D-glycerate.

Mycobacterium hassiacum recovers from nitrogen starvation with up-regulation of a novel glucosylglycerate hydrolase and depletion of the accumulated glucosylglycerate

Some microorganisms accumulate glucosylglycerate (GG) during growth under nitrogen deprivation. However, the molecular mechanisms underlying the role of GG and the regulation of its levels in the nitrogen stress response are elusive. Since GG is required for biosynthesis of mycobacterial methylglucose lipopolysaccharides (MGLP) we examined the molecular mechanisms linking replenishment of assimilable nitrogen to nitrogen-starved M. hassiacum with depletion of GG accumulated during nitrogen deficiency. To probe the involvement of a newly identified glycoside hydrolase in GG depletion, we produced the mycobacterial enzyme recombinantly and confirmed the specific hydrolysis of GG (GG hydrolase, GgH) in vitro. We have also observed a pronounced up-regulation of GgH mRNA in response to the nitrogen shock, which positively correlates with GG depletion in vivo and growth stimulation, implicating GgH in the recovery process. Since GgH orthologs seem to be absent from most slowly-growing mycobacteria including M. tuberculosis, the disclosure of the GgH function allows reconfiguration of the MGLP pathway in rapidly-growing species and accommodation of this possible regulatory step. This new link between GG metabolism, MGLP biosynthesis and recovery from nitrogen stress furthers our knowledge on the mycobacterial strategies to endure a frequent stress faced in some environments and during long-term infection.

A Unique Pool of Compatible Solutes on Rhodopirellula baltica, Member of the Deep-Branching Phylum Planctomycetes

The intracellular accumulation of small organic solutes was described in the marine bacterium Rhodopirellula baltica, which belongs to the globally distributed phylum Planctomycetes whose members exhibit an intriguing lifestyle and cell morphology. Sucrose, α-glutamate, trehalose and mannosylglucosylglycerate (MGG) are the main solutes involved in the osmoadaptation of R. baltica. The ratio and total intracellular organic solutes varied significantly in response to an increase in salinity, temperature and nitrogen content. R. baltica displayed an initial response to both osmotic and thermal stresses that includes α-glutamate accumulation. This trend was followed by a rather unique and complex osmoadaptation mechanism characterized by a dual response to sub-optimal and supra-optimal salinities. A reduction in the salinity to sub-optimal conditions led primarily to the accumulation of trehalose. In contrast, R. baltica responded to salt stress mostly by increasing the intracellular levels of sucrose. The switch between the accumulation of trehalose and sucrose was by far the most significant effect caused by increasing the salt levels of the medium. Additionally, MGG accumulation was found to be salt- as well as nitrogen-dependent. MGG accumulation was regulated by nitrogen levels replacing α-glutamate as a K⁺ counterion in nitrogen-poor environments. This is the first report of the accumulation of compatible solutes in the phylum Planctomycetes and of the MGG accumulation in a mesophilic organism.

Characterization of NaCl tolerance in Desulfovibrio vulgaris Hildenborough through experimental evolution

Desulfovibrio vulgaris Hildenborough strains with significantly increased tolerance to NaCl were obtained via experimental evolution. A NaCl-evolved strain, ES9-11, isolated from a population cultured for 1200 generations in medium amended with 100 mM NaCl, showed better tolerance to NaCl than a control strain, EC3-10, cultured for 1200 generations in parallel but without NaCl amendment in medium. To understand the NaCl adaptation mechanism in ES9-11, we analyzed the transcriptional, metabolite and phospholipid fatty acid (PLFA) profiles of strain ES9-11 with 0, 100- or 250 mM-added NaCl in medium compared with the ancestral strain and EC3-10 as controls. In all the culture conditions, increased expressions of genes involved in amino-acid synthesis and transport, energy production, cation efflux and decreased expression of flagellar assembly genes were detected in ES9-11. Consistently, increased abundances of organic solutes and decreased cell motility were observed in ES9-11. Glutamate appears to be the most important osmoprotectant in D. vulgaris under NaCl stress, whereas, other organic solutes such as glutamine, glycine and glycine betaine might contribute to NaCl tolerance under low NaCl concentration only. Unsaturation indices of PLFA significantly increased in ES9-11. Branched unsaturated PLFAs i17:1 ω9c, a17:1 ω9c and branched saturated i15:0 might have important roles in maintaining proper membrane fluidity under NaCl stress. Taken together, these data suggest that the accumulation of osmolytes, increased membrane fluidity, decreased cell motility and possibly an increased exclusion of Na(+) contribute to increased NaCl tolerance in NaCl-evolved D. vulgaris.

Role of glutamate metabolism in bacterial responses towards acid and other stresses

Glutamate plays a central role in a wide range of metabolic processes in bacterial cells. This review focuses on the involvement of glutamate in bacterial stress responses. In particular, it reviews the role of glutamate metabolism in response against acid stress and other stresses. The glutamate decarboxylase (GAD) system has been implicated in acid tolerance in several bacterial genera. This system facilitates intracellular pH homoeostasis by consuming protons in a decarboxylation reaction that produces γ-aminobutyrate (GABA) from glutamate. An antiporter system is usually present to couple the uptake of glutamate to the efflux of GABA. Recent insights into the functioning of this system will be discussed. Finally, the intracellular fate of GABA will also be discussed. Many bacteria are capable of metabolizing GABA to succinate via the GABA shunt pathway. The role and regulation of this pathway will be addressed in the review.

Biosynthesis of mycobacterial methylglucose lipopolysaccharides

Mycobacterial pathogenesis is closely associated with a unique cell envelope rich in complex carbohydrates and unique lipids, among which are the mycolic acids. Mycobacteria also synthesize unique intracellular polymethylated polysaccharides (PMPSs), namely methylglucose lipopolysaccharides (MGLPs), which are acylated with short-chain fatty acids, and methylmannose polysaccharides (MMPs). Since PMPSs modulate the synthesis of long-chain fatty acids in vitro, the possibility of a similar role in vivo and the regulation of mycolic acids assembly have been anticipated. Unlike MGLPs, MMPs have been identified in M. smegmatis and other fast-growing mycobacteria but not in M. tuberculosis, implying an essential role for MGLPs in this pathogen and turning the biosynthetic enzymes into attractive drug targets. The genome of M. tuberculosis was decoded 14 years ago but only recently has the identity of the genes involved in MGLPs biosynthesis been investigated. Two gene clusters (Rv1208-Rv1213 and Rv3030-Rv3037c) containing a few genes considered to be essential for M. tuberculosis growth, have initially been proposed to coordinate MGLPs biosynthesis. Among these genes, only the product of Rv1208 for the first step in the MGLPs pathway has, so far, been crystallized and its three-dimensional structure been determined. However, recent results indicate that at least three additional clusters may be involved in this pathway. The functional assignment of authentic roles to some of these M. tuberculosis H37Rv genes sheds new light on the intricacy of MGLPs biogenesis and renewed interest on their biological role.

Mycobacterium tuberculosis Rv2419c, the missing glucosyl-3-phosphoglycerate phosphatase for the second step in methylglucose lipopolysaccharide biosynthesis

Mycobacteria synthesize intracellular methylglucose lipopolysaccharides (MGLP) proposed to regulate fatty acid synthesis. Although their structures have been elucidated, the identity of most biosynthetic genes remains unknown. The first step in MGLP biosynthesis is catalyzed by a glucosyl-3-phosphoglycerate synthase (GpgS, Rv1208 in Mycobacterium tuberculosis H37Rv). However, a typical glucosyl-3-phosphoglycerate phosphatase (GpgP, EC3.1.3.70) for dephosphorylation of glucosyl-3-phosphoglycerate to glucosylglycerate, was absent from mycobacterial genomes. We purified the native GpgP from Mycobacterium vanbaalenii and identified the corresponding gene deduced from amino acid sequences by mass spectrometry. The M. tuberculosis ortholog (Rv2419c), annotated as a putative phosphoglycerate mutase (PGM, EC5.4.2.1), was expressed and functionally characterized as a new GpgP. Regardless of the high specificity for glucosyl-3-phosphoglycerate, the mycobacterial GpgP is not a sequence homolog of known isofunctional GpgPs. The assignment of a new function in M. tuberculosis genome expands our understanding of this organism's genetic repertoire and of the early events in MGLP biosynthesis.

Diversity, biological roles and biosynthetic pathways for sugar-glycerate containing compatible solutes in bacteria and archaea

A decade ago the compatible solutes mannosylglycerate (MG) and glucosylglycerate (GG) were considered to be rare in nature. Apart from two species of thermophilic bacteria, Thermus thermophilus and Rhodothermus marinus, and a restricted group of hyperthermophilic archaea, the Thermococcales, MG had only been identified in a few red algae. Glucosylglycerate was considered to be even rarer and had only been detected as an insignificant solute in two halophilic microorganisms, a cyanobacterium, as a component of a polysaccharide and of a glycolipid in two actinobacteria. Unlike the hyper/thermophilic MG-accumulating microorganisms, branching close to the root of the Tree of Life, those harbouring GG shared a mesophilic lifestyle. Exceptionally, the thermophilic bacterium Persephonella marina was reported to accumulate GG. However, and especially owing to the identification of the key-genes for MG and GG synthesis and to the escalating numbers of genomes available, a plethora of new organisms with the resources to synthesize these solutes has been recognized. The accumulation of GG as an 'emergency' compatible solute under combined salt stress and nitrogen-deficient conditions now seems to be a disseminated survival strategy from enterobacteria to marine cyanobacteria. In contrast, the thermophilic and extremely radiation-resistant bacterium Rubrobacter xylanophilus is the only actinobacterium known to accumulate MG, and under all growth conditions tested. This review addresses the environmental factors underlying the accumulation of MG, GG and derivatives in bacteria and archaea and their roles during stress adaptation or as precursors for more elaborated macromolecules. The diversity of pathways for MG and GG synthesis as well as those for some of their derivatives is also discussed. The importance of glycerate-derived organic solutes in the microbial world is only now being recognized. Their stress-dependent accumulation and the molecular aspects of their interactions with biomolecules have already fuelled several emerging applications in biotechnology and biomedicine.

Compatible solute biosynthesis in cyanobacteria

Compatible solutes are a functional group of small, highly soluble organic molecules that demonstrate compatibility in high amounts with cellular metabolism. The accumulation of compatible solutes is often observed during the acclimation of organisms to adverse environmental conditions, particularly to salt and drought stress. Among cyanobacteria, sucrose, trehalose, glucosylglycerol and glycine betaine are used as major compatible solutes. Interestingly, a close correlation has been discovered between the final salt tolerance limit and the primary compatible solute in these organisms. In addition to the dominant compatible solutes, many strains accumulate mixtures of these compounds, including minor compounds such as glucosylglycerate or proline as secondary or tertiary solutes. In particular, the accumulation of sucrose and trehalose results in an increase in tolerance to general stresses such as desiccation and high temperatures. During recent years, the biochemical and molecular basis of compatible solute accumulation has been characterized using cyanobacterial model strains that comprise different salt tolerance groups. Based on these data, the distribution of genes involved in compatible solute synthesis among sequenced cyanobacterial genomes is reviewed, and thereby, the major compatible solutes and potential salt tolerance of these strains can be predicted. Knowledge regarding cyanobacterial salt tolerance is not only useful to characterize strain-specific adaptations to ecological niches, but it can also be used to generate cells with increased tolerance to adverse environmental conditions for biotechnological purposes.

Functional and structural characterization of a novel mannosyl-3-phosphoglycerate synthase from Rubrobacter xylanophilus reveals its dual substrate specificity

Rubrobacter xylanophilus is the only actinobacterium known to accumulate the organic solute mannosylglycerate (MG); moreover, the accumulation of MG is constitutive. The key enzyme for MG synthesis, catalysing the conversion of GDP-mannose (GDP-Man) and D-3-phosphoglycerate (3-PGA) into the phosphorylated intermediate mannosyl-3-phosphoglycerate and GDP, was purified from R. xylanophilus cell extracts and the corresponding gene was expressed in E. coli. Despite the related solute glucosylglycerate (GG) having never been detected in R. xylanophilus, the cell extracts and the pure recombinant mannosyl-3-phosphoglycerate synthase (MpgS) could also synthesize glucosyl-3-phosphoglycerate (GPG), the precursor of GG, in agreement with the higher homology of the novel MpgS towards GPG-synthesizing mycobacterial glucosyl-3-phosphoglycerate synthases (GpgS) than towards MpgSs from hyper/thermophiles, known to accumulate MG under salt or thermal stress. To understand the specificity and substrate ambiguity of this novel enzyme, we determined the crystal structure of the unliganded MpgS and of its complexes with the nucleotide and sugar donors, at 2.2, 2.8 and 2.5 Å resolution respectively. The first three-dimensional structures of a protein from this extremely gamma-radiation-resistant thermophile here reported show that MpgS (GT81 family) contains a GT-A like fold and clearly explain its nucleotide and sugar-donor specificity. In the GDP-Man complex, a flexible loop ((254) RQNRHQ(259) ), located close to the active site moves towards the incoming sugar moiety, providing the ligands for both magnesium ion co-ordination and sugar binding. A triple mutant of R. xylanophilus MpgS, mimicking the (206) PLAGE(210) loop stabilizing hydrogen bond network observed for mycobacterial GpgSs, reduces significantly the affinity to GDP-Man, implicating this loop in the sugar-donor discrimination.

Expressed sequence tag (EST) profiling in hyper saline shocked Dunaliella salina reveals high expression of protein synthetic apparatus components

The unicellular, halotolerant, green alga, Dunaliella salina (Chlorophyceae) has the unique ability to adapt and grow in a wide range of salt conditions from about 0.05 to 5.5M. To better understand the molecular basis of its salinity tolerance, a complementary DNA (cDNA) library was constructed from D. salina cells adapted to 2.5M NaCl, salt-shocked at 3.4M NaCl for 5h, and used to generate an expressed sequence tag (EST) database. ESTs were obtained for 2831 clones representing 1401 unique transcripts. Putative functions were assigned to 1901 (67.2%) ESTs after comparison with protein databases. An additional 154 (5.4%) ESTs had significant similarity to known sequences whose functions are unclear and 776 (27.4%) had no similarity to known sequences. For those D. salina ESTs for which functional assignments could be made, the largest functional categories included protein synthesis (35.7%), energy (photosynthesis) (21.4%), primary metabolism (13.8%) and protein fate (6.8%). Within the protein synthesis category, the vast majority of ESTs (80.3%) encoded ribosomal proteins representing about 95% of the approximately 82 subunits of the cytosolic ribosome indicating that D. salina invests substantial resources in the production and maintenance of protein synthesis. The increased mRNA expression upon salinity shock was verified for a small set of selected genes by real-time, quantitative reverse-transcription-polymerase chain reaction (qRT-PCR). This EST collection also provided important new insights into the genetic underpinnings for the biosynthesis and utilization of glycerol and other osmoprotectants, the carotenoid biosynthetic pathway, reactive oxygen-scavenging enzymes, and molecular chaperones (heat shock proteins) not described previously for D. salina. EST discovery also revealed the existence of RNA interference and signaling pathways associated with osmotic stress adaptation. The unknown ESTs described here provide a rich resource for the identification of novel genes associated with the mechanistic basis of salinity stress tolerance and other stress-adaptive traits.

Two alternative pathways for the synthesis of the rare compatible solute mannosylglucosylglycerate in Petrotoga mobilis

The compatible solute mannosylglucosylglycerate (MGG), recently identified in Petrotoga miotherma, also accumulates in Petrotoga mobilis in response to hyperosmotic conditions and supraoptimal growth temperatures. Two functionally connected genes encoding a glucosyl-3-phosphoglycerate synthase (GpgS) and an unknown glycosyltransferase (gene Pmob_1143), which we functionally characterized as a mannosylglucosyl-3-phosphoglycerate synthase and designated MggA, were identified in the genome of Ptg. mobilis. This enzyme used the product of GpgS, glucosyl-3-phosphoglycerate (GPG), as well as GDP-mannose to produce mannosylglucosyl-3-phosphoglycerate (MGPG), the phosphorylated precursor of MGG. The MGPG dephosphorylation was determined in cell extracts, and the native enzyme was partially purified and characterized. Surprisingly, a gene encoding a putative glucosylglycerate synthase (Ggs) was also identified in the genome of Ptg. mobilis, and an active Ggs capable of producing glucosylglycerate (GG) from ADP-glucose and d-glycerate was detected in cell extracts and the recombinant enzyme was characterized, as well. Since GG has never been identified in this organism nor was it a substrate for the MggA, we anticipated the existence of a nonphosphorylating pathway for MGG synthesis. We putatively identified the corresponding gene, whose product had some sequence homology with MggA, but it was not possible to recombinantly express a functional enzyme from Ptg. mobilis, which we named mannosylglucosylglycerate synthase (MggS). In turn, a homologous gene from Thermotoga maritima was successfully expressed, and the synthesis of MGG was confirmed from GDP-mannose and GG. Based on the measurements of the relevant enzyme activities in cell extracts and on the functional characterization of the key enzymes, we propose two alternative pathways for the synthesis of the rare compatible solute MGG in Ptg. mobilis.

Global transcriptional, physiological, and metabolite analyses of the responses of Desulfovibrio vulgaris hildenborough to salt adaptation

The response of Desulfovibrio vulgaris Hildenborough to salt adaptation (long-term NaCl exposure) was examined by performing physiological, global transcriptional, and metabolite analyses. Salt adaptation was reflected by increased expression of genes involved in amino acid biosynthesis and transport, electron transfer, hydrogen oxidation, and general stress responses (e.g., heat shock proteins, phage shock proteins, and oxidative stress response proteins). The expression of genes involved in carbon metabolism, cell growth, and phage structures was decreased. Transcriptome profiles of D. vulgaris responses to salt adaptation were compared with transcriptome profiles of D. vulgaris responses to salt shock (short-term NaCl exposure). Metabolite assays showed that glutamate and alanine accumulated under salt adaptation conditions, suggesting that these amino acids may be used as osmoprotectants in D. vulgaris. Addition of amino acids (glutamate, alanine, and tryptophan) or yeast extract to the growth medium relieved salt-related growth inhibition. A conceptual model that links the observed results to currently available knowledge is proposed to increase our understanding of the mechanisms of D. vulgaris adaptation to elevated NaCl levels.

Osmosensitive response of glutamate in the substantia nigra

Previous studies have suggested the increase of extracellular glutamate (GLU) in the substantia nigra (SN) as a cause of dopamine-cell degeneration (excitotoxicity) in Parkinson's disease (PD). However, the mechanisms involved in this increase remain unknown. The present work studied osmoregulation as a cause of GLU release in the SN. Microdialysis was used to change extracellular osmolarity, to administer drugs and to quantify the extracellular non-synaptic GLU (EnS-GLU). Two osmolarity modifications were performed, a moderate decrease (5%) resembling physiological modifications and a substantial decrease (>or=20% decrease) similar to that observed under pathological conditions. Hypo-osmolarity induced a dose-response (285-80 mOsm) increase of EnS-GLU which was detected after small osmolarity modifications (15 mOsm) and which was very marked (>1000%) after more intense osmolarity changes. This response disappeared after pre-treating rats with a P2 purinergic-receptor antagonist (pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid; 1 mM) suggesting ATP involvement in the osmosensitive EnS-GLU response. The EnS-GLU increase observed after administration of ATP (0.1-100 microM) and 2-methylthioadenosine triphosphate tetrasodium (P2-receptor agonist; 100 microM) and the lack of effects of adenosine administration (1 mM) suggest that the ATP action on P2 receptors is an amplificatory mechanism in the osmosensitive EnS-GLU response. The marked action of osmolarity on extracellular Glu suggests osmolarity regulation as a possible source for excitotoxicity in the SN.

Single-step enzymatic synthesis of (R)-2-O-alpha-D-glucopyranosyl glycerate, a compatible solute from micro-organisms that functions as a protein stabiliser

Regioselective glucosylation of R-glycerate catalysed by sucrose phosphorylase in the presence of sucrose as the donor substrate provided the natural compatible solute (R)-2-O-alpha-D-glucopyranosyl glycerate with complete regioselectivity in an optimised synthetic yield of 90% R-glycerate converted and a concentration of about 270 mM.

Crystallization and preliminary crystallographic analysis of mannosyl-3-phosphoglycerate synthase from Rubrobacter xylanophilus

Rubrobacter xylanophilus is the only Gram-positive bacterium known to synthesize the compatible solute mannosylglycerate (MG), which is commonly found in hyperthermophilic archaea and some thermophilic bacteria. Unlike the salt-dependent pattern of accumulation observed in (hyper)thermophiles, in R. xylanophilus MG accumulates constitutively. The synthesis of MG in R. xylanophilus was tracked from GDP-mannose and 3-phosphoglycerate, but the genome sequence of the organism failed to reveal any of the genes known to be involved in this pathway. The native enzyme was purified and its N-terminal sequence was used to identify the corresponding gene (mpgS) in the genome of R. xylanophilus. The gene encodes a highly divergent mannosyl-3-phosphoglycerate synthase (MpgS) without relevant sequence homology to known mannosylphosphoglycerate synthases. In order to understand the specificity and enzymatic mechanism of this novel enzyme, it was expressed in Escherichia coli, purified and crystallized. The crystals thus obtained belonged to the hexagonal space group P6(5)22 and contained two protein molecules per asymmetric unit. The structure was solved by SIRAS using a mercury derivative.

Alpha-D-mannopyranosyl-(1-->2)-alpha-D-glucopyranosyl-(1-->2)-glycerate in the thermophilic bacterium Petrotoga miotherma--structure, cellular content and function

The intracellular accumulation of low molecular mass organic compounds in response to stressful conditions was investigated in the thermophilic bacterium Petrotoga miotherma, a member of the order Thermotogales. This led to the discovery of a new solute, whose structure was established as alpha-D-mannopyranosyl-(1-->2)-alpha-D-glucopyranosyl-(1-->2)-glycerate (MGG) by MMR spectroscopy and MS. Under optimum growth conditions (3% NaCl; 55 degrees C), MGG was the major solute [up to 0.6 micromol.(mg protein)(-1)]; alpha-glutamate and proline were also present but in minor amounts [below 0.08 micromol.(mg protein)(-1)]. The level of MGG increased notably with the salinity of the growth medium up to the optimum NaCl concentration. At higher NaCl concentrations, however, the level of MGG decreased, whereas the levels of proline and alpha-glutamate increased about five-fold and 10-fold, respectively. MGG plays a role during low-level osmotic adaptation of Petrotoga miotherma, whereas alpha-glutamate and, to a lesser extent, proline are used for osmoprotection under salt stress. MGG is not part of the cell strategy for coping with heat or oxidative stress. Nevertheless, MGG was an efficient protector of pig heart malate dehydrogenase against heat inactivation and freeze-drying, although mannosylglycerate was better. This is the first report on the occurrence of MGG in living systems.

Single-step pathway for synthesis of glucosylglycerate in Persephonella marina

A single-step pathway for the synthesis of the compatible solute glucosylglycerate (GG) is proposed based on the activity of a recombinant glucosylglycerate synthase (Ggs) from Persephonella marina. The corresponding gene encoded a putative glycosyltransferase that was part of an operon-like structure which also contained the genes for glucosyl-3-phosphoglycerate synthase (GpgS) and glucosyl-3-phosphoglycerate phosphatase (GpgP), the enzymes that lead to the synthesis of GG through the formation of glucosyl-3-phosphoglycerate. The putative glucosyltransferase gene was expressed in Escherichia coli, and the recombinant product catalyzed the synthesis of GG in one step from ADP-glucose and d-glycerate, with K(m) values at 70 degrees C of 1.5 and 2.2 mM, respectively. This glucosylglycerate synthase (Ggs) was also able to use GDP- and UDP-glucose as donors to form GG, but the efficiencies were lower. Maximal activity was observed at temperatures between 80 and 85 degrees C, and Mg(2+) or Ca(2+) was required for catalysis. Ggs activity was maximal and remained nearly constant at pH values between 5.5 and pH 8.0, and the half-lives for inactivation were 74 h at 85 degrees C and 8 min at 100 degrees C. This is the first report of an enzyme catalyzing the synthesis of GG in one step and of the existence of two pathways for GG synthesis in the same organism.

Substantia nigra osmoregulation: taurine and ATP involvement

An extracellular nonsynaptic taurine pool of glial origin was recently reported in the substantia nigra (SN). There is previous evidence showing taurine as an inhibitory neurotransmitter in the SN, but the physiological role of this nonsynaptic pool of taurine has not been explored. By using microdialysis methods, we studied the action of local osmolarity on the nonsynaptic taurine pool in the SN of the rat. Hypoosmolar pulses (285-80 mosM) administered in the SN by the microdialysis probe increased extrasynaptic taurine in a dose-dependent way, a response that was counteracted by compensating osmolarity with choline. The opposite effect (taurine decrease) was observed when osmolarity was increased. Under basal conditions, the blockade of either the AMPA-kainate glutamate receptors with 6-cyano-7-nitroquinoxaline-2,3-dionine disodium or the purinergic receptors with pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid modified the taurine concentration, suggesting that both receptors modulate the extrasynaptic pool of taurine. In addition, these drugs decreased the taurine response to hypoosmolar pulses, suggesting roles for glutamatergic and purinergic receptors in the taurine response to osmolarity. The participation of purinergic receptors was also supported by the fact that ATP (which, under basal conditions, increased the extrasynaptic taurine in a dose-dependent way) administered in doses saturating purinergic receptors also decreased the taurine response to hypoosmolarity. Taken together, present data suggest osmoregulation as a role of the nonsynaptic taurine pool of the SN, a function that also involves glutamate and ATP and that could influence the nigral cell vulnerability in Parkinson's disease.