A gene from the mesophilic bacterium Dehalococcoides ethenogenes encodes a novel mannosylglycerate synthase

Exploring the Potential of Corynebacterium glutamicum to Produce the Compatible Solute Mannosylglycerate

The compatible solute mannosylglycerate (MG) has exceptional properties in terms of protein stabilization and protection under salt, heat, and freeze-drying stresses as well as against protein aggregation. Due to these characteristics, MG possesses large potential for clinical and biotechnological applications. To achieve efficient MG production, Corynebacterium glutamicum was equipped with a bifunctional MG synthase (encoded by mgsD and catalyzing the condensation of 3-phosphoglycerate and GDP-mannose to MG) from Dehalococcoides mccartyi. The resulting strain C. glutamicum (pEKEx3 mgsD) intracellularly accumulated about 111 mM MG (60 ± 9 mg gCDW -1) with 2% glucose as a carbon source. To enable efficient mannose metabolization, the native manA gene, encoding mannose 6-phosphate isomerase, was overexpressed. Combined overexpression of manA and mgsD from two plasmids in C. glutamicum resulted in intracellular MG accumulation of up to ca. 329 mM [corresponding to 177 mg g cell dry weight (CDW) -1] with glucose, 314 mM (168 mg gCDW -1) with glucose plus mannose, and 328 mM (176 mg gCDW -1) with mannose as carbon source(s), respectively. The product was successfully extracted from cells by using a cold water shock, resulting in up to 5.5 mM MG (1.48 g L-1) in supernatants. The two-plasmid system was improved by integrating the mgsD gene into the manA-bearing plasmid and the resulting strain showed comparable production but faster growth. Repeated cycles of growth/production and extraction of MG in a bacterial milking-like experiment showed that cells could be recycled, which led to a cumulative MG production of 19.9 mM (5.34 g L-1). The results show that the newly constructed C. glutamicum strain produces MG from glucose and mannose and that a cold water shock enables extraction of MG from the cytosol into the medium.

Production of mannosylglycerate in Saccharomyces cerevisiae by metabolic engineering and bioprocess optimization

BACKGROUND

Mannosylglycerate (MG) is one of the most widespread compatible solutes among marine microorganisms adapted to hot environments. This ionic solute holds excellent ability to protect proteins against thermal denaturation, hence a large number of biotechnological and clinical applications have been put forward. However, the current prohibitive production costs impose severe constraints towards large-scale applications. All known microbial producers synthesize MG from GDP-mannose and 3-phosphoglycerate via a two-step pathway in which mannosyl-3-phosphoglycerate is the intermediate metabolite. In an early work, this pathway was expressed in Saccharomyces cerevisiae with the goal to confirm gene function (Empadinhas et al. in J Bacteriol 186:4075-4084, 2004), but the level of MG accumulation was low. Therefore, in view of the potential biotechnological value of this compound, we decided to invest further effort to convert S. cerevisiae into an efficient cell factory for MG production.

RESULTS

To drive MG production, the pathway for the synthesis of GDP-mannose, one of the MG biosynthetic precursors, was overexpressed in S. cerevisiae along with the MG biosynthetic pathway. MG production was evaluated under different cultivation modes, i.e., flask bottle, batch, and continuous mode with different dilution rates. The genes encoding mannose-6-phosphate isomerase (PMI40) and GDP-mannose pyrophosphorylase (PSA1) were introduced into strain MG01, hosting a plasmid encoding the MG biosynthetic machinery. The resulting engineered strain (MG02) showed around a twofold increase in the activity of PMI40 and PSA1 in comparison to the wild-type. In batch mode, strain MG02 accumulated 15.86 mg_MG g _DCW ^-1 , representing a 2.2-fold increase relative to the reference strain (MG01). In continuous culture, at a dilution rate of 0.15 h^-1, there was a 1.5-fold improvement in productivity.

CONCLUSION

In the present study, the yield and productivity of MG were increased by overexpression of the GDP-mannose pathway and optimization of the mode of cultivation. A maximum of 15.86 mg_MG g _DCW ^-1 was achieved in batch cultivation and maximal productivity of 1.79 mg_MG g _DCW ^-1  h^-1 in continuous mode. Additionally, a positive correlation between MG productivity and growth rate/dilution rate was established, although this correlation is not observed for MG yield.

Effect of engineered environment on microbial community structure in biofilter and biofilm on reverse osmosis membrane

Four dual media filters (DMFs) were operated in a biofiltration mode with different engineered environments (DMF I and II: coagulation with/without acidification and DMF III and IV: without/with chlorination). Designed biofilm enrichment reactors (BERs) containing the removable reverse osmosis (RO) coupons, were connected at the end of the DMFs in parallel to analyze the biofilm on the RO membrane by DMF effluents. Filtration performances were evaluated in terms of dissolved organic carbon (DOC) and assimilable organic carbon (AOC). Organic foulants on the RO membrane were also quantified and fractionized. The bacterial community structures in liquid (seawater and effluent) and biofilm (DMF and RO) samples were analyzed using 454-pyrosequencing. The DMF IV fed with the chlorinated seawater demonstrated the highest reductions of DOC including LMW-N as well as AOC among the other DMFs. The DMF IV was also effective in reducing organic foulants on the RO membrane surface. The bacterial community structure was grouped according to the sample phase (i.e., liquid and biofilm samples), sampling location (i.e., DMF and RO samples), and chlorination (chlorinated and non-chlorinated samples). In particular, the biofilm community in the DMF IV differed from the other DMF treatments, suggesting that chlorination exerted as stronger selective pressure than pH adjustment or coagulation on the biofilm community. In the DMF IV, several chemoorganotrophic chlorine-resistant biofilm-forming bacteria such as Hyphomonas, Erythrobacter, and Sphingomonas were predominant, and they may enhance organic carbon degradation efficiency. Diverse halophilic or halotolerant organic degraders were also found in other DMFs (i.e., DMF I, II, and III). Various kinds of dominant biofilm-forming bacteria were also investigated in RO membrane samples; the results provided possible candidates that cause biofouling when DMF process is applied as the pretreatment option for the RO process.

Genome sequence of the organohalide-respiring Dehalogenimonas alkenigignens type strain (IP3-3(T))

Dehalogenimonas alkenigignens IP3-3^T is a strictly anaerobic, mesophilic, Gram negative staining bacterium that grows by organohalide respiration, coupling the oxidation of H₂ to the reductive dehalogenation of polychlorinated alkanes. Growth has not been observed with any non-polyhalogenated alkane electron acceptors. Here we describe the features of strain IP3-3^T together with genome sequence information and its annotation. The 1,849,792 bp high-quality-draft genome contains 1936 predicted protein coding genes, 47 tRNA genes, a single large subunit rRNA (23S-5S) locus, and a single, orphan, small unit rRNA (16S) locus. The genome contains 29 predicted reductive dehalogenase genes, a large majority of which lack cognate genes encoding membrane anchoring proteins.

Experimental validation of in silico model-predicted isocitrate dehydrogenase and phosphomannose isomerase from Dehalococcoides mccartyi

Gene sequences annotated as proteins of unknown or non‐specific function and hypothetical proteins account for a large fraction of most genomes. In the strictly anaerobic and organohalide respiring Dehalococcoides mccartyi, this lack of annotation plagues almost half the genome. Using a combination of bioinformatics analyses and genome‐wide metabolic modelling, new or more specific annotations were proposed for about 80 of these poorly annotated genes in previous investigations of D. mccartyi metabolism. Herein, we report the experimental validation of the proposed reannotations for two such genes (KB1_0495 and KB1_0553) from D. mccartyi strains in the KB‐1 community. KB1_0495 or DmIDH was originally annotated as an NAD⁺‐dependent isocitrate dehydrogenase, but biochemical assays revealed its activity primarily with NADP⁺ as a cofactor. KB1_0553, also denoted as DmPMI, was originally annotated as a hypothetical protein/sugar isomerase domain protein. We previously proposed that it was a bifunctional phosphoglucose isomerase/phosphomannose isomerase, but only phosphomannose isomerase activity was identified and confirmed experimentally. Further bioinformatics analyses of these two protein sequences suggest their affiliation to potentially novel enzyme families within their respective larger enzyme super families.

Mannosylglycerate: structural analysis of biosynthesis and evolutionary history

Halophilic and halotolerant microorganisms adapted to thrive in hot environments accumulate compatible solutes that usually have a negative charge either associated with a carboxylic group or a phosphodiester unit. Mannosylglycerate (MG) has been detected in several members of (hyper)thermophilic bacteria and archaea, in which it responds primarily to osmotic stress. The outstanding ability of MG to stabilize protein structure in vitro as well as in vivo has been convincingly demonstrated. These findings led to an increasingly supported link between MG and microbial adaptation to high temperature. However, the accumulation of MG in many red algae has been known for a long time, and the peculiar distribution of MG in such distant lineages was intriguing. Knowledge on the biosynthetic machinery together with the rapid expansion of genome databases allowed for structural and phylogenetic analyses and provided insight into the distribution of MG. The two pathways for MG synthesis have distinct evolutionary histories and physiological roles: in red algae MG is synthesised exclusively via the single-step pathway and most probably is unrelated with stress protection. In contrast, the two-step pathway is strongly associated with osmoadaptation in (hyper)thermophilic prokaryotes. The phylogenetic analysis of the two-step pathway also reveals a second cluster composed of fungi and mesophilic bacteria, but MG has not been demonstrated in members of this cluster; we propose that the synthase is part of a more complex pathway directed at the synthesis of yet unknown molecules containing the mannosyl-glyceryl unit.

New insights into Dehalococcoides mccartyi metabolism from a reconstructed metabolic network-based systems-level analysis of D. mccartyi transcriptomes

Organohalide respiration, mediated by Dehalococcoides mccartyi, is a useful bioremediation process that transforms ground water pollutants and known human carcinogens such as trichloroethene and vinyl chloride into benign ethenes. Successful application of this process depends on the fundamental understanding of the respiration and metabolism of D. mccartyi. Reductive dehalogenases, encoded by rdhA genes of these anaerobic bacteria, exclusively catalyze organohalide respiration and drive metabolism. To better elucidate D. mccartyi metabolism and physiology, we analyzed available transcriptomic data for a pure isolate (Dehalococcoides mccartyi strain 195) and a mixed microbial consortium (KB-1) using the previously developed pan-genome-scale reconstructed metabolic network of D. mccartyi. The transcriptomic data, together with available proteomic data helped confirm transcription and expression of the majority genes in D. mccartyi genomes. A composite genome of two highly similar D. mccartyi strains (KB-1 Dhc) from the KB-1 metagenome sequence was constructed, and operon prediction was conducted for this composite genome and other single genomes. This operon analysis, together with the quality threshold clustering analysis of transcriptomic data helped generate experimentally testable hypotheses regarding the function of a number of hypothetical proteins and the poorly understood mechanism of energy conservation in D. mccartyi. We also identified functionally enriched important clusters (13 for strain 195 and 11 for KB-1 Dhc) of co-expressed metabolic genes using information from the reconstructed metabolic network. This analysis highlighted some metabolic genes and processes, including lipid metabolism, energy metabolism, and transport that potentially play important roles in organohalide respiration. Overall, this study shows the importance of an organism's metabolic reconstruction in analyzing various "omics" data to obtain improved understanding of the metabolism and physiology of the organism.

Genome sequencing of a single cell of the widely distributed marine subsurface Dehalococcoidia, phylum Chloroflexi

Bacteria of the class Dehalococcoidia (DEH), phylum Chloroflexi, are widely distributed in the marine subsurface, yet metabolic properties of the many uncultivated lineages are completely unknown. This study therefore analysed genomic content from a single DEH cell designated 'DEH-J10' obtained from the sediments of Aarhus Bay, Denmark. Real-time PCR showed the DEH-J10 phylotype was abundant in upper sediments but was absent below 160 cm below sea floor. A 1.44 Mbp assembly was obtained and was estimated to represent up to 60.8% of the full genome. The predicted genome is much larger than genomes of cultivated DEH and appears to confer metabolic versatility. Numerous genes encoding enzymes of core and auxiliary beta-oxidation pathways were identified, suggesting that this organism is capable of oxidising various fatty acids and/or structurally related substrates. Additional substrate versatility was indicated by genes, which may enable the bacterium to oxidise aromatic compounds. Genes encoding enzymes of the reductive acetyl-CoA pathway were identified, which may also enable the fixation of CO2 or oxidation of organics completely to CO2. Genes encoding a putative dimethylsulphoxide reductase were the only evidence for a respiratory terminal reductase. No evidence for reductive dehalogenase genes was found. Genetic evidence also suggests that the organism could synthesise ATP by converting acetyl-CoA to acetate by substrate-level phosphorylation. Other encoded enzymes putatively conferring marine adaptations such as salt tolerance and organo-sulphate sulfohydrolysis were identified. Together, these analyses provide the first insights into the potential metabolic traits that may enable members of the DEH to occupy an ecological niche in marine sediments.

Organic cofactors in the metabolism of Dehalococcoides mccartyi strains

Dehalococcoides mccartyi strains are strictly anaerobic organisms specialized to grow with halogenated compounds as electron acceptor via a respiratory process. Their genomes are among the smallest known for free-living organisms, and the embedded gene set reflects their strong specialization. Here, we briefly review main characteristics of published Dehalococcoides genomes and show how genome information together with cultivation and biochemical experiments have contributed to our understanding of Dehalococcoides physiology and biochemistry. We extend this approach by the detailed analysis of cofactor metabolism in Dehalococcoides strain CBDB1. Dehalococcoides genomes were screened for encoded proteins annotated to contain or interact with organic cofactors, and the expression of these proteins was analysed by shotgun proteomics to shed light on cofactor requirements. In parallel, cultivation experiments testing for vitamin requirements showed that cyanocobalamin (vitamin B12), thiamine and biotin were essential supplements and that cyanocobalamin could be substituted by dicyanocobinamide and dimethylbenzimidazole. Dehalococcoides genome analysis, detection of single enzymes by shotgun proteomics and inhibition studies confirmed the expression of the biosynthetic pathways for pyridoxal-5-phosphate, flavin nucleotides, folate, S-adenosylmethionine, pantothenate and nicotinic acids in strain CBDB1. Haem/cytochromes, quinones and lipoic acids were not necessary for cultivation or dechlorination activity and no biosynthetic pathways were identified in the genomes.

Complete genome sequence of Dehalogenimonas lykanthroporepellens type strain (BL-DC-9(T)) and comparison to "Dehalococcoides" strains

Dehalogenimonas lykanthroporepellens is the type species of the genus Dehalogenimonas, which belongs to a deeply branching lineage within the phylum Chloroflexi. This strictly anaerobic, mesophilic, non spore-forming, Gram-negative staining bacterium was first isolated from chlorinated solvent contaminated groundwater at a Superfund site located near Baton Rouge, Louisiana, USA. D. lykanthroporepellens was of interest for genome sequencing for two reasons: (a) an unusual ability to couple growth with reductive dechlorination of environmentally important polychlorinated aliphatic alkanes and (b) a phylogenetic position that is distant from previously sequenced bacteria. The 1,686,510 bp circular chromosome of strain BL-DC-9(T) contains 1,720 predicted protein coding genes, 47 tRNA genes, a single large subunit rRNA (23S-5S) locus, and a single, orphan, small subunit rRNA (16S) locus.

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.

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.

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.

Effects of the nonionic surfactant tween 80 on microbial reductive dechlorination of chlorinated ethenes

Recent field studies have indicated synergistic effects of coupling microbial reductive dechlorination with physicochemical remediation (e.g., surfactant flushing) of dense nonaqueous phase liquid (DNAPL) source zones. This study explored chlorinated ethene (e.g., tetrachloroethene [PCE]) dechlorination in the presence of 50-5000 mg/L Tween 80, a nonionic surfactant employed in source zone remediation. Tween 80 did not inhibit dechlorination by four pure PCE-to-cis-1,2-dichloroethene (cis-DCE) or PCE-to-trichloroethene (TCE) dechlorinating cultures. In contrast, cis-DCE-dechlorinating Dehalococcoides isolates (strain BAV1 and strain FL2) failed to dechlorinate in the presence of Tween 80. Bio-Dechlor INOCULUM (BDI), a PCE-to-ethene dechlorinating consortium, produced cis-DCE in the presence of Tween 80, further suggesting that Tween 80 inhibits dechlorination by Dehalococcoides organisms. Quantitative real-time PCR analysis applied to BDI revealed that the number of Dehalococcoides cells decayed exponentially (R(2) = 0.85) according to the Chick-Watson disinfection model (pseudo first-order decay rate of 0.13+/-0.02 day(-1)) from an initial value of 6.6 +/-1.5 x 10(8) to 1.3+/-0.8 x 10(5) per mL of culture after 58 days of exposure to 250 mg/L Tween 80. Although Tween 80 exposure prevented ethene formation and reduced Dehalococcoides cell numbers, Dehalococcoides organisms remained viable, and dechlorination activity pist cis-DCE was recovered following the removal of Tween 80. These findings suggest that sequential Tween 80 flushing followed by microbial reductive dechlorination is a promising strategy for remediation of chlorinated ethene-impacted source zones.

Glucosylglycerate biosynthesis in the deepest lineage of the Bacteria: characterization of the thermophilic proteins GpgS and GpgP from Persephonella marina

The pathway for the synthesis of glucosylglycerate (GG) in the thermophilic bacterium Persephonella marina is proposed based on the activities of recombinant glucosyl-3-phosphoglycerate (GPG) synthase (GpgS) and glucosyl-3-phosphoglycerate phosphatase (GpgP). The sequences of gpgS and gpgP from the cold-adapted bacterium Methanococcoides burtonii were used to identify the homologues in the genome of P. marina, which were separately cloned and overexpressed as His-tagged proteins in Escherichia coli. The recombinant GpgS protein of P. marina, unlike the homologue from M. burtonii, which was specific for GDP-glucose, catalyzed the synthesis of GPG from UDP-glucose, GDP-glucose, ADP-glucose, and TDP-glucose (in order of decreasing efficiency) and from d-3-phosphoglycerate, with maximal activity at 90 degrees C. The recombinant GpgP protein, like the M. burtonii homologue, dephosphorylated GPG and mannosyl-3-phosphoglycerate (MPG) to GG and mannosylglycerate, respectively, yet at high temperatures the hydrolysis of GPG was more efficient than that of MPG. Gel filtration indicates that GpgS is a dimeric protein, while GpgP is monomeric. This is the first characterization of genes and enzymes for the synthesis of GG in a thermophile.

A combined shotgun and multidimensional proteomic analysis of the insoluble subproteome of the obligate thermophile, Geobacillus thermoleovorans T80

To further our understanding of the biology of the thermophilic bacterium Geobacillus thermoleovorans T80, we now report the first proteomic analysis of the insoluble subproteome of this isolate. A combination of both shotgun and multidimensional methodologies were utilized, and a total of 8628 peptides was initially identified by automated MS/MS identification software. Curation of these peptides led to a final list of 184 positive protein identifications. The proteins from this insoluble subproteome were functionally classified, and physiochemical characterization was carried out. Of 15 hypothetical conserved proteins identified, we have assigned function to all but four. A total of 31 proteins were predicted to possess signal peptides. In silico investigation of these proteins allowed us to identify four of the five bacterial classes of signal peptide, namely, (i) twin-arginine translocation; (ii) Sec-type; (iii) lipoprotein, and (iv) ABC transport. In addition, a number of proteins were identified that are known to be involved in the transport of compatible solutes, known to be important in microbial stress responses.

Extremolytes: Natural compounds from extremophiles for versatile applications

Extremophilic microorganisms have adopted a variety of ingenious strategies for survival under high or low temperature, extreme pressure, and drastic salt concentrations. A novel application area for extremophiles is the use of "extremolytes," organic osmolytes from extremophilic microorganisms, to protect biological macromolecules and cells from damage by external stresses. In extremophiles, these low molecular weight compounds are accumulated in response to increased extracellular salt concentrations, but also as a response to other environmental changes, e.g., increased temperature. Extremolytes minimize the denaturation of biopolymers that usually occurs under conditions of water stress and are compatible with the intracellular machinery at high (>1 M) concentrations. The ectoines, as the first extremolytes that are produced in a large scale, have already found application as cell protectants in skin care and as protein-free stabilizers of proteins and cells in life sciences. In addition to ectoines, a range of extremolytes with heterogenous chemical structures like the polyol phosphates di-myoinositol-1,1'-phosphate, cyclic 2,3-diphosphoglycerate, and alpha-diglycerol phosphate and the mannose derivatives mannosylglycerate (firoin) and mannosylglyceramide (firoin-A) were characterized and were shown to have protective properties toward proteins and cells. A range of new applications, all based on the adaptation to stress conditions conferred by extremolytes, is in development.

Characterization of the biosynthetic pathway of glucosylglycerate in the archaeon Methanococcoides burtonii

The pathway for the synthesis of the organic solute glucosylglycerate (GG) is proposed based on the activities of the recombinant glucosyl-3-phosphoglycerate synthase (GpgS) and glucosyl-3-phosphoglycerate phosphatase (GpgP) from Methanococcoides burtonii. A mannosyl-3-phosphoglycerate phosphatase gene homologue (mpgP) was found in the genome of M. burtonii (http://www.jgi.doe.gov), but an mpgS gene coding for mannosyl-3-phosphoglycerate synthase (MpgS) was absent. The gene upstream of the mpgP homologue encoded a putative glucosyltransferase that was expressed in Escherichia coli. The recombinant product had GpgS activity, catalyzing the synthesis of glucosyl-3-phosphoglycerate (GPG) from GDP-glucose and d-3-phosphoglycerate, with a high substrate specificity. The recombinant MpgP protein dephosphorylated GPG to GG and was also able to dephosphorylate mannosyl-3-phosphoglycerate (MPG) but no other substrate tested. Similar flexibilities in substrate specificity were confirmed in vitro for the MpgPs from Thermus thermophilus, Pyrococcus horikoshii, and "Dehalococcoides ethenogenes." GpgS had maximal activity at 50 degrees C. The maximal activity of GpgP was at 50 degrees C with GPG as the substrate and at 60 degrees C with MPG. Despite the similarity of the sugar donors GDP-glucose and GDP-mannose, the enzymes for the synthesis of GPG or MPG share no amino acid sequence identity, save for short motifs. However, the hydrolysis of GPG and MPG is carried out by phosphatases encoded by homologous genes and capable of using both substrates. To our knowledge, this is the first report of the elucidation of a biosynthetic pathway for glucosylglycerate.

Compatible solutes of the hyperthermophile Palaeococcus ferrophilus: osmoadaptation and thermoadaptation in the order thermococcales

The effect of salinity and growth temperature on the accumulation of intracellular organic solutes was examined in the hyperthermophilic archaeon Palaeococcus ferrophilus. The genus Palaeococcus represents a deep-branching lineage of the order Thermococcales, which diverged before Thermococcus and Pyrococcus. Palaeococcus ferrophilus accumulated mannosylglycerate, glutamate, and aspartate as major compatible solutes. Unlike members of the genera Pyrococcus and Thermococcus, Palaeococcus ferrophilus did not accumulate di-myo-inositol phosphate, a canonical solute of hyperthermophiles. The level of mannosylglycerate increased in response to both heat and salt stress; glutamate increased at supraoptimal growth temperatures, whereas aspartate increased at supraoptimal salt concentration. Proline, alanine, and trehalose were also found in lesser amounts, but their levels did not respond significantly to any of the stresses imposed. Additionally, the genes involved in the synthesis of mannosylglycerate in Palaeococcus ferrophilus and Thermococcus litoralis were identified. In both organisms the synthesis proceeds via the two-step pathway comprising mannosyl-3-phosphoglycerate synthase (MPGS) (EC 2.4.1.217) and mannosyl-3-phosphoglycerate phosphatase (MPGP) (EC 3.1.3.70). The mpgS and mpgP genes of Palaeococcus ferrophilus were expressed in Escherichia coli and the proteins were characterized. MPGS had maximal activity at 90 degrees C and pH near 7.0, and was strictly dependent on Mg2+. MPGP had optimal activity at 90 degrees C and pH 6.0 and was barely dependent on Mg2+. The half-life values for inactivation of MPGS and MPGP at 83 degrees C were 18 and 25 min, respectively. A comparative discussion of the osmo- and thermoadaptation strategies in these three genera of the Thermococcales is presented.

Stress response by solute accumulation in archaea

The accumulation of organic solutes is a prerequisite for osmotic adjustment of all organisms. Archaea synthesize unusual solutes such as beta-amino acids, Nepsilon-acetyl-beta-lysine, mannosylglycerate and di-myo-inositol phosphate but, as in other cells, uptake of solutes such as glycine betaine is preferred over de novo synthesis. Study of the molecular basis of osmoadaptation and its regulation in archaea is still in its infancy, but genomics and functional genome analyses combined with classical biochemistry shed light on the processes that confer osmoadaptation in archaea. Most interestingly, some solutes are not only produced in response to salt but also to temperature stress.