Role of Ngamma-acetyldiaminobutyrate as an enzyme stabilizer and an intermediate in the biosynthesis of hydroxyectoine

The ups and downs of ectoine: structural enzymology of a major microbial stress protectant and versatile nutrient

Ectoine and its derivative 5-hydroxyectoine are compatible solutes and chemical chaperones widely synthesized by Bacteria and some Archaea as cytoprotectants during osmotic stress and high- or low-growth temperature extremes. The function-preserving attributes of ectoines led to numerous biotechnological and biomedical applications and fostered the development of an industrial scale production process. Synthesis of ectoines requires the expenditure of considerable energetic and biosynthetic resources. Hence, microorganisms have developed ways to exploit ectoines as nutrients when they are no longer needed as stress protectants. Here, we summarize our current knowledge on the phylogenomic distribution of ectoine producing and consuming microorganisms. We emphasize the structural enzymology of the pathways underlying ectoine biosynthesis and consumption, an understanding that has been achieved only recently. The synthesis and degradation pathways critically differ in the isomeric form of the key metabolite N-acetyldiaminobutyric acid (ADABA). γ-ADABA serves as preferred substrate for the ectoine synthase, while the α-ADABA isomer is produced by the ectoine hydrolase as an intermediate in catabolism. It can serve as internal inducer for the genetic control of ectoine catabolic genes via the GabR/MocR-type regulator EnuR. Our review highlights the importance of structural enzymology to inspire the mechanistic understanding of metabolic networks at the biological scale.

Physiological response of the moderately halophilic psychrotolerant strain Chromohalobacter sp. N1 to salinity change and low temperature

The available information on de novo synthesized compatible solutes in response to high medium salinity by bacteria of the Chromohalobacter genus is limited to studies of the mesophilic moderately halophilic strain Chromohalobacter salexigens DSM 3043^T. Therefore, there is a need for studies of representatives of other species of the Chromohalobacter genus of the Halomonadaceae family. A moderately halophilic psychrotolerant bacterium, strain N1, closely related to the species Chromohalobacter japonicus was isolated from the salt crust of a rock salt waste pile in Berezniki, Perm Krai, Russia. An intracellular pool of compatible solutes of strain N1 was investigated by NMR spectroscopy. Cells grown in the presence of 5% NaCl at optimal growth temperature (28 °C) accumulated ectoine, glutamate, N(4)-acetyl-l-2,4-diaminobutyrate (NADA), alanine, trehalose, hydroxyectoine, and valine. Such a combination of compatible solutes is unique and distinguishes the strain from C. salexigens DSM 3043^T. Hyperosmotic stress induced by 15% NaCl caused the accumulation of ectoine, NADA, and hydroxyectoine but led to a decrease in the amount of alanine, valine, and trehalose. The intracellular pool of glutamate was not significantly changed. A reduction of the growth temperature from 28 to 5 °C led to an increase in the amount of ectoine, NADA, trehalose, and hydroxyectoine. Ectoine was the major compatible solute.

Study on the osmoregulation of "Halomonas socia" NY-011 and the degradation of organic pollutants in the saline environment

"Halomonas socia" NY-011, a new species of moderately halophilic bacteria isolated and identified in our laboratory, can grow in high concentrations of salt ranging from 0.5 to 25%. In this study, the whole genome of NY-011 was sequenced and a detailed analysis of the genomic features was provided. Especially, a series of genes related to salt tolerance and involved in xenobiotics biodegradation were annotated by COG, GO and KEGG analyses. Subsequently, RNA-Seq-based transcriptome analysis was applied to explore the osmotic regulation of NY-011 subjected to high salt stress for different times (0 h, 1 h, 3 h, 6 h, 11 h, 15 h). And we found that the genes related to osmoregulation including excluding Na⁺ and accumulating K⁺ as well as the synthesis of compatible solutes (alanine, glutamate, ectoine, hydroxyectoine and glycine betaine) were up-regulated, while the genes involved in the degradation of organic compounds were basically down-regulated during the whole process. Specifically, the expression trend of genes related to osmoregulation increased firstly then dropped, which was almost opposite to that of degrading organic pollutants genes. With the prolongation of osmotic up-shock, NY-011 survived and gradually adapted to osmotic stress, the above-mentioned two classes of genes slowly returned to normal expression level. Then, the scanning electron microscope (SEM) and transmission electron microscope (TEM) were also utilized to observe morphological properties of NY-011 under hypersaline stress, and our findings showed that the cell length of NY-011 became longer under osmotic stress, at the same time, polyhydroxyalkanoates (PHAs) were synthesized in the cells. Besides, physiological experiments confirmed that NY-011 could degrade organic compounds in a high salt environment. These data not only provide valuable insights into the mechanism of osmotic regulation of NY-011; but also make it possible for NY-011 to be exploited for biotechnological applications such as degrading organic pollutants in a hypersaline environment.

Role of N,N-Dimethylglycine and Its Catabolism to Sarcosine in Chromohalobacter salexigens DSM 3043

Chromohalobacter salexigens DSM 3043 can grow on N,N-dimethylglycine (DMG) as the sole C, N, and energy source and utilize sarcosine as the sole N source under aerobic conditions. However, little is known about the genes and enzymes involved in the conversion of DMG to sarcosine in this strain. In the present study, gene disruption and complementation assays indicated that the csal_0990, csal_0991, csal_0992, and csal_0993 genes are responsible for DMG degradation to sarcosine. The csal_0990 gene heterologously expressed in Escherichia coli was proven to encode an unusual DMG dehydrogenase (DMGDH). The enzyme, existing as a monomer of 79 kDa with a noncovalently bound flavin adenine dinucleotide, utilized both DMG and sarcosine as substrates and exhibited dual coenzyme specificity, preferring NAD⁺ to NADP⁺ The optimum pH and temperature of enzyme activity were determined to be 7.0 and 60°C, respectively. Kinetic parameters of the enzyme toward its substrates were determined accordingly. Under high-salinity conditions, the presence of DMG inhibited growth of the wild type and induced the production and accumulation of trehalose and glucosylglycerate intracellularly. Moreover, exogenous addition of DMG significantly improved the growth rates of the four DMG^- mutants (Δcsal_0990, Δcsal_0991, Δcsal_0992, and Δcsal_0993) incubated at 37°C in S-M63 synthetic medium with sarcosine as the sole N source. ¹³C nuclear magnetic resonance (¹³C-NMR) experiments revealed that not only ectoine, glutamate, and N-acetyl-2,4-diaminobutyrate but also glycine betaine (GB), DMG, sarcosine, trehalose, and glucosylglycerate are accumulated intracellularly in the four mutants.IMPORTANCE Although N,N-dimethylglycine (DMG) dehydrogenase (DMGDH) activity was detected in cell extracts of microorganisms, the genes encoding microbial DMGDHs have not been determined until now. In addition, to our knowledge, the physiological role of DMG in moderate halophiles has never been investigated. In this study, we identified the genes involved in DMG degradation to sarcosine, characterized an unusual DMGDH, and investigated the role of DMG in Chromohalobacter salexigens DSM 3043 and its mutants. Our results suggested that the conversion of DMG to sarcosine is accompanied by intramolecular delivery of electrons in DMGDH and intermolecular electron transfer between DMGDH and other electron acceptors. Moreover, an unidentified methyltransferase catalyzing the production of glycine betaine (GB) from DMG but sharing no homology with the reported sarcosine DMG methyltransferases was predicted to be present in the cells. The results of this study expand our understanding of the physiological role of DMG and its catabolism to sarcosine in C. salexigens.

Heterologous Ectoine Production in Escherichia coli: Optimization Using Response Surface Methodology

Introduction

A halophilic bacterium of the Halomonas elongata BK-AG25 has successfully produced ectoine with high productivity. To overcome the drawbacks of high levels of salt in the production process, a nonhalophilic bacteria of Escherichia coli (E. coli) was used to express the ectoine gene cluster of the halophilic bacteria, and the production of ectoine by the recombinant cell was optimized.

Methods

The ectoine gene cluster from the halophilic bacterium was isolated and inserted into an expression plasmid of pET30(a) and subsequently transformed into E. coli BL21 (DE3). Production of ectoine from the recombinant E. coli was investigated and then maximized by optimizing the level of nutrients in the medium, as well as the bioprocess conditions using response surface methodology. The experimental designs were performed using a central composite design.

Results

The recombinant E. coli successfully expressed the ectoine gene cluster of Halomonas elongata BK-AG25 under the control of the T7 promoter. The recombinant cell was able to produce ectoine, of which most were excreted into the medium. The optimization of ectoine production with the response surface methodology showed that the level of salt in the medium, the incubation temperature, the optical density of the bacteria before induction, and the final concentration of the inducer gave a significant effect on ectoine production by the recombinant E. coli. Interestingly, the level of salt in the medium and the incubation temperature showed an inverse effect on the production of intracellular and extracellular ectoine by the recombinant cell. At the optimum conditions, the production yield was about 418 mg ectoine/g cdw (cell dry weight) after 12 hours of incubation.

Conclusion

This study is the first report on the expression of an ectoine gene cluster of Halomonas elongata BK-AG25 in E. coli BL21, under the control of the T7 promoter. Optimization of the level of nutrients in the medium, as well as the bioprocess condition using response surface methodology, has successfully increased the production of ectoine by the recombinant bacteria.

Stabilization of bovine lactoperoxidase in the presence of ectoine

Lactoperoxidase (LPO) is a heme peroxidase with various applications in industry and medicine. In this study, the effects of ectoine, as a compatible solute, on the structure, thermal stability, thermodynamic parameters, activity, and stability of LPO have been investigated. The results showed that the catalytic activity of LPO was improved by increasing ectoine concentration. The UV-visible absorption spectroscopy and FTIR spectra studies indicated that ectoine could bind to the LPO spontaneously. Moreover, ectoine increased the enzyme Tm and Gibbs free energy. The fluorescence measurements showed that LPO fluorescence was quenched in the presence of ectoine. The quenching mechanism was probably a static quenching by forming a ground state complex. The thermodynamic parameters indicated that hydrogen bonding and Vander Waals forces played a key role in the LPO-ectoine interaction process. The findings suggest that ectoine could be used as a lactoperoxidase stabilizing agent for industrial or medical purposes.

Role of the Extremolytes Ectoine and Hydroxyectoine as Stress Protectants and Nutrients: Genetics, Phylogenomics, Biochemistry, and Structural Analysis

Fluctuations in environmental osmolarity are ubiquitous stress factors in many natural habitats of microorganisms, as they inevitably trigger osmotically instigated fluxes of water across the semi-permeable cytoplasmic membrane. Under hyperosmotic conditions, many microorganisms fend off the detrimental effects of water efflux and the ensuing dehydration of the cytoplasm and drop in turgor through the accumulation of a restricted class of organic osmolytes, the compatible solutes. Ectoine and its derivative 5-hydroxyectoine are prominent members of these compounds and are synthesized widely by members of the Bacteria and a few Archaea and Eukarya in response to high salinity/osmolarity and/or growth temperature extremes. Ectoines have excellent function-preserving properties, attributes that have led to their description as chemical chaperones and fostered the development of an industrial-scale biotechnological production process for their exploitation in biotechnology, skin care, and medicine. We review, here, the current knowledge on the biochemistry of the ectoine/hydroxyectoine biosynthetic enzymes and the available crystal structures of some of them, explore the genetics of the underlying biosynthetic genes and their transcriptional regulation, and present an extensive phylogenomic analysis of the ectoine/hydroxyectoine biosynthetic genes. In addition, we address the biochemistry, phylogenomics, and genetic regulation for the alternative use of ectoines as nutrients.

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.

Osmoadaptative Strategy and Its Molecular Signature in Obligately Halophilic Heterotrophic Protists

Halophilic microbes living in hypersaline environments must counteract the detrimental effects of low water activity and salt interference. Some halophilic prokaryotes equilibrate their intracellular osmotic strength with the extracellular milieu by importing inorganic solutes, mainly potassium. These "salt-in" organisms characteristically have proteins that are highly enriched with acidic and hydrophilic residues. In contrast, "salt-out" halophiles accumulate large amounts of organic solutes like amino acids, sugars and polyols, and lack a strong signature of halophilicity in the amino acid composition of cytoplasmic proteins. Studies to date have examined halophilic prokaryotes, yeasts, or algae, thus virtually nothing is known about the molecular adaptations of the other eukaryotic microbes, that is, heterotrophic protists (protozoa), that also thrive in hypersaline habitats. We conducted transcriptomic investigations to unravel the molecular adaptations of two obligately halophilic protists, Halocafeteria seosinensis and Pharyngomonas kirbyi Their predicted cytoplasmic proteomes showed increased hydrophilicity compared with marine protists. Furthermore, analysis of reconstructed ancestral sequences suggested that, relative to mesophiles, proteins in halophilic protists have undergone fewer substitutions from hydrophilic to hydrophobic residues since divergence from their closest relatives. These results suggest that these halophilic protists have a higher intracellular salt content than marine protists. However, absence of the acidic signature of salt-in microbes suggests that Haloc. seosinensis and P. kirbyi utilize organic osmolytes to maintain osmotic equilibrium. We detected increased expression of enzymes involved in synthesis and transport of organic osmolytes, namely hydroxyectoine and myo-inositol, at maximal salt concentration for growth in Haloc. seosinensis, suggesting possible candidates for these inferred organic osmolytes.

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.

Dimethylglycine provides salt and temperature stress protection to Bacillus subtilis

Glycine betaine is a potent osmotic and thermal stress protectant of many microorganisms. Its synthesis from glycine results in the formation of the intermediates monomethylglycine (sarcosine) and dimethylglycine (DMG), and these compounds are also produced when it is catabolized. Bacillus subtilis does not produce sarcosine or DMG, and it cannot metabolize these compounds. Here we have studied the potential of sarcosine and DMG to protect B. subtilis against osmotic, heat, and cold stress. Sarcosine, a compatible solute that possesses considerable protein-stabilizing properties, did not serve as a stress protectant of B. subtilis. DMG, on the other hand, proved to be only moderately effective as an osmotic stress protectant, but it exhibited good heat stress-relieving and excellent cold stress-relieving properties. DMG is imported into B. subtilis cells primarily under osmotic and temperature stress conditions via OpuA, a member of the ABC family of transporters. Ligand-binding studies with the extracellular solute receptor (OpuAC) of the OpuA system showed that OpuAC possesses a moderate affinity for DMG, with a Kd value of approximate 172 μM; its Kd for glycine betaine is about 26 μM. Docking studies using the crystal structures of the OpuAC protein with the sulfur analog of DMG, dimethylsulfonioacetate, as a template suggest a model of how the DMG molecule can be stably accommodated within the aromatic cage of the OpuAC ligand-binding pocket. Collectively, our data show that the ability to acquire DMG from exogenous sources under stressful environmental conditions helps the B. subtilis cell to cope with growth-restricting osmotic and temperature challenges.

Role of central metabolism in the osmoadaptation of the halophilic bacterium Chromohalobacter salexigens

Bacterial osmoadaptation involves the cytoplasmic accumulation of compatible solutes to counteract extracellular osmolarity. The halophilic and highly halotolerant bacterium Chromohalobacter salexigens is able to grow up to 3 m NaCl in a minimal medium due to the de novo synthesis of ectoines. This is an osmoregulated pathway that burdens central metabolic routes by quantitatively drawing off TCA cycle intermediaries. Consequently, metabolism in C. salexigens has adapted to support this biosynthetic route. Metabolism of C. salexigens is more efficient at high salinity than at low salinity, as reflected by lower glucose consumption, lower metabolite overflow, and higher biomass yield. At low salinity, by-products (mainly gluconate, pyruvate, and acetate) accumulate extracellularly. Using [1-(13)C]-, [2-(13)C]-, [6-(13)C]-, and [U-(13)C6]glucose as carbon sources, we were able to determine the main central metabolic pathways involved in ectoines biosynthesis from glucose. C. salexigens uses the Entner-Doudoroff pathway rather than the standard glycolytic pathway for glucose catabolism, and anaplerotic activity is high to replenish the TCA cycle with the intermediaries withdrawn for ectoines biosynthesis. Metabolic flux ratios at low and high salinity were similar, revealing a certain metabolic rigidity, probably due to its specialization to support high biosynthetic fluxes and partially explaining why metabolic yields are so highly affected by salinity. This work represents an important contribution to the elucidation of specific metabolic adaptations in compatible solute-accumulating halophilic bacteria.

Synthesis and release of the bacterial compatible solute 5-hydroxyectoine in Hansenula polymorpha

Ectoine and 5-hydroxyectoine belong to the family of compatible solutes which are known to mainly contribute to the adaptation of the cell to osmotic stress by mediation of a constant turgor. In addition the cell's essential functions are maintained under stress conditions like high salinity, heat or aridity stress. Hansenula polymorpha was engineered to catalyze the transformation of monomeric substrates to 5-hydroxyectoine. For this purpose four genes encoding the enzymes of the 5-hydroxyectoine biosynthesis pathway of Halomonas elongata, EctA, EctB, EctC, and EctD, were inserted into the genome of H. polymorpha. Subsequently the syntheses of ectoine and 5-hydroxyectoine were analyzed and optimized. We showed that H. polymorpha is a suitable system for recombinant 5-hydroxyectoine synthesis in gram per liter scale (2.8 g L⁻¹ culture supernatant, 365 μmol/g dcw) in which almost 100% conversion of ectoine to 5-hydroxyectoine without necessity of high salinity were achieved.

Role of trehalose in salinity and temperature tolerance in the model halophilic bacterium Chromohalobacter salexigens

The disaccharide trehalose is considered as a universal stress molecule, protecting cells and biomolecules from injuries imposed by high osmolarity, heat, oxidation, desiccation and freezing. Chromohalobacter salexigens is a halophilic and extremely halotolerant γ-proteobacterium of the family Halomonadaceae. In this work, we have investigated the role of trehalose as a protectant against salinity, temperature and desiccation in C. salexigens. A mutant deficient in the trehalose-6-phosphate synthase gene (otsA::Ω) was not affected in its salt or heat tolerance, but double mutants ectoine- and trehalose-deficient, or hydroxyectoine-reduced and trehalose-deficient, displayed an osmo- and thermosensitive phenotype, respectively. This suggests a role of trehalose as a secondary solute involved in osmo- (at least at low salinity) and thermoprotection of C. salexigens. Interestingly, trehalose synthesis was osmoregulated at the transcriptional level, and thermoregulated at the post-transcriptional level, suggesting that C. salexigens cells need to be pre-conditioned by osmotic stress, in order to be able to quickly synthesize trehalose in response to heat stress. C. salexigens was more sensitive to desiccation than E. coli and desiccation tolerance was slightly improved when cells were grown at high temperature. Under these conditions, single mutants affected in the synthesis of trehalose or hydroxyectoine were more sensitive to desiccation than the wild-type strain. However, given the low survival rates of the wild type, the involvement of trehalose and hydroxyectoine in C. salexigens response to desiccation could not be firmly established.

Genes and enzymes of ectoine biosynthesis in halotolerant methanotrophs

Ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidine carboxylic acid) is a widely distributed compatible solute accumulated by halophilic and halotolerant microorganisms to prevent osmotic stress in highly saline environments. Ectoine as a highly water keeping compound stabilizing biomolecules and whole cells can be used in scientific work, cosmetics, and medicine. Detailed understanding of the organization/regulation of the ectoine biosynthetic pathway in various producers is an active area of research. Here we review current knowledge on some genetic and enzymatic aspects of ectoine biosynthesis in halophilic and halotolerant methanotrophs. By using PCR methodology, the genes coding for the specific enzymes of ectoine biosynthesis, diaminobutyric acid (DABA) aminotransferase (EctB), DABA acetyltransferase (EctA), and ectoine synthase (EctC), were identified in several methanotrophic species. Organization of these genes in either ectABC or ectABC-ask operons, the latter additionally encoding aspartate kinase isozyme (Ask), correlated well with methanotroph halotolerance and intracellular ectoine level. A new gene, ectR1 encoding the MarR-like transcriptional regulatory protein EctR1, negatively controlling transcription of ectoine biosynthetic genes was found upstream of ectABC-ask operon in Methylomicrobium alcaliphilum 20Z. The ectR-like genes were also found in halotolerant methanol utilizers Methylophaga alcalica and Methylophaga thalassica as well as in several genomes of nonmethylotrophic species. The His(6)-tagged DABA acetyltransferases from Mm. alcaliphilum, M. alcalica, and M. thalassica were purified and the enzyme properties were found to correlate with the ecophysiologies of these bacteria. All these discoveries should be very helpful for better understanding the biosynthetic mechanism of this important natural compound, and for the targeted metabolic engineering of its producers.

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.

Accumulation and role of compatible solutes in fast-growing Salinivibrio costicola subsp. yaniae

The moderately halophilic bacterium Salinivibrio costicola subsp. yaniae showed an extremely fast growth rate. Optimal growth was observed in artificial seawater containing 1.4 mol/L NaCl and in MM63 media containing 0.6 mol/L NaCl. We analyzed a variety of compatible solutes that had accumulated in this strain grown in the media. The supplementation effect of the compatible solutes glycine betaine, glutamate, and ectoine to the growth of S. costicola subsp. yaniae was examined. Glycine betaine and glutamate had no supplementation effect on the fast growth rate. Growth of salt-sensitive mutants MU1 and MU2, both of which were defective in the ability to synthesize ectoine, was not observed in MM63 medium in the presence of more than 1.0 mol/L NaCl. From these data, we conclude that ectoine was the predominant compatible solute synthesized in this bacterium that effected an extremely fast growth rate.

Ectoines in cell stress protection: uses and biotechnological production

Microorganisms produce and accumulate compatible solutes aiming at protecting themselves from environmental stresses. Among them, the wide spread in nature ectoines are receiving increasing attention by the scientific community because of their multiple applications. In fact, increasing commercial demand has led to a multiplication of efforts in order to improve processes for their production. In this review, the importance of current and potential applications of ectoines as protecting agents for macromolecules, cells and tissues, together with their potential as therapeutic agents for certain diseases are analyzed and current theories for the understanding of the molecular basis of their biological activity are discussed. The genetic, biochemical and environmental determinants of ectoines biosynthesis by natural and engineered producers are described. The major limitations of current bioprocesses used for ectoines production are discussed, with emphasis on the different microorganisms, environments, molecular engineering and fermentation strategies used to optimize the production and recovery of ectoines. The combined application of both bioprocess and metabolic engineering strategies, allowing a deeper understanding of the main factors controlling the production process is also stated. Finally, this review aims to summarize and update the state of the art in ectoines uses and applications and industrial scale production using bacteria, emphasizing the importance of reactor design and operation strategies, together with the metabolic engineering aspects and the need for feedback between wet and in silico work to optimize bioproduction.

Interplay between iron homeostasis and the osmotic stress response in the halophilic bacterium Chromohalobacter salexigens

In this study, the connection between iron homeostasis and the osmostress response in the halophile Chromohalobacter salexigens was investigated. A decrease in the requirement for both iron and histidine and a lower level of siderophore synthesis were observed at high salinity, and these findings were correlated with a lower protein content in salt-stressed cells. A six-gene operon (cfuABC-fur-hisI-orf6 operon) located downstream of the ectABC ectoine synthesis genes was characterized. A fur strain (in which the ferric iron uptake regulator Fur was affected) had the Mn resistance phenotype typical of fur mutants, was deregulated for siderophore production, and displayed delayed growth under iron limitation conditions, indicating that fur encodes a functional iron regulator. hisI was essential for histidine synthesis, which in turn was necessary for siderophore production. Fur boxes were found in the promoters of the cfuABC-fur-hisI-orf6 and ectABC operons, suggesting that Fur directly interacts with DNA in these regions. Fur mediated the osmoregulated inhibition of cfuABC-fur-hisI-orf6 operon expression by iron and functioned as a positive regulator of the ectABC genes under high-salinity conditions, linking the salt stress response with iron homeostasis. Excess iron led to a higher cytoplasmic hydroxyectoine content, suggesting that hydroxyectoine protects against the oxidative stress caused by iron better than ectoine. This study provides the first evidence of involvement of the iron homeostasis regulator Fur as part of the complex circuit that controls the response to osmotic stress in halophilic bacteria.

Synthesis and uptake of the compatible solutes ectoine and 5-hydroxyectoine by Streptomyces coelicolor A3(2) in response to salt and heat stresses

Streptomyces coelicolor A3(2) synthesizes ectoine and 5-hydroxyectoine upon the imposition of either salt (0.5 M NaCl) or heat stress (39 degrees C). The cells produced the highest cellular levels of these compatible solutes when both stress conditions were simultaneously imposed. Protection against either severe salt (1.2 M NaCl) or heat stress (39 degrees C) or a combination of both environmental cues could be accomplished by adding low concentrations (1 mM) of either ectoine or 5-hydroxyectoine to S. coelicolor A3(2) cultures. The best salt and heat stress protection was observed when a mixture of ectoine and 5-hydroxyectoine (0.5 mM each) was provided to the growth medium. Transport assays with radiolabeled ectoine demonstrated that uptake was triggered by either salt or heat stress. The most effective transport and accumulation of [(14)C]ectoine by S. coelicolor A3(2) were achieved when both environmental cues were simultaneously applied. Our results demonstrate that the accumulation of the compatible solutes ectoine and 5-hydroxyectoine allows S. coelicolor A3(2) to fend off the detrimental effects of both high salinity and high temperature on cell physiology. We also characterized the enzyme (EctD) required for the synthesis of 5-hydroxyectoine from ectoine, a hydroxylase of the superfamily of the non-heme-containing iron(II)- and 2-oxoglutarate-dependent dioxygenases (EC 1.14.11). The gene cluster (ectABCD) encoding the enzymes for ectoine and 5-hydroxyectoine biosynthesis can be found in the genome of S. coelicolor A3(2), Streptomyces avermitilis, Streptomyces griseus, Streptomyces scabiei, and Streptomyces chrysomallus, suggesting that these compatible solutes play an important role as stress protectants in the genus Streptomyces.

Unravelling the adaptation responses to osmotic and temperature stress in Chromohalobacter salexigens, a bacterium with broad salinity tolerance

Chromohalobacter salexigens, a Gammaproteobacterium belonging to the family Halomonadaceae, shows a broad salinity range for growth. Osmoprotection is achieved by the accumulation of compatible solutes either by transport (betaine, choline) or synthesis (mainly ectoine and hydroxyectoine). Ectoines can play additional roles as nutrients and, in the case of hydroxyectoine, in thermotolerance. A supplementary solute, trehalose, not present in cells grown at 37°C, is accumulated at higher temperatures, suggesting its involvement in the response to heat stress. Trehalose is also accumulated at 37°C in ectoine-deficient mutants, indicating that ectoines suppress trehalose synthesis in the wild-type strain. The genes for ectoine (ectABC) and hydroxyectoine (ectD, ectE) production are arranged in three different clusters within the C. salexigens chromosome. In order to cope with changing environment, C. salexigens regulates its cytoplasmic pool of ectoines by a number of mechanisms that we have started to elucidate. This is a highly complex process because (i) hydroxyectoine can be synthesized by other enzymes different to EctD (ii) ectoines can be catabolized to serve as nutrients, (iii) the involvement of several transcriptional regulators (σ^S, σ³², Fur, EctR) and hence different signal transduction pathways, and (iv) the existence of post-trancriptional control mechanisms. In this review we summarize our present knowledge on the physiology and genetics of the processes allowing C. salexigens to cope with osmotic stress and high temperature, with emphasis on the transcriptional regulation.

Regulation of osmoadaptation in the moderate halophile Halobacillus halophilus: chloride, glutamate and switching osmolyte strategies

The moderate halophile Halobacillus halophilus is the paradigm for chloride dependent growth in prokaryotes. Recent experiments shed light on the molecular basis of the chloride dependence that is reviewed here. In the presence of moderate salinities Halobacillus halophilus mainly accumulates glutamine and glutamate to adjust turgor. The transcription of glnA2 (encoding a glutamine synthetase) as well as the glutamine synthetase activity were identified as chloride dependent steps. Halobacillus halophilus switches its osmolyte strategy and produces proline as the main compatible solute at high salinities. Furthermore, Halobacillus halophilus also shifts its osmolyte strategy at the transition from the exponential to the stationary phase where proline is exchanged by ectoine. Glutamate was found as a "second messenger" essential for proline production. This observation leads to a new model of sensing salinity by sensing the physico-chemical properties of different anions.

Crystal structure of the ligand-binding protein EhuB from Sinorhizobium meliloti reveals substrate recognition of the compatible solutes ectoine and hydroxyectoine

In microorganisms, members of the binding-protein-dependent ATP-binding cassette transporter superfamily constitute an important class of transport systems. Some of them are involved in osmoprotection under hyperosmotic stress by facilitating the uptake of "compatible solutes". Currently, the molecular mechanisms used by these transport systems to recognize compatible solutes are limited to transporters specific for glycine betaine and proline betaine. Therefore, this study reports a detailed analysis of the molecular principles governing substrate recognition in the Ehu system from Sinorhizobium meliloti, which is responsible for the uptake of the compatible solutes ectoine and hydroxyectoine. To contribute to a broader understanding of the molecular interactions underlying substrate specificity, our study focused on the substrate-binding protein EhuB because this protein binds the ligand selectively, delivers it to the translocation machinery in the membrane and is thought to be responsible for substrate specificity. The crystal structures of EhuB, in complex with ectoine and hydroxyectoine, were determined at a resolution of 1.9 A and 2.3 A, respectively, and allowed us to assign the structural principles of substrate recognition and binding. Based on these results, site-directed mutagenesis of amino acids involved in ligand binding was employed to address their individual contribution to complex stability. A comparison with the crystal structures of other binding proteins specific for compatible solutes revealed common principles of substrate recognition, but also important differences that might be an adaptation to the nature of the ligand and to the demands on protein affinity imposed by the environment.

Salinity-dependent switching of osmolyte strategies in a moderately halophilic bacterium: glutamate induces proline biosynthesis in Halobacillus halophilus

The moderately halophilic bacterium Halobacillus halophilus copes with the salinity in its environment by the production of compatible solutes. At intermediate salinities of around 1 M NaCl, cells produce glutamate and glutamine in a chloride-dependent manner (S. H. Saum, J. F. Sydow, P. Palm, F. Pfeiffer, D. Oesterhelt, and V. Müller, J. Bacteriol. 188:6808-6815, 2006). Here, we report that H. halophilus switches its osmolyte strategy and produces proline as the dominant solute at higher salinities (2 to 3 M NaCl). The proline biosynthesis genes proH, proJ, and proA were identified. They form a transcriptional unit and encode the pyrroline-5-carboxylate reductase, the glutamate-5-kinase, and the glutamate-5-semialdehyde dehydrogenase, respectively, catalyzing proline biosynthesis from glutamate. Expression of the genes was clearly salinity dependent and reached a maximum at 2.5 M NaCl, indicating that the pro operon is involved in salinity-induced proline biosynthesis. To address the role of anions in the process of pro gene activation and proline biosynthesis, we used a cell suspension system. Chloride salts lead to the highest accumulation of proline. Interestingly, chloride could be substituted to a large extent by glutamate salts. This unexpected finding was further analyzed on the transcriptional level. The cellular mRNA levels of all three pro genes were increased up to 90-fold in the presence of glutamate. A titration revealed that a minimal concentration of 0.2 M glutamate already stimulated pro gene expression. These data demonstrate that the solute glutamate is involved in the switch of osmolyte strategy from glutamate to proline as the dominant compatible solute during the transition from moderate to high salinity.

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.

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.

Uncoupling of choline-O-sulphate utilization from osmoprotection in Pseudomonas putida

The genomic context of the recognized bet genes for choline-O-sulphate (COS) utilization in Pseudomonas putida KT2440 is such that betC (choline sulphatase) lies adjacent to an ATP-binding cassette transporter and a LysR type regulator, but well away from betBA, encoding enzymes for transformation of choline into glycine betaine. The consequences of such genetic layout of the functions for COS metabolism have been examined with a suite of genetic and biochemical approaches. An early clue of the utilities of the betencoded products was exposed by the phenotypes of a betC deletion. This mutant still accumulated intact COS but failed to use this compound as carbon or nitrogen source. Furthermore, betC expression was downregulated at high salt concentrations, showing that the principal role of this gene lied in COS metabolism, not in osmoprotection. In contrast, the betBA genes were required for choline transformation into the highly effective compatible solute glycine betaine (and the concomitant endurance to high salt) and also for its utilization as carbon or nitrogen source. Thus, unlike in the cases of Bacillus subtilis and Sinorhizobium meliloti, betC is unrelated to osmoprotection in Pseudomonas putida while the betBA genes are required for both betaine synthesis and tolerance to high osmotic pressure.

Osmoprotection of Salmonella enterica serovar Typhimurium by Nγ-acetyldiaminobutyrate, the precursor of the compatible solute ectoine

N(gamma)-acetyl-2,4-diaminobutyrate (NADA), the precursor of the compatible solute ectoine, was shown to function as an osmoprotectant for the non-halophilic bacterium Salmonella enterica serovar Typhimurium. The addition of NADA-containing extracts of an ectoine synthase mutant of the broad salt-growing halophile Chromohalobacter salexigens DSM 3043(T) could alleviate the inhibitory effects of high salinity in S. enterica, which lacks the ectoine biosynthetic pathway. NADA, purified from extracts of the mutant, protected S. enterica against salinity stress. This osmoprotective effect was slightly lower than that of ectoine, but more potent than that of hydroxyectoine. Accumulation of purified NADA by S. enterica was demonstrated by (13)C-NMR spectroscopy and HPLC analysis. In addition, it was shown that NADA was taken up by S. enterica via the ProP and ProU transport systems, which are known to transport glycine betaine and proline. This finding provides evidence that these permeases can recognize a diaminoacid that carries an unsubstituted alpha-amino group. This is the first time that NADA has been connected with osmoprotective functions in non-halophilic bacteria.

Viscosity B-coefficients and standard partial molar volumes of amino acids, and their roles in interpreting the protein (enzyme) stabilization

This review systematically surveys the viscosity B-coefficients and standard partial molar volumes of amino acids at various temperatures as these data are quite important for interpreting the hydration and other properties of peptides and proteins. The effect of organic solutes and various ions on the viscometric and volumetric properties of amino acids has also been discussed in terms of their kosmotropic ('structure-making') effects on the hydration of amino acids. The comparison of these effects on the amino acid hydration enables us to have a better understanding of the influence of organic solute and salt on the protein stabilization. In addition, the viscometric and volumetric behaviors of amino acid ions (cations and anions) are also summarized because these ions have recently been incorporated as part of novel ionic liquids, which have wide applications in biocatalysis and protein stabilization.

The ectD gene, which is involved in the synthesis of the compatible solute hydroxyectoine, is essential for thermoprotection of the halophilic bacterium Chromohalobacter salexigens

The halophilic bacterium Chromohalobacter salexigens synthesizes and accumulates compatible solutes in response to salt and temperature stress. (13)C-nuclear magnetic resonance analysis of cells grown in minimal medium at the limiting temperature of 45 degrees C revealed the presence of hydroxyectoine, ectoine, glutamate, trehalose (not present in cells grown at 37 degrees C), and the ectoine precursor, Ngamma-acetyldiaminobutyric acid. High-performance liquid chromatography analyses showed that the levels of ectoine and hydroxyectoine were maximal during the stationary phase of growth. Accumulation of hydroxyectoine was up-regulated by salinity and temperature, whereas accumulation of ectoine was up-regulated by salinity and down-regulated by temperature. The ectD gene, which is involved in the conversion of ectoine to hydroxyectoine, was isolated as part of a DNA region that also contains a gene whose product belongs to the AraC-XylS family of transcriptional activators. Orthologs of ectD were found within the sequenced genomes of members of the proteobacteria, firmicutes, and actinobacteria, and their products were grouped into the ectoine hydroxylase subfamily, which was shown to belong to the superfamily of Fe(II)- and 2-oxoglutarate-dependent oxygenases. Analysis of the ectoine and hydroxyectoine contents of an ectABC ectD mutant strain fed with 1 mM ectoine or hydroxyectoine demonstrated that ectD is required for the main ectoine hydroxylase activity in C. salexigens. Although in minimal medium at 37 degrees C the wild-type strain grew with 0.5 to 3.0 M NaCl, with optimal growth at 1.5 M NaCl, at 45 degrees C it could not cope with the lowest (0.75 M NaCl) or the highest (3.0 M NaCl) salinity, and it grew optimally at 2.5 M NaCl. The ectD mutation caused a growth defect at 45 degrees C in minimal medium with 1.5 to 2.5 M NaCl, but it did not affect growth at 37 degrees C at any salinity tested. With 2.5 M NaCl, the ectD mutant synthesized 38% (at 37 degrees C) and 15% (at 45 degrees C) of the hydroxyectoine produced by the wild-type strain. All of these data reveal that hydroxyectoine synthesis mediated by the ectD gene is thermoregulated and essential for thermoprotection of C. salexigens.

Compatible solutes as protectants for zymogens against proteolysis

Compatible solutes are small organic osmoprotectants that have the capability to stabilize proteins. In coupled assays, the effect of the solutes ectoine, hydroxyectoine and betaine on the activation of the zymogens trypsinogen and chymotrypsinogen, catalyzed by enteropeptidase and trypsin, respectively, was studied. To different extents, all solutes protected the zymogens against activation. Ectoine (800 mM) was the most potent solute in reducing the formation of trypsin to 4% of the control value and of chymotrypsin to 23%. In separate experiments, the ability of the solutes to preserve proteolytic activity during incubation was investigated. After 4 h, trypsin and chymotrypsin completely lost their activity, but in the presence of ectoine, approximately 50% residual activity was maintained. It is proposed that a conformational shift of the protein towards folded, native-like states induced by preferential exclusion of the solute is responsible for the stabilizing and chaperone-like effects.

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.

High-yield cultivation of Marinococcus M52 for production and recovery of hydroxyectoine

Ectoine and hydroxyectoine, produced by Halomonas ssp. and Marinococcus ssp., have been extensively characterized and proposed for biotechnological applications. Large-scale production of ectoine was achieved by implementing the so-called "milking process" by Bitop (Witten, Germany) and commercializing compatible solutes as stabilizers for biological systems. However, there is a continuing interest in optimization of the production process for hydroxyectoine alone, as the latter was shown to have greater stabilization capacity. Recently, Marinococcus strain M52 was selected for its ability to convert most ectoine into hydroxyectoine during a prolonged stationary phase. This study reports on the correlation between growth conditions and hydroxyectoine production in Marinococcus M52 cultivation. We demonstrated that a dissolved oxygen content higher than 10% during cultivation leads to more rapid accumulation of hydroxyectoine than of ectoine (with hydroxyectoine up to 1.6 g l-1). In addition, we employed a microfiltration bioprocess to improve biomass and yield of products (reaching 3.6 g l-1 of hydroxyectoine). Finally, we developed a novel extraction method based on osmotic down-shock coupled with thermal permeabilization to recover the desired products from the biomass.

Supplementation effects of hydroxyectoine on proline uptake of downshocked Brevibacterium sp. JCM 6894

Downshock treatment of the halotolerant Brevibacterium sp. JCM 6894 was a prerequisite for proline uptake which is a function for cell survival. Hydroxyectoine served as an effective stimulator for the proline uptake and cell survival of the downshocked cells of this strain. Duration of osmotic downshock, downshock strength, and the kinds of osmolyte affected the efficient rate of growth (ERG) and the uptake of proline. A shorter duration of osmotic downshock, that is <or=1 h, induced higher ERG values and stimulated proline uptake, compared with those of non-downshocked cells when incubated with hydroxyectoine and proline. These activities, however, were gradually suppressed with the prolongation of the duration of osmotic downshock. When the cells grown in KCl-containing medium were subjected to downshock treatment for 1 h, their activities were enhanced more remarkably than those of cells grown in NaCl-containing medium.

Organic compatible solutes of halotolerant and halophilic microorganisms

Microorganisms that adapt to moderate and high salt environments use a variety of solutes, organic and inorganic, to counter external osmotic pressure. The organic solutes can be zwitterionic, noncharged, or anionic (along with an inorganic cation such as K(+)). The range of solutes, their diverse biosynthetic pathways, and physical properties of the solutes that effect molecular stability are reviewed.

Contribution of chemical changes in membrane lipids to the osmoadaptation of the halophilic bacterium Chromohalobacter salexigens

The long-term response of the broad-salt growing halophile Chromohalobacter salexigens DSM 3043T to salt stress has been investigated with respect to adaptive changes in membrane lipid composition. This study included the wild-type and three salt-sensitive, ectoine-deficient strains: CHR62 (ectA::Tn1732, unable to grow above 0.75 M NaCl), CHR63 (ectC::Tn1732, unable to grow above 1.5 M NaCl), and CHR64, which was able to grow in minimal medium M63 up to 2.5 M NaCl, but its growth was slower than the wild-type strain at salinities above 1.5 M NaCl. This mutant accumulated ectoine and hydroxyectoine as major compatible solutes, but also the ectoine precursor, N-gamma-acetyldiaminobutyric acid, and was found to be affected in the ectoine synthase gene ectC. The main phospholipids of the wild-type strain were phosphatidylethanolamine, phosphatidylglycerol (PG), and cardiolipin (CL). Major fatty acids were detected as 16:0, 18:1, and 16:1, including significant amounts of cyc-19:0, and cyc-17:0. CL and cyclopropane fatty acids (CFA) levels were elevated when the wild-type strain was grown at high salinity (2.5 M NaCl). Membranes of the most salt-sensitive trains CHR62 and CHR63, but not of the less salt-sensitive strain CHR64, contained lower levels of CL. The proportion of cyc-19:0 in CHR64 was three-fold (at 2.0M NaCl) and 2.5-fold (at 2.5 M NaCl) lower than that of the wild type, suggesting that this mutant has a limited capacity to incorporate CFA into phospholipids at high salt. The addition of 1 mM ectoine to cultures of the wild-type strain increased the ratio PG/CL from 1.8 to 3.3 at 0.75 M NaCl, and from 1 to 6.5 at 2.5 M NaCl, and led to a slight decrease in CFA content. Addition of 1 mM ectoine to the mutants restored the steady-state levels of CL and CFA found in the wild-type strain supplemented with ectoine. These findings suggest that exogenous ectoine might attenuate the osmostress response involving changes in membrane lipids.

Complex regulation of the synthesis of the compatible solute ectoine in the halophilic bacterium Chromohalobacter salexigens DSM 3043T

The synthesis of the compatible solute ectoine, mediated by the ectABC gene products, is the main mechanism used by the halophilic bacterium Chromohalobacter salexigens to cope with osmotic stress. Evidence was found that this process is regulated at the transcriptional level. S1 protection analyses performed with RNA extracted from cells grown in minimal medium at low (0.75 M NaCl) or high (2.5 M NaCl) osmolarity suggested the existence of four promoters upstream of ectA. Two of these (PectA1 and PectA2) might be recognized by the main vegetative sigma factor sigma(70), and one (PectA3) might be dependent on the general stress sigma factor sigma(S). The S1 protection assays suggest that PectA1 and PectA3 may be osmoregulated promoters. In addition, an internal promoter showing sequences homologous to promoters dependent on the heat-shock sigma factor sigma(32) was found upstream of ectB. Transcription from PectA in C. salexigens followed a pattern typical of sigma(S)-dependent promoters, and was reduced by 50 % in an E. coli rpoS background. These data strongly suggest the involvement of the general stress sigma factor sigma(S) in ectABC transcription in C. salexigens. Expression of PectA-lacZ and PectB-lacZ trancriptional fusions was very high at low salinity, suggesting that ectABC may be a partially constitutive system. Both transcriptional fusions were induced during continuous growth at high temperature and their expression was reduced in cells grown in the presence of osmoprotectants (ectoine or glycine betaine) or the DNA gyrase inhibitor nalidixic acid. Moreover, PectA-lacZ expression was negatively modulated in cells grown with an excess of iron (FeCl(3)). Measurement of ectoine levels in the presence of glycine betaine at different NaCl concentrations suggests that an additional post-transcriptional control may occur as well.