Osmoregulation in Escherichia coli by accumulation of organic osmolytes: betaines, glutamic acid, and trehalose

Unexpected property of ectoine synthase and its application for synthesis of the engineered compatible solute ADPC

A new cyclic amino acid was detected in a deletion mutant of the moderately halophilic bacterium Halomonas elongata deficient in ectoine synthesis. Using mass spectroscopy (MS) and nuclear magnetic resonance (NMR) techniques, the substance was identified as 5-amino-3,4-dihydro-2H-pyrrole-2-carboxylate (ADPC). We were able to demonstrate that ADPC is the product of a side reaction of lone ectoine synthase (EC 4.2.1.108), which forms ADPC by cyclic condensation of glutamine. This reaction was shown to be reversible. Subsequently, a number of ectoine derivatives, in particular 4,5-dihydro-2-methylimidazole-4-carboxylate (DHMICA) and homoectoine, were also shown to be cleaved by ectoine synthase, which is classified as a hydro-lyase. This study thus reports for the first time that ectoine synthase accepts more than one substrate and is a reversible enzyme able to catalyze both the intramolecular condensation into and the hydrolytic cleavage of cyclic amino acid derivatives. As ADPC supports growth of bacteria under salt stress conditions and stabilizes enzymes against freeze-thaw denaturation, it displays typical properties of compatible solutes. As ADPC has not yet been described as a natural compound, it is presented here as the first man-made compatible solute created through genetic engineering.

Natural and engineered hydroxyectoine production based on the Pseudomonas stutzeri ectABCD-ask gene cluster

We report on the presence of a functional hydroxyectoine biosynthesis gene cluster, ectABCD-ask, in Pseudomonas stutzeri DSM5190(T) and evaluate the suitability of P. stutzeri DSM5190(T) for hydroxyectoine production. Furthermore, we present information on heterologous de novo production of the compatible solute hydroxyectoine in Escherichia coli. In this host, the P. stutzeri gene cluster remained under the control of its salt-induced native promoters. We also noted the absence of trehalose when hydroxyectoine genes were expressed, as well as a remarkable inhibitory effect of externally applied betaine on hydroxyectoine synthesis. The specific heterologous production rate in E. coli under the conditions employed exceeded that of the natural producer Pseudomonas stutzeri and, for the first time, enabled effective hydroxyectoine production at low salinity (2%), with the added advantage of simple product processing due to the absence of other cosolutes.

A blueprint of ectoine metabolism from the genome of the industrial producer Halomonas elongata DSM 2581 T

The halophilic γ-proteobacterium Halomonas elongata DSM 2581^T thrives at high salinity by synthesizing and accumulating the compatible solute ectoine. Ectoine levels are highly regulated according to external salt levels but the overall picture of its metabolism and control is not well understood. Apart from its critical role in cell adaptation to halophilic environments, ectoine can be used as a stabilizer for enzymes and as a cell protectant in skin and health care applications and is thus produced annually on a scale of tons in an industrial process using H. elongata as producer strain. This paper presents the complete genome sequence of H. elongata (4 061 296 bp) and includes experiments and analysis identifying and characterizing the entire ectoine metabolism, including a newly discovered pathway for ectoine degradation and its cyclic connection to ectoine synthesis. The degradation of ectoine (doe) proceeds via hydrolysis of ectoine (DoeA) to Nα-acetyl-l-2,4-diaminobutyric acid, followed by deacetylation to diaminobutyric acid (DoeB). In H. elongata, diaminobutyric acid can either flow off to aspartate or re-enter the ectoine synthesis pathway, forming a cycle of ectoine synthesis and degradation. Genome comparison revealed that the ectoine degradation pathway exists predominantly in non-halophilic bacteria unable to synthesize ectoine. Based on the resulting genetic and biochemical data, a metabolic flux model of ectoine metabolism was derived that can be used to understand the way H. elongata survives under varying salt stresses and that provides a basis for a model-driven improvement of industrial ectoine production.

Preferential osmolyte accumulation: a mechanism of osmotic stress adaptation in diazotrophic bacteria

A common cellular mechanism of osmotic-stress adaptation is the intracellular accumulation of organic solutes (osmolytes). We investigated the mechanism of osmotic adaptation in the diazotrophic bacteria Azotobacter chroococcum, Azospirillum brasilense, and Klebsiella pneumoniae, which are adversely affected by high osmotic strength (i.e., soil salinity and/or drought). We used natural-abundance C nuclear magnetic resonance spectroscopy to identify all the osmolytes accumulating in these strains during osmotic stress generated by 0.5 M NaCl. Evidence is presented for the accumulation of trehalose and glutamate in Azotobacter chroococcum ZSM4, proline and glutamate in Azospirillum brasilense SHS6, and trehalose and proline in K. pneumoniae. Glycine betaine was accumulated in all strains grown in culture media containing yeast extract as the sole nitrogen source. Alternative nitrogen sources (e.g., NH(4)Cl or casamino acids) in the culture medium did not result in measurable glycine betaine accumulation. We suggest that the mechanism of osmotic adaptation in these organisms entails the accumulation of osmolytes in hyperosmotically stressed cells resulting from either enhanced uptake from the medium (of glycine betaine, proline, and glutamate) or increased net biosynthesis (of trehalose, proline, and glutamate) or both. The preferred osmolyte in Azotobacter chroococcum ZSM4 shifted from glutamate to trehalose as a consequence of a prolonged osmotic stress. Also, the dominant osmolyte in Azospirillum brasilense SHS6 shifted from glutamate to proline accumulation as the osmotic strength of the medium increased.

Role of Glycine Betaine and Related Osmolytes in Osmotic Stress Adaptation in Yersinia enterocolitica ATCC 9610

Yersinia enterocolitica is a gram-negative, food-borne pathogen that can grow in 5% NaCl and at refrigerator temperatures. In this report, the compatible solutes (osmolytes) which accumulate intracellularly and confer the observed osmotic tolerance to this pathogen were identified. In minimal medium, glutamate was the only detectable osmolyte that accumulated in osmotically stressed cells. However, when the growth medium was supplemented with glycine betaine, dimethylglycine, or carnitine, the respective osmolyte accumulated intracellularly to high levels and the growth rates of the osmotically stressed cultures improved from 2.4- to 3.5-fold. Chill stress also stimulated the intracellular accumulation of glycine betaine, but the growth rate was only slightly improved by this osmolyte. Both osmotic upshock and temperature downshock stimulated the rate of uptake of [(sup14)C]glycine betaine by more than 30-fold, consistent with other data indicating that the osmolytes are accumulated from the growth medium via transport.

Genome-driven investigation of compatible solute biosynthesis pathways of Pseudomonas syringae pv. syringae and their contribution to water stress tolerance

The foliar pathogen Pseudomonas syringae pv. syringae exhibits an exceptional ability to survive on asymptomatic plants as an epiphyte. Intermittent wetting events on plants lead to osmotic and matric stresses which must be tolerated for survival as an epiphyte. In this study, we have applied bioinformatic, genetic, and biochemical approaches to address water stress tolerance in P. syringae pv. syringae strain B728a, for which a complete genome sequence is available. P. syringae pv. syringae B728a is able to produce the compatible solutes betaine, ectoine, N-acetylglutaminylglutamine amide (NAGGN), and trehalose. Analysis of osmolyte profiles of P. syringae pv. syringae B728a under a variety of in vitro and in planta conditions reveals that the osmolytes differentially contribute to water stress tolerance in this species and that they interact at the level of transcription to yield a hierarchy of expression. While the interruption of a putative gene cluster coding for NAGGN biosynthesis provided the first experimental evidence of the NAGGN biosynthetic pathway, application of this knockout strain and also a gfp reporter gene fusion strain demonstrated the small contribution of NAGGN to cell survival and desiccation tolerance of P. syringae pv. syringae B728a under in planta conditions. Additionally, detailed investigation of ectC, an orphan of the ectoine cluster (lacking the ectA and ectB homologs), revealed its functionality and that ectoine production could be detected in NaCl-amended cultures of P. syringae pv. syringae B728a to which sterilized leaves of Syringa vulgaris had been added.

Transient Accumulation of Glycine Betaine and Dynamics of Endogenous Osmolytes in Salt-Stressed Cultures of Sinorhizobium meliloti

The fate of exogenously supplied glycine betaine and the dynamics of endogenous osmolytes were investigated throughout the growth cycle of salt-stressed cultures of strains of Sinorhizobium meliloti which differ in their ability to use glycine betaine as a growth substrate, but not as an osmoprotectant. We present (sup13)C nuclear magnetic resonance spectral and radiotracer evidence which demonstrates that glycine betaine is only transiently accumulated as a cytoplasmic osmolyte in young cultures of wild-type strains 102F34 and RCR2011. Specifically, these strains accumulate glycine betaine as a preferred osmolyte which virtually prevents the accumulation of endogenous osmolytes during the lag and early exponential phases of growth. Then, betaine levels in stressed cells decrease abruptly during the second half of the exponential phase. At this stage, the levels of glutamate and the dipeptide N-acetylglutaminylglutamine amide increase sharply so that the two endogenous solutes supplant glycine betaine in the ageing culture, in which it becomes a minor osmolyte because it is progressively catabolized. Ultimately, glycine betaine disappears when stressed cells reach the stationary phase. At this stage, wild-type strains of S. meliloti also accumulate the disaccharide trehalose as a third major endogenous osmolyte. By contrast, glycine betaine is always the dominant osmolyte and strongly suppresses the buildup of endogenous osmolytes at all stages of the growth cycle of a mutant strain, S. meliloti GMI766, which does not catabolize this exogenous osmoprotectant under any growth conditions.

Heterologous ectoine production in Escherichia coli: by-passing the metabolic bottle-neck

Transcription of the ectoine biosynthesis genes ectA, ectB and ectC from Marinococcus halophilus in recombinant Escherichia coli DH5α is probably initiated from three individual σ⁷⁰/σ^A-dependent promoter sequences, upstream of each gene. Consequently, mRNA-fragments containing the single genes and combinations of the genes ectA and ectB or ectB and ectC, respectively, could be detected by Northern blot analysis. Under the control of its own regulatory promoter region (ectUp) a seemingly osmoregulated ectoine production was observed. In addition, aspartate kinases were identified as the main limiting factor for ectoine production in recombinant E. coli DH5α. Co-expression of the ectoine biosynthesis genes and of the gene of the feedback-resistant aspartate kinase from Corynebacterium glutamicum MH20-22B (lysC) led to markedly increased production of ectoine in E. coli DH5α, resulting in cytoplasmic ectoine concentrations comparable to those reached via ectoine accumulation from the medium.

Carbohydrate storage in anaerobic sequencing batch reactors

This study demonstrates the accumulation and degradation of trehalose as a storage compound in a glucose-fed anaerobic sequencing batch reactor (ASBR). One hour after substrate addition, only 40% of the added organic matter (as chemical oxygen demand, COD) was accounted for by the cumulative methane production and soluble COD remaining in the reactor. All influent COD was accounted for by methane and biomass production by the end of the 24-h ASBR cycle. These dynamics can be explained by the production of an intracellular storage product. Total carbohydrate analysis showed that 26% of the glucose added to the reactor transiently accumulated within the biomass. Based on (13)C-nuclear magnetic resonance (NMR) analysis, trehalose (alpha-D-glucopyranosyl-(D-glucopyranoside)) was identified as the main carbohydrate produced. Mathematical modeling was performed and the IWA Anaerobic Digestion Model No. 1 (ADM1) was modified to include microbial storage. The modified model adequately described the ASBR dynamics during a 24-h cycle.

Effects of trehalose co-incubation on in vitro matured prepubertal ovine oocyte vitrification

Our aim was to evaluate if loading prepubertal ovine oocyte with trehalose would impact on their further developmental potential in vitro and if it would improve their survival to vitrification procedures. COCs matured in vitro with (TRH) or without (CTR) 100mM trehalose were tested for developmental potential after in vitro fertilization and culture. Trehalose uptake was measured by the antrone spectrophotometric assay. No differences were recorded between the two experimental groups in fertilization rates (91.1 CTR vs 92.5% TRH), cleavage rates calculated on fertilized oocytes (96.1 CTR vs 95.4% TRH), first cleavage kinetic (56.1 CTR vs 51% TRH), and blastocyst rates (14.3 CTR vs 13.0% TRH). Anthrone assay revealed that in TRH group trehalose concentration/oocyte was 2.6microM. MII oocytes were then vitrified using cryoloops in TCM 199 containing 20% FCS, sucrose 0.5M, 16.5% Me(2)SO, 16.5% EG and plunged in LN(2). After warming, oocytes from TRH and CTR groups were tested for membrane integrity using the propidium iodide (PI)/Hoechst differential staining, and for developmental ability after in vitro fertilization. Trehalose in maturation medium affected membrane resistance (P<0.01) to vitrification/warming but not fertilization and cleavage rates. The differential staining showed a lower number of PI positive cells in TRH group compared to CTR one (14.3 vs 24.7%, respectively). Fertilization rates and cleavage rates did not differ between the two groups (55.3 and 41% for TRH and 47.7 and 41.7% for CTR, respectively). In conclusion trehalose in maturation medium stabilizes cell membranes during vitrification/warming of prepubertal ovine oocytes but does not affect fertilization and cleavage rates after warming.

Endogenously synthesized (-)-proto-quercitol and glycine betaine are principal compatible solutes of Schizochytrium sp. strain S8 (ATCC 20889) and three new isolates of phylogenetically related thraustochytrids

We report that endogenously synthesized (-)-proto-quercitol (1D-1,3,4/2,5-cyclohexanepentol) and glycine betaine were the principal compatible solutes of Schizochytrium sp. strain S8 (ATCC 20889) and three new osmotolerant isolates of thraustochytrids (strains T65, T66, and T67). The compatible solutes were identified and quantified by use of nuclear magnetic resonance spectroscopy, and their identity was confirmed by mass spectroscopy and measurement of the specific optical rotation. The cellular content of compatible solutes increased with increasing NaCl concentration of a defined medium. (-)-proto-Quercitol was the dominating solute at all NaCl concentrations tested (0.25 to 1.0 M), e.g., cells of S8 and T66 stressed with 1.0 M NaCl accumulated about 500 micromol (-)-proto-quercitol and 100 micromol glycine betaine per g dry weight. To our knowledge, (-)-proto-quercitol has previously been found only in eucalyptus. The 18S rRNA gene sequences of the four (-)-proto-quercitol-producing strains showed 99% identity, and they displayed the same fatty acid profile. The only polyunsaturated fatty acids accumulated were docosahexaenoic acid (78%) and docosapentaenoic acid (22%). A less osmotolerant isolate (strain T29), which was closely phylogenetically related to Thraustochytrium aureum (ATCC 34304), did not contain (-)-proto-quercitol or glycine betaine. Thus, the level of osmotolerance and the osmolyte systems vary among thraustochytrids.

Identification of proteins involved in osmotic stress response in Enterobacter sakazakii by proteomics

Enterobacter sakazakii is considered an opportunistic food-borne pathogen, causing rare but significant illness especially in neonates. It has been proposed that the organism is relatively resistant to osmotic and dry stress compared to other species of the Enterobacteriaceae group. To understand the mechanisms involved in osmotic stress response, 2-DE protein analysis coupled to MALDI-TOF MS was employed to investigate changes in the protein profiles of E. sakazakii cells in response to two different types of osmotic stress (physical desiccation and growth in hyperosmotic media). In total, 80 differentially expressed protein spots corresponding to 53 different protein species were identified. Affiliation of proteins to functional categories revealed that a considerable number of the differentially expressed proteins from desiccated and hyperosmotic grown samples belonged to the same functional category but were regulated in opposite directions. Our data show that the protein pattern of NaCl-grown cultures reflect more or less a general down-regulation of central metabolic pathways, whereas adaptation of (non-growing) cells in a desiccated state represents an accumulation of proteins that serve some structural or protective role. The most striking effects observed for both types of osmotic stress in E. sakazakii were a significant down-regulation of the motility apparatus and the formation of filamentous cells.

Escherichia coli AspP activity is enhanced by macromolecular crowding and by both glucose-1,6-bisphosphate and nucleotide-sugars

Escherichia coli ADP-sugar pyrophosphatase (AspP) is a "Nudix" hydrolase that catalyzes the hydrolytic breakdown of ADP-glucose linked to glycogen biosynthesis. Moderate increases of AspP activity in the cell are accompanied by significant reductions of the glycogen content. In vitro analyses showed that AspP activity is strongly enhanced by macromolecular crowding and by both glucose-1,6-bisphosphate and nucleotide-sugars, providing a first set of indicative evidences that AspP is a highly regulated enzyme. To our knowledge, AspP is the sole bacterial enzyme described to date which is activated by both G1,6P(2) and nucleotide-sugars.

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.

NhaD type sodium/proton-antiporter of Halomonas elongata: a salt stress response mechanism in marine habitats?

Background

Sodium/proton-antiporters (Nha) are known to play an important role in pH- and Na⁺-homeostasis. In microorganisms several types with different capacity, affinity and selectivity for Na⁺ and Li⁺ exist. The homeostasis system of E. coli, NhaA and NhaB, is well researched, but the function of other types of Na⁺/H⁺-antiporters like NhaD is yet to be fully understood. Since several antiporters play an important role at various points in the physiology of higher organisms, one can speculate that the main functions of some of those procaryotic antiporters differ from pH- and Na⁺-homeostasis.

Results

This study investigates the function and regulation of a gene encoding for a NhaD type antiporter which was discovered in the halophilic eubacterium Halomonas elongata.

The deduced primary amino acid sequence of the abovementioned gene showed more than 60% identity to known antiporters of the NhaD type from Alkalimonas amylolytica, Shewanella oneidensis and several other marine organisms of the γ-Proteobacteria. Evidence was found for a dual regulation of H. elongata NhaD expression. The gene was cloned and expressed in E. coli. Antiporter deficient NaCl and LiCl sensitive E. coli mutants EP432 and KNabc were partially complemented by a plasmid carrying the H. elongata nhaD gene. Surprisingly the LiCl sensitivity of E. coli strain DH5α having a complete homeostasis system was increased when NhaD was co-expressed.

Conclusion

Since NhaD is an antiporter known so far only from halophilic or haloalcaliphilic Proteobacteria one can speculate that this type of antiporter provides a special mechanism for adaptation to marine habitats. As was already speculated – though without supporting data – and substantiated in this study this might be active Na⁺-import for osmoregulatory purposes.

Transposon mutations in the 5' end of glnD, the gene for a nitrogen regulatory sensor, that suppress the osmosensitive phenotype caused by otsBA lesions in Escherichia coli

GlnD of Escherichia coli is a bifunctional signal-transducing enzyme (102.4 kDa) which uridylylates the allosteric regulatory protein PII and deuridylylates PII-UMP in response to growth with nitrogen excess or limitation, respectively. GlnD catalyzes these reactions in response to high or low levels of cytoplasmic glutamine, respectively, and indirectly directs the expression of nitrogen-regulated genes, e.g., the glnK-amtB operon. We report that chromosomal mini-Tn10 insertions situated after nucleotide number 997 or 1075 of glnD partially suppressed the osmosensitive phenotype of DeltaotsBA or otsA::Tn10 mutations (defective osmoregulatory trehalose synthesis). Strains carrying these glnD::mini-Tn10 mutations either completely repressed the expression of trp::(glnKp-lacZ) or induced this reporter system to nearly 60% of the wild-type glnD level in response to nitrogen availability, an essentially normal response. This was in contrast to the much-studied glnD99::Tn10 mutation, which carries its insertion in the 3' end of the gene, causes a complete repression of glnKp-lacZ expression under all growth conditions, and also confers leaky glutamine auxotrophy. When expressed from the Pm promoter in plasmid constructs, the present glnD mutations produced proteins with an apparent mass of 39 or 42 kDa. These proteins were deduced to comprise 344 or 370 N-terminal residues, respectively, harboring the known nucleotidyltransferase domain of GlnD, plus a common C-terminal addition of 12 residues encoded by IS10. They lacked three other domains of GlnD. Apparently, the transferase domain by itself enabled the cells to catalyze the uridylylation reaction and direct nitrogen-regulated gene expression. Our data indicate that there exists a link between osmotic stress and the nitrogen response.

Glutathione in bacteria

Glutathione metabolism and its role in vital functions of bacterial cells are considered, as well as common features and differences between the functions of glutathione in prokaryotic and eukaryotic cells. Particular attention is given to the recent data for the role of glutathione in bacterial redox-regulation and adaptation to stresses.

Osmolyte-induced transcription: -35 region elements and recognition by sigma38 (rpoS)

In order to meet osmotic challenges in the gastrointestinal tract, enteric bacteria rapidly accumulate salts of glutamate and other weak organic acids. The ensuing transcriptional activation is mediated by unknown elements at sigma38 (rpoS)-dependent promoters. Here we identify DNA elements needed for high levels of transcription in the presence of salt and acetate and show that they are associated with the -35 regions of target promoters. Unrelated -35 region sequences are shown to specify maximal salt-challenged transcription at the otsB promoter and maximal acetate-challenged transcription at the cfa promoter. Mutants in sigma38 are isolated that contribute to bypassing the salt response and most of these cluster in a small segment corresponding to the presumptive -35 DNA recognition determinant of the protein. Overall, the data suggest that an ensemble of -35 region elements exists at sigma38 promoters and these can help mediate responsiveness to physiological challenges through interactions involving region 4 of the sigma38 protein.

Characterization of the ectoine biosynthesis genes of haloalkalotolerant obligate methanotroph “Methylomicrobium alcaliphilum 20Z”

The genes involved in biosynthesis of the major compatible solute ectoine (1,4,5,6-tetrahydro-2-methylpyrimidine carboxylic acid) in halotolerant obligate methanotroph "Methylomicrobium alcaliphilum 20Z" were studied. The complete nucleotide sequences of the structural genes encoding L: -aspartokinase (Ask), L-2,4-diaminobutyric acid transaminase (EctB), L-2,4-diaminobutyric acid acetyltransferase (EctA), and L-ectoine synthase (EctC) were defined and shown to be transcribed as a single operon ectABCask. Phylogenetic analysis revealed high sequence identities (34-63%) of the Ect proteins to those from halophilic heterotrophs with the highest amino acid identities being to Vibrio cholerae enzymes. The chromosomal DNA fragment from "M. alcaliphilum 20Z" containing ectABC genes and putative promoter region was expressed in Escherichia coli. Recombinant cells could grow in the presence of 4% NaCl and synthesize ectoine. The data obtained suggested that despite the ectoine biosynthesis pathway being evolutionary well conserved with respect to the genes and enzymes involved, some differences in their organization and regulation could occur in various halophilic bacteria.

Gadolinium ion inhibits loss of metabolites induced by osmotic shock and large stretch-activated channels in bacteria

Bacteria subjected to a hypotonic osmotic shock lose internal ions and also metabolites, without lysis of the cells. We show that the presence in the shock medium, at submillimolar concentrations, of the ion gadolinium, recently shown to block stretch-activated channels in Xenopus oocytes [Yang, X.-C. & Sachs, F. (1989) Science 243, 1068-1071], was sufficient to inhibit shock-induced release of metabolites such as lactose and ATP in Escherichia coli and ATP in Streptococcus faecalis. Moreover, gadolinium was observed, in patch-clamp experiments, to inhibit the giant stretch-activated channels of E. coli, S. faecalis. and Bacillus subtilis. Taken together, these data suggest that stretch-activated channels are localized in the cytoplasmic membrane of Gram-negative and Gram-positive bacteria, where they control the efflux of osmotic solutes, thus probably playing a major role in the response to hypotonic osmotic shock.

Dehydration-induced tps gene transcripts from an anhydrobiotic nematode contain novel spliced leaders and encode atypical GT-20 family proteins

Accumulation of the non-reducing disaccharide trehalose is associated with desiccation tolerance during anhydrobiosis in a number of invertebrates, but there is little information on trehalose biosynthetic genes in these organisms. We have identified two trehalose-6-phosphate synthase (tps) genes in the anhydrobiotic nematode Aphelenchus avenae and determined full length cDNA sequences for both; for comparison, full length tps cDNAs from the model nematode, Caenorhabditis elegans, have also been obtained. The A. avenae genes encode very similar proteins containing the catalytic domain characteristic of the GT-20 family of glycosyltransferases and are most similar to tps-2 of C. elegans; no evidence was found for a gene in A. avenae corresponding to Ce-tps-1. Analysis of A. avenae tps cDNAs revealed several features of interest, including alternative trans-splicing of spliced leader sequences in Aav-tps-1, and four different, novel SL1-related trans-spliced leaders, which were different to the canonical SL1 sequence found in all other nematodes studied. The latter observation suggests that A. avenae does not comply with the strict evolutionary conservation of SL1 sequences observed in other species. Unusual features were also noted in predicted nematode TPS proteins, which distinguish them from homologues in other higher eukaryotes (plants and insects) and in micro-organisms. Phylogenetic analysis confirmed their membership of the GT-20 glycosyltransferase family, but indicated an accelerated rate of molecular evolution. Furthermore, nematode TPS proteins possess N- and C-terminal domains, which are unrelated to those of other eukaryotes: nematode C-terminal domains, for example, do not contain trehalose-6-phosphate phosphatase-like sequences, as seen in plant and insect homologues. During onset of anhydrobiosis, both tps genes in A. avenae are upregulated, but exposure to cold or increased osmolarity also results in gene induction, although to a lesser extent. Trehalose seems likely therefore to play a role in a number of stress responses in nematodes.

Osmosensitivity associated with insertions in argP (iciA) or glnE in glutamate synthase-deficient mutants of Escherichia coli

An ampicillin enrichment strategy following transposon insertion mutagenesis was employed to obtain NaCl-sensitive mutants of a gltBD (glutamate synthase [GOGAT]-deficient) strain of Escherichia coli. It was reasoned that the gltBD mutation would sensitize the parental strain even to small perturbations affecting osmotolerance. Insertions conferring an osmosensitive phenotype were identified in the proU, argP (formerly iciA), and glnE genes encoding a glycine betaine/proline transporter, a LysR-type transcriptional regulator, and the adenylyltransferase for glutamine synthetase, respectively. The gltBD+ derivatives of the strains were not osmosensitive. The argP mutation, but not the glnE mutation, was associated with reduced glutamate dehydrogenase activity and a concomitant NH4+ assimilation defect in the gltBD strain. Supplementation of the medium with lysine or a lysine-containing dipeptide phenocopied the argP null mutation for both osmosensitivity and NH4+ assimilation deficiency in a gltBD background, and a dominant gain-of-function mutation in argP was associated with suppression of these lysine inhibitory effects. Osmosensitivity in the gltBD strains, elicited either by lysine supplementation or by introduction of the argP or glnE mutations (but not proU mutations), was also correlated with a reduction in cytoplasmic glutamate pools in cultures grown at elevated osmolarity. We propose that an inability to accumulate intracellular glutamate at high osmolarity underlies the osmosensitive phenotype of both the argP gltBD and glnE gltBD mutants, the former because of a reduction in the capacity for NH4+ assimilation into glutamate and the latter because of increased channeling of glutamate into glutamine.

Osmo-regulation of bacterial transcription via poised RNA polymerase

Adaptation to high-salt environments is critical for the survival of a wide range of cells, especially for pathogenic bacteria that colonize the animal gut and urinary tract. The adaptation strategy involves production of the salt potassium glutamate, which induces a specific gene expression program that produces electro-neutral osmolytes while inhibiting general sigma(70) transcription. These data show that in Escherichia coli potassium glutamate stimulates transcription by disengaging inhibitory polymerase interactions at a sigma(38) promoter. These occur in an upstream region that is marked by an osmotic shock promoter DNA consensus sequence. The disruption activates a poised RNA polymerase to transcribe. This transcription program leads to the production of osmolytes that are shown to have only minor effects on transcription and therefore help to restore normal cell function. An osmotic shock gene expression cycle is discussed.

Desiccation and heat tolerance of Enterobacter sakazakii

AIMS

Enterobacter sakazakii is an opportunistic pathogen which has been isolated at low levels from powdered infant formulas. This study was performed to demonstrate that Ent. sakazakii is not particularly thermotolerant, but can adapt to osmotic and dry stress.

METHODS AND RESULTS

We determined the heat, osmotic and dry stress resistance of Ent. sakazakii. The D-value at 58 degrees C ranged from 0.39 to 0.60 min, which is comparable with that of other Enterobacteriaceae, but much lower than reported previously (Nazarowec-White and Farber 1997, Letters in Applied Microbiology 24: 9-13). However, stationary phase Ent. sakazakii cells were found to be more resistant to osmotic and dry stress than Escherichia coli, Salmonella and other strains of Enterobacteriaceae tested. Further analysis indicated that the dry resistance is most likely linked to accumulation of trehalose in the cells.

CONCLUSIONS

The high tolerance to desiccation provides a competitive advantage for Ent. sakazakii in dry environments, as found in milk powder factories, and thereby increases the risk of postpasteurization contamination of the finished product.

SIGNIFICANCE AND IMPACT OF THE STUDY

Understanding of the physiology and survival strategies of Ent. sakazakii is an important step in the efforts to eliminate this bacterium from the critical food production environments.

Quantitative microinjection of trehalose into mouse oocytes and zygotes, and its effect on development

Sugars such as trehalose are effectively used by various organisms as protective agents to undergo anhydrobiosis and cryobiosis. The objective of this study was first to establish a method for quantitative delivery of trehalose as a model sugar into oocytes, and then to evaluate its effect on development of mouse zygotes. To this end, a quantitative microinjection technique was developed using volumetric response of microdroplets suspended in dimethylpolysilaxene. To verify accuracy of this technique, both microdroplets and oocytes were microinjected with fluorophore-labeled dextran. Thereafter, injection volumes were calculated from fluorescence intensity, and volumetric responses of both microdroplets and oocytes. Comparison of calculated injection volumes revealed that this technique reflects microinjection into oocytes with pL-accuracy. The next series of experiments focused on toxicity of injection buffers (i.e., 10mM Tris and 15mM Hepes) and trehalose. Microinjection of Hepes and Tris buffer in the presence of 0.1M trehalose resulted in blastocyst rates of 86 and 72%, respectively, without a significant difference when compared to controls (86%). In subsequent experiments, Hepes was used as the injection buffer, and embryonic development of zygotes was studied as a function of intracellular trehalose concentrations. Microinjection of trehalose up to 0.15M resulted in development to blastocyst stage similar to controls (85 and 87%, respectively) while the blastocyst rate was significantly decreased (43%) in the presence of 0.20M intracellular trehalose. When transferred to foster mothers, trehalose-injected zygotes (0.1M) implanted and developed to day 16 fetuses similar to controls, healthy pups were born. The findings of this study suggest that trehalose at effective intracellular concentrations does not impair development of mouse zygotes.

Hydroxyectoine is superior to trehalose for anhydrobiotic engineering of Pseudomonas putida KT2440

Anhydrobiotic engineering aims to increase the level of desiccation tolerance in sensitive organisms to that observed in true anhydrobiotes. In addition to a suitable extracellular drying excipient, a key factor for anhydrobiotic engineering of gram-negative enterobacteria seems to be the generation of high intracellular concentrations of the nonreducing disaccharide trehalose, which can be achieved by osmotic induction. In the soil bacterium Pseudomonas putida KT2440, however, only limited amounts of trehalose are naturally accumulated in defined high-osmolarity medium, correlating with relatively poor survival of desiccated cultures. Based on the enterobacterial model, it was proposed that increasing intracellular trehalose concentration in P. putida KT2440 should improve survival. Using genetic engineering techniques, intracellular trehalose concentrations were obtained which were similar to or greater than those in enterobacteria, but this did not translate into improved desiccation tolerance. Therefore, at least for some populations of microorganisms, trehalose does not appear to provide full protection against desiccation damage, even when present at high concentrations both inside and outside the cell. For P. putida KT2440, it was shown that this was not due to a natural limit in desiccation tolerance since successful anhydrobiotic engineering was achieved by use of a different drying excipient, hydroxyectoine, with osmotically preconditioned bacteria for which 40 to 60% viability was maintained over extended periods (up to 42 days) in the dry state. Hydroxyectoine therefore has considerable potential for the improvement of desiccation tolerance in sensitive microorganisms, particularly for those recalcitrant to trehalose.

The substrate-binding protein TeaA of the osmoregulated ectoine transporter TeaABC from Halomonas elongata: purification and characterization of recombinant TeaA

TeaABC is a novel, osmoregulated secondary transport system for compatible solutes found in the halophilic proteobacterium Halomonas elongata DSM 2581(T). Sequence comparisons have confirmed that TeaABC belongs to the family of TRAP transporters and as such it consists of two putative transmembrane proteins (TeaB, TeaC) and a putative periplasmic substrate-binding protein (TeaA). TeaABC is the only osmoregulated transporter for ectoines found in H. elongata. By overexpressing TeaA-(6)His tag in Escherichia coli we demonstrated that TeaA is processed as predicted and exported to the periplasm. Furthermore, it was proven that TeaA is indeed a periplasmic ectoine-binding protein. The functionality of the purified protein as an ectoine-binding protein was tested employing a modified binding assay.

Role of trehalose biosynthesis in environmental survival and virulence of Salmonella enterica serovar typhimurium

The otsA and otsB genes, encoding trehalose-6-phosphate synthase and trehalose-6-phosphate phosphatase respectively, have been isolated from Salmonella enterica serovar typhimurium and nucleotide-sequenced. Induction of trehalose biosynthesis by exposure of bacteria to high osmotic strength resulted in the intracellular accumulation of trehalose. An otsA mutant of S. enterica serovar typhimurium was more susceptible to killing by heat, and grew poorly under conditions of high osmolarity. The wild-type and otsA mutant strains showed similar abilities to colonise spleen tissues after oral dosing of mice. These findings suggest that the otsBA gene products play a role in environmental survival, but not in virulence, of S. enterica serovar typhimurium.

New type of osmoregulated solute transporter identified in halophilic members of the bacteria domain: TRAP transporter TeaABC mediates uptake of ectoine and hydroxyectoine in Halomonas elongata DSM 2581(T)

The halophilic bacterium Halomonas elongata synthesizes as its main compatible solute the aspartate derivative ectoine. We constructed a deletion mutant of H. elongata, KB1, defective in ectoine synthesis and tolerating elevated salt concentrations only in the presence of external compatible solutes. The dependency of KB1 on solute uptake for growth in high-salt medium was exploited to select insertion mutants unable to accumulate external solutes via osmoregulated transporters. One insertion mutant out of 7,200 failed to accumulate the osmoprotectants ectoine and hydroxyectoine. Genetic analysis of the insertion site proved that the mutation affected an open reading frame (ORF) of 1,281 bp (teaC). The nucleotide sequence upstream of teaC was determined, and two further ORFs of 603 bp (teaB) and 1,023 bp (teaA) were identified. Deletion of teaA and teaB proved that all three genes are mandatory for ectoine uptake. Sequence comparison showed significant identity of TeaA, TeaB, and TeaC to the transport proteins of the recently identified tripartite ATP-independent periplasmic transporter family (TRAP-T). The affinity of the cells for ectoines was determined (K(s) = 21.7 microM), suggesting that the transporter TeaABC exhibits high affinity for ectoines. An elevation of the external osmolarity resulted in a strong increase in ectoine uptake via TeaABC, demonstrating that this transporter is osmoregulated. Deletion of teaC and teaBC in the wild-type strain led to mutants which excreted significant amounts of ectoine into the medium when cultivated at high salt concentrations. Therefore, the physiological role of TeaABC may be primarily to recover ectoine leaking through the cytoplasmic membrane.

Osmotic stress signaling and osmoadaptation in yeasts

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

Osmoregulation and its importance to food-borne microorganisms

The control of water activity has been used as a means of preserving foods for thousands of years. This preservation strategy presents food-borne microorganisms with serious problems, many of which relate to the management of water flow. Although the specific details of how each organism deals with these problems are different, several common themes have emerged. Bacteria induce specific responses. both physiological and genetic, to respond to either the loss or the gain of water, triggered by changes in the osmolarity of the environment. Many of the key systems have now been identified and the mechanisms of their regulation are beginning to be understood. Here we review recent developments in the field of bacterial osmoregulation with emphasis on key food-borne genera.

Analysis of the kefA2 mutation suggests that KefA is a cation-specific channel involved in osmotic adaptation in Escherichia coli

Mechanosensitive channels play an essential role in the regulation of turgor pressure in bacteria. In Escherichia coli, there are multiple mechanosensitive channels that have been characterized genetically: MscL, YggB and KefA. In this report, we describe the cloning of the kefA gene, the organization of the KefA protein and the phenotype of a missense mutation, kefA, which affects the KefA mechanosensitive channel. The altered function of the channel is manifest through increased sensitivity to K+ during growth at low osmolarity and complete inhibition of growth in media containing high K+ concentrations (0.6 M) in the presence of betaine or proline. Growth in high Na+ medium (0.6 M NaCl plus 20 mM K+) is normal. Analysis of the cytoplasmic pools shows that the mutant cannot regulate the K+ content of the cytoplasm when grown in high K+ medium. However, regulation of pools of amino acids is essentially normal and the mutant can accumulate high pools of proline during growth inhibition. The mutant shows increased sensitivity to acid hypo-osmotic shock (transition from neutral to acid pH combined with a reduction in osmolarity). The data are consistent with abnormal regulation of KefA in the presence of high K+ concentrations and either betaine or proline.

Role of trehalose in growth at high temperature of Salmonella enterica serovar Typhimurium

Moderate osmolality can stimulate bacterial growth at temperatures near the upper limit for growth. We investigated the mechanism by which high osmolality enhances the thermotolerance of Salmonella enterica serovar Typhimurium, by isolating bacteriophage MudI1734-induced insertion mutations that blocked the growth-stimulatory effect of 0.2 M NaCl at 45 degrees C. One of these mutations proved to be in the seqA gene (a regulator of initiation of DNA synthesis). Because this gene is cotranscribed with pgm (which encodes phosphoglucomutase), it is likely to be polar on the expression of the pgm gene. Pgm catalyzes the conversion of glucose-6-phosphate to glucose-1-phosphate during growth on glucose, and therefore loss of Pgm results in a deficiency in a variety of cellular constituents derived from glucose-1-phosphate, including trehalose. To test the possibility that the growth defect of the seqA::MudI1734 mutant at high temperature in medium of high osmolality is due to the block in trehalose synthesis, we determined the effect of an otsA mutation, which inactivates the first step of the trehalose biosynthetic pathway. The otsA mutation caused a growth defect at 45 degrees C in minimal medium containing 0.2 M NaCl that was similar to that caused by the pgm mutation, but otsA did not affect growth rate in this medium at 37 degrees C. These results suggest that the growth defect of the seqA-pgm mutant at high temperature could be a consequence of the block in trehalose synthesis. We found that, in addition to the well-known osmotic control, there is a temperature-dependent control of trehalose synthesis such that, in medium containing 0.2 M NaCl, cells grown at 45 degrees C had a fivefold higher trehalose pool size than cells grown at 30 degrees C. Our observations that trehalose accumulation is thermoregulated and that mutations that block trehalose synthesis cause a growth defect at high temperature in media of high osmolality suggested that this disaccharide is crucial for growth at high temperature either for turgor maintenance or for protein stabilization.

Synthesis, accumulation and hydrolysis of trehalose during growth of peanut rhizobia in hyperosmotic media

We examined and compared the activities of synthetic and hydrolytic enzymes involved in trehalose metabolism, in three peanut rhizobia strains grown in control, hypersaline, and non-ionic hyperosmotic media. Results indicated that the effects of hyperosmolarity on the synthesis and the degradation of the disaccharide were diverse. In the salt-tolerant slow-growing strain Bradyrhizobium sp. ATCC 10317, we observed increased synthesis and accumulation of trehalose under hyperosmolarity imposed by either NaCl or PEG-8000. In the other two peanut rhizobia strains, the disaccharide level did not change under hypersalinity. In the salt-sensitive slow-growing strain Bradyrhizobium sp. USDA 3187, intracellular trehalose diminished in late stationary phase-cells grown with PEG, this reduction was accompanied by both an increased activity of synthetic enzymes and a decreased activity of trehalase. In the salt-tolerant fast-growing strain Rhizobium sp. TAL 1000, we also observed a reduction of intracellular trehalose under PEG-mediated growth, this decrease was early and transiently accompanied by an enhancement of trehalase activity, afterwards, the activity of synthetic enzymes augmented.

Intracellular trehalose improves osmotolerance but not desiccation tolerance in mammalian cells

Trehalose has been shown to play a role in osmotolerance or desiccation tolerance in some microorganisms, anhydrobiotic invertebrates and resurrection plants. To test whether trehalose could improve stress responses of higher eukaryotes, a mouse cell line was genetically engineered to express bacterial trehalose synthase genes. We report that the resulting levels of intracellular trehalose ( approximately 80 mM) are able to confer increased resistance to the partial dehydration resulting from hypertonic stress, but do not enable survival of complete desiccation due to air drying.