In vitro reconstitution of osmoregulated expression of proU of Escherichia coli

Bacterial stimulus perception and signal transduction: response to osmotic stress

When exposed to osmotic stress from the environment, bacteria act to maintain cell turgor and hydration by responding both on the level of gene transcription and protein activity. Upon a sudden decrease in external osmolality, internal solutes are released by the action of membrane embedded mechanosensitive channels. In response to an osmotic upshift, the concentration of osmolytes in the cytoplasm is increased both by de novo synthesis and by active uptake. In order to coordinate these processes of osmoregulation, cells are equipped with systems and mechanisms of sensing physical stimuli correlated to changes in the external osmolality (osmosensing), with pathways to transduce these stimuli into useful signals which can be processed in the cell (signal transduction), and mechanisms of regulating proper responses in the cell to recover from the environmental stress and to maintain all necessary physiological functions (osmoregulation). These processes will be described by a number of representative examples, mainly of osmoreactive transport systems with a focus on available data of their molecular mechanism.

The gene ssl3076 encodes a protein mediating the salt-induced expression of ggpS for the biosynthesis of the compatible solute glucosylglycerol in Synechocystis sp. strain PCC 6803

Acclimation to high salt concentrations involves concerted changes in gene expression. For the majority of salt-regulated genes, the mechanism underlying the induction process is not known. The gene ggpS (sll1566), which encodes the glucosylglycerol-phosphate synthase responsible for the synthesis of the compatible solute glucosylglycerol (GG), is specifically induced by salt in the cyanobacterial model strain Synechocystis sp. strain PCC 6803. To identify mechanisms mediating this salt-specific gene regulation, the ggpS promoter was analyzed in more detail. 5' rapid amplification of cDNA ends (5'-RACE) experiments revealed that the adjacent open reading frame (ORF), which is annotated as unknown protein Ssl3076, overlaps with the transcriptional start site of the ggpS gene. Reporter gene expression analyses indicated an essential role for the intact ssl3076 gene in the salt-regulated transcription of a gfp reporter gene. Promoter fragments containing a mutated ssl3076 lost the salt regulation; similarly, a frameshift mutation in ssl3076 resulted in a high level of ggpS expression under low-salt conditions, thereby establishing this small ORF, named ggpR, as a negative regulator of ggpS. Interestingly, small ORFs were also found adjacent to ggpS genes in the genomes of other GG-accumulating cyanobacteria. These results suggest that the GgpR protein represses ggpS expression under low-salt conditions, whereas in salt-shocked and salt-acclimated cells a stress-proportional ggpS expression occurs, leading to GG accumulation.

Differential responses of nitrogen-fixing cyanobacteria to salinity and osmotic stresses

Two nitrogen-fixing Anabaena strains were found to be differentially tolerant to salinity and osmotic stresses. Anabaena torulosa, a brackish-water, salt-tolerant strain, was relatively osmosensitive. Anabaena sp. strain L-31, a freshwater, salt-sensitive strain, on the other hand, displayed significant osmotolerance. Salinity and osmotic stresses affected nitrogenase activity differently. Nitrogen fixation in both of the strains was severely inhibited by the ionic, but not by the osmotic, component of salinity stress. Such differential sensitivity of diazotrophy to salinity-osmotic stresses was observed irrespective of the inherent tolerance of the two strains to salt-osmotic stress. Exogenously added ammonium conferred significant protection against salinity stress but was ineffective against osmotic stress. Salinity and osmotic stresses also affected stress-induced gene expression differently. Synthesis of several proteins was repressed by salinity stress but not by equivalent or higher osmotic stress. Salinity and osmotic stresses induced many common proteins. In addition, unique salt stress- or osmotic stress-specific proteins were also induced in both strains, indicating differential regulation of protein synthesis by the two stresses. These data show that cyanobacterial sensitivity and responses to salinity and osmotic stresses are distinct, independent phenomena.

A novel potassium deficiency-induced stimulon in Anabaena torulosa

Potassium deficiency enhanced the synthesis of fifteen proteins in the nitrogen-fixing cyanobacterium Anabaena torulosa and of nine proteins in Escherichia coli. These were termed potassium deficiency-induced proteins or PDPs and constitute hitherto unknown potassium deficiency-induced stimulons. Potassium deficiency also enhanced the synthesis of certain osmotic stress-induced proteins. Addition of K+ repressed the synthesis of a majority of the osmotic stress-induced proteins and of PDPs in these bacteria. These proteins contrast with the dinitrogenase reductase of A. torulosa and the glycine betaine-binding protein of E. coli, both of which were osmo-induced to a higher level in potassium-supplemented conditions. The data demonstrate the occurrence of novel potassium deficiency-induced stimulons and a wider role of K+ in regulation of gene expression and stress responses in bacteria

Repression by binding of H-NS within the transcription unit

H-NS inhibits transcription by forming repressing nucleoprotein complexes next to promoters. We investigated repression by binding of H-NS within the transcription unit using the bgl and proU operons. Repression of both operons requires a downstream regulatory element (DRE) in addition to an upstream element (URE). In bgl, H-NS binds to a region located between 600 to 700 bp downstream of the transcription start site, whereas in proU the DRE extends up to position +270. We show that binding of H-NS to the bgl-DRE inhibits transcription initiation at a step before open complex formation, as shown before for proU. This was shown by determining the occupancy of the bgl transcription unit by RNA polymerases, expression analysis of bgl and proU reporter constructs, and chloroacetaldehyde footprinting of RNA polymerase promoter complexes. The chloroacetaldehyde footprinting also revealed that RNA polymerase is "poised" at the osmoregulated sigma70-dependent proU promoter at low osmolarity, whereas at high osmolarity poising of RNA polymerase and repression by H-NS are reduced. Furthermore, repression by H-NS via the URE and DRE is synergistic, and the efficiency of repression by H-NS via the DRE inversely correlates with the promoter activity. Repression is high for a promoter of low activity, whereas it is low for a strong promoter. Inefficient repression of strong promoters by H-NS via a DRE may account for high induction levels of proU at high osmolarity and for bgl upon disruption of the URE.

Identification of genes regulated by changing salinity in the deep-sea bacterium Shewanella sp. WP3 using RNA arbitrarily primed PCR

The differential gene transcription of a deep-sea bacterium Shewanella sp. WP3 in response to changing salinity was analyzed by RNA fingerprinting using arbitrarily primed PCR (RAP-PCR). Ninety primer sets were used to scan two different RNA pools derived from cultures of 1% and 7% NaCl concentrations. Forty-three putative differential-expressed fragments were identified, cloned, and sequenced. Six out of the 43 fragments were confirmed to be truly differentially transcribed in terms of changing salinity. The deduced amino acid sequences of the six gene fragments showed highest identities (66-96%) with ribosomal protein L24, ATP binding protein, and chaperon protein HscA of Shewanella oneidensis MR-1 (Y6, Y9, and Y29); isocitrate lyase of Pseudomonas aeruginosa (Y15); peptidylprolyl cis-trans isomerase of Shewanella sp. SIB1 (Y21), glutamine synthetase of Shewanella violacea (Y25), respectively. Four genes (Y6, Y15, Y21, and Y25) were up regulated in 7% NaCl, while the other two (Y9 and Y29) contained more abundant transcripts in 1% NaCl. The data suggested that strategies involved in controlling protein synthesis, protein folding and/or trafficking, glutamate concentration, fatty acid metabolism, and substance transporting were used for salt adaptation in Shewanella sp. WP3. The expression patterns of the six genes in response to transient stress shocks including salt shock (3% NaCl shift to 12%), cold shock (15 degrees C shift to 0 degrees C), and high-hydrostatic pressure shock (0.1 MPa shift to 50 MPa) were further examined. Y29 encoding the putative HscA chaperon protein was indicated to be involved in adaptation of all the stresses tested.

Osmoregulation in Lactococcus lactis: BusR, a transcriptional repressor of the glycine betaine uptake system BusA

The busA (opuA) locus of Lactococcus lactis encodes a glycine betaine uptake system. Transcription of busA is osmotically inducible and its induction after an osmotic stress is reduced in the presence of glycine betaine. Using a genetic screen in CLG802, an Escherichia coli strain carrying a lacZ transcriptional fusion expressed under the control of the busA promoter, we isolated a genomic fragment from the L. lactis subsp. cremoris strain MG1363, which represses transcription from busAp. The cloned locus responsible for this repression was identified as a gene present upstream from the busA operon, encoding a putative DNA binding protein. This gene was named busR. Electrophoretic mobility shift and footprinting experiments showed that BusR is able to bind a site that overlaps the busA promoter. Overexpression of busR in L. lactis reduced expression of busA. Its disruption led to increased and essentially constitutive transcription of busA at low osmolarity. Therefore, BusR is a major actor of the osmotic regulation of busA in L. lactis.

Bacterial osmoadaptation: the role of osmolytes in bacterial stress and virulence

Two general strategies exist for the growth and survival of prokaryotes in environments of elevated osmolarity. The 'salt in cytoplasm' approach, which requires extensive structural modifications, is restricted mainly to members of the Halobacteriaceae. All other species have convergently evolved to cope with environments of elevated osmolarity by the accumulation of a restricted range of low molecular mass molecules, termed compatible solutes owing to their compatibility with cellular processes at high internal concentrations. Herein we review the molecular mechanisms governing the accumulation of these compounds, both in Gram-positive and Gram-negative bacteria, focusing specifically on the regulation of their transport/synthesis systems and the ability of these systems to sense and respond to changes in the osmolarity of the extracellular environment. Finally, we examine the current knowledge on the role of these osmostress responsive systems in contributing to the virulence potential of a number of pathogenic bacteria.

The downstream regulatory element of the proU operon of Salmonella typhimurium inhibits open complex formation by RNA polymerase at a distance

The intracellular concentration of K(+)-glutamate, chromatin-associated proteins, and a downstream regulatory element (DRE) overlapping with the coding sequence, have been implicated in the regulation of the proU operon of Salmonella typhimurium. The basal expression of the proU operon is low, but it is rapidly induced when the bacteria are grown in media of high osmolarity (e.g. 0.3 M NaCl). It has previously been suggested that increased intracellular concentrations of K(+)-glutamate activate the proU promoter in response to increased extracellular osmolarity. We show here that the activation of the proU promoter by K(+)-glutamate in vitro is nonspecific, and the in vivo regulation cannot simply be mimicked in vitro. In vivo specificity requires both the chromatin-associated protein H-NS and the DRE; they are both needed to maintain repression of proU expression at low osmolarity. How H-NS and the DRE repress the proU promoter in vivo has so far been unclear. We show that, in vivo, the DRE acts at a distance to inhibit open complex formation at the proU promoter.

Regulation of compatible solute accumulation in bacteria

In their natural habitats, microorganisms are often exposed to osmolality changes in the environment. The osmotic stress must be sensed and converted into an activity change of specific enzymes and transport proteins and/or it must trigger their synthesis such that the osmotic imbalance can be rapidly restored. On the basis of the available literature, we conclude that representative gram-negative and gram-positive bacteria use different strategies to respond to osmotic stress. The main focus of this paper is on the initial response of bacteria to hyper- and hypo-osmotic conditions, and in particular the osmosensing devices that allow the cell to rapidly activate and/or to synthesize the transport systems necessary for uptake and excretion of compatible solutes. The experimental data allow us to discriminate the transport systems by the physicochemical parameter that is sensed, which can be a change in external osmotic pressure, turgor pressure, membrane strain, internal osmolality and/or concentration of specific signal molecule. We also evaluate the molecular basis for osmosensing by reviewing the unique structural features of known osmoregulated transport systems.

Role of DNA supercoiling and rpoS sigma factor in the osmotic and growth phase-dependent induction of the gene osmE of Escherichia coli K12

Transcription of the gene osmE of Escherichia coli is osmotically inducible and regulated by the growth phase. In a medium of low osmotic pressure, expression of osmE is induced at the onset of stationary phase. At elevated osmotic pressure, a biphasic induction pattern is observed. The first step occurs during exponential phase, and this is followed by a strong induction at the onset of stationary phase. Both steps appear to result from stimulation of transcription at the same promoter, osmEp. In the absence of sigma s, the stationary phase sigma factor encoded by rpoS, osmEp stationary phase induction is abolished, while the osmotic effect is still observed. Mutations that compensate for the absence of sigma s mapped to the gene topA. The effect of such mutation and of novobiocin, an inhibitor of DNA gyrase, suggest that changes in DNA supercoiling are involved in the osmotic induction of osmEp. In addition, modulation of the supercoiling level of a reporter plasmid was observed during growth in rich media. The kinetics of osmEp transcription are discussed in light of the variations of DNA supercoiling.

Osmotically induced response in representatives of halophilic prokaryotes: the bacterium Halomonas elongata and the archaeon Haloferax volcanii

Haloferax volcanii and Halomonas elongata have been selected as representatives of halophilic Archaea and Bacteria, respectively, to analyze the responses to various osmolarities at the protein synthesis level. We have identified a set of high-salt-related proteins (39, 24, 20, and 15.5 kDa in H. elongata; 70, 68, 48, and 16 kDa in H. volcanii) whose synthesis rates increased with increasing salinities. A different set of proteins (60, 42, 15, and 6 kDa for H. elongata; 63, 44, 34, 18, 17, and 6 kDa for H. volcanii), some unique for low salinities, was induced under low-salt conditions. For both organisms, and especially for the haloarchaeon, adaptation to low-salt conditions involved a stronger and more specific response than adaptation to high-salt conditions, indicating that unique mechanisms may have evolved for low-salinity adaptation. In the case of H. volcanii, proteins with a typical transient response to osmotic shock, induced by both hypo- and hyperosmotic conditions, probably corresponding to described heat shock proteins and showing the characteristics of general stress proteins, have also been identified. Cell recovery after a shift to low salinities was immediate in both organisms. In contrast, adaptation to higher salinities in both cases involved a lag period during which growth and general protein synthesis were halted, although the high-salt-related proteins were induced rapidly. In H. volcanii, this lag period corresponded exactly to the time needed for cells to accumulate adequate intracellular potassium concentrations, while extrusion of potassium after the down-shift was immediate. Thus, reaching osmotic balance must be the main limiting factor for recovery of cell functions after the variation in salinity.

The DNA-directed in vitro protein synthesizing system of Salmonella typhimurium: the effect of glutamate substitution

The effect of potassium glutamate was examined on the DNA-directed in vitro protein synthesizing system of Salmonella typhimurium which conventionally contained acetate as a sole counter anion. The glutamate replacement increased the potassium optimum by about 70% and improved the expression of different DNA templates, but selectively. The biggest improvements in expression (about 8-fold) were seen with a lacUV5 (from Escherichia coli) template and with a mutant promoter his operon (from S. typhimurium) template. In contrast, the expression of a leuV promoter (from Escherichia coli) template was relatively unaffected by the glutamate replacement. The chain-growth-rate of mRNA and polypeptide syntheses in the DNA-directed in vitro protein synthesizing system were unaffected by the glutamate replacement. It was concluded that at least a part of the effect of glutamate replacement is on RNA polymerase-promoter interaction, and most likely the association step. Glutamate replacement did not alter the ppGpp-mediated positive and negative regulation of the his and leuV promoter, respectively, in the in vitro system. Taken together, the results suggest that the use of potassium glutamate in place of potassium acetate in DNA-directed in vitro synthesis provides a physiologically more relevant approximation of the ionic environment in vivo.

"Compensatory" organic osmolytes in high osmolarity and dehydration stresses: history and perspectives

As stated in the conclusion, "life is a thing of macromolecular cohesion in salty water." This brief historical overview shows that "compensatory" organic osmolytes take an essential place in this cohesion. It reviews the major steps of the study of these compounds over more than 100 years, from the early beginnings of 1885 until now, showing some of its fascinating developments and ending on the idea that the most fascinating is still to come. This study can be taken as an example of the richness of the comparative approach.

How is osmotic regulation of transcription of the Escherichia coli proU operon achieved? A review and a model

The proU operon in enterobacteria encodes a binding-protein-dependent transporter for the active uptake of glycine betaine and L-proline, and serves an adaptive role during growth of cells in hyperosmolar environments. Transcription of proU is induced 400-fold under these conditions, but the underlying signal transduction mechanisms are incompletely understood. Increased DNA supercoiling and activation by potassium glutamate have each been proposed in alternative models as mediators of proU osmoresponsivity. We review here the available experimental data on proU regulation, and in particular the roles for DNA supercoiling, potassium glutamate, histone-like proteins of the bacterial nucleoid, and alternative sigma factors of RNA polymerase in such regulation. We also propose a new unifying model, in which the pronounced osmotic regulation of proU expression is achieved through the additive effects of at least three separate mechanisms, each comprised of a cis element [two promoters P1 and P2, and negative-regulatory-element (NRE) downstream of both promoters] and distinct trans-acting factors that interact with it: stationary-phase sigma factor RpoS with P1, nucleoid proteins HU and IHF with P2, and nucleoid protein H-NS with the NRE. In this model, potassium glutamate may activate proU expression through each of the three mechanisms whereas DNA supercoiling has a very limited role, if any, in the osmotic induction of proU transcription. We also suggest that proU may be a virulence gene in the pathogenic enterobacteria.

Erwinia chrysanthemi at high osmolarity: influence of osmoprotectants on growth and pectate lyase production

The mechanism of osmotic stress adaptation was investigated in the phytopathogen Erwinia chrysanthemi. Growth of the bacterium was inhibited by elevated medium osmolarity, and exogenous glycine betaine, proline, ectoine or pipecolate permitted recovery of growth at inhibitory osmolarity. Osmoprotectants were taken up by transporters induced by elevated osmolarity, and their level of accumulation within the cell was dependent on the osmolarity of the growth medium. The influence of osmolarity and osmoprotectants on the production of pectate lyases (PLs) was investigated. Increased medium osmolarity resulted first in an induction of PL activity, followed by a shift to the basal level at higher osmolyte concentrations. This induction was reversed by osmoprotectants in the medium. The increased PL activity was attributed in part to the induced transcription of the main PL gene, pelE, and all the osmoprotectants that were analysed were found to prevent pelE induction. PL activity was partially inhibited in vitro by high ionic strength but not by elevated concentrations of sugars, and the addition of osmoprotectants at 1 mM had no effect on PL activity in vitro.

Promoter selectivity control of Escherichia coli RNA polymerase by ionic strength: differential recognition of osmoregulated promoters by E sigma D and E sigma S holoenzymes

Transcription in vitro of two osmoregulated promoters, for the Escherichia coli osmB and osmY genes, was analysed using two species of RNA polymerase holoenzyme reconstituted from purified core enzyme and either sigma D (sigma 70, the major sigma in exponentially growing cells) or sigma S (sigma 38, the principal sigma at stationary growth phase). Under conditions of low ionic strength, the osmB and osmY promoters were transcribed by both E sigma D and E sigma S. Addition of up to 400 mM potassium glutamate (K glutamate), mimicking the intracellular ionic conditions under hyperosmotic stress, specifically enhanced transcription at these promoters by E sigma S but inhibited that by E sigma D. At similar high concentrations of potassium chloride (KCl), however, initiation at both these promoters was virtually undetectable. These data suggest that the RNA polymerase, E sigma S, itself can sense osmotic stress by responding to changes in intracellular K glutamate concentrations and altering its promoter selectivity in order to recognize certain osmoregulated promoters.

Two different Escherichia coli proP promoters respond to osmotic and growth phase signals

proP of Escherichia coli encodes an active transport system for proline and glycine betaine which is activated by both hyperosmolarity and amino acid-limited growth. proP DNA sequences far upstream from the translational start site are strongly homologous to the promoter of proU, an operon that specifies another osmoregulated glycine betaine transport system. Mutation and deletion analysis of proP and primer extension experiments established that this promoter, P1, was responsible for proP's strong expression in minimal medium and its response to osmotic signals. When cells were grown in complex medium, expression from a proP-lacZ fusion was induced three- to fourfold as growth slowed and cells entered stationary phase. Stationary-phase induction was dependent on rpoS, which encodes a stationary-phase sigma factor. Deletion of 158 bp of the untranslated leader sequence between P1 and the proP structural gene abolished rpoS-dependent stationary-phase regulation. Transcription initiation detected by primer extension within this region was absent in an rpoS mutant. proP is therefore a member of the growing class of sigma S-dependent genes which respond to both stationary-phase and hyperosmolarity signals.

The accumulation of glutamate is necessary for optimal growth of Salmonella typhimurium in media of high osmolality but not induction of the proU operon

Synthesis of glutamate can be limited in bacterial strains carrying mutations to loss of function of glutamate synthase (2-oxoglutarate:glutamine aminotransferase) by using low concentrations of NH4+ in the growth medium. By using such gltB/D mutant strains of Salmonella typhimurium, we demonstrated that: (i) a large glutamate pool, previously observed to correlate with growth at high external osmolality, is actually required for optimal growth under these conditions; (ii) the osmoprotectant glycine betaine (N,N,N-trimethylglycine) apparently cannot substitute for glutamate; and (iii) accumulation of glutamate is not necessary for high levels of induction of the proU operon in vivo. Expression of the proU operon, which encodes a transport system for the osmoprotectants proline and glycine betaine, is induced > 100-fold in the wild-type strain under conditions of high external osmolality. Ramirez et al. (R. M. Ramirez, W. S. Prince, E. Bremer, and M. Villarejo, Proc. Natl. Acad. Sci. USA 86:1153-1157, 1989) observed and we confirmed that in vitro expression of the lacZ gene from the wild-type proU promoter is stimulated by 0.2 to 0.3 M K glutamate. However, we observed a very similar stimulation for lacZ expressed from the lacUV5 promoter and from the proU promoter when an important negative regulatory element downstream of this promoter (the silencer) was deleted. Since the lacUV5 promoter is not osmotically regulated in vivo and osmotic regulation of the proU promoter is largely lost as a result of deletion of the silencer, we conclude that stimulation of proU expression by K glutamate in vitro is not a specific osmoregulatory response but probably a manifestation of the optimization of in vitro transcription-translation at high concentrations of this solute. Our in vitro and in vivo results demonstrate that glutamate is not an obligatory component of the transcriptional regulation of the proU operon.

Osmoadaptation in rhizobia: ectoine-induced salt tolerance

After having shown that ectoine (a tetrahydropyrimidine) displays osmoprotective properties towards Escherichia coli (M. Jebbar, R. Talibart, K. Gloux, T. Bernard, and Blanco, J. Bacteriol. 174:5027-5035, 1992), we have investigated the involvement of this molecule in the osmotic adaptation of Rhizobium meliloti. Ectoine appeared almost as effective as glycine betaine in improving the growth of R. meliloti under adverse osmotic conditions (0.5 M NaCl). Moreover, improvement of growth of rhizobial strains insensitive to glycine betaine was also observed. Ectoine transport proved inducible, periplasmic protein dependent, and, as shown by competition experiments, distinct from the transport of glycine betaine. Medium osmolarity had little effect on the uptake characteristics, since the rate of influx increased from 12 to only 20 nmol min-1 mg of protein-1 when NaCl concentrations were raised from 0 to 0.3 or 0.5 M, with a constant of transport of 80 microM. Natural-abundance 13C-nuclear magnetic resonance and radiolabelling assays showed that ectoine, unlike glycine betaine, is not intracellularly accumulated and, as a consequence, does not repress the synthesis of endogenous compatible solutes (glutamate, N-acetylglutaminylglutamine amide, and trehalose). Furthermore, the strong rise in glutamate content in cells osmotically stressed in the presence of ectoine suggests that, instead of being involved in osmotic balance restoration, ectoine should play a key role in triggering the synthesis of endogenous osmolytes. Hence, we believe that there are at least two distinct classes of osmoprotectants: those such as glycine betaine or glutamate, which act as genuine osmolytes, and those such as ectoine, which act as chemical mediators.

The Escherichia coli proU promoter element and its contribution to osmotically signaled transcription activation

The proU operon of Escherichia coli encodes a high-affinity glycine betaine transport system which is osmotically inducible and enables the organism to recover from the deleterious effects of hyperosmotic shock. Regulation occurs at the transcriptional level. KMnO4 footprinting showed that the preponderance of transcription initiated at a single primary promoter region and that proU transcription activation did not occur differentially at alternate promoters in response to various levels of salt shock. Mutational analysis confirmed the location of the primary promoter and identified an extended -10 region required for promoter activity. Specific nucleotides within the spacer, between position -10 and position -35, were important for maximal expression, but every mutant which retained transcriptional activity remained responsive to osmotic signals. A chromosomal 90-bp minimal promoter fragment fused to lacZ was not significantly osmotically inducible. However, transcription from this fragment was resistant to inhibition by salt shock. A mutation in osmZ, which encodes the DNA-binding protein H-NS, derepressed wild-type proU expression by sevenfold but did not alter expression from the minimal promoter. The current data support a model in which the role of the proU promoter is to function efficiently at high ionic strength while other cis-acting elements receive and respond to the osmotic signal.

Adaptation of Escherichia coli to high osmolarity environments: osmoregulation of the high-affinity glycine betaine transport system proU

A sudden increase in the osmolarity of the environment is highly detrimental to the growth and survival of Escherichia coli and Salmonella typhimurium since it triggers a rapid efflux of water from the cell, resulting in a decreased turgor. Changes in the external osmolarity must therefore be sensed by the microorganisms and this information must be converted into an adaptation process that aims at the restoration of turgor. The physiological reaction of the cell to the changing environmental condition is a highly coordinated process. Loss of turgor triggers a rapid influx of K+ ions into the cell via specific transporters and the concomitant synthesis of counterions, such as glutamate. The increased intracellular concentration of K(+)-glutamate allows the adaptation of the cell to environments of moderately high osmolarities. At high osmolarity, K(+)-glutamate is insufficient to ensure cell growth, and the bacteria therefore replace the accumulated K+ ions with compounds that are less deleterious for the cell's physiology. These compatible solutes include polyoles such as trehalose, amino acids such as proline, and methyl-amines such as glycine betaine. One of the most important compatible solutes for bacteria is glycine betaine. This potent osmoprotectant is widespread in nature, and its intracellular accumulation is achieved through uptake from the environment or synthesis from its precursor choline. In this overview, we discuss the properties of the high-affinity glycine betaine transport system ProU and the osmotic regulation of its structural genes.

Molecular characterization of the promoter of osmY, an rpoS-dependent gene

The osmY gene, which encodes a periplasmic protein with an apparent M(r) of 22,000, is induced by both osmotic and growth phase signals. We demonstrate here that osmY expression is regulated at the level of transcription and that transcription initiates 242 nucleotides upstream of the osmY open reading frame. Relative to the transcriptional start site, 5' deletions up to -36 did not inhibit osmY expression. 3' deletions that extended into the untranslated leader region affected the overall level of osmY::lacZ expression but did not affect inducibility. 5' and 3' deletions that extended past the transcriptional start region essentially abolished osmY expression, suggesting that there is a single promoter region. A putative promoter was identified, and its -10 region, TATATT, closely resembles the sigma 70 consensus -10 sequence, TATAAT. However, we show that osmY is not absolutely dependent on a functional sigma 70 for its expression. Since osmY expression does require rpoS (R. Hengge-Aronis, R. Lange, N. Henneberg, and D. Fischer, J. Bacteriol. 175:259-265, 1993), which encodes a stationary-phase sigma factor, sigma S (K. Tanaka, Y. Takayanagi, N. Fujita, A. Ishihama, and H. Takahashi, Proc. Natl. Acad. Sci. USA 90:3511-3515, 1993), E sigma S may be the form of RNA polymerase responsible for transcription of osmY.

Osmolar regulation of endothelin-1 production by rat inner medullary collecting duct

Recent evidence has implicated endothelin-1 (ET-1) as an autocrine inhibitor of inner medullary collecting duct (IMCD) sodium and water transport. The regulators of IMCD ET-1 production are, however, largely unknown. Because of the unique hypertonic environment of the IMCD, the effect of varying extracellular tonicity on IMCD ET-1 production was evaluated. Increasing media osmolality from 300 to 450 mosmol with NaCl or mannitol but not urea caused a marked dose- and time-dependent reduction in ET-1 release by and ET-1 mRNA in cultured rat IMCD cells. In contrast, increasing osmolality had no effect on ET-1 production by rat endothelial or mesangial cells. To see if ET-1 varies in a similar manner in vivo, ET-1 production was assessed in volume expanded (lower medullary tonicity) or volume depleted (high medullary tonicity) rats. Urinary ET-1 excretion and inner medulla ET-1 mRNA were significantly reduced in volume depleted as compared to volume expanded animals. These results indicate that extracellular sodium concentration inhibits ET-1 production specifically in IMCD cells. We speculate that extracellular sodium concentration, via regulation of ET-1 production, provides a link between volume status and IMCD sodium and water reabsorption.

Osmotic repression of anaerobic metabolic systems in Escherichia coli

The influence of the osmolarity of the growth medium on anaerobic fermentation and nitrate respiratory pathways was analyzed. The levels of several enzymes, including formate dehydrogenase, hydrogenase, and nitrate reductase, plus a nickel uptake system were examined, as was the expression of the corresponding structural and regulatory genes. While some functions appear to be only moderately affected by an increase in osmolarity, others were found to vary considerably. An increase in the osmolarity of the medium inhibits both fermentation and anaerobic respiratory pathways, though in a more dramatic fashion for the former. fnr expression is affected by osmolarity, but the repression of anaerobic gene expression was shown to be independent of FNR regulatory protein, at least for hyd-17 and fdhF. This repression could be mediated by the intracellular concentration of potassium and is reversed by glycine betaine.

Osmoprotection of Escherichia coli by ectoine: uptake and accumulation characteristics

Ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid) is a cyclic amino acid, identified as a compatible solute in moderately halophilic bacteria. Exogenously provided ectoine was found to stimulate growth of Escherichia coli in media of inhibitory osmotic strength. The stimulation was independent of any specific solute, electrolyte or nonelectrolyte. It is accumulated in E. coli cells proportionally to the osmotic strength of the medium, and it is not metabolized. Its osmoprotective ability was as potent as that of glycine betaine. The ProP and ProU systems are both involved in ectoine uptake and accumulation in E. coli. ProP being the main system for ectoine transport. The intracellular ectoine pool is regulated by both influx and efflux systems.

osmY, a new hyperosmotically inducible gene, encodes a periplasmic protein in Escherichia coli

A new osmotically inducible gene in Escherichia coli, osmY, was induced 8- to 10-fold by hyperosmotic stress and 2- to 3-fold by growth in complex medium. The osmY gene product is a periplasmic protein which migrates with an apparent molecular mass of 22 kDa on sodium dodecyl sulfate-polyacrylamide gels. A genetic fusion to osmY was mapped to 99.3 min on the E. coli chromosome. The gene was cloned and sequenced, and an open reading frame was identified. The open reading frame encoded a precursor protein with a calculated molecular weight of 21,090 and a mature protein of 18,150 following signal peptide cleavage. Sequencing of the periplasmic OsmY protein confirmed the open reading frame and defined the signal peptide cleavage site as Ala-Glu. A mutation caused by the osmY::TnphoA genetic fusion resulted in slightly increased sensitivity to hyperosmotic stress.

Osmotic adaptation of Escherichia coli with a negligible proton motive force in the presence of carbonyl cyanide m-chlorophenylhydrazone

It has been reported that Escherichia coli is able to grow in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP) when ATP is produced by glycolysis (N. Kinoshita et al., J. Bacteriol. 160:1074-1077, 1984). We investigated the effect of CCCP on the osmotic adaptation of E. coli growing with glucose. When E. coli growing in rich medium containing CCCP was transferred to medium containing sucrose, its growth stopped for a while and then started again. This lag time was negligible in the absence of CCCP. The same results were obtained when the osmolarity was increased by N-methylglucamine-maleic acid. In addition to adapting itself to the hyperosmotic rich medium, E. coli adapted itself to hyperosmolarity in a minimal medium containing CCCP, again with a lag time. Hyperosmotic shock decreased the internal level of potassium ion rather than causing the accumulation of external potassium ion in the presence of CCCP. The internal amount of glutamic acid increased in cells growing in hyperosmotic medium in the presence and absence of CCCP. Large elevations in levels of other amino acids were not observed in the cells adapted to hyperosmolarity. Trehalose was detected only in hyperosmosis-stressed cells in the presence and absence of CCCP. These results suggest that E. coli can adapt to changes in the environmental osmolarity with a negligible accumulation of osmolytes from the external milieu but that the accumulation may promote the adaptation.

Staphylococcus aureus osmoregulation: roles for choline, glycine betaine, proline, and taurine

Choline, glycine betaine, and L-proline enhanced the growth of Staphylococcus aureus at high osmolarity (i.e., they acted as osmoprotectants) on various liquid and solid defined media, while an osmoprotective effect of taurine was shown only for cells growing on high-NaCl solid medium that lacked other osmoprotectants. Potassium pool levels were high, and there was little difference in levels in cells grown at different osmolarities. Glycine betaine accumulated to high levels in osmotically stressed cells, and choline was converted to glycine betaine. Proline and taurine also accumulated in response to osmotic stress but to lower levels than glycine betaine.

Proline transport in Staphylococcus aureus: a high-affinity system and a low-affinity system involved in osmoregulation

L-Proline enhanced the growth of Staphylococcus aureus in high-osmotic-strength medium, i.e., it acted as an osmoprotectant. Study of the kinetics of L-[14C]proline uptake by S. aureus NCTC 8325 revealed high-affinity (Km = 1.7 microM; maximum rate of transport [Vmax] = 1.1 nmol/min/mg [dry weight]) and low-affinity (Km = 132 microM; Vmax = 22 nmol/min/mg [dry weight]) transport systems. Both systems were present in a proline prototrophic variant grown in the absence of proline, although the Vmax of the high-affinity system was three to five times higher than that of the high-affinity system in strain 8325. Both systems were dependent on Na+ for activity, and the high-affinity system was stimulated by lower concentrations of Na+ more than the low-affinity system. The proline transport activity of the low-affinity system was stimulated by increased osmotic strength. The high-affinity system was highly specific for L-proline, whereas the low-affinity system showed a broader substrate specificity. Glycine betaine did not compete with proline for uptake through either system. Inhibitor studies confirmed that proline uptake occurred via Na(+)-dependent systems and suggested the involvement of the proton motive force in creating an Na+ gradient. Hyperosmotic stress (upshock) of growing cultures led to a rapid and large uptake of L-[14C]proline that was not dependent on new protein synthesis. It is suggested that the low-affinity system is involved in adjusting to increased environmental osmolarity and that the high-affinity system may be involved in scavenging low concentrations of proline.

Salt-inducible betaine aldehyde dehydrogenase from sugar beet: cDNA cloning and expression

Members of the Chenopodiaceae, such as sugar beet and spinach, accumulate glycine betaine in response to salinity or drought stress. The last enzyme in the glycine betaine biosynthetic pathway is betaine aldehyde dehydrogenase (BADH). In sugar beet the activity of BADH was found to increase two- to four-fold in both leaves and roots as the NaCl level in the irrigation solution was raised from 0 to 500 mM. This increase in BADH activity was paralleled by an increase in level of translatable BADH mRNA. Several cDNAs encoding BADH were cloned from a lambda gt10 library representing poly(A+) RNA from salinized leaves of sugar beet plants, by hybridization with a spinach BADH cDNA. Three nearly full-length cDNA clones were confirmed to encode BADH by their nucleotide and deduced amino acid sequence identity to spinach BADH; these clones showed minor nucleotide sequence differences consistent with their being of two different BADH alleles. The clones averaged 1.7 kb and contained an open reading frame predicting a polypeptide of 500 amino acids with 83% identity to spinach BADH. RNA gel blot analysis of total RNA showed that salinization to 500 mM NaCl increased BADH mRNA levels four-fold in leaves and three-fold in the taproot. DNA gel blot analyses indicated the presence of at least two copies of BADH in the haploid sugar beet genome.

Multiple mechanisms contribute to osmotic inducibility of proU operon expression in Escherichia coli: demonstration of two osmoresponsive promoters and of a negative regulatory element within the first structural gene

Transcription of the proU operon in Escherichia coli is induced several hundredfold upon growth of cells in media of elevated osmolarity. A low-copy-number promoter-cloning plasmid vector, with lacZ as the reporter gene, was used for assaying the osmoresponsive promoter activity of each of various lengths of proU DNA, generated by cloning of discrete restriction fragments and by an exonuclease III-mediated deletion approach. The results indicate that expression of proU in E. coli is directed from two promoters, one (P2) characterized earlier by other workers with the start site of transcription 60 nucleotides upstream of the initiation codon of the first structural gene (proV), and the other (P1) situated 250 nucleotides upstream of proV. Furthermore, a region of DNA within proV was shown to be involved in negative regulation of proU transcription; phage Mu dII1681-generated lac fusions in the early region of proV also exhibited partial derepression of proU regulation, in comparison with fusions further downstream in the operon. Sequences around promoter P1, sequences around P2, and the promoter-downstream negative regulatory element, respectively, conferred approximately 5-, 8-, and 25-fold osmoresponsivity on proU expression. Within the region genetically defined to encode the negative regulatory element, there is a 116-nucleotide stretch that is absolutely conserved between the proU operons of E. coli and Salmonella typhimurium and has the capability of exhibiting alternative secondary structure. Insertion of this region of DNA into each of two different plasmid vectors was associated with a marked reduction in the mean topological linking number in plasmid molecules isolated from cultures grown in high-osmolarity medium. We propose that this region of DNA undergoes reversible transition to an underwound DNA conformation under high-osmolarity growth conditions and that this transition mediates its regulatory effect on proU expression.

Osmotic induction of the periplasmic trehalase in Escherichia coli K12: characterization of the treA gene promoter

The Escherichia coli treA gene encodes an osmotically inducible periplasmic trehalase. A strain carrying a treA-lacZ transcriptional fusion was constructed. The beta-galactosidase activity produced in this strain growing exponentially in a medium of high osmotic pressure was 10-fold higher than that produced in a medium of low osmotic pressure, demonstrating that treA transcription is osmotically inducible. treA transcriptional induction depends neither on the presence of trehalase itself nor on the synthesis of cytoplasmic trehalose which occurs in response to osmotic stress in wild-type E. coli strains. The treA promoter was identified by S1 nuclease protection. Deletion analysis demonstrated that sequences sufficient for the osmotic induction lie downstream from nucleotide -40 with respect to the transcription start. Transcription initiation at treAp required the presence of a functional sigma 70 subunit of RNA polymerase. treA expression was increased in the presence of a mutation in osmZ, which was previously identified as leading to a partially constitutive expression of the osmotically inducible proU operon.

Characterization of mutations affecting the osmoregulated proU promoter of Escherichia coli and identification of 5' sequences required for high-level expression

Expression of the Escherichia coli proU operon, which encodes an efficient uptake system for the osmoprotectant glycine betaine, is strongly increased in cells grown at high osmolarity. We isolated 182 independent spontaneous mutants with elevated expression of the chromosomal phi(proV-lacZ) (Hyb2) fusion at low osmolarity. Genetic analysis demonstrated that eight of these mutant strains carried mutations closely linked to the fusion, whereas all others carried mutations that appeared to be in osmZ. All of the mutations resulted in increased but still osmoregulated expression of the phi(proV-lacZ)(Hyb2) fusion. The proU-linked mutants carried an identical point mutation (proU603) which changes the -35 sequence of the proU promoter from TTGCCT to TTGACT and thereby increases the homology of the -35 region to the consensus sequence (TTGACA) of E. coli promoters. We also selected for mutants with decreased expression of the plasmid pOS7-encoded phi(proV-lacZ)(Hyb2) fusion and isolated a plasmid with an IS1 insertion (proU607) between the proU -10 and -35 regions. This insertion creates a hybrid promoter and drastically reduces expression of the fusion but does not abolish its osmotic regulation. Deletion analysis of chromosomal sequences 5' to the proU promoter revealed that sequences located approximately 200 bp upstream of the -35 region were required for high-level expression. Removal of these sequences resulted in a 10-fold decline of phi(proV-lacZ)(Hyb2) expression. Osmotic regulation was retained in deletion constructs carrying just 19 bp of chromosomal DNA 5' of the promoter, showing that no sequences further upstream are required for the proper osmoregulation of proU transcription. Experiments with himA and fis mutant strains indicated that the IHF and FIS proteins are not required for the normal osmoregulation of proU expression.

Osmotic signal transduction to proU is independent of DNA supercoiling in Escherichia coli

proU expression has been proposed to form part of a general stress response that is regulated by increased negative DNA supercoiling brought about by environmental signals such as osmotic or anaerobic stress (N. Ni Bhriain, C. J. Dorman, and C. F. Higgins, Mol. Microbiol. 3:933-944, 1989). However, we find that although proU-containing plasmids derived from cells grown in media of elevated osmolarity were more supercoiled than plasmids from cells grown in standard media, they did not activate proU expression in vitro. The gyrA96 mutation and anaerobic conditions are known to affect DNA supercoiling but did not alter proU expression. Finally, the gyrase inhibitors coumermycin and novobiocin did not reduce in vitro proU expression. Therefore, this evidence rules out regulation by changes in DNA superhelicity for proU in Escherichia coli.

Intracellular accumulation of potassium and glutamate specifically enhances survival of Escherichia coli in seawater

The high resistance of Escherichia coli grown in saline media to seawater was suppressed by an osmotic down-shock. The shock released several molecules into the medium, including potassium, glutamate, and glycine betaine when cells were previously grown in the presence of this osmolyte. Incubation of such sensitized cells in a solution containing K+ (80 mM) and glutamate (50 mM) at pH 7.4 restored their resistance to seawater up to a level close to that observed initially. The protective effect was partly due to the rapid accumulation of K+; a significant exponential relationship between intracellular concentration of K+ and resistance to seawater was observed. Glutamate was accumulated more slowly and progressively completed the action of K+. These data emphasize the specific influence of potassium glutamate on osmotically stressed E. coli cells. They confirm that regulation of osmotic pressure and, probably, of intracellular pH strongly enhances survival of E. coli in seawater. Osmotic fluctuations in waters carrying enteric bacteria from intestines to seawater, together with variations in their K+ and amino acid contents, could modify the ability of cells to survive in marine environments. These results demonstrate the need to strictly control conditions (K+ content, temperature) used to wash cells before their transfer to seawater microcosms. They suggest that the K+ and glutamate contents of media in which E. coli cells are transported to the sea can influence their subsequent survival in marine environments.

Characterization of the osmoregulated Escherichia coli proU promoter and identification of ProV as a membrane-associated protein

The Escherichia coli proU operon encodes a high-affinity, binding-protein-dependent transport system for the osmoprotectant glycine betaine. Expression of proU is osmoregulated, and transcription of this operon is greatly increased in cells grown at high osmolarity. Characterization of the proU operon and its promoter provided results similar to those published elsewhere (Gowrishankar, 1989; Stirling et al., 1989). The previously identified proU601 mutation, which leads to increased proU expression both at low- and high osmolarity, is a G to A transition in the Pribnow box of the proU promoter, which increases the homology of the -10 region to the consensus sequence of E. coli promoters. Using an antiserum raised against a ProV-beta-galactosidase hybrid protein, we have identified ProV as a protein associated with the cytoplasmic membrane. This cellular location is consistent with its proposed role as the energy-coupling component of the ProU transport system.

Nucleotide sequence of the transcriptional control region of the osmotically regulated proU operon of Salmonella typhimurium and identification of the 5' endpoint of the proU mRNA

Southern blot analysis of 15 proU transposon insertions in Salmonella typhimurium indicated that this operon is at least 3 kilobase pairs in length. The nucleotide sequence of 1.5-kilobase-pair fragment that contains the transcriptional control region of the proU operon and the coding sequences specifying 290 amino acids of the first structural gene of the operon was determined. The predicted amino acid sequence of the product of this gene shows extensive similarity to the HisP, MalK, and other proteins that are inner membrane-associated components of binding protein-dependent transport systems. S1 mapping and primer extension analysis of the proU mRNAs revealed several species with different 5' ends. Two of these endpoints are sufficiently close to sequences that have weak similarities to the consensus -35 and -10 promoter sequences that they are likely to define two transcription start sites. However, we cannot rule out the possibility that some or all of the 5' endpoints detected arose as a result of the degradation of a longer mRNA. The expression of proU-lacZ operon fusions located on plasmids was normal in S. typhimurium regardless of the plasmid copy number. The sequences mediating normal, osmoregulated expression of the proU operon were shown by subcloning to be contained on an 815-base-pair fragment. A 350-base-pair subclone of this fragment placed onto a lacZ expression vector directed a high-level constitutive expression of beta-galactosidase, suggesting that there is a site for negative regulation in the proU transcriptional control region which has been deleted in the construction of this plasmid.